Increased screw backout with non–inlay design retrograde femoral nails

Abstract Purpose To compare the rate of distal interlocking screw backout between inlay-design and non-inlay design retrograde intramedullary femoral nails in the treatment of distal femur fractures. Methods Adults with AO/OTA 33 distal femur fractures treated with a retrograde ultra-high molecular weight polyethylene (UHMWPE) inlay-design nail (RFNA; DePuy Synthes) or a non-inlay design nail (T2 SCN; Stryker) between January 2015 and June 2025 at a single Level I academic trauma center with a minimum follow-up of 3 months were retrospectively reviewed. The primary outcome was distal interlocking screw backout. Secondary outcomes included time to backout diagnosis, backout distance, and rate of symptomatic screw removal. All outcomes were compared between the inlay-design and the non-inlay design cohorts. Results A total of 134 patients were included: 76 in the inlay cohort and 58 in the non-inlay cohort. The inlay cohort demonstrated a significantly lower rate of distal interlocking screw backout compared to the non-inlay cohort (5.3% vs. 19.0%; RR 0.28, 95% CI 0.09–0.83; p = 0.02), corresponding to a 72.2% relative risk reduction. Time to backout diagnosis and backout distance did not differ significantly between designs. The rate of symptomatic screw removal was significantly lower in the inlay cohort (1.3% vs. 10.3%; p = 0.04). The distal transverse screw was the most common screw with backout in both cohorts. Conclusion The ultra-high molecular weight polyethylene (UHMWPE) inlay at the distal interlocking interface of a retrograde femoral nail was associated with significantly lower rates of distal interlocking screw backout and symptomatic screw removal. When backout did occur, polyethylene inlay did not impact time to diagnosis and backout distance.