The influence of external fixators on fracture motion during simulated walking

1350-4533( 95)00056-9 ELSEVIER

The influence of external fixators motion during simulated walking T. N. Gardner,

on fracture

M. Evans and J. Kenwright

Oxford Orthopaedic Engineering Centre, University of Oxford, Orthopaedic Centre, Windmill Road, Headington, Oxford, UK

Nuffield

ABSTRACT This expamental study examines the relative influence ofJive uniluteral external jixntors on tibia1 fructure .stnbility during simulated walking. Stability during routine patient uctivity i.c important, because cyclic inter-fragmentary motion, or strain, has been shown to affect ji-arlure healing. In model stable fractures simulating early healing (six weeks), it was found thatjxators do little to constrain aguinst uxial interfraCgmentq strains us great as 100% at on[y nominal weight-bearing (6.0 kg). ?‘he,$e strains may orcur repeatably at peak nmp?itudes motion d7cringwalking Similurly, peak angular movements may &ad to additional axial strains of up to 25% at the external cortex and sheur movrmunt.r may lead to shear strains OJ up to 100%. Such strains nre greut enough to yield and possibly refracture the intm gap fracture tissue that may be composed of a combination of gvanulation tissue, Jibrou~~ rartiluCge, cartilage and bone. It was also shown that the prowdure releasing the jxator column to trlesrope (dynamizr) hnr little inJluence on peak cyclic axial motion and on loading at the frarturr. although inrrenses orrurrrd in peak transverse and torsional shear strains of up to 100%. Since permanent interJiiqmentan trarrskation (dso arises Jirom thr consequent compaction of the intra g@ tissue, it may be permanent displacement rathm th,nn nny rhnngr in the amplitude motion that is responsible for the benejrial rffert on healing claimed for the drnnmizini poruduru. In unstnblP fractures that are unable to .support tibia1 load at the fracture. the peak ampl~tude.~ of ryrlir movement were as
of

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Keywords: healing

Fracture,

external

Med. Brig. Phys., 1996, \‘ol.

fixator,

fracture

model.

inter

fragmentary

strain, fracture

mov~mcn~,

18, 305-313, June

INTRODUCTION The biological process and speed of fracture healing is influenced by the inter fragmentary motion permitted by the fracture fixation device. This is because fractures usually heal by a combination of two different processes. The ‘direct’ process of repair involves relatively rigid fracture fixation and primarily osteonal remodelling at the fragment ends’. The more rapid ‘indirect’ process, involves a more flexible fixation permitting some degree of inter fragmentary motion and results primarily in the formation of callus, usually initiated at the periosteum’. During mainly indirect healing with external fixation, it has been shown that the nature of inter fragmentary motion influences the speed of restoration of mechanical integrity at the fracture, and therefore of limb function.‘. Since the fixation frame pro-

vides the main constraint against this motion during regular motion-inducing activities such as walking, the performance of the fixator must have a profound influence on the process and speed of healing. Therefore, this study examines experimentally the influence of external fixators on fracture motion during simulated walking. The properties of inter fragmentary motion that influence healing (amplitude, direction, frequency and strain rate) Kenwright and Goodship showed that passive cyclic axial movements, that are applied mechanically to tibia1 fractures, can be either inhibitory or stimulator-y to the process of indirect healing. For example amplitudes of 2.0 mm axially in experimental fractures (corresponding to 67% strain in the 3.0 mm gaps used) was shown to delay callus formation in comparison with ‘rigid fixation’, and 0.5 mm (corresponding to 17% strain) to produce

