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 °C saline solution produced a complete loss of bending strength at 18 months. An accelerated test at 70 °C led to a complete loss of strength after only 96 h. Degradation of the gamma-sterilized pin at 70 °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.