椎弓根螺釘於自然骨及仿生植入假體的拉拔強度與鑽鑿強度關聯性

Pedicle screw pullout strength and its relationship with maximum insertion force for bone and TPMS implants

蔡亞芸(Ya-Yun Tsai)

指導教授 ： 張書瑋

國立臺灣大學/工學院/土木工程學研究所/碩士(2022年)

https://doi.org/10.6342/NTU202201037

摘要

植入椎弓根螺釘是脊椎固定手術中最為廣泛應用的方法。然而，植入過程時常發生椎弓根損傷破壞之情形，因而導致螺釘固定脊椎的效力降低。傳統評估椎弓根螺釘之穩定度常以拉拔強度為標準，但僅能應用於體外試驗。因此，臨床上仍缺乏確認椎弓根螺釘穩固性的方法。另一方面，3D列印金屬孔洞骨植入假體近年來十分盛行，基於孔洞金屬的各項優勢，包含可控的物性、更佳的骨整合性，以及客製化的形狀。然而，關於螺釘在這種孔洞骨植入物的穩固性，在文獻上仍未被探討。本研究提出一個理論預測模型，稱作「加權骨頭埋置體積模型」，利用X光片來分析擷取特徵，預測骨釘拉拔強度。此模型考慮到了骨頭的異質性，以及骨釘的圍束情形。本研究以模擬的方式驗證了此模型，也與文獻上的生物力學試驗做了比較，得到的相關係數為0.94。本研究證實只要有X-ray影像，「加權骨頭埋置體積模型」的預測結果可作為椎弓根螺釘穩定度之另一評估指標。此外，為了釐清鑽鑿強度與拉拔強度之關聯，本研究探討了33個三度週期最小曲面的孔洞狀骨頭結構與4個塊狀無孔洞骨頭結構，模擬其釘子鑽鑿與拉拔之完整過程。我們發現鑽鑿強度與拉拔強度呈現高度線性相關，相關係數超過0.98。然而，兩者的比例在三度週期最小曲面結構與塊狀結構並不相同。在三度週期最小曲面結構中，釘子較容易鑽鑿進入，但具有較高拉拔強度，表示在被釘子鑽鑿之後，三度週期最小曲面結構較塊狀結構來說，具有較佳的結構穩固性。本研究的發現可以幫助理解骨頭在受到釘子鑽入和拉拔時的破壞模式，以及三度最小曲面結構設計孔洞骨植體有何力學上的優勢。

關鍵字

椎弓根螺釘 ； 拉拔強度 ； 電腦模擬 ； 三度週期最小曲面 ； 骨植入物 ； 骨支架

並列摘要

The gold standard in the surgical correction of spinal deformities, fractures, and instability is spinal fusion surgery with posterior pedicle screws. Pedicle screw loosening is the main concern, which may result from pedicle fracture after inserting the pedicle screw. In vitro biomechanical tests such as pullout tests are the main approach to assess the stability of a screw; however, direct evidence is lacking for the human spine clinically. On the other hand, metallic porous bone implants using additive manufacturing techniques are thriving due to advantages including the controllable physical properties, better osseointegration, and customized morphologies. However, the fixation of screws in such bone implants has yet to be discussed. Here, we aim to provide a model that can predict the pullout strengths of pedicle screws in various pedicle conditions from X-ray images. A weighted embedded bone volume (EBV) model is proposed for pullout strengths prediction by considering the bone heterogeneity and confinement of the screw. We showed that the pullout strength is proportional to the EBV for homogeneous bone and the weighted EBV for layered composite bone. The proposed weighted EBV model is validated with in vitro Sawbones® pullout experiments, and the coefficient of determination is 0.94. The proposed weighted EBV model can help assess the stability of a pedicle screw in a broken pedicle by simply examining 2D X-ray images. To find out the relationship between the forces during screw insertion and pullout, we simulated the screw insertion and pullout processes of 33 TPMS and four solid bone-screw models. We found that the pullout strength is correlated with the maximum insertion force with coefficients of determination higher than 0.98, but the ratio is different for TPMS and solid bone. TPMS bone is easier to be penetrated, but the screw inside TPMS bone is more difficult to be pulled out, indicating better fixation stability of TPMS bone than solid bone. The findings in this work can help understand the failure mechanisms of such porous implants and acknowledge the advantages of adopting TPMS architectures in bone implants.

並列關鍵字

Pedicle screw ； Pullout strength ； Computational modeling ； Triply periodic minimal surface ； Porous implants ； Orthopedic scaffolds

參考文獻

1. Rezaii, P.G., et al., Conventional versus stereotactic image guided pedicle screw placement during spinal deformity correction: a retrospective propensity score-matched study of a national longitudinal database. International Journal of Neuroscience, 2021. 131(10): p. 953-961.

2. Hsieh, M.-K., et al., Biomechanical Comparison of Fixation Stability among Various Pedicle Screw Geometries: Effects of Screw Outer/Inner Projection Shape and Thread Profile. Applied Sciences, 2021. 11(21): p. 9901.

3. Liang, W., et al., 3D-printed drill guide template, a promising tool to improve pedicle screw placement accuracy in spinal deformity surgery: a systematic review and meta-analysis. European Spine Journal, 2021. 30(5): p. 1173-1183.

4. Wang, L., et al., Posterior short segment pedicle screw fixation and TLIF for the treatment of unstable thoracolumbar/lumbar fracture. BMC musculoskeletal disorders, 2014. 15(1): p. 1-11.

5. Hoppe, S. and M. Keel, Pedicle screw augmentation in osteoporotic spine: indications, limitations and technical aspects. European journal of trauma and emergency surgery, 2017. 43(1): p. 3-8.