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a greater stimulus to callus formation and fracture stabilization. Cast-brace studies have indicated that tibia1 fracture movements can be more substantial although non-unions are uncommon5, and in plastercasted patients shear and angulation was measured at between 3.0 mm and 2” (for three stable fractures) and 2 cm and 21” (for an unstable fracture)6. Although the fractures may have all subsequently united, it is not clear whether the healing rate could have been improved by reducing the movement in either study to the approximate strain shown to be beneficial by Goodship and Kenwright3. With regard to the direction of movement, Goodship and Kenwright and Kenwright and Goodship showed that axial movement can be a stimulus for healing in both humans and sheep, and, although it may be difficult to control, angular movement has also been shown to stimulate substantial callus in rat tibiae’. The disadvantage of angular movement is that it may cause random shear and axial movement, when both bone fragments do not pivot precisely about the fracture centre. The effect of shear on healing is unclear. Although significant shear movements have been shown to cause non-union in experimental fractures in the rabbit tibia*, it is suggested that cells in healing bone may respond to intermittent shear strains by significant tissue proliferationg. In addition to displacement, other factors such as frequency, strain rate and the time of commencement of movement after injury have all been shown to influence healing by varying degrees. The frequencies and strain rates used successfully to stimulate osteogenesis have been physiological, both in experimental fractures’ and in intact bonelo, while the time to commence this movement would seem to be as early as pain and discomfort allows”. In practise however, fractures are regularly exposed to the random active movements that arise from the routine activities of weight-bearing and muscle flexion. Healing response is therefore a consequence of this mechanical environment, as well as the biological one, and not of the controlled mechanical environment discussed above. Although active movement will probably provide the desired frequency and strain rate for an optimum healing response, the amplitude and direction of this motion is not expected to be appropriate. The influence on motion of the load-bearing properties of the fracture site

The structural and material properties of the fracture site influence the performance of the fixation system and therefore the amplitude and direction of inter fragmentary motion. Any evaluation of the performance of unilateral external fixators must therefore encompass the ranges of variation possible in these two properties. Owing to the morphology of healing, there is continuous change in the tissue composition and geometric structure of the fracture, and this affects the control of load-bearing movement by

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the fixator framei2. During the initial stages the fracture is stiffened rapidly by the arching and bridging of the callus at the periosteum, formed by enchondral ossification13, and subsequently through the process of uniting the inter fragmentary surfaces by fibrous connective tissue, culminating in directly formed bone by intamembranous or enchondral ossification13. Since motion at the fracture becomes more constrained by the stiffening callus, the amplitude and direction of movement will reduce gradually and the demand on the fixator to provide constraint is expected to change. Since the nature of motion will vary, this effect should be considered in the performance evaluation of a fixator. Of equal importance is the influence on inter fragmentary motion of the structural support to load offered at the fracture site by the interposing fragment ends brought into contact by tibia1 load. Here, the distinction must be made between the inter fragmentary motion of fractures where tibia1 load is ‘well-supported’ at the fragment ends and of fractures where load is largely ‘unsupported!. Owing to the different degrees of support, the inter fragmentary motion of such fractures during patient activity is expected to be substantially different. Also, many forms of unilateral external fixators now incorporate an option for changing the degree of axial support provided by the frame, referred to as the ‘dynamizing’ function. Consideration must be given to the influence this may have on load-bearing motion in well-supported fractures, but it is not generally used for unsupported fractures. The influence of the fixator on inter fragmentary motion (past work and present s-59

Numerous studies have been reported on the performance of different unilateral external fixators, using a variety of models and loading configurations14-17, in an attempt to characterize the mechanical properties of each device. The difficulty in assessing the importance of these studies arises from the uncertainty as to whether the experimental conditions were realistic. As a consequence, it is uncertain whether the observed behaviour is representative of real fractures of variable bone-end support that develop an increasing constraint to movement throughout the healing process. Also, in the natural environment of a fracture, axial, bending and shear (transverse and torsional) loads arise through weight-bearing and through muscle, tendon and ligament activity. The loading is therefore very and it would require considerable complex, resources to generate and validate an experimental, analytical or numerical model that was able to predict, for example, the forces or movements at a mid-diaphyseal tibia1 fracture site. A more realistic approach has been attempted for this study. Here, axial, bending and shear loads have been applied cyclically to model fractures at physiological frequency and strain rate. The magnitude of the loading ensured that the

