鎖定式骨板(Locking plate)目前被廣泛地運用於治療身體各部位骨折，原因是鎖定式骨板多了鎖定式系統，可增強骨板結構與增加穩定度。然而，在鎖入及拔除螺絲的時候，螺絲頭滑牙(Slippage of screw head)的情形卻經常發生，此會造成無法轉動螺絲。滑牙(Slippage)的發生主要是因為螺絲與骨頭間咬合太緊固，使用者沒有正確的傳遞扭力，此種現象經常出現在小尺寸的六角形(Hexagonal)鈦合金(Titanium Alloy)螺絲身上，目前還沒有任何研究針對不同螺絲頭的設計進行完整比較。因此本研究目的為探討最佳化的螺絲頭設計以預防滑牙的發生。
本研究設計出四種不同的螺絲頭形狀，分別為六角形、星形(Torx)、十字(Cross)以及鐵十字形(Iron-cross)，且對此四種螺絲頭進行有限元素法分析(Finite Element Method)及生物力學測試，以驗證數值與實驗結果是否一致，評估方式為比較螺絲頭在去除同樣面積及體積的條件下，哪種螺絲頭形狀可產生較大的扭力，以避免螺絲頭滑牙的現象產生，此外並模擬螺絲起子沒有準確插入螺絲頭時，其中鎖緊及拔除動作之扭力結果，針對容易產生滑牙的螺絲頭凹槽設計進行最佳化的改良設計。本實驗所選用的螺絲頭材料為鈦合金，螺絲起子材料則是選用淬火後的SUS420不銹鋼。由於螺絲滑牙大部分是由於螺絲頭與螺絲起子產生的最大扭力小於骨頭與骨螺絲之咬合強度所造成，因此本研究均使用扭力測試進行不同設計之評估。
本研究結果發現，四種小尺寸形狀在轉到0.1度時，可以發現六角形可以產生最大的扭力，當螺絲起子與螺絲頭完全密合時，四種形狀螺絲起子會變形斷裂，而在螺絲起子與螺絲頭無完全密合時，六角形會發生滑牙的現象，當螺絲起子的面積擴大時，可以產生更高的扭力，也使得螺絲起子不斷裂，增強螺絲起子的強度。
六角形螺絲頭雖然可以產生較高的扭力，但是在尺寸小的時候，由於形狀太接近圓，使得螺絲起子在長期使用下易產生磨損，達成力的傳遞較不容易，另外亦可能因為施力過大導致螺絲起子插不準確造成滑牙。另外從有限元素的分析中，設計形狀的角數量越少，可以增加滑牙所需去除的體積，減少滑牙發生，但有限元素分析中，無法模擬滑牙情形，只能以實驗進行確認。未來可以繼續探討新形狀以減少及預防滑牙的產生。

Locking plates have widely been used to treat different kinds of fractures in the body during surgical operation, for it can strengthen the bone-plate structures to make the fixation more rigid. However, the screw head slippage happened frequently during insertion and removal of the screws. Slippage may occur when the screws are well bonded with the bone. Slippage occurs when a screw cannot be driven since the user’s power or torque is not transmitted properly and is commonly seen in small size hexagonal screws made of titanium. There have been no studies with comparisons of different kinds of screw head designs on the slippage of screw head. Therefore, the purpose of this study is to optimal socket design to prevent screw head stripping.
In this study, four types of screw head designs were investigated, including Hexagonal, Torx, Cross, Iron Cross using Finite Element Method (FEM) and Biomechanical experimental testing to validate the results in order to evaluate which type of screw head produces better torque to avoid screw slippage under the condition of cutting the same area and volume. Moreover, what kind of screw head design may have a greater torque to avoid screw head stripping. We simulation in incomplete engagement screw driver during the insertion and removal of screw was also observed. In this study, Titanium and SUS420 stainless steel were utilized in the manufacturing of screw head and screw driver, and torques of each designs were measured and calculated after torsion tests were applied to all specimens.
The result of FEM reveals that Hexagonal screw head has the best performance at the screw driver rotated 0.1 degrees to the screw head, In small screw, the mechanical testing shows that Hexagonal has the best performance. In well engaged conditions, hexagonal screws had the highest torque. All the drivers warped and broken at the end of testing. In the mathematical simulations, the driver with the structure as close as round shape could achieve higher torque. In incomplete engagement, hexagonal screws stripped with more decrease of the torque. Torx and Cross screws warped only with less decrease of torque. When the screwdriver area expansion, we can obtain greater torque and don’t make driver broken.
Although Hexagonal screw head can produce highest torque among all the other, its shape approaches to circle as the screw size gets smaller. Having screw driver being used for a couple of period, wear and tear could cause inefficient force transmission, which eventually bring about slippage easier in incomplete engagement. There are difficulties simulating the slippage of screw head with FEM, and can only be determined by experiment. In the future, this study could be extended to design new socket reduction and avoiding slippage of screw head.

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