POLY(L-LACTIDE) - A LONG-TERM DEGRADATION STUDY IN-VIVO .2. PHYSICOMECHANICAL BEHAVIOR OF IMPLANTS

Three different poly(L-lactide) rods (25 x 3 x 2 mm) were produced eit her by injection moulding or machined out of a solid as-polymerized po lylactide block and were implanted for 1-116 months into the dorsal mu scle of rats. After recovery, the polylactide specimens were carefully cleaned, dried, photographed and weighed. Bending strength and Young' s modulus of elasticity were determined. The surfaces of the broken ro ds were examined by scanning electron microscopy. Block polylactide sa mples initially looked milky. They;became friable and broke into white or brownish fragments during the implantation period, whereas total d isintegration could not be observed. Electron scanning microscopy reve aled a porous surface with crystalline elements persisting for the who le time. Mechanical stability fell from 127 +/- 3 MPa at implantation time to about half after 3 wk (61 +/- 4 MPa) and about a quarter (32 /- 4 MPa) after 6 wk. Both injection-moulded polylactides (A1 and A2) were clear and transparent initially. After implantation they graduall y became whitish, fragmented after about 64 wk and disintegrated 90 wk later into small parts and powder. Electron scanning microscopy at fi rst showed a homogeneous surface. A kind of cortex developed after abo ut 4 wk and deep cracks ran through the rod after 32 wk. Round pores o f 1.5-10 mu m diameter developed after 1 yr of implantation. Bending s trengths were 130 +/- 8 MPa (A1) and 115 +/- 14 MPa (A2); these remain ed nearly stable over about 12 wk, then declined linearly. Although a higher initial mechanical strength is desirable for use in osteosynthe tic devices, mechanical stability of amorphous injection-moulded polyl actides over the first 12 wk and total disintegration thereafter appro aches the requirements for their use as a material for osteosynthesis.