OPTIMAL LOCATION OF A SINGLE DISTAL INTERLOCKING SCREW IN INTRAMEDULLARY NAILING OF DISTAL 3RD FEMORAL-SHAFT FRACTURES

Objective: This biomechanical study was done to determine the effect o f the level of a single distal screw in a static intramedullary (IM) f emoral nail on the stability of fixation of a fracture in the distal t hird of the femur. Design: Fifteen composite fiberglass femora were os teotomized transversely in the distal third of the femur. A Grosse-Kem pf nail was implanted into the femurs, which were divided into three g roups of five specimens. Single screw distal nail locking was varied d istal to the osteotomy site for each group at 2.5, 5.0, and 7.5 centim eters, respectively. Intervention: All instrumented femurs were mounte d on a servohydraulic testing machine and fitted with transducers to m easure axial, rotational, and bending displacements. Specimens were cy clically loaded (one hertz) in simultaneous torsion (moment: +/-10 new ton-meters) and axial compression (amplitude: 2,000 newtons) for 500 c ycles with a 250-pound abductor force. Main Outcome Measurement: Data from Linear and rotational transducers were sampled at 100 hertz for f ive cycles before cycling, every 100 cycles of loading,and immediately after cycling. Custom computer software was developed to convert tran sducer signals into static and dynamic measurements of axial motion (i n millimeters), rotation (in degrees), and angulation (in degrees). Re sults: Osteotomy site dynamic rotation increased significantly in spec imens locked at 7.5 centimeters when compared with the 2.5-centimeter group. There was minimal difference between the stability of the 5.0-c entimeter and 7.5-centimeter groups. There was no significant change i n position at the fracture site before or after cyclic loading with re spect to axial shortening, rotation, or bending. Both dynamic axial an d angular displacements were also unaffected by screw position. Conclu sion: 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.