摘要：目前可降解式固定器所面临最大的难题在于强度的大小及降解时间的维持。为了评估可降解式固定器的降解速率，可建立一套体外强度衰减测试模式，以评估材料的机械强度。然而文献报道的测试流程及方法并不一致。本研究参考国外文献的报道及ASTM相关标准，以二种不同材料制成的可降解骨螺钉为对象，建立一套可降解固定器的体外强度衰减测试模式。使用恒温水槽，提供回旋摇晃运动，使磷酸盐缓冲液(PBS)与骨螺钉表面产生相对流动，pH值维持在7.4±0.2，温度控制在37±2℃，在设定的时间点职出骨螺钉进行强度测试。采用剪力及三点弯曲测试，每次测试五根骨螺钉，以得到强度衰减与时间的关系。结果表明，PLLA骨螺钉在第6～7周时强度产生急速衰减，PLGA骨螺钉在第4周强度已几乎消失。通过本研究建立的体外强度衰减测试模式，可得到可降解骨螺钉机械强度随时间衰减的曲线，此套测试模式的建立可作为研发可降解固定器的体外测试评估及参考依据。

THE MECHANICAL tESTING MODEL OF A BIODEGRADABLE SCREW IN VITRO DEGRADATION

Hung wen Wei ,Ruey yug tsay ,Chengkung Cheng(Institute of Biomedical Engineering, National Yang Ming University, Taipei, taiwan)

Abstract：The strength and decay rate of biodegradable fixation device can be monitored and improved by means of in vitro degradation tests. However, from literature review, it is found that the testing procedure and controlled degradation environment applied in various vitro tests are not the same. Therefore, the purpose of this study is to establish an in vitro degradation sysytem by monitoring the degradation system of mechanical strength of a biodegradable bone screw. Conditions set for in vitro degradation and mechanical tests refer to aSTM F1635 and relevant papers. The sample screws, offered by Industrial technology Research Institute, are made of two different materials, ie, Poly-L-lactic(PLLA) acid and the copolymer of Poly-L-lactide and Poly-glycolide(PLGA).They were put in several containers and immersed in isotonic PBS(pH=7.4±0.2, 0.15M NaC1). The temperature of buffer solution was maintained at 37±2℃ by a shake bath. Both of the shear and three points bending tests were carried out to monitor the decay of the mechanical strength of the screws. The resulting mechanical strength reported here in is an average of 5 samples. The present results indicate that the strength of PLLA screws decreased significantly among week 6 to week 7 and PLGA screws lost almost all its strenght in about 4 week. The in vitro degradation system developed here in could be applied to help evaluation the performance of a biodegradable screw.

Keywords：Biodegradable screw, in vitro, Mechanical Test

作者简介：蔡瑞莹通讯作者参考文献：

［1］“Standsrd Test Method for in vitro Degradation testing of poly(L-lactic Acid)resin and fabricated from for surgical implants” aSTM F1635～95, 1998.

［2］Mainil-Varlet, P., Curtis, R., and Gogolewski, S., Effect of in vivo and in vitro degradation on molecular and mechanical properties of various low-molecular-weight poly-lactides, Journal of Biomedical Materials Research.36(3): 360～80, 1997 Sep 5.

［3］Suuronen, R. et al., Strength retention of self-reinforced poly-L-lactide screws and plates: an in vivo and in vitro study, Journal of Materials Science: materials in Medicine, 3; 426～31, 1992.

［4］Tormala, P., Vasenius, J., Vainionpa, S., Laiho, J., Pohjonen, T., and rokkanen, P., Ultra-high-strenght absorbable self-reinforce polyglycolide (SR-PGA) composite rods for internal fixation of bone fractures: in vitro and in vivo study., Journal of Biomedical Materials Research. 25(1):1～22, 1991 Jan.

［5］Matsusue, Y., Yamamuro, T., Oka, M., Shikinami, Y., Hyon, SH., and Ikada, y., In vitro and in vivo studies on bioabsorbable ultra-high-strength poly(L-lactide)rods. Journal of Biomedical Materials Research. 26(12): 1553～67,1992 Dec.