Locked plate fixation of osteoporotic humeral shaft fractures: are two locking screws per segment enough?

OBJECTIVE The purpose of this study was to compare the biomechanical behavior of using two versus three locking screws per bone segment in a cadaveric humerus fracture gap model. METHODS Six matched pairs of elderly osteoporotic fresh-frozen human cadaveric humerii were used. An eight-hole locking compression plate was placed posteriorly on the humeral shaft and secured with either four or six bicortical locking screws. A 5-mm middiaphyseal gap osteotomy was created to simulate a comminuted fracture without bony contact. Specimens were tested in offset axial compression, four-point anteroposterior bending, four-point medial-lateral bending, and torsion. After the initial testing in each of these modalities, the constructs were cyclically loaded in torsion and again tested in the four loading modalities. Lastly, the fixation constructs were then tested to failure in torsion. RESULTS There were no significant differences in stiffness between the group fixed with two screws per segment and the group fixed with three screws per segment. The peak torque to failure was higher in the four-screw construct compared with the six-screw construct. The mean torque to failure was 23.5 +/- 3.7 Nm in the construct with two locking screws per segment compared with 20.4 +/- 2.8 Nm in the construct with three locking screws per segment (P = 0.030). CONCLUSIONS The addition of a third screw in the locked plate construct did not add to the mechanical stability in axial loading, bending, or torsion. In testing to failure, the addition of a third screw resulted in lower load to failure.