Posterior Inferior Comminution Significantly Influences Torque to Failure in Vertically Oriented Femoral Neck Fractures: A Biomechanical Study

Objectives:To evaluate axial fracture obliquity and posterior inferior comminution in vertically oriented femoral neck fractures (FNFs) in the physiologically young patient. A biomechanical investigation was designed to evaluate the impact of these fracture elements on torque to failure using cannulated screw (CS) and sliding hip screw fixation.Methods:Four Pauwels III FNF models were established in synthetic femurs: (1) vertically oriented in the coronal plane (COR), (2) coronal plane with axial obliquity (AX), (3) coronal plane with posterior inferior comminution (CCOM), and (4) coronal plane with axial obliquity and posterior inferior comminution (ACOM). In each group (n = 10), specimens were fixed using either 3 CSs or a sliding hip screw with supplemental antirotation screw (SHS). Quasistatic cyclic ramp-loading to failure was performed using a custom testing jig combining axial preloading and torsional ramp-loading. The primary outcome was torque to failure, defined as angular displacement ≥5 degrees.Results:In the CS group, torque to failure was 40.2 ± 2.6 Nm, 35.0 ± 1.4 Nm, 29.8 ± 1.5 Nm, and 31.8 ± 2.2 Nm for the COR, AX, CCOM, and ACOM fracture groups, respectively (P < 0.05). In the SHS group, torque to failure was 28.6 ± 1.3 Nm, 24.2 ± 1.4 Nm, 21.4 ± 1.2 Nm, and 21.0 ± 0.9 Nm for the COR, AX, CCOM, and ACOM fracture groups, respectively (P < 0.05). In both constructs, groups with posterior inferior comminution demonstrated significantly lower torque to failure compared to the COR group (P < 0.05). The CS construct demonstrated higher torque to failure in all groups when compared to the SHS construct (P < 0.01).Conclusions:Posterior inferior comminution significantly affects torque to failure in vertically oriented FNFs. Three peripherally placed CSs may resist combined axial and torsional loading better than a sliding hip screw construct.