Optimal blocking screw placement for retrograde IM nail fixation of distal femur fracture: a standardized biomechanical study of "osteoporotic" synthetic bone.

Abstract

Intramedullary (IM) nail use for metaphyseal fracture management is problematic in long bones like the femur because the larger medullary cavity allows increased fracture motion and potentially increased implant failure and malunion/nonunion risk; Achieving effective distal femur fracture fixation is even more difficult in osteoporotic bone. Blocking screws to control IM nail movement are known to be effective for tibia fracture management. Few studies have evaluated IM nail and blocking screw use efficacy for distal femur fracture fixation in osteoporotic bone. This study used an osteoporosis simulation synthetic bone model to evaluate retrograde IM nail fixation of a standardized non-comminuted, extra-articular distal femur fracture with and without blocking screws. The hypothesis was that use of one or two blocking screws would increase IM nail fixation stability as evidenced by greater peak IM nail load and lesser movement. A servohydraulic device under displacement control collected loading force versus movement deflection data. Shapiro-Wilk tests confirmed data normality. One-way analysis of variance and Tukey post hoc tests were used to compare condition specific loading force and movement differences. In the coronal plane, blocking screw conditions displayed greater loading ranges (38.3 ± 2 vs. 19.1 ± 2N, 100.5% difference) (p < 0.0001) and lesser movement (2.0 ± 0.3 vs. 6.86 ± 0.4mm, 243% difference) (p < 0.0001). In the sagittal plane, the two blocking screw condition displayed a significantly greater loading magnitude (29.9 ± 6 vs. 20.8 ± 2.3N, 43.8% difference) than the identical control condition without blocking screws (p = 0.018); however, movements were comparable (p = 0.82). Long-axis rotational loading failed to reveal load magnitude differences (p = 0.28); however, two different blocking screw conditions displayed decreased movement (1.32 ± 0.2 vs. 2.05 ± 0.3mm, 53.8% difference) compared to other conditions (p ≤ 0.018). Use of one or two blocking screws on the medial and lateral sides of the IM nail decreased coronal plane movement in the intramedullary canal. Combining retrograde IM nail implantation with blocking screws reduced medial-lateral IM nail movement and increased fracture stability. These characteristics may help prevent fixation failure, malunion, and even nonunion in patients with a distal femur fracture in osteoporotic bone.