A Biomechanical Comparison of Locked and Unlocked Long Cephalomedullary Nails in a Stable Intertrochanteric Fracture Model

Objectives: This study compared the torsional properties of stable intertrochanteric femur fractures in a cadaveric bone model using 2 different distal fixation strategies: unlocked long cephalomedullary nailing versus dynamically locked nailing. Methods: Fourteen matched pairs of cadaveric femora were randomly assigned to 1 of 2 distal fixation treatment groups: a single distal interlock screw placed in the dynamic orientation or no distal screw fixation. A stable 2-part intertrochanteric fracture was produced. Specimens were potted and mounted in a double gimbal fixture, facilitating unconstrained motion in the sagittal and coronal planes. Specimens were cyclically loaded dynamically in both internal and external rotation. Range of motion, internal and external rotation stiffness, torsion stiffness, torsion yield, and ultimate torsion magnitude were calculated. Results: The samples instrumented with a distal locking screw reported statistically significantly greater internal (1.54 +/- 0.81 N.m per degree vs. 1.08 +/- 0.35 N$ m per degree; P = 0.026) and external rotational stiffness (1.42 +/- 0.72 N$ m per degree vs. 0.86 +/- 0.36 N$ m per degree; P = 0.009). Samples with locked distal fixation were statistically stiffer and displayed statistically less displacement at the yield and peak torque. The yield torque was statistically significantly higher in the samples without distal fixation (14.2 +/- 3.3 N.m per degree vs. 10.6 +/- 3.8 N.m per degree; P = 0.037). The peak torque was comparable between locked and unlocked samples (15.0 +/- 4.6 N.m per degree vs. 16.2 +/- 4.2 N.m per degree; P = 0.492). Conclusions: Distal locking of femoral intramedullary nails increases the stiffness of the nail-femur construct. Unlocked samples displayed statistically significant higher yield torque while maintaining comparable peak torque as the locked samples. This study indicates that treating stable intertrochanteric fractures with unlocked long intramedullary nails may be an acceptable option, although further clinical study will be needed to test this assertion.