Study On The Behavior Of Biodegradation Of UHMWPE Artificial Joint

Ultra-high molecular weight polyethylene(UHMWPE) has excellent physical-chemical properties and biocompatibility. UHMWPE is a kind of ideal medical polymer that is widely used in the area of artificial joint. Accompany with the improvement of design, manufactory and operating level, the life expectancy of UHMWPE artificial joint is twenty years or more. It is estimated that 200,000 or so joint replacements are performed in 2011 in our country, and this number reaches 240,000 in 2012 and is still expected to increase. Although failure rates of artificial joint are declining, annual number of failure case of artificial joints keeps rising that attracts general attention among doctors, manufacturers and researchers.By systematizing and summarizing biodegradation mechanism of UHMWPE in this paper, a conclusion is made that changes in condensed matter structure are largely responsible for evolution of performance of UHMWPE artificial joint which is essentially the effect of relaxation of polymer chain segments. Individual and synergy mechanism of temperature, stress, water molecular in solvent for biodegradation of UHMWPE artificial joint is derived theoretically using basic theory of physical chemistry and polymer physics. A mathematical model is established and verified by in vitro accelerated biodegradation experiment.Among changes in mechanical properties of biodegraded UHMWPE, relationship of creep behavior and degradation time have good stability so that it can be used for evaluation and prediction of degradation.According to ball indentation creep curve of biodegraded UHMWPE, Burgers model which describes viscoelastic of linear polymers and constitutive equations are obtained using mathematical fitting. During biodegradation, elasticity of UHMWPE decreases and viscosity increases. Analysis of its variation and reason is made.Simulation rheological test conditions, dynamic viscoelasticity of biodegraded UHMWPE is acquired by studying Burgers model or constitutive equations. With biodegradation time rising, entanglement molecular weight goes up, number of entanglement points goes down; weight-average molecular weight rises, molecular weight distribution broadens, numbers of oligomer decreases while content of long branched chain on the contrary. Storage energy improved and loss energy reduced in single vibration cycle provide a possible theoretical explanation for the phenomenon that dislocation is a high-occurrence failure within one month after joint implant surgery and there is a high wear rate within six months. A new kind of testing method for weight average molecular weight has been brought up according to ball indentation depth.