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Biomechanical analysis of foot structure and function
2 Biomechanical analysis of foot structure and function JOHN
FOULSTON
The classical texts on anatomy discuss the foot and leg in great detail with the original material derived mainly from dissection and observation of the tissues. To an anatomist, therefore, those structures found to be enclosed by a single synovial capsule are named as a joint. However, when the function of this anatomical joint is discussed, it is sometimes very difficult to understand the precise mechanics of the system because the bones belonging to the articular surfaces enclosed by the joint capsule do not necessarily act as a unified structural mechanism. The exciting aspect of a study of biomechanics is that the movements and the function of the foot and of the whole organism are considered by using concepts which are more generally applied to mechanical systems. Thus, the normal function of the human locomotory apparatus becomes more readily appreciated and pathological conditions are better understood. In the study of the biomechanics of the human foot, a major difficulty is soon encountered. This is the varying terminology used to describe the structure and function of the foot by the different disciplinary groups who have dealings with patients or who study the subject academically. There is a tendency among some to use a fixed set of terms exclusively and to ignore information presented in an alternative linguistic style. It is the intention here to take data from a wide selection of papers and to try to interpret this information into a common set of descriptive terms that may easily be understood by the clinician.
MECHANICAL CONCEPTS A bony articulation with one degree of motion may be considered to be a simple hinge. As can be seen in Figure 1, movement can only occur at right angles to the axis of the hinge. The position and orientation of the axis is genetically determined while minor variations may be produced by developmental factors. Therefore it is the position and orientation of the hinge axis that governs the movement that can take place in the joint. Either element of the joint may move relative to the other, sometimes one element is fixed and sometimes both elements are free to move. A good anatomical Bailli~re's Clinical Rheumatology--Vol. 1, No. 2, August 1987
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Figure 1. The mechanism of a hinge allows movement to occur only in one plane and at right angles to the axis. The hinge has one degree of freedom of motion. Some human joints act as a simple hinge so that their articulation must be at right angles to the axis. The position and orientation of the axis therefore determines the direction of motion at the joint. example is an interphalangeal joint where the distal element is free to move in flexion or in extension relative to the proximal element. No abduction, adduction or rotation of the distal part relative to the proximal part is possible. A n articulation with two degrees of freedom may be considered to act as a mechanical universal joint. A good model for this sort of articulation is a flexible drinking straw (Figure 2). If one element of the unit is held static, relative to that, the second element may move up and down or it may move from side to side. Movement may also take place in two axes at once by motion occurring from side to side and up and down at the same time. The best anatomical example is the metacarpo-phalangeal joint which allows the proximal phalanx of the finger to move up and down in extension and flexion or from side to side as abduction and adduction. Movement in both axes at once is circumduction. No rotation of one element of the articulation relative to the other element is possible. Three degrees of freedom of movement in an articulation are allowed if the joint is of the ball and socket type. Mechanical models seem sometimes to be more complicated than nature and in this case the human hip joint itself serves to describe the mechanism of this type of joint. It is represented by a simple line diagram in Figure 3. In addition to movement up, down and sideways, it is possible for one element to rotate relative to the other. Thus a hip joint may flex and extend, abduct and adduct and also may allow the thigh to rotate internally and externally. Texts on biomechanics usually refer to movements of parts of the body relative to the three cardinal planes. These are the median or sagittal plane, the coronal or frontal plane and the horizontal or transverse plane. In each case the alternative name of the plane has been given because these words are
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Figure 2. The bending point in a flexible drinking straw is a good model for an articulation with two degrees of freedom of motion. If one arm of the straw is held static then the other arm may only move up and down or sideways. O n e arm may not rotate relative to the other arm. A circular motion of the end of an arm is possible if it moves in two planes at once. A h u m a n metatarso-phalangeal joint is a good example of this sort of articulation where the distal portion of the joint may m o v e laterally or medially, may flex or extend or may combine these m o v e m e n t s in circumduction.
Rotation. Horizontal plane motion Coronal plane motion
Sagittal plane motion Figure 3. The h u m a n hip joint may bc represented by a simple line diagram. Here the joint has three degrees of freedom of motion. These are flexion/extension, abduction/adduction and internal/external rotation. Each of these terms describes m o v e m e n t in one of the three cardinal planes of the body. T h u s extension/flexion occurs in the sagittal plane, abduction/adduction occurs in the coronal plane and internal/external rotation occurs in the horizontal plane. A single m o v e m e n t of the leg so that the hip joint flexes, abducts and externally rotates at the same time is termed triplanar motion because it takes place in all three planes of the body at the same time.
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often used synonymously. It is convenient to define the meeting point of these three planes as the position of the centre of mass of the upright figure standing in the anatomical position. An example of the use of this terminology is to say that hip extension is movement of the thigh in the sagittal plane, hip abduction is movement of the thigh in the coronal plane and internal rotation of the thigh is m o v e m e n t in the horizontal plane. A movement of part of the body might involve motion in all three planes at once as in pronation of the foot at the subtalar joint. These concepts are useful in describing motion in the human body in topographical or mathematical disciplines. Computerized imaging techniques are being used to develop an understanding of normal human movement and the changes which occur in pathological conditions such as arthritis. M o v e m e n t at a joint is produced by muscle power and is limited in the normal joint by ligaments and to some extent by the joint capsule and by contraction of opposing muscle groups during activity. It is self-evident that a muscle can only exert a pull. The direction of action is determined by the anatomical position of the muscle and the course taken by its tendon. The strength of contraction of a muscle is greater near its rest-length and decreases as the muscle length shortens (Inman et al, 1981). It has been found that such mechanical factors influence human activity. Organisms tend to move at a comfortable rate unless forced by circumstances of urgency such as eating or being eaten. This comfortable rate has been found to be the most efficient in energetics terms and a faster or slower rate of work tends to be less efficient (Inman et al, 1981). A quadruped will change its style of locomotion from walk to trot, from trot to canter and from canter to gallop according to the energy required at any given time. In a similar way, a human subject will choose to change from walk to run at a particular velocity in order to be efficient mechanically, a fast walk being less efficient than a slow run. The importance of this concept is that if any part of the locomotory system has been damaged, the resulting limp or change of gait or posture is less efficient energetically and the subject uses available energy more quickly. The angle of insertion of a tendon into the bone influences the turning effect or moment that is produced on a joint by a muscle. This m o m e n t is greater when the insertion of the tendon is at right angles to the bone and decreases as the tendon becomes parallel. The moment of a joint is also higher if insertion of a tendon is further away from the joint thus making a longer level. Human lever systems tend to be of the third order kind which have a mechanical disadvantage but which gives great speed of movement to the distal part (Figure 4). Muscles are nearly always placed proximal to the joint that they move which allows the muscle mass to be further away from the moving distal segment thus reducing the energy needed to start and to stop any motion in the limb. A maximization of efficiency also occurs in stair climbing. H e r e there is an additional factor which is that of the optimum angle reached by the hip and knee joints. This angle is influenced by the power of leg muscles at their optimum length and at the optimum angle of insertion of muscle into pelvis, femur and tibia (Inman et al, 1981). This comfortable stair climbing activity has influenced the height of tread in stair
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) t t I Figure 4. This diagram shows a representation of a typical h u m a n joint lever system. T h e muscle body is proximal to the joint and inserts into the distal bone not far from the articulation. T h e muscle on contraction therefore exerts a large force through a small m o v e m e n t which is translated at the distal end of the limb into a smaller force but the range of motion is larger and much quicker. This is a third order lever.
design and therefore the standard ratio of riser to tread width. A muscle may be active while it is shortening (isotonic contraction), while it remains the same length (isometric contraction) or while it is increasing in length. Electromyographical studies of muscle action in conjunction with video analysis of body movement help in the understanding of muscular activity in the normal subject and in patients with pathological changes in the locomotory system. CENTRE OF MASS The centre of mass of the human subject standing upright in the anatomical position is found to be at about 56 % of body height in the female and at about 57 % of body height in the male. The centre of mass lies on the centre line of the body within the abdomen below the level of the navel. The position of the centre of mass moves as the body changes shape but for ordinary purposes of locomotion the position within the abdomen may be considered to be accurate enough. An imaginary line connecting the centre of mass of an object to the centre of the earth must pass within the base of support of the object if it is to remain in a stable position. Once the imaginary line moves outside the base of support, the object becomes unstable and will fall to a new position. In the human, the line must pass within the supporting footprint otherwise the person will fall over. In normal walking, this means that there is a lateral body sway to bring the centre of mass successively over
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each stance phase foot. in patients with a painful limp because of an arthritic joint, the gait must be modified within the constraints of this fundamental physical taw in order for the subject to remain upright. The size of the effective base of support may be increased considerably by the use of a walking frame or by holding onto nearby objects so giving greater stability to the patient. In a static position, the centre of mass-centre of earth imaginary line is upright, but during movement, lateral and front to back forces also occur. At heel strike in walking, there is a force exerted by the subject onto the ground from a posterior to anterior direction. This force is resisted by an equal and opposite force by the ground because of frictional effects (a banana skin stepped on at heel strike reduces friction!). It is possible to resolve by vector addition these two forces of body weight acting downwards and foot contact force acting forwards into a single component which is one force having its own magnitude and direction. There may at times be three forces acting between the foot and the ground during locomotion. These occur in stance phase and consist of the large force of body weight and the smaller forces from front to back and from side to side. Ground reaction force
Fy If" . . . .
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Figure 5. Force-plate studies show that the large force of body weight acting downwards at one moment during the stance phase of walking may be added by vector addition to the force acting between the foot and the ground from posterior to anterior and to the smaller force acting from medial to latcral. The diagonal line acting downwards is the resolved vector of these three original forces and represents the direction and magnitude of the action of the body on the ground. An equal and opposite force is shown acting upwards, backwards and medially. This is the ground reaction force and its position relative to the joints in the lower limb is fundamental in determining their action. F• =Positior to anterior force; Fy - Lateral force; F~ =Downwards force of bodyweight.
