The double torsion test for evaluation of fracture toughness of PLLA bioresorbable polymer during degradation

Poly-l-lactic acid (PLLA) is widely used in the fabrication of bioresorbable orthopaedic and bone fixation devices. These implants degrade over time within the in vivo environment increasing the risk of potential mechanical failure. Therefore it is necessary to better understand the fracture behaviour of PLLA under physiological conditions during the degradation process. The aim of this work was to estimate how its fracture properties would change once a PLLA device has been implanted in a patient’s body and commenced degradation. PLLA specimens were compression moulded and degraded at 70 °C in PBS (the elevated temperature ensured accelerated degradation). As PLLA had rarely been studied using the double torsion (DT) test, some preliminary validation tests were also conducted in order to optimise the specimen geometry to achieve consistent behaviour. Tests were conducted after 1, 3 and 7 days of accelerated in vitro degradation. It was possible to determine the fracture toughness from the DT technique for up to 3 days of degradation. Fracture toughness deteriorated dramatically between 1 and 3 days of degradation from 4.45±0.42 to 2.06±0.20 MPam1/2. Beyond 3 days the extent of degradation prevented testing by the DT technique.