Biodegradation and strength retention of poly-L-lactide screws in vivo. An experimental long-term study in sheep.

BACKGROUND AND AIMS: Poly-L-lactide implants have gained popularity in the fixation of fractures and osteotomies in the past decade. The aim of the present experimental long-term study was to examine the degradation and strength retention of self-reinforced poly-L-lactide (SR-PLLA) lag-screws and the bone tissue response. MATERIAL AND METHODS: A total of 27 young adult sheep were used. Self-reinforced poly-L-lactide (SR-PLLA) lag-screws of 6.3 mm were implanted in the left proximal femur of nine sheep. At two, three and five years three of the sheep were sacrificed and the degradation was studied radiologically, microradiographically and histologically. For the strength retention measurements five SR-PLLA lag-screws of 6.3 mm and five lag-screws of 4.5 mm were implanted in the subcutaneous tissue of the five sheep and lag-screws of 6.3 mm for the pull-out test in the left proximal femur of 20 sheep. At 0, 12, 18, 24, 32, and 36 weeks bending and shear strength, molecular weight and pull-out measurements were performed. RESULTS: At five years no SR-PLLA material could be seen. The implant area was surrounded by high density bone with bone ingrowth in the screw area. At 36 weeks the bending strength of the 6.3 mm screws had decreased from 257.9 MPa to 36.4 MPa and the shear strength from 131.8 MPa to 19.8 MPa. The pull-out strength of the lag-screws of 6.3 mm in diameter decreased from 1507 N to 331 N in 24 weeks. CONCLUSIONS: SR-PLLA lag-screws showed high initial values, a controlled strength retention and gradual degradation process making the use of them safe also in demanding fixations.