amplitude and direction of the resulting inter fragmentary motion was representative of the spectrum of movements commonly occurring in patients during walking at two to four weeks post fixation. Inter fragmentary motion in patients has been characterized in previous studies that have used a displacement transducer to monitor the 3dimensional movements of well-supported tibia1 fractures during y ical daily activities over the full period of healing 8. For these patients, weightbearing ground loads were frequently over 200 N from as early as 2 weeks, and the corresponding peak displacements throughout the healing period were not always predominantly axial. In the early stages of healing, angulations in a vertical plane and transverse shear movements (of up to 1.0” and 0.7 mm respectively) were frequently as significant as peak axial motion (of up to 1 .S mm) and often greater lg. During the healing period (around 2-20 weeks), the stiffening and strengthening of the fracture provided an increasing resistance to three-dimensional movement. To model the increasing resistance, four fracture simulation materials of increasing stiffness were fixed to the opposing fragment ends across the fracture site. In this way, the relative performances of the five frames were evaluated throughout a simulated period of healing. The above tests were carried out using a model of a well-supported fracture and by applying the loads that simulated the spectrum of movement found in a group of patients with stable fractures. The same load combination was then applied to a model of an unsupported fracture (without fracture simulation material), to evaluate the relative performance of fixators with unstable fractures. Unfortunately, the means of appraising fixator performance in relation to the control of inter fragmentary motion are not readily available. Although the merits of frame strength may be assessed easily, frame stiffness, and its influence on inter fragmentary movement under load, may not. The stiffness performances of fixators should be evaluated only in relation to their ability to control inter fragmentary motion during normal activity to that which is required for an optimum healing response. Some flexibility is required to induce the indirect process of healing desired for external fixation; but how stiff must the fixator frames be or what is the motion required? As the means of appraisal, a criterion of performance was needed against which the performance of each fixator could be compared. Although an exact prescription for motion could not be obtained from the literature, it was possible to form a general guide from what is already known. Since the effect of shear strain on healing is unclear, it seems prudent to expect fixators to avoid transverse and torsional movement at the fracture. Also, as angular motion causes non-uniform axial strain around the cortex and the pivot point can be difficult to control, it seems prudent to avoid angular movement. This leaves axial motion, which can be applied successfully to stimulate callus osteogenesis, provided that it is controlled in amplitude to avoid axial strains

reaching yield level in the intra gap tissue”. Therefore, the preferred pattern of movement would be controlled axial motion, with the avoidance of angular and shear motion. If fixators control motion in this way, patients may be encouraged to walk as early as discomfort allows, since walking will provide the physiological frequencies and strain rates also found to be optimum for healing. If they do not, the effect of load bearing movement on healing is likely to be more inhibitory than stimulator-y. The relative performance of each fixator was therefore evaluated from the disparity between the actual fracture site motion of a model during simulated walking, and the preferred motion. METHOD Fixators A group of unilateral external fixators currently used clinically were selected for the study of inter fragmentary motion. -These were the Dynabrace (Smith and Nephew) j Bi-roll (Hoffman), Modulsystem DAF, (Orthofix), the red and blue Monotubes (Howmedica International). Where appropriate, tests were carried out using both the ‘dynamizing’ and ‘nondynamizing’ mode of operation of each fixator, where the individual dynamizing actions are generally different. For the Dynabrace, tibia1 load causes the groups of screw clamps either side of the fracture site to slide on the column towards each other, resisted by spring pressure. With the other four fixators this is achieved by the column sliding telescopically. Here, the Monotubes provide the possibility of using an adjustable spring pressure offering variable resistance to axial loading, whereas the Bi-role transfers all the tibia1 axial load to the fracture site, and the Modulsystem is used with or without a compressible ring, (the ‘Dyno-ring’) to resist axial movement. The fixator operations are therefore referred to as either locked (‘nondynamizing’) or unlocked (‘dynamizing’). For the tests in the unlocked mode, the Monotubes and Dynabrace were operated with zero spring return pressure and the Modulsystem was used with and without the ‘Dyno-ring’. Experimental

models

In the laboratory model of the stabilized fracture (Figure I) each fixator frame was arranged in a standard geometric configuration, except for some variation in the lateral spacing between bone screws as a result of the individual clamp designs. Two 135 mm lengths of glass fibre tubing of 25 mm diameter were arranged in line to model the bone each side of a 60 mm gap. Two 6 mm diameter screws were fixed into each section at 200 mm between pair centres, and at a tube/screw clamp clearance of 70 mm. In this first condition (the unsupported frature model), only the fixator framework provided stability at the fracture site; no contribution to the support of tibia1 load was made from the fracture site. This

