Computed tomography-based three-dimensional visualisation of bone corridors and trajectories for screws in open reduction and internal fixation of symphysis diastasis: a retrospective radiological study.

INTRODUCTION: Typical stabilisation of pelvic open book injuries consists of plate fixation of the symphysis. No previous literature has been published about the evaluation of screw placement and their trajectory with four oblique 4.5 mm screws using a four-hole plate in symphysis diastasis. The aim of this study was to define insertion points and angles of trajectory for crossed screw placement regardless of any plate design based on an analysis of three-dimensional computed tomography data sets. METHODS: One hundred human pelvic CT data sets were collected. Unilateral and bilateral placements of crossed 4.5 mm screws were simulated. Primary outcome measure was successful simulated screw placement without cortical breach. Secondary outcome measures included the anatomical measurements of the screw positions. RESULTS: Simulated screw placement of two oblique screws on each side of the pubic symphysis without cortical breach was achieved in all (100 %) cases. There were a total of 400 screw simulations. Medial screws were longer, lateral screws had higher coronal angles, and the distance between both screws was higher on the right side (p < 0.001 each). The lengths of the right lateral, right medial, left lateral, and left medial screws were 44.9, 65.8, 45.4, and 67.4 mm, respectively. The sagittal angles to the dorsal surface area of the pubic rami were 10.5°, 11.1°, 9.0°, and 11.0°. The coronal angles to the vertical axis of the symphysis measured 39.5°, 16.0°, 33.8°, and 16.8°. The distances between these screws and the medial edge of the pubic crest were 33.5, 8.6, 29.5, and 7.3 mm. Furthermore, certain sex- and side-related differences were noted. CONCLUSIONS: This series provides results about the feasibility and a detailed anatomical description of crossed screw placement. This is of special interest in pelvic surgery for choosing the entry points, safe screw channel parameters, and trajectories.