On the enhancement of thermo-mechanical properties of poly(L-lactide) by solid-state extrusion for biodegradable spinal fixation devices

Abstract

Poly(L-lactide) (PLLA) has been receiving attention as a material for bone fixation devices because of its good biocompatibility and biodegradability. However, its mechanical properties are weaker than those of natural bone. Interestingly, solid-state extrusion can enhance the mechanical properties by orienting the polymer chains, and molecular weight is one of the key factors in this process. In this study, we optimized the heating conditions for vacuum compression molding to make a PLLA block for the solid-state extrusion process. We confirmed that the molecular weight of the material decreased with increasing heating temperature and time. In the case of a low molecular weight such as 120 kDa, the heating temperature and time were optimized to 190 oC and 90 min, respectively. Moreover, we fabricated a plate, a rod, and a square pole of PLLA by solid-state extrusion for the development of a bone plate, a screw, and an interbody cage. PLLA plates having various molecular weights (150-250 kDa) were extruded to various draw ratios to investigate the effect of molecular weight on the solid-state extrusion process. The mechanical properties such as flexure stress and flexure modulus increased with increasing draw ratio, which in turn increased with increasing molecular weight of the polymer. In addition, we determined that the crystallinity of PLLA dramatically increased when the draw ratio was more than 4. In addition, the orientation of the crystals by solid-state extrusion was confirmed by using X-ray diffraction, and the orientation effect increased with increasing draw ratio of the extruded PLLA. PLLA plates made by solid-state extrusion could be manufactured into bone plates. Furthermore, we fabricated a PLLA rod by solid-state extrusion and vacuum compression molding. The flexural properties of the extruded rod were better than those of the molded rod. We could, therefore, fabricate a screw using the extruded PLLA rod. Finally, interbody cages were made by using poly(L,D/L-lactide) (PLDLLA), PLLA of various molecular weights (150-250 kDa), and PLLA enhanced by solid-state extrusion. The results of the compression tests indicated that PLLA was generally stronger than PLDLLA and that the extruded PLLA had the best mechanical properties. Consequently, it is possible to successfully manufacture biodegradable spinal fixation devices, including plates, screws, and interbody cages, by solidstate extrusion. Moreover, their mechanical properties could be improved by increasing the draw ratio and molecular weight.