Increased Risk of Fixation Failure With Two 3.5 Versus 4.5 Millimeter Solid Bicortical Screws for Tibial Tubercle Osteotomy

PURPOSE: To assess the association between screw size and fixation failure after tibial tubercle osteotomy (TTO). METHODS: Patients undergoing primary TTO from 2015 to 2024 by 2 patellofemoral surgeons fixed with two 3.5 mm or two 4.5 mm solid bicortical screws were compared. The primary outcome was fixation failure (i.e., fracture, nonunion, or symptomatic delayed union requiring operative intervention) by 6 months. Patient demographic and surgical factors were collected. Univariate and multivariable adjusted analyses controlling for known and/or theoretical risk factors for overall complications after TTO were performed. Secondary analysis assessed the association between other patient/surgical factors and fixation failure. RESULTS: There were 398 patients with a mean follow-up of 2.2 years (mean age 28.3 ± 9.6 years; 74.4% women). The overall rate of fixation failure was 2.0%, occurring at a mean of 3.3 ± 1.8 months postoperatively. Patients undergoing TTO with 3.5 mm screws were significantly more likely to experience fixation failure than patients undergoing TTO with 4.5 mm screws (7.9% vs 1.4%, P = .03); results were even more pronounced when limiting analysis to the 296 female patients (7.9% vs 0.78%, P = .02). Results held in all adjusted analyses, including when controlling for age, sex, smoking, concomitant procedures, and TTO distalization (which was more common in the 3.5 mm group), among other factors (odds ratio 8.1, 95% confidence interval 1.1-59.8, P = .04). No other patient or surgical factor, including distalization, was associated with fixation failure (P ≥ .15 for all). CONCLUSIONS: Compared with two 4.5 mm screws, two 3.5 mm screws were associated with a significantly increased risk of fixation failure after TTO, even after adjusting for patient and surgical factors. Although fixation failure overall remains rare after TTO, screw size may be a modifiable risk factor to reduce the risk of this devastating complication. LEVEL OF EVIDENCE: Level III, retrospective comparative case series.