Talocalcaneonavicular joint

Articular surfaces

As the name suggests, the talocalcaneonavicular joint is a synovial ball and socket joint formed between three tarsal bones (talus, calcaneus and navicular) and the adjacent ligamentous structures.

There are five articular facets on the talus that participate in the formation of this joint. They include:

• Middle facet for calcaneus located on the talar neck
• Anterior facet for calcaneus located on the talar head
• Navicular articular facet
• Facet for plantar calcaneonavicular ligament
• Facet for calcaneonavicular part of bifurcate ligament

There are articular facets articulate with the socket formed by the navicular bone, calcaneus, the plantar calcaneonavicular ligaments and calcaneonavicular part of bifurcate ligament.

This socket in the foot is also referred to as the acetabulum pedis. The anterior aspect of the acetabulum is formed by the concave proximal articular facet of the navicular bone. The posterior part is formed by the middle and anterior talar articular facets found on the anterior aspect of the calcaneus. The middle parts are the plantar calcaneonavicular ligament and the calcaneonavicular fibers of the bifurcate ligament. They are located medially and laterally, respectively.

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Ligaments and joint capsule

The joint capsule of the talocalcaneonavicular joint is present on the dorsal and proximal aspects of the articulation. The dorsal component of the capsule extends from the neck of talus to the dorsal margin of the proximal articular surface of the navicular bone, blending medially with the medial collateral and plantar calcaneonavicular ligaments, and laterally with the calcaneonavicular part of bifurcate ligament (described below). The proximal part of the joint capsule is better  developed and found within the tarsal sinus. This part (also known as the "true" joint capsule) forms the strong talocalcaneal interosseous ligament, together with the anterior part of the talocalcaneal joint capsule.

The joint capsule is lined with the synovial membrane which helps to lubricate the joint to facilitate movements of the bones.

There are three ligaments that support this joint: dorsal talonavicular ligament, the plantar calcaneonavicular (spring) ligament, and calcaneonavicular part of bifurcate ligament.

Innervation

The innervation of the inferomedial aspect of the talocalcaneonavicular joint is provided by the medial plantar nerve, while the dorsomedial, dorsal and lateral aspects receive branches of the deep (peroneal) fibular nerve. The medial plantar nerve is a branch of the tibial nerve, while the deep fibular nerve is the branch of the common fibular (peroneal) nerve.

Blood supply

Arterial supply for the subtalar joint comes from the articular branches of following arteries:

• Posterior tibial artery
• Fibular (peroneal) artery

Movements

Movement at the talocalcaneonavicular joint is complex due to the fact that its talocalcaneal and talonavicular components are considered as belonging to different functional joints. Therefore, it is helpful to remember that the anatomical subtalar joint and talocalcaneal element of the talocalcaneonavicular joint are collectively described as the ‘functional’ subtalar joint, with the talonavicular part of the talocalcaneonavicular joint and calcaneocuboid joint are to be considered as the transverse tarsal joint (Chopart’s joint). Movement between the functional subtalar and transverse tarsal joints however is mechanically linked, meaning that weight-bearing motion at the subtalar joint always results in simultaneous motion at the talonavicular and calcaneocuboid articulations, albeit at a lesser magnitude.

Movements at the subtalar and transverse tarsal joint joints occur around different axes, none of which align with the orthogonal axes of the foot, and are subject to a high degree of interindividual variation.
In the case of the subtalar joint, studies have suggested that the average axis of rotation is orientated 42° upwards and anterior to the transverse plane, and 16° from the sagittal plane.

For the transverse tarsal joint, two axes of rotation have been described; a longitudinal axis which likes 15° upwards and anterior to the transverse plane, and 9° medial to the sagittal plane, and an oblique axis which is inclined 52° superior to the transverse plane and 57° medial to the sagittal plane.
Motion around all of these axes will always be triplanar in nature (i.e. occur in all three cardinal planes), and occurs in the form of supination or pronation.

Supination is a composite movement which combines inversion, adduction and plantarflexion at these joints. Pronation, on the other hand, is the opposite movement resulting from eversion, abduction and dorsiflexion.
Although these cardinal movements (inversion/eversion, adduction/abduction and plantar/dorsiflexion) will be described individually below, it’s important to emphasize that they always occur synchronously with each other, and never in isolation.

Inversion and eversion, if considered in isolation, occur around a longitudinal axis in the frontal plane and are considered to be the primary movements in the subtalar and transverse tarsal joints. The range of motion at the subtalar joint varies across sources; the RoM in inversion ranges from 25° to 30°, while in eversion it ranges from 5° to 10°. The RoM at the transverse tarsal joint is said to be about half or one third of that seen at the subtalar joint.

Inversion is the movement in which the sole of the foot rotates towards the midline, while the lateral border of the foot is directed inferiorly. Eversion is the movement in which the sole of the foot is laterally orientated while the medial border of the foot is directed inferiorly.

Abduction and adduction (also described as external and internal rotation, respectively) occur around a vertical axis in the transverse plane. In the subtalar joint, since the axis of rotation is roughly halfway between the vertical and longitudinal orthogonal axes, the range of motion for adduction/abduction is usually similar to that mentioned for inversion/eversion. This is subject to interindividual orientation of the axis of rotation for the joint i.e. an inclination less than 42° will result in greater RoM for inversion/eversion, and less adduction/abduction, and while the opposite will occur the closer the axis gets to the long axis of the leg. In the transverse tarsal joint, the RoM for adduction/abduction is much less than that of eversion, due to the fact that the long axis of rotation is closer to being longitudinal than vertical.

Dorsiflexion and plantarflexion of the subtalar and transverse tarsal joints occur around a mediolateral axis within the sagittal plane. Due to the orientation of the mentioned axes of rotation for the subtalar and transverse tarsal joints, the RoM for these movements is minor relative to inversion/eversion and abduction/adduction, except in the case of rotation around the oblique axis of the transverse tarsal joint. One study by Lundberg et al., found that 12% of the first 30 degrees of total foot plantarflexion occurs at these joints.

The close packed position of the talocalcaneonavicular joint is full supination, while the open (resting) packed position is slight supination (midway between the extremes of RoM). A capsular pattern is present when supination is more limited than pronation.

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Muscles acting on the talocalcaneonavicular joint

• Supination at the talocalcaneonavicular joint is primarily produced by tibialis anterior and tibialis posterior, with assistance from extensor hallucis longus, flexor hallucis longus and flexor digitorum longus.
• Pronation is mainly produced by fibularis longus, fibularis brevis and fibularis tertius with assistance from the extrinsic muscles that extend the toes (extensor digitorum longus and extensor hallucis longus).