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 ass ociated with higher loads in the adjacent segments, seven lumbar cadaver sp ines were mounted in a spine tester and loaded with pure moments of flexion /extension, left and right lateral bending, and left and right axial rotati on. 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 int ersegmental motion were measured at all levels. In the bridged region, thes e parameters were strongly affected by an internal fixator. In most cases, the effect was small in the regions above and below the fixators. Highly si gnificant differences in these regions (P < 0.01) were far below the inter- specimen range. We did not find any case where both intradiscal pressure ch anges and intersegmental motion showed highly significantly differences in the regions adjacent to the bridged one. Our results suggest that disc dege neration, which is sometimes found at the level directly above and below th e fixators, is not caused by mechanical factors.