A stress analysis of the patella, and how it relates to patellar articular cartilage lesions

J. Biomechanics Vol. 12, pp. 69!-711. 0 Pergamon Press Ltd. 1979. Printed in Great Britain

0021-9290/7910901-0699

W2.00/0

A STRESS ANALYSIS OF THE PATELLA, AND HOW IT RELATES TO PATELLAR ARTICULAR CARTILAGE LESIONS R. J. MINNS Department of Engineering Science, University of Durham, Durham, U.K. A. J. M. BIRNIE Department of Orthopaedics, Dryburn Hospital, Durham, U.K.

and P. J.

ABERNETHY

Princess Margaret Rose Orthopaedic Hospital, Edinburgh, U.K. Abstract - The pathogenesis of three types of lesion of the articular cartilage of the patella are described.

Stress analysis of the patella mid-sections show that for large values of‘Q’ angle, a wide area of the medial articular surface is under tensile stress. In those patients who have short medial and ‘odd’ facets, the tensile stress generated could lead to fatigue failure of the cartilage producing open chondromalacic lesions commonly observed on the medial aspect of the patella. Vertical tensile stress can occur over the medial side of the lateral facet. If tensile stress is combined with high contact stresses applied normal to the surface, there exists a situation of very high shear stress which may fatigue the deeper layers of the articular cartilage, leading to the closed chondromalacic type-of lesion centred on the median region and commonly extending on to the lateral facet. A third rare type of lesion, termed chondrosclerosis, may be caused by an extreme compression phenomenon as a result of the high bending stress combined with the high contact stress over the lateral facet. The effect of surgical procedures on these stresses is considered.

INTRODUCTION

Numerous explanations have been proposed to account for the destruction of human articular cartilage. In the patellofemoral joint, such lesions have been particularly related to the mechanics of articulation, to contact stresses and to the contact surface topography. Wiberg (1941) proposed one of the earliest explanations for the pathogenesis of articular cartilage change occurring on the medial patellar facet. He described certain positions of high joint loading in which a convex surface of the facet opposes a convex surface on the underlying femoral condyle, producing high contact stresses. This applied particularly to those patellae in which there was a distinct ridge between the medial and odd facets. One argument against this hypothesis is that fewer cartilage changes occur on the medial femoral condyle, even though it is subjected to the same surface stresses. Wiberg’s explanation was that the articular cartilage was thicker on the patellar facet than on the condyle and as a result probably receives less nutriment and consequently becomes more prone to degenerative disease. A further argument is that there are patellae which have concave surfaces and yet still undergo degeneration of the articuiar cartilage on the medial and odd facets, even though these do not contact the condylar surface except in certain positions of extreme flexion. l

Received 19 November 1978.

Outerbridge (1961) described a rim on the upper border of the medial femoral condyle which abraded the medial patellar facet leading to chondromalacic changes on this facet. However, patients without this ridge may also have similar lesions on the medial patellar facet (Emery and Meachim, 1973). Bandi and Brennwald (1974) noted that both extremely high contact pressures and surface friction may lead to patellofemoral arthrosis. They advocated anterior displacement of the tibia1 tuberosity to reduce the contact pressure. Ficat and Maroudas (1975) cited high contact pressures combined with lower glycosaminoglycan content of the articular cartilage, as important factors in the frequency of observed destructive changes of the patellar cartilage compared with the hip joint. There appears to be no relationship between patellar height (Blackburne and Peel, 1977) and chondromalacia. Insall et nl. (1976) support the relationship between patella alta and chondromalacia whilst Marks and Bentley (1978) disclaim such a relationship. Although trauma has been implicated as a possible cause of cartilage destruction, some feel it is of more importance as an aggravating factor than as a primary cause (Wiles et al., 1956). Structural changes in the underlying bone and its effects on the loading of the trabeculae have been suggested to result in a concentration of cartilaginous shear stresses on the central medial facet (Townsend et al., 1977). If stiff or weakened bone is adjacent to 699

