Long-term systemic metal distribution in patients with stainless steel spinal instrumentation: a case-control study

Case-control study.

To verify whether metal ions in the serum of patients bearing spinal stainless steel instrumentation were elevated over the long-term period after implantation of stainless steel prostheses and to determine whether these levels could predict potential unfavorable outcomes.

Instrumented spinal arthrodesis, the standard procedure to correct scoliosis, routinely remains in situ for the lifetime of the patient. Elevated metal ion levels have been reported at short-term follow-up, but the long-term status, possibly related to systemic toxic effects, is unknown.

Twenty-two patients treated for scoliosis with posterior spinal arthrodesis using stainless steel instrumentation were included. Minimum follow-up was 10 years. Oswestry Disability Index and visual analog scale were recorded. Chromium (Cr) and nickel (Ni) levels were measured (ng/mL) and compared with levels in a control group including 30 healthy subjects. A receiver-operating characteristic curve was calculated on the basis of the clinical assessment (pain and disability) and the x-ray picture; the cutoff values for the parameters were settled, and the ion-testing potential was considered as a surrogate marker for failure.

The level of Cr was significantly increased in patients, compared with controls (P=0.018). A remarkable Cr release without any clinical-radiologic sign was recorded in some female patients. A high specificity (93%), positive likelihood ratio (7.00), and overall accuracy (77%) were calculated for Cr; these indicate a high risk of failure when the levels exceeded the cutoff value, which was 0.6 ng/mL. No significant difference between the groups was found for Ni (P=0.7).

Cr testing is suggested as a reliable marker for the malfunctioning assessment and as a support for standard procedures, especially with doubtful diagnosis. Furthermore, high levels of Cr ions were observed in female patients. This finding deserves attention especially when counseling young fertile women.