Investigations of the properties of blockpolymerized and injection-moulded PLLA

The blockpolymerization and injection-moulding procedure are two main techniques used to produce PLLA for medical devices. Until now there has been no comparison of the well known data of blockpolymerized material and data of injection-moulded material. Test rods (2×3×25 mm) were made from block polymerized balls by the injection-moulding procedure. The rods were incubated in tris-buffer-saline for different periods (0, 2, 4, 6, and 8 weeks) at a pH of 7.4 and a temperature of 37°C. The test methods we used were the three-point-bending, the fatigue behaviour and the calculation of the decrease of molecular weight. Both materials under investigation are chemically similar but show very different mechanical properties, in particular the injection moulded material has a very decreased behaviour and a very low loss of molecular weight. Blockpolymerized PLLA rods show a very fast lost bending strength and after 6 weeks a low resistance against cyclic loads in the fatigue behaviour test. Injection moulded PLLA will be used in future for internal fixation of fractures.