Computed tomography analysis of sacropelvic parameters in relation to anterior access to the lumbosacral disc

Purpose: Anecdotally a higher pelvic incidence (PI) confers a 'deeper' pelvis with the potential for challenging access, yet this is ill-defined in the existing literature. The aim of this study was to assess the relationship of sacropelvic sagittal parameters and their relationship with the projection angle (PA), an indicator of access to and orientation of the lumbosacral disc with respect to the pelvis and identify a threshold PI value beyond which more difficult surgical access may be anticipated.Materials and methods: Computed tomography (CT) scans taken for trauma were studied. Measures including the PI, sacral kyphosis (SK), sacral table angle (STA), PA and anterior pelvic angle (APA) were taken. The PA is the angle subtended by a line from the apex of the pubic symphysis to the sacral promontory and a line running along the sacral endplate. A positive value is obtained when the line from the endplate runs superior to the symphysis.Results: 168 scans were reviewed, mean age 44.2 years (s.d. 18.4). The mean PI was 50.0 (s.d. 10.2), SK 24.4 (s.d. 12.3), and STA 102.0 (s.d. 6.1). The mean PA was 20.1 (range -14 to 46; s.d. 10.3). PA correlated with PI (R = -0.892; p < .001) and also SK (R = -0.760; p < .001). With PI values above 73 the PA is likely to be negative with the lumbosacral disc orientation falling behind the pubic symphysis. One-way analysis of variance showed differences in PA according the six subclasses of PI.Conclusion: The PA, as an indicator of the orientation of the lumbosacral disc with respect to the pelvis, correlated strongly with the PI. Patients with high PI are more likely to have a lumbosacral disc with trajectory falling behind the pubic symphysis and surgeons should closely analyse pelvic anatomy, particularly in patients with high PI.