Biomechanics of spinal fixation and fusion

Recent experiments have shown that in the sheep spine a displacement of 5.2 mm and a strain of 36% was present at the lumbosacral joint, where fusion almost never occurred when multiple, small (5 mm x 14 mm) cancellous and corticocancellous bone grafts were placed in an interlocking fashion across the decorticated lamina of the lumbar spine and sacrum. A displacement of 1.2 mm and a strain of 10% was found at the L5-L6 joint, where fusion always occurred when the same type of bone grafts were applied in the same manner (described above) over the decorticated lamina of L5 and L6. Firm fixation of the lumbosacral spine and bone graft has brought about an arthrodesis in this model, presumably because the displacement and strain had been limited to acceptable levels. Data are presented to support the concept that, because the spine is composed of bony elements separated by a disc with viscoelastic properties, the biomechanics of spinal fixation and fusion are different than the biomechanics of long bone fixation and fracture healing. These differences should be considered in the design and use of spinal fixation devices.