Biomechanics
The ankle joint is the least understood weight-bearing joint of the lower limb. The function of the ankle cannot be understood without knowing the planes and axes of the ankle movements. The three cardinal planes of ankle movement include sagittal plane, coronal plane and transverse plane.
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Movement in the sagittal plane is known as dorsiflexion and plantar flexion. Movement in the frontal plane is known as eversion and inversion. Movement in the transverse plane is known as abduction and adduction.
Rotation in any given plane occurs about an axis perpendicular to that plane. So, the movement in the sagittal plane (flexion-extension) occurs about a medial-lateral axis, movement in the frontal plane (abduction-adduction) occurs about an anterior-posterior axis, and movement in the transverse plane (medial- lateral [internal-external] rotation) occurs about a longitudinal axis (Oatis 1988).
Inman described the axis of motion at the ankle as passing just distal to the medial and lateral malleoli (Inman, 1976). This description means that the axis of motion is oblique to all of the cardinal planes of motion. Yet, the ankle joint is undoubtedly a hinge joint (ie, the motion is uniaxial).
Ankle pronation (and its opposite, supination) refers to the calcaneal motion with respect to the talus orientation at the subtalar joint. The ankle pronates by dorsiflexing, abducting, and everting. Supination occurs by planterflexion, adduction and inversion. The normal plantar flexion varies from 40 to 65 degrees and dorsiflexion varies between 10 and 30 degrees (Ellis, 2006).
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In heel-toe walking and running, ankle pronation is accompanied by knee flexion and internal tibial rotation. At heel strike, pronation of subtalar joint unlocks the midtarsal joints and allows the foot to absorb shock and adapt to uneven terrains. In take off, subtalar joint supinates and relocks the midtarsal joints, which turns the foot into a rigid lever for push-off (Cheung et al., 2006).
The axis orientation of the subtalar joint is about 42 and 23 degrees to the human anatomical transverse plane and sagittal plane respectively. Since the axis does not coincide with the human anatomical reference frame, the subtalar joint movement is often described as a tri-planar motion (Fong et al., 2008).
There are five tri-plane joints within the foot that allow pronation and supination to occur. These include the talocrural, the subtalar, the midtarsal, the first metatarsophalangeal and the fifth metatarsophalangeal joints (Root, 1977).​
The movements of ankle joint complex are controlled by the moments due to ground reaction, constraints due to joint surfaces and ligaments, and moments produced by leg muscles, which pass over the ankle. The understanding of the load transfer across the ankle joint is an essential component of its biomechanics.
As compared to the knee and hip joints, the ankle is subjected to larger stresses across the joint (5x compared to 3x in the knee). This is thought to be due to a relatively smaller load-bearing surface area of the ankle (11–13 cm2) (Stauffer et al., 1977). The stress distribution exerted on the talus by the body is determined by not only the position of the ankle joint but also the integrity of its ligamentous attachments.
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An experiment conducted by Earll showed that in a loaded cadaver model where the tibiocalcaneal fascicle of the superior deltoid ligament was sectioned demonstrated a 43% decrease in talar contact area and a 30% increase in peak pressures.
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In a weight-bearing position, 77–90% of the load is transmitted to the dome of the talus via the tibial plafond. The remainder is transmitted through the medial and lateral talar facets with a further increase in load during inversion and eversion respectively.
A cadaveric experiment by Calhoun et al. has shown that with the ankle in dorsiflexion or neutral flexion both inversion and eversion cause a decrease in total contact area and an associated increase in the average high pressure (Calhoun et al., 1994).
In the same experiment, it was noticed that planterflexion also has a lower articular surface contact area and therefore higher average pressures than neutral flexion and dorsiflexion, but with inversion and eversion having little change. Furthermore, increasing the axial load from 490 N to 980 N increased the contact area without significantly changing the pressure across the joint. Calhoun et al. also demonstrated how the centroid of the contact moves anteriorly to posteriorly on the talus as the joint moved from dorsiflexion to plantarflexion. This suggests different stress distribution patterns varying with ankle position.​
Pronation occurs in stance phase of the gait. From heel-strike to toe-strike; there are four basic forces acting on the ankle joint and the rest of the lower limb. Upon heel strike, 80% of the body weight is directly over calcaneus, producing a vertical force against the ground. Due to its inherent nature, bone tries to reduce the compressive forces.
The alignment of the tibia, talus and calcaneus at heel-strike are important to distribute the vertical compressive forces safely. From heel-strike to toe-strike; the compressive force of weight bearing is distributed between the calcaneus and metatarsals. The midfoot does not carry any weight during the stance phase (Hutton, 1981).
There is also an anterior shearing force of tibia on the talus, which is mainly decelerated by gastrocnemius-soleus group (Root, 1977). The subtalar joint responds to internal rotation and medial shear by allowing the calcaneus to move laterally or into valgus. As a consequence, the talus rolls medially to complete the articulation with calcaneus. This rotation of talus and calcaneus is known as the torque converter of the lower limb (Inman, 1976).
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Supination occurs at the end of stance phase of gait. It results from the activity of extrinsic muscles (toe-strike to push-off). The lower limb then externally rotates, the contralateral limb swings forward initiating an external rotation force, which causes a lateral shearing force within the foot promoting supination (Mann, 1982). The midtarsal joint locks upon the supination of subtalar joint. Dorsiflexion of the first metatarsophalangeal joint produces increased tension of planter apponeurosis assisting subtalar joint supination, the mechanism known as ‘windlass effect’ (Mann, 1982).
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References
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Calhoun et al. 1994. A comprehensive study of pressure distribution in the ankle joint with inversion and eversion. Foot Ankle Int, 15, 125-33.
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Cheung et al. 2006. Association of footwear with patellofemoral pain syndrome in runners. Sports Med, 36, 199-205.
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Earll et al. 1996. Contribution of the deltoid ligament to ankle joint contact characteristics: a cadaver study. Foot Ankle Int, 17, 317-24.
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Ellis, H. 2006. Clinical Anatomy, Blackwell Publishing.
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Hutton, W. 1981. The mechanics of m l and halux valgus feet-A quantitive study. Clinical Orthopaedics, 157, 7-13.
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Inman 1976. The joints of ankle, Williams and Wilkins.
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Mann, R. A. 1982. Biomechanics of running. Symposium on the Foot and Leg in running sports, 1-29.
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Root 1977. Normal and abnormal function of foot. Clinical Biomechanics.