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These three forces may also be resolved by vector addition into one singular force acting mostly downwards but with a slight inclination anteriorally or posteriorally and laterally or medially depending on the exact part of stance phase in the gait cycle. A force which is exactly equal and opposite to this singular downwards force is termed the ground reaction force (Figure 5). The ground reaction force may be visualized or printed out onto hard copy through merging data from a video camera and a force plate (Tait and Rose, 1979). In this way it is possible to see what forces occur in the lower limb during locomotion and in other activities such as sitting or rising from a chair. For maximum efficiency of a joint with the minimum of wear and tear, the ground reaction vector should pass through a weight bearing joint in the optimum position. If the ground reaction force passes outside a joint, then there will be a turning effect or moment exerted onto the joint with a magnitude depending on the size of the force and the vertical distance of the line of the vector to the joint pivoting point. This may be helpful to the organism as in the case of the extended human knee joint in standing where the line of the ground reaction force falls anterior to the joint so exerting an extending moment onto the knee. This keeps the knee fully extended and stable so reducing the need for muscular activity. During walking, the ground reaction vector is sometimes posterior to the knee so producing a flexing moment of the joint (Figure 6). Centre of mass entre of mass
I Ground reaction I
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force
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- ~1 I-
Extending
Ground reaction force
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moment at knee
Flexing moment at knee
II I Figure 6. The diagram shows the dotted line representing the ground reaction force in standing.
In normal standing (left), there is a turning effect or moment at the knee which tends to extend the knee joint against the stop at full extension or slight hyperextension so reducing the need for muscular activity in maintaining the posture. If there is a flexion deformity at the knee (right), the line of the ground reaction force falls posterior to the joint so causing a flexing moment at the knee joint and the requirement for muscle action to maintain the erect posture which is tiring for the patient.
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This concept is very important in the understanding of the effect of body forces acting on arthritic joints. If the joint becomes deformed or is limited in its range of motion, then the forces which occur during standing, walking or in other activities may not pass through the correct anatomical alignment or weight bearing surfaces of the joint so leading to further deformation or abnormal wear. Similarly, a door hinge will wear quickly if its alignment is not carefully managed. If, for example, the subtalar joint is limited in pronation with the heel slightly inverted because of the effects of arthritis (Figure 7), then the load line will fall medial to the pivoting point of the joint. This will induce a turning moment at the joint and there will be further supination and the eventual collapse of the foot in the lateral direction if the process is allowed to continue. If there is t h e additional problem of neuropathy because of diabetes mellitus or some other systemic disorder affecting sensation, the resulting rapid joint damage can be massive and very severe. In the arthritic patient with normal sensation, the damage to surrounding soft tissues and to the joint structure because of inappropriate mechanical stresses, can be very painful and cause much distress to the patient.
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Centre of mass
I Ground reaction force
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Figure 7. This diagram shows a posterior viewof an unstable left foot. The pivotingpoint of the subtalar joint is lateral to the load line (ground reaction force) so that a turning effect is produced in the lateral direction so tending to increase the existing deformity.
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LOAD AND P R E S S U R E ON THE FOOT
So far in this discussion, there has been a consideration of load on the joints and the lower limb. A force-plate like a weighing machine only measures the total load exerted by the whole foot and does not identify forces under anatomical landmarks such as individual metatarsal heads. In order to obtain data about pressure under the foot in walking or in standing, other methods are necessary. Clinical observation
A close look at the plantar skin surface quickly reveals the pressures falling onto parts of the foot. In a way similar to that which may identify a person's hands as belonging to a manual worker or to an office worker, normal weight bearing plantar skin may be distinguished from adjacent smooth skin which may be underloaded or from skin showing slight thickening from extra loading or from skin with overt pressure lesions such as corn and callus or ulceration. An understanding of the biomechanical function of the foot may be gained from interpretation of these pressure patterns. A close look should also be made of the shoes worn by the patient. Typical wear marks on the sole of the shoe and abnormal shaping of the upper are indicative of the underlying pathological manifestations of arthritis, of the gait pattern and of the load being applied to various parts of the foot. Direct visualization
Direct visualization of the weight bearing pattern on the plantar surface of the foot is possible if the patient stands barefoot on a glass sheet. It is not possible to identify quantitative or qualitative loading on parts of the foot but the overall shape of the weight bearing area may be determined. Photographs of this pattern through the glass are useful clinical records. The Harris and Beath footprinting mat
The Harris and Beath footprinting mat (Harris and Beath, 1947) is the cheapest method of obtaining plantar pressure data. The inked mat is covered by a clean piece of semi-absorbent paper and placed on a fiat surface. The subject stands to give a static footprint or walks across the paper to give a cumulative image of the pressure under the foot in one stance phase. It is possible to obtain a good qualitative image of foot pressure which may form a useful clinical record. Work has been done to quantify these prints (Silvino, 1980) and to take measurements from them in order to determine the amount of pronation which may be represented by the valgus index (Rose, 1985). The divided force plate
The divided force plate (Dhanendran et al, 1978; Cavanagh et al, 1985) and
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other methods give pressure data under the foot in barefoot walking. These methods rely on small load cells or other devices which give load o v e r a discrete area. Information from apparatus for determining pressure measurements is mostly confined to research rather than to clinical application in a number of centres (Lord et al, 1986). Pressure transducers or load cells have been used on direct application to the surface of the foot but these are mainly confined to research rather than to clinical application.
The pedobarograph
The pedobarograph is a visualization method (Lord et al, 1986) which shows pressure under the foot in good definition and is in use clinically in a number of centres. The static version gives the pressure pattern under the foot whilst standing and in addition, the clinician using the machine is able to ask the patient to move the foot and leg and to observe the changes in plantar pressure pattern in order to check on biomechanical function. For instance, on weight bearing, internal rotation of the leg at the hip is translated into eversion of the heel by pronation at the subtalar joint. This process may be observed by looking at the shifting pattern of pressure under the heel and the consequent effects on loading at the forefoot indicates the functional aspects of the mid-tarsal articulation. Latest versions of the pedobarograph are able to show the dynamic pressure pattern under the foot. These graphs of toad against time for identifiable parts of the weight bearing foot are useful in determining both the biomechanical function within the foot and lower limb and in showing areas of high loads with prolonged contact. It is the time of contact as well as direct pressure onto the soft tissues that is responsible for the development of pressure lesions. Although most work has been done on the barefoot during walking and standing, it is possible to use these pressure sensing devices to visualize pressure between the shoe and their contact surface providing that the shoe has a sole that is not too thick and is without a large tread pattern. The pressure of an enlarged metatarso-phalangeal joint that is visible in the barefoot pressure image is also visible through the sole of the shoe. Work is being undertaken at a number of centres to produce pressure transducers that fit into the shoe and which are not invasive and which do not act as foreign bodies on the surface of the skin.
Results of scientific studies on the arthritic foot
Some scientific research has been carried out on the changes in gait parameters in patients with arthritic conditions and on the clinical implications of foot pressure in the arthritic or rheumatoid foot (Lord et al, 1986). Dimonte and Light (1982) listed the five most common foot problems associated with rheumatoid arthritis. These are hallux valgus, foot pronation, prominence of the metatarsal heads, fixed and buckled toes and the presence of pain associated with calcaneal spur or from retrocalcaneal bursitis. Other workers have described the effects of these diseases on gait because of pain
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and limitation of motion at the subtalar joint (Locke et al, 1984). Enlarged joints in the foot lead to high plantar pressures (Sharma et al, 1979), particularly under the second and third metatarso-phalangeal joints (Barrett, 1976). This high plantar pressure does not always follow the same pattern in all patients. Because of the biomechanical influences from the rest of the foot and because joint enlargement and pain varies from patient to patient, it is difficult to make a classical description of the plantar pressures. High lateral plantar metatarsal pressures (Coltis and Jayson, 1972) are found in a number of arthritic feet and are likely to be related to an inversion of the forefoot relative to the heel (varus forefoot) or to a fixed and maybe flexed first metatarso-phalangeal joint. These reports of high plantar pressures are for both high static pressure which may be more than three times that in the normal foot (Minns and Craxford, 1984) and also for dynamic pressures during walking (Duckworth et al, 1982). Another use of scientific investigation associated with the arthritic conditions is in the validation of surgical techniques. A good example is the use of the dynamic pedobarograph associated with a radiological investigation reported by Grace et al (1986) where Hohmann and Wilson osteotomies have been compared retrospectively. THE MECHANICS OF JOINT MOVEMENTS IN THE FOOT Although the emphasis is on arthritic problems of the foot, it is important to recognize that the foot must not be considered in isolation from the rest of the body. Mechanical difficulties are transmitted in both directions. Problems within the foot may well cause an alteration in gait which in turn may give rise to pain higher in the locomotory system because of abnormal stresses on soft tissues or abnormal loading through other joints. In a similar way, arthritic problems in the back, hip or knee may well lead to overloading within the foot or to an abnormal foot pressure distribution. A good example is that a flexion deformity of the knee will lead to overloading of the forefoot during stance phase. The anatomy of the foot has been described in Chapter 1. From a biomechanical point of view, these structures may now be defined in mechanical terms. Plantarflexion of the ankle may be defined as a movement causing the toes to move further away from the knee and dorsiflexion is movement in the opposite direction. Although anatomy books usually describe the free movement of the foot on the leg, in the stance phase of walking it is the foot which is fixed to the ground and the leg which swings forwards over it and this movement is still defined as dorsiflexion. Investigations leading to an understanding of the biomechanics of the foot have been carried out by a number of workers in different disciplines. The best description of normal human walking and the mechanisms of the lower limb is that given by Inman (1981). Anatomists (Hicks, 1953, 1954), orthopaedic surgeons (Klenerman, 1982; Rose, 1986), podiatrists (Root et al, 1977) and others (Yale and Yale, 1984) have all contributed to this field of work. The study is tending to become interdisciplinary and a very recent
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approach to a further understanding of the biomechanics of the normal foot is the work being done by Lundberg and Goldie (1986) who are carrying out rather heroic in-vivo radiological studies of the normal foot with implanted small titanium spheres at strategic anatomical bony sites.
