Ankle and Foot Biomechanics 2- Ankle joint function

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Biomechanics of the Ankle and Foot Complex : 2 Ankle Joint Function : 

Biomechanics of the Ankle and Foot Complex : 2 Ankle Joint Function Dr. Dibyendunarayan Bid [PT] The Sarvajanik College of Physiotherapy, Rampura, Surat

Ankle Joint Function: 

Ankle Joint Function The primary motions allowed at the ankle joint are dorsiflexion and plantarflexion. Normal ankle joint ranges of motion are reported to be: 10° to 20° for dorsiflexion and 20° to 50 ° for plantarflexion.

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The large variation in magnitudes of ankle motion is due to differences in : measurement techniques, subject populations, and even which joints are included in the measure of dorsiflexion or plantarflexion.

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If ankle joint range of motion measurement includes other joints of the foot (i.e., subtalar joint or transverse tarsal joints), greater ROM values will be obtained. Isolating motion to the tibia and talus will yield lower ROM values: 10° dorsiflexion and 20° plantarflexion.

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Ten degrees of ankle dorsiflexion often is considered the minimal amount needed to ambulate without deviations or injury.

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During ankle joint dorsiflexion / plantarflexion , the shape of the body of the talus facilitates joint stability. The trochlear (superior) surface of the talus is wider anteriorly than posteriorly (see Fig. 12-10B). When the foot is weight-bearing, dorsiflexion occurs by the tibia’s rotating over the talus. As the tibia rotates over the talus, the concave tibiofibular segment slides forward on the trochlear surface of the talus.

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Therefore, the wider anterior portion of the talus “wedges” into the mortise formed by the spreading tibia and fibula, enhancing stability of the ankle joint. The enhanced stability at the ankle joint in dorsiflexion allows the ankle to withstand compression forces of as much as 450% of body weight, with little incidence of primary ( nontraumatic ) degenerative arthritis over time.

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The sliding of the tibia on the talus during ankle motion contributes to a changing instantaneous center of rotation and also changes contact areas across the joint surfaces. This motion between the mortise and the talus, including some incongruence in the ankle joint, may be necessary for: normal load distribution, cartilage nutrition, and lubrication of the ankle joint.

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The loosepacked position of the ankle joint is in plantarflexion when only the relatively narrow posterior body of the talus is in contact with the mortise. The ankle is considered to be less stable when in plantarflexion; there is a higher incidence of ankle sprains when the ankle is plantarflexed than when dorsiflexed .

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The asymmetry in size and orientation of the lateral and medial facets of the ankle joint contribute to changes in the ankle mortise that occur during ankle dorsiflexion. The lateral (fibular) facet is substantially larger than the medial ( tibial ) facet, and its surface is oriented slightly obliquely to that of the medial facet (see Fig. 12-10).

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Inman and Mann proposed that the body of the talus can be thought of: as a segment of a cone lying on its side with its base directed laterally. The cone should be visualized as “truncated” or cut off on either end at slightly different angles (Fig. 12-11).

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The asymmetry in size and orientation of the facets means that the distal fibula moving on the larger lateral facet of the talus must undergo a greater displacement (in a slightly different plane) than the tibial malleolus as the tibia and fibular move together during dorsiflexion.

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The greater arc of motion for the fibula malleolus than for the tibial malleolus results in superior/inferior motion and medial/lateral rotation of the fibula that requires mobility of the fibula at both the proximal and the distal tibiofibular joints.

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Johnson, in reviewing the research literature, found the motions to be consistently small in magnitude but variable in direction among individuals and with different loading conditions.

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Individual differences in fibular motion may be related to orientation of the proximal tibiofibular facet, with more mobility available in the facets that are more vertical, or to factors such as tibiofibular ligamentous elasticity. Such individual differences may account for the variations in effect on ankle dorsiflexion/plantarflexion ROM that are seen when surgical tibiofibular fixation is necessary.

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Effectively, however, mobility of the fibula at the tibiofibular joints should be considered a component of normal ankle motion.

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One might also expect that the magnitude of proximal tibiofibular joint motion should exceed that of the distal tibiofibular joint, given that small motion at the distal fibula would be magnified at the opposite (proximal) end. This presumably accounts for the proximal joint’s being synovial, whereas the distal joint is a comparatively less mobile syndesmosis joint.

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Ankle dorsiflexion and plantarflexion movements are limited primarily by soft tissue restrictions. Active or passive tension in the triceps surae ( gastrocnemius and soleus muscles) is the primary limitation to dorsiflexion.

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Dorsiflexion is more limited typically with the knee in extension than with the knee in flexion (as demonstrated in the patient case) because the gastrocnemius muscle is lengthened over two joints when the knee is extended.

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Tension in the tibialis anterior , extensor hallucis longus , and extensor digitorum longus muscles is the primary limit to plantarflexion. Although the ligaments of the ankle assist in checking dorsiflexion and plantarflexion, a more important function appears to be in minimizing side-to-side movement or rotation of the mortise on the talus.

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The ligaments are assisted in that function by the muscles that pass on either side of the ankle. The tibialis posterior, flexor hallucis longus, and flexor digitorum longus muscles help protect the medial aspect of the ankle; the peroneus longus and peroneus brevis muscles protect the lateral aspect.

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Bony checks of any of the potential ankle motions are rarely encountered unless there is extreme hypermobility (as may be found among gymnasts or dancers) or a failure of one or more of the other restraint systems.

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A more complete analysis of the function of the muscles crossing the ankle will be presented later, because all muscles of the ankle cross at least two and generally three or more joints of the ankle and foot.

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End of part - 2