第四章 综合討論
4.5 實驗限制
本研究使用的人體胸椎椎骨來進行實驗，用來模擬椎弓根螺釘在椎骨內受疲 勞負載後的破壞型態，但臨床上後方脊椎融合術常用於腰椎部位的融合，而胸椎 幾何型態和大小與腰椎不盡相同，且腰椎試樣取得不易，以及本實驗所需試樣數 量過於龐大，因此本研究仍採用胸椎進行實驗。實驗是屬於體外實驗，自然有一 些體內才有的機制是體外無法模擬，Burr(1998)6等人指出，當椎弓根螺釘在體內受 疲勞負載同個時間點上，骨頭本身會對破壞的骨質進行再生(regrowth)和重建 (remodeling)的功能，代表椎骨在微破壞期間又不斷修復，儘管重建過後的骨質可 能沒有原先來的緻密，至少在椎弓根螺釘和椎骨間的咬合強度上有一定程度的修 補。
由本實驗的 X 光片結果發現，當對螺釘頭進行疲勞負載時，椎弓根螺釘在椎 骨內以外層較硬皮質骨為支點，經槓桿原理，導致螺釘尾端在椎骨內部晃動破壞 並掏空椎骨，造成螺釘下方黑影的產生(見圖 3.6)，但由於本研究使用的 X 光機屬 於低解析的傳統洗片式，洗片的結果會因浸泡顯影劑和定影劑的時間長短而有不 同，比起臨床用的數位式(高解析)X 光機，黑影的判斷自然較為不易。加上本研究 僅以單向疲勞負載(caudal loading)模擬椎弓根螺釘在椎骨內走路時的破壞型態，但 人體在生活中有會有前傾(flexion)和後彎(extension)的動作，椎弓根螺釘在體內應 是屬於雙向疲勞負載(cranio-caudal loading)的破壞型態。基於以上原因，本研究 X 光影像無法建立與臨床上關聯性。
另外本研究在比較骨質密度與椎弓根螺釘的拉出強度、剛性強度和震盪頻率 的關聯性上，皆無顯著性的相關。但Halvorson(1994)14等人研究指出骨質密度和拉 出強度上是有高度相關，由於本實驗使用的是人體椎骨，在試樣準備中可以發現
部分椎骨周圍有明顯的骨刺生成，骨刺是椎間盤軟骨損傷後的自行修補反應，導 致椎骨周圍形成一層厚且硬的高密度新生骨質，由於本研究的骨質密度是由平面 骨質密度儀掃瞄得知，因此骨刺的多寡會影響掃描結果。且使用的是椎骨尺寸是 相對於腰椎較小的胸椎椎骨(T9-T12)。加上本實驗試樣的骨質密度範圍和樣本數量 相 較 於 Halvorson 等 人 並 不 算 廣 ( 本 研 究 ： 0.665-1.039g/cm2 n=10, Halvorson ： 0.73-1.32 g/cm2 n=32)，以上原因皆有可能導致骨質密度與拉出強度關聯性建立上 的誤差。
第五章 結論與未來展望
5.1 結論
本研究證明了當椎弓根螺釘與人工仿骨和椎骨間介面間隙分別到達 1mm 時，
經拉出測試後發現拉出強度明顯下降，代表螺釘和試樣間的咬合減弱，以此資訊 提供臨床上在判斷螺釘鬆脫時的依據。本研究的 X 光影像，因體外椎骨的破壞型 態和體內不同，加上 X 光影像的解析度不足，無法提供有效的影像作為比較。
另外在人工仿骨或人體椎骨的實驗中，成功建立椎弓根螺釘在試樣內剛性強 度與拉出強度，以及震盪頻率與拉出強度之間的關係，經統計分析發現，兩種非 破壞性測試的結果與拉出強度有顯著性的中高關聯性，證明利用椎弓根螺釘在試 樣內剛性強度及震盪頻率，可以用來代表體外拉出測試的結果。
5.2 未來展望
本研究僅探討當椎弓根螺釘和椎骨間間隙為 0mm、1mm 和 2mm 三組別之間 三種測試方法的關聯性，假若要建立更完整架構，應該進行更多不同間隙的組別，
另外本研究只對胸椎椎骨進行實驗，往後會對腰椎椎骨做更進一步的實驗，探討 其結果和本研究是否有相同趨勢。
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