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normal bone, there exists a difference in resistance to deformation, producing high local shear strains within the cartilage which may be responsible for cartilage damage (Abernethy et al., 1978). Variations in the location of cartilage lesions and their association with certain clinical syndromes has been described by Ficat and Hungerford (1977). Three main types of lesion have been established. The first type, termed open chondromalacia, is usually confined to the medial and odd facet and involves the secondary ridge between these two facets. Goodfellow et al. (1976b) claim that these changes can be almost limited to the odd facet. The pathological features of this lesion comprise a proliferation of cartilaginous material sprouting above the surface and consisting of massive bundles of collagen fibres arranged in tufts and whorls and associated with an abundance of hyperactive cells. A second lesion, usually developing on the central medial aspect of the patellar surface and particularly associated with the median ridge, is termed closed chondromalacia. This consists of a Iocalised softening of the cartilage. In this instance, electron microscopy reveals an area ofabnormality which is localized to the intermediate zone of the uncalcified cartilage and in which the collagen fibres undergo abrupt changes of direction and disintegration. A third lesion, termed chondrosclerosis, is usually associated with an extreme lateral pressure syndrome involving the lateral facet. Its gross pathological appearance comprises areas of hard and yellowish cartilage. The collagen fibres within this lesion are usually dense and compact throughout all the zones of the cartilage. With increasing age, particularly in women, the patellar cartilage invariably becomes thinner and is associated with narrowing of thejoint space (Meachim et al., 1977). This leads to a secondary stage of patellofemoral arthrosis, accompanied by the usual underlying bony changes and in which the three cartilage lesions previously described cannot be isolated (Ficat, 1974). Bony changes in chrondromalacia have been described by Darracott and Vernon-Roberts (1971). Large areas of oesteoporotic bone, predominantly deep to the medial facet, were associated with discrete nodular aggregates of woven bone attached to the trabeculae. They reasoned that these nodules arose as a result of some form of disordered new bone formation producing a buttressing effect at sites of high trabecular stress. Stougard (1975) observed that the bone was less dense medially and that bone sclerosis did not occur even in the presence of medial cartilage damage. Bony trabeculae are modified when subjected to high compression forces, as observed radiologically in the extreme lateral pressure syndrome (Ficat and Hungerford, 1977). Haasters (1974) wrote that the spongiosa of the patella, as observed on lateral X-ray of the patella, responds to loads applied through the quadriceps mechanism. Clearly, the localization of the lesions appears to support the hypothesis that stress

P. J. ABERNETHY

and the evolution of chrondromalacia are associated (Wiles et al., 1956). The purpose of this study was to determine the effect of changes in ‘Q’ angle, patellar tendon length and medial-A/P patellar angle on patellar surface stresses in normal knees, and correlate the stress levels and patterns with a small number of patellae from patients who had retro-patellar pain. MATERIALS AND METHODS

Source of patellae

Fifty-six normal patellae were examined. These comprised : (1) sixteen were sections which had been produced for a previous study conducted by one of us (P. J. A.) and were post-mortem horizontal mid-sections, prepared as described by Abernethy et al. (1978). The negatives of these patellae sections included grids marked in centimetres for scaling purposes; (2) twelve patellae were obtained post-mortem sawn horizontally through the mid-section, laid on a photographic plate with a scale and X-rayed; (3) skyline radiographs were also taken of 28 patients, of average age 20 yr, who on clinical examination were free of patello-femoral symptoms. Skyline radiographs of 11 patellae were also studied from 11 patients who underwent surgery for the relief of patello-femoral pain. There were 2 men and 9 women in this group with a mean age of 20 yr. Of these, 9 were considered to have the extreme lateral pressure syndrome (ELPS), of whom 2 had Wiberg type I patellae and 7 had Wiberg type II patellae. Of the 2 patients who were considered to have medial chondromalacia patellae, one had a Wiberg type III patella and the other a Baumgartl(l964) type patella. One patient with Wiberg type II patella had bilateral fractures of the lateral facets. In this group of patients undergoing surgery, physical dimensions were taken from the patient for scaling purposes and only true skyline views were analysed. The X-ray cassette was positioned at the same distance from the mid-patellar axis in all cases as was the X-ray tube at the distal end of the leg. Stress analysis

To obtain a stress analysis of the section, it is necessary to know both the loads encountered and the geometrical properties of the section, including the position of the centre of the plan area, the principal second moments of the area and the angle of the principal axes to the width and depth of the patella. The geometrical properties were determined by the analysis used by Minns et al. (1975) and are summarized for both normal and chondromalacic patellae in Table 1. The loads considered were those determined by Smidt (1973) for isometric quadriceps contracture (Fc) at knee joint angles (01)of5,15,30,45, 60 and 90”. The force F, was expressed as a function of body weight and at 90” of knee flexion was 3.63 times body weight, which is close to the value for patello-

