The effect of cobalt chrome and stainless steel particles generated by Total Disc Replacements on the viability of cells of the CNS

摘要

Introduction: Degeneration of the intervertebral disc causes significant pain to patients, surgical interventions include spinal fusion or total disc replacement (TDR). Fusion procedures reduce spinal mobility and are commonly associated with adjacent level effects (hastened degeneration of the functional spinal unit neighbouring the immobilized region). Devices aiming to preserve the spinal motion, in particular metal on metal TDR's have been introduced clinically. There is a growing concern regarding wear of these devices and the adverse biological effects this will have on the spinal cord. There is evidence that nanoscale metallic debris may breach the meninges, the protective barrier around the cord, and could lead to pseudotumor formation, potentially impinging upon and altering the functionality of the cord. This study aims to understand the toxicity of cobalt chrome and stainless steel wear debris and ions to cells of the CNS. Materials and Methods: A six-station pin on plate wear rig was used to generate clinically relevant cobalt chrome (CoCr) and stainless steel wear debris which was similar in size and morphology to that generated by Metal on Metal Cervical Total Disc Replacements.To determine the effect of these metal particles on the viability of CNS cells a 3D collagen hydrogel seeded with primary astrocytes and microglia in co-culture (isolated from the cortex of P2 rat pups) and astrocytes in isolation (microglia shaken off) were used. The cells were dosed with increasing particle volumes; 0.5-50μm~3 debris per cell for 28 hours and 5 days. The particle doses were selected based on MOM TDR wear rates and scaled down accordingly. The effect on viability was determined using a Live Dead assay. A comet assay was used to determine the effect of metallic wear particles on the integrity of primary astrocyte and microglia DNA after 48 hours and 5 days in culture. Results and Discussion: A significant reduction in viability was observed after 48 hours when primary astrocytes were cultured in 3D with the highest particle dose of CoCr (50μm~3 debris per cell). After 5 days in culture a significant reduction in viability was observed at the highest particle dose and the mid dose of 5μm~3 CoCr debris per cell (Figure 1). Similar results were seen when astrocytes and microglia were cultured with CoCr debris.vSignificant effects on viability were not observed when primary astrocytes were cultured with stainless steel wear particles, at any volume dose. A significant reduction in viability was seen when primary astrocytes in isolation were cultured with cobalt chrome ions from the 50μm3 and 5μm3 particle doses for 48 hours and 5 days. No significant effects on viability were observed when primary astrocytes with microglia were cultured with cobalt chrome ions or on any cells with stainless steel ions. Interestingly similar levels of DNA damage were seen between the two biomaterials when primary astrocytes and microglia in co-culture and in isolation were cultured with metallic wear debris. Conclusion: The two different biomaterials have differing effects in the viability of primary astrocytes and microglia in co-culture and astrocytes in isolation. As these materials are introduced into relatively young patients and the wear debris has potential to build up and disseminate throughout the body over long periods of time additional research is necessary to further understand the response of the body to metal particles.