K. Yeung, W. W. Lu, K. D. Luk, K. M. Cheung, The University of Hong Kong, Hong Kong, Hong Kong; J. C. Y. Chung, City University of Hong Kong, Hong Kong, Hong Kong
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 and 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: The results reveal that the couplings survived at dynamic axial compression between 330-370N. Conventional couplings failed between 570-740N at static compression, while the new couplings reached 800N without loosening. Similarly in axial rotation, the conventional devices failed between 1.8-5.3Nm, 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.
Discussion and conclusion: The ability of an implant coupling to self-tighten once fretting occurs is a new concept. This is made possible by use of the superelasticity, so that it is able to exert a continuous and constant force on the rod even after tightening.
Summary: 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).