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Stress analysis of the patella Table 1. Geometrical properties of normal and chondromalacic patellae

Chandromalacic potelloe

6.354

femoral joint reaction force obtained by Reilly and Martens (1972) for climbing and descending stairs. The average load patello-femoral contact area was taken from Matthews et al. (1977), as was the angle of the patellar mechanism (4) which relates the angle of the patellar tendon to the tibia ($) and to the knee flexion angle (a). The physiologic valgus of the knee gives an angle between the line of pull of the quadriceps and the patellar tendon, the so-called ‘Q angle (Ficat and Hungerford, 1977). This angle is normally 15”, and is considered abnormal if greater than 20” (Insall et al., 1976). This ‘Q’ angle produces a lateral bending movement about the patellar centroid related to the quadriceps/patellar tendon force passing through the mid-section. The open medial angle of the

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patellar (fl in Fig. 1) was determined by the method of

Laurin et al. (1978) with slight modification, in that the width of the patellar axis was related to the mediolatera1 plane. Because of the lateral obliqueness of the force F,, two resolved bending moments occur on the section considered, ML and MAP,which produces tension medially and compression laterally (see Figs. 2a-c). The magnitudes of the surface stress are rdated to the geometrical properties of the section and to the direct bending loads applied to that section. Using the analysis described by Minns et al. (1977), the total stress at any point on the surface can be determined. This analysis required data on the cross-sectional geometrical properties, the position of the centre of area of that section, the direct forces and the bending moments acting through this centre of area and the displacement of the surface about the centre of sectional area. The surface stress at 10” intervals around the centre of area was determined using conventional planar beam theory. As this was a stress analysis and did not include a strain analysis of the surface, the material (bone) properties were not needed. However, the material was considered to act as a homogeneous, isotropic material throughout the section analysed, which is obviously a gross assumption, but which for a comparative stress analysis between sections is justified. The pateilar length and the patellar tendon length were determined by the method of Insall and Salvati (1971) and as these dimensions affect the magnitude and the direction of the force resultants, these were both varied to determine their effect on the stress distribution of each section. Scanning electron

Fig. 1. Loads and angles used in the stress analysis of the pateilag.

!

microscopy

Samples ofcartilage were collected from the patients who had undergone chondrectomy. The samples were stored in distilled water until ready for preparation for scanning electron microscopy. The specimens were cut into samples approximately 5 mm by 5 mm (thickness was typically 2 mm) and were then dried by using a critical point drying apparatus with liquid carbon dioxide as the drying medium. The specimens were then mounted on marked aluminium stubs and coated with gold/palladium in a sputter coating unit to produce a uniform coating approximately 200 A thick. They were then examined in a ‘Cambridge’ MOO Stereoscan electron microscope at an accelerating

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Fig. 2. Loading system on the three views of the patella and method of representation of surface stress variation. The angle between the pull of the quadriceps mechanism (Fo) and the patellar tendon (FPT) is; viewed from the front, termed the ‘Q’ angle.

voltage of 15 kV and views were recorded on FP4 135 film in an Exacta camera attached to the microscope.

stress shown within the patellar profile. The scales are shown on each figure. The c$l%ctof patellar tendon length on patellar stresses

RESULTS Our method of presenting the stress variation around each contour is that the magnitude of the compressive stress at any point on the surface is shown on the outside of the patellar profile with the tensile

To study the effect of patella alta, the resultant stress variations of patellar length and patellar tendon to the tibia1 angle must be considered together. Figure 3 shows the variation of surface fibre vertical stress for different patellae lengths between 40 and 50 mm. There is no lateral ‘Q’ angle, i.e. the loading has been applied

Fig. 3. Variation of vertical surface stress around a horizontal section of a normal patella at 30” of knee flexion, with patellar length.

