Biomechanics of stand-alone cages and cages in combination with posterior fixation: a literature review

Interbody cages in the lumbar spine have met with mixed success in clinical studies. This has led many investigators to supplement cages with posterio r instrumentation. The objective of this literature review is to address th e mechanics of interbody cage fixation in the lumbar spine with respect to three-dimensional stabilization and the strength of the cage-vertebra inter face. The effect of supplementary posterior fixation is reviewed. Only thre e-dimensional stabilization evaluations in human cadaveric models are inclu ded. These studies involve the application of different loads to the spine and the measurement of vertebral motion in flexion-extension, axial rotatio n, and lateral bending. There are no published studies which detected any d ifferences between different cage designs. However, it does seem that cages inserted from an anterior direction provide better stabilization to the sp ine than those inserted from a posterior direction. In general, anterior ca ges stabilize better than posterior cages in axial rotation and lateral ben ding. Cages from both directions stabilized well in flexion, but not in ext ension. Supplementary posterior fixation with pedicle or translaminar screw s substantially improves the stabilization in all directions. The strength of the cage-vertebra interface from studies using human cadaveric specimens is also reviewed. The axial compressive strength of this interface is high ly dependent upon vertebral body bone density. Other factors such as preser vation of the subchondral bony end-plate and cage design are clearly less i mportant in the compressive strength. Supplementary posterior instrumentati on does not enhance substantially the interface strength in axial compressi on.