Degradation rates of oral resorbable implants (polylactates and polyglycolates): Rate modification with changes in PLA/PGA copolymer ratios

Abstract

The difference in rate of degradation between pure polymers of lactic acid (PLA), glycolic acid (PGA), and various ratios of copolymers of these 2 substances were determined. Fast-cured and slow-cured polyglycolide was compared with copolymers of glycolide/lactide intermixed in ratios of 75:25, 50:50, and 25:75, as well as pure polylactide. A total of 420 rats were implanted with 14C and [3H]-labeled polymers in bone and soft tissue. At intervals of 1, 2, 3, 5, 7, 9, and 11 mo., groups of 5 animals with the implants in bone and 5 with the implants in the abdominal wall were sacrificed. The implant area as well as tissue from the liver, spleen, kidney, lung and some muscle tissue was analyzed for radioactivity along with the urine and feces collected throughout the experiment. Half-lives of the different polymers and copolymers were calculated from the radioactivity present in the implant area for each time interval. Half-life of the polymers and copolymers decreased from 5 mo. for 100% PGA to 1 wk with 50:50 PGA:PLA copolymer and rapidly increased to 6.1 mo. for 100% PLA. Fast-cured PGA had a half-life in tissue of 0.85 mo. No difference in rate of degradation was seen in soft tissue or bone. No significant radioactivity was detected in urine, feces, or tissue samples. Control of degradation rate of the implant could apparently best be attained by varying the composition of PLA and PGA between 75% and 100% PLA along with a corresponding 25%-0% PGA. This provides a half-life range of the implant of from 2 wk to 6 mo.