Comparison of Stress Characteristics and Tribological Optimization of Articular Surface Design for Artificial Cervical Disc with Fixed or Sliding Nucleus Pulposus

Two kinds of artificial cervical discs were designed with fixed or sliding nucleus pulposus, based on the database of physiological parameters of Chinese cervical spine. The stress and deformation characteristics of the two products were compared by the finite element analysis method. Three key parameters of ball-on-socket structure were selected including ball radius, radial clearance and hemispherical height. The orthogonal test method was used to optimize the tribological design on the articular surface, according to the maximum contact stress and the λ value (Ratio of minimum film thickness to equivalent roughness). The results showed that the stress concentration of the ball-on-socket prosthesis occurred on the articular surface and the connection location between different components. The maximum stress on the upper surface of nucleus pulposus presented a circular distribution. Compared with the prosthesis with fixed nucleus pulposus, the prosthesis with sliding nucleus pulposus showed the advantages of richer movement mode, more uniform stress transmission, while the disadvantages of increasing stress from 1.36 MPa to 2.43 MPa due to complex movement of nucleus pulposus. The total deformation of fixed type was much smaller than that of sliding type. Taking nucleus pulposus component as an example, the former was only 22.5% of the latter. However, both types showed the phenomenon of bigger total deformation on the front and back end of the prosthesis. The order of the influence weight on contact stress was hemispherical height > ball radius > radial clearance for articular surface parameters. It revealed lower stress with bigger values of hemispherical height and ball radius as well as smaller value of radial clearance. The order of the influence weight on lubrication state was radial clearance > ball radius. It revealed better lubrication state under smaller value of radial clearance and bigger value of ball radius. The best parameter combination was 16.00 mm, 10.0 μm and 1.2 mm for ball radius, radial clearance and hemispherical height, respectively. Under this combination, the maximum contact stress of the prosthesis with sliding nucleus pulposus was the smallest with a value of 2.79 MPa, and the lubrication state was the best with a λ value of 0.573.