The ankle joint The ankle joint (talocrural joint) may be considered to be a simple hinge with a coronal plane axis which passes from the higher medial malleolus through to the lower lateral malleolus (Figure 8). The wedge-shaped talus articulates within the mortise of the distal end of the tibia and fibular. The angle of coronal plane axis to the horizontal varies among individuals (lnman, 1976). The larger this angle, the greater the rotation of the foot relative to the lower leg in flexion and extension of the ankle. With the foot fixed to the ground in stance phase, the lower leg rotates internally as the ankle joint dorsiflexes. The neutral position of the ankle joint is defined as the position where the foot is at right angles to the lower leg. This angle is better seen on the lateral side of the foot where a good datum is the line of the lateral edge of the plantar surface. It is essential to test both passive and active movements of the ankle joint with the knee extended and also with the knee flexed as ankle plantarflexion
Medial
Posterior [ Figure 8. This diagram shows the mechanical representation of the human ankle joint (After Inman, 1981). The joint may be considered to be a hinge with a coronal plane axis at an angle to the horizontal. The medial malleolus is higher than the lateral malleolus. When the foot is on the ground in the stance phase of walking, there is dorsiflexion of the joint as the lower leg moves forwards and a subsequent internal rotation of the leg because of the angle of the hinge axis.
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is sometimes limited when the knee is fully extended because of tight posterior lower leg musculature. The wedge shape of the ankle joint structure means that the joint is tightly fixed on dorsiflexion but in plantarflexion, there are other movements possible. These are a slight side-to-side gliding, a rotation, and abduction or adduction of the foot about the lower leg. These movements may be implicated in arthritic joint pain along with pain which arise from the articulation between the distal ends of the tibia and the fibular. By holding the foot in dorsiflexion while the patient is sitting with the knee extended, it is possible to feel these small movements and to identify the source of any pain or limitation of movement. The normal range of motion from the neutral position described above at the ankle is 20 degrees of dorsiftexion and 50 degrees of plantarflexion (American Academy of Orthopedic Surgeons, 1965). This figure will vary from the average figure quoted because of variations with age and with the results of the wearing of high-heeled shoes which tends to reduce the available plantarflexion. The subtalar joint The subtalar joint may be defined in biomechanical terms as the articulation between the talus and the calcaneous. There may be some dispute about the use here of the word 'joint' but this terminology is widely used in the literature. The subtalar joint may be considered to be a simple hinge with the axis of the hinge almost in the sagittal plane and at an angle of about 45 degrees from the horizontal. As with the ankle joint there are differences in the exact axis of the subtalar joint from individual to individual. It is thought that inherited and developed differences in this important articulation may account for the incidence of foot problems in some subjects. A detailed and prolonged discussion of these mechanisms is not in place here in a text on the arthritic foot except to say that abnormal forces falling through a joint will lead to premature or unusual wear and tear with the consequent pathological changes due to osteo-arthrosis. The action of the subtalar joint is to act as a torque-converter between the leg and the foot. Because of the angle of the axis of the hinge joint (Figure 9), internal rotation in the leg is translated to eversion of the foot because of pronation at this joint. In the opposite direction, an external rotation of the leg becomes inversion of the foot through supination at the joint. In the normal foot, there is about twice as much range of motion in supination as there is in pronation at this joint when measured from the neutral position which is most conveniently defined as the position in which there is neither pronation nor supination at the joint when detected respectively by the presence of a bony lump or of a hollow at the site of the head of the talus on the dorsum of the foot. It ispossible to take measurements of the line of the posterior aspect of the calcaneous in relation to the lower third of the posterior muscle group on the lower leg but this is tedious and is more suitable for the young patient requiring corrective orthoses for a pronating foot. By definition, arthritis in the foot implies that protection and support is needed rather than active correction of foot position. The range of motion at
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Figure 9. This is a mechanical representation of the subtalar joint which may be considered to be a hinge ahnost in the sagittal plane. The axis of the hinge lies at about 45 degrees from the horizontal. The action of the joint is to allow the foot to cope with rough or sloping terrain and to act as a torque-converter between the foot and the leg. Internal rotation of the leg is translated into eversion of the foot (pronation) and external rotation of the leg into inversion of the foot (supination).
the s u b t a l a r j o i n t s h o u l d b e i d e n t i f i e d by i s o l a t i n g m o v e m e n t c a r e f u l l y to e x c l u d e a n y m o t i o n in the a n t e r i o r a r t i c u l a t i o n s of the f o o t a n d to k e e p t h e ankle joint immobile.
The midtarsal articulation T h e m i d t a r s a l a r t i c u l a t i o n consists o f the a r t i c u l a t i o n s b e t w e e n t h e talus a n d t h e n a v i c u l a r on t h e m e d i a l side a n d t h e c a l c a n e o u s a n d t h e c u b o i d on the l a t e r a l side. M o v e m e n t at this p a r t o f the f o o t is c o m p l e x with two axes of m o t i o n ( R o o t et al, 1977) b u t t h e j o i n t can b e m o d e l l e d s i m p l y b y suggesting t h a t t h e a r t i c u l a t i o n w o r k s as t w o j o i n e d hinges. T h e n e t effect is t h a t the f r o n t p a r t o f the f o o t is a b l e to r o t a t e r e l a t i v e to the b a c k p a r t of the foot. A l t h o u g h t h e t e r m s p r o n a t i o n a n d s u p i n a t i o n h a v e b e e n u s e d to d e s c r i b e this m o v e m e n t , it is s i m p l e r to r e s e r v e t h e s e t e r m s for m o v e m e n t at t h e s u b t a l a r j o i n t a n d to use t h e t e r m s i n v e r s i o n a n d e v e r s i o n of t h e f o r e f o o t w h e n discussing m o t i o n at t h e m i d t a r s a l a r t i c u l a t i o n . A close e x a m i n a t i o n of t h e f u n c t i o n of t h e m i d t a r s a l a r t i c u l a t i o n a n d its r e l a t i o n s h i p with the s u b t a l a r j o i n t has led to t h e view t h a t at h e e l s t r i k e , t h e m i d t a r s a l a r t i c u l a t i o n is flexible a n d u n l o c k e d b e c a u s e of s u b t a l a r p r o n a t i o n .
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The forefoot can then contact the ground at whatever attitude it wishes depending on the slope or shape of the terrain. Once mechanical loading ont O the forefoot takes place and the heel starts to rise, the subtalar joint moves from pronation to supination and this tends to lock the midtarsal articulation into the position in which it finds itself. The large mechanical forces of forefoot phase may then be transmitted through a rigid structure of the foot to the leg. It is thought that if pronation is prolonged into the latter part of stance phase because of the architecture of the subtalar joint, this locking process of the midtarsal articulation does not occur and that forefoot deformities such as hallux valgus and hallux rigidus will follow. It might be worth considering at this point an engineering design specification for the human foot based on a retrospective analysis of its present biomechanical features. The foot is an ideal locomotory mechanism. It is non-specialized in evolutionary terms which means that it is able to cope with a large variety of terrains such as rocks, rough ground, sand, mud, snow, trees and many others. The mechanism is such that the foot normally stabilizes itself during the process of one stance without the requirements for active intellectual control. The human foot has a large amount of engineering redundancy which means that many defects may occur in its mechanism before it ceases to work totally. An arthritic patient who is only ambulant on flat surfaces is not using the inbuilt design features which enable a young fit person to run quickly across rough and uneven ground barefoot without too much conscious thought on the detailed position of parts of the foot. Articulations in the forefoot
The forefoot has a number of functional articulations. There is the metatarso-phalangeal break line (inman et al, 1981). This straight line extends from the head of the fifth metatarsal through the head of the second metatarsal to the medial aspect of the foot. In normal action after heel raise during stance phase, the forefoot bends by dorsiflexion of the toes. Because the angle of the metatarso-phalangeal break line is at about 60 degrees from the coronal plane, the foot inverts during forefoot phase and this inversion becomes external rotation of the leg via supination at the subtalar joint. This line is usually evident also on the dorsal aspect of the upper of an old shoe as a transverse diagonal crease. The first ray of the foot may be thought of as the first toe, the first metatarsal and the medial cuneiform. At the proximal end of the first ray, there are two articulations; between the first metatarsal and the medial cuneiform and between the medial cuneiform and the navicular. Essentially, the head of the first metatarsal moves up and down with a slight rotation in extension and flexion because of this double articulation at the base of the first ray. On the lateral side of the foot, a similar hinge motion occurs at the proximal end of the fifth metatarsal where there is movement at the metatarso-cuboidal joint and slight motion between the cuboid and the calcaneus. The fifth ray moves up and down in flexion and extension with a slight rotation because of the angle of its hinge.
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The toes The next series of articulations in the foot distally are individual joints rather than large complexes of joints. The metatarso-phalangeal joints are condyloid joints with two degrees of freedom of motion. Movement is lateral or medial and dorsiftexion or plantarflexion. A combination of these movements is circumduction. The interphalangeal joints are simple hinges with their axes in the coronal plane giving an up-and-down movement of flexion and extension. If the toe should become rotated pathologically, the axis of the hinge will also rotate and the consequent motion of the phalanges iwill be at right angles to the new axis.
Biomechanical implications It should be realised that the description given above of the biomechanics of the foot is only part of the biomechanics of the locomotory and supportive system as a whole. There is a long chain of interlinked mechanisms from spine to distal phalanx. Should there be a fault in this system proximally, then it is possible that pathological changes will occur distally. In addition, the extraordinary amount of engineering redundancy in this system means that compensation usually occurs elsewhere to maintain the mobility of the person. This compensation itself may well cause later mechanical stress because forces are being transmitted through the wrong parts of the anatomy or may be larger than the design limits of the structure. Bone and soft tissue increase in strength in response to extra loading but joint surfaces tend to be damaged by these unusual forces. Pain results from the inflammatory process following tissue damage and is an essential element is limiting further damage. However, an altered gait or activity because of pain elsewhere may~often lead to unusual stresses being placed on soft tissues so causing additional pain from damage to these. A patient will often present because of this secondary pain so that the clinician's diagnostic skills must be used to uncover the original mechanical problem.