Stress analysis of the patella

Lateral

Fig. 4. Variation of vertical surface stress around a normal patella at 30” of knee flexion, with patella tendon/tibia angle.

in the sagittal

plane

at 30” of knee flexion. The

maximum compressive stress occurs posteriorly and at the greatest patellar length. Figure 4 shows the variation of surface stress at 30” of knee flexion, with patellar tendon to tibia1 angle. This is equivalent to varying the length of the patellar tendon if the patella is considered to remain in the same position. Increasing the angle, or decreasing the patellar tendon length, has the effect of increasing the maximum compressive vertical stress on the articular surface by 15% when the range of equivalent patellar tendon (PT) to patellar length (P) ratio varies from 0.7 to 1.3. Sections were also taken at 20, 30 and 70% levels of patellar length and its variation of surface stress as a result of axial tension and bending were analysed. The bending moments are reduced accordingly, but the geometrical properties reduce to give the same stress variations shown in Fig. 5. Surprisingly, the maximum compressive stresses on the surface vary only slightly throughout the length of the patella; 100°? being the most distal point of the apex. The stresses are shown for a ‘Q angle of 15” and are predominantly compressive on the lateral articular surfaces with a small area in tension medially. The maximum compressive stresses vary from 3.1 MN/m2 at 20”/, of the patellar length to 4.5 MN/m’ at mid-section. Five normal age-matched patallae were also analysed in a vertical plane through the middle. With a ‘Q’ angfe of 15”, the variation of surface stress was analysed for the three angles of knee flexion most corn-monly used for everyday functional activities. This variation is shown on the lateral view of the patella in Fig. 6; the largest compressive stresses occurring at mid-plane between 50 and 60”/, of the length distally, the maximum values rising from

3 MN/m2 at 5” of knee flexion to 4.5 MN/m’. The maximum tensile stress of 4.3 MN/m2 at 30” of knee flexion occurs anteriorly at mid-section level. 7he

&ct

of the ‘Q’ angle on patellar

stresses

The ‘Q’ angle has been considered relevant to the lateral loading of the patella, giving rise to a lateral component of force resisted by the medial structures. A stress analysis was conducted on the patella of a young female patient who had chondromalacia of the medial and ‘odd’ facets of her left patella. It was noted that she had a high ‘Q’ angle of approximately 20”, assuming that the ‘Q’ angle remained constant at 20” and at 15”, i.e. that no derotation at the tibia occurred during knee flexion. The analysis was repeated on the assumption that the ‘Q’ angle reduced from 15 to 5”, at 30” of knee flexion. The results are shown in Fig. 7 and show a dramatic increase in maximum vertical surface stress, which is tensile on the medial aspect of the patel$ as the ‘Q’angle is increased from 15 to 20”. The maximum tensile stress of 24.8 MN/m2 at the higher angles of knee flexion falls to 15.5 MN/m’ when the ‘Q’ angle is reduced by 5”. Even more dramatic is the drop in maximum tensile stress as the knee is flexed, if the ‘Q’ angle falls to 5” as a result of internally rotating the tibia. The stress is reduced by 50% when the ‘Q’ angle reduces from 15 to 5” at a knee flexion angle of 30”. The effect of the ‘Q’ angle was further studied by analysing normal patellae while the knee was flexed. The a;ngle was reduced from 15 to 5” and its effect on the surface stress variation was shown on patellar mid-sections. In Fig. 8 the variation of stress with ‘Q’angle shows little fluctuation, with the values at all points on the surface moving medially as the knee is flexed, obviously due to

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R. J. MINNS,A. J. M. BIRNIEand P. J. ABERNETHY

I PROXIMAL.

DISTAL

2#

70%

I

Fig. 5. Variation of vertical surface stress around a normal patella at different horizontal sections down the patella at 30” knee flexion.

a decrease in the lateral bending moment as the ‘Q’ angle reduces. The cflect of pateflar axis angie on pateflar stresses In the patient mentioned earlier with a characteristic Wiberg Type III patella contour, the medial ‘j?’angle was changed to determine this effect on the stress variation around the surface. The stress variation around this patella is shown in Fig. 9. An increase in this medial angle ‘r has the effect of increasing the tensile stress on the odd facet and the position at which the stress becomes compressive moves laterally: For this patient the ‘Q’ angle was kept constant at 20”, and for a medial open ‘/?’angle of 20”, the maximum tensile stress on the articular surface apppears to be as high as 36.0 MN/m’. This reduces to 30.0 MN/m2 when the ‘$ angle was zero. This patient had an extensive open

