椎間盤凸出症對椎間盤性能與脊椎生物力學的影響

Abstract

前言. 椎間盤凸出（Herniated intervertebral disc；在此簡稱為HIVD ）的病因，ㄧ般認為是因為椎間盤受到連續性機械外力後，椎間核向後外側方向凸出壓迫到椎間孔中的神經根所引起的。而凸出後的椎間盤除了本身性能發生改變之外，必定會影響其鄰近椎骨的動態反應。因此，本研究擬透過椎間盤凸出症的生物力學模型，探討鄰近椎骨受衝擊時加速度的變化情形與椎間盤性能的改變，進而建構出正常與凸出後的椎間盤其吸震能力的反應理論。 材料與方法. 本計畫使用豬胸椎的三節運動單元為試樣（T2~T5、T6~T9），共16組（前導實驗使用4組、生物力學測試使用12組）。前導實驗階段，我們先對椎間盤注入0.2ml稀釋過的染液（染液與水的比例為1：2），並且於試樣的左前方施加5Hz、425N下壓之疲勞負載五小時（此程序主要在製造椎間盤凸出），然後作切片觀察。生物力學測試階段，我們先針對中間位置的椎間盤其上、下脊骨架設加速規，以便測量正常狀況下脊骨的加速度情形，然後對於試樣進行HIVD處理（目的在製造椎間盤突出的生物力學模型），然後再測量推間盤凸出後的脊骨加速度，最後利用椎間盤性能測試系統量測退化後的椎間盤其壓力-體積曲線，以便與正常的椎間盤做比較。 結果. 實驗結果顯示，椎間盤凸出的方向與椎體彎曲方向相反；椎間盤凸出會導致軸向椎骨加速度降低、椎骨加速度由上往下衰減趨勢變小、力量傳輸率減少、椎間盤所承擔的力量變大；正常椎間盤的初始壓力大於凸出後椎間盤的初始壓力；相同注水量對凸出的椎間盤所造成的壓力較小。此外，凸出後椎間盤可承擔的最大壓力為2.28MPa，大大地小於正常椎間盤可承擔的極限。 討論. 椎間盤凸出導致椎骨軸向加速度變小，此乃來自下方組織的抵抗力增加所致，因此會限制椎骨的運動狀態，並且增加椎骨的破壞機會。此外，由於椎間盤已喪失緩衝能力，因此造成椎骨整體加速度由上往下衰減的趨勢降低，而且由於椎骨變得容易受到衝擊高度與緩衝件的影響而產生大變動，會使得椎間盤常常處在不穩定狀態，加上凸出後的椎間盤具備低強度以及高液體損耗率之特性，這些都會加速盤體本身的破壞。 結論. 本實驗中加速度衰減值不會受到衝擊能量與接觸時間的影響，但卻在椎間盤凸出後減少，因此，加速度衰減值是一個良好的指標，可以用來評估椎間盤凸出的程度；此外，由於本身量測儀器固定流速的特性，因此在椎間盤完整性測試結果中的直線斜率可以用來評估纖維環的鎖水能力。透過這些指標也在在顯示椎間盤凸出後會降低其功能的完整性。本研究最後亦提出一個數學模型來演算椎間盤承擔之力量，幫助臨床人員預測椎間盤的受力狀況，藉此預防椎間盤破壞的發生。

Introduction. The herniation of intervertebral disc（HIVD）is referred to that the protruded nucleus pulposus compresses the nerve root when the disc is subjected to continuous load. HIVD affects not only the performance of the disc but also the motion response of adjacent vertebra. The purpose of the study is to investigate the effect of HIVD on the disc performance and spine biomechanics by analyzing HIVD of porcine produced by an artificial procedure. Methods. Sixteen (16) porcine thoracic 3-unit-motion-segments were used in this study: four (4) for HIVD procedure establishment and twelve (12) for HIVD study. In HIVD procedure establishment, 0.2 ml dilute dye (33.3％ black dye and 66.7% water) was first injected to the nucleus pulposus to track the direction of protrusion. Then, each specimen underwent an HIVD procedure with the 425N loading on the left-anterior position at a rate of 5Hz for 5 hours. The disc was cut to verify the occurrence of the disc. In HIVD study, the specimens were first subjected to the HIVD procedure, then an impact test with controlled impact energy and contact period were performed. Shock attenuation properties, including the acceleration attenuation and force transmissibility of motion segment, were calculated from the acceleration and force responses. Finally, we used disc-integrity testing apparatus to quantify the disc integrity pre and post HIVD procedures. Results. The direction of progressive posterior-lateral herniation was opposite to the direction of vertebra bending. The shock attenuation properties and the disc integrity decreased post HIVD procedure. The averaged acceleration attenuation was -1.4 dB for pre HIVD procedure and -0.3 dB for post HIVD procedure, respectively. These values were independent of impact energy and contact period of the test. However, the force transmissibility increased for long contact period test and decreased for short contact period test. The intrinsic pressure, leakage pressure, and saturation pressure decreased post HIVD procedure. Discussion. HIVD reduces the cushioning effect of the disc and causes the adjacent vertebra to fracture. After HIVD procedure, the disc suffers the lost of the strength and liquid retaining capability, which may lead to the rupture of the disc. Conclusion. The acceleration attenuation decreases after HIVD occurs, and is independent of impact energy and contact period. Therefore, the acceleration attenuation can provide an index to evaluate the degree of disc herniated injury. Besides, the linear slope of the disc-integrity test can be used to evaluate the liquid retaining capability of annulus fibrosus. These test results show that HIVD decreases the functional integrity of the disc and the disc is at higher risk of injury when HIVD occurs.