Tumorigenicity of poly-L-lactide (PLLA) plates compared with medical-grade polyethylene.

Poly-L-lactide (PLLA) plates, 20 x 10 x 1 mm in size, with flat surfaces, were implanted into 50 young male Wistar rats subcutaneously, and the incidence of tumors for 2 years under specific pathogen-free (SPF) conditions was evaluated. Tumors arose in 22 rats (at the implant sites in 20, and distant from the implantation site in 2). As a control, plates of medical-grade polyethylene of the same shape were implanted into 50 rats. Tumors appeared in 23 of these rats (21 at the sites of the plates, and 2 ectopically). In 30 rats given sham operations, no tumors appeared in the 2 year observation period. Histologically, all the tumors were mesenchymal malignant tumors resembling fibrosarcoma or malignant fibrous histocytoma (MFH) in humans. In 6 of the PLLA tumors and 3 of the PE tumors, bone formation was observed. There was no significant difference between the incidence of tumors caused by PLLA plates and that of tumors caused by medical-grade polyethylene plates. Macroscopically, PLLA plates retained their initial shape for 2 years. Scanning electron microscopy revealed small holes on the surfaces of the plates, and their molecular weight was reduced to 8% of the initial value.