Abstract
The main objective of joint arthroplasty is to improve activities of daily living of the patient. However, normal daily activities may lead to separation of articular surfaces of an artificial joint, possibly as a result of a combined impact and sliding motion. Therefore, the properties of articular surfaces define the durability of implant materials. Modification of bearing surfaces with poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) increases the hydration of the surfaces and decreases the wear of the substrates. Hence, a PMPC layer can potentially cushion the impact and improve the resistance of cross-linked polyethylene (CLPE). This study aimed to explore the fatigue and wear resistance of PMPC-grafted hydrated CLPE under impact-to-wear conditions using a pin-on-disk tester.
The surfaces of a CLPE disk (3- or 6-mm thick) were modified with PMPC by photoinduced polymerization and were sterilized using gamma rays. The wear resistance of PMPC-grafted CLPE disks against a Co-Cr-Mo alloy pin was evaluated and compared to that of untreated disks. The disks were fixed to the tester with a metal plate (Ti-6Al-4V alloy) that had a central hole. The test was performed for 2 × 106 cycles of repetitive impact and unidirectional sliding with the maximum load of 150 N, sliding distance of 10 mm, and frequency of 1 Hz [Fig. 1]. Gravimetric wear was determined by weighing the disks, and soak controls were used to compensate for fluid absorption. Volumetric changes in the surfaces of the disks were evaluated using a three-dimensional non-contact optical profiler.
The average gravimetric wear (mg) after 2 × 106 cycles was 0.000/0.120 for CLPE (3/6 mm) and −0.073/–0.137 for PMPC-CLPE (3/6 mm). The weight gain of the PMPC-CLPE disks was due to their greater fluid absorption compared to that of the soak controls under the impact-to-wear conditions, as judged from the fact that during the load-soak in the lubricant this gain was observed for all the disks irrespectively of PMPC grafting. PMPC-grafting decreased the gravimetric wear of CLPE (p < 0.01) in the 6-mm group, whereas the thickness of the CLPE disks had no substantial effect on the wear resistance [Fig. 2]. In all cases, three-dimensional measurements detected a remarkable volumetric penetration in the impact-sliding surfaces and an extrusion of CLPE from the backside surfaces into the hole in the metal plate. Both the volumetric penetration and backside extrusion were smaller in the 6-mm group. The PMPC grafting had no discernible effect on these volumetric changes [Fig. 3]. Even after 2 × 106 cycles of impact loads, mechanical fracture or delamination of the impact-sliding or backside surfaces were hardly observed in all the groups.
The results of this study revealed that: (1) PMPC-grafting of CLPE surfaces decreased the gravimetric wear irrespectively of the disk thickness; and (2) thinner CLPE increased the risk of volumetric changes, including penetration in the impact-sliding surface and extrusion of the backside surface. In conclusion, PMPC grafting can potentially improve the wear resistance of the bearing surface of biomaterials even under impact-to-wear conditions, increasing the longevity of artificial joints.