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Fixator

Figure 1 The model fracture configuration, Axial, bending and torsional loads are applied to the model bone cyclically and in phase, to simulate physiological loading during walking

model simulates the immediate post-operation behaviour of cornminuted fractures or unreduced fractures with substantial gaps owing to bone loss. In the second condition, additional support was provided at the fracture site to simulate tibia1 loadbearing across the fracture, between the interposing fragment ends (the well-supported fracture model). This model simulates the behaviour of non-comminuted fractures well-reduced, that sub sequently develop only a marginal gap. A 30 mm diameter polyurethane disk of thickness 12.5 mm was bonded to aluminium rods, that were inserted into the ends of the two tubes across the fracture site, to simulate the resistance to inter fragmentary motion provided by the callus of a healing fracture. Four grades of simulation material were used that had axial stiffnesses of 50, ,385, 526 and 1430 N/mm. The 50 N/mm material modelled the contribution to stiffness provided by a wellsupported fracture at about four weeks post fixationzo, where a soft cartilaginous callus may be formed prior to ossification. This period of low fracture stiffness is important because, at this early stage in healing, the fixator frame is expected to have a greater mfluence on movement at the fracture. Also, since there is potential for greater movement during the initial stages of fracture repair, it is suspected that movement during early healing may have a greater effect on the outcome of healing. After this period the contribution of the fixator frame to axial stability of the fracture begins to reduce significantly*l, as the fracture heals and stiffens. During this later phase, the gap tissue and periosteal callus mineralize form bone and remodel, leading to the removal of the fixation device at an axial fracture stiffness of around 1000 N/mm **. This secondary phase was modelled using the 385, 526 and 1430 N/mm fracture simulation materials.

of 2 Nm was applied by the piston thruster to the bottom (distal) end of the model tibia, in phase with the vertical load. This simulated a torsional loading of the tibia owing to a rotation of the foot about the long axis of the bone. A bending moment of 1.75 Nm was applied to the fracture site about an axis parallel to the screws, by offsetting the line of action of the axial load at the top end of the model bone (proximal) by 16 mm. This arrangement simulated a laterally eccentric loading of the tibia, applied through the knee. The loads were combined as shown in Figure I and were applied cyclically using compressed air pulses from a pneumatic diaphragm thruster and cylinder thruster, installed at the lower end of the model tibia. The combination of loads on the unsupported fracture model produced approximately the spectrum of inter fragmentary movement observed in the group of fracture patients.

Measurements

The Oxford Micromovement Transducer (OMT)” was clamped between the inner pair of screws parallel to the tibia and immediately adjacent to the fracture, to measure 3dimensional inter fragmentary movement under loading for both fracture models. Movements at the transducer in response to the loading were measured in 6 degrees of freedom (three linear orthogonal directions, and three angular rotations about the linear axes). These movements were then translated to obtain the inter fragmentary motion at the fracture site, and were finally reduced to the four directions of movement shown in Figure 2. Diagrams (a) and (6) illustrate the two linear movements of transverse shear and axial compression, while (c) and (d) illustrate angular movement and torsional shear movement. (Angular movement was calculated in the plane for which the angle was the maximum). A load cell at the base of the model measured vertical reaction, and a computer was used to acquire load vs displacement data from the load cell and transducer over 6 s test periods.

(a)

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An axial load of 220 N was applied along the longitudinal axis of the model tibia by the diaphragm thruster at the base. A clockwise torsion

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Inter fragmentary motion was examined in comparison with the desired motion performance already discussed. The control of axial displacement was considered to be beneficial and the constraint imposed upon shear (both transverse and torsional) and angulation as being desirable. Also, it was assumed that the object of unlocking the fixator is to alter the dynamic load at the fracture, without affecting the non-axial constraint (against angulation and transverse or torsional shear movement) ’ ’ . Since well-supported and unsupported fractures have substantially different behaviour, they were addressed separately.

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I:@Lw 3 shows the peak amplitudes of inter fragmentary motion seen during simulated walkiqg, for an unsupported fracture model with the fixator in the locked mode. Here, movements are substantially greater than will be seen later with the well-supported fracture model. The greatest peak axial movement at the fracture site occurred with the blue Monotube and the Bi-roll, with the Bi-roll also offering the least constraint against transverse shear and angular movement. The Modulsystem offered the greatest constraint against transverse shear, the Dynabrace against torsional shear, and the red Monotube against angular movement.

