Mechanical characteristics of an absorbable copolymer internal fixation pin

Absorbable internal fixation is gaining acceptance among foot and ankle surgeons. While absorbable pins made of poly-L-lactic acid, polyglycolic acid, or poly-p-dioxanone are generally effective as applied in the foot, their strength loss profiles and degradation characteristics may not be optimally matched to the healing process. This study investigated a novel absorbable oriented copolymer pin, with unique absorption characteristics, made of 82% poly-L-lactic acid and 18% polyglycolic acid, to determine its suitability for use in fixation in the foot. The pins were incubated in a 37°C buffer bath that simulated in vivo conditions and were mechanically tested in four-point bend and shear at time intervals up to 12 weeks. In vitro strength loss profiles demonstrated peak strength retention (flexural and shear) for about 8 weeks, with 50% of properties remaining by 12 weeks. The initial Young's modulus of the pins was approximately 7 GPa. The mathematical relationship between pin strength and pin diameter was discussed, providing the surgeon with helpful criteria for making an implant selection. The degradation time course of these pins appears to compliment the known healing dynamics of bone, making them a suitable choice for use in foot surgery.