LONG-TERM SOFT-TISSUE REACTION TO VARIOUS POLYLACTIDES AND THEIR IN-VIVO DEGRADATION

Cylindrical pins made from poly(L-lactide), poly(L/D-lactide) 95/5% an d poly(L/DL-lactide) 95/5% were implanted in the subcutaneous tissue o f sheep. The tissue reaction to the implanted materials and their in v ivo degradation was investigated at 1, 3, 6 and 12 months. The capsule formed around the polylactide implants consisted of fibroblasts, fibr ocytes, phagocytes, a few foreign body giant cells and polymorphonucle ar cells. For all three polylactides used, the cellular response was m ost intensive during the first 6 months of implantation and significan tly subsided at 1 year. The thickness of the capsule was 200 mu m at 1 month, increased to 200-600 mu m at 6 months, and decreased to 100 to 200 mu m at 1 year, depending on the material used. The tissue reacti on was more intense for poly(L/D-lactide) than for poly(L/DL-lactide) and poly(L-lactide). The drop in molecular weight of the implants was highest after 1 month of implantation (70 to 95%). Irrespective of the extensive reduction of the molecular weight at 1 month, none of the p olymers used was completely resorbed at 1 year. The most advanced reso rption was observed for poly(L/D-lactide). Despite molecular weight re duction, the poly(L-lactide) implants had maintained 70% of their init ial bending strength and 95% of their shear strength at 3 months. The poly(L/D-lactide) and poly(L/DL-lactide) had maintained only 26 to 27% of their initial bending strength and 26 to 31% of the initial shear strength, respectively. The crystallinity of all the materials increas ed after implantation as compared with nonimplanted materials. The ove rall crystallinity increase and the final crystallinity reached by the materials at 1 year was, however, lowest for poly(L/DL-lactide) as co mpared with the other two polylactides.