The Effect of Spinal Implant Rigidity on Vertebral Bone Density A Canine Mode

An animal model of anterior and posterior cojumn instability was developed to allow in vivo observation of bone remodeling and arthrodesis following spinal instrumentation. After an initial anterior and posterior destabilizing lesion was created at the L5-L6 vertebral levels in 63 adult beagles, various spinal reconstructive surgical procedures were performed—with or without bilateral posterolateral bone grafting, with or without bilateral oophorectomies, and with or without spinal instrumentation (Harrington distraction, Luque rectangular, Cotrel- Dubousset pedicular, or Steffee pedicular implants). Observation 6 months after surgery revealed a significantly jmproved probability of achieving a spinal fusion if spinal instrumentation had been used (X2=5.84, P=.016). Nondestructive mechanical testing after removal of all metal instrumentation in torsion, axial compression, and flexion revealed that the fusions performed in conjunction with spinal instrumentation were more rigid (P<.05). Quantitative histomorphometry showed that the volumetric density of bone was significantly lower (ie, device-related osteoporosis occurred) for fused versus unfused spines. In addition, a linear correlation occurred between decreasing volumetric density of bone and increasing rigidity of the spinal implant (r=.778); ie, device- related osteoporosis occurred secondary to Harrington, Cotrel-Dubousset, and Steffee pedicular instrumentation. Oophorectomized dogs became more osteoporotic than their surgically matched controls (posterolateral bone grafting alone, Cotrel-Dubousset pedicular instrumentation, and Steffee pedicular instrumentation); device-related osteoporosis added to the degree of hormonally induced osteoporosis (t=5.0, P<.0001). This is the first study to date documenting the occurrence of stress shielding in the spine secondary to spinal instrumentation. Device-related osteoporosis occurred secondary to pedicular instrumentation and to a lesser degree with conventional Harrington and Luque instrumentation. Overall, vertebral osteoporosis secondary to spinal fixation does not appear deleterious. There was an increased chance of achieving a successful arthrodesis, and the biomechanical rigidity of the fusion was more favorable because of an increased cross-sectional area of the posterolateral fusion mass.