Insight on the in vivo wear characteristics of goat artificial cervical disc implanted for 6 months

Since the prosthesis may suffer overload or extreme motion during the daily activities, some special failure modes may be found in service. In order to give an insight on the in vivo stability of artificial cervical disc, the wear characteristics of goat prosthesis were studied after implanted in goat animal for 6 months. The prosthesis was designed with a ball-on-socket structure under the material combination of PE-on-TC4. The X-ray examination was performed to monitor the in vivo wear process. The worn morphology and wear debris were analyzed in detail by EDX and SEM. The result indicated that goat prosthesis revealed good safety and effectiveness during 6-month in vivo wear test. The wear damage occurred only on nucleus pulposus component with the dominant failure mode of surface fatigue and deformation. The damage distribution and wear severity was seriously uneven with a trend that the closer to the edge, the more severe the wear. For example, slippage phenomenon caused a wide and curved severe ploughing damage on the edge. Three kinds of debris were found including bone debris, carbon-oxygen compound debris and PE wear debris. Both bone debris and carbon-oxygen compound debris came from superior endplate while PE wear debris came from nucleus pulposus. The debris proportion for endplate was 82% for bone debris, 15% for carbon-oxygen compound debris and 3% for PE debris while for nucleus pulposus it was 8% for carbon-oxygen compound debris and 92% for PE debris. The size range of PE debris for nucleus pulposus was 0.1-100 μm, with an average size of 9.58 ± 16.34 μm. For the bone debris of endplate components, the size range was 0.1-600 μm, with an average size of 49.18 ± 94.54 μm. After wear test, the equivalent elastic modulus of nucleus pulposus increased from 28.55 MPa to 38.25 MPa. The results of FT-IR spectrum showed that the functional groups on the surface of polyethylene have not changed significantly after wear test. The results indicated that there were some differences in wear characteristics of wear morphology and wear debris between in vivo wear and in vitro wear.