A Novel “Smart” Spinal Implant Locking Mechanism Based on Nickel-Titanium Alloy

Summary: A new locking mechanism based on the use of a nickel-titanium alloy is described. It has shape-memory and superelastic properties. It can lock tighter than conventional locking mechanisms, and has the ability to retighten once fretting occurs. In all mechanical tests performed, the new locking mechanism is superior to four commonly used conventional ones. Recent advances in nanotechnology surface treatment makes the use of such materials as spine implants a real possibility.

Introduction: All current implant locking involves tightening of a nut against the rod and screw head to form a coupling. With spinal motion, wear between the rod and the locking mechanism (fretting) can occur. Particulate debris is generated and the coupling becomes loose. To avoid this, a new locking mechanism based on the superelastic and shape memory properties of nickel-titanium (NiTi) alloy has been developed; it has the ability to automatically re-tighten once fretting occurs. This new mechanism is tested against current locking mechanisms.

Methodology: The new locking mechanism is based on a head made from NiTi alloy, this head will tightly lock the rod when its temperature is raised to 50 deg C (shape memory effect). If fretting occurs, the head will further tighten itself around the rod (super-elastic effect). This new coupling is mechanically tested against 4 commonly used implant coupling.

Results: All 5 couplings survived dynamic axial compression between 330 and 370N. However, under axial compression, conventional couplings failed between 570 to 740N, while the new memory coupling reached 800N without loosening. Similarly in axial rotation, all conventional devices failed between 1.8 to 5.3Nm, while the new coupling reached 6.5Nm without failure. A saw-tooth pattern to the mechanical testing readout could be seen with the new coupling, and is indicative of the re-tightening effect once fretting occurs. This is not seen with conventional couplings.

Discussion and Conclusion: The ability of an implant coupling to self-tighten once fretting occurs is a new concept not previously described. This is made possible by use of the special properties of NiTi alloys. While this material is not new, recent advances in nanotechnology surface treatment described by the authors is able to completely suppress nickel release, and makes their use in spine implants a real possibility.