The effect of instrumentation on human spinal fusion mass

This study is a prospective histomorphometric evaluation of human spinal fusion bone using video dimensional analysis. Little information exists regarding the biology of fusion mass (FM) and the effect that instrumentation has on FM quality. Concerns regarding potential for "stress shielding" of FM with rigid implants have arisen. The goal of this study was to determine what effect spinal implants have on the quality and metabolism of FM bone. Fifty-six patients underwent surgeries to remove spinal implants or extend a fusion after pulse-dose labeling with fluorochrome. Twelve patients had undergone fusions without instrumentation. Duplicate biopsies of FM and iliac crest (IC) were obtained and evaluated blindly for mineralized volume, trabecular thickness, mineralization rate, and bone formation rate. Iliac crest, instrumented FM, and noninstrumented FM were compared. Instrumental FM had superior material properties relative to noninstrumented FM or IC. No significant difference in metabolic activity was present. Instrumentation does not lead to FM "stress-shielding."