Soft layer bearing joints for spine arthroplasty

This paper investigates the application of a soft layer in one of the bearing surfaces of cervical and lumbar total disc arthroplasty devices. The disc arthroplasty was analytically modelled as a ball and socket joint, with the ball made from a metal and the socket made from an elastomer. The peak contact pressure and the minimum film thickness between the bearing surfaces were calculated, as the parameters of ball radius, radial clearance, angular velocity, soft layer Young’s modulus and thickness were varied. For the cervical and lumbar models the peak contact pressures were in the range 0.6–3.7 and 2.2–13.3 MPa, respectively. Values of minimum film thickness in the ranges 0.21–0.57 and 0.11–0.31 μm were determined for the cervical and lumbar models, respectively. The results indicate that lower contact pressures may be achieved using a soft layer, compared to conventional disc arthroplasty devices that have hard against hard bearing surfaces. The cervical disc may be able to operate with fluid film lubrication under some conditions, while the lumbar disc will have a mixed lubrication regime. The results of this study will help with design parameters for the design of future disc arthroplasty devices.