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The contact stress values found in this study were a lower than those reported in other studies dealing with joint contact stress in the talocrural joint. Bertsch et al. (2001) reported maximal pressures of 2.3 MPa when loading the joint with 740 N. Calhoun et al. (1994) reported about 3.5 MPa in the same joint at 686 N axial loading. Those two studies used pressure-sensitive films to determine joint contact stress. The specimens tested in Calhoun’s study were clearly younger (38.5 yrs mean) than in the present study. Individual differences in morphologies and material properties of the biological structures as well as methodological differences might explain the variations in results. The results of the present study indicate that the congruence of the talocrural joint surface is not particularly influenced by additionally applied axial lower leg loading: The axial load influenced the contact stress magnitude in the talocrural joint, but to a lesser extent its distribution. This is in accordance with results the of Calhoun et al. (1994) who found only small variations in the contact area of the talocrural joint with increasing axial load (490–980 N) while the principal contact pattern stayed unchanged. In the present study the application of a 200 N force through the triceps surae maintained the principle pattern of a pronounced anterior stress. The application of forces through smaller foot muscles have a greater effect on the joint contact stress distribution even though the applied force was smaller compared to the triceps surae load. As expected, increasing force transmitted by the eversion muscles (peroneus longus, peroneus brevis) stressed the lateral joint aspects more and the inverters (tibialis posterior, flexor hallucis longus, flexor digitorum longus) stressed the medial aspects more. Consequently co-actuating of synergists had a bigger effect on changing the contact stress distribution than single muscles. However the small eversion and inversion angles (<1.5 ) between talus and tibia cannot explain the changes in stress distribution. Some of the investigated muscles (peroneus longus, peroneus brevis, tibialis anterior, flexor digitorum longus, tibialis posterior) have an eccentric line of action with a relatively small lever arm about the ankle joint centre. They appear to act like a tensile bracing about the talocrural joint and therefore had a large effect on the joint contact stress. The fact that the tibialis posterior is the strongest inverter muscle and that it’s tendon crosses the talo-calcaneo- navicular joint and the sustentaculum tali may explain its effect on the medial shift of the joint stress. Since the tendon of the flexor hallucis longus crosses the talocrural joint centrally, force application by the flexor hallucis longus led to a similar stress increase in all areas. Calhoun et al. (1994) found shifts of the pressure centroids to the lateral joint areas when the feet were axially loaded and 5 everted and medial shifts at the same axial load with 10 inversion. However these shifts were not quantified. Similarly, to that Tochigi et al. (2006) found medial increases in contact stress when applying 0.15 Nm and 0.3 Nm external inversion torques and lateral increases with the same magnitudes of eversion torques. Once again, the changes in contact stress were not quantified. The results of this study highlight the role of the flexor hallucis longus on the joint contact stress and stress distribution. It can be assumed that a reduced muscular capacity, or even the surgical removal of the flexor hallucis longus would have a significant impact on the integrity of the foot and on stress in the talocrural joint. Although the effect of torn ligaments on contact stress was small, the role of the flexor hallucis longus on joint contact stress changed remarkably when the ligaments were cut.
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- bakansky
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- wathavy
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- DIooggooID
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