RESUMO
INTRODUCTION: One of the main failure mechanisms of modern plate fixateurs is the tearing of the whole plate-screw construct out of the osteoporotic bone. The aim of this study was to show whether an oblique screw placement can improve the fixation of the plate to the bone. METHODS: A steel probe was fixed to a synthetic bone (Fa. Sawbones) with standard titanium fixed-angle screws in parallel and 55° oblique positioning. We tested the static tear force and the dynamic stability (force distance 1.4 or 1.6 mm, 610 and 900 N, frequency of force shift of 560 or 380/min). Endpoint was a visible tear of the artificial bone. In addition we performed a morphological analysis of the torn fragments. RESULTS: The maximal tear force was 2.04 kN (1.95-2.13) for oblique and 2.66 kN (2.55-2.77) for parallel placement (p < 0.05). With a parallel positioning a higher number of force shifts were performed before a visual tear appeared. With parallel positioning the screw canal was torn with an intact surrounding corticalis; in oblique positioning the threads remained intact, the corticalis was torn out with a wedge. DISCUSSION: An oblique screw positioning does not result in a higher tear force in modern plate fixateurs.
