前位頸椎融合術鄰近節段之生物力學研究

Abstract

臨床研究顯示前位頸椎融合術後使骨融合部位相鄰節段加速退化，亦即，前位頸椎融合術會影響其長期臨床效果，故本研究為探討前位頸椎融合術後上相鄰節段的負荷變化。本研究使用12個羊頸椎(C3-C6)進行試驗，並根據以下程序進行試驗：完整頸椎、椎間支架植入(Cage)、椎間支架加上前位鋼板 (Cage+Plate)使用於融合節段(C5-C6)。試件使用位移控制以1 °/s速度在前屈20°和後伸15°間循還運動，本研究之試驗結果包含在前屈20°和後伸15°運動時的各節段活動度、整體力矩、融合節段上相鄰節段的椎體和面關節應變、應力中性軸位置之偏移量。

在前屈運動時，植入椎間支架程序時的力矩、融合節段的活動度和面關節應變相較於完整頸椎的數值為小；在融合部位上相鄰兩節段的活動度、椎體應變、中性軸位置的偏移相較於完整頸椎狀態的數值為大。而在椎間支架加上前位鋼板程序時，其力矩相較於完整頸椎狀態的力矩為大，並且融合部位上相鄰兩節段的椎體和面關節應變也相較為大。在後伸運動時，植入椎間支架程序時的融合節段的活動度相較於完整頸椎程序的數值為大，其力矩相較於完整頸椎狀態的力矩為小，並且融合部位上相鄰兩節段的活動度、椎體和面關節應變也相較為小。在椎間支架加上前位鋼板程序時，其融合節段的活動度小於完整頸椎程序的活動度，而其力矩相較於完整頸椎狀態的力矩為大，而且融合部位上相鄰兩節段的椎體和面關節應變也相較於完整頸椎狀態的數值為大。前位頸椎融合術後需要較大的力矩於頸椎運動，表示肌肉及韌帶組織需要更大的力量來使頸椎運動，且融合部位相鄰節段的活動度、椎體和面關節應力增加非常明顯。

本研究達成二項重要結果，第一為椎間支架植入在前屈運動時，植入部位之相鄰兩節段的應力中性軸向後偏移會導致活動度和椎骨應力重新分佈和相鄰節段過度活動。從生物力學的角度來看，椎間支架可能增加椎體的剛性和造成應力遮蔽效應。應力由後方面關節傳遞至前方椎體，並使得骨融合部位相鄰節段的應力增加。因此，中性軸的應力與應變規的力學模型，可用於測量椎骨的應力分佈。第二為椎間支架加上前位鋼板在前屈運動時，相鄰節段的應力中性軸向後偏移造成骨融合部位的相鄰椎體的應力增加。相鄰節段的應力和活動度的增加在長期植骨融合後可能使相鄰節段加速退化，或加重其退化程度。

總結，剛性強的前位鋼板雖可以提高頸椎融合術後短期穩定度，但是長期可能加速鄰近節段的退化，因此，建議患者可考慮使用剛性低或動態鋼板以減少前位頸椎骨融合後鄰近節段退化的發生率。

Clinical studies have established that anterior cervical spine fusion accelerates the degeneration of segments adjacent to the fused bone. Consequently, anterior cervical spine fusion reduces its effect of the long-term clinical results. Hence, this investigation studies the changes in loading of the superior segments adjacent to the fused bone following anterior cervical spine fusion. Anterior cervical spine fusion thus diminishes in terms of its effect for a long-term clinical outcome. Therefore, this study examines the changes in loading of the superior segments adjacent to fused bone following anterior cervical spine fusion. Twelve sheep cervical spine specimens (C3-C6) were tested using the following surgical procedure: the spine was maintained intact; a cage was inserted (Cage); and an anterior plate was fixated using cage (Cage+Plate) at the fusion segments (C5-C6). Specimens were cycled between 20° flexion and 15° extension with displacement control at 1 °/s. The measured outcomes in this study were the range of motion (ROM) for each segment. Additionally, the torque, strains of both the body and facet joint at superior segments (C3-C4) were adjacent to the fused bone. Moreover, the position of the neutral axis of stress was under 20° flexion and 15° extension.

Under flexion, the change in the torque, ROM, and strain of the facet of the fused segment were lower than those of the intact spine. Also, the change in ROM, strain of the body, and position offset of the neutral axis of superior segments adjacent to the fused bone were higher than those of the intact spine after the Cage procedure. Additionally, the change in torque, ROM, and strains of both the body and the facet of superior segments adjacent to the fused bone were higher than those of the intact spine after the Cage+Plate procedure. Under extension, the change in the ROM of fused segments was higher than those of the intact spine. Meanwhile, the changes in the torque, ROM, and strains of both the body and the facet of superior segments adjacent to the fused bone were lower than those of the intact spine after the Cage procedure. Following the Cage+Plate procedure, change in the ROM of the fused segment was less than that of the intact spine. Meanwhile, changes in the torque, ROM, and stress of both the body and the facet of superior segments adjacent to the fused bone were higher than those of the intact spine. Moreover, cervical motion required a greater torque following anterior cervical spine fusion, indicating that the muscle and ligament have to exert a larger force to cause cervical motion. Furthermore, the ROM and stress of the body and facet joint on segments adjacent to the fused bone increased significantly.

This study has two major findings. First, this study demonstrated redistribution of ROM and vertebrae stress, and hypermobility on adjacent segments, due to the neutral axis of stress on adjacent segments moving backwards. Meanwhile the cage insertion occurred under flexion. From a biomechanical perspective, a cage may increase the stiffness of a body and produce a stress-shielding effect. The stress may transmit from the back facet to the anterior body, subsequently increasing the segments adjacent to fusion bones. Therefore, vertebral stress distribution can be determined using the mechanical model of neutral axis of stress with a strain gauge. Second, this study demonstrated increased stress of the body of the segments adjacent to the fused bone, due to the neutral axis of stress on adjacent segments moving backwards. Meanwhile, anterior plate fixation using cage occurred under flexion. Furthermore, the increased stress and ROM of the adjacent segment after long-term bone fusion may accelerate degeneration of the adjacent segment, or even increase its degree.

In sum, although the anterior plate with high stiffness can increase the stability for a short time period after cervical spine fusion, degeneration of adjacent level may accelerate in the long term. Therefore, we recommend that patients consider using low stiffness or a dynamic plate to decrease mobility of the adjacent level after anterior cervical spine fusion.