MECHANICAL AND CHEMICAL-STABILITY OF BIOD EGRADABLE BLOCK-POLYMERIZEDAND INJECTION-MOLDED POLY-L-LACTIC ACID IN-VITRO

Biodegradable poly-L-lactic acid rods made of block-polymerized materi al (BP; molecular weight 550000) and injection-moulded rods with high (SGI; molecular weight 121 000) and low (SGA, molecular weight 118 000 ) molecular orientation were compared 2, 4 and 6 weeks after incubatio n in enzyme solutions with high hydrolytic activity (esterase, alpha c hymotrypsin and peptidase) and in buffer solution (TRIS buffer). The m olecular weight, modulus of elasticity, bending strength (three-point bending test), and cyclic bending load to failure applied to the rods in a newly developed testing machine (1 Hz, maximum 100000 cycles) wer e compared. The molecular weight of BP material decreased to 36% after 2 weeks, in contrast to the injection-moulded materials, in which it decreased only to 66% even after 6 weeks. The bending strength of all specimen decreased significantly faster in alpha chymotrypsin than in the other media (MANOVA, P<0.001). SGI had a significantly higher bend ing strength than SGA, and SGA a higher strength than BP. There was no difference after incubation in the other two enzymatic solutions. BP lost 80% of its initial bending strength (140 N/mm2) after 6 weeks, an d SGI and SGA (120 N/mm2) only 20%. Under permanent cyclic loading BP initially resisted 100 000 cycles with an applied cyclic load of 12.5 N/mm2, decreasing after 6 weeks to only 9 500 cycles, in contrast to S GI and SGA, which resisted to 46 000 cycles. There was, however, no re levant difference in the mechanical characteristics of the two injecti on-moulded rods. These results confirm that BP is degraded significant ly faster than SGA and SGI. Clinical relevance: The better mechanical stability of the injection-moulded poly-L-lactic acid will probably al low the use of small implants in future than are already available.