ABSTRACT
STUDY DESIGN: Biomechanical test. OBJECTIVE: To summarize the preclinical tests performed to assess the durability of a novel fusionless dynamic device for the treatment of adolescent idiopathic scoliosis (AIS). SUMMARY OF BACKGROUND DATA: The minimal invasive deformity correction (MID-C) system is a distractible posterior dynamic deformity correction device designed to reduce scoliosis for AIS patients, to maintain curve correction, and to preserve spinal motion. To overcome the challenges of wear and fatigue of this procedure, the system has two unique features: polyaxial joints at the rod-screw interface and a ceramic coating of the moving parts. METHODS: Five biomechanical tests were performed: Static compression to failure, fatigue loading per ASTM F 1717 with 5.5-mm screws for 10 million cycles (MC) at 5 Hz, wear assessment, wear test of the polyaxial joint under 100 N load for 10 MC, and wear particle implantation in rabbits. RESULTS: The system failed through buckling of the rod with loads over 3000 N (400% of human body weight). Dynamically, the system maintained 700 N for 10 MC with 5.5 mm screws. The maximum total steady-state wear rate was 0.074 mg/MC (0.03 per polyaxial joint and 0.014 mg/MC for the ratchet mechanism). Histologic evaluation of the particle injection sites indicated no difference in the local tissue response between the control and test articles. At 3 and 6 months postinjection, there were neither adverse local effects nor systemic effects observed. CONCLUSIONS: The unique design features of the MID-C system, based on polyaxial joints and ceramic coating, resulted in favorable static, fatigue, and wear resistance properties. Wear properties were superior to those published for artificial spinal discs. Long-term outcomes from clinical use will be required to correlate these bench tests to the in vivo reality of clinical use. LEVEL OF EVIDENCE: Level V.
