BACKGROUND: An unstable valgus ankle with an incompetent medial ligament complex is still treated by many surgeons with isolated tibiotalar (TT) arthrodesis. To date, it is unknown whether rigid fixation of the talus within the ankle mortise sufficiently corrects and stabilizes the hindfoot complex. The purpose of this study was to critically analyze patients with this problem and to assess the underlying causes for their acquired peritalar instability. METHODS: This series included 4 male patients (ages 55, 70, 72, and 79 years). Preoperatively, all patients presented with an unstable valgus ankle associated with stage IV adult acquired flatfoot deformity (AAFD), with a valgus talar tilt averaging 12 degrees (ranging from 9 to 14 degrees). All patients had a successful TT fusion, with screws and plates used for fixation. The position of the TT fusion was estimated at the intersection of a line drawn defining the talar shoulders and the longitudinal tibial axis on the anteroposterior weight-bearing radiograph. The hindfoot deformity was measured via the calcaneal offset on the Saltzman view. RESULTS: All patients had a healed TT fusion with the talus remaining in valgus averaging 4.8 degrees (ranging from 4 to 6 degrees). At the latest follow-up (1.2-18 years), all patients showed a progressive destabilization of the hindfoot complex that resulted in a valgus pronation deformity with flattening of the arch and a highly unstable foot that was not manageable with corrective shoes and braces. Key findings were a floppy hindfoot that turned into extreme valgus misalignment while loading. Radiographically, the calcaneal offset was in valgus misalignment of 16 to 54 mm related to the tibial axis on the Saltzman alignment view. In 2 cases, there was a complete medial dislocation of the talus. CONCLUSIONS: Even with rigid stabilization of the talus in the ankle mortise, peritalar instability may persist and allow calcaneus and navicular subluxation around the talus, which can result in progressive destabilization of the hindfoot complex. In stage IV AAFD, incompetent peritalar ligaments may not be able to withstand the increased mechanical load after TT fusion. Persistent valgus talar tilt after fusion may promote this unfavorable process. Therefore, isolated TT fusion should be performed with caution for treatment of valgus tilted ankles in stage IV AAFD. If this is considered, we recommend that fusion in neutral or even slightly varus talar positions be attempted. LEVEL OF EVIDENCE: Level IV, retrospective case series.