0

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50 N/mm), contribute almost equally to resisting tibia1 load (in the ratio of 60:50). That is, the mechanical properties of the fixator are as important as those of the fracture material in influencing inter fragmentary motion at around two to four weeks post fixation. The Bi-roll allows the greatest axial movement with the fixator locked. Unusually, with the Dynabrace the expected change in overall stiffness, caused by unlocking the fixator, appears to have little influence on peak axial displacement; here friction may be limiting the axial sliding at the fixator column. The greatest difference in axial movement was produced by the blue Monotube, (increasing when unlocked by 1.1 mm), followed by the Modulsystem (0.34 mm), and the red Monotube (0.28 mm). Transverse shear displacements were resisted better in the Dynabrace, red Monotube and BiRoll fixators, than the blue Monotube. Although the Modulsystem Provided some resistance to transverse shear in the locked mode, it increased significantly in the unlocked mode, as did torsional shear. This was caused by the looseness of the telescoping mechanisms in both the Monotube and Modulsystem, when operating in the unlocked mode”. Peak angular movement appeared to reduce overall when the red Monotube was unlocked, but either remained unchanged or increased with the other fixators. It is worth noting that peak move-

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Figure4 Peak inter fragmentary motion at the ‘well-supported’ fracture model during simulated walking for the five fixators. The fixator columns are either ‘locked’ or ‘unlocked’. Four fracture simulation materials of different stiffnesses are used to model the mechanical properties of the different stages of healing from the initial growth of the callus to its ossification (a) 50 N/mm, (b) 385 N/mm, (c) 526 N/mm, and (d) 1430 N/mm

ments in all directions were lowered by 50 to 100% by fitting the ‘Dyno-ring’ to the Modulsystern; this also reduced the additional transverse and torsional shear arising from the looseness of the telescoping mechanism.

The 385 N/mm simulation maternal. With the 385 N/mm material (Figure4(b)), peak axial movements were substantially reduced for all the fixators. This material simulated the rapid increase in stiffness and the corresponding reduction in movement associated with the callus mineralization stage. Again, axial movement was not significantly changed by unlocking the Dynabrace and Bi-roll fixators,.and also on this occasion there was no sign of the increase in axial movement observed with the 50 N/mm material when unlocking the Monotubes and the Modulsystem. Transverse shear movements were constrained to less than 0.3 mm for the red Monotube and Biroll in both locked and unlocked modes, and for the Modulsystem in the locked mode. Again, owing to rotational looseness, it increased after unlocking with the Modulsystem, and only slightly with the blue Monotube. Peak angular movements were best constrained

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with both Monotubes, but the previous reduction in angular movement caused by unlocking the red Monotube was not apparent with this simulation material. The Monotube fixators provided the greatest constraint against torsional shear movement, but shear.was substantial with the Bi-roll and the Modulsystem; this was again owing to the fixator looseness when unlocked. In general, the model indicates that axial movement is reducing proportionately more than angular and shear movement during the initial stiffening of the fracture site simulating calcification. The influence on healing of angular and shear movement may therefore become more important than axial movement, as fractures heal.

The 526 N/mm simulation material. With the 526 N/mm material (Figure 4( c)), representing a fracture, perhaps at the half-way sta e to fixator removal at a stiffness of 1000 N/mm i5 , peak axial movement did not appear to be influenced at all by the type of fixator or its mode of operation. Movements were between 0.4 and 0.5 mm for all fixators in both the locked and unlocked modes. Transverse shear movement was constrained to below 0.2 mm with the Dynabrace, but was less constrained in the Modulsystem; again this was due to fixator looseness. Similarly, with torsional shear, movements were constrained to within 0.2” with the blue Monotube but less so with the Modulsystem (locked and unlocked). Angular movement was below 0.25” with the blue Monotube, but was again less constrained by the Modulsystern, and generally movement was increased by unlocking the fixator. The 1430 N/mm simulation material. With the 1430 N/mm material, representing a fracture at the fixator removal stage, the fracture movements were small (Figure 4(d) ) . Axial tibia1 load was resisted by the combined fracture and frame system in proportion to the individual stiffnesses of the system (1430:60). Therefore, axial movements were again largely controlled by the fracture material, rather than by the fixator frames of comparatively low average stiffness, and peak axial displacements (of around 0.15 mm for all frames) were largely unaffected by the type of fixator or its mode of operation. However, both shear movements were again enhanced with the Modulsystem, by unlocking the fixator and initiating fixator looseness. DISCUSSION Inter fragmentary fracture

motion

in the well-supported

Validity of the model. The validity of the model was first established before conclusions were drawn from the results. This was demonstrated by comparing movements in the model, using the low stiffness simulation material, with those measured in patients during early healing. Peak axial movements with the fixators locked were generally

