The subtalar joint

In a simplified way, one can divide the subtalar joint into two parts, an anterior and a posterior part. Anteriorly, the talar head is located on the anterior and middle facets of the calcaneus, forming the acetabulum pedis with the posterior surface of the navicular bone.1 However, it is important to mention that the talar head is not only supported by the articulating surfaces of the calcaneus and the navicular bone, but also by the ‘spring’ ligament. This ligament complex plays a key role in stabilising the talar head. Insufficiency of this structure can lead to acquired flat foot deformity. Posteriorly, the concave facet of the talus lies on the convex posterior facet of the calcaneus.1,2 The size and shape of the three calcaneal facets vary between individuals. Both the anterior and middle facets are concave, while the posterior facet is convex.1,3 The posterior facet is larger compared with the middle and the anterior facets and is separated from the other two facets by the interosseous calcaneal ligament.3-5 For the anterior and middle calcaneal facets, different anatomical variations have been described in the literature. Studies found that 42% have a combined anterior and middle facet in an ovoid form, 22% a ‘bean’ form and 36% have a complete separation.6 The sustentaculum tali is formed by the middle calcaneal facet (dorsal surface) and provides a sliding surface for three tendons (plantar surface): the tibialis posterior, flexor hallucis longus and flexor digitorum longus tendons.3 Subtalar joint anatomy is shown in Figure 1. The ligaments around the subtalar joint can be distinguished as intrinsic (cervical ligament, interosseous talocalcaneal ligament) and extrinsic ligaments (calcaneo-fibular ligament, tibio-calcaneal part of the deltoid ligament).1 Malfunction of the interosseous talo-calcaneal ligament, especially in combination with failure of the anterior talofibular ligament, leads to an unphysiological anterolateral rotation of the talus during gait.7 As a result, subtalar joint and secondary ankle joint instability may occur.7 The importance of the calcaneo-fibular ligament for subtalar stabilisation is still being debated. In both cadaveric and clinical studies, where the calcaneo-fibular ligament had failed, an increase of subtalar movement and instability was observed.8-12 In contrast, Michelson et al13 could not find any changes in subtalar joint stability during the stance phase, after transection of all lateral ligaments, including the calcaneo-fibular ligament. A further structure which affects the subtalar joint in terms of stability and movement is the inferior extensor retinaculum. Acting like a pulley, movement of the extensor tendons influences the subtalar stability and movement.3,14