Background: Given that the pelvis is the pedestal on which the spine lies, its morphology has been observed to be associated with specific sagittal spinal shapes and should therefore be taken into account when dealing with pathologic conditions of the spine. However, the exact relationship between the pelvic morphology and lumbar lordosis still remains poorly defined. We hypothesized that the shape of the lumbar lordosis and its relationship with the pelvis could be described using anatomic parameters, independently of posture.
Questions/purposes: (1) What is the variation of lumbar segmental lordosis in an asymptomatic adult population? (2) Is there an association between increasing magnitude of pelvic incidence (PI) and segmental lordosis? (3) How does the position of the apex of lordosis change with increasing magnitude of PI value?
Methods: This retrospective study used data drawn from a longitudinally maintained database; between March 2014 and January 2015, 119 asymptomatic volunteers between 18 and 79 years old were enrolled in the study. Mean age was 51 years; there were 81 women and 38 men. Both segmental and cumulative lordosis were measured and used to describe the shape of the lumbar spine. We defined cumulative lordosis as the angle between L1 and S1, proximal lordosis as the angle between L1 and the superior endplate of L4, and distal lordosis as the angle between the superior endplates of L4 and S1. PI is defined as the angle between the line passing through the center of the femoral head and the center of the sacral endplate and a line perpendicular to the sacral endplate. Pearson's correlation was performed to analyze the relationship among PI, proximal and distal lordosis. Stratification by PI was performed (low, < 45°; average, 45°-60°; and high, > 60°) and the proportions of distal and proximal lordosis were then compared using an analysis of variance test.
Results: In the whole cohort, proximal lordosis accounted for 38% of total lordosis, whereas distal lordosis accounted for 62%. PI revealed a positive correlation with proximal lordosis (r = 0.546; p < 0.001). However, there was no correlation with distal lordosis (r = 0.087; p = 0.346). Stratification by PI showed that proximal lordosis increased across PI groups (16.6° [± 10] versus 21.6° [± 9] versus 30.1° [± 9]; p < 0.001), whereas distal lordosis remained relatively constant (34.8° [± 8] versus 36.7° [± 7] versus 35.9° [± 10]; p = 0.581). Apex position was overall more proximal as PI increased (r = -0.199; p = 0.034).
Conclusions: Our study demonstrated that PI influences only the proximal part of the lordosis, but not the distal part in an asymptomatic adult population. The proximal part of the lumbar spine had the most variability across individuals and appeared to accommodate to pelvic morphology (incidence). Further studies using a larger cohort size are encouraged not only to refine this relationship, but also to investigate the effect of restoration of normal lordotic shape of the lumbar spine on the functional outcomes after spinal fusion.
Clinical relevance: Our findings may be useful for surgical planning in an era of patient-specific care. The findings suggest that surgeons should aim for a patient-specific lumbar shape rather than simple global lordosis matched to the PI.