骨髓內釘為封閉式的固定法，是目前用來治療股骨骨折的標準方法，相對於骨板開放式的固定法，骨髓內釘可為整體骨折帶來較佳的穩定性，並避免過多的軟組織損傷及感染問題產生。針對股骨骨髓內釘的治療法，許多文獻皆指出優先採用靜態固定法，然而卻可能因骨折間隙過大，導致骨折不癒合的情況發生，因此可考慮使用另一種有效的動態固定法，即移除某一端的鎖定螺絲來減少骨折間隙，幫助骨癒合。然而過去的動態固定法大多依靠醫生在臨床上的經驗判斷，並沒有力學上的證據來證明移除何處的鎖定螺絲為最佳的動態固定法，因此本研究的目的是利用有限元素法，將市售的鎖定式骨髓內釘進行靜態固定法與動態固定法的比較，進而得到最佳的骨髓內釘治療方法。
　　本研究使用目前被廣泛接受的股骨模型及市售的鎖定式骨髓內釘，以有限元素分析來評估股骨植入順向性骨髓內釘或逆向性骨髓內釘後，其靜態固定法與移除近端或遠端鎖定螺絲兩種動態固定法的生物力學特性，並探討植入物在五種不同股骨骨幹骨折位置上的情況。判斷其生物力學性能的方法為畸變能準則，主要是觀察植入物的最大應力值及應力集中的位置，以防此植入物的過度變形及損壞；而骨折斷面上的接觸面積和接觸壓力則是用來評估幫助骨癒合的重要因素。
　　分析結果顯示動態固定法的應力值均小於靜態固定法，且發現最大von Mises應力會發生在螺絲孔洞及骨髓內釘上靠近骨折處的位置，因此可考慮使用動態固定法來降低整體植入物的應力，避免植入物損壞的風險。而本研究藉由有限元素分析的結果，亦可發現移除離骨折處最遠的鎖定螺絲時，骨折斷面上會有最大的接觸面積發生，與文獻和臨床所使用的方法相同，因此可證實此方法確實為最佳的動態固定法。然而動態固定法雖可幫助骨癒合，但卻存在不穩定和肢骨縮短的可能性，因此建議優先使用靜態固定法，若仍無明顯骨癒合發生時，才考慮改用動態固定法。
　　希望藉由本研究所得出的結果，可以幫助骨科醫師了解動態固定法的生物力學特性，並做為在臨床上選擇使用動態固定法的參考依據。

A closed interlocking nail is the gold standard treatment for femoral shaft fracture. Relative to the plate, interlocking nail has biomechanical superiority because of its better fixation, less soft tissue stripping, and less infection. For the treatment of interlocking nail, the static fixation technique is often selected to treat fractured femurs. However, the static fixation may result in a gap between fracture fragments and fracture nonunion. Immediate or subsequent dynamization of an interlocking nail is regarded as an effective treatment for patients with fracture nonunion. The principle of this technique is to remove a screw or screws to decrease the gap between the interlocked fracture fragments. In addition, the characteristics of dynamic fixation techniques are unclear, and the surgical rule, involving the removal of the locking screws farthest from the fracture site, is based mainly on surgeons’ experiences. The purpose of this study is to investigate the characteristics of the fixation techniques and validity of the surgical rule by finite element method.
The commercial interlocking nails and the standardized femur model were used in this study. The biomechanical characteristics of conventional static fixation technique and two types of dynamic fixation techniques applied to five types of femoral shaft fractures with an antegrade nail and retrograde nail were compared. To determine the biomechanical characteristics of different fixation techniques, the maximum von Mises stress of the implants, the contact area on the fracture surfaces, and the contact pressure on the fracture surfaces were calculated.
The results show that the dynamic fixation techniques could significantly reduce the maximum von Mises stress. The maximum von Mises stress occurred at either the nail hole or the nail near the fracture site. Therefore, the dynamic fixation techniques could reduce the stress of the nail and avoid the risk of implant failure. In this study, the numerical results obtained in this study had the same outcomes of clinical applications. It could be found that the dynamic fixation technique by removing the locking screws farthest from the fracture site had the largest contact area at the fracture surface of the femoral fragment. Although the dynamic fixation techniques could improve bone healing rate, but it might cause instability and limb shortening. We suggested that the priority fixation techniques were the static fixation. However, the dynamic fixation technique should be applied when the patients with nonunion occurred.
The results of this study could help surgeons to understand the biomechanical characteristics of dynamic fixation techniques and provide evidence to support the use of dynamic fixation techniques.

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