脊椎後方融合術之生物力學研究

Biomechanical Study of Posterior Lumbar Interbody Fusion

周文凱(Wen-Kai Chou)

指導教授 ： 王兆麟

國立臺灣大學/工學院/醫學工程學研究所/博士(2015年)

https://doi.org/10.6342/NTU.2015.02139

摘要

脊椎後方融合術是目前廣泛應用於治療脊椎變形、退化及不穩定等脊椎相關疾病的手術方式。由於生物力學的發展、先進醫療器材的開發及民眾對於生活品質的要求，脊椎後方融合術實施數量及整體醫療花費與年俱增。雖能有效治療病患症狀，但其相關術後併發症常需再次手術以維持治療效果，將降低臨床成效並提高醫療花費。本研究使用人體屍骨及豬隻試樣進行體外生物力學研究，目的為探討脊椎後方融合術的併發症，針對常見的短期的植入物鬆脫及長期的鄰近節椎間盤提早退化的問題進行探討，並提出未來植入物的設計方向。
本研究第一部分，探討椎弓根螺釘與周圍骨組織間隙的拉出強度關係，研究使用疲勞負載來模擬日常生活中脊椎的受力情形，並藉由影像分析椎弓根的幾何結構與拉出強度間的關係。研究結果顯示，當椎弓根螺釘與周圍骨組織間隙達1mm時其拉出強度已顯著下降，此研究成果將有助於臨床醫師判斷植入物鬆脫的狀態。研究也發現植入6mm的椎弓根螺釘於人體骨質疏鬆的胸椎時，若植入的椎弓根緻密骨於椎弓根內的比例大於0.73時，能降低植入物鬆脫的機率，讓醫師能在術前規劃適合螺釘植入的椎弓根區域。
本研究第二部分，探討彈性連接桿與剛性連接桿植入系統對於脊椎椎間盤高度及椎間核壓力的影響，以及兩種植入物的受力及對周圍骨組織的受力影響。研究結果顯示，使用彈性連接桿植入系統相較於傳統的剛性連接桿能使脊椎的生物力學特性更接近健康時的狀態。另外使用彈性連接桿能降低植入物本身及螺釘附近的骨組織應力，證明使用彈性連接桿有可能降低鄰近節提早退化及降低植入物損壞的風險，以提供臨床醫師使用不同器械時的參考方向。
本研究第三部分，探討植入節的活動度變化對於其鄰近節的生物力學代償情形。研究結果指出，隨著植入節的活動度增加，則鄰近節的活動角度及椎間核壓力代償現象則有下降的趨勢，且於前彎動作下植入節的最大活動角度為4.04°時，以及在後仰動作下活動角度為2.41°時，可以提供植入節達到穩定的情形，同時並不會造成鄰近節過度的代償情形，因此可以避免鄰近節提早退化的發生，以提供未來新型固定器設計的參考方向。
本論文對脊椎後方融合術的併發症進行一系列的生物力學研究，並提出可能的解決方向。研究成果期能提供醫事人員施行脊椎後方融合術時的參考，以降低併發症發生及再次手術的機率，使患者有更好的復原情形及生活品質，增進此手術的長期效益。

關鍵字

脊椎後方融合術 ； 生物力學 ； 植入物鬆脫 ； 鄰近節退化

並列摘要

Posterior Lumbar Interbody Fusion (PLIF) is one of the most common surgical techniques employed to treat spinal deformity, degeneration and other instability related conditions. With the improved understanding of the spine biomechanics combined with the advancement of medical devices as well as the heightened expectations of the general public in pursuing better health and quality of life, the demand for PLIF is ever increasing in the modern society. Despite its well-published effectiveness in relieving clinical symptoms, the frequent development of secondary complications requiring revision surgery associated with PLIF inevitably reduces its clinical efficacy. It is therefore the primary aim of the current project to investigate the mechanisms underlying the development of secondary complications associated with PLIF through a series of in-vitro studies using both porcine and human cadaver specimens. The secondary aim of the project focused on the issues of implant failure and the development of adjacent segment disease after instrumentation and more importantly, discussed some plausible design modifications for future implant development.
The first study of the project was an in-vitro cadaveric study designed to determine the exact role of the pedicular cortical bone composition and the screw-bone gap on the screw fixation failure. The results of the study identified a significant correlation between cortical bone area ratio and the screw pullout strength. Presented results also demonstrated that the fatigue loading induced screw-bone gap of 1 mm was sufficient to cause a significant decrease in the pullout strength. However, a cortical bone area ratio of 0.73 or higher in this group was able to preserve most of the screw-bone interfacial strength and subsequently may play an important role in preventing a complete implant failure in clinical practice.
The second study in the series was an in-vitro fatigue-loading test utilizing porcine specimens to comparatively analyze the biomechanical performance of PEEK and Titanium rods construct subjected to a battery of fatigue loading testing. Based on the results, it was determined that the differences in biomechanical characteristics of PEEK and Titanium rods construct when subjected to fatigue loading. More specifically, the result is indicative of the potential benefits of the PEEK rods construct in reducing the risks of adjacent segment disease and implant failure rates.
The final study in this series investigated the effect of altered mobility of the instrumented level and its consequential impact on the biomechanical characteristics of the adjacent segment levels. The outcome of the study demonstrated that the increasing range of motion at the instrumented level is associated with decreased compensatory motion and intra-discal pressure at the adjacent segments. It was also determined that a maximum of 4.04° into flexion and 2.41° into extension achieved maximum stability of the instrumented level coupled with minimal biomechanical compensatory at the adjacent levels. Such finding is of significant clinical value as it provides a guideline for the future design and development of more advanced spinal implants.
Overall, the series of studies comprised in the current project investigated the possible mechanisms associated with the development of secondary complications post PLIF surgery and proposed plausible solutions to overcome each of the shortcomings by identifying more appropriate medical material or through modification of device design. The outcomes of the project provided valuable knowledge base and guidelines for clinicians performing PLIF and it is hoped that the presented information will ultimately assist in the reduction of secondary complications and revision surgery associated with PLIF and subsequently improve the long-term quality of life for the patients.

並列關鍵字

Posterior lumbar interbody fusion ； Biomechanics ； Implant failure ； Adjacent segment disease

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