Loss of the normal lumbar lordosis from lumbar fusion surgery is a common source of sagittal imbalance seen in clinical practice. Such loss of lordosis is commonly referred to as a flat back deformity due to the flattening of the lumbar curvature. This commonly results in the patient compensating by extending at the hips and the mobile portions of the unfused spine as well as flexing at the knees in order to maintain balance in an erect posture. This compensation results in excessive mechanical strain on the mobile portions of the spine as well as the hips and knees; it also commonly results in complaints of difficulty standing upright, difficulty maintaining balance, back pain, leg pain, and fatigue.
Surgical treatment of symptomatic flat back deformities requires addressing the underlying biomechanical imbalance to restore a neutral sagittal balance. This may be accomplished in multiple ways largely dependent on the preexisting condition of the patient’s spine such as the length of prior fusions but the goal remains to restore a neutral sagittal balance so that the spine has a stable foundation. Failure to adequately correct a sagittal imbalance leaves the spine with an unstable foundation which can perpetuate excessive mechanical stress on the knees, hips, and spine and lead to progressive deformity, pain, and disability.