The Load Bearing Capacity of Vertebral Cancellous Bone in Interbody Fusion of the Lumbar Spine

Abstract

Interbody fusion involves the grafting of bone between the vertebral bodies themselves. The bone graft is considered to be load bearing post-operatively, and it is necessary to know how large the transverse sectional area of the graft should be in order to sustain the loads on the spine, post-operatively, before bony union is attained. The cancellous bone of the vertebral bodies has previously been shown to be the weakest element of the grafted spinal segment and a series of compression tests on cadaveric material was undertaken in order to determine the size of graft required to distribute the compressive loads so as not to exceed the failure strength of the vertebral bone. The results suggest that at least 0.4 of the transverse sectional area of the vertebral bodies should be covered by the graft for the fused segment to sustain axial loading post-operation. Local trabecular fracture due to incongruency of the graft/vertebra surfaces and slight movements at the grafted joint is proposed as a mechanical reason for delayed union.