Spinal sagittal balance is influenced by thoracic kyphosis, lumbar lordosis, as well as the position and angle of the pelvis. The abnormal position of sagittal imbalance causes easy fatigue and pain on the back muscles, so that the body voluntarily takes action to compensate for the imbalance.
Compensatory mechanism occurs over several steps. It begins primarily in the mobile lumbar segments. When sagittal imbalance occurs due to decreased lumbar lordosis, the disc space at the mobile segments is hyperextended, allowing the C7 plumb to be restored within near normal range. As patients are older, subsequent degenerative disc changes develops at the hyperlordotic segments leading to loss of lumbar lordosis, which results in sagittal imbalance again. If the compensation in the spine is no longer able to restore the sagittal imbalance, posterior rotation of pelvis can reestablishes sagittal balance. This finding shows that the rotation of pelvis is important in the compensatory mechanisms.
Another thing to consider is that compensation gets involved in the actual lumbar lordosis. During the compensation process rotation of pelvis determines the sagittal imbalance. This is also very difficult to estimate how much the rotation of pelvis is, because the rotation is variable depending on the position.
Pelvic incidence (PI) is a suitable index to determine how much correction of lumbar lordosis requires, as this does not change depending on the position of the pelvis. Patients with high PI are able to compensate the sagittal imbalance more than them with low PI. As PI is the sum of pelvic tilt and sacral slope, PI is closely related to lumbar lordosis. Patients with high PI needs more correction of lumbar lordosis to restore sagittal imbalance than the patients with low PI.
