Long-term tissue response to bioabsorbable poly-L-lactide and metallic screws: an experimental study.

Late, clinically manifest, adverse inflammatory reactions have sometimes occurred after the use of slowly degrading bioabsorbable poly-l-lactide (PLLA) devices in clinical series of bone fixation. In this study, long-term tissue response to bioabsorbable fracture fixation screws made of poly-l-lactide and to similar metallic screws in cancellous bone was examined and compared with intact bone. The postoperative evaluation of the rabbit femora was performed by using plain radiography, microradiography, histology, histomorphometry, and oxytetracycline labeling studies. The follow-up times were 36 and 51 months in groups of 15 and 14 rabbits, respectively. A walling-off response by formation of trabecular bone which outlined the screw profile was observed in the PLLA and metallic groups both. Connective tissue between this bone front and the implant was seen only in the PLLA group. There was no difference in the thickness of the layer between the two follow-up groups. Between the surgically handled femora and the intact control bone, there was no statistically significant difference in the amount of trabecular bone. The osteoid formation activity in the tissue-implant interface showed no differences between the groups. However, active osteoblasts were visible only in the PLLA group. The amount of birefringent PLLA material diminished between the 36-month and the 51-month follow-up groups. Within the follow-up times of this study, both the PLLA screws and the metallic screws were rather inert. Also, long-term walling-off was a typical response to both PLLA and metallic screws in cancellous bone. PLLA screws did not evoke any osteostimulatory reaction over the long-term follow-up. The findings clearly demonstrated that the overall degradation process of PLLA was very slow and accompanied by fibrous tissue formation. Macrophage activity seemed to be related to the slow degradation process of PLLA and might be associated with the formation of connective tissue replacing the original implant. The results of this study showed no significant differences between the bioabsorbable PLLA screws and the metallic screws in biocompatibility, and no signs of inflammatory foreign-body reactions occurred during the follow-up.