THE RELATIVE IMPORTANCE OF VERTEBRAL BONE-DENSITY AND DISC DEGENERATION IN SPINAL FLEXIBILITY AND INTERBODY IMPLANT PERFORMANCE - AN IN-VITRO STUDY

Study Design. An in vitro biomechanical investigation in the human lum bar spine focuses on the functional significance of vertebral bone den sity and intervertebral disc degeneration. Objective. To determine the interrelationship between vertebral bone density and intervertebral d isc degeneration, their effect on normal spine motion and their signif icance in biomechanical performance of interbody fixation techniques. Summary of Background Data. A relationship between vertebral bone dens ity and intervertebral disc degeneration has been suggested, but a def initive relationship has not been established. The effect of vertebral bone density and intervertebral disc degeneration on interbody stabil ization remains unknown despite the rapidly increasing use of this sur gical method for patients with chronic low back pain. Methods. The ver tebral bone density and intervertebral disc degeneration of 72 functio nal spinal units were determined using dual energy x-ray absorptiometr y scans and macroscopic grading, respectively. A three-dimensional fle xibility test was performed on 24 functional spinal units in the intac t and stabilized conditions. The compressive behavior of the bone-impl ant interface was evaluated in 48 functional spinal units. Results. Th e vertebral bone density in moderately degenerated discs was significa ntly lower than at all other levels of intervertebral disc degeneratio n. Increasing intervertebral disc degeneration resulted in more axial rotation and less lateral bending. In flexion-extension and lateral be nding, better vertebral bone resulted in significantly better stabiliz ation. This trend was observed also in axial compression, in which hig her failure loads were observed with greater bone densities. Conclusio n. The authors conclude a significant relationship exists between bone density and disc degeneration, bone density is a highly important fac tor in the performance of interbody stabilization, and disc degenerati on is of moderate importance in spinal motion.