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Foot and Ankle Complex 3.4 PPT (iRAT week 3)

Terms in this set (70)

The ankle joint is considered to be triplanar because its axis for dorsiflexion-plantarflexion crosses all three planes. The ankle joint axis passes through the malleoli of the tibia and fibula. Rather than being a true coronal axis, the ankle joint axis is rotated laterally about 20-25°. It is also inclined downwardly 10-15° on the lateral side. The laterally rotated location of the axis gives the foot its apparent toe-out seen in stance. An ankle joint axis rotated laterally in excess of 25° from the frontal plane indicates the existence of lateral tibial torsion. If there is substantially less than 20° of lateral torsion, it would be considered medial tibial torsion. Although not immediately obvious from examining the inclination, the triplanar orientation of the ankle joint axis results in dorsiflexion of the ankle around this oblique hinge to simultaneously result in inclination of the tibia medial to the weight-bearing foot, with what appears as a small amount of medial rotation (or movement of the foot laterally and into some eversion relative to the tibia when the distal lever is free).

The triplanar nature of the ankle joint noted above does not take into consideration that there are also two additional motions at the ankle that may be available in very small amounts at the ankle joint, or when there is ligamentous laxity. Talar tilt (inversion/eversion) is motion of the talus around an ~A-P axis, while talar rotation (abduction/adduction) is a motion of the talus around a vertical axis.
In weight-bearing, the calcaneus is not free to perform either its abduction/adduction component or its dorsiflexion/plantarflexion component. Consequently, while it continues to invert and evert, the other two obligatory components take place as movements of the talus. The talus, as the proximal joint segment, can produce the same joint movement that the distal calcaneal segment produces by moving in the opposite direction. Thus, in weight-bearing, supination becomes inversion of the calcaneus with concomitant dorsiflexion and abduction of the head of the talus. As the head of the talus (lying at the apex of the medial arch of the foot) moves up and out, it draws the arch of the weight-bearing foot up. Weight-bearing pronation occurs as calcaneal eversion with adduction and plantarflexion of the head of the talus. As the head of the talus in the weight-bearing foot moves down and in, the medial arch of the foot flattens.
As the head of the talus moves into adduction and abduction in weight-bearing pronation-supination, the talus must carry the superimposed mortise (that hugs the body of the talus) with it. This imposes a rotatory force on the tibia during weight-bearing supination and pronation of the subtalar joint. In supination, the head of the talus abducts, which creates a lateral rotatory force on the leg. In pronation, the head of the talus adducts, which creates a medial rotatory force on the leg. Similarly, a medial or lateral rotatory force imposed on the leg in weight-bearing will cause the subtalar joint to move into pronation (with accompanying flattening of the arch) or supination (with accompanying increase in the arch) respectively.
"Subtalar neutral" is, of course, supposed to be the point at which the subtalar joint is neither pronated or supinated. It is largely a clinical concept, however; it is used to attempt to discriminate between positional faults of the hindfoot and the forefoot. While subtalar neutral is conceptually intriguing, confirming its location is controversial. The focus of subtalar neutral is on the calcaneus because the position of the calcaneus in inversion (varus) or eversion (valgus) is the only readily observable component of complex subtalar supination/pronation and the only component movement that is consistent in weight-bearing and non-weight-bearing. Subtalar neutral is considered by some to be the point where the calcaneus can invert twice as much as it can evert although this presumes the inclination of the subtalar axis is similar for everyone. Some consider subtalar neutral to the be midpoint of the palpated swing of the talar head between full calcaneal inversion and full calcaneal eversion (allowing for individual variability in axis inclination). Using these techniques, subtalar neutral may be anywhere from 2º of calcaneovalgus to 1.5º of calcaneovarus. If it is assumed (as it commonly is) that the subtalar joint passes through subtalar neutral during the midstance phase of gait, the position would be approximately 3.5º of calcaneovalgus in the normal foot. This approximates the angle between the leg and posterior calcaneus at the normal subtalar joint in static bilateral stance.
The transverse tarsal joint (TT) is a composite s-shaped joint that lies transversely across the foot. It is formed by the anatomically distinct but functionally related talonavicular joint medially and calcaneocuboid joint laterally. The talonavicular joint is a fairly mobile plane joint which has already been described as part of the TCN. The calcaneocuboid is also a plane synovial joint, formed by two reciprocally convex-concave facets on the calcaneus and the cuboid. This facet shape requires that the moving segment move in opposite directions intra-articularly, limiting the motion that can take place.

Although the movements of the two portions of the transverse tarsal joint are more complex than those of the subtalar joint, the combination of talonavicular and calcaneocuboid motion together produce the net effect of pronation/supination, although the range of these motions at the transverse tarsal joint is more restricted (30-50% of ROM) than at the subtalar. The functional task of the transverse tarsal joint is to compensate for or absorb the rotations (movements) of the leg and hindfoot with minimal disruption to the forefoot. That is, it is the transverse tarsal joint's task to maintain the most even weight-bearing distribution in the forefoot that is possible based on the demands of the weight-bearing surface. The transverse tarsal joint may increase the pronation or supination occurring at the subtalar joint or may move in a direction opposite to that of the subtalar joint. The ability of the transverse tarsal joint to add to or reverse subtalar motion, however, is restricted by the fact that locking of the subtalar joint in supination also causes locking of the transverse tarsal joint in supination (not surprising because the talonavicular joint is functionally related to both composite joints). Consequently, the more the subtalar joint supinates, the less able the transverse tarsal joint is able to reverse into pronation. When the subtalar joint is fully supinated, the transverse tarsal joint can not pronate at all, but is also locked in supination.