Intervertebral disc composition in neuromuscular scoliosis: changes in cell density and glycosaminoglycan concentration at the curve apex.

STUDY DESIGN: An analysis of the variation in glycosaminoglycan, water content, and cell density with disc level in patients with neuromuscular scoliosis. OBJECTIVES: To determine whether the composition of the apical disc differed from that of adjacent discs in the same spine. SUMMARY OF BACKGROUND DATA: Compositional differences between the convex and concave sides of scoliotic discs have been noted and are thought to be secondary to altered loading. However, there is little information on changes relative to the apex. METHODS: Intact wedges of disc obtained during anterior fusion procedures were taken from 23 discs of 6 patients with neuromuscular scoliosis. Radial profiles of glycosaminoglycan, water content, and cell density were measured. Concentrations were compared at a standard distance (5 mm) into the disc and plotted versus spinal level. RESULTS: Glycosaminoglycan and water content were lowest in the outer annulus and increased steadily toward the disc center, whereas the cell density was highest in the outer 2 mm, fell steeply and then remained constant. At 5 mm from the annulus edge, cell density was lowest in apical discs and, in most cases, was noticeably higher in adjacent discs of the same spine. At the same point, there was no consistent change in glycosaminoglycan/dry weight from disc to disc, indicating no significant proteoglycan loss. However, glycosaminoglycan/tissue water, and therefore swelling pressure, was highest in the apical discs, suggesting that these discs were the most heavily loaded. CONCLUSIONS: The loss of cells from the disc at the curve apex probably arose because this disc experiences greater mechanical stress or is more deformed than its neighbors. The decrease in cell density was not associated with major changes in tissue composition, possibly because rates of degradation and of synthesis were reduced, leaving the matrix largely unchanged.