Locked plating : biomechanics and biology

Conventional plate techniques have been widely used in the treatment of humeral fractures. The controversial clinical outcome is believed to be related to the reduced blood supply and to the presence of osteoporotic bone substance. In fact, the compressive force under the plate prevents periosteal perfusion resulting in periosteum and bone necrosis deep to the plate and adjacent to the fracture site, localized bone resorption at the screw threads, and loosening of the implant. Furthermore, in the presence of osteoporotic bone, it may become impossible to develop sufficient screw torque to generate sufficient screw force to prevent micromotion. In light of the encouraging results achieved in the management of metaphyseal fractures of the tibia and femur, locking plates have recently been introduced for the treatment of humeral fractures. These systems allow the screws to rigidly lock into the plate hole. Locking plate systems decrease gap strain by minimizing motion whereas tolerating an increased gap length. Strain at the fracture site is optimized so that secondary bone healing with callus formation is favored over fibrosus nonunion or primary bone healing. As internal fixators, locking plates no longer rely on frictional force between the plate and bone thus allowing the local blood supply. Promising initial clinical results have been reported. Regarding proximal humerus, biomechanical tests have shown better fatigue resistance and stiffness of locked plates as compared with conventional ones. Interestingly, in the case of distal humerus, comparative tests have shown that the biomechanical behavior depends more on plate configuration than plate type. Anyway, further clinical and biomechanical investigations are needed to understand the indications for locked plating as opposed to conventional plating in the treatment of humeral fracture.