Segmental Pelvic Correlation (SPeC): a novel approach to understanding sagittal plane spinal alignment

Background ContextLumbar lordosis (LL) correlates with pelvic morphology, and it has been demonstrated that as LL increases, the inflection point and apex of lordosis move cranially. This suggests that each segment of the lumbar spine relates to pelvic morphology in a unique way.ObjectivesThis study aimed to establish whether there is a direct relationship between pelvic morphology and lumbar segmental angulation in the sagittal plane.Study DesignA retrospective analysis of 41 patient radiographs was carried out.Patient SampleInclusion criteria included patients with full length standing anterioposterior and lateral radiographs of the spine from base of occiput to proximal femora, with clearly visible vertebral end plates from T12 to S1 and a thoracic kyphosis (TK) and LL within the normal range. Patients were excluded if they had a coronal spinal deformity affecting the lumbar spine, chronic back pain, spondylolisthesis, spondylolysis, congenital scoliosis, or skeletal dysplasia.Outcome MeasuresSpinopelvic radiographic parameters of pelvic incidence (PI), LL, TK, and segmental angulation at each level from L1 to the sacrum were the outcome measures.MethodsForty-one lateral whole spine radiographs with normal sagittal profiles from the spinal deformity clinic were retrospectively reviewed. Pelvic incidence, LL, TK, and segmental angulation at each level from L1 to the sacrum were measured (from end plate to end plate), distinguishing the vertebral body and intervertebral disc contribution. Pearson correlation coefficients were used to analyze any relationship between pelvic parameters and segmental angulation.ResultsA strong correlation was found between PI and LL. Pelvic incidence correlated strongly with the L1 and L2 motion segments (p=.0001, p=.03), notably at the intervertebral discs but not the L4 and L5 motion segments. The proportion of total LL contributed at L4–L5 and L5–S1 reduced as PI increased.ConclusionsPelvic incidence can predict segmental angulation. Although the majority of LL is produced at the L4 and L5 motion segments, cephalad lumbar segments sequentially become increasingly important as PI increases. This describes a continuum where the L1 and L2 motion segments crucially fine-tune total LL according to PI. This allows segmental abnormalities to be identified when compensation in adjacent segments maintain normal total LL. It also paves the way for anatomical segmental reconstruction in degenerative adult deformity based on pelvic morphology.