Setscrew distal locking for intramedullary nails: A biomechanical study

Objective: This biomechanical study was undertaken to examine the effective nes of setscrew distal locking id a static intramedullary (IM) femoral nail on the stability of fixation of femoral shaft fractures. Design: Fifteen fresh-frozen cadeveric femora were randomly separated into three groups of five bones and transversely sectioned immediately distal to the isthmus. After the insertion of the large-diameter nails, distal locki ng was obtained by conventional method in the first group. In the second gr oup, setscrew design was used in which two transverse screws penetrated onl y the lateral cortex of the femur and compressed the nail in the intramedul lary canal. No distal locking was used in the third group. Intervention: All instrumented femurs were mounted on a servohydraulic test ing machine and tested in both rotations (20 degrees) and axial compression (amplitude: 1,000 Newton). Main Outcome Measurement: Loading-versus-displacement data, acquired at a t en-Hertz sampling rate, were calibrated and used to calculate maximum torqu e, stiffness, and energy capacity to failure. Maximum displacement and axia l stiffness also were determined. Results: Mean maximum torque at 10 degrees for each group were 15.3 +/- 4.8 newton-meters for the interlocking group, 8.5 +/- 1.2 newton-meters for th r setscrew group, and 3.6 +/- 2.7 newton-meters for the nonlocked femora. A t 20 degrees of rotational displacement, the torque measured 37.4 +/- 2.6 n ewton-meters; 15.0 +/- 4.0 newton-meters; and 5.3 +/- 3.1 newton-meters, re spectively (p < 0.05). Mean torsional stiffness was 1.17 +/- 0.76 newton-me ters per degree for the setscrew group and 1.34 +/- 0.83 newton-meters per degree for the interlocking group (p > 0.05). The setscrew design provided 87 percent of the torsional rigidity of the interlocking group. In the axia l compression test, mean maximum shortening was 1.1 +/- 0.3 millimeters in the interlocking group and 1.4 +/- 0.6 millimeters in the setscrew group (p > 0.05). The mean stiffness on longitudinal compression provided by the in terlocking screws and the setscrews was 918 and 860 newton-meters per milli meter, respectively. Conclusion: The distal setscrew design provides adequate distal fixation of intramedullary nail for patients in the postoperative rehabilitation perio d of the femoral shaft fractures treated with intramedullary nailing.