S. Bhatia et al., THE EFFECT OF SITE OF IMPLANTATION AND ANIMAL AGE ON PROPERTIES OF POLYDIOXANONE PINS, Journal of biomaterials science. Polymer ed., 6(5), 1994, pp. 435-446
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.