Distal tibiofibular synostosis and ankle function: the critical role of reduced distal tibiofibular interval in limiting motion after surgical fixation of distal lower leg fractures

The impact of distal tibiofibular synostosis (DTS) on ankle range of motion (ROM) following surgical fixation of distal lower leg fractures remains controversial. Crucially, the potential influence of a reduced distal tibiofibular interval (DTI) on ROM in patients with DTS has been overlooked in previous studies. This study aimed to investigate the role of reduced DTI in limiting motion in patients with DTS. We conducted a retrospective review of 61 patients diagnosed with DTS after internal fixation for distal lower leg fractures (43 males, 18 females; mean age 47.3 years). A reduced DTI was defined as ≤ 2 mm. Active/passive dorsiflexion/plantarflexion ROM was measured. Between-group comparisons and Spearman rank bivariate correlation analysis were performed. Functional outcomes were assessed using the Olerud–Molander Ankle Score (OMAS) and the American Orthopaedic Foot & Ankle Society (AOFAS) score. At the 1-year follow-up, significant reductions in active/passive dorsiflexion/plantarflexion were observed on the affected side compared to the healthy side (P < 0.05). Among the patients, 33 exhibited reduced dorsiflexion, with 6 of these also showing reduced plantarflexion. Spearman rank bivariate correlation analysis demonstrated that DTI was positively correlated with active dorsiflexion and plantarflexion (P < 0.01). The group with reduced DTI showed greater reductions in active and passive dorsiflexion and plantarflexion than the group with normal DTI (P < 0.05). However, there were no significant differences in OMAS, AOFAS, or baseline characteristics between the two groups (P > 0.05). The formation of DTS after surgical fixation of distal lower leg fractures poses a risk of limited ankle dorsiflexion and plantarflexion. DTI was positively correlated with dorsiflexion and plantarflexion, i.e., a critical role of reduced DTI in limiting ankle motion. The primary limitation is dorsiflexion, whereas plantarflexion is affected only in cases of severe dorsiflexion restriction. As the reduction in ROM is generally mild, the functional impact on most patients is not significant.