between 1 and 2 mm for the model. This correlates reasonably well with measurements obtained from patients with initially fully reduced fractures, that had axial movements of up to 1.8 mm while walking at 2 to 4 weeks post fixation’“. Also peak transverse shear was constrained to within 1.0 mm, and angular movement to within 1” for the model, which showed reasonable correlation with transverse shears generally below 0.7 mm and angular movements below 1.0” recorded in patients. However, a slight weakness in the model is exposed by unlocking the fixators. Peak cyclic axial movement was increased significantly with the low stiffness material by unlocking the Monotubes, but this was not the case in the clinical condition for a patient fitted with a blue Monotube”. Here, contrary to general expectation, a trend of reducing axial movement was seen after unlocking the fixator. Therefore, the loaddisplacement response of the simulated fracture material, which is fully elastic, is not the same as fracture tissue which probably has a viscoelasticplastic response. That is, full recovery of the initial gap size on unloading occurs immediately in the model, but in the clinical situation recovery is timedependan t because of the viscoelastic response, and remains incomplete because of the plastic response. Fix&or ptyformance. If it is considered that controlled axial movement may be applied to fractures to provide the desired mechanical regime for healing, then do currently available unilateral external fixators enable the clinician to control this movement? The answer must be no, since fracture movement arises from the combined flexibility of the frame and the fracture material, and is a consequence of the degree of weight-bearing, the fracture gap and the support to tibia1 load provided by the section of fractured tibia. Clinicians may only influence to a degree the magnitude and orientation of movement by providing support to tibia1 load through the fracture site (for example by reducing non-cornminuted fractures), and by unlocking the fixator where and when it is desirable. Since at present no correlation has been made between the extremes of axial, shear and angular movements measured in patients and their effect on healing, a conservative approach to fixator design should be adopted. The frames tested here should constrain against movements not universally accepted to be beneficial. Therefore, those that do not fUy COIIstrair, against shear and angular movement need to be stiffened. For angular movement in the locked mode, the E-roll and the Modulsystem provided the least constraint at 1.4” and 1.15”, with the greatest constraint provided by the Monotube and Dynabrace fixators. In addition to the possibility of an inhibitory affect on healing, there is a risk of refracture at the external cortex with this degree of angular movement during walking. If, for example, it is assumed that the fracture gap is 1 mm, a 1.4” angular movement would cause an axial gap strain

of around 25% in line with the external cortex of the bone. Although the distribution of strain in the non-homogeneous gap tissue will be complex, maximum axial strains are unlikely to be much lower than 25%. This amplitude of strain will be sufficient to refracture all but the spongy granulation tissue formed in the first stage of healing”“. If refracture occurs regularly through walking, at some point the capacity of the fractures to heal will be exceeded by the continual challenge to the physiological repair processes. This will result in an inhibitory affect on healing, which may contribute to the delayed union of some fractures, and may cause an increase in the incidence of non-unions arising from inappropriate mechanical conditions. In the same example, the transverse shears of up to 1.0 mm discussed earlier would cause a gap shear strain of 100%. The overall reduction in peak angular movement for the red Monotube that occurred through unlocking the fixator column would reduce the risk of refracture in the healing callus. Here the fixator column is allowed to shorten telescopically, rather than bend, because of compression at the fracture site. Since bending of the column imposes angular movement on the fi-dcture, the reduction in bending of the Monotube leads to a reduction in angular movement at the fracture. With the other fixators, unlocking caused the angular movement at the fracture to be either unchanged or to increase. possibly because of a combination between the column not sliding telescopically (sticking) and the looseness in the column (slack) assisting angular movement. This was not the case with the Modulsystems in the clinical situation, where angular movement for the group of 10 patients reduced by an average of 28% through unlocking the columns”‘. although the number of patients measured was insufficient to provide statistical significance at f> > 0.05 for the difference. However, healing may also be affected detrimentally by the unlocking of the column, since this reduces the constraint against. transverse and torsional shear. The least constraint against torsional shear was provided by the Modulsystem, and this increase in shear has also been observed in tibia1 fracture patients stabilized by Modulsystems. At 6 weeks post fixation, the average increase in torsional and transverse shear at the fracture site through unlocking was fotlnd to be around 100% I”. For the three stiffer fracture materials, simulating all but the initial stage of healing, axial movement was influenced only by the the stiffness of these materials; it tias only slight9 influenced by the contribution to combined stiffness made by any of’ the fixation devices. Therefore, the choice of’fixator is expected to have little influence on the peak axial movement and the healing of well supported fractures that are additionally stabilized b!. the formation and calcification of callus. Also, unlocking the fixators had little effect on axial movement. although with the Modulsystem it again reduced significantly the constraint against other directions of movement (particularly trans-