chondromalacic lesion of the medial surface approximately 1.5 cm in diameter centred on the medial/‘odd’ facet ridge, and it is this region which appears to be under considerable tensile stress. Mathematically altering the same patella by reducing the ‘Q’ angle during knee flexion produces a dramatic reduction in the surface stresses, in particular the tensile stresses in the region of the medial and ‘odd’ facets (see Fig. 10). During flexion, the maximum tensile stresses on the ‘odd’ facet fall from 17.0 MN/m2 to 9.0 MN/m2 when the knee is flexed to 30”, with the ‘Q’ angle reducing from 15 to 5” through this range of flexion. Specimens of excised cartilage were examined in the scanning electron microscope The chondromalacic articular cartilage displayed the characteristic ‘tuft’ appearance of proliferative collagen (Fig. 11) in the

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Stress analysis of the patella

5 Pax

t Mhl/WP

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5

10

15

Knee

I

-----

JO-

Fig. 6. Lateral variation of vertical surface stress for a normal patella at three angles of knee flexion. region of the medial facet, in particular

on the ridge separating the medial and ‘odd’ facet. Large areas within the lesions showed massive areas of torn

11

11

/Ill 0

20

25

flexlon

30

35

40

!

45

1

50

’

55

1

60

ongle, degrees

Fig. 7. Variation of maximum tensile stress with knee flexion angle, at different ‘Q’ angles.

collagen bundles predominantly torn parallel to the articular surface (Fig. 12), and through the superficial and middle layers. These characteristics were similar to those seen in lesions associated with osteoarthrosis (Minns et al., 1977), the main difference being that the torn fibre bundles seen on the femoral heads of

Fig. 8. Variation of vertical surface stress around a normal patella with knee flexion and decreasing ‘Q’angle.

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Fig. 9. Variation of vertical surface stress around a patella on a patient with chondromalacia patellae, with the medial angle ‘/?‘.Knee flexion angle 15” and ‘Q’angle maintained constant at 20”.

osteoarthritic cartilage articulating surface.

were perpendicular

to the

DlSCUS!SlON The application of conventional beam theory to the problem of bending of the patella is limited. It does not require a knowledge of the material properties of bone and cartilage. This simplified analysis can give an indication of the magnitude and areas of stress at the surface of the patella. The technique can be used as a comparative method of assessment for both normal

and chondromalacic patellae when subjected to different values of bending moments in different planes. Over its entire articulating surface the patella is subjected to stress that may be either compressive or tensile, due to the bending which the patella undergoes during contraction of the quadriceps mechanism, and the resulting reaction through the patellar tendon. This is an additional stress to the high contact stresses that have been evaluated as occurring in certain activities (Seedhom and Tsubuku, 1977). It will occur in areas that are not in contact with the opposing femoral sur-

5 MN/m’ 0

Fig. 10. Variation of vertical surface stress around the same patella as Fig. 9, but the ‘Q’angle decreases from 15” with knee fkxion angle.

-

.-

Fig. 11. Scanning electron micrograph of articular cartilage removed from a patient with open chondroma lacia of the medial facet with fasciculation and tufts dominating the whole defect.

Fig. 12. Scanning electron micrograph of articular cartilage removed from another chondromalacia of the medial facet, showing a large number of torn horizontal collagen whole lesion. 707

patient with open bundles within the

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Stress analysis of the patella (b) Shear fatigue lesion

(a) Tensile fatigue

lesion

(Open chondmmalacia

I

(Closed

chondromolocio)

Wlberg type II and UI E.L.P.S. .’ Taut loteml retinaculum 0 angle large- M lorge

Wiberp type l.lI, &lurngartl large ‘6’ angle, constant through flexion

Fig. 13. Diagram of the features associated with the three lesions seen in the articular cartilage of patients with patello-femoral disorders.

