Subtalar Joint

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https://www.ncbi.nlm.nih.gov/pubmed/28775922?dopt=Abstract

Consist of two joints on plantar surface of talus and dorsal surface of calcaneus – form one functional unit.

Articulation is between one, two or three separate facets – large variations among individuals:
Type A: three facets
Type B: two facets – anterior and middle facets  become one facet
Type C: one facet – all facets are continuous

Calcaneal facets:
Posterior facet – usually convex and rectangular; seperated form anterior and middle facets by sinus tarsi
Middle facet – usually slightly concave; on superior surface of sustentaculum tali (articulate with inferior aspect of neck of talus)
Anterior facet – most variable;

Functional subtalar joint = all facets of the talus and calcaneus (share joint capsule with midtarsal joint)
Anatomical subtalar joint = posterior facets of talus and calcaneus (have own joint capsule)

Four ligaments:
• Medial – from medial tubercle of talus to back of sustentaculum tali – fibres blend with deltoid ligament
• Lateral – from lateral process of talus to lateral surface of calcaneus
• Interosseous – crosses sinus tarsi - broad flat band – tightens during pronation – but is considered relatively weak
• Cervical – over lateral entrance to sinus tarsi – prevents inversion; strongest ligament between talus and calcaneus; fibres are directed dorsally and medially from the calcaneus to the talus

Sinus tarsi – tunnel formed by sulci calcanei (inferior) ad sulci tali (superior); wider laterally, narrower medially; Hoke’s tonsil – fibrofatty tissue (plug) in sinus tarsi; other contents – arteries (deltoid, artery of sinus tarsi, nerves, interosseous talocalcaneal ligament, medial attachment of inferior extensor retinaculum; palpate lateral side one finger width inferior and anterior to edge of lateral malleolus

Function of subtalar joint:
Previously considered to function as a single axis similar to a mitred oblique hinge , but function is more complicated than this.
Axis – mean position 42 degrees in sagittal plane from transverse and 16 degrees in the transverse plane from the sagittal plane  joints functions relative to all three body planes (triplane motion). Clinically, a wide variation is noted.
Function of joint is to allow the leg to undergo rotational movements in response to foot to changes in foot positions. Original conception of axis was to function as a hinge, but the axis is now considered to move during joint motion.
Large individual variation is seen in axis location. Axes that are greater than the average deviation (>42 degrees) from the transverse plane show more adduction/abduction and less inversion/eversion.
Axes that are closer to the transverse plane (<42 degrees) demonstrate more inversion/eversion and more adduction/abduction.
Open kinetic chain function – Pronation = dorsiflexion, abduction, eversion. Supination = plantarflexion, adduction, inversion
Closed kinetic chain function – Pronation = calcaneal eversion, talar adduction and plantarflexion. Supination = calcaneal inversion, talar abduction and dorsiflexion.
Average range of motion assumed to be 25 to 30 degrees with more inversion than eversion

Primary function of subtalar joint is to absorb the transverse plane rotations of the leg during the stance phase. It has been widely assumed that subtalar joint pronation at contact is used to absorb shock, but two studies that investigated shock absorption when STJ motion was restricted did not support this (Yingling; road camber study).

Compensatory motion at STJ:
Normal – in response to changes in the surface or posture
Abnormal – for structural deformities in the foot in other body planes

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