Mechanisms, Predisposing Factors and Prognosis of Intra-Operative Vertebral Subluxation During Pedicle Subtraction Osteotomy in Surgical Correction of Thoracolumbar Kyphosis Secondary to Ankylosing Spondylitis.

STUDY DESIGN: A retrospective study.

OBJECTIVE: To analyze the mechanisms, predisposing factors and prognosis of the intraoperative vertebral subluxation (VS) during pedicle subtraction osteotomy (PSO) for thoracolumbar kyphosis secondary to ankylosing spondylitis (AS).

SUMMARY OF BACKGROUND DATA: VS is one of the most daunting challenges that surgeons encounter during PSO closure, especially in AS patients with ankylosed and mostly osteoporotic spine. Unfortunately, there is a paucity of research designed to conceptualize the mechanisms, predisposing factors and discuss the complication-avoidance strategies and prognosis.

METHODS: A retrospective single-center review was performed for a consecutive series of 153 AS patients with rigid thoracolumbar kyphosis who underwent one-level PSO from April 2000 to December 2013. The incidence of the VS at the level of PSO during correction was analyzed and the potential causative factors were investigated.

RESULTS: VS occurred in six patients with the incidence being 3.9% in this patient cohort. The predisposing factors were: (1) early fracture of the anterior cortex of the osteotomized vertebra (OV); (2) excessive decancellation from vertebral body causing parallel collapse of the vertebral column with significant loss of the ability to create local lordosis; (3) improper manual osteoclasis due to insufficient decancellation of the OV; and (4) inappropriate application of cantilever technique plus concomitant long instrumentation. The early surgical complication involved one patient with cerebrospinal fluid leakage at the osteotomized site, but no devastating neurological deficits. During follow-up, bone healing and adaptive vertebral remodeling with no rod breakage were observed for all these six patients.

CONCLUSION: Intra-operative VS was a rare occurrence associated with inappropriate manual manipulation of osteotomy, gap closure and rod insertion. Neurological complication was a potential risk, but could be well prevented with extensive laminectomy and emergency actions favoring partial subluxation reduction. Moreover, adapative bone remodeling and fusion at the level of vertebral subluxation ensured the maintenance of kyphosis correction and avoidance of instrumentation failure.

LEVEL OF EVIDENCE: 4.