Torsion test method for mechanical characterization of PLDLA 70/30 ACL interference screws

This study developed a test method for determining the mechanical properties of poly-L/D-lactide (PLDLA) 70/30 cannulated bioabsorbable anterior cruciate ligament (ACL) interference screws. We proposed a method that defines three characteristic lengths associated with the screw void space, which was designed to receive the screwdriver during insertion of the implant: (1) the clamping length Lc, (2) the screwdriver insertion length Lb, and (3) the gauge length Lg. We tested twenty completely fabricated and finished screws that had been submitted to different levels of hydrolytic degradation. In torsion tests, the variables measured were stiffness, yield torque, yield torque angle, maximum torque and maximum torque angle. These values were correlated with the fracture surface, numerical simulation and molecular weight. The results of the degradation testing, lasting from 0 to 240 days, show a clear transition in the screws' mechanical behavior from ductile to brittle. We concluded that the proposed torsion testing method is appropriate for determining the mechanical properties of cannulated polymeric screws.