THE EFFECT OF SCREW POSITION AND SCREW NUMBER ON THE “CUTOUT” BEHAVIOUR OF MODERN LOCKED IMPLANTS IN THE PROXIMAL HUMERUS – A CADAVER STUDY

Abstract

Introduction: Since the introduction of locked implants new possibilities in the treatment of proximal humeral fractures have evolved. Despite the success using locked plates recent publications report the cutting of screws through the humeral head in up to 30% of the cases. The distribution of the bone strength in the humeral head is not linear. Can polyaxial screw positioning in areas with higher bone strength reduce the “cutout” rate? Which effect has an inferomedial screw if the medial hinge is not restored?

Methods: 4 groups were formed from 31 fresh frozen proximal humeral cadavers. A polyaxial proximal humeral locking plate was used to perform the tests. A standardised unstable intraarticular fracture was created. Main Outcome measure was the load and cycle where at least one screw was cutting through the cartilage.

Results: Polyaxial screw placement in areas of incresead bone strength compared to random screw placement had no effect on the cutout behaviour (p=0.7). Increased screw number (3 vs. 5) significantly increases the resistance against cutout (p< 0.04). An inferomedial screw significantly increases the resistance to develop a cutout compared to the control groups(p=0.03 and p< 0.05).

Discussion: The placement of an inferomedial screw significantly increases the resistence to develop a cutout in proximal humeral fractures without a medial hinge independently of the total screw number. In addition we could also show that the number of screws in the humeral head has a significant effect on cutout resistance in a human cadaver setup.