動態式腰椎固定系統穩定度之生物力學探討

Abstract

背景簡介:人體因為年齡增長以及姿勢不良造成脊椎不正常受力，椎間盤會逐漸產生椎間不穩定的情形，同時會壓迫到椎間孔的神經造成許多神經性的疾病，臨床上較常見的治療方法為脊椎減壓手術，其原理為將活動度過大的退化節椎板切除減少神經的壓迫，同時配合後方脊椎融合術將退化節融合，而在臨床上和生物力學測試上皆發現，脊椎融合手術會造成鄰近節過度的代償現象，進而造成鄰近節提早退化，於是許多非融合式的裝置設計與手術逐漸產生，其目的為提升手術節的活動度，以減少鄰近節的代償現象，但是在臨床上仍然有許多的報告顯示非融合手術仍然有鄰近節提早退化情形的發生，這說明在非融合裝置的設計中，手術節的活動度似乎仍須進一步的研究。 目的: 藉由改變操作節所允許的活動角度，觀察在操作節與鄰近節的生物力學變化情形，希望能找到一個適合非融合手術裝置的固定椎節區間，以提供操作節足夠的穩定度，並且不會造成鄰近節過多的代償現象。 材料與方法:本實驗利用豬隻腰椎進行前彎後仰動作下的力學測試，將試樣依序由健康、模擬受傷、模擬固定、控制活動範圍四個階段進行測試，並於各椎節置入旗標，利用攝影機量測各節的相對活動角度，同時在各節椎間盤置入針型壓力感測器，量測椎間核壓力，並於各節椎間孔上黏貼直徑1.2mm的鋼珠，在各狀態拍攝X光影像，以分析椎間孔的面積變化。 結果:在前彎動作下，隨著操作節限制角度逐漸放寬，操作節的活動角度有上升的趨勢，而上、下鄰近節代償性活動角度則有下降的趨勢，在操作節的限制活動範圍為60%(4.04°)時，在操作節與上、下鄰近節活動角度相對於健康狀態皆沒有顯著性差異；操作節椎間核壓力變化隨著控制角度放寬而有逐漸下降的趨勢，但是皆沒有顯著差異，上下鄰近節椎間核壓力有下降的趨勢，在限制活動範圍為40%、60%、80%時，鄰近節的椎間核壓力變化皆比健康狀態的椎間盤壓力小；椎間孔面積變化並不顯著。在後仰動作中，隨著操作節控制角度的逐漸放寬，操作節的活動角度有上升的趨勢，而上、下鄰近節代償性活動角度則有下降的趨勢。在操作節限制活動範圍為40%(2.41°)時，操作節，上、下鄰近節相對於健康狀態皆沒有顯著性差異；操作節椎間孔面積隨著控制角度放寬而有逐漸下降的趨勢，但是皆沒有顯著差異，上下鄰近節椎間孔面積有下降的趨勢，且發現在限制活動範圍為60%、80%時鄰近節的椎間孔面積皆比健康狀態時的椎間孔面積小，在後仰動作中椎間核壓力變化並不顯著。於前彎與後仰活動中可發現隨著操作節限制角度的逐漸放寬操作節的中性區間有逐漸上升的趨勢，當限制角度範圍為20%、40%時相對於健康狀態的操作節沒有顯著性差異，而上下鄰近節則有代償性下降的趨勢，但是相對於健康狀態皆沒有顯著性差異。 結論: 操作節的前彎活動角度最大限制範圍為60% (4.04°)時，後仰動作活動角度限制範圍為40% (2.41°) 時，不會造成鄰近節過度代償，推測可以避免鄰近節提早退化的發生。

Summary of Background Data: Fusion surgery is often used to treat unstable spinal diseases. Fusion surgery usually decreases the motion of the implanted levels and induces compensation behaviors at the adjacent levels. It is wildly believed that the excess motions at the adjacent levels cause disc degeneration. Some dynamic devices, ex. Dynesys system, have been developed to solve the problems by preserving motion at the implanted levels. However, the flexibility of these products varies and their performances on reducing adjacent disc degeneration are still absurd. Objective: The purpose of this study is to find the proper flexibility of posterior lumbar dynamic stabilizers by evaluating the neutral zone, range of motion, intradiscal pressure and intervertebral foramen area of the implanted and adjacent motion segments. Materials and methods: Eight 4-level (L2-L5) lumbar spine were dissected from 6-month old pigs. All soft tissues except the surrounding ligaments and facet capsule were carefully removed. Specimens were wrapped in saline-soaked gauze and stored in the freezer until the experiment. The flexion / extension angular displacement of the specimen in the intact status were measured under 8 Nm of pure moment. Then the specimen was injured at L3-L4 level by damaging the facet joints and surrounding ligaments. The angular flexion/extension displacement obtained from the specimen in the intact status under 8 Nm of pure moment was applied to the specimens in the injured status and consecutively in 5 levels of constrained status which was controlled by a self-designed adjustable dynamic stabilizer implanted at L3-L4. The flexion / extension motion of L3-4 was constrained to be 0%, 20%, 40%, 60%, and 80 % of the flexion / extension motion of the injured status. The intersegmental neutral zone (NZ), range of motion (ROM), changes of intradiscal pressure (det IDP) and changes of intervertebral foramen area (det IFA) of the implanted and adjacent cranial/caudal motion segments were calculated. The IDP was measured by in-house 20 G needle pressure transducers inserted into the disc. The intervertebral foramen area was calculated based on lateral radiographys. The “det IDP” and “det IFA” was defined as the difference of IDP / IFA before and after the angular displacement loading. Results: (1) During flexion. The NZ, ROM and the det IDP of the implanted and the adjacent cranial / caudal motion segments decreased with the increase of motion at the implanted level. The ROMs of the implanted and adjacent cranial/caudal motion segments in the status of 60% constrained motion at the implanted level were the same as those in the intact status. The det IDPs of the implanted and adjacent cranial/caudal motion segments in the status of 40%, 60% and 80% constrained motions were less than those in the intact status. The det IFAs of all motion segments in the injured and 5 constrained status were similar to those in the intact status. (2) During extension. The NZ, ROM and det IFA of the adjacent cranial and caudal motion segments decreased with the increase of motion at the implanted segment. The ROMs of the implanted and adjacent cranial/caudal motion segments in the status of 40% constrained motion at the implanted segment were the same as those in the intact status. The det IFAs of the implanted and adjacent cranial/caudal motion segments in the status of 60% and 80% constrained motions of the implanted segments were less than those in the intact status. The det IDPs of all motion segments in the injured and 5 constrained status were similar to those in the intact status. Conclusions: It is found that 60% constrained flexion motion and 40% constrained extension motion at the implanted level induce least compensation ROM, IDP change or IFA change at the adjacent cranial and caudal levels without violating the stability at the implanted level. The result of this study is expected to be helpful for the development of new dynamic stabilization systems.