疲勞負載與休息對椎間盤避震功能的影響

Abstract

前言：椎間盤除了提供脊椎活動的能力，在身體內也扮演吸震器的角色。一般來說，它被視為是黏彈性體。在過去的研究中發現椎間盤的傷害會對其減震性造成影響，而透過適當的休息讓水分流回椎間盤內，可以恢復其性能。但當恢復性質後的椎間盤再受到疲勞負載時，椎間盤減震性的變化卻少有人探討。本實驗希望藉長時間(3hr)及短時間(0.5hr)的疲勞負載，以及對椎間盤休息前後兩次的疲勞負載，了解疲勞負載對椎間盤減震性的影響。 材料與方法：實驗中使用六個月大的豬隻腰椎做為試樣，以兩節椎骨加上一個椎間盤作為一運動單元，長時間疲勞負載有9組，短時間疲勞負載有9組。將椎骨做第一次循環負載，分別在每個不同的時間點做衝擊測試。再將椎間盤浸泡在生理食鹽水中12個小時後做第二次疲勞負載。也在不同時間點在做衝擊測試，比較前後兩次疲勞負載椎間盤的變化。我們將椎間盤視為一彈簧阻尼系統，使用的參數為剛性係數K(kN/mm)、阻尼比ξ、阻尼係數C(Ns/m)。另外我們也量測當上下節椎骨受到外力衝擊時，所產生的力量大小。 結果：在長時間疲勞負載測試，第一次的疲勞負載中K、ξ、C值變化需要一個小時才能達到穩定，而第二次的疲勞負載只要0.5小時就能達到穩定。比較前後兩次的疲勞負載測試，則在第0.5小時會有顯著性的差異。短時間的疲勞負載測試，K、ξ、C值的主要差異主要是在0分鐘跟30分鐘的兩個時間點才有。而前後二次的疲勞負載在每個時間點都沒有顯著性的差異。另外不管在長時間或短時間的疲勞負載中，其上下節椎骨間的受力，力量衰減的百分比都沒有顯著差異。 結論：長時間的疲勞負載會使椎間盤的水分流失，造成其喪失緩衝的功能。雖然透過浸泡食鹽水讓水分流回椎間盤內可恢復其性能，推測因椎間環破裂影響其鎖水的功能，使得再做第二次疲勞負載時，水分流失速度會比第一次快(第一次需要1小時，第二次需要0.5小時)，便會很快的再度喪失其緩衝能力。而短時間的疲勞負載卻不會有這種現象，經過短時間疲勞負載的椎間盤，會喪失些許水分。只要讓水分可以回到椎間盤，便可跟完整時一模一樣，不影響其鎖水功能。在力傳比的比較，試樣為只有一個椎間盤的運動單元，其減震能力有限，所以力量衰減百分比才會沒有顯著性差異。這些數據可以提供從事復建工作之醫護人員，在替病人做復健治療時，時間長短依據。

Introduction: The intervertebral disc is a viscoelastic material, which provides flexibility and acts as a shock absorber for the spine. In previous studies, injury of the disc affects its shock attenuation. During recovery, fluid flows into the intervertebral disc, allowing full recovery of the mechanical properties. However, information on the alterations of the discs shock attenuation, after fatigue loading with recovery, is lacking in the literature. This study evaluated the effects of short(0.5hr) and long(3hr) term fatigue loading. The purpose of this study is to investigate the effect of fatigue loading on the shock attenuation of the intervertebral disc by fatigue loading before and after rest. Material and method: Six month old porcine lumbar 2-unit motion segments were used in this study and divided into short and long term fatigue loading groups(n=9 for each). Impact loading was performed on the samples prior to fatigue loading. For the short term fatigue loading group, impact test was performed after each ten minutes of fatigue loading. For the long term fatigue loading group, impact test was performed after a half hour and each hour of fatigue loading. The change in the disc mechanical properties between first and second fatigue loading cycles, were compared. The intervertebral disc was modeled as a spring damping system. There are three parameters used to describe the performance of disc, including: stiffness, K (kN/mm), damping ratio, ξ, and damping coefficient, C(Ns/m). The input and output force were measured for the impact loading tests. Result: In the first cycle of long term fatigue loading, the values of K, C and ξ reached steady state after one hour. In the second loading cycle, the values of K, C and ξ reached steady state in a half hour. Comparing the first and second cycle of long term fatigue loading, there was significant differences within the first half hour. In the short term fatigue loading group, there was significant differences between 0min and 30min. There was no significant difference in the force attenuation, input and output force parameters, for any time point, when comparing the time points of the first and second loading cycles. Conclusion: The lack of significant difference in the force transmitted during the impact loading tests, for all time points, suggests that the amount of load absorbed by the intervertebral disc was small. This may be due to only having one intervertebral disc in the specimen, compared to 26 in the full human spine. The intervertebral disc will lose water during long term of fatigue loading and cause it lose its function as a shock absorber. In the long term fatigue loading group, the annulus fibrosus was damaged during the first testing cycle, preventing full recovery of the discs mechanical properties. Therefore, fluid flowed out of the disc more readily during the second cycle of fatigue loading. If the disc loses water, then its shock absorption properties will be altered. However, this phenomenon was not abserved in the short term fatigue loading group. Because the annulus fibrosus was not injured. In conclusion, the short term fatigue loading group was able to retain the recovered water, as seen by the recovery of the meachanical properties in the second set of fatigue loading ; however, the annulus fibrosus was damaged in the long term fatigue loading group.