A prospective and quantitative animal study was performed to evaluate the production of wear particles from a spinal fixation device, and to test the hypothesis that the concentration of wear debris particles adjacent to spinal fixation hardware is correlated with the stiffness of the spinal fusion construct and local bone formation at the fusion site. An established canine segmental spinal fusion model with three interfacet fusions was used in this study. Several bone substitute materials were grafted to the area of the interfacet fusion. Internal fixation was performed on both sides of the spinous processes at each site using a stainless steel plate system in 19 dogs. After 12 weeks, spinal segments were excised, then 3-dimensional computerized tomography was used to measure bone volume and bone area of the individual fusion sites. The stiffness of each segment was tested using a servohydraulic materials testing machine. Biopsies were obtained from the soft tissues immediately around the plate system, and wear particles were collected and characterized using an electrical resistance particle analyzer, light and scanning electron microscopy (SEM) with energydispersive X-ray spectroscopy (EDX). Biopsies from para-spinal tissue from adjacent, unoperated spinal levels served as negative controls. Histologically, 24 of 57 specimens (42.1%) showed only fibrous tissue with no recognizable macrophages, inflammation, or debris. Fourteen of 57 specimens (24.6%), however, contained many particles that were composed of Fe, Cr, and Ni, corresponding to elements found in the fixation hardware. Another 19 specimens showed only occasional particles. The mean concentration of particles from the tissue around the plate system was 2.8 Ψ 10 9 per gram dry tissue weight, compared to 0.5 Ψ 10 9 particles per gram for controls (p < 0.05). Statistical analyses showed significant inverse correlation between the log particle number and stiffness (r β«Ψβ¬ Ψ0.41, p < 0.01), bone volume (r β«Ψβ¬ Ψ0.28, p < 0.05), and bone area (r β«Ψβ¬ Ψ0.34, p < 0.05) of the corresponding segments. The concentration of particles in the tissue showed a significant inverse correlation with stiffness, bone volume, and bone area of the fusion constructs.