Distal femur nonunion: Risk factors and validation of RUST scores

Abstract Objectives: (1) The purpose of this study was to investigate immediate postoperative weight-bearing status and other potential risk factors for nonunion after locked plating of distal femur fractures and (2) to compare clinical and radiographic union using the previously validated Radiographic Union Scale for Tibia (RUST) and modified RUST scores. Design: This is a retrospective cohort analysis. Setting: Academic Level-1 trauma center. Patients/Participants: Ninety patients with distal femur fractures (OTA/AO 33-A, including periprosthetic) treated with lateral locked plating over a 10-year period. Intervention: Distal femur fracture fixation with lateral locked plating; surgical intervention to facilitate bone healing in cases of nonunion. Main Outcome Measurements: Nonunion rate, construct rigidity scores, RUST, and modified RUST scores. Results: Seventy-eight of 90 patients (87%) achieved clinical union (13% nonunion rate). Rigidity score was significantly associated with risk of nonunion (P = 0.003). No significant association was detected between nonunion and postoperative weight-bearing status (P = 0.77) or other previously identified risk factors. Patients who achieved fracture union had significantly higher mean (SD) RUST (10.67 [1.37] vs. 6.53 [1.48], P < 0.001) and modified RUST (13.47 [2.20] vs. 6.94 [1.79], P < 0.001) scores than patients who developed nonunion. Sensitivity (Sn) analyses identified a RUST score threshold of 9 for diagnosing clinical union (Sn 93.6% and specificity [Sp] 91.7%) and a modified RUST score threshold of 8 (Sn 93.6%, Sp 91.7%). Conclusions: Immediate postoperative weight-bearing status does not seem to affect nonunion rates. We observed a statistically significant association between rigidity score and nonunion. This study supports the utility of the RUST and modified RUST scores in distal femur metaphyseal fractures. Level of Evidence: Therapeutic Level IV.