Screw position affects dynamic compression plate strain in an in vitro fracture model

Objective: This investigation considers the effect of a variety of screw po sitions on plate strain in three fracture models. Design: Dynamic compression plate fixation of in vitro fracture models. Methods: To model a fracture, a plastic pipe was cut transversely and a twe nty-hole dynamic compression plate was attached by screws. Eighteen stacked , rectangular, rosette strain gauges were installed on the plate to evaluat e strain. Three models were evaluated: two constructs in which there was no contact between the cut ends of the pipe under the fixation plate (small- and large-gap models) and a construct in which there was direct apposition of the cut ends (no-gap model). The pattern and magnitude of strains were a ssessed as a function of varying combinations of screw position for each mo del. Results: Maximal plate strain in the gap models was lowest with screws plac ed closest to the gap, compared with screws placed away from the gap or spa ced apart. The no-gap model showed significantly lower strains when screws were placed further from the osteotomy site than when screws were positione d close together or spaced apart. In all cases, maximal plate strain occurr ed adjacent to the most central screw holes and rapidly dissipated along th e length of the plate. Conclusion: In a model simulating a comminuted fracture (gap), this study f ound that screws should be placed as close to the fracture site as possible to minimize plate strain. In an anatomically reduced two-part fracture mod el (no gap), widely spaced screws or those placed away from the fracture re sulted in lower strains.