使用頸椎參數化有限元素模型評估頸椎融合術對椎節力學之影響

Abstract

簡介：頸椎退化性脊髓病變(Cervical spondylosis with myelopathy, CSM)好發於中老年人，患者具有肩頸痠痛、動作感覺異常等症狀，其發生原因為椎孔狹窄或神經壓迫。治療方法依照手術位置可以分為前路減壓手術(Anterior cervical decompression fusion, ACDF)以及後路減壓手術(Posterior cervical decompression fusion, PCDF)。其中前路減壓手術，常使用椎間籠植入物(Intervertebral fusion cage)以提供椎體前側支撐與穩定，但長期追蹤發現該手術易加速鄰近節退化，使患者可能需要再次手術，故近年來許多學者針對鄰近節退化現象(Adjacent level degeneration)加以探討。 目的：本研究設計一參數化頸椎有限元素模型，輸入個別病患X光影像的量測參數，建立客製化模型。將術前與術後的客製化模型進行活動度(Range of motion, ROM)與椎間盤壓力(Intervertebral disc pressure, IDP)分佈比較，了解個別病患在術後對鄰近節椎間盤的影響。 材料與方法：本研究分兩部分：第一部分為參數化模型設計，參考側向X光影像上的特徵長度與相對位置，使用簡單的立體幾何建構出可隨參數改變外型的頸椎模型。第二部分為客製化模型應用，本研究回顧了5位使用單節椎間籠的病患進行前路頸椎減壓融合手術，平均年齡為56歲。使用每位病患之術前與術後三個月的X光影像，產生正立姿勢下的客製化模型，並量測各節活動度與椎間盤高度的資訊。為了模擬融合手術下的活動範圍，並使模型與真實運動行為一致，用各節位移控制的方式讓正立姿勢模型進行前彎後仰運動，計算完整頸椎模型、術前模型與術後模型三種姿勢下的椎間盤壓力以觀察鄰近節在術後的狀況。 結果：模型驗證方面，參數化有限元素模型使用73.6牛頓的預壓(preload)與1.8牛頓米的純彎矩(pure moment)施力下，前彎活動度為32.4度，後仰活動度為35.6度，總活動度為68度；椎間盤壓力落在正常範圍內，符合文獻結果，證實模型能重現頸椎生物力學特性。客製化模型的應用方面，5位病患中有4位(病患#1~4)術後總活動度顯著下降(p=0.004)，有1位術後總活動度上升視為特例討論(病患#5)。各節前彎活動度在術後皆有顯著降低的趨勢，上鄰近節術後降低卻無顯著變化，且其活動度下降量最少，是術後各節活動度中最高者；上鄰近節的椎間盤高度在術後皆有降低的趨勢，且其下降量最多；各節前彎椎間盤壓力術後降低，但其上鄰近節有上升的趨勢，無顯著可能來自於樣本數的不足；椎間環壓力在術後皆有降低的趨勢，上鄰近節在前彎時具有顯著降低。進行迴歸分析後發現，椎間盤壓力與椎間盤高度變化在前彎時具有負相關；椎間盤壓力與活動度在前彎後仰時具有正相關，椎間盤壓力變化會同時受到椎間盤高度變化與活動度影響。 結論：本研究設計之頸椎參數化有限元素模型能用來評估個人頸椎術前與術後的力學特性。實際使用在5位使用單節椎間籠的病患身上時，發現椎間盤壓力變化同時受到椎間盤高度變化與活動度兩者的影響，當椎間盤高度變化越小，則椎間盤壓力越大；當活動度越大，則椎間盤壓力越大。病患在術後的頸椎各節活動度皆有降低的現象。相對各節，上鄰近節活動度降低的幅度較少，術後活動度最高，同時發生椎間盤壓力上升的現象，可能導致鄰近節提早退化。

Objective: The purpose of this study is to build a patient-specific finite element model (FEM) to investigate how a single-cage implant may lead to adjacent segment degeneration (ASD) in patients diagnosed with cervical spondylotic myelopathy. Introduction: Cervical spondylotic myelopathy (CSM) is becoming one of the most common cervical disorders in elderly individuals as a result of direct spinal cord compression. CSM is associated with neck/shoulder pain and progressive neurological symptoms and surgical interventions are often required. The two most commonly employed surgical approaches are: (1) anterior cervical decompression fusion (ACDF) and (2) posterior cervical decompression fusion (PCDF). Recent literature suggests that despite the positive surgical outcomes associated with ACDF, it has also been linked to accelerating ASD in long-term follow-ups. In order to investigate this issue further, current pilot study focused on CSM patients undergone ACDF with a single level cage. Material and Method: (a). Model construction. Based on the geometric parameters obtained from lateral radiographs, the lower cervical model (C3-7) was constructed by selecting pre-determined geometry parameters from the individual patients and subsequently with a customized geometric model constructed for each patient. (b). Patient-specific model. Lateral cervical spine radiographs in neutral, flexion and extension views in the standing position were obtained for five CSM patients (average age: 56 years; range: 36-74 years) undergone single level ACDF with cage. Radiographs were obtained pre-operation and again at the three months follow-up. For each patient, the geometric parameters of the individual vertebra, the cervical range of motion (ROM) as well as the disc height change were obtained from radiographs and used to build the preoperative and postoperative FEMs. In order to utilize the constructed model to obtain the intervertebral disc pressure changes under different loading conditions, models were simulated into flexion and extension with the degree of range of motion replicating the range observed on the radiographs. Result: (a). Model validation. For the simulation of displacements under the condition of 73.6N preload and 1.8Nm pure moment for flexion and extension, the ROM was found to be 32.4 degrees and 35.6 degrees for flexion and extension respectively. These data were in good agreement with the published data. (b). Application of the patient-specific model. The total ROM significantly decreased (p=0.004) in all cases except case #5. In flexion, the segmental ROM for all levels were significantly decreased post-operatively with the exception of the level above the operated level, which showed the least amount of change and with the highest segmental ROM compared with the other levels. Furthermore, the upper adjacent level also showed a trend of decrease in height post-operatively. In terms of the IDP post-operatively, in the flexion position, the simulated model illustrated an increase in the upper adjacent level IDP and in contrast, all other levels demonstrated a decrease in IDP. Moderate positive correlation was found between IDP and ROM (R=0.57, p<0.05), and low negative correlation was found between IDP and disc height change (R=0.36, p<0.05). Conclusion: This study indicates that the patient-specific finite element model presented here could be used to estimate the biomechanical effect of a single level anterior cervical fusion. After applying the patient-specific finite element model into the 5 human cases, it has demonstrated that the IDP change is associated with disc height and ROM changes. More specifically, the IDP increases when the segmental ROM is increased and/or disc height is decreased. It has also been found that the segmental ROMs decreased post-operatively for all levels with the exception of the adjacent upper level. In summary, the identified increase in mobility and IDP of the upper adjacent level may explain the association between ASD and CSM.