疲勞負載對胸椎皮質骨螺釘生物力學特性之影響

Abstract

研究目的: 探討椎弓根螺釘之胸椎皮質骨路徑與傳統路徑的固定強度，以及經過疲勞負載測試後椎弓根螺釘之固定強度。 背景介紹: 皮質骨路徑為近年來提出的椎弓根螺釘植入路徑，其目的在使螺釘與骨密度較高的皮質骨有更多的接觸，以便增強螺釘的咬合。目前已有許多文獻探討腰椎皮質骨路徑的生物力學特性，然而僅有極少數的研究討論疲勞負載後對皮質骨路徑的影響，本研究以體外生物力學測試探討在疲勞負載測試前後的胸椎皮質骨路徑與傳統路徑的固定強度。 材料與方法: 本研究使用 39節人體屍骨胸椎試樣(T7-T12)，由骨質密度測定儀測得試樣之平均骨質密度為0.961g/cm2(範圍：0.634-1.273g/cm2)。將人體試樣之椎骨分離並去除軟組織，由神經外科醫師在脊椎左右兩側分別施打皮質骨路徑與傳統路徑。實驗以螺釘種類分為兩組，組別一在兩個路徑都使用傳統多軸椎弓根螺釘(4.35 mm x 35 mm)；組別二則是在皮質骨路徑使用皮質骨螺釘(5.0mm x 40 mm)，傳統路徑使用的是傳統多軸椎弓根螺釘(5.0 mm x 40 mm)。再將所有試樣離散分配至控制組、疲勞負載組共兩組，疲勞負載參數為10-100 N、1 Hz、10,000循環，記錄疲勞負載時的位移變化量並定義「疲勞破壞量」，「疲勞破壞量」又可細分為「剛性破壞量」和「潛變破壞量」。由拉出測試之力與位移結果，可計算出「拉出強度」與「拉出剛性」。 結果: 本研究在疲勞負載部分，組別一和組別二的總破壞量在皮質骨路徑中不論是使用傳統螺釘或是皮質骨螺釘，其總破壞量皆小於傳統路徑(組別一：0.29(0.17) mm vs. 0.49(0.34) mm, p=0.061; 組別二：0.27(0.11) mm vs. 0.37(0.11) mm, p=0.008)。 在拉出強度的部分，在疲勞負載前組別一中皮質骨路徑的拉出強度小於傳統路徑(438(188) N vs. 536(226) N, p=0.061)，但是在組別二皮質骨路徑的拉出強度則大於傳統路徑(799(369) N vs. 683(210) N, p=0.092)，而經過10,000循環疲勞負載測試後，不論是組別一或組別二的皮質骨路徑與傳統路徑之拉出強度皆趨於接近(組別一：337(122) N vs. 394(103) N, p=0.193；組別二：765(465) N vs. 735(397) N, p=0.446)。 結論： 本研究結果顯示在疲勞破壞前，當使用傳統螺釘植入兩種路徑時，傳統路徑較皮質骨路徑會有較強的固定強度，因此不建議將傳統螺釘植入皮質骨路徑；但是若使用傳統螺釘植入傳統路徑對比於皮質骨螺釘植入皮質骨路徑時，皮質骨骨釘和路徑會有較強的固定強度。但是經過疲勞負載後，我們發現不論螺釘的種類，皮質骨路徑與傳統路徑之拉出強度會趨於接近。依據本研究之結果推論，當進行胸椎固定手術時，若沒有急性期的強度要求，兩種路徑和骨釘在穩定期的強度差異不大，醫師可根據病人的需求給予適當的路徑。

Objective: To compare the fixation strengths of pedicle screws inserted via the cortical bone trajectory and the traditional trajectory, and to investigate the effect of fatigue loading on fixation strength. Introduction: Cortical bone trajectory is a relatively new technique for pedicle screw insertion. The cortical bone trajectory aims to improve fixation strength by increasing the contact between the screw and the denser cortical bone. Many papers studied the biomechanical characteristics of insertion using the cortical bone trajectory. However, the effect of fatigue loading on fixation strength of the cortical bone trajectory has received comparatively less attention. This study investigated the fixation strengths of thoracic cortical bone trajectory and traditional trajectory with the use of cortical pedicle screw and traditional pedicle screw by human cadaveric biomechanical tests both before and after fatigue loading. Materials and methods: Thirty-nine human cadaveric thoracic spine vertebrae (T7-T12) were used. The mean bone mineral density (BMD) of the specimens was 0.961 g/cm2 (range: 0.634- 1.273 g/cm2). The experiment consisted of two groups. In both groups, the cortical bone trajectory and traditional trajectory were used in the left and right pedicles of each vertebra, respectively. Group 1 used traditional poly-axial pedicle screws (4.35 mm x 35 mm) in both trajectories. Group 2 used traditional poly-axial pedicle screws (5.0mm x 40 mm) in the traditional trajectory and cortical screws (5.0mm x 40 mm) in the cortical bone trajectory. All specimens were randomly distributed into the control group and fatigue group. The peak-to-peak force and loading frequency of fatigue loading were 10-100 N and 1 Hz. The loading was vertically applied at the screw end to simulate the axial body force. The displacement history along the loading line during fatigue loading were recorded. The increase of the displacement at the end of fatigue loading was defined as the 'fatigue damage'. The 'fatigue damage' was further divided into 'stiffness damage' and 'creep damage'. From the force and displacement curve of the pullout test, the 'pullout strength' and 'pullout stiffness' were calculated. Results: The results of fatigue loading showed; both group 1 and group 2 exhibited less fatigue damage in the cortical bone trajectory than in the traditional trajectory (group 1: 0.29(0.17) mm vs. 0.49(0.34) mm, p=0.061; group 2: 0.27(0.11) mm vs. 0.37(0.11) mm, p=0.008). Before the fatigue loading, the pullout strength of the cortical bone trajectory was lower than that of the traditional trajectory in group 1 (438(188) N vs. 536(226) N, p=0.061), and the pullout strength of the cortical bone trajectory was greater than that of the traditional trajectory in group 2 (799(369) N vs. 683(210) N, p=0.092). After 10,000 cycles fatigue loading, no difference in pullout strength was observed between the cortical bone trajectory and the traditional trajectory within either group 1 or group 2 (group 1: 337(122) N vs. 394(103) N, p=0.193；group 2: 765(465) N vs. 735(397) N, p=0.446). Conclusion: The results of this study showed that the use of traditional screw with traditional trajectory reached better strength than the traditional screw with cortical bone trajectory before fatigue loading; hence it is not suggested to use the traditional screw in the cortical bone trajectory. The pull out strength of cortical screw with cortical bone trajectory was better than the one of traditional screw with traditional trajectory before fatigue loading. However, we found both screw types at both trajectories reached similar fixation strength after fatigue loading. This may imply that the surgeon may choose the screw type and trajectory at their convenience if the fixation strength at the acute stage may not be a critical issue.