Wear Behaviors of Cp-Ti and UHMWPE Artificial Lumbar Disc Designed with Various Center of Radius

The center of radius (CoR) of the lumbar spinal disc unit is usually thought to be below the center of the natural disc. The purpose of this research is to determine the appropriateness of CoR design toward wear rate, wear scars, and surface roughness of artificial lumbar disc model. The wear behaviors of a ball-on-socket (UHMWPE-on-Cp-Ti) artificial lumbar disc were studied under dynamic load (50–150 N) for 1 MC (million cycles). The wear test was done under lateral/bending, flexion/extension, and axial rotation motion for different CoR (7.5, 9, 11, and 13 mm). The wear rates and surface roughness were measured, while the wear scars were observed. The CoR affected the wear behaviors of the artificial lumbar disc. The wear rate of the Cp-Ti socket increases from $0.812\pm 0.078$ to $2.524\pm 0.119$ mm3/MC; on the other hand, the surface roughness decreases from $0.512 \pm 0.039$ to $0.329\pm$ 0.048 μm. Both changes happened along with the increase of CoR from 7.5 to 13 mm. The wear mechanisms of the Cp-Ti socket for different CoR were similar. The dominant wear mechanisms were plowing/scratching and abrasive wear. Hence, the CoR 11 mm is recommended to be applied for a lumbar disc implant.