Long Adult Deformity Fusions to L5: The Fate of the L5-S1 Disc

Introduction: The goal of adult deformity surgery is to halt progression, alleviate pain and restore global balance. In order to achieve satisfactory outcomes, long thoracolumbar fusions are required. Multiple studies have demonstrated the efficacy of these operations, however, the decision to end the fusion at L5 or to include the sacrum remains controversial. The obvious advantages of ending at L5 are retention of lumbosacral motion and less operative time. However, are these benefits outweighed by potential complications such as L5-S1 disc degeneration, resultant loss of balance, and reoperation in order to extend the fusion? We report the continued follow up on 46 patients fused to L5, with particular attention to the fate of the L5-S1 disc and the percentage of patients requiring extension of the fusion to the sacrum.

Methods: Fifty-one consecutive patients were identified with adult deformity that underwent long fusions to L5 over a 7-year period (1993-2000). Five patients were lost to follow up leaving a study sample of forty-six patients. There were 5 men and 41 women, with an average age of 47.5 and an average preoperative Cobb angle of 65 degrees. Each patient underwent long fusions from the thoracic spine to L5. The determination to end instrumentation at L5 was based on MRI evidence of disc health. The average clinical follow-up was 4 years (range: 2 to 9 years). Complete radiographs, consisting of full-length AP and lateral standing films (preop, immediate postop, and most recent followup) were evaluated. Coronal and sagittal balance, the negative inclination of T12 in relation to the vertical, thoracic kyphosis, lumbar lordosis, pelvic incidence, depth of L5 in the pelvis, and the L5-S1 disc height were measured. L5- S1 disc degeneration was also assessed.

Results: Six (13%) of the patients required extension of their fusion to the sacrum at a mean postoperative time of 35 months (range: 16 to 56 months). Of the remaining 41 patients, 51% demonstrated radiographic signs of L5-S1 progressive disc degeneration, however, were doing well clinically. Subsequent extension of the fusion to the sacrum was associated with increased sagittal balance (86mm ± 58mm for the revision group versus 24mm ± 43mm for the non-revised, p=0.01), loss of the negative inclination of T12 with respect to the vertical (-2 degrees ± 11 degrees for the revision group versus -14 degrees ± 9 degrees for the non-revised, p=0.01), and the presence of radiographic preoperative disc degeneration (p=0.01). None of the other radiographic parameters reached statistical significance when comparing the two groups.

Conclusion: Long fusions in adult spinal deformity, which end at L5, result in good intermediate term outcome with a relatively low revision rate. The factors which predispose to extension of the fusion to the sacrum include loss of sagittal balance and radiographic evidence of preoperative degenerative disc disease. Although it is well known that the remaining lumbosacral disc will eventually degenerate in long fusions ending at L5, radiographic evidence of L5- S1 disc degeneration at follow up did not always correlate with symptoms nor, more importantly, with a high revision rate.