Biodegradation and tissue-reaction in a long-term implantation study of poly(L-lactide)

Three poly(L-lactides) with different molecular weights were synthesized as solid blocks from the melt. Two of those were ground and samples were produced by injection moulding. Specimens, cubes (2×3×3 mm) and rods (2×3×25 mm), were machined out of the samples, yielding the amorphous parts. Specimens of the crystalline parts were directly machined out of the third block. All were implanted into the paravertebral muscle of 70 rats to explore the biodegradation of poly(L-lactide) in vivo and the tissue changes at the implantation site. The rats were sacrificed after 1 to 116 weeks and the implants recovered. Histological sections of the cubes including the surrounding tissue were prepared by the cutting-grinding technique according to Donath. The three different materials were incorporated well, forming a collagenous layer. The crystalline poly(L-lactide) (M _vis 429 000) remained almost stable in form and structure over the whole observation period. No signs of inflammation or foreign-body reaction were observed. The amorphous poly(L-lactide) of higher molecular weight (M _vis 203 000) degraded nearly completely, whereas the amorphous poly(L-lactide) of lower molecular weight (M _vis 120 000) was totally resorbed. After about 8 weeks both injection-moulded materials degraded progressively, subsequently accompanied by a mild to moderate foreign-body reaction. The degradation in the inner part of the implants proceeded faster than in the cortex. The final biodegradation appeared accompanied by a resorptive histiocytic inflammation. The degradation rate and velocity of the amorphous poly(L-lactides) did not overtax the absorption capacity of the surrounding tissue. These properties of biodegradation seem to meet the requirements for a biodegradable material in osteosynthesis.