Tribological Characterization Of Uhmwpe Loaded With Alendronate Sodium In Ball-on-Disc Test

Ultra-high molecular weight polyethylene (UHMWPE) has been used as a major bearing materials in total joint replacement with satisfactory clinical results due to its superior wear resistance and excellent biocompatibility. However, the wear debris of UHMWPE after implantation which would result in osteolysis and aseptic loosening has become one of the main issues that determine the longevity of artificial joint replacement. In our previous studies, UHMWPE was respectively loaded with 17β-estradiol (E2) or alendronate sodium (ALN) which were anti-osteolysis drugs in clinic. Both of UHMWPE-E2 and UHMWPE-ALN possessed satisfactory physical properties. The percentage of drug released from UHMWPE-E2 and UHMWPE-ALN during 90 days was 40% and 34%, respectively. The results of cell culture indicated that the wear debris of drug-loaded UHMWPE could prevent debris-induced osteolysis. In vivo, UHMWPE serves as one part of weight-bearing surfaces in artificial joints and guarantees the joint mobility. Thus, the wear performance of drug-loaded UHMWPE plays an important role in the human comfort and longevity of artificial joints. Furthermore, the drug release of UHMWPE-ALN in vivo would probably be affected by the stress on weight bearing joints, the relative motion-induced friction, and the wear mechanism of materials as well as the lubricants existing in peri-implants. Therefore, it is necessary to investigate the effect of loaded drug on the wear performance of UHMWPE-ALN, and the influence of friction and wear of UHMWPE-ALN on drug release rate needs to be further studied.ALN was loaded into UHMWPE with different concentration 0.25,0.5 and 1.0 wt% (named UHMWPE-ALN 0.25 wt%,0.5wt%,1.0wt%, respectively), which were processed by the solvent evaporation and hot-pressing. The Fourier Transform Infrared Spectroscopy (FTIR) and thermal analysis (TA) were carried out to investigate the thermalstability of ALN. The functional groups, crystal structure, crystallinity, contact angle, mechanical properties of UHMWPE-ALN were investigated after hot-pressing. Ball-slide-on-disc wear testing was carried out using 25 vol% calf serum or deionized water as lubricants and zirconia (ZrO2) ball as friction pair. The frictional coefficient, volume of wear scar and morphology of wear surface of UHMWPE-ALN was compared with the UHMWPE. The Secondary Electron Microscopy (SEM), Back-Scattered Electron Microscopy (BSEM) and Energy Dispersive X-Ray (EDX) revealed the distribution of ALN in UHMWPE-ALN and the effect of agglomeration of ALN on wear performance of UHMWPE-ALN. The wear performance of UHMWPE-ALN 1.0 wt% after adding surfactant (F68) was preliminarily studied. The ALN release of UHMWPE-ALN 1.0 wt% under wear process was investigated in a reciprocating mode using deionized water as lubricant.The FTIR and TA results demonstrated that the hot-pressing would not alter the functional groups of UHMWPE-ALN. X-Ray Diffraction (XRD) revealed that UHMWPE-ALN and UHMWPE had similar crystal structure. Differential Scanning Calorimetry (DSC) and contact angle testing showed that the crystallinity of UHMWPE-ALN decreased and tended to be hydrophile with the increasing concentration of ALN. UHMWPE-ALN and UHMWPE possessed similar surface hardness. The results of tensile test, small punch test and nano-scratch test indicated that the mechanical properties and scratch-resistance of UHMWPE-ALN 0.25 and 0.5wt% were similar with UHMWPE whereas those of UHMWPE-ALN 1.0 wt% decreased significantly.The friction coefficient, volume of wear scar and morphology of UHMWPE-ALN 0.25 and 0.5wt% were similar with those of UHMWPE under identical lubricant and normal load. Whereas, the friction coefficient and volume of wear scar of UHMWPE-ALN 1.0 wt% increased significantly. A lot of non-oriented loose fibrils accumulated on the worn surfaces of UHMWPE-ALN 1.0 wt% while some sheets-like peelings appeared on UHMWPE. BSE-SEM revealed that the agglomeration of ALN could probably caused the different morphology and the decrease in wear resistance of UHMWPE-ALN 1.0 wt%. By adding surfactant (F68), the friction coefficient, volume of wear scar and morphology of UHMWPE-ALN 1.0 wt% showed no significant difference as compared with UHMWPE.The ALN release during wear testing results showed that the mass of ALN released into lubricant gradually increased with the time and the concentration varied between 0.79 and 1.16 mol/L prior to 200h; and the ALN release rate increased significantly and the concentration was between 3.66 to 2.33 mol/L in the followed 200h to 310h. The ALN concentration of the control group which was soaked (without wear process) under the same lubricant was undetectable. The drug release mechanism should be further studied.The present study demonstrated that UHMWPE-ALN 0.25 and 0.5 wt% possessed similar material properties and wear performance as compared with the UHMWPE. The agglomeration of ALN caused the decrease of wear resistance of UHMWPE-ALN 1.0 wt% but it could be compensated with adding the surfactant (F68). The wear condition promoted the ALN release of UHMWPE-ALN 1.0 wt%.