| Ultra-high molecular weight polyethylene (UHMWPE) has been used extensively as one of acetabular components for more than50years due to its excellent biocompatibility and wear resistance. However, UHMWPE wear debris produced from articulating surface may cause osteolysis, which results in aseptic loosening of the prosthesis and its failure. Recently, UHMWPE loaded with alendronate sodium (ALN) was developed in our previous study. ALN as a potential drug will be released with wear debris to prevent osteolysis. The wear performance of UHMWPE-ALN0.5wt.%showed no significant difference compared with that of UHMWPE in25vol.%calf serum when the zirconia (ZrO2) ball was applied. Nevertheless, the lubricating propertis of conventional lubricants were vastly different from those of natural synovial fluid. In addition, the transformation of wear mechanisms of UHMWPE-ALN was not understood. Sodium hyaluronate (NaHA) is the major component of natural synovial fluid, which can reduce wear of the hip joint. Therefore, NaHA was incorporated in biomimetic lubricant, and the wear performance, mechanism and transformation of wear debris of UHMWPE-ALN in this lubricant were investigated in this study. Meanwhile, the wear performance of UHMWPE-ALN under variable load was also investigated.In this study, deionized water (DIW), solutions of sodium hyaluronate (NaHA,0.3wt.%), of calf serum (CS,25vol.%) and of both components (NaHA+CS,0.3wt.%+25vol.%) which was used as biomimetic lubricant. UHMWPE-ALN0.5%was prepared by hot press moulding and sliding against316L stainless steel ball by a reciprocating ball-on-plate apparatus. The wear performance of UHMWPE-ALN under different lubricants and the wear evolution with the time under the same lubricant were studied. In addition, the wear performance of UHMWPE-ALN in same contact areas was investigated in different lubricants by using316L steel ball with diameter of28mm and load with30N, and then with diameter of20mm and load with10N. The main evaluation of wear including friction coefficient, wear profile and morphology of wear surface. The viscosity, contact angle, surface tension and work of adhesion were used to characterize the influence of lubricants on wear performance of UHMWPE-ALN.The lubrication of biomimetic lubricants was better than that of conventional lubricants, which could reduce wear of UHMWPE-ALN. This study showed that the friction coefficients of CS, NaHA+CS, DIW and NaHA decreased successively, and the volumetric losses and wear factors of CS, NaHA, NaHA+CS and DIW decreased successively. The feature of wear surface of UHMWPE-ALN in NaHA+CS and NaHA was irregular ripples which perpendicular to the sliding direction, and the wear mechanism was the plastic deformation. Cracks were observed on the edge of ripples in CS, indicating that the wear mechanism was micro-fatigue wear. In addition, the wear debris and the transfer of UHMWPE film were observed on the wear surface of316L ball, which due to the higher wear factor of UHMWPE-ALN in CS. The ploughs which paralleled to sliding direction were observed on the wear surface of UHMWPE-ALN in DIW, indicating the main wear mechanism was abrasive wear. The works of adhesion of DIW, NaHA, NaHA+CS and CS decreased gradually, which calculated by contact angles and surface tension coefficient. The lower work of adhesion may cause higher friction coefficient and lower wear. The most serious wear in CS, which probably be caused by the adsorption of proteins in serum.The wear mechanism changed with time and various mechanisms existed during different wear stage. When CS was lubricant, it is better to characterize wear with different samples by increasing wear time respectively. UHMWPE-ALN was in the running-in stage of wear before the first80h, and then wear entered the steady wear stage after80h. The wear profile changed from the initial "V" type to obvious "U" type at the same time. The microscopic wear morphology in different period showed that the main wear mechanisms of the middle of wear scar were plastic deformation, fatigue wear, abrasive wear and adhesive wear successively. The first two wear mechanisms mainly appeared in the running-in stage, and the latter two wear mechanisms mainly appeared in steady wear stage. Wear debris in the middle of wear scar transformed from early lamellar and fibrous into later granulated. The main wear mechanism was fatigue wear due to the directional shear stress in the ends of the wear scar changed suddenly. Ribbon layer debris produced by the propagation of fatigue crack might be transform into separate plate-like debris after tearing. Corrugate plastic deformation and small debris were observed on UHMWPE-ALN processed by variable load. The wear morphology were corrugated plastic deformation and small ploughs by observing the wear scar, which indicated that the main wear mechanism were plastic deformation and abrasive wear. The plate-like debris appeared in NaHA+CS at the ends of wear scar, whereas ribbon layer with the edge of tear appeared in NaHA and CS. The debris were mainly plate-like and composed of small debris by bonding in DIW. The gap on the edge of wear scar in different lubricants had very different in the size and morphology of wear debris. The former was mainly because316L stainless steel ball deviated from the center slightly, and the latter has a great relationship with the wear mechanism. The main wear mechanism was plastic deformation in NaHA+CS, which showed the wear debris was long strips. The main wear mechanism was abrasive wear around the gap and the wear debris was fibrous in DIW The volume of wear of HUMWPE-ALN in CS was the biggest, and then it decreased in order with DIW, NaHA and NaHA+CS. Therefore, NaHA+CS possessed the best lubrication in the condition of variable load.This study showed that biomimetic lubricant can effectively reduce the wear of UHMWPE-ALN compared with that of conventional lubricant, and it can availably simulate the lubricating condition in vivo. The wear mechanism and the morphology of wear debris was different from the middle of wear scar to the edge, which was related to lubricant, properties of the friction pairs, friction mode and working condition. Therefore, it is more meaningful to study and evaluate the friction performance of prosthesis in vitro by using biomimetic lubricant which was similar to synovial fluid and variable load which simulated the hip joint contact force during walking. |
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