CLINICAL EXAMINATION The clinical examination must include an assessment of the patient as a whole and must also include a good investigation of an ability to walk and to carry out other simple tasks such as sitting or rising from a chair and getting up and down a step or stairway. An arthritic patient may well have difficulty walking barefoot so observation of walking with outdoor or indoor shoes should also be undertaken. Data about normal range of joint movements is best taken from the American Academy of Orthopedic Surgeons text--Joint Motion: Method of Measuring and Recording (1965). The range of motion at a joint will diminish because of the normal ageing pattern or because of a lack of exercise of that joint as well as from pathology of the arthritic condition
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present. In addition, care should be taken to assess the general level of joint laxity in a subject. There is wide variation of mobility from person to person. A useful test is to ask the subject to place their hand on a fiat surface and for the examiner to lift the subject's middle finger upwards in extension at the metacarpo-phalangeal joint. A large movement is usually indicative of general ligamentous laxity. Of course, if rheumatoid arthritic changes have also occurred at this joint, this test is not valid and a good indication of general bodily mobility may be gained by observing passive hyperextension at the elbow. As well as the range of motion at a joint, the quality of motion should also be assessed by feeling for smooth movement and a good firm stop at the end of the range of movement in both directions. The patient should be asked about discomfort during joint assessment. It is important too to isolate the joint under investigation by holding the proximal bone with one hand and the distal bone with the other hand, care being also taken not to allow two or more articulations to be moved at once because this causes confusion about the exact site of motion. This clinical examination and history taking should give a good impression of the state of the foot architecture. There are a number of investigative techniques developed by various professional groups. Standard orthopaedic practice (Klenerman, 1982) may be supplemented by the techniques described by chiropodists (Neale, 1981), osteopaths (Hartman, 1983), podiatrists (Sgarlato, 1971) and others. The clinician interested in the assessment of patients should seek advice about techniques from a wide variety of professional groups, all of whom have developed interesting methods. The clinical examination of the arthritic patient should be carried out with as much of the patient visible as possible. After the check on function as outlined above, a careful check should be made on the posture in normal standing. The load-line in standing is vertical from the position of the centre of mass. Standing on one foot, should this be possible for the patient, will indicate how the load-line lies in relation to the lower limb joints. A load-line passing to one side of a joint indicates that a turning moment is being applied to the joint by the action of body weight and that if this is not corrected by an appropriate orthosis, extra wear on the joint and soft tissue damage will occur. An example is instability of an arthritic knee with lateral or medial deviation of the joint on weight bearing. In the foot, the joint at risk is the subtalar joint where a lateral or medial deviation in arthritic disease is equally likely (Klenerman, personal communication). In either case the mechanism is similar. A subtalar joint which starts to deform because of arthritic changes will cause the load line to be displaced to one side so leading to a moment on the joint which further tends to increase the deformity. This positive feedback mechanism needs correcting by an appropriate ankle-foot orthosis at an early stage. While the patient is standing, any discrepancy in leg length should be noted by palpating both anterior superior spines of the iliac crests at the same time and checking that these are parallel to the ground. If there is a difference in leg length of more than about 2 centimetres, this also tends to overload lower limb joints because of a consequent alteration in load line.
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The clinical examination should proceed with the patient sitting. A check should be made on the range and quality of motion at the hip, knee and ankle. A note should also be taken of any valgus or varus deviation at the knee as well as any twisting or angular deviation of the lower leg. The non-weight bearing foot should next be checked for movements at the subtalar joint. There is normally a greater range of inversion of the hindfoot from subtalar supination as the joint moves from the neutral position (where the head of the talus is neither causing a lump nor a hollow on the dorsum of the foot) than eversion of the hindfoot because of pronation at the joint. The midtarsal articulation is the next to be examined. With the subtalar joint in the neutral position and with the midtarsal articulation fully locked at its most everted position, it can be seen whether the forefoot lies in the same plane as the heel or if it is inverted relative to the heel. It can also be determined if there is an adequate free rotation of forefoot relative to heel. Emphasis should next be given to a check of up and down movement of the joints at the base of the first ray. It is fairly common to find a limitation of movement at this site because of articular changes. Similarly, the base of the fifth ray may be painful or limited in movement. Both of these sets of joints may be hypermobile because they are producing compensation for loss of movement elsewhere in the foot. Attention should next be focused on the metatarso-phalangeal joints and the toes. The shape of the forefoot is very important in the choice of footwear and any fixed deformity must be noted. Particular care should be taken to check on toe deviations that lead to a requirement for extra depth at the front of a shoe. The first metatarso-phalangeal joint is nearly always limited in dorsiflexion in arthritic patients. This hallux rigidus has knock-on effects on the biomechanics of the rest of the foot because of the alteration in gait as the first ray is unable to bend after heel-off in stance phase. Pain is also c o m m o n at this site on movement df the joint in the sagittal plane.
CONCLUSION The foot is a very complex piece of engineering and it is remarkable how much damage can occur to it before the whole patient is completely incapacitated. Because of this ability to compensate for damage, the foot needs a very careful examination to establish the true site of pathological change so that remedial action can be then taken. The main clinical skill needed is one of observation and careful questioning of the patient. The observation should never just be a passive look at the foot but must include palpation to establish the range and quality of motion at articulations, the presence of pain on active and passive movement and on direct pressure and how any change from the normal foot biomechanics is leading to further problems. Hands-on experience is essential if the clinician is to develop the necessary skills in the examination of the arthritic foot. Arthritic conditions nearly always cause difficulties with locomotion. If the patient is then unable to get about or is limited in activity, this affects their whole life-style and is for them an important problem. A routine part of
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the clinical assessment during a consultation for any rheumatic or arthritic problem must include a question or two about the feet and the patient's shoes and socks or tights must be removed for further investigation of the feet and legs should there be the likelihood that a problem exists or is developing. The appropriate referral of the patient to chiropodist, physiotherapist, orthotist, surgical footwear fitter or orthopaedic surgeon for specialist care and treatment then can be made. REFERENCES American Academy of Orthopedic Surgeons (1965) Joint Motion: Method of Measuring and Recording. Reprinted by the British Orthopaedic Association (1966). Distributed in Edinburgh: Churchill Livingstone. Barrett JP Jr (1976) Plantar pressure measurements,. Journal of the American Medical Association 235: 1138-1139. Cavanagh PR, Hennig EM, Rodgers MM & Sanderson DJ (1985) The measurement of pressure distribution on the plantar surface of diabetic feet. In Whittle M & Harris D (eds) Biornechanical Measurement in Orthopaedic Practice, pp 159-166. Oxford: Clarendon Press. Collis JMF & Jayson MIV (1972) Measurement of pedal pressures. Annals of Rheumatic Diseases 31: 215-217. Dhanendran M, Hutton WC & Parker Y (1978) The distribution of force under the foot--an on-line measuring system. Measurement and Control 11: 261-264. Dimonte P & Light H (1982) Pathomechanics, gait deviations and treatment of the rheumatoid foot, Physical Therapy 62: 1148-1156. Duckworth T, Betts RP, Franks CI & Burke J (1982) The measurement of pressures under the foot. Foot and Ankle 3: 130-141. Grace D, Hughes J & Klenerman L (1986) Comparison of Hohman v. Wilson Metatarsal osteomaties. Fifth Annual Meeting of the British Orthopaedic Foot Surgery Society. To be reported in the Journal of Bone and Joint Surgery. Harris RJ & Beath T (1947) Army Foot Survey--An Investigation into Foot Ailments in Canadian Soldiers. Ottawa: National Research Council of Canada. Hartman LS (1983) Handbook of Osteopathic Technique. Hadley Wood, Herts, UK: N.M.K. Publications. Hicks JH (1953) The mechanics of the foot. I. The joints. Journal of Anatomy 87: 345-357. Hicks JH (1954) The mechanics of the foot. II. Journal of Anatomy 88: 25-30. Inman VT (1976) The Joints of the Ankle. Baltimore: Williams & Wilkins. Inman VT, Ralston HJ & Todd F (1981) Human Walking. Baltimore/London: Williams & Wilkins. 154 pp. Klenerman L (ed.) (t982) The Foot and its Disorders. 2rid edn Oxford: Blackwell Scientific Publications. Locke M, Perry J, Campbell J & Thomas L (1984) Anklc and subtalar motion during gait in arthritic patients. Physical Therapy 64: 504-509. Lord M, Reynolds DP & Hughes JR (1986) Foot pressure measurement: a review of clinical findings. Journal of Biomedical Engineering 8: 283-294. Lundberg A & Goldie I (1986) In vivo CinematicaIAnalysis of the Foot. Fifth Annual Meeting of the British Orthopaedic Foot Surgery Society. To be reported in thc Journal of Bone and Joint Surgery. Minns RJ & Craxford AD (1984) Pressure under the forefoot in rheumatoid arthritis. Clinical Orthopaedics 187: 235-242. Neale D (ed.) (1981) Common Foot Disorders': Diagnosis and Management, pp 252. London: Churchill Livingstone. Root ML, Orien WP & Weed JH (1977) Clinical Biomechanics Volume 11: Normal and Abnormal Function of'the Foot. Los Angeles: Clinical Biomechanics Corporation. Rose GK (1985) Orthotics: Principles and Practice. London: tteinemann.
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Sharma M, Dhanendran M, Hutton WC & Corbett M (1979) Changes in load bearing in the rheumatoid foot. Annals of Rheumatic Diseases 38: 549-552. Silvino N (1980) The Harris and Beath footprinting mat. Clinical Orthopaedics and Related Research 151: 265-269. Sgarlato TE (1971) A Compendium o f Podiatric Biomechanics. California College of Podiatric Medicine. Tait JH & Rose GK (1979) The real time video vector display of ground reaction forces during ambulation. Journal of Medical Engineering and Technology 3: 252-255. Yale I & Yale JF (1984) The Arthritic Foot and Related Tissue Disorders. Baltimore: Williams and Wilkins.
