椎弓足骨螺絲彎曲強度之生物力學測試與有限元素分析

Abstract

椎弓足骨螺絲在脊椎外科手術中廣泛地被應用。從臨床上發現，病人在手術後經過一段時間，由於上半身體重的反覆負載，椎弓足骨螺絲有可能發生斷裂的情形。因此，增加彎曲強度是設計椎弓足骨螺絲的重點之一。 在本研究中設計並製造十種椎弓足骨螺絲，利用生物力學測試與有限元素分析兩種方法來評估這些椎弓足骨螺絲的彎曲強度。在生物力學測試部分，把椎弓足骨螺絲分別鎖入塑膠鋼（高分子聚乙烯 ultrahigh molecular polyethylene）中並且鎖入深度為40mm，測試包括靜態負載降伏測試和反覆負載疲勞測試，記錄每支骨螺絲之降伏強度、勁度、疲勞壽命及多週期勁度。在有限元素分析部分，利用ANSYS套裝軟體建立椎弓足骨螺絲之三維有限元素分析模型，然後模擬生物力學測試之情況，有限元素分析模型包括椎弓足骨螺絲和塑膠鋼圓柱，在塑膠鋼上施予點力220N並且把螺絲頭邊緣完全拘束住，椎弓足骨螺絲與塑膠鋼之界面均為接觸界面，經有限元素分析運算過程後會得到最大張應力、最大位移量和總應變能，然後評估有限元素分析模型的收斂性並且探討生物力學測試與有限元素分析之相關性。 由生物力學測試結果中，同一內徑之椎弓足骨螺絲，其錐度長度越長者，其降伏強度、勁度、多週期勁度與疲勞壽命均越大。圓錐形骨螺絲的彎曲強度性能均比圓柱形骨螺絲來的好，且增加內徑可以增加彎曲強度性能。在有限元素分析結果中，同一內徑之椎弓足骨螺絲，其錐度長度越長者，最大張應力、最大位移量，總應變能均越小。較大內徑的圓錐形骨螺絲有較小的最大張應力、最大位移量和總應變能。全部的有限元素分析模型都已收斂。在生物力學測試和有限元素分析之相關性分析中，有限元素分析的最大位移量或總應變能與降伏強度之相關性高達–0.914，而有限元素分析中的最大張應力與疲勞壽命的對數之相關性高達–0.928。最後，在本研究中所建立的有限元素分析模型可以反應出生物力學測試之結果，此可用來幫助臨床醫師為病人選擇最適當的固定器。

Pedicle screws have been used popularly in spinal surgery. In clinical point of view, the pedicle screw may break after a short span of the surgical operation because of cyclic loading of upper body weight. Therefore, designing the pedicle screw with a good bending strength is becoming an important issue. Ten types of pedicle screws were designed and manufactured in this study. The bending strength of those pedicle screws was investigated by biomechanical tests and finite element analysis. In biomechanical tests, the pedicle screws were inserted into the ultrahigh molecular polyethylene cylinder and the insertion length of pedicle screw was 40 mm. Static-load yielding tests and cyclic-load fatigue tests were conducted. The yielding load, stiffness, fatigue life, and multi-cycle stiffness of each screw were recorded. In finite element analysis, three-dimensional finite element models were established by ANSYS to simulate the biomechanical tests. The finite element models consisted of the pedicle screw and polyethylene cylinder. A point load of 220 N was applied on the polyethylene cylinder and the peripheral margin of the screw head was fully constrained. The interfaces between pedicle screw and polyethylene cylinder were contact. In post processing, the maximal tensile stress, maximal deflection, and total strain energy of finite element models were derived. The convergent study of finite element models was evaluated and the correlation study between biomechanical tests and finite element analysis was assessed. From the results of biomechanical tests, the screw with larger taper length has higher yielding strength, stiffness, multi-cycles stiffness, and fatigue life. The conical screw has higher bending strength performance than the cylindrical screw. Increasing the inner diameter could increase the bending strength. From the results of finite element analysis, the screw with larger taper length has smaller maximal tensile stress, maximal deflection, and total strain energy. The conical screw with larger inner diameter has smaller maximal tensile stress, maximal deflection, and total strain energy. All of the finite element models were convergent. In correlation study, the maximal deflection or total strain energy in finite element analysis was closely related to the yielding strength with a high correlation coefficient of –0.914. The maximal tensile stress in finite element simulation was closely related to the logarithm of fatigue life with a high correlation coefficient of –0.928. In conclusion, developing the finite element models in this study could be used to reflect the results of biomechanical tests. This result can assist surgeons in selecting suitable devices for patients.