Research

INTRODUCTION: Most studies published on sagittal spinal balance have focused on the cervical, thoracic, lumbar and sacral spine without evaluating its relationship to the pelvis and femoral heads. Measures of sagittal balance of 148 asymptomatic volunteers from two countries have been studied. The purpose of this study was to evaluate the relationship between the measures of pelvic equilibrium and lumbar lordosis.
MATERIAL AND METHOD: Volunteers were required to have had no previous spinal surgery, no low back pain, no lower limb length inequality and no scoliotic deformity. A force plate provided the ground coordinates of the central axis of gravity (CAG). Morphological and positional data of spine and pelvis were measured using custom software. Spinal measures collected included thoracic kyphosis and lumbar lordosis. Pelvic angles measured included one constant shape parameter, pelvic incidence (PI: angle between the line uniting the middle of the femoral heads and middle of the sacral end plate and the perpendicular to the sacral end in its middle), and two positional parameters: sacral slope (SS: angle between the sacral end and horizontal) and pelvic tilt (PT: angle between the line uniting the middle of the femoral heads and middle of the sacral end and vertical). These three parameters are related in that PI=PT+SS.
RESULTS: Distance between the CAG and the center of the femoral heads (femoral gravity offset) averaged 10.0 mm posterior. The anatomic parameter, PI, varied from 30.7° to 89.8° (mean: 52.2°, SD: 8.2 °). The average lumbar lordosis was 44.6°. The average thoracic kyphosis was 47.4°. We found a statistical correlation between PI and lumbar lordosis (r=0.60, p<0.01) and between sacral inclination and lumbar lordosis (r=0.67, p<0.01).
DISCUSSION AND CONCLUSION: The spine and pelvis balance around the hip axis in order to position the CAG over the femoral heads. Spinal-pelvic sagittal balance in normals is a combination between spinal and pelvic shape parameters. The pelvic shape, quantified by PI, determines the position of the sacrum, and in this way the magnitude of lumbar lordosis. PT controls the position of CAG. Sagittal balance of the spine seems to be linked to the pelvic shape. The relationship between the femoral heads and the sacrum indexed with the PI correlates generally well with the lumbar lordosis. An adverse relationship between PI and lumbar lordosis could be a factor in producing low back pain.