Subtalar Arthrodesis

Indication. Symptomatic, end-stage subtalar osteoarthritis — lateral hindfoot pain, stiffness and functional limitation that has failed at least 3–6 months of conservative care (orthotics or insoles, activity modification, NSAIDs, and at least one image-guided corticosteroid injection into the subtalar joint) — with the talonavicular and calcaneocuboid joints preserved on weight-bearing radiographs and CT. When those adjacent joints are also arthritic, a triple arthrodesis is preferred instead. The operation is chosen for five broad indication groups. In every case the decision to fuse only the subtalar joint (rather than extend to a triple fusion) rests on confirming the talonavicular and calcaneocuboid joints are clinically and radiographically preserved.

Contraindications. Active infection (absolute); subtalar pain not attributable to subtalar OA with preserved cartilage; significant talonavicular or calcaneocuboid OA (triple arthrodesis instead); peripheral arterial disease with critical ischaemia (relative — vascular opinion first); and severe osteoporosis (relative — plan augmented fixation). Consent specifically for sural nerve injury or a painful lateral-foot neuroma (the most common neurological complication, 5–15%), wound dehiscence or infection (higher in post-traumatic cases with prior scars and in inflammatory arthritis), non-union (5–16%, higher in smokers and diabetics), heel malposition requiring revision, and the realistic long-term risk of symptomatic adjacent joint arthrosis that may need conversion to triple arthrodesis. Setup. Lateral decubitus with the operative foot uppermost (excellent sinus tarsi access and easy fluoroscopy), or prone with both feet hanging over the table edge. Thigh or calf tourniquet for a bloodless field during cartilage preparation. C-arm available for lateral and Harris-Heath axial hindfoot views throughout.

The goal is to expose the subtalar joint through the lateral sinus tarsi approach, remove all articular cartilage from the posterior and anterior-middle facets down to bleeding cancellous bone while protecting the sural nerve, the peroneal tendons and the medial neurovascular bundle, position the heel in 0–5° valgus, and compress the prepared surfaces with one or two large cannulated screws. The exposure is laid out in full below (and in depth on the sinus tarsi approach to the subtalar joint page).

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Critical structures to protect. Five danger zones account for almost all the morbidity of this operation:

Rehabilitation | Phase | Timing | Immobilisation | Therapy | |-------|--------|----------------|---------| | 1 | 0–2 weeks | Below-knee plaster backslab; strict non-weight-bearing; foot elevated above the heart for the first 48–72 hours | Finger and toe active range of motion only; wound check and suture or staple removal at 10–14 days | | 2 | 2–6 weeks | Convert to a fibreglass cast or removable walker boot; remain non-weight-bearing | Ankle dorsiflexion and plantarflexion to prevent tibiotalar stiffness; multimodal analgesia (avoid NSAIDs for six weeks — they impair bone healing) | | 3 | 6–12 weeks | Radiographic review at six weeks; if satisfactory callus, protected weight-bearing in the boot, progressing to full weight-bearing as tolerated | Calf strengthening once full weight-bearing is established; gait re-education for reduced hindfoot motion | | 4 | 3–6 months | Wean from the boot into supportive footwear | Graded return to function; CT at six months if union is in doubt | Most patients return to sedentary work at 6–8 weeks, light duties at 10–12 weeks, and manual labour or prolonged standing at 4–6 months; running and sport resume at 6–12 months depending on union and strength. AOFAS hindfoot scores typically improve from the mid-20s to around 70 (Easley 2000) or 75 (Flemister 2000 for calcaneal-fracture sequelae), with 93% satisfaction (Mann 1998). Patients should be counselled pre-operatively that walking on uneven ground remains difficult: transverse tarsal motion is reduced by about 40%, dorsiflexion by 30%, and plantarflexion by only 9% (Mann 1998) — useful motion is retained, not abolished. Complications

Non-union workup. Obtain a CT at six months if examination suggests failed union (persistent pain on subtalar stress or palpation). Add a metabolic and infection screen (vitamin D, calcium, HbA1c, ESR, CRP) and a bone scan if the CT is equivocal. Revision re-prepares the surfaces through the same approach with iliac crest autograft and improved fixation (add a second screw or change configuration) after correcting modifiable risk factors. Malposition revision. For a solidly fused but malpositioned heel, correct through an osteotomy of the fusion mass with a Gigli or oscillating saw, re-position to 0–5° valgus under fluoroscopy, supplement with bone graft, and revise fixation. Results are generally satisfactory when performed before secondary adjacent joint changes develop — no re-fusion is required because the joint is already united.

Epidemiology and indication profile. Isolated subtalar arthrodesis is performed most often for post-traumatic osteoarthritis after a displaced intra-articular calcaneal fracture, which destroys posterior facet cartilage in a substantial proportion of patients — Sanders Type III and IV fractures carry the highest risk of symptomatic subtalar OA. Primary subtalar OA is less common and typically affects older patients; talocalcaneal coalition, adult acquired flatfoot deformity (Stage II posterior tibial tendon dysfunction), inflammatory arthritis, and talar avascular necrosis make up the remainder. The operation is selected only when the talonavicular and calcaneocuboid joints are spared, preserving useful transverse tarsal and ankle motion and avoiding the greater stiffness of a triple arthrodesis. Hindfoot kinematics after fusion. The subtalar joint provides inversion and eversion (about 20–30° total range) with coupled internal and external rotation. Fusion abolishes this motion, but the midtarsal (Chopart) joints partially compensate. Mann (1998) quantified the motion lost after isolated fusion: transverse tarsal motion reduced by 40%, dorsiflexion by 30%, and plantarflexion by only 9% — useful motion is retained, which is why an isolated fusion is preferred over a triple whenever the adjacent joints allow. Post-traumatic calcaneal deformity classification (Zwipp). When subtalar fusion is combined with malunion correction, the Zwipp classification of post-traumatic calcaneal deformity guides the choice between in-situ fusion, distraction bone-block arthrodesis, and corrective osteotomy:

Union rates and the evidence base. Primary isolated fusion achieves union in roughly 84–95% of patients depending on selection. The landmark figures to quote: Easley (2000) reported an overall union rate of 84% (154 of 184 feet), rising to 96% only after excluding smokers, revisions, structural grafts, and fusions adjacent to a prior ankle fusion — examiners reward the real overall figure rather than the best-case subgroup. Smoking is the dominant modifiable risk factor: union fell from 92% in non-smokers to 73% in smokers, and every non-union occurred where more than 2 mm of avascular bone was present. For calcaneal-fracture sequelae specifically, Flemister (2000) reported 96% union across 86 fusions with a mean AOFAS of 75, independent of graft type — supporting local lateral-wall bone graft to avoid iliac-crest donor morbidity. Where a malunion has lost height and produced anterior ankle impingement, a distraction bone-block arthrodesis (Rammelt 2004) restores talocalcaneal height rather than a simple in-situ fusion. CT is the gold standard for assessing union — plain radiographs substantially overestimate the percentage fused (Coughlin 2006).