Sacral Dysmorphism and its Implication on the Size of the Sacroiliac Joint Surface

STUDY DESIGN This is a retrospective data analysis. OBJECTIVE The aim of this study was to analyze the prevalence of sacral dysmorphism and its correlation to the size of the sacroiliac joint (SIJ) surface based on computed tomography (CT) scans. SUMMARY OF BACKGROUND DATA Sacroiliac screw fixation is a widely accepted technique for stabilization of posterior pelvic ring injuries. Safe sacral screw placement may be impaired by sacral dysmorphism. The prevalence and impact of sacral dysmorphism on the size of the SIJ surface is unknown. MATERIALS AND METHODS In total, 269 CT scans were evaluated for the presence of the 5 signs of sacral dysmorphism (mammillary bodies, tongue-in-groove, residual upper sacral disk space, colinearity, and dysmorphic sacral neural foramina). The size of the SIJ surface was calculated by measuring the sacral joint line of the SIJ on each axial CT slice. Logistic regression analyses were conducted to reveal sex-related or age-related differences and correlations between the presence of the dysmorphic signs and the size of the SIJ surface. RESULTS Prevalence rates of the dysmorphic signs ranged from 5% (colinearity) to 70% (residual sacral disk space). Only 15% did not show any sign of sacral dysmorphism. The average size of the SIJ surface was 7.36 cm; it was significantly larger in male (8.46 cm) than in female (6.11 cm) patients (P<0.001). The presence of tongue-in-groove morphology was associated with a significantly larger SIJ surface (P<0.001), the presence of a residual upper sacral disk space with a significantly smaller joint surface (P=0.006). CONCLUSIONS The prevalence of sacral dysmorphism is remarkably high in a normal population and it is questionable if the respective signs should be called dysmorphic after all. The possibility of a smaller joint surface in female patients and patients with a residual upper sacral disk space should be considered in the planning of iliosacral screw placement.