Factors Predictive of Blocking Screw Placement in Retrograde Nailing of Distal Femur Fractures.

Objectives: (1) Identify factors that predict blocking screw placement in the treatment of a distal femur fracture with retrograde nail fixation and (2) determine whether acceptable alignment and stability were achieved in fractures that received blocking screws. Design: Retrospective Comparative Study. Setting: Level I Trauma Center. Patients/Participants: Between 2011 and 2017, we identified 84 patients with distal third femur fractures treated with a retrograde femoral nail. Data were analyzed according to those who did (BLOCK, n = 30) and did not (NO BLOCK, n = 54) receive blocking screws. Patients in both groups were treated by orthopaedic trauma surgeons; postoperative weight-bearing radiographs were obtained of every patient. Intervention: Fixation using a retrograde femoral nail with or without blocking screws. Blocking screws were placed at the discretion of the treating surgeon to reduce malaligned fractures or improve stability. Main Outcome Measurements: (1) Demographics, radiographic apparent bone gap (RABG), space available for the nail (SAFN), and distal segment length [as a ratio of bicondylar width (BCW)]; and (2) post-operative alignment and stability (change in alignment over time). Results: Patients treated with blocking screws had a higher body mass index (BMI) (BLOCK: 29.0, NO BLOCK 25.7, P = 0.03). In addition, the BLOCK group had larger RABGs (BLOCK: 8.2 mm, NO BLOCK: 3.6 mm, P = 0.02), more SAFN (BLOCK: 51.4 mm, NO BLOCK: 39.8 mm, P = 0.02), and shorter distal segments (BLOCK: 1.7 x BCW, NO BLOCK: 2.0 x BCW, P = 0.01). In a multivariable logistic regression, the combination of these factors was significantly predictive of blocking screw placement with a large effect size (R-2 = 0.36, P < 0.01). A distal segment length <= x2 BCW was 77% sensitive for blocking screw placement, and a BMI >= 25 kg/m(2) was 70% sensitive. Negative predictive values for blocking screw placement were distal segment length > x2 BCW (79%), BMI <25 kg/m(2) (77%), RABG <4 mm (76%), and SAFN <50 mm (71%). Patients that received blocking screws had acceptable postoperative alignment and stability, similar to fractures that did not receive blocking screws (P > 0.05). Conclusions: This retrospective study of distal femur fractures treated with retrograde nails identified several factors that can be used to predict when blocking screw placement may be useful for increasing stability and maintaining alignment in distal third femur fractures treated with retrograde IM nails. Patients treated with blocking screws had a higher BMI, greater cortical bone loss, more SAFN, and shorter distal segments. There was no difference in postoperative alignment or stability between the 2 groups.