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verse and torsional shear), which increases the potential for refracture. There was no reduction in angular movement through unlocking the Monotubes as seen with the least stiffest simulation material. This is because the axial compression of the stiffer materials was substantially lower than with the 50 N/mm material. Therefore, in the initially locked mode, there was a reduction in the angular movement imposed at the fracture site by column bending. Inter fragmentary fracture

motion

in the unsupported

Movements are substantial at around 3 mm (axial), 2 mm (transverse shear) and 1.5” (angular movement), and are similar to those that occur with largely well-supported fractures using more flexible forms of stabilization such as plaster castP. They are about twice the movements of the well-supported fracture during early healing (O-6 weeks). Therefore the healing response for unsupported externally fixated fractures may be closer to that of a fracture stabilized by plaster. The influence on inter fragmentary motion mechanical conditions at the fracture site

of

For well-supported fractures, the control of inter fragmentary motion during patient activity and the consequent effect on healing should be of great concern. Here, there is more restriction to axial movement which is potentially beneficial, while the shear and angular movements, that may be disruptive, remain possible. Again, for largely unsupported fractures, fracture movement is important since little resistance to movement is offered at the fragment ends, and in place of this the frame is comparatively flexible. Here, the dominant influence on fracture motion is the degree of weight-bearing. The results demonstrate the load-bearing interplay between the fixator frame and the fracture site, during the progressive stiffening of a fracture as it heals. This can be explained using JQUW 5 showing the proportion of axial tibia1 load supported bv the bone ends across the fracture. The curve of increasing fracture stiffness is obtained from the work of Cunningham et aLz3 against which an average frame stiffness of 60 N/mm can be compared. As soon as the fracture site shows

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General

observations

A criticism may be made of this study in relation to the measurement of inter fragmentary ‘movement’ at the fracture, and the prediction of ‘strain’ as movement in proportion to gap size. It has been necessary to take this simplistic view of what is really a non-isotropic strain field within a material for two reasons. non-homogeneous Initially it was to provide legitimate mechanical conditions for the performance comparison, and subsequently it was to provide some means of predicting the consequences of the mechanical An accurate characteristics of each fixator. detailed distribution of strain may only be predicted by using comprehensive S-dimensional finite element models of the complex geometry of a real fracture, in conjunction with the measurements of inter fragmentary movement. This may not be a practical solution, in view of the difficulty of this approach. However, it has become apparent from this limited study that fixator design does not make the best use of the little that is known about the influence of mechanical conditions on fracture healing. A more informed approach would be to avoid the reduced constraint in directions for which the effect of movement on healing is unclear. REFERENCES

fixator ----stiffness

VVeLs d&t-fiGtion

5 The proportion during progressive

I

1100

fracture loa

signs of stabilizing (at 4 weeks in this example) almost all the tibia1 load is very quickly transferred from the fixator to the fracture, because of a comparatively low frame stiffness. However, during the initial stage of healing (O-4 weeks), there is very little tissue solidity at the fracture and it is then that the degree of support from the interposing fragment ends is critical; this period may be prolonged in the case of an unsupported fracture. For well-supported fractures, it has been found from clinical studies that the mean peak cyclic compression is of the order of 1.0 mm before most of the axial tibia1 load is transferred across the fracturelg. This means that the fixator will sup port only the initial 60 N (6 kg) of any tibia1 load before the gap is compressed and further movement is restricted; any additional load thereafter is transferred between the fragment ends. Here the fracture may be loaded at almost full body weight and, although inter fragmentary movement at the fracture may be small, inter fragmentary strain may be substantial. For largely unsupported fractures during this period, load equates directly with movement. In the example, each 60 N of axial tibia1 load produces 1.0 mm of compression; therefore movement at the fracture may be substantial although inter fragmentary strain may be small.

