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，這樣會造成骨板固定不牢靠及拔除螺絲的困難。滑牙(Slippage)的發生主要是因為使用者沒有正確的傳遞扭力，而這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，且還沒有任何研究針對不同螺絲頭的設計進行完整比較。因此本研究目的為探討不同螺絲頭設計對於骨螺絲頭滑牙之影響。
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(Finite Element Method)分析及生物力學測試以驗證結果是否一致，目的為比較螺絲頭在去除同樣面積及體積的條件下，哪種螺絲頭形狀可以產生較大的扭力來避免螺絲頭滑牙的現象產生，並額外觀察螺絲起子用不同角度進行鎖緊及拔除的動作，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50度得到的扭力值來做討論。
本研究由有限元素法分析結果可以發現，若只比較螺絲頭及螺絲起子角度為0.1度時，鐵十字形螺絲頭是最強的，其次是十字螺絲頭，然後是星形螺絲頭，六角形螺絲頭則是四種螺絲頭設計中最弱的，而鐵十字形的扭力值是六角形的5倍之多，然而在機械測試部分發現六角形的表現是最好的，其次是星形，然後是鐵十字形，最後則是十字形，而六角形的扭力是十字形的2倍之多。從中發現數值分析與生物力學測試的結果有異，原因是因為沒有將起子的扭轉考慮進去。而在觀察傾斜角度及無完全密合的情況下，發現六角形的螺絲頭會產生滑牙的現象。
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，力的傳遞較不容易，因此容易滑牙，對於六角形在傾斜角度以及沒有確實密合的情形下，也容易產生滑牙，另外，從有限元素的分析中，無法模擬出滑牙的現象，只能以實驗進行確認。未來也可以繼續探討如何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 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 investigate the effects of a variety screw head designs on screw slippage.
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, the effect on using different angle of twisting 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 for up to 50 degrees rotation.
The result of FEM reveals that Iron Cross screw head has the best performance at the screw driver rotated 0.1 degrees to the screw head, then Cross, Torx , and Hexagonal screw head . The results show that Iron cross provides for up to 5 times higher moment than Hexagonal. However, the mechanical testing shows that Hexagonal has the best performance, then Torx, and Iron Cross. The moment Hexagonal has provided is 2 times higher than that of Cross. The differences between numerical analysis and biomechanical experimental tests was caused by not considering the torsion of screw driver in numerical computation, and the dimensions of Iron Cross cannot be manufactured exactly the same as defined in numerical model. During the experiment , we also see that in different angle and the driver is not all engage to the screw head can lead Hexagonal slippage.
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 during different angle of insertion and removal of screws. 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 reduction and avoiding slippage of screw head.

[1] G. Pattison, J. Reynolds, and J. Hardy, "Salvaging a stripped drive connection when removing screws," Injury-International Journal of the Care of the Injured, vol. 30, pp. 74-75, Jan 1999.
[2] G. M. Georgiadis, N. K. Gove, A. D. Smith, and I. P. Rodway, "Removal of the Less Invasive Stabilization System," Journal of Orthopaedic Trauma, vol. 18, pp. 562-564, Sep 2004.
[3] J. H. Bae, J. K. Oh, C. W. Oh, and C. R. Hur, "Technical difficulties of removal of locking screw after locking compression plating," Arch Orthop Trauma Surg, vol. 129, pp. 91-5, Jan 2009.
[4] M. Ehlinger, P. Adam, P. Simon, and F. Bonnomet, "Technical difficulties in hardware removal in titanium compression plates with locking screws," Orthop Traumatol Surg Res, vol. 95, pp. 373-6, Sep 2009.
[5] T. Suzuki, W. R. Smith, P. F. Stahel, S. J. Morgan, A. J. Baron, and D. J. Hak, "Technical Problems and Complications in the Removal of the Less Invasive Stabilization System," Journal of Orthopaedic Trauma, vol. 24, pp. 369-373, Jun 2010.
[6] L. C. Haug, C. Deml, M. Blauth, and R. Arora, "Dorsal screw penetration following implant removal after volar locked plating of distal radius fracture," Arch Orthop Trauma Surg, vol. 131, pp. 1279-82, Sep 2011.
[7] N. Mitsukawa, "Clinical experience with the screw extraction set for broken screw," J Craniofac Surg, vol. 22, pp. 226-9, Jan 2011.
