STRENGTH RETENTION OF SELF-REINFORCED POLY-L-LACTIDE SCREWS - A COMPARISON OF COMPRESSION MOLDED AND MACHINE CUT SCREWS

The effect of the manufacturing method on the strength retention of se lf-reinforced poly-L-lactide (SR-PLLA) screws was studied in vitro and in vivo from 3 up to at least 15 weeks. SR-PLLA screws were manufactu red from axially oriented SR-PLLA billets by the conventional compress ion moulding process and an in-house developed machine cutting techniq ue. New machined SR-PLLA screws (thread diameters 4.5 mm and 3.5 mm) w ere significantly stronger than older compression moulded SR-PLLA scre ws (4.5 mm and 3.5 mm) in bending and torque strength tests but signif icantly weaker in shear strength tests. In pull out tests there were n ot significant strength differences between the screws. Mechanical ana lysis and molecular weight measurements confirmed earlier observations that SR-PLLA degrades faster in vivo than in vitro. These results sug gest that the new screws could be suitable for clinical use.