The Development Of Bioresorbale,High-Strength Poly-L-Lactide(PLLA)Osteosynthesis.--Degradation Properties,Biocopatibility,Experimental Osteotomy Fixation,and Clinical Study

Objective Poly-L-lactide (PLLA) is the main material making for biodegradable osteosynthesis. But the commonly used processes make the PLLA's initial mechanical properties too low to sufficient for the need for clinical osteosynthesis. Then it must be to develop some special process to enhance the mechanical properties. The techniques using for enhancement today are very complex and need expensive equipments. Even though, the processes are not fit for objects with large cross-section. We are developing a new technique named solid-state compression (SC) to harvest high-strength PLLA to fabricate PLLA osteosynthesis. The initial mechanical strengths are as same as those made by the enhancement techniques today. The aims of the study are to evaluate the properties of SC-PLLA and obtain some experimental materials needed for the ftirther fabrication and clinical implantation of SC-PLLA. The study items are the physical and chemical properties of SC-PLLA in vitro and in vivo degradation; the mechanical strengths retention during the degradation; the tissue reaction surrounding the implant; the fixation of experimental osteotomy, and pilot clinical study. Material and method By comprehensively using the methods of the viscosity measurement and differential thermal property measurement, mechanical tests, light microscopy(LM), electron microscopy, undecalcif7ted section and surface stain, and histomorphometric measurement, the research details were: (1) The degradation and mechanical strength retention. The in vitro study: SC-PLLA rods (3.2mmX2Omm) were immersed in a solution of phosphate buffered saline (PBS, pH 7.4 37 1 C), and removed at 4, 8,12, 24, 36, 48, and 72 weeks. The in vivo study: SC-PLLA rods (3.2mmX4Omm) were implanted in New Zealand rabbits dorsal subcutaneous tissue and in femoral intramedullary, and removed at 4, 8, 12, 24, 36, and 48 weeks. The degradation properties of SC-PLLA were studied as the tokens of the macroscopic changes, the weight loss, the mechanical strength retention (bending strength and shear strength), the viscosity-average molecular weight degradation, and the changes of the melting temperature and crystallinity. The surface and inner structure were studied by scanning electron microscopy (SEM). (2) The tissue biocompatibility of SC-PLLA rods. SC-PLLA rods (3.2mm X 20mm) were implanted in New Zealand rabbits dorsal subcutaneous tissue and SC LLA rods (3.2mmX4Omm) in femoral intramdullary, and removed at 4, 8, 12, 24, 36, 48 and 72 weeks. By using the technique of LM, transmission electron microscopy (TEM), and histomorphometric measurement, the tissue reaction was studied. (3) Osteotomies fixed by SC--PLLA screws and nails. A PLLA screw or metal screw (4.5mm) was used to fix the fenioral distal transcondylar ostertomy of rabbit. The rabbits were killed at 4, 8, 12, 36 weeks post-operation. A PLLA nail or metal nail (3.2mmX4Omm) was used to fix the femoral supracondylar osteotomy of rabbit. The rabbits were killed at 16 weeks post-operation. LM and bone histomorphometric measurement was used to value the effectiveness of the fixation. (4) The pilot clinical study of SC-PLLA screws. 4.5 mm SC-PLLA screws fixed 4 newly closed fractures (2 ankle, I hand scaphoid and...