Technical note: biomechanical analysis of two absorbable fracture fixation pins after long-term canine implantation

The design requirements for bioabsorbable fracture fixation devices for specific applications are as yet unknown. Therefore, a range of initial mechanical properties and degradation kinetics may provide developers with additional choices for the design of absorbable fracture fixation devices. This study evaluated the changes in push-out strength, polymer mechanical properties, and bone mechanical properties of self-reinforced poly(glycolide) (SR-PGA) and poly(ortho ester) (POE) fracture fixation pins implanted into the canine femoral canal for 18 months. Mechanical testing indicated that SR-PGA pins had degraded to a pasty consistency by 3 months, showing complete loss of all mechanical properties. Meanwhile, POE pins showed a simultaneous linear decrease in both compressive strength and stiffness to almost zero by the end of the study period, suggesting that these devices were undergoing surface erosion. However, changes in specimen diameter, which would support this mechanism, were not apparent. The decrease in polymer density after 12 months suggests that there was an increase in bulk erosion for POE devices. This was further supported by the observation of internal polymer resorption noticed in specimen cross-sections after 18 months. This observation appears to be related to the method of polymer processing; hot-compression molding of fine powdered polymer. The appearance of grain boundaries would provide a path for water to penetrate into the bulk polymer and cause autocatalysis in the interior of the implant.