Internal Fixation of the Unstable Anterior Pelvic Ring: A Biomechanical Comparison of Standard Plating Techniques and the Retrograde Medullary Superior Pubic Ramus Screw

Abstract

Summary: The purpose of this study was to evaluate pubic ramus fracture fixation. This biomechanical evaluation compared standard plating techniques with retrograde medullary screw fixation of a superior pubic ramus fracture in a pelvic fracture model. Six fresh-frozen, cadaveric pelvic specimens with a mean age of 79 years were harvested. These specimens were physiologically loaded according to the following modifications and instrumentations: (a) intact; (b) an APC-II unstable pelvic injury, specifically, unilateral superior and inferior rami osteotomies combined with ipsilateral anterior sacroiliac (SI) joint, sacrospinous, and sacrotuberous ligamentous disruptions, without fixation; (c) disrupted as in (b) but fixed anteriorly with a 10-hole 3.5-mm reconstruction plate contoured to the superior ramus and secured with four 3.5-mm cortical screws; (d) disrupted as in (b) but fixed anteriorly with a 10-hole 3.5-mm reconstruction plate contoured to the superior ramus and secured with six 3.5-mm cortical screws; (e) disrupted as in (b) but fixed anteriorly with a 4.5-mm retrograde medullary superior pubic ramus cortical screw 80 mm long (medial to the hip joint); and (f) disrupted as in (b) but fixed anteriorly with a 4.5-mm retrograde medullary superior pubic ramus cortical screw 130 mm long that was extraarticular and engaged the lateral iliac cortex cephalad to the ipsilateral hip joint. The posterior disruptions of the pelvic ring were not fixed. The APC-II injury created in this study resulted in significant (p < 0.05) motion at the disrupted rami and the injured SI joint, compared with the intact pelvic specimen. When compared with the disrupted specimen without fixation, displacement at the superior ramus was significantly (p < 0.05) decreased by all forms of ramus fixation evaluated. Plate fixation decreased pubic ramus and sacroiliac deflections slightly better than retrograde screw fixation did, yet not significantly better. The number of screws in the plate did not significantly affect displacement measurements at either the disrupted ramus or the disrupted SI joint. Similarly, the length of the retrograde ramus screw did not significantly alter displacements at either the injured pubic ramus or the disrupted SI joint. Sacroiliac joint deflections were not significantly (p < 0.05) decreased by any of the forms of anterior pelvic fixation. Flexion at the disrupted SI joint was slightly, but not significantly (p < 0.05), decreased with all forms of fixation when compared with the disrupted specimen. The long retrograde screw and the plate with six screws decreased flexion slightly, but not significantly, better than the short retrograde screw and the plate with four screws.