Nickel Ion Level in Scoliotic Patients Implanted with Nitrogen Plasma Surface Modified Nickel-titanium Superelastic Spinal Implant: Preliminary Clinical Trial Results

Nickel-titanium (NiTi) shape memory alloys have been used as surgical implants in orthopaedic procedures. However, nickel ion release remains a concern. Therefore, an advanced surface technology using plasma immersion ion implantation (PIII) has been developed to address this issue. This paper describes serum nickel ion level monitoring of patients with scoliosis operated upon using nitrogen plasma implanted NiTi spinal rods compared with standard titanium alloy rods in a randomized human clinical trial. NiTi spinal rods with 50.8% Ni were treated by nitrogen PIII at 40kV with 100Hz. Seventeen patients with spinal scoliosis were surgically treated by either Ti alloy rods or nitrogen plasma implanted NiTi rods. The mean age at operation was 16.6 years. The mean number of treated spinal segments was 9. Blood samples were collected as baseline before surgery and up to 1 year post-surgery. Nickel levels were assayed by inductively coupled plasma mass spectrometry. Twelve patients (7M and 5F, 6=NiTi and 6=Ti) were followed up for 6 months, and five of them (2M and 3F, 2=NiTi and 3=Ti) had completed 1-year follow-up. In all cases, at Day 1 after surgery, patients who had NiTi rods implanted had a 2.5 times increase in the Ni level, compared with before surgery. However, all were within recommended safe limits. The Ni levels returned to baseline in all subjects between 7 days to 1 month postsurgery. No clinical signs and symptoms of Ni allergy or toxicity was observed.