當病人的骨質密度很低時，椎弓根螺釘(椎弓足螺絲)的咬合能力會大幅的降低，同時也提高鬆脫的風險性。近來已經有研究將椎體成型術(Vertebroplasty)或氣球椎體成型術(Balloon Kyphoplasty) 與椎弓根螺釘結合達到增加螺絲咬合能力的效果。目前為止，尚未有文獻以有限元素分析的方式，探討骨螺絲與不同骨水泥的大小及位置所得到的拉出強度。本文將利用有限元素分析建立一套模擬骨螺絲骨水泥拉出強度的方法論，並與先前的文獻結果比對驗證本研究之可信度。
於有限元素分析中，吾人先進行脊椎CT影像的量測得到椎體相關尺寸，再經由這些椎體尺寸，設計分析模擬的幾何尺寸。以ANSYS建立三維有限元素分析的模型，包含鬆質骨、硬質骨、骨螺絲、骨水泥。模型建立後則進行參數化分析及靈敏度分析，探討於不同情況下的最大骨水泥拉出強度位置是否相同。其主要的模型分為椎體考慮鬆質骨且螺絲為平滑螺絲、椎體考慮硬質骨且螺絲為平滑螺絲、椎體考慮鬆質骨且螺絲為有螺紋等三種情況下進行探討。
從分析的結果中，發現鬆質骨的楊氏係數及骨頭的大小，在研究中是非常重要的參數。未來若以人造假骨進行穿孔骨螺絲骨水泥擴張術的機械測試，其固定端面積沒有大於骨水泥大小時，必須避免骨水泥位置過於靠近固定端。最後，與先前的文獻結果比較後，本研究之骨螺絲包含骨水泥拉出強度模型，是一套準確度高且可信任的方法論。

Due to very low BMD (Bone mineral density), it is a challenge to fix pedicle screw on osteoporotic spine, therefore the risk of screw loosening will increase. Recently, there are some studies using Vertebroplasty and Balloon Kyphoplasty to increase strength of pedicle screw fixation. However, using Finite Element Analysis (FEA) method to simulate the pullout strength of pedicle screw with bone cement augmentation has not been established. Hence the purpose of this study is creating a Finite Element Analysis methodology to simulate the pullout strength of pedicle screw with bone cement augmentation, and then compare with the previous studies in order to prove our methodology being reliable.
In finite element analysis, we measure the vertebral body dimensions from CT-scan imagine first, and then we will design our finite element model using the imagine data. The ANSYS software is used to create our 3-D finite element model. The model includes cancellous bone, cortical bone, pedicle screw, and bone cement. After creating finite element model, we do the parametric and sensitivity analysis to make sure that the maximum bone cement pullout position is the same or not under different boundary conditions. The finite element model study is divided into three parts, first we consider smooth screw and only cancellous bone, second we consider smooth screw and cancellous bone with cortical bone, third we consider screw with thread and only cancellous bone.
According the FEA result, there are two important parameters that can affect the pullout strength obviously in our study. First is the young’s modulus of cancellous bone and second is the bone size. If we will do the mechanical test in the future, the bone cement is suggested should not very close to constrained side. Finally, we compare our simulation result with the previous result. It shows that Finite Element Model for the pullout strength of pedicle screw with bone cement augmentation is a feasible and reliable methodology.

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