Introduction: Current orthopaedic implant locking involves tightening a nut against the rod and screw head to form a coupling. With body motion, fretting (wear) at coupling interface can occur, particulate debris is generated and the fixation loosens. The debris is also likely associated with numbers of post-operative complications. To overcome these problems a new locking coupling based on the special properties of nitinol is invented. Use a spinal implant as the first example of its application. The new spinal locking coupling will tightly close and lock the rod when its temperature is raised to 50ºC (memory effect). If fretting occurs, the head will further tighten itself around the rod (super-elastic effect).

Methodology: Two versions of coupling have been developed. These new couplings are tested against 4 commonly used spinal implant locking mechanisms using Material Testing System (MTS) testing machine. The coupling is embedded to the setup with its screw perpendicular to the compression axis. The testing protocols include dynamic axial compression, static axial compression (load to failure or reach to the maximum point of 800N), and axial rotation in 20 and 40 degree.

Results: All the couplings survived dynamic axial compression between 330 and 370N. Under axial compression, conventional couplings failed between 570-740N, while the new couplings reached 800N without loosening. Similarly in axial rotation, all conventional devices failed between 1.8 to 5.3Nm, while the new couplings 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.

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1. New Spinal Implant Using Super-Elastic Nickel-Titanium Alloy For Progressive Deformity Correction: In-Vivo Demonstration In Goat Model
2. Superelastic Porous NiTi with Adjustable Porosities Synthesized by Capsule-Free Hot Isostatic Pressing