Processing and Characterization of Poly(Lactic Acid) Screw

Abstract

Usage of the conventional bioabsorbable plastic bone fixation devices have been limited, because the mechanical characteristics of them are lower. In this study, poly-L-lactide ( PLA ) which is one of the bioabsorbable plastic was molded into the screw shape and the mechanical characteristics of the PLA screw was investigated. In order to strengthen by orientation of molecular chain, a PLA screw was prepared by extruding a rectangular PLA block followed by forging. The molding conditions were varied to evaluate the formability of PLA. The mechanical properties, such as pulled out strength, shearing strength and the twist strength were measured. The cross-sectional surface of the thread was observed and the crystallinity was measured. Decrease in void in the thread for the screw extruded using lubricants were observed. This results suggested void formation in the thread was caused at higher extrusion ratio. Decrease in shear strength and increase in twist strength were observed for the screw extruded using lubricants, which is due to lower extrusion ratio. Same tendency is observed for the screws molded at higher temperature. These results suggested the lubrication and higher molding temperature have similar effects. Decrease in twist strength at molding temperature of 120°C was obtained, which was caused by embrittlement due to crystallization.