SERUM AND URINARY LEVELS OF COMPONENT METAL IONS IN PATIENTS WITH SPINAL IMPLANTS

Abstract

Introduction Patients who have undergone total joint arthroplasty have demonstrated elevated postoperative levels of the component metal ions (cobalt, chrome, titanium) in blood and urine. The metal ion release is due to wear particles produced by fretting and corrosion of the metal surfaces and interfaces. Postoperative levels may be many-fold greater than preoperative and normal population levels. The postoperative levels depend on a number of factors including component metals, implant design, and fixation.

Fretting corrosion of spinal implants has been previously demonstrated. Elevated metal ion levels in tissue fluids might be expected, however there are significant differences in stress on the spinal implant when compared with a mobile joint bearing. The aim of this study is to determine whether component metal ions can be measured in the blood and urine of patients who have previously undergone surgery with spinal instrumentation.

Methods Patients who had undergone surgery which included spinal instrumentation were recruited to the study. All had stainless steel implants. These cohorts were subdivided into two groups: those who had retained implants and those who had spinal implants which had been removed. A cohort of volunteers who had no metal implants served as controls. All subjects provide blood for serum nickel and serum chromium assays and (random) urine for measurement of chromium and creatinine. Serum chromium and nickel and urinary chromium/creatinine ratio were compared between groups. Non-parametric statistical analysis was used to test differences (MWU) and correlations (Spearman). A p-value less than 0.05 was considered significant.

Results The study group consisted of 44 patients who had undergone surgery 5–25 years previously. The implants had been removed in 12 patients. The mean number of spinal levels at operation was 10 (range 3–18) and the mean number of couplings was 10 (range 2–18). The average age at follow-up was 36 years (range18–75).

There was no difference in levels of serum nickel and blood chromium between controls and study cohorts. A significant difference between the controls and study groups was noted with the mean urinary chromium/ creatinine ratio being in the order of 100 times greater in the implant group. The mean level in the implant-retained group was1.6 times greater than in patients in whom the implants had been removed. The random urinary chromium/creatinine ratios correlated with the length of the instrumentation and the number of couplings in the spinal instrumentation.

Conclusions A raised level of chromium excretion in patients with spinal implants suggests that metal ions are released from the implants by electrolytic and fretting-corrosion. Removal of the implant reduces the excretion levels but not to base-line levels. The blood levels of both metal ions tested were no different from controls suggesting that the kidney was able to adequately handle the release of chromium from the implant without a build-up of chromium in the blood.