Effect of an internal fixator and a bone graft on intersegmental spinal motion and intradiscal pressure in the adjacent regions.

Stabilizing a lumbar spine with an implant alters the mechanical properties of the bridged region. In order to determine whether this procedure is associated with higher loads in the adjacent segments, seven lumbar cadaver spines were mounted in a spine tester and loaded with pure moments of flexion/extension, left and right lateral bending, and left and right axial rotation. The material studied comprised intact lumbar spines, intact spines with bisegmental internal spinal fixators, and postcorpectomy spines both with a graft and fixators and with fixators alone. Intradiscal pressures and intersegmental motion were measured at all levels. In the bridged region, these parameters were strongly affected by an internal fixator. In most cases, the effect was small in the regions above and below the fixators. Highly significant differences in these regions (P<0.01) were far below the interspecimen range. We did not find any case where both intradiscal pressure changes and intersegmental motion showed highly significantly differences in the regions adjacent to the bridged one. Our results suggest that disc degeneration, which is sometimes found at the level directly above and below the fixators, is not caused by mechanical factors.