The Role of Subtalar Motion and Ankle Contact Pressure Changes from Angular Deformities of the Tibia

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Abstract
It is a well known entity that fractures of the tibia heal with some component of angular deformity. Ankle and subtalar joints may compensate for small degrees of angular deformities, but the exact amount of malunion that can be accepted without development of late sequalae has yet to be determined. Two recent studies from this institution have concluded that (1) contact changes at the tibiotalar joint tend to be greater with distal third tibial fracture deformities compared to proximal and middle with the ankle in neutral, 5° dorsiflexion, and 20° of plantar flexion. (2) Anterior and posterior bow deformities produced a greater change in contact area of the tibiotalar joint than with valgus or varus deformities. This phenomena may be possibly explained by the subtalar motion in the horizontal plane which averages 23°. Thus, it was the primary purpose of this paper to determine the exact role, if any, in subtalar motion on tibiotalar contact in angular deformities of the tibia. To achieve this objective the subtalar joint was transfixed thereby eliminating its perceived compensatory movement. Six cadaveric lower extremities were disarticulated at the knee joint and stripped of soft tissue preserving capsular and ligamentous structures. A custom universal joint was used to create various angulatory deformities at proximal, middle, and distal third levels of the tibia. Contact pressure across the tibiotalar joint was recorded using pressure-sensitive film and analyzed quantitatively in terms of contact area as well as pattern. The same combinations of angular deformities were then run with the subtalar joint transfixed in neutral. The results indicated that as in the two previous studies distal third deformities resulted in the greatest amount of change in ankle contact pressure area. The data also demonstrated that when subtalar motion was restricted ankle contact area decreased significantly in all planes of angulatory deformity. (1) The data collected agree with the results of two previous studies which showed that there was a decreased in total ankle contact area consistently at the distal third level with posterior angulatory deformities of the tibia. (2) By defining the resultant fracture angle and the foot axis angle a geometric explanation can be given to demonstrate a distal level fracture of the tibia has a greater effect on the ankle articulation than one more proximal. (3) The ankle joint has been shown by others to be less congruent as it moves away from its neutral position. This was found to affect and therefore cause a decrease in ankle contact area with tibial angulatory deformities. (4) The ankle joint is more adapted for weightbearing in neutral and in dorsiflexion. The anterior portion of the talar dome is probably more adapted to weightbearing than the posterior portion. This accounted for greater changes in ankle contact area during plantarflexion than in dorsiflexion. (5) The subtalar joint was found to play a very significant role in maintaining the talus in its normal relationship to the tibia. Restriction of the subtalar joint affected all deformities of the tibia as the resultant fracture angle increased. (6) The data supports Inman's concept of the subtalar joint acting as a torque transmitter and compensates for tibial varus and valgus deformities. (7) Subtalar joint restriction affected varus deformities more than valgus deformities probably due to shifting of the talar dome therefore significantly altering its normal biomechanics.

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