FOULSTON
The classical texts on anatomy discuss the foot and leg in great detail with the original material derived mainly from dissection and observation of the tissues. To an anatomist, therefore, those structures found to be enclosed by a single synovial capsule are named as a joint. However, when the function of this anatomical joint is discussed, it is sometimes very difficult to understand the precise mechanics of the system because the bones belonging to the articular surfaces enclosed by the joint capsule do not necessarily act as a unified structural mechanism. The exciting aspect of a study of biomechanics is that the movements and the function of the foot and of the whole organism are considered by using concepts which are more generally applied to mechanical systems. Thus, the normal function of the human locomotory apparatus becomes more readily appreciated and pathological conditions are better understood. In the study of the biomechanics of the human foot, a major difficulty is soon encountered. This is the varying terminology used to describe the structure and function of the foot by the different disciplinary groups who have dealings with patients or who study the subject academically. There is a tendency among some to use a fixed set of terms exclusively and to ignore information presented in an alternative linguistic style. It is the intention here to take data from a wide selection of papers and to try to interpret this information into a common set of descriptive terms that may easily be understood by the clinician.
MECHANICAL CONCEPTS A bony articulation with one degree of motion may be considered to be a simple hinge. As can be seen in Figure 1, movement can only occur at right angles to the axis of the hinge. The position and orientation of the axis is genetically determined while minor variations may be produced by developmental factors. Therefore it is the position and orientation of the hinge axis that governs the movement that can take place in the joint. Either element of the joint may move relative to the other, sometimes one element is fixed and sometimes both elements are free to move. A good anatomical Bailli~re's Clinical Rheumatology--Vol. 1, No. 2, August 1987
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Figure 1. The mechanism of a hinge allows movement to occur only in one plane and at right angles to the axis. The hinge has one degree of freedom of motion. Some human joints act as a simple hinge so that their articulation must be at right angles to the axis. The position and orientation of the axis therefore determines the direction of motion at the joint. example is an interphalangeal joint where the distal element is free to move in flexion or in extension relative to the proximal element. No abduction, adduction or rotation of the distal part relative to the proximal part is possible. A n articulation with two degrees of freedom may be considered to act as a mechanical universal joint. A good model for this sort of articulation is a flexible drinking straw (Figure 2). If one element of the unit is held static, relative to that, the second element may move up and down or it may move from side to side. Movement may also take place in two axes at once by motion occurring from side to side and up and down at the same time. The best anatomical example is the metacarpo-phalangeal joint which allows the proximal phalanx of the finger to move up and down in extension and flexion or from side to side as abduction and adduction. Movement in both axes at once is circumduction. No rotation of one element of the articulation relative to the other element is possible. Three degrees of freedom of movement in an articulation are allowed if the joint is of the ball and socket type. Mechanical models seem sometimes to be more complicated than nature and in this case the human hip joint itself serves to describe the mechanism of this type of joint. It is represented by a simple line diagram in Figure 3. In addition to movement up, down and sideways, it is possible for one element to rotate relative to the other. Thus a hip joint may flex and extend, abduct and adduct and also may allow the thigh to rotate internally and externally. Texts on biomechanics usually refer to movements of parts of the body relative to the three cardinal planes. These are the median or sagittal plane, the coronal or frontal plane and the horizontal or transverse plane. In each case the alternative name of the plane has been given because these words are
243
BIOMECHANICAL ANALYSIS
~
~
SaO~tta~p~ane ~ r o n a l plane -. ~'-~-l- ~ / motion
Figure 2. The bending point in a flexible drinking straw is a good model for an articulation with two degrees of freedom of motion. If one arm of the straw is held static then the other arm may only move up and down or sideways. O n e arm may not rotate relative to the other arm. A circular motion of the end of an arm is possible if it moves in two planes at once. A h u m a n metatarso-phalangeal joint is a good example of this sort of articulation where the distal portion of the joint may m o v e laterally or medially, may flex or extend or may combine these m o v e m e n t s in circumduction.
Rotation. Horizontal plane motion Coronal plane motion
Sagittal plane motion Figure 3. The h u m a n hip joint may bc represented by a simple line diagram. Here the joint has three degrees of freedom of motion. These are flexion/extension, abduction/adduction and internal/external rotation. Each of these terms describes m o v e m e n t in one of the three cardinal planes of the body. T h u s extension/flexion occurs in the sagittal plane, abduction/adduction occurs in the coronal plane and internal/external rotation occurs in the horizontal plane. A single m o v e m e n t of the leg so that the hip joint flexes, abducts and externally rotates at the same time is termed triplanar motion because it takes place in all three planes of the body at the same time.
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often used synonymously. It is convenient to define the meeting point of these three planes as the position of the centre of mass of the upright figure standing in the anatomical position. An example of the use of this terminology is to say that hip extension is movement of the thigh in the sagittal plane, hip abduction is movement of the thigh in the coronal plane and internal rotation of the thigh is m o v e m e n t in the horizontal plane. A movement of part of the body might involve motion in all three planes at once as in pronation of the foot at the subtalar joint. These concepts are useful in describing motion in the human body in topographical or mathematical disciplines. Computerized imaging techniques are being used to develop an understanding of normal human movement and the changes which occur in pathological conditions such as arthritis. M o v e m e n t at a joint is produced by muscle power and is limited in the normal joint by ligaments and to some extent by the joint capsule and by contraction of opposing muscle groups during activity. It is self-evident that a muscle can only exert a pull. The direction of action is determined by the anatomical position of the muscle and the course taken by its tendon. The strength of contraction of a muscle is greater near its rest-length and decreases as the muscle length shortens (Inman et al, 1981). It has been found that such mechanical factors influence human activity. Organisms tend to move at a comfortable rate unless forced by circumstances of urgency such as eating or being eaten. This comfortable rate has been found to be the most efficient in energetics terms and a faster or slower rate of work tends to be less efficient (Inman et al, 1981). A quadruped will change its style of locomotion from walk to trot, from trot to canter and from canter to gallop according to the energy required at any given time. In a similar way, a human subject will choose to change from walk to run at a particular velocity in order to be efficient mechanically, a fast walk being less efficient than a slow run. The importance of this concept is that if any part of the locomotory system has been damaged, the resulting limp or change of gait or posture is less efficient energetically and the subject uses available energy more quickly. The angle of insertion of a tendon into the bone influences the turning effect or moment that is produced on a joint by a muscle. This m o m e n t is greater when the insertion of the tendon is at right angles to the bone and decreases as the tendon becomes parallel. The moment of a joint is also higher if insertion of a tendon is further away from the joint thus making a longer level. Human lever systems tend to be of the third order kind which have a mechanical disadvantage but which gives great speed of movement to the distal part (Figure 4). Muscles are nearly always placed proximal to the joint that they move which allows the muscle mass to be further away from the moving distal segment thus reducing the energy needed to start and to stop any motion in the limb. A maximization of efficiency also occurs in stair climbing. H e r e there is an additional factor which is that of the optimum angle reached by the hip and knee joints. This angle is influenced by the power of leg muscles at their optimum length and at the optimum angle of insertion of muscle into pelvis, femur and tibia (Inman et al, 1981). This comfortable stair climbing activity has influenced the height of tread in stair
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BIOMECHANICAL ANALYSIS I
) t t I Figure 4. This diagram shows a representation of a typical h u m a n joint lever system. T h e muscle body is proximal to the joint and inserts into the distal bone not far from the articulation. T h e muscle on contraction therefore exerts a large force through a small m o v e m e n t which is translated at the distal end of the limb into a smaller force but the range of motion is larger and much quicker. This is a third order lever.
design and therefore the standard ratio of riser to tread width. A muscle may be active while it is shortening (isotonic contraction), while it remains the same length (isometric contraction) or while it is increasing in length. Electromyographical studies of muscle action in conjunction with video analysis of body movement help in the understanding of muscular activity in the normal subject and in patients with pathological changes in the locomotory system. CENTRE OF MASS The centre of mass of the human subject standing upright in the anatomical position is found to be at about 56 % of body height in the female and at about 57 % of body height in the male. The centre of mass lies on the centre line of the body within the abdomen below the level of the navel. The position of the centre of mass moves as the body changes shape but for ordinary purposes of locomotion the position within the abdomen may be considered to be accurate enough. An imaginary line connecting the centre of mass of an object to the centre of the earth must pass within the base of support of the object if it is to remain in a stable position. Once the imaginary line moves outside the base of support, the object becomes unstable and will fall to a new position. In the human, the line must pass within the supporting footprint otherwise the person will fall over. In normal walking, this means that there is a lateral body sway to bring the centre of mass successively over
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each stance phase foot. in patients with a painful limp because of an arthritic joint, the gait must be modified within the constraints of this fundamental physical taw in order for the subject to remain upright. The size of the effective base of support may be increased considerably by the use of a walking frame or by holding onto nearby objects so giving greater stability to the patient. In a static position, the centre of mass-centre of earth imaginary line is upright, but during movement, lateral and front to back forces also occur. At heel strike in walking, there is a force exerted by the subject onto the ground from a posterior to anterior direction. This force is resisted by an equal and opposite force by the ground because of frictional effects (a banana skin stepped on at heel strike reduces friction!). It is possible to resolve by vector addition these two forces of body weight acting downwards and foot contact force acting forwards into a single component which is one force having its own magnitude and direction. There may at times be three forces acting between the foot and the ground during locomotion. These occur in stance phase and consist of the large force of body weight and the smaller forces from front to back and from side to side. Ground reaction force
Fy If" . . . .
-.~
~,_.1\\'-.'I.:. ",~ Resolved force Fz
Figure 5. Force-plate studies show that the large force of body weight acting downwards at one moment during the stance phase of walking may be added by vector addition to the force acting between the foot and the ground from posterior to anterior and to the smaller force acting from medial to latcral. The diagonal line acting downwards is the resolved vector of these three original forces and represents the direction and magnitude of the action of the body on the ground. An equal and opposite force is shown acting upwards, backwards and medially. This is the ground reaction force and its position relative to the joints in the lower limb is fundamental in determining their action. F• =Positior to anterior force; Fy - Lateral force; F~ =Downwards force of bodyweight.
