The components of metallosis generated by magnetically controlled growing rods and their implications

Abstract

Introduction: It has been a decade since magnetically-controlled growing rods (MCGRs) have been in use for treatment of early onset scoliosis (EOS). Unique complications like metallosis have come to our attention. This manifests as pseudo-capsule formation around the barrel opening (BO) with an accumulation of black/grey particles. The MCGR wear particles’ components and concentrations in human tissues is unclear. This study aims to systematically investigate metallosis to reveal the main metal particle profile of tissues surrounding MCGR and the phagocytic immune response. Material and Methods: All consecutive patients with EOS and MCGRs implanted undergoing rod exchange were recruited between 02/2019 to 01/2020. Two biopsies were taken of the paraspinal muscles surrounding the portion of the barrel housing the magnets (BM) and the BO due to the location of metallosis and its relationship to the MCGR’s distraction mechanism. 5mm3 sized normal tissues 3cm distal to the piston rod were controls. Each biopsy sample was used for spectrum analyses of metal concentrations and histology. Metal concentrations at wear marks and human tissues were studied. The concentrations of metals (mg/kg) in the sample were measured three times by inductively coupled plasma optical emission spectrometers (ICP-OES). For each metal element, the ICP produced excited atoms and ions that emitted electromagnetic radiation at different wavelengths to show the intensity of the emissions. Results: Biopsy samples were collected from 10 patients (average age 12 ± 1.3 years, 80% females). The majority of metals within the piston rod were common Titanium (Ti) alloy with Aluminum (Al), Vanadium (V) and Carbon (C). Six samples were tested and the average concentration (wt/wt) of Ti was 88 ± .6%; Al was 4.9 ± .2%; V was 4 ± 1.0%. In the samples taken at the BO, the concentrations of Ti (54361.34 ± 17467.98 vs control: 301.56 ± 110.39 mg/kg; P = .017) and V (1287.65 ± 407.02 vs control:17.28 ± 4.37 mg/kg; P = .016) were significantly higher than controls and from BM (Ti:24141.55 ± 10585.42mg/kg; P = .028 and V:587.27 ± 271.44 mg/kg; P = .043). For Al, no significant differences in the concentrations were discovered between samples taken at different sites of the implant. A significantly increased concentration of Nd was detected from the samples taken at the BO (34.30 ± 3.94mg/kg; P = .0003) compared to BM (17.03 ± 5.22 mg/kg; P = .024) and to controls (9.50 ± 2.52 mg/kg). Samples were mainly fibromuscular tissues with some atrophic skeletal muscle. Accumulation of black and fine particles was discovered in the dense fibrotic areas. A mild active and moderate chronic inflammatory cell infiltration was seen in the most slides near the black particles. Macrophages containing black pigmentations were observed under high magnification. No observations of local neoplasia were made. Conclusion: This is the first study systematically examining metallosis caused by MCGRs. Vastly increased Ti, V, and Nd concentration with mild to moderate chronic inflammation was found. Ti and V are generated mainly at the barrel openings due to metal-on-metal contact, whereas the Nd from the magnet rotor within the barrel is likely released from the BO during distractions. There is potential toxicity of magnetic metal particles (Nd) in adults with embolisms and liver damage. There may be a concern for child-bearing and teratogenic risks with long-term exposure to these metals.