近年來。椎弓根骨螺絲已廣泛用於治療退化性脊椎病變與創傷，目前有關於新的椎弓根骨螺絲的設計已漸漸被開發出來，但在骨螺絲完全植入與退牙植入之拉出強度特性，目前還未有完整的研究。本研究的目的是提出一種新設計的雙直徑骨螺絲，並與其它的設計作比較，包括圓柱型、圓錐型、雙內直徑和雙外直徑之設計。在椎弓根骨螺絲外徑為7mm條件下，利用有限元素法田口方法。類神經網路和遺傳演算法來尋找最佳骨螺絲之參數。分析結果顯示，在椎弓根骨螺絲外徑為7mm尺寸下，最佳尺寸分別如下:近端根部半徑為0.4mm、螺蚜傾角為5°、節距為3.3mm、螺蚜寬度為0.1mm。
此外，進行市售椎弓根骨螺絲和雙內外直徑骨螺絲之生物力學實驗，彎曲實驗之結果顯示，在承受50-500 N循環負荷下，只有固定外徑之圓錐型骨螺絲和新型雙內外直徑骨螺絲可承受100萬次。在剛性方面，新型骨螺絲相較於錐型骨螺絲低，此可允許患者更多的活動度。另外，在穩定度方面，新型骨螺絲相似於錐型骨螺絲與其他骨螺絲有較顯著的穩定度。在拉出實驗結果，具有小內徑之第一型錐型骨螺絲，在完全植入與退牙植入之拉出強度分別為2,402 N和2,170 N，兩者相差了 9.7％。在退牙植入方面，具有小內徑之第一型錐型骨螺絲與雙內外直徑骨釘並無明顯的差異（P> 0.01），具有小內徑之錐型骨螺絲、雙內直徑和雙內外直徑。在完全植入（分別為2,115 N，2,183 N和2,227 N)與退牙植入（分別為2,066 N，2,014 N和1,941 N ）均提供了良好的效果，錐型設計增加拉出強度的原因，主要是由於骨壓縮的效果。然而，錐型設計的骨螺絲於退牙植入條件時，將流失較多的拉出強度，因此，錐型設計的骨螺絲需準確的植入到所需的位置，且避免骨螺絲進行退牙植入，尤其病人具有骨質疏鬆病症。雙內直徑和雙內外直徑骨螺絲在完全植入與退牙植入物提供足夠的穩定性，具有較小螺牙深的圓柱型骨螺絲與雙外直徑骨螺絲於臨床應用時需特別注意。

Pedicle screws have been used over the last several decades for treating degenerative spine diseases and trauma. Recently, new pedicle screw designs have been developed. However, the performances of these screws are still unclear, especially their pullout strengths when backed out after insertion. The objective of this study was to present a novel design, called double dual core screw, and compared its performances with the other designs, including cylindrical, conical, dual inner core and dual outer core designs. Taguchi’s techniques were used together with finite element analyses, artificial neural network and genetic algorithm to find the optimal screw’s parameters for pedicle screws with outer diameter of 7 mm. The optimal proximal root radius, proximal half angle, pitch and thread width of 7mm-pedicle screw were 0.4 mm, 5°, 3.3 mm, and 0.1 mm, respectively.
The biomechanical tests of 7 available commercial pedicle screws and the double dual core screws were conducted. In bending tests, the results showed that only the conical screws with fixed outer diameter and the new double dual core screw could withstand 1,000,000 cycles of a 50–500 N cyclic load. The new screw, however, exhibited lower stiffness than the conical screw, indicating that it could afford patients more flexible movements. Moreover, the new screw produced a level of stability comparable to that of the conical screw, and it was also significantly stronger than the other screws.
In pullout tests, the conical type 1 screw with a small inner diameter provided the highest pullout strength in both full insertion and backed-out insertion (2,402 N and 2,170 N, respectively). However, this screw’s pullout strength significantly decreased (9.7%) when backed out from full insertion. There was no significant difference between the conical type 1 screw with a small inner diameter and double dual core screw (P > 0.01) in backed-out insertion. The cylindrical screw with a small diameter, dual inner core screw and double dual core screw also provided good results in both full insertion (2,115 N, 2,183 N and 2,227 N, respectively) and backed out conditions (2,066 N, 2,014 N and 1,941 N, respectively). The increased pullout strength of the conical design could be due to the effect of bone compaction. However, the screw exhibited less consistent pullout strength when backed out when compared with the other designs. The conical screw should be inserted to the precise position without turning back, especially in osteoporosis patients. The dual inner core screw and double dual core screw could provide great stability in both conditions. Care should be taken when using both the cylindrical screw with a small thread depth and the dual outer core screw.

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