THE EFFECT OF SITE OF IMPLANTATION AND ANIMAL AGE ON PROPERTIES OF POLYDIOXANONE PINS

Absorbable polymeric orthopaedic pins (Orthosorb) of 2.0 mm diameter w ere implanted at different sites in mature (3.5 kg, >5 months) and imm ature (5 weeks old) rabbits (total 36) for 2, 4, and 5 weeks. The site s of implantation were the medullary canal of the femur, muscles of th e thigh and subcutaneous tissue of the dorsum. In mature rabbits, 1.3 mm diameter pins were also implanted in the medullary canal of the fem ur. The shear strength of the pins harvested from the rabbits, was mea sured at each time period using a fixture that shears the pins into th ree parts symmetrically about the load axis. In both mature and immatu re rabbits the rate of degradation in mechanical properties was higher in the medullary canal of bone than in the muscle and in the subcutan eous tissue (p < 0.05). The strength retention was lower in immature t han in mature rabbits after 4 and 5 weeks. The 1.3-mm pins had higher initial strength (174.7 +/- 7 MPa), higher strength retention and slow er degradation within the medullary canal of femur of mature rabbits a s compared to the 2.0-mm pins (157.5 +/- 4.8). DSC and X-ray diffracti on results of control and implanted pins showed higher initial crystal linity and a wider range of crystallite size in the 1.3-mm pins. After 5 weeks in vivo, the crystallinity increased indicating degradation w ithin the amorphous phase. The smaller crystallites underwent recrysta llization to form larger crystallites. The results indicate that site of implantation and age of recipient influence the degradation and ass ociated effects on mechanical properties of absorbable implants. The s ize of the implant, though important in determining its properties, sh ould be considered in association with its microstructure, which also plays an important role in determining strength and strength retention of absorbable polymeric systems.