Transmission electron microscopic visualization of the degradation and phagocytosis of a poly-L-lactide screw in cancellous bone: a long-term experimental study

The increasing clinical use of biodegradable implants in orthopedic surgery makes it necessary to determine their long-term behavior in tissues. In this study, a biodegradable screw made of poly-L-lactide (PLLA) was inserted axially into the right distal femur in 18 rabbits. The degradation and phagocytosis process of PLLA was assessed histologically and by transmission electron microscopy (TEM). The follow-up times were 3 and 4.5 years for groups of nine and eight animals, respectively. Abundant birefringent polymeric material was still present in the center of the implant channel in all specimens in both follow-up groups. The PLLA material studied appeared to be a biologically relatively inert material, with only sparse reactive cellular activity at the tissue-implant boundary. In the TEM specimens, polymeric particles of an average area of 2 microm2 were seen to be located intracellularly within phagocytic cells. The spheric and polygonal particles were membrane-bound and to a great extent filled up each phagocyte. In the 4.5-year specimens, the size of the polymeric particles, measured as area and perimeter, was significantly smaller (p < 0.02) than that of the 3-year specimens. The findings indicate that the ultimate degradation process of PLLA is much longer than it previously was thought to be. Complete degradation probably still would have taken years after the 4.5-year span of this study.