The effect of site of implantation and animal age on properties of polydioxanone pins.

Absorbable polymeric orthopaedic pins (Orthosorb) of 2.0 mm diameter were implanted at different sites in mature (3.5 kg, > 5 months) and immature (5 weeks old) rabbits (total 36) for 2, 4, and 5 weeks. The sites of implantation were the medullary canal of the femur, muscles of the thigh and subcutaneous tissue of the dorsum. In mature rabbits, 1.3 mm diameter pins were also implanted in the medullary canal of the femur. The shear strength of the pins harvested from the rabbits, was measured at each time period using a fixture that shears the pins into three parts symmetrically about the load axis. In both mature and immature rabbits the rate of degradation in mechanical properties was higher in the medullary canal of bone than in the muscle and in the subcutaneous tissue (p < 0.05). The strength retention was lower in immature than 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 slower degradation within the medullary canal of femur of mature rabbits as compared to the 2.0-mm pins (157.5 +/- 4.8). DSC and X-ray diffraction results of control and implanted pins showed higher initial crystallinity and a wider range of crystallite size in the 1.3-mm pins. After 5 weeks in vivo, the crystallinity increased indicating degradation within the amorphous phase. The smaller crystallites underwent recrystallization to form larger crystallites. The results indicate that site of implantation and age of recipient influence the degradation and associated effects on mechanical properties of absorbable implants. The size of the implant, though important in determining its properties, should be considered in association with its microstructure, which also plays an important role in determining strength and strength retention of absorbable polymeric systems.