疲勞負載對脊骨整型術後脊椎之靜態與動態反應的影響

Abstract

目的：探討脊骨整型術後之椎骨經疲勞負載後之力學反應變化 背景簡介：骨質疏鬆症是導致椎骨發生壓迫性骨折的因子，而脊骨整型手術是治療此病症的方法。在臨床結果中，這些病患在術後經過一段時間，有可能再次發生骨折。每天各式的活動都會對脊椎造成疲勞負載，這是導致椎骨壓迫性骨折的原因之一，也是脊骨整型手術後再次骨折的重要因素之一。我們希望能從脊椎受到疲勞負載過程的力學反應變化，了解造成椎骨再破壞的原因，或許能夠用來預測及預防椎骨的再破壞。 材料與方法：採用人體胸椎之運動元共十組，每一試樣為四節椎骨。在去除軟組織後，將試樣第一與第四節椎骨包覆強化樹脂並架設加速度計。完成後將試樣置於實驗機台進行衝擊測試，量測試樣之剛性與振動加速度。試樣在不同的階段接受衝擊測試，首先模擬椎骨的壓迫性骨折，接下來注射骨水泥對破壞節椎骨補強。等待骨水泥硬化後，試樣進行疲勞負載的階段。實驗中共模擬三組不同載重情況下的疲勞負載，試樣依序接受650 牛頓、950牛頓及1150 牛頓的疲勞破壞，每一組負載時間皆為5 小時，各組之間會有12 小時的休息。 結果：在椎骨模擬壓迫性骨折後，椎骨試樣剛性下降13.5% (P = 0.14)，第二節椎骨加速度上升25.8% (P = 0.00)，而使用骨水泥補強後，可以椎骨的剛性及加速度回復至接近破壞前。在5 小時的疲勞負載後，椎骨運動元之剛性增加26% (P = 0.00)，而第一節椎骨加速度下降了14% (P = 0.00)，休息可以使之回復。在負載大的組別中，加速度與剛性的變化速度較快，在第1 小時疲勞負載造成的變化最大。 結論：本研究顯示破壞會造成椎骨受衝擊時的剛性降低而加速度上升，破壞節椎骨處於不穩定的狀態，而經皮脊椎整型手術能夠恢復椎骨的穩定度。隨著疲勞負載的時間，脊椎有剛性增加與加速度下降的現象，而休息可使脊椎性能回復。從數學模型計算脊椎受力變 化，結果顯示在經過5 小時疲勞負載後，脊椎受力增加了1.46%，而在負重較高的情況中，脊椎受力增加的比較快速。因此，長時間疲勞負載是造成脊椎再破壞的因子，在負重較高的情況中，休息顯得更為重要。

Objective: To investigate the effect of fatigue loading on the biomechanical responses of spinal column post vertebroplasty. Summary of Background Data: Fatigue loading is one of the risk factors for vertebral compression fracture. Vertebroplasty is a minimal invasive surgery for spinal compression fracture. It is believed that the fatigue loading is also an important factor for the vertebral re-fracture post vertebroplasty. We hope we can explore the mechanical response of augmented vertebra secondary to the fatigue loading. The results may help to predict or prevent the vertebral refracture. Methods: Ten human thoracic motion segments (T4-T7, T5-T8, T9-T12) were used and set up accelerometer on 1st and 2nd level. Specimens were applied with impulse testing to measure the stiffness and acceleration after fractured, augmented, and fatigue loading. The fatigue loadings were 650N, 950N, and 1150N at 5Hz for 1, 2, 3, 4, and 5 hours (18,000, 36,000, 54,000, 72,000, and 90,000 cycles), and 12-hours rest was applied between different loadings. The paired-t test was respectively performed to evaluate the variations of stiffness and acceleration after a series of fatigue loading. The independent t-test was performed to evaluate the variation of acceleration on 1st and 2nd level. Results: After fractured, the stiffness decreased 13.5% (P = 0.14), and acceleration of level 2 increased 25.8% (P = 0.00). The variations of these properties were recovered after cement injection. After 5-hours fatigue loading, the stiffness increased 26% (P = 0.00) but acceleration of level 1 decreased 14% (P = 0.00), and both recovered after 12-hours rest. Under high loading condition, the changing rates of acceleration and stiffness is higher, especially in the first hour. Conclusion: Spinal stability was decreased after vertebral compression fracture, and recovered post vertebroplasty. The stiffness increased but acceleration decreased after fatigue loadings, and rest could restore these mechanical properties. The results of numerical model indicated the internal force of spinal column increased 1.46% after 5-hours fatigue loading. The internal force of spinal column increased rapidly under high loading condition. Therefore, a long-term activity is a risk factor for vertebral refracture.