Studies On The Effect Of Continuous Stress On The Degradation Of Poly(D, L-lactic Acid) (PDLLA) Foam-gasket And The Stress Shielding Effect Of Internal Fixation System With PDLLA

Biodegradable materials are a kind of polymer that can degrade and disappear in the organism. Currently, applications for these biomaterials cover a broad range of clinical and scientific disciplines. Undoubtedly, the most significant members of the aliphatic polyester family are the poly lactide acid (PLA). PLA has excellent biocompatibility, biodegradability, mechanical strength, and thermoplasticity. Since it degrades gradually and has few negative effects to human body, PLA has become the researching focus in clinical fields.In this dissertation, the effect of continuous stress on the degradation rate of poly(D, L-lactic acid) (PDLLA) foam-gasket, with initial molecular weight as 2.0×10~5, was investigated in detail by using a special designed stresses-providing device. The degradation behaviors of PDLLA in the PBS solution(pH, 7.4) at 37℃were evaluated with the changes of surface morphology, molecular weight, elastic modulus, ultimate stress and weight loss.The stress shielding effect of a new type internal fixation plate, compounded with Tc4 (Ti- 6Al- 4V) metal plate and the degradable PDLLA foam-gasket of different thickness, was investigated in the rabbit shankbone fracture experiment in vivo and in vitro. The thickness of added PDLLA foam-gasket separately was 1.0 mm,0.8 mm,0.6 mm,0.4 mm and 0.2 mm. And the internal fixation modelings were divided into compared group of non-PDLLA and experimental group with different thickness PDLLA.The results showed that:1. The degradation rates of PDLLA under continuous stress aider 12 weeks were obviously quicker than those of without stress. The influences of tensile stress and complex stress were greater than that of compressive stress. It was indicated that the in vivo degradation of PDLLA would not only be influenced by the local solution, but also by the surrounding stress circumstance.2. It can be indicated from experiment in vivo that the PDLLA began to degrade at 4th week after operation. While after 8 weeks, it gradually split and the degradation degree was obvious. At 12th week, the PDLLA had degraded greatly, there was only a few fragments left. The result of x-ray, body inspect and histology observing showed that both of comparison and experiment group could healed up favorably, while the growth of bone scab and the rebuild of bone skinniness in experiment group was better than that of comparison group.3. The biomechanical testing in vitro showed that at the initial stages, both of two groups had rigid internal fixation effects and large stress shielding rate. They respectively were 41.27 %,30.98%,25.04%,75.89%,54.43%,37.09%. When the PDLLA foam-gasket was entirely degraded, the stress shielding rate of experimental groups were gradually decreased, while that of compared group didn't change evidently. Bring to bear circular impact of 1.5×10~4, 5×10~4 , 1×10~5 times on the experimental group of 0.6 mm PDLLA, with the loading frequency 60 HZ and load value 30N, the results showed that the stress shielding rates were nonlinearly decreased as the loading times increasing.4. The result of finite analysis proved that models with metal plates had serious stress shielding, especially areas between the two internal screws, the application of PDLLA was capable of transferring pressure force from the bone to the plate, thus decrease stress shielding effect.