An evaluation of poly-D,L-lactide bio-absorbable intramedullary plugs

Poly-D,L-lactide has been evaluated as a material for the manufacture of intramedullary plugs to be used in total hip arthoplasty. Raw materials with weight-average molecular weights between 207000 and 105000 were used to compression mould plugs and plaques for the production of standard test specimens, and the effects of drying, compression moulding and sterilisation by gamma irradiation on molecular weight were determined. The cumulative effect of the manufacturing and sterilising processes is a reduction in molecular weight of between a third and a half depending upon irradiation dose. The mechanical strength of the plugs was assessed by a specially developed test designed to produce stresses similar to those experienced in insertion and the failure load was found to depend on molecular weight. In vitro hydrolysis was carried out by immersion in phosphate-buffered saline at 37 degrees C and rates of degradation also showed a dependence upon molecular weight; this is considered to arise from the easier diffusion of small molecules into and out of lower-molecular-weight matrices. Although material taken from the centre of plaque mouldings exposed to saline buffer shows evidence of the development of a bimodal molecular weight distribution, indicating some trapping of oligomers, material sampled from different parts of the plugs showed more uniform degradation. The long-term stability of the products was assessed by storing plugs in standard packaging under conditions of controlled humidity and temperature, and monitoring mechanical strength and molecular weight over a period of 3 years. Storage temperature was found to be more important than humidity. (C) 1998 Published by Elsevier Science Limited. All rights reserved.