The position and number of screws influence screw perforation of the humeral head in modern locking plates: a cadaver study.

OBJECTIVES: Screw perforation of the humeral head in locking plate osteosynthesis occurs in up to 30% of cases. The current study compared different fixation possibilities (eg, number and position of screws) to reduce screw perforation in the humeral head. METHODS: A humeral head fracture with a missing medial support was created in 30 fresh-frozen cadavers and fixed with a polyaxial locking plate (NCB PH; Zimmer, Warsaw, IN). The constructs were loaded with increasing force and the number of cycles until screw perforation was recorded. Four different fixation methods were tested: group 1 five screws with fixed angle, group 2 five screws in polyaxial position according to bone strength, group 3 three screws, and group 4 five screws with 1 as an inferomedial support screw. RESULTS: More screws in the humeral head significantly increased the number of cycles before screw perforation. An inferomedial support screw further increased the number of cycles. Polyaxial screw placement compared with fixed-angle placement had no effect on the screw perforation phenomenon. CONCLUSIONS: We recommend to position an inferomedial support screw, and at least 5 screws in the head fragment, when using a locking plate in proximal humerus fractures with disrupted medial hinge.

The treatment of periprosthetic fractures with locking plates: effect of drill and screw type on cement mantles: a biomechanical analysis.

INTRODUCTION: Periprosthetic fractures after cemented hip replacement are a challenging problem to manage. Biomechanical studies have suggested the benefit of using locking screws for plate fixation, but there are concerns whether screws damage the cement mantle and promote crack propagation leading to construct failure. METHOD: In this biomechanical study, different screw types were implanted into the cement mantle after pre-drilling holes of different sizes, in unicortical and bicortical configuration. The presence of cracks and the pull-out resistance of these screws were then evaluated. RESULTS: No unicortical screw induced cracks. Screws with a shortened tip, smaller flutes and double threads were significantly better for pull-out resistance. Bicortical screws were associated with a risk of local cement mantle damage, but also with a significantly greater holding power. By increasing the drill diameter, the onset of cracks decreased, but so does the pull-out resistance.