Investigation of wear in cervical total disc replacement under loads and motions that represent activities of daily living

There are several articulating cervical disc implants available commercially with different mechanical properties. Although their short- and medium-term clinical outcomes have been positive, the long-term impact of this innovation is still unknown, given it has only been introduced into clinical practice relatively recently. Protocols for wear simulation in testing the device do not adequately reflect the loading and motion of the cervical spine in daily life. This two-phase research project aims to investigate the wear of the implant under the loads and motions during activities of daily living.

Ethical approval was granted to recruit research participants performing daily activities involving neck using motion capture equipment. Data were processed and simulated using musculoskeletal multibody modelling to estimate the cervical intervertebral joint angles and contact forces. These outputs were fashioned into a novel wear testing profile and fed into a mechanical joint implant simulator. Wear simulation, based on activities of daily living, was carried out for five million cycles, on top of a separate standard wear test recommended by ISO18192-1. Gravimetric wear rate and surface roughness were assessed.

Results from Phase I have shown that, on average, cervical joint excursions are less than the prescribed angles in ISO18192-1 but joint loading is higher, in general, for all activities studied. Outcomes from Phase II revealed that the wear rate based on parameters from activities of daily living is comparable to wear rate under ISO18192-1 parameters, and the difference is statistically insignificant. Nonetheless, tribological regime during activities of daily living is harsher than the standard test condition based on the wear coefficient estimation. To truly evaluate implant’s wear performance in vivo, several wear profiles based on activities of daily living must be employed in tandem with an appropriately considered duty cycle, which is estimated between three to five million cycles per year.