Degeneration of non-fused segments after floating lumbar fusion.

Degeneration of the disc or discs between two fused spinal segments has been termed "floating disc disease". The purpose of this retrospective study was to show the radiological evolution of the floating disc(s) and the relationship between floating disc degeneration and segmental lordosis, lumbar lordosis and pelvic incidence. Twenty patients, with a mean age of 49.9 years, with symptomatic lumbar degenerative disc disease or low grade spondylolisthesis, who failed non-operative treatment and underwent fusion of 2 or more noncontiguous spinal segments, were included in this study. The radiographs of the floating discs were graded with the modified Gore System. The mean follow-up was 4.2 years. Forty-seven levels were fused and 27 floating discs were studied (13 single, 7 double). Five out of 27 floating discs (18%), in 4 patients, progressively degenerated. None of the floating discs degenerated more than two radiographic grades and none needed additional surgery. Postoperatively, 3 out of 5 degenerated floating discs had decreased segmental lordosis, while the other two had no change; this difference was not significant (p = 0.08). Neither was there any significant correlation between floating disc degeneration and lumbar lordosis L1-S1 (p > 0.10) or pelvic incidence (p > 0.10). This study shows that the effect of floating fusion on floating discs is the same as the effect of a contiguous fusion on adjacent discs.