24

tibia1 load supported as it heals

by the

frac-

1. Rahn BA, Gallinaro P, Baltensperger A and Peren SM. Primary bone healing: an experimental study in the rabbit. J Bone andJoint Surg. 1971; 53: 783-786. 2. McKibbin B. The biology of fracture healing in long bones. J. BoneJoint Surg. 1978; 60B: 150.

3. Goodship AE and KenwrightJ. The influence of induced micromovement upon the healing of experimental fractures. ]. Bcwze,Joint Surg. 1985; 67-B/4: 650-655. 4. Kenwright J and Goodship AE. Controlled mechanical stimulation in the treatment of tibia1 fractures. Clin. Orth. and Rel. Res. 1989; 241: 36-47 .i. Sarmiento A. Functional bracing of tibia1 fractures. Clin. Orthop. 1974; 105: 202. 6. Lippert FG and Hirsch C. Three dimensional measurcment of tibia fracture motion by photogrammetry. Cli. OtThop., 1974; 105: 130-143. 7. Lindholm RV, Lindholm TS. Toikkanen S and Leino. The effect of forced inter-fragmental movements on the healing of tibia1 fractures in rats. A&L Orthop. Scund., 1970; 40: 721-728. 8. Yamagishi M and Yoshimura Y. The biomechanics of fracture healing. J. BoneJoinl Sung. 1955; 37A: 1035-1068. 9. Carter DR. Blenman PR and Beaupre GS. Correlations between mechanical stress history and tissue differentiation in initial fracture healing. ,J. Orthop. I&s. 1988; 6: 736-748. 10. Lanyon LE, Rubin CT, O’Connor JA and Goodship AE. The stimilus for mechanically adaptive bone remodelling. In: O.~teopm-osis. Menczel J, Robin GC, Makin M and Steinberg R, eds. Wiley, UK, 1982; 135-147. 11. De Bastiani G, Aldegheri R and Renzi Brivio L. The treatment of fractures with a dynamic axial fixator. j. Bow .Joint Surg 1984; 66B: 538-545. 12. Gardner TN and Evans M. Relative stiffness, transverse displacement and dynamisation in comparable external fixators. Qiniral Biomrchanirs 1992; 7: 231-239. 13. Brighton CT. In: Principle.? o/j-acture healing: Instructional rour.Te IpCture,y. Murray JA, ed. C. V. Mosby Co., St. Louis, 1984; 60-82.

14. Behrens Bending

F, Johnson WD, Koch TW and Kovacevic N. stiffness of unilateral and bilateral frames. Clin. Orth. and Rel. Res. 1983; 178: 103-110. 1.5, Kristiansen T. Fleming B, Neale G, Reinecke S and Pope MH. Comparative study of fracture gap motion in external fixation. Clin. Biomech. 1987; 2: 191-195. 16. McCoy MT, Chao YS and Kasman RA. Comparisons of mechanical performance in four types of external fixators. Clin. Orth. and Rel. Res. 1983; 180: 23-33. 17. Paley D, Fleming BS, Catagni M, Kristianssen ‘I’ and Pope M. Mechanical evaluation of external fixators used in limb lengthing. C/in. Orth. and I&l. REX 1990; 250: 50-57. 18. Gardner TN, Evans M, Simpson AHRW and TurnerSmith AR. 3-Dimensional movement at externally fixated tibia1 fractures and osteotomies dllring normal patient function. Clinical Biomechanics 1994; 9: 51-59. 19. Gardner TN, Evans M, Simpson AHRW, Kcnwright .J, Hardy JRW and Richardson JB. Can walking heal fractures. Prof. 2nd. Meeting OJ Combined Orthopmdir Research .Societies, San Diego, 1995, in press. 20. Perren SM and Cordey J. Die Gewebsdifferenzierung in der Fracturheilung. Monatsschrift f. I?lfalZhrzlkunde 1977: 80: 161-164. 21. Beaupre GS, Hayes WC, Jofe MH and White AA. Monitoring fracture site properties with external fixation. Trans ASME 1983; 105: 120-126. 22. Evans M, Kenwright J, Cunningham JL. I)esign and performance of a fracture monitoring transducer. j. Biomed. lhg. 1988; 10: 6469. 23. Cunningham JL, Evans M and Kenwright J. Measurement of fracture movement in patients treated with unilateral external fixation. J Biomed. Rng. 1989; 11: 118-122.

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