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BIOMECHANICAL ANALYSIS
These three forces may also be resolved by vector addition into one singular force acting mostly downwards but with a slight inclination anteriorally or posteriorally and laterally or medially depending on the exact part of stance phase in the gait cycle. A force which is exactly equal and opposite to this singular downwards force is termed the ground reaction force (Figure 5). The ground reaction force may be visualized or printed out onto hard copy through merging data from a video camera and a force plate (Tait and Rose, 1979). In this way it is possible to see what forces occur in the lower limb during locomotion and in other activities such as sitting or rising from a chair. For maximum efficiency of a joint with the minimum of wear and tear, the ground reaction vector should pass through a weight bearing joint in the optimum position. If the ground reaction force passes outside a joint, then there will be a turning effect or moment exerted onto the joint with a magnitude depending on the size of the force and the vertical distance of the line of the vector to the joint pivoting point. This may be helpful to the organism as in the case of the extended human knee joint in standing where the line of the ground reaction force falls anterior to the joint so exerting an extending moment onto the knee. This keeps the knee fully extended and stable so reducing the need for muscular activity. During walking, the ground reaction vector is sometimes posterior to the knee so producing a flexing moment of the joint (Figure 6). Centre of mass entre of mass
I Ground reaction I
I
force
I
I
- ~1 I-
Extending
Ground reaction force
-If /
moment at knee
Flexing moment at knee
II I Figure 6. The diagram shows the dotted line representing the ground reaction force in standing.
In normal standing (left), there is a turning effect or moment at the knee which tends to extend the knee joint against the stop at full extension or slight hyperextension so reducing the need for muscular activity in maintaining the posture. If there is a flexion deformity at the knee (right), the line of the ground reaction force falls posterior to the joint so causing a flexing moment at the knee joint and the requirement for muscle action to maintain the erect posture which is tiring for the patient.
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This concept is very important in the understanding of the effect of body forces acting on arthritic joints. If the joint becomes deformed or is limited in its range of motion, then the forces which occur during standing, walking or in other activities may not pass through the correct anatomical alignment or weight bearing surfaces of the joint so leading to further deformation or abnormal wear. Similarly, a door hinge will wear quickly if its alignment is not carefully managed. If, for example, the subtalar joint is limited in pronation with the heel slightly inverted because of the effects of arthritis (Figure 7), then the load line will fall medial to the pivoting point of the joint. This will induce a turning moment at the joint and there will be further supination and the eventual collapse of the foot in the lateral direction if the process is allowed to continue. If there is t h e additional problem of neuropathy because of diabetes mellitus or some other systemic disorder affecting sensation, the resulting rapid joint damage can be massive and very severe. In the arthritic patient with normal sensation, the damage to surrounding soft tissues and to the joint structure because of inappropriate mechanical stresses, can be very painful and cause much distress to the patient.
"•"
Centre of mass
I Ground reaction force
1
Ii
/'
Lateral
I
Medial
II
\ ,I' Lateral m o m e n \
I.
t
Heel inversion
J
Figure 7. This diagram shows a posterior viewof an unstable left foot. The pivotingpoint of the subtalar joint is lateral to the load line (ground reaction force) so that a turning effect is produced in the lateral direction so tending to increase the existing deformity.
BIOMECHANICAL ANALYSIS
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LOAD AND P R E S S U R E ON THE FOOT
So far in this discussion, there has been a consideration of load on the joints and the lower limb. A force-plate like a weighing machine only measures the total load exerted by the whole foot and does not identify forces under anatomical landmarks such as individual metatarsal heads. In order to obtain data about pressure under the foot in walking or in standing, other methods are necessary. Clinical observation
A close look at the plantar skin surface quickly reveals the pressures falling onto parts of the foot. In a way similar to that which may identify a person's hands as belonging to a manual worker or to an office worker, normal weight bearing plantar skin may be distinguished from adjacent smooth skin which may be underloaded or from skin showing slight thickening from extra loading or from skin with overt pressure lesions such as corn and callus or ulceration. An understanding of the biomechanical function of the foot may be gained from interpretation of these pressure patterns. A close look should also be made of the shoes worn by the patient. Typical wear marks on the sole of the shoe and abnormal shaping of the upper are indicative of the underlying pathological manifestations of arthritis, of the gait pattern and of the load being applied to various parts of the foot. Direct visualization
Direct visualization of the weight bearing pattern on the plantar surface of the foot is possible if the patient stands barefoot on a glass sheet. It is not possible to identify quantitative or qualitative loading on parts of the foot but the overall shape of the weight bearing area may be determined. Photographs of this pattern through the glass are useful clinical records. The Harris and Beath footprinting mat
The Harris and Beath footprinting mat (Harris and Beath, 1947) is the cheapest method of obtaining plantar pressure data. The inked mat is covered by a clean piece of semi-absorbent paper and placed on a fiat surface. The subject stands to give a static footprint or walks across the paper to give a cumulative image of the pressure under the foot in one stance phase. It is possible to obtain a good qualitative image of foot pressure which may form a useful clinical record. Work has been done to quantify these prints (Silvino, 1980) and to take measurements from them in order to determine the amount of pronation which may be represented by the valgus index (Rose, 1985). The divided force plate
The divided force plate (Dhanendran et al, 1978; Cavanagh et al, 1985) and
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other methods give pressure data under the foot in barefoot walking. These methods rely on small load cells or other devices which give load o v e r a discrete area. Information from apparatus for determining pressure measurements is mostly confined to research rather than to clinical application in a number of centres (Lord et al, 1986). Pressure transducers or load cells have been used on direct application to the surface of the foot but these are mainly confined to research rather than to clinical application.
The pedobarograph
The pedobarograph is a visualization method (Lord et al, 1986) which shows pressure under the foot in good definition and is in use clinically in a number of centres. The static version gives the pressure pattern under the foot whilst standing and in addition, the clinician using the machine is able to ask the patient to move the foot and leg and to observe the changes in plantar pressure pattern in order to check on biomechanical function. For instance, on weight bearing, internal rotation of the leg at the hip is translated into eversion of the heel by pronation at the subtalar joint. This process may be observed by looking at the shifting pattern of pressure under the heel and the consequent effects on loading at the forefoot indicates the functional aspects of the mid-tarsal articulation. Latest versions of the pedobarograph are able to show the dynamic pressure pattern under the foot. These graphs of toad against time for identifiable parts of the weight bearing foot are useful in determining both the biomechanical function within the foot and lower limb and in showing areas of high loads with prolonged contact. It is the time of contact as well as direct pressure onto the soft tissues that is responsible for the development of pressure lesions. Although most work has been done on the barefoot during walking and standing, it is possible to use these pressure sensing devices to visualize pressure between the shoe and their contact surface providing that the shoe has a sole that is not too thick and is without a large tread pattern. The pressure of an enlarged metatarso-phalangeal joint that is visible in the barefoot pressure image is also visible through the sole of the shoe. Work is being undertaken at a number of centres to produce pressure transducers that fit into the shoe and which are not invasive and which do not act as foreign bodies on the surface of the skin.
Results of scientific studies on the arthritic foot
Some scientific research has been carried out on the changes in gait parameters in patients with arthritic conditions and on the clinical implications of foot pressure in the arthritic or rheumatoid foot (Lord et al, 1986). Dimonte and Light (1982) listed the five most common foot problems associated with rheumatoid arthritis. These are hallux valgus, foot pronation, prominence of the metatarsal heads, fixed and buckled toes and the presence of pain associated with calcaneal spur or from retrocalcaneal bursitis. Other workers have described the effects of these diseases on gait because of pain
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and limitation of motion at the subtalar joint (Locke et al, 1984). Enlarged joints in the foot lead to high plantar pressures (Sharma et al, 1979), particularly under the second and third metatarso-phalangeal joints (Barrett, 1976). This high plantar pressure does not always follow the same pattern in all patients. Because of the biomechanical influences from the rest of the foot and because joint enlargement and pain varies from patient to patient, it is difficult to make a classical description of the plantar pressures. High lateral plantar metatarsal pressures (Coltis and Jayson, 1972) are found in a number of arthritic feet and are likely to be related to an inversion of the forefoot relative to the heel (varus forefoot) or to a fixed and maybe flexed first metatarso-phalangeal joint. These reports of high plantar pressures are for both high static pressure which may be more than three times that in the normal foot (Minns and Craxford, 1984) and also for dynamic pressures during walking (Duckworth et al, 1982). Another use of scientific investigation associated with the arthritic conditions is in the validation of surgical techniques. A good example is the use of the dynamic pedobarograph associated with a radiological investigation reported by Grace et al (1986) where Hohmann and Wilson osteotomies have been compared retrospectively. THE MECHANICS OF JOINT MOVEMENTS IN THE FOOT Although the emphasis is on arthritic problems of the foot, it is important to recognize that the foot must not be considered in isolation from the rest of the body. Mechanical difficulties are transmitted in both directions. Problems within the foot may well cause an alteration in gait which in turn may give rise to pain higher in the locomotory system because of abnormal stresses on soft tissues or abnormal loading through other joints. In a similar way, arthritic problems in the back, hip or knee may well lead to overloading within the foot or to an abnormal foot pressure distribution. A good example is that a flexion deformity of the knee will lead to overloading of the forefoot during stance phase. The anatomy of the foot has been described in Chapter 1. From a biomechanical point of view, these structures may now be defined in mechanical terms. Plantarflexion of the ankle may be defined as a movement causing the toes to move further away from the knee and dorsiflexion is movement in the opposite direction. Although anatomy books usually describe the free movement of the foot on the leg, in the stance phase of walking it is the foot which is fixed to the ground and the leg which swings forwards over it and this movement is still defined as dorsiflexion. Investigations leading to an understanding of the biomechanics of the foot have been carried out by a number of workers in different disciplines. The best description of normal human walking and the mechanisms of the lower limb is that given by Inman (1981). Anatomists (Hicks, 1953, 1954), orthopaedic surgeons (Klenerman, 1982; Rose, 1986), podiatrists (Root et al, 1977) and others (Yale and Yale, 1984) have all contributed to this field of work. The study is tending to become interdisciplinary and a very recent
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approach to a further understanding of the biomechanics of the normal foot is the work being done by Lundberg and Goldie (1986) who are carrying out rather heroic in-vivo radiological studies of the normal foot with implanted small titanium spheres at strategic anatomical bony sites.
