Dynamic Sagittal Imbalance of the Spine in Degenerative Flat Back: Significance of Pelvic Tilt in Surgical Treatment

Choon–Sung Lee,
Yung–Tae Kim,
Jeong–Hyun Yoo,
Choon–Ki Lee
University of Ulsan and Seoul National University, Seoul, Korea

BACKGROUND:
Degenerative flat back, a common spinal deformity in oriental countries, is a typical example of sagittal spinal imbalance causing serious disability due to stooping of the trunk. Sagittal imbalance in degenerative flat back is usually more evident on walking, suggesting its dynamic nature. Surgical correction of degenerative flat back is often accompanied by various complications. The most puzzling is postoperative persistent stooping despite restoration of sufficient lumbar lordosis by corrective surgery. The purpose of this study was to elucidate the cause of the postoperative persistent stooping.

METHODS :
A tool practically available for evaluation of dynamic imbalance was gait analysis. Twenty seven patients, who had been operated after gait analysis from 1995 to 1997, were divided into two groups according to the level of postoperative improvement in stooping: twenty two patients with marked improvement in stooping and five patients with persistent stooping. In order to determine the cause of the persistent stooping, various radiographic and gait (sagittal kinematic and kinetic) parameters were compared between the two groups.

RESULTS:
All the radiographic parameters were not significantly different between the two groups (p > 0.05). On the other hand, in kinematic gait data, anterior pelvic tilt was significantly increased in those patients with persistent stooping, along with greater hip and knee flexion (p < 0.05). In kinetic gait data, hip extensor moment was also significantly increased in these patients.

DISCUSSION:
Marked degeneration and weakness of lumbar extensor muscles were demonstrated in most patients with degenerative flat back. In these patients, posterior pelvic tilt has been considered to be the most important compensatory mechanism to overcome stooping from sagittal imbalance. We observed that this compensatory mechanism was lost in some patients. Extensive weakness of pelvic as well as lumbar extensor muscles was presumed to be responsible for this loss of pelvic compensatory mechanism, i.e., posterior pelvic tilt.

CONCLUSION:
Degenerative flat back could be classified into two types based on pelvic position during walking : one with posterior pelvic tilt, and the other with marked anterior pelvic tilt. In the former type, corrective surgery could improve the stooping. In the latter, in which pelvic compensatory mechanism was lost, corrective surgery was usually ineffective, resulting in postoperative persistent stooping. This study demonstrated that not all the patients with degenerative flat back were candidates for corrective surgery.