Impact of Prior Ipsilateral Arthrodesis on Subsequent Ankle and Subtalar Fusion Outcomes: A Propensity-Matched Cohort Study

Background: Although prior ankle or subtalar arthrodesis is thought to affect outcomes at adjacent joints, previous studies have not distinguished between successful and failed prior fusions. This study examines whether prior successful vs failed ipsilateral arthrodesis influences nonunion risk in subsequent ankle or subtalar fusion. The primary objective of this study is to examine nonunion rates after subtalar and ankle arthrodesis in patients with and without prior ipsilateral arthrodesis, and vice versa. Methods: A retrospective study using the TriNetX Research Network compared nonunion rates in patients who underwent subtalar (ST) arthrodesis with (ankle-ST) or without (ST-only) prior ankle arthrodesis. Patients were stratified by whether the prior arthrodesis was successful (no nonunion diagnosis/revision) or failed (nonunion requiring revision Ankle-ST patients were stratified by successful or failed prior ankle arthrodesis. A secondary analysis evaluated ankle arthrodesis in patients with (ST-ankle) or without (ankle-only) prior subtalar arthrodesis, similarly stratified. Propensity score matching (1:1) adjusted for age, sex, body mass index (BMI), and comorbidities. The primary outcome is nonunion rates. Results: 144 patients were in the successful ankle-ST cohort and 12,635 in the ST-only cohort. After propensity score matching, successful ankle-ST patients had no difference in nonunion rates (16.7% vs 16.0%, P = .873). However, patients with failed prior ankle arthrodesis had a 3-fold higher risk of subtalar nonunion compared to those with successful ankle fusion (risk ratio [RR] 3.0, 95% CI 1.79-5.02). Furthermore, 109 patients were in the successful ST-Ankle group and 6,801 in the ankle-only group. After matching, there was no difference in rate of nonunion (11.1% vs 19.4%, P = .089) between the successful ST-ankle and the ankle-only groups. Conversely, patients with failed prior subtalar arthrodesis had a 2.4-fold higher risk of ankle nonunion compared to those with successful subtalar fusion (RR 2.4, 95% CI 1.30-4.42). Conclusion: Our analysis of the TriNetX Research Network database suggests that when the primary ankle or subtalar arthrodesis is successful, performing a subsequent adjacent fusion does not significantly increase the risk of nonunion compared with an isolated fusion. However, failed prior arthrodesis substantially increases nonunion risk, highlighting the importance of distinguishing between successful and failed prior procedures in clinical decision-making.