Biomechanical evaluation of locking plate fixation with hybrid screw constructs in analogue humeri

Locking plates are well suited to complex fracture patterns and weak bone. In the study reported here, we compared the structural stability of 3 different locking compression plate (LCP) constructs using composite analogue humeri. Eighteen analogue composite humeri were used as bone models. A 6.5-mm osteotomy gap was stabalized with a 9-hole 3.5-mm narrow LCP using four 3.5-mm self-tapping screws on each side of the fracture with the middle hole empty. Three construct configurations were studied: B (all screws bicortical), BU (bicortical screw on each side of fracture gap and remaining screws unicortal), and U (all screws unicortal). Each bone model was fixed in a customized jig and subjected to mediolateral and anteroposterior 4-point bending and external rotational torque to assess rigidity, stiffness, and failure. There was significant (P<.05) differences in torsional stiffness but no significant differences in terms of flexural rigidity between each of the constructs. The results also indicated that construct BU provided as much stability as the other constructs. Therefore, consideration should be given to type of fixation construct, especially when torsional stability is required. Replacing a single set of unicortal locking screws with bicortical locking screws closer to the fracture site improved construct stability compared with any unicortal screw construct. A hybrid fixation construct that provides bicortical screws at any location may provide equivalent construct stability in this model. Hybrid fixation constructs may provide adequate fracture stabilization for a fracture pattern that would typically be considered unstable.