Introduction: Evaluation of sagittal sacropelvic morphology and its association with lumbopelvic alignment is an important and clinically relevant issue. Radiographic assessment of this morphology is often difficult due to anatomic variation of the proximal sacrum. An efficient and reliable technique to assess this morphology, which avoids registration of the entire sacral endplate, is needed.
Methods: 200 adult volunteers had standing 36″ radiographs of the spine and pelvis that included both hips. The Pelvic Radius Angle (PRA) was measured from the PR line (as defined in the literature) to the horizontal. Vertebral endplate slopes (VES) were measured from the horizontal. PRA = sacral endplate slope + the fixed pelvic contribution to lordosis (Pelvic Lordosis). Lumbo‐Pelvic Lordosis (LPL) = PRA ± a specific VES. PI was determined. A clinically relevant group of 24 high grade lytic spondylolisthesis patients was used to determine reliability and efficiency of the PRA and PI methodologies.
Results: PRA was always <90°, but >55°. L4 slope was approximately neutral (range: 16° to ‐24°). T12 slope was always positive (4° to 37°). Lordosis varied greatly (34° to 80°), but total LPL (PR line to T12 slope) was approximately 90° and always >70°. Distal LPL (PR line to L4 slope) was always >45°. PRA was better correlated with the lordosis measurements (r = ‐.72) compared to PI (r = ‐.52). In the volunteers, intra‐ and inter‐observer reliabilities were very high for both PRA and PI measurements (r ≥ .95 for all). But when applied clinically in the patients, intra‐ and inter‐observer reliabilities were better for PRA vs. PI (r = .93 and .89 vs. r = .79 and .68, respectively). Measurement times for PRA were faster vs. those for PI (i.e. 50% less time to measure PRA).
Conclusion: The PRA and VES can determine minimum lordosis requirements, which are dependent on individual pelvic morphology, without having to register the entire sacral endplate. This methodology can be both efficiently and reliably applied within the clinical setting. When fixing lordosis by surgical fusion, the minimum requirements should be met.
Significance: Assessment and understanding of sagittal balance is fundamental in our management of spinal disorders.