Mechanisms of failure of locked-plate fixation of the proximal humerus: acoustic emissions as a novel assessment modality.

Objectives: Locking plates are the predominant implants used for proximal humerus fractures. Despite a preponderance of good clinical outcomes, failures continue to occur. The purpose of this study was to investigate the failure mechanism of locked proximal humeral plate fixation and its relationship with bone density and screw length.

Methods: Human cadaveric humeri were subjected to cyclic bending loads after an unstable 2-part fracture (Orthopedic Trauma Association classification 11 A-3) was created and stabilized with a locking proximal humeral plate. Acoustic emission (AE) sensors were mounted on the specimens to detect fracture displacement and generation of microcracks. The data were analyzed to evaluate construct failure.

Results: Eight of 10 locking plate constructs in cadaver specimens failed in varus collapse. The primary influences on failure were cancellous bone density and cancellous bone screw length. AE monitoring demonstrated patterns of microcrack progression, predominantly along the inferior screws. The progression trends according to AE were similar to their respective actuator displacement versus time curves.

Conclusions: Cancellous bone density and total cancellous screw depth penetration seem to be critical variables. Although the patients' bone density cannot be controlled, surgeons may decrease the risk of failure by maximizing the length of the screws within the cancellous bone. Analysis of microcrack formation revealed that failures begin at the midportion and tips of the inferior screws and at the bone-plate interface of the inferior screws.