[8] D. Hontzsch and F. M. Stuby, "[Removal of plates and screws. Tips and tricks for problematic cases]," Unfallchirurg, vol. 115, pp. 291-8, Apr 2012.
[9] T. Maehara, S. Moritani, H. Ikuma, K. Shinohara, and Y. Yokoyama, "Difficulties in removal of the titanium locking plate in Japan," Injury, vol. 44, pp. 1122-6, Aug 2013.
[10] G. Kumar and C. Dunlop, "Case report: A technique to remove a jammed locking screw from a locking plate," Clin Orthop Relat Res, vol. 469, pp. 613-6, Feb 2011.
[11] N. R. Gopinathan, M. S. Dhillon, and R. Kumar, "Surgical Technique: Simple Technique for Removing a Locking Recon Plate With Damaged Screw Heads," Clinical Orthopaedics and Related Research, vol. 471, pp. 1572-1575, May 2013.
[12] J. W. Brubacher, T. M. Owen, and M. S. Vrahas, "Use of Surgilube to minimize metal debris in removal of jammed titanium locking screws," Injury, vol. 44, pp. 1648-50, Nov 2013.
[13] S. Ahmad, L. Azura, S. Duski, and M. Y. Aziz, "Paravertebral Abscess Caused by Bukholderia Pseudomallei in," Malaysian Orthopaedic Journal, vol. 3, pp. 88-90, 2009.
[14] A. Ansari and R. Twyman, "Removal of screws with damaged heads—A technical tip," Injury Extra, vol. 39, pp. 345-346, 2008.
[15] S. J. Kim and M. U. Kim, "A Simple Technique for Removing a Locking Compression Plate With a Stripped Locking Screw," Journal of Orthopaedic Trauma, vol. 26, pp. E51-E53, Jun 2012.
[16] S. Raja, A. M. Imbuldeniya, S. Garg, and G. Groom, "Difficulties encountered removing locked plates," Ann R Coll Surg Engl, vol. 94, pp. 502-5, Oct 2012.
[17] J. J. Kim, J. W. Kim, H. S. Yu, H. S. Lee, and H. K. Oh, "Factors affecting accurate drill sleeve insertion in locking compression plates," Orthop Traumatol Surg Res, vol. 99, pp. 823-7, Nov 2013.
[18] N. Schwarz, S. Euler, M. Schlittler, T. Ulbing, P. Wilhelm, G. Fronhofer, et al., "Technical complications during removal of locking screws from locking compression plates: a prospective multicenter study," European Journal of Trauma and Emergency Surgery, vol. 39, pp. 339-344, 2013.
[19] J. S. Hayes and R. G. Richards, "The use of titanium and stainless steel in fracture fixation," Expert Review of Medical Devices, vol. 7, pp. 843-853, Nov 2010.
[20] J. K. Behring, N. R. Gjerdet, and A. Molster, "Slippage between screwdriver and bone screw," Clinical Orthopaedics and Related Research, pp. 368-372, Nov 2002.
[21] K. R. Spencer, J. W. Ferguson, A. C. Smith, and J. E. A. Palamara, "Screw head design: an experimental study to assess the influence of design on performance," Journal of Oral and Maxillofacial Surgery, vol. 62, pp. 473-478, 2004.
[22] S. A. Klein, N. A. Kenney, J. A. Nyland, and D. Seligson, "Evaluation of cruciate and slot auxiliary screw head design modifications for extracting stripped screw heads," Acta Orthopaedica Belgica, vol. 73, pp. 772-777, Dec 2007.
[23] Q. Liu, D. R. Olson, F. W. Tiley, M. Shea, M. Smits, and R. A. Hart, "Biomechanical comparison of a novel multilevel hex-head pedicle screw design with a conventional head design," J Orthop Res, vol. 25, pp. 1115-20, Aug 2007.
[24] D. S. Jun, H. Kou, Y. C. Kim, C.-S. Jun, and S. C. Lee, "Evaluation of a cloverleaf screw head to minimize the slippage of medical screws," International Journal of Precision Engineering and Manufacturing, vol. 12, pp. 703-709, 2011.
[25] C. Y. Chien, W. H. Chuang, W. C. Tsai, S. C. Lin, Y. P. Luh, and Y. J. Chen, "A Finite Element Study about CAM-Out Failure of the Recess-Screwdriver Interfaces for the Cold-Welded Periarticular Fixation," Journal of Mechanics, vol. 29, pp. N1-N7, 2012.
[26] AO Foundation images.