faces. The ‘odd’ facet in particular appears to be a noncontact area at knee flexion angles less than 90” (Goodfellow et al., 1976a). It has been suggested that lesions which occur in this area arise because of high shear and compressive stresses which occur during extreme flexion. The analysis we have conducted indicates quite clearly that if the quadriceps force is located laterally, there is a resulting large area of the medial patellar surface which can be under tension during the whole range of knee flexion in both normal and in chondromalacic patellae. The values of this tensile stress and its localisation on the medial aspect are dependent on the proximal and distal forces acting on the patellar periphery and on the geometrical properties of the section. In those patients who had chondromalacia of the medial facet, it was noted that their sectional geometric properties were lower (see Table l), creating less resistance to the high bending loads that occur. They also had a high ‘Q’ angle, which was maintained through large values of flexion. This high ‘Q’ angle has been observed in many patients with chondromalacia patellae undergoing surgical correction (Insall et al., 1976). It has also been observed that some patients with chondromalacia patellae have persistent external tibia] torsion which also maintains a larger ‘Q’angle (Ficat and Hungerford, 1977). In our stress analysis, the ‘Q’ angle has emerged as the most important factor in the production of high tensile stresses on the medial and, in particular, the odd facets. The values of tensile stress which occur in this region have been noted to be as high as 36.0 MN/m’, which is greater than any of the threshold values which Weightman et al. (1978) obtained for tensile fatigue failure of cartilage collagen framework. A further factor in the production of high medial tensile stresses is the medial ‘/Yangle. If this angle is large and is maintained during flexion, this indicates that the patella does not rotate

the vertical axis. If a combination of high unaltering ‘Q’ and ‘p’ angles co-exist in an overweight patient with a patella of small sectional geometrical properties, the patella will be subjected to repetitive high tensile stresses dangerously close to the values that Weightman et al. (1978) predict for the fatigue failure of collagen. We suggest that the open chondromalacic lesion located on the medial aspect of the patella is the result of tensile fatigue of the collagen framework occurring particularly in the superficial zone of the cartilage (see Fig. 13a). Any resulting patellofemoral pain which may be associated with this lesion must arise because of stimulation of nerve endings in the subchondral bone (Goodfellow et al., 1976b), because they are no longer protected from the cyclical tensile stresses. The dense osteosclerotic bone seen laterally and the osteopenic bone commonly seen medially may be a response to the high compressive stresses laterally and tensile stresses medially. The mechanism of the second type of lesion, in which softening of the cartilage about the median region is seen, may occur because the collagen framework fails due to a shear fatigue phenomenon (Minns, 1976). This occurs particularly in the middle and deeper bundles which are angled to the surface (Minns and Steven, 1977). The shear stresses could be a result of high compressive stresses normal to the surface, combined with tension due to the external lateral forces (Fig. 13b) at high values of the ‘Q’ and ‘b’ angles (see Appendix). The internal shearing of the cartilage would produce the microscopic appearances shown by Ficat and Hungerford (1977) of disorganized and broken collagen fibres in the intermediate zone matrix. The third type of lesion, in which areas of cartilage on the lateral surface appear ‘glossy’ and hard and microscopically chondrosclerotic, could be the result of extreme compressive stress due to bending of the around

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patella and high compressive of the lateral constraints geometry.

R. J.

MINNS, A. J.

M. BIRNIEand P. J.

contact stresses because and femoral trochlear

The matrix of the cartilage responds to this extreme compressive stress by producing masses of dense bundles of collagen fibres and thickening of the subchondral bone plate. The occasional stress fractures that occur (Devas, 1960), as we have observed in one instance, could be the result of combined compression due to the lateral bending of the patella, high contact pressure on the lateral surface and high tensile stresses due to the lateral retinacular mechanism. This would produce high vertical shear stresses in the lateral bone similar to that which produces the closed chondromalacic lesion already described. If these mechanisms exist, they can provide an explanation of the etiology of certain types of patellofemoral arthropathies. Most workers claim that the observed cartilage changes are responsible for the patient’s symptoms, although Abernethy et al. (1978) have recently questioned this relationship. A variety of surgical operations has been suggested in the management ofchondromalacia patellae and it is of interest to consider the mechanical effects of some of these procedures and how they might modify the observed patellar stresses. One of the ways of reducing the contact stresses is to remove some or all of the articular cartilage, but this has limited clinical success (Bentley, 1970; Goymann and Muller, 1974). It would have little effect on the tensile stresses that occur mediaily, but would lower the possible harmful central shear stresses that appear to exist in the extreme lateral pressure syndrome. Prosthetic replacement of the surface of the patella provides no solution, as high tensile stresses are still present. Not only is the anatomy altered by the introduction of the prosthesis, but the section geometrical properties are reduced, leading to even higher tensile stresses within the remaining bone. This may lead to loosening of the prosthesis and/or fracture of the supporting bone. If prosthetic replacement is used, it would appear essential that these harmful stresses must be reduced by either carrying out an osteotomy (Hanslik, 1974) above the tibia1 tuberosity to correct the ‘Q’angle or by releasing the lateral retinaculum in combination with plication of the medial capsule. Lateral shift of the patella is more likely to occur with an increase in the lateral vector of the forces. One of the possible factors in this is wide spacing of the hips (Outerbridge, 1964) producing a high ‘Q’ angle. This effect presumably accounts to a large extent for the increased frequency of patellofemoral problems in young women. This lateral vector is counteracted by the vastus medialis and the maintenance of adequate strength in this muscle must be an important factor in reducing the lateral subluxation and dislocation of the patella (Hughston, 1968; Fischer et al., 1978). Develonine the vastus medialis is also likely to reduce

