Generally, it is agreed that the basic deformity of pes planus is that of a peritalar subluxation. The talus remains seated within the ankle mortise until later in the disease, when deltoid insufficiency leads to valgus talar tilt. When insufficiency of the posterior tibialis tendon is combined with an equinus deformity, the calcaneus subluxes posteriorly, creating a valgus position relative to the talus. With hindfoot valgus, the alignment of the axes of the calcaneocuboid and talonavicular joints become more parallel, leading to increased flexibility at the midtarsal joint. The anterior subluxation of the talus on the calcaneus pushes the midfoot into abduction and factors into the joint incongruity and abnormal biomechanics of the subtalar joint; this often leads to compromise of the posterior tibialis tendon and spring ligament. Posterior displacement of the calcaneus leads to the lateral column appearing short relative to the medial column, and abduction occurs at the midtarsal joint level as a result of the length mismatch between the talus and the calcaneus (17).

Unless subluxation of the subtalar joint is corrected, the abnormal relationship, as described above, will persist. Therefore, with correction of the hindfoot valgus by means of a subtalar joint arthrodesis, the normal axes of the calcaneocuboid and talonavicular joints are restored, providing improved stability and alignment to the midtarsal joints.

Astion et al (18) determined the motion of the hindfoot and transverse tarsal joints after various arthrodesis procedures. After talonavicular joint arthrodesis, only 8% to 9% of subtalar joint motion remains. After subtalar joint arthrodesis, approximately 25% of talonavicular joint and 55% of calcaneocuboid joint motions remain.

Catanzariti (19) reported that an isolated subtalar joint arthrodesis preserves approximately 50% of the midtarsal joint motion that is typically lost with a triple arthrodesis. Mann and Baumgarten (20), advocates of isolated subtalar joint fusions, stated that the triple arthrodesis eliminates the necessary motion in the rearfoot that allows for proper compensation.

The patient is positioned supine with a bump placed under the ipsilateral hip—this allows for internal rotation of the lower extremity, providing ease of access to the subtalar joint. General anesthesia is most commonly performed. Prior to the arthrodesis, appropriate release of any equinus contracture will be performed. Exposure of the subtalar joint begins with a standard lateral incision, which runs from the tip of the fibula to the calcaneocuboid joint (Fig. 59.1A). It is the author’s opinion that too extensive an incision is all too commonly performed for this procedure, which creates unnecessary soft tissue stripping, predisposing the patient to delayed or nonunion. Alternatively, a vertical incision may be made over the lateral subtalar joint, in the event that a bone block arthrodesis is being performed (21,22). This modification is performed so as to limit tension on the incision when an increase in the hindfoot height is created with insertion of the bone graft.

Sharp and blunt dissection is performed down to the level of the deep fascia. An L-shaped incision is then performed through the deep fascia (Fig. 59.1B). This fascial incision is performed just superior to the course of the peroneal tendons and sural nerve and inferior to the course of the intermediate
dorsal cutaneous nerve. On occasion, a communicating branch between these neural structures may require excision due to the risk of traction-induced neuropraxia or chronic neuritis (23). The horizontal arm of the L-shaped incision is placed just superior to the peroneal tendons and the vertical arm is made just proximal to the extensor digitorum muscle belly (Fig. 59.1C). Reflection of the extensor digitorum muscle belly is then performed to the level of the calcaneocuboid joint (Fig. 59.1D). The contents of the sinus tarsi are then evacuated utilizing a rongeur; this will include transaction of the interosseous talocalcaneal ligament (Fig. 59.1E). While retracting the peroneal tendons inferiorly, the deep incision is carried posteriorly, incising the lateral talocalcaneal and calcaneofibular ligaments. At this point, a Crego elevator is inserted along the lateral aspect of the talocalcaneal joint and directed posteriorly—this aids in release and exposure of the posterior aspect of the subtalar joint (Fig. 59.1F and G).