Study Design. A prospective study of tissue surrounding spinal instrumentation was performed using histologic and chemical analysis.
Objectives. To identify and quantify the amount of metal debris generated by titanium pedicle screw instrumentation and to evaluate the histologic response in the spinal tissues.
Summary of Background Data. Microscopic metal particles from the soft tissue surrounding joint arthroplasties have been shown to activate a macrophage response that leads to bone resorption and increased inflammation. The use of titanium spinal implants for spine surgery projects the possibility of generating wear debris in the spine.
Methods. Nine patients with titanium instrumentation from a prior lumbar decompression and fusion procedure who were undergoing reoperation were entered into this study. Tissue samples were collected from areas near the pedicle screw-rod junction, the scar tissue overlying the dura, and the pedicle screw holes. Metal levels for titanium were determined by electrothermal atomic absorption spectroscopy, and histologic analysis was performed by light and electron microscopy.
Results. Tissue concentrations of titanium were highest in patients with a pseudarthrosis (30.36 μg/g of dry tissue). Patients with a solid fusion had low concentrations of titanium (0.586 μg/g of dry tissue). Standard light microscopy identified metal particles in the soft tissues. Transmission electron microscopy demonstrated macrophages with numerous secondary lysosomes containing electron-dense bodies and collagenous stroma with electron-dense rod-like profiles consistent with metal debris.
Conclusions. Wear debris is generated by the use of titanium spinal instrumentation in patients with a pseudarthrosis. These particles activate a macrophage cellular response in the spinal tissues similar to that seen in surrounding joint prostheses. Patients with a solid spinal fusion have negligible levels of particulate matter.
Metal implants are used commonly in prosthetic joint replacements with excellent results. Long-term studies, however, demonstrate that microscopic metal particles (wear debris) are generated by these metal implants and deposited in the periprosthetic soft tissues. 1,2,8 Microscopic metal particles generated by the use of metal implants have been isolated from the periprosthetic soft tissue surrounding joint arthroplasties. These have been shown to activate a macrophage response that leads to bone resorption and increased inflammation. 1,3,5,8
The use of metal instrumentation, particularly titanium, for spinal fusion has introduced the possibility of generating microscopic metal particles that may be deposited in the paraspinal soft tissues or on the neural elements. Previous studies in vitro have shown that these metal particles can be phagocytized intracellularly, leading to the release of inflammatory cytokines. 1,3-5,8 This inflammatory cascade may lead in turn to resorption of bone and cellular death.
The potential for bone resorption is particularly distressing in spinal surgery because much of the operation's success depends on obtaining a spinal fusion. The toxic effects of these metal particles also are of great concern because the neural elements are widely exposed during spinal decompressions.
No previous studies have identified metal particles generated by the use of spinal instrumentation and deposited in the paraspinal soft tissues. The purpose of this study was to identify and quantify the amount of metal debris generated by spinal instrumentation and then to evaluate the histologic response to these microscopic particles in the soft tissues surrounding the spine.