New bioresorbable pin for the reduction of small bony fragments: design, mechanical properties and in vitro degradation.

The design, material properties, and in vivo degradation characteristics of a new resorbable pin for the reductions of small bony fragments are described. The Polypin, made of 70:30 poly (L, DL-lactide), had an initial bending strength of 155-163 MPa, as measured by a three-point bending test. Ethylene oxide (EO)- and gamma-sterilization did not substantially affect its initial mechanical properties. The initial molecular weight (Mw) of 523,000 to 600,000, however, decreased 60-75% after gamma-sterilization. Incubation of the EO-sterilized pins in 37 degrees C saline solution produced a complete loss of bending strength at 18 months. An accelerated test at 70 degrees C led to a complete loss of strength after only 96 h. Degradation of the gamma-sterilized pin at 70 degrees C was about 30% faster than that of the EO-sterilized pin. Bending strength and molecular weight were unaffected by storage at room temperature for 46 months. The relatively slow strength loss rate of the Polypin potentially extends the application of resorbable devices to slow-healing fractures. The new pin design allows application of light interfragmentary compression, thus reducing the risk of pin loosening, and an X-ray marker is provided.