The ankle joint The ankle joint (talocrural joint) may be considered to be a simple hinge with a coronal plane axis which passes from the higher medial malleolus through to the lower lateral malleolus (Figure 8). The wedge-shaped talus articulates within the mortise of the distal end of the tibia and fibular. The angle of coronal plane axis to the horizontal varies among individuals (lnman, 1976). The larger this angle, the greater the rotation of the foot relative to the lower leg in flexion and extension of the ankle. With the foot fixed to the ground in stance phase, the lower leg rotates internally as the ankle joint dorsiflexes. The neutral position of the ankle joint is defined as the position where the foot is at right angles to the lower leg. This angle is better seen on the lateral side of the foot where a good datum is the line of the lateral edge of the plantar surface. It is essential to test both passive and active movements of the ankle joint with the knee extended and also with the knee flexed as ankle plantarflexion
Medial
Posterior [ Figure 8. This diagram shows the mechanical representation of the human ankle joint (After Inman, 1981). The joint may be considered to be a hinge with a coronal plane axis at an angle to the horizontal. The medial malleolus is higher than the lateral malleolus. When the foot is on the ground in the stance phase of walking, there is dorsiflexion of the joint as the lower leg moves forwards and a subsequent internal rotation of the leg because of the angle of the hinge axis.
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253
is sometimes limited when the knee is fully extended because of tight posterior lower leg musculature. The wedge shape of the ankle joint structure means that the joint is tightly fixed on dorsiflexion but in plantarflexion, there are other movements possible. These are a slight side-to-side gliding, a rotation, and abduction or adduction of the foot about the lower leg. These movements may be implicated in arthritic joint pain along with pain which arise from the articulation between the distal ends of the tibia and the fibular. By holding the foot in dorsiflexion while the patient is sitting with the knee extended, it is possible to feel these small movements and to identify the source of any pain or limitation of movement. The normal range of motion from the neutral position described above at the ankle is 20 degrees of dorsiftexion and 50 degrees of plantarflexion (American Academy of Orthopedic Surgeons, 1965). This figure will vary from the average figure quoted because of variations with age and with the results of the wearing of high-heeled shoes which tends to reduce the available plantarflexion. The subtalar joint The subtalar joint may be defined in biomechanical terms as the articulation between the talus and the calcaneous. There may be some dispute about the use here of the word 'joint' but this terminology is widely used in the literature. The subtalar joint may be considered to be a simple hinge with the axis of the hinge almost in the sagittal plane and at an angle of about 45 degrees from the horizontal. As with the ankle joint there are differences in the exact axis of the subtalar joint from individual to individual. It is thought that inherited and developed differences in this important articulation may account for the incidence of foot problems in some subjects. A detailed and prolonged discussion of these mechanisms is not in place here in a text on the arthritic foot except to say that abnormal forces falling through a joint will lead to premature or unusual wear and tear with the consequent pathological changes due to osteo-arthrosis. The action of the subtalar joint is to act as a torque-converter between the leg and the foot. Because of the angle of the axis of the hinge joint (Figure 9), internal rotation in the leg is translated to eversion of the foot because of pronation at this joint. In the opposite direction, an external rotation of the leg becomes inversion of the foot through supination at the joint. In the normal foot, there is about twice as much range of motion in supination as there is in pronation at this joint when measured from the neutral position which is most conveniently defined as the position in which there is neither pronation nor supination at the joint when detected respectively by the presence of a bony lump or of a hollow at the site of the head of the talus on the dorsum of the foot. It ispossible to take measurements of the line of the posterior aspect of the calcaneous in relation to the lower third of the posterior muscle group on the lower leg but this is tedious and is more suitable for the young patient requiring corrective orthoses for a pronating foot. By definition, arthritis in the foot implies that protection and support is needed rather than active correction of foot position. The range of motion at
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Internalrotatioofnlowerleg
)
~,
~
eversioofnthfiot e
Figure 9. This is a mechanical representation of the subtalar joint which may be considered to be a hinge ahnost in the sagittal plane. The axis of the hinge lies at about 45 degrees from the horizontal. The action of the joint is to allow the foot to cope with rough or sloping terrain and to act as a torque-converter between the foot and the leg. Internal rotation of the leg is translated into eversion of the foot (pronation) and external rotation of the leg into inversion of the foot (supination).
the s u b t a l a r j o i n t s h o u l d b e i d e n t i f i e d by i s o l a t i n g m o v e m e n t c a r e f u l l y to e x c l u d e a n y m o t i o n in the a n t e r i o r a r t i c u l a t i o n s of the f o o t a n d to k e e p t h e ankle joint immobile.
The midtarsal articulation T h e m i d t a r s a l a r t i c u l a t i o n consists o f the a r t i c u l a t i o n s b e t w e e n t h e talus a n d t h e n a v i c u l a r on t h e m e d i a l side a n d t h e c a l c a n e o u s a n d t h e c u b o i d on the l a t e r a l side. M o v e m e n t at this p a r t o f the f o o t is c o m p l e x with two axes of m o t i o n ( R o o t et al, 1977) b u t t h e j o i n t can b e m o d e l l e d s i m p l y b y suggesting t h a t t h e a r t i c u l a t i o n w o r k s as t w o j o i n e d hinges. T h e n e t effect is t h a t the f r o n t p a r t o f the f o o t is a b l e to r o t a t e r e l a t i v e to the b a c k p a r t of the foot. A l t h o u g h t h e t e r m s p r o n a t i o n a n d s u p i n a t i o n h a v e b e e n u s e d to d e s c r i b e this m o v e m e n t , it is s i m p l e r to r e s e r v e t h e s e t e r m s for m o v e m e n t at t h e s u b t a l a r j o i n t a n d to use t h e t e r m s i n v e r s i o n a n d e v e r s i o n of t h e f o r e f o o t w h e n discussing m o t i o n at t h e m i d t a r s a l a r t i c u l a t i o n . A close e x a m i n a t i o n of t h e f u n c t i o n of t h e m i d t a r s a l a r t i c u l a t i o n a n d its r e l a t i o n s h i p with the s u b t a l a r j o i n t has led to t h e view t h a t at h e e l s t r i k e , t h e m i d t a r s a l a r t i c u l a t i o n is flexible a n d u n l o c k e d b e c a u s e of s u b t a l a r p r o n a t i o n .
BIOMECHANICAL ANALYSIS
255
The forefoot can then contact the ground at whatever attitude it wishes depending on the slope or shape of the terrain. Once mechanical loading ont O the forefoot takes place and the heel starts to rise, the subtalar joint moves from pronation to supination and this tends to lock the midtarsal articulation into the position in which it finds itself. The large mechanical forces of forefoot phase may then be transmitted through a rigid structure of the foot to the leg. It is thought that if pronation is prolonged into the latter part of stance phase because of the architecture of the subtalar joint, this locking process of the midtarsal articulation does not occur and that forefoot deformities such as hallux valgus and hallux rigidus will follow. It might be worth considering at this point an engineering design specification for the human foot based on a retrospective analysis of its present biomechanical features. The foot is an ideal locomotory mechanism. It is non-specialized in evolutionary terms which means that it is able to cope with a large variety of terrains such as rocks, rough ground, sand, mud, snow, trees and many others. The mechanism is such that the foot normally stabilizes itself during the process of one stance without the requirements for active intellectual control. The human foot has a large amount of engineering redundancy which means that many defects may occur in its mechanism before it ceases to work totally. An arthritic patient who is only ambulant on flat surfaces is not using the inbuilt design features which enable a young fit person to run quickly across rough and uneven ground barefoot without too much conscious thought on the detailed position of parts of the foot. Articulations in the forefoot
The forefoot has a number of functional articulations. There is the metatarso-phalangeal break line (inman et al, 1981). This straight line extends from the head of the fifth metatarsal through the head of the second metatarsal to the medial aspect of the foot. In normal action after heel raise during stance phase, the forefoot bends by dorsiflexion of the toes. Because the angle of the metatarso-phalangeal break line is at about 60 degrees from the coronal plane, the foot inverts during forefoot phase and this inversion becomes external rotation of the leg via supination at the subtalar joint. This line is usually evident also on the dorsal aspect of the upper of an old shoe as a transverse diagonal crease. The first ray of the foot may be thought of as the first toe, the first metatarsal and the medial cuneiform. At the proximal end of the first ray, there are two articulations; between the first metatarsal and the medial cuneiform and between the medial cuneiform and the navicular. Essentially, the head of the first metatarsal moves up and down with a slight rotation in extension and flexion because of this double articulation at the base of the first ray. On the lateral side of the foot, a similar hinge motion occurs at the proximal end of the fifth metatarsal where there is movement at the metatarso-cuboidal joint and slight motion between the cuboid and the calcaneus. The fifth ray moves up and down in flexion and extension with a slight rotation because of the angle of its hinge.
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The toes The next series of articulations in the foot distally are individual joints rather than large complexes of joints. The metatarso-phalangeal joints are condyloid joints with two degrees of freedom of motion. Movement is lateral or medial and dorsiftexion or plantarflexion. A combination of these movements is circumduction. The interphalangeal joints are simple hinges with their axes in the coronal plane giving an up-and-down movement of flexion and extension. If the toe should become rotated pathologically, the axis of the hinge will also rotate and the consequent motion of the phalanges iwill be at right angles to the new axis.
Biomechanical implications It should be realised that the description given above of the biomechanics of the foot is only part of the biomechanics of the locomotory and supportive system as a whole. There is a long chain of interlinked mechanisms from spine to distal phalanx. Should there be a fault in this system proximally, then it is possible that pathological changes will occur distally. In addition, the extraordinary amount of engineering redundancy in this system means that compensation usually occurs elsewhere to maintain the mobility of the person. This compensation itself may well cause later mechanical stress because forces are being transmitted through the wrong parts of the anatomy or may be larger than the design limits of the structure. Bone and soft tissue increase in strength in response to extra loading but joint surfaces tend to be damaged by these unusual forces. Pain results from the inflammatory process following tissue damage and is an essential element is limiting further damage. However, an altered gait or activity because of pain elsewhere may~often lead to unusual stresses being placed on soft tissues so causing additional pain from damage to these. A patient will often present because of this secondary pain so that the clinician's diagnostic skills must be used to uncover the original mechanical problem.
