A cohort study of union following subtalar fusion dependent on the presence of an adjacent ankle fusion

T. Jennison, J. Dalgleish, S. Taher, C. Chadwick, C. Blundell, M. Davies, H. Davies

Introduction: Subtalar fusion is the surgical treatment of subtalar arthritis. There is a concern that subtalar fusions may have a higher non-union rate if there is an adjacent ankle fusion. The aim of this cohort study was to determine the rates of union for subtalar fusions with and without an adjacent ankle fusion. Secondary outcomes were to assess risk factors for non-union.

Methods: A retrospective cohort study of all patients that underwent a subtalar fusion in a single centre between 2010 and 2020. The primary outcome of union was determined based on bridging callous on radiographs. Chi squared test and Mann-Whitney tests compared differences in demorgaphics and risk factors for non-union between groups. A logistical regression model was performed to determine risk factors for non-union.

Results: There were 18 patients with an adjacent ankle fusion and 53 patients without. There were no significant differences in demographics between the two groups. Overall 46 (86.8%) of those without an ankle fusion fused, compared to 8 (44.4%) of those with an ankle fusion above (p=0.000). On multivariate logistic regression an adjacent ankle fusion was the only significant risk factor for non-union. When taking account of other risk factors for non-union the odds ratio of union of the subtalar joint with an ankle fusion present 0.1753 (95% CI 0.0449 - 0 .6843) compared to a subtalar fusion with a native ankle joint (P=0.012). 9.4% of patients without an ankle fusion underwent a revision subtalar fusion compared to 44.4% of those with an ankle fusion (P=0.001).

Conclusions: Patients undergoing a subtalar arthrodesis with an adjacent ankle fusion have a significantly increased risk of non-union compared to those with a native ankle. Patients with a previously fused ankle need counselling about the high risk of non-union and additional surgery.