Mechanical testing of a smart spinal implant locking mechanism based on nickel-titanium alloy

Abstract

STUDY DESIGN. Development and testing of a new spinal implant-locking mechanism based on the special properties of nickel-titanium alloy. OBJECTIVE. To develop a new self-tightening locking mechanism to reduce fretting corrosion at implant junctions. SUMMARY OF BACKGROUND DATA. All current implant locking involves tightening of a nut against the rod and screw head to form a coupling. Particulate debris is generated, and the coupling becomes loose because of wear between the rod and locking mechanism (fretting). To avoid this fretting, a new locking mechanism with an automatic retightening effect based on the superelastic and shape-memory properties of nickel-titanium alloy has been developed. METHOD. The new coupling made of nickel-titanium alloy will tightly lock the rod when temperature increases to 50°C (shape-memory effect). If fretting occurs, the coupling will further tighten itself around the rod (superelastic effect). This new coupling is mechanically tested against 4 current implant couplings. RESULTS. In axial compression, conventional couplings failed between 570 and 740 N, while the new coupling reached 800 N without loosening. In axial rotation, conventional devices failed between 1.8 and 5.3 Nm, while the new coupling reached 6.5 Nm without failure. During testing, the retightening effect could be seen on the force versus displacement plot. CONCLUSIONS. To our knowledge, the self-tightening coupling is a new concept not previously described and is attributable to the superior superelastic effect of the new coupling. This implant coupling has the potential to be used as a very low profile system and also in nonfusion technologies in which demands on the coupling would not higher without the protection of spinal fusion. ©2006, Lippincott Williams & Wilkins, Inc.