The pelvis plays an important role as a mediator of sagittal balance in humans, and the sacrum is the junction which transfers loads from the spine to the lower extremity13. The anatomical characteristics of the sacrum must allow for the maintenance of bipedal posture as well as dynamic movements with minimal energy expenditure14. A more curved sacrum is considered to be a distinctive feature of homo sapiens15, 16, and sacral curvature is minimal in tailed mammals and human infants, but increases with age in humans until reaches its skeletal maturity17. As pelvic incidence has also been reported to develop and change in a similar manner during human growth, which is thought as a result of erect posture and linked with numerous spinopelvic pathologies18, 19, thus it’s natural to relate the sacral curvature to PI together. There has been many reports showing that global sacral kyphosis correlates closely with increasing lumbosacral translation, sacral horizontal angle, lumbar lordosis, and lumbar index6. However, because of deficiencies in the conventional radiographic examination, the sacrum, acetabulum and hips were not included on spine radiograph in patients with lumbar spine disorders, and the association between pelvic incidence and sacral morphology is largely unknown.
As a parameter of the sagittal spine profile, pelvic incidence describes the angulation of the sacrum in the pelvis in relation to the hip joints, and influences force transmission thus has been associated with spondylolisthesis. Abola et al. 10 measured 120 cadaveric samples and found that a more curved sacrum, decreased sacral-ala width, and a more linear SI joint were related to an increased pelvic incidence. However, we only found a weak correlation between sacrum curvature (SCA) and PI by the correlation coefficient analysis, and further regression analysis were not able to confirm the significance. Rather than the sacrum curvature, morphological parameters of S1: S1I, SSVA1 and STA were found to be significantly correlated to PI. Morphological parameters of other sacral segments were also included in this study, but none of them were correlated to PI either.
STA is an anatomical parameter describing S1 rather than the curve of the sacrum. Inoue et al.20 identified STA as a key anatomical sacral parameter in patients with spondylolisthesis. Compared to the control group, Wang et al. 21 revealed a significant decrease in STA in patients with L5-S1 spondylolisthesis, and that the slip grade is related to the decrease in STA. Whitesides et al. 22 indicated that STA may play a more important role than PI in the etiology of spondylolisthesis. We confirmed that STA is negatively correlated with PI, which corroborate the previous findings and may serve as an explanation. For patients with high-grade spondylolisthesis, whose sacral endplate is always dome shaped, PI can be difficult to measure12, 23. The newly suggested parameter (S1I) may have great potential in defining the shape of the sacrum as well as the sagittal balance of the spine and may serve as an indirect parameter for the prediction of PI. However, more work is needed to validate the reliability of this new method.
Previously, two methods have been reported in the literature to measure, one using the standard Cobb angle method and the other with the Ferguson technique. Because the anterior and posterior aspects of the sacrum often appear eroded in spondylolisthesis, the Ferguson method has been thought more reliable than the Cobb method. Even so, manual tracing of standing lateral radiograph still have inherent error and flaws, and reconstructed computed tomography scans would have given more information24. In this study, we introduced a new circle fitting method based on the morphology of S1, S2, S3 to simulate the sacral curvature, and three parameters: SCA, SAL and SAR were used to describe the curvature of sacrum. Since SI joint involves primarily the first three sacral vertebra and does not involve the last two sacral vertebra which curve is highly variable, we did not include S4 or S5. ICC analysis confirmed that this method has satisfactory intraobserver and interobserver reliabilities.