ABERNETHY

the ‘/I’ angle, but full correction may require lateral retinacular release with or without medial plication (Merchant and Mercer, 1974). In order to reduce the apparently harmful high tensile stresses that can lead to faiIure of articular cartilage, it would appear that, in mechanical terms, the most effective factor appears to be reduction of the ‘Q’ angle by medial displacement of the tibia1 tuberosity. If the ‘Q’ angle is reduced as far as possible, the resulting tensile stresses due to lateral bending will dramatically fall and reduce the chance of further fatigue damage to both the cartilage and bone of the patella. CONCLUSIONS A stress analysis of normal patellae suggests that areas of high tensile stress on the medial aspect may exist during flexion of the knee. The values of stress calculated may exceed those values which can produce tensile fatigue failure of the collagen framework of normal articular cartilage in subjects over 9 years of age. This high tensile stress is a function of quadriceps/ patellar tendon angle (‘Q’ angle), reduction of which will dramatically reduce the values of combined stress within the cartilage, reducing the chance of fatigue failure of the cartilage collagen.

Acknowledgements

- We wish to thank Professor G. R. Higginson for helpful technical advice and Messrs. R. L. Orme and P. Brown for analysis of the patellar sections, Mrs. M. Tinkler, Radiographer at Dryburn Hospital, Durham, for providing the skyline radiographs, and Dr. K. Robinson, Consultant Pathologist, Dryburn Hospital, Durham.

REFERENCES Abernethy, P. J., Townsend, P. R., Rose, R. M. and Radin, E. L. (1978) Is chondromalacia patellae a separate clinical entity? J. Bone Jt Surg. (B) 60, 205-210. Bandi, W. and Brenwald, J. (1974) The significance of femoropatellar pressure in the pathogenesis and treatment of chondromalacia patellae and femoropatellar arthrosis. The Knee Joint (Edited by Ingwersen, 0. S. et al.). Excerpta Medica, Amsterdam. Baumgartl, F. (1964) Das Kniegelenk; Erkrankungen Verletzungen und ihre Behandlung mit Hinweisen fir die Begutachtung, p. 452. Springer, Berlin. Bentley, G. (1970) Chondromalacia patellae. J. Bone Jt Surg. (A) 52,221-232. Blackburne. J. S. and Peel. T. E. (1977) A new method of measuring patellar height. J. Bone Jr Surg. (B) 59,241-242. Darracott, J. and Vernon-Robert, B. (1971) The bony changes in “Chondromalacia Patellae”. Rheum. phys. Med. 11, 175-179. Devas, M. B. (1970) Stress fractures of the patella. J. Bone Jt Surg. (B) 42, 7 l-74. Emery, I. H. and Meachim, G. (1973) Surface morphology and topography of patello-femoral cartilage fibrillation in Liverpool necropsies. J. Anat.. Land. 116, 103-120. Ficat, C. and Maroudas, A. (1975) Cartilage of the patella. Topographical variation of glycosaminoglycan content in normal and fibrillated tissue. Ann. rheum. Dis. 34,515-519. Ficat, R. P. (1974) Degeneration of the patello-femoral joint. The Knee Joint (Edited by Ingwersen,O. S. et of.). Excerpta Medica, Amsterdam.

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and rr2 is tensile, the

Clearly, this is greater when one of the stresses will be large in compression and the other perpendicular stress large in tension (Fig. 14b).

(a)

(b) Fig. 14.