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% and poly(L/DL-lactide) 95/5% were implanted in the subcutaneous tissue of sheep. The tissue reaction to the implanted materials and their in vivo degradation was investigated at 1, 3, 6 and 12 months. The capsule formed around the polylactide implants consisted of fibroblasts, fibrocytes, phagocytes, a few foreign body giant cells and polymorphonuclear cells. For all three polylactides used, the cellular response was most intensive during the first 6 months of implantation and significantly 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 reaction 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 polymers used was completely resorbed at 1 year. The most advanced resorption was observed for poly(L/D-lactide). Despite molecular weight reduction, the poly(L-lactide) implants had maintained 70% of their initial 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 increased after implantation as compared with nonimplanted materials. The overall crystallinity increase and the final crystallinity reached by the materials at 1 year was, however, lowest for poly(L/DL-lactide) as compared with the other two polylactides.