骨科手術中，鎖定式骨板(Locking plate)目前被廣泛地運用於治療身體各個部位的骨折，原因是由於鎖定式骨板多了鎖定式系統，可以增強骨板的結構，使得固定更加穩健。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，這樣會造成拔除螺絲的困難。螺絲頭滑牙的發生主要是因為螺絲與骨頭間咬合太緊固，使用者沒有正確地傳遞扭力，而這種現象經常出現在小尺寸的六角形(Hexagon)鈦合金(Titanium Alloy)鎖定式螺絲，且還沒有任何研究針對不同螺絲頭的設計進行完整比較，因此本研究目的為改善鎖定式螺絲頭凹槽設計以防滑牙的發生。
本研究設計出七種不同的螺絲頭形狀，分別為小尺寸的六角形、星形(Torx)、十字形(Cross)以及大尺寸的3.93mm六角形、星形、八星形與星形PLUS，且進行有限元素分析(Finite Element Analysis)及生物力學測試以驗證結果是否一致，目的為得到一個可以產生較大的扭力來避免螺絲頭滑牙現象產生的螺絲頭設計，並模擬螺絲起子沒有準確插入螺絲頭時，進行拔除螺絲的動作，對螺絲頭是否有影響。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。由於臨床螺絲頭的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，因此本研究的實驗均使用扭力測試將不同螺絲頭試片扭轉得到的扭力值來做討論。
本研究由有限元素分析結果可以發現，在模型一的情況下，大尺寸星形、八星形與星形PLUS扭力的模擬與生物力學測試結果最一致，但是卻與3.93mm六角形的生物力學測試結果不一致；小尺寸設計的生物力學測試結果可以用有限元素分析模型二~四的扭力結果來預測；由生物力學測試結果可以發現，大尺寸八星形的扭力最大，接著依序為大尺寸星形與星形PLUS，當螺絲起子與螺絲頭完全密合時，此三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，此三種形狀皆會因為螺絲起子葉片(Lobe)斷裂而產生滑牙。
在尺寸放大後，比起先前研究的小尺寸螺絲頭凹槽，可以產生更大的扭力，而且大尺寸星形、八星形與星形PLUS皆不會產生螺絲頭滑牙。星形設計比六角形設計還要好，因為星形設計不會產生螺絲頭滑牙，在尺寸放大後，扭力可以有效提升。而且我們也提出了一個比星形還要好的設計-八星形。

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, and 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 improve the socket design of locking screw head to prevent screw head stripping.
In this study, seven types of screw head designs were investigated, including small Hexagon, small Torx, small Cross, 3.93mm-Hexagon, big Hexa-torx, big Octa-torx and Torx PLUS. We used 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. We simulated 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 torque of each designs were measured and calculated after torsion tests were applied to all specimens.
The results of FEM revealed that the torque obtained from model one had the best consistency with the torque of the big Hexa-torx, Octa-torx and Torx PLUS obtained from the biomechanical test, but not in the case of the big Hexagon and the small screw. The results of the biomechanical test revealed that Octa-torx screws head had the highest torque in both engaged conditions, while all the drivers warped and broke during the biomechanical test in well-engaged condition and the lobes of most of the drivers broke during the biomechanical test in incomplete-engaged condition. In the mathematical simulations, the driver with the structure as close as round shape could achieve higher torque.
After enlarging the size of the socket of the screws, the torque is much higher than the torque of the small ones obtained from previous studies. Furthermore, the big Hexa-torx, Octa-torx and Torx PLUS screws can actually prevent screw head stripping. We proved that Torx is a better design than Hexagon is and that there is a better design than Torx, which is Octa-torx.

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