CLINICAL EXAMINATION The clinical examination must include an assessment of the patient as a whole and must also include a good investigation of an ability to walk and to carry out other simple tasks such as sitting or rising from a chair and getting up and down a step or stairway. An arthritic patient may well have difficulty walking barefoot so observation of walking with outdoor or indoor shoes should also be undertaken. Data about normal range of joint movements is best taken from the American Academy of Orthopedic Surgeons text--Joint Motion: Method of Measuring and Recording (1965). The range of motion at a joint will diminish because of the normal ageing pattern or because of a lack of exercise of that joint as well as from pathology of the arthritic condition
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present. In addition, care should be taken to assess the general level of joint laxity in a subject. There is wide variation of mobility from person to person. A useful test is to ask the subject to place their hand on a fiat surface and for the examiner to lift the subject's middle finger upwards in extension at the metacarpo-phalangeal joint. A large movement is usually indicative of general ligamentous laxity. Of course, if rheumatoid arthritic changes have also occurred at this joint, this test is not valid and a good indication of general bodily mobility may be gained by observing passive hyperextension at the elbow. As well as the range of motion at a joint, the quality of motion should also be assessed by feeling for smooth movement and a good firm stop at the end of the range of movement in both directions. The patient should be asked about discomfort during joint assessment. It is important too to isolate the joint under investigation by holding the proximal bone with one hand and the distal bone with the other hand, care being also taken not to allow two or more articulations to be moved at once because this causes confusion about the exact site of motion. This clinical examination and history taking should give a good impression of the state of the foot architecture. There are a number of investigative techniques developed by various professional groups. Standard orthopaedic practice (Klenerman, 1982) may be supplemented by the techniques described by chiropodists (Neale, 1981), osteopaths (Hartman, 1983), podiatrists (Sgarlato, 1971) and others. The clinician interested in the assessment of patients should seek advice about techniques from a wide variety of professional groups, all of whom have developed interesting methods. The clinical examination of the arthritic patient should be carried out with as much of the patient visible as possible. After the check on function as outlined above, a careful check should be made on the posture in normal standing. The load-line in standing is vertical from the position of the centre of mass. Standing on one foot, should this be possible for the patient, will indicate how the load-line lies in relation to the lower limb joints. A load-line passing to one side of a joint indicates that a turning moment is being applied to the joint by the action of body weight and that if this is not corrected by an appropriate orthosis, extra wear on the joint and soft tissue damage will occur. An example is instability of an arthritic knee with lateral or medial deviation of the joint on weight bearing. In the foot, the joint at risk is the subtalar joint where a lateral or medial deviation in arthritic disease is equally likely (Klenerman, personal communication). In either case the mechanism is similar. A subtalar joint which starts to deform because of arthritic changes will cause the load line to be displaced to one side so leading to a moment on the joint which further tends to increase the deformity. This positive feedback mechanism needs correcting by an appropriate ankle-foot orthosis at an early stage. While the patient is standing, any discrepancy in leg length should be noted by palpating both anterior superior spines of the iliac crests at the same time and checking that these are parallel to the ground. If there is a difference in leg length of more than about 2 centimetres, this also tends to overload lower limb joints because of a consequent alteration in load line.
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The clinical examination should proceed with the patient sitting. A check should be made on the range and quality of motion at the hip, knee and ankle. A note should also be taken of any valgus or varus deviation at the knee as well as any twisting or angular deviation of the lower leg. The non-weight bearing foot should next be checked for movements at the subtalar joint. There is normally a greater range of inversion of the hindfoot from subtalar supination as the joint moves from the neutral position (where the head of the talus is neither causing a lump nor a hollow on the dorsum of the foot) than eversion of the hindfoot because of pronation at the joint. The midtarsal articulation is the next to be examined. With the subtalar joint in the neutral position and with the midtarsal articulation fully locked at its most everted position, it can be seen whether the forefoot lies in the same plane as the heel or if it is inverted relative to the heel. It can also be determined if there is an adequate free rotation of forefoot relative to heel. Emphasis should next be given to a check of up and down movement of the joints at the base of the first ray. It is fairly common to find a limitation of movement at this site because of articular changes. Similarly, the base of the fifth ray may be painful or limited in movement. Both of these sets of joints may be hypermobile because they are producing compensation for loss of movement elsewhere in the foot. Attention should next be focused on the metatarso-phalangeal joints and the toes. The shape of the forefoot is very important in the choice of footwear and any fixed deformity must be noted. Particular care should be taken to check on toe deviations that lead to a requirement for extra depth at the front of a shoe. The first metatarso-phalangeal joint is nearly always limited in dorsiflexion in arthritic patients. This hallux rigidus has knock-on effects on the biomechanics of the rest of the foot because of the alteration in gait as the first ray is unable to bend after heel-off in stance phase. Pain is also c o m m o n at this site on movement df the joint in the sagittal plane.
CONCLUSION The foot is a very complex piece of engineering and it is remarkable how much damage can occur to it before the whole patient is completely incapacitated. Because of this ability to compensate for damage, the foot needs a very careful examination to establish the true site of pathological change so that remedial action can be then taken. The main clinical skill needed is one of observation and careful questioning of the patient. The observation should never just be a passive look at the foot but must include palpation to establish the range and quality of motion at articulations, the presence of pain on active and passive movement and on direct pressure and how any change from the normal foot biomechanics is leading to further problems. Hands-on experience is essential if the clinician is to develop the necessary skills in the examination of the arthritic foot. Arthritic conditions nearly always cause difficulties with locomotion. If the patient is then unable to get about or is limited in activity, this affects their whole life-style and is for them an important problem. A routine part of
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the clinical assessment during a consultation for any rheumatic or arthritic problem must include a question or two about the feet and the patient's shoes and socks or tights must be removed for further investigation of the feet and legs should there be the likelihood that a problem exists or is developing. The appropriate referral of the patient to chiropodist, physiotherapist, orthotist, surgical footwear fitter or orthopaedic surgeon for specialist care and treatment then can be made. REFERENCES American Academy of Orthopedic Surgeons (1965) Joint Motion: Method of Measuring and Recording. Reprinted by the British Orthopaedic Association (1966). Distributed in Edinburgh: Churchill Livingstone. Barrett JP Jr (1976) Plantar pressure measurements,. Journal of the American Medical Association 235: 1138-1139. Cavanagh PR, Hennig EM, Rodgers MM & Sanderson DJ (1985) The measurement of pressure distribution on the plantar surface of diabetic feet. In Whittle M & Harris D (eds) Biornechanical Measurement in Orthopaedic Practice, pp 159-166. Oxford: Clarendon Press. Collis JMF & Jayson MIV (1972) Measurement of pedal pressures. Annals of Rheumatic Diseases 31: 215-217. Dhanendran M, Hutton WC & Parker Y (1978) The distribution of force under the foot--an on-line measuring system. Measurement and Control 11: 261-264. Dimonte P & Light H (1982) Pathomechanics, gait deviations and treatment of the rheumatoid foot, Physical Therapy 62: 1148-1156. Duckworth T, Betts RP, Franks CI & Burke J (1982) The measurement of pressures under the foot. Foot and Ankle 3: 130-141. Grace D, Hughes J & Klenerman L (1986) Comparison of Hohman v. Wilson Metatarsal osteomaties. Fifth Annual Meeting of the British Orthopaedic Foot Surgery Society. To be reported in the Journal of Bone and Joint Surgery. Harris RJ & Beath T (1947) Army Foot Survey--An Investigation into Foot Ailments in Canadian Soldiers. Ottawa: National Research Council of Canada. Hartman LS (1983) Handbook of Osteopathic Technique. Hadley Wood, Herts, UK: N.M.K. Publications. Hicks JH (1953) The mechanics of the foot. I. The joints. Journal of Anatomy 87: 345-357. Hicks JH (1954) The mechanics of the foot. II. Journal of Anatomy 88: 25-30. Inman VT (1976) The Joints of the Ankle. Baltimore: Williams & Wilkins. Inman VT, Ralston HJ & Todd F (1981) Human Walking. Baltimore/London: Williams & Wilkins. 154 pp. Klenerman L (ed.) (t982) The Foot and its Disorders. 2rid edn Oxford: Blackwell Scientific Publications. Locke M, Perry J, Campbell J & Thomas L (1984) Anklc and subtalar motion during gait in arthritic patients. Physical Therapy 64: 504-509. Lord M, Reynolds DP & Hughes JR (1986) Foot pressure measurement: a review of clinical findings. Journal of Biomedical Engineering 8: 283-294. Lundberg A & Goldie I (1986) In vivo CinematicaIAnalysis of the Foot. Fifth Annual Meeting of the British Orthopaedic Foot Surgery Society. To be reported in thc Journal of Bone and Joint Surgery. Minns RJ & Craxford AD (1984) Pressure under the forefoot in rheumatoid arthritis. Clinical Orthopaedics 187: 235-242. Neale D (ed.) (1981) Common Foot Disorders': Diagnosis and Management, pp 252. London: Churchill Livingstone. Root ML, Orien WP & Weed JH (1977) Clinical Biomechanics Volume 11: Normal and Abnormal Function of'the Foot. Los Angeles: Clinical Biomechanics Corporation. Rose GK (1985) Orthotics: Principles and Practice. London: tteinemann.
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Sharma M, Dhanendran M, Hutton WC & Corbett M (1979) Changes in load bearing in the rheumatoid foot. Annals of Rheumatic Diseases 38: 549-552. Silvino N (1980) The Harris and Beath footprinting mat. Clinical Orthopaedics and Related Research 151: 265-269. Sgarlato TE (1971) A Compendium o f Podiatric Biomechanics. California College of Podiatric Medicine. Tait JH & Rose GK (1979) The real time video vector display of ground reaction forces during ambulation. Journal of Medical Engineering and Technology 3: 252-255. Yale I & Yale JF (1984) The Arthritic Foot and Related Tissue Disorders. Baltimore: Williams and Wilkins.