Optimal Location of a Single Distal Interlocking Screw in Intramedullary Nailing of Distal Third Femoral Shaft Fractures

Objective: This biomechanical study was done to determine the effect of the level of a single distal screw in a static intramedullary (IM) femoral nail on the stability of fixation of a fracture in the distal third of the femur. Design: Fifteen composite fiberglass femora were osteotomized transversely in the distal third of the femur. A Grosse-Kempf nail was implanted into the femurs, which were divided into three groups of five specimens. Single screw distal nail locking was varied distal to the osteotomy site for each group at 2.5, 5.0, and 7.5 centimeters, respectively. Intervention: All instrumented femurs were mounted on a servohydraulic testing machine and fitted with transducers to measure axial, rotational, and bending displacements. Specimens were cyclically loaded (one hertz) in simultaneous torsion (moment: ±10 newton-meters) and axial compression (amplitude: 2,000 newtons) for 500 cycles with a 250-pound abductor force. Main Outcome Measurement: Data from linear and rotational transducers were sampled at 100 hertz for five cycles before cycling, every 100 cycles of loading, and immediately after cycling. Custom computer software was developed to convert transducer signals into static and dynamic measurements of axial motion (in millimeters), rotation (in degrees), and angulation (in degrees). Results: Osteotomy site dynamic rotation increased significantly in specimens locked at 7.5 centimeters when compared with the 2.5-centimeter group. There was minimal difference between the stability of the 5.0-centimeter and 7.5-centimeter groups. There was no significant change in position at the fracture site before or after cyclic loading with respect to axial shortening, rotation, or bending. Both dynamic axial and angular displacements were also unaffected by screw position. Conclusion: The location of a single distal interlocking screw in static IM nail fixation of distal third femur fractures can significantly affect rotational stability but not axial or angular fixation.