Adjacent segment hypermobility after lumbar spine fusion: no association with progressive degeneration of the segment 5 years after surgery

Abstract

Background and purpose: Increased intradiscal pressure and relative segmental hypermobility are in vitro observations supporting the idea of increased postoperative load being a reason for progressive degeneration of the free mobile segment adjacent to a lumbar fusion. These mechanisms have been difficult to confirm in clinical studies, and an alternative theory claims instead that the adjacent segment degeneration follows a natural degenerative course in patients who are predisposed. We examined 9 patients 5 years after lumbar fusion, to assess whether relative hypermobility of the segment adjacent to fusion could be correlated to progressive degeneration of the same segment.

Patients and methods: The 9 patients, all of whom had been treated with a lumbar fusion after a preoperative intervertebral mobility assessment by spinal RSA, were re-examined 5 years after surgery. The intervertebral translations of the vertebra proximal to the fusion were determined by RSA and compared to the mobility of the same lumbar segment before fusion. The disc height and any progressive reduction at the two levels proximal to the one fused were measured on conventional radiographs.

Results: Adjacent segment mobility 5 years after fusion--expressed as mean transverse, vertical, and sagittal translation of the vertebra proximal to fusion--was not significantly changed compared to the mobility measured before surgery. Increased mobility of the segment seen in 5 individual patients was not associated with progressive degeneration of the same segment or to a poor clinical outcome.

Interpretation: Hypermobility of the segment adjacent to fusion is not a general finding. Increased mobility that can be seen in certain individuals does not impair the 5-year result. The significance of mechanical alterations in adjacent segment degeneration is uncertain, and it is possibly overestimated.