The effect of thermal treatment on sterility, molecular and mechanical properties of various polylactides. I. Poly(L-lactide)

Resorbable pins for the treatment of osteochondral defects produced by injection-moulding from poly(L-lactide) were subjected to the action of heat at 105-150 degrees C under a low moisture and low oxygen atmosphere for a predetermined time. Standard procedures were used to evaluate the sterility, mechanical properties, molecular weight, polydisperity and crystallinity of the heat-treated samples. All of the heat-treated samples were sterile after 2 h of exposure to heat. There was an 10% increase in the shearing strength of the samples treated thermally for up to 10 h. This was attributed to the increase in overall crystallinity and crystal dimensions in the samples which enhanced the propagation of cracks formed upon testing through areas of high ductility. There was, however, a 20-35% drop in flexural strength which might result from the decreased concentration of the amorphous regions in the heat-treated implants. A 20% increase in molecular weight of the heat-treated implants was assigned to the transesterification process between the -OH and -COOH groups present initially in the polymer and/or formed due to chain scission upon exposure of the material to heat.