過大直徑之椎弓根螺釘經疲勞負載後之生物力學特性探討

Abstract

研究目的： 探討過大直徑之椎弓根螺釘經疲勞負載後對其固定強度之影響。 背景介紹： 椎弓根螺釘系統目前廣泛應用於脊椎手術中，盡管其技術已相當成熟但椎弓根螺釘鬆脫現象仍是脊椎手術中常見的術後併發症。普遍認為挑選適當的螺釘直徑會降低此現象發生的機率，問題在於使用大直徑螺釘會造成椎弓破壞的風險而使用小直徑螺釘則有穩定性不足的考量，故本研究欲探討過大直徑之椎弓根螺釘與較小直徑之螺釘在經過疲勞負載之後對其固定強度的影響為何，希望能藉由實驗的結果提供臨床醫師一系列的生物力學數據，以作為未來在臨床施打椎弓根螺釘時，選擇適當螺釘尺寸的考量依據。因此，本研究假說有二：一、大直徑之椎弓根螺釘於植入當下時，固定強度較強；二、經疲勞負載後，其固定強度下降，但仍較小直徑之椎弓根螺釘來得強。 材料與方法： 本研究使用5副高加索人種人體屍骨胸椎試樣(共27節，T3-T8，2位男性、3位女性，平均年齡67.4歲，範圍：52-83歲)，由骨質密度測定儀測得試樣之平均骨質密度為0.645g/cm2(範圍：0.353-0.848g/cm2)，可知試樣皆屬於骨質疏鬆的界定範圍內。將人體試樣之椎體分離並將軟組織分離後留下完整椎骨部分，由富經驗之神經外科醫師將兩種直徑之多軸椎弓根螺釘(5.0x35mm，以下簡稱大螺釘； 4.35x35mm，以下簡稱小螺釘)隨機植入至同一椎節之左右兩邊椎弓，接著將27節試樣隨機分配至控制組、疲勞負載5,000循環組(短載組)、疲勞負載10,000循環組(長載組)共三組，疲勞負載參數為10-100 N、1 Hz，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」、「潛變破壞量」和「總破壞量」三項。控制組於測量螺釘自然頻率後即進行拉出測試，疲勞負載組於疲勞負載前後均測量螺釘自然頻率，之後再進行拉出測試，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」。每組隨機挑選三節試樣拍攝微電腦斷層掃描(Micro CT scan)影像，其他試樣則拍攝X光影像。 統計方法分成平均數檢定與相關性分析兩部分。平均數檢定的部分使用配對樣本t檢定(paired t-test)檢測疲勞負載前後之螺釘自然頻率、個別椎體左右兩邊不同大小的螺釘之自然頻率、疲勞破壞量、拉出強度、拉出剛性是否有顯著性差異；使用單因子變異數分析(One-way ANOVA)來檢測控制組與短載組與長載組之骨質密度、疲勞破壞量、拉出強度、拉出剛性彼此之間是否有顯著性差異，若有顯著差異，則使用事後檢定(post-hoc test)來檢測各組別間的差異關係為何。 相關性分析的部分使用皮爾森相關係數(Pearson correlation)來檢測骨質密度、自然頻率、疲勞破壞量(包含剛性破壞量、潛變破壞量及總破壞量)與拉出強度、拉出剛性之間是否有顯著相關。所有統計結果在p-value<0.05視為有顯著性差異。 結果： 本研究利用兩不同大小直徑之椎弓根螺釘於中上段胸椎進行植入，由X光和micro-CT影像發現大部分均造成椎弓側向破壞。 三組之骨質密度彼此無顯著差異(p=0.492)。大螺釘自然頻率有高於小螺釘自然頻率之趨勢，顯示大螺釘與椎骨似乎有較高的咬合度。經疲勞負載後，螺釘自然頻率有減少的趨勢，顯示螺釘與椎骨之咬合度有降低的趨勢。 經過10,000循環的疲勞負載之後，長載組有部分試樣產生微結構破壞，故將此組再細分為「微結構完整組」與「微結構破壞組」，並進行比較。經疲勞負載後，大螺釘與小螺釘之疲勞破壞量均無顯著差異。但於5,000循環時，微結構破壞組相較於微結構完整組有較高且顯著的疲勞破壞量(p<0.032)。拉出強度和拉出剛性於控制組、短載組、長載組間皆無顯著差異。在控制組，大螺釘有較高之拉出強度與剛性的趨勢，但是在短載組與長載組，大螺釘與小螺釘之拉出強度趨於相同。由迴歸模型，我們發現拉出強度相對於疲勞總破壞量會有先陡後緩之下降趨勢，表示術後一段時間內螺釘固定強度會急遽下降，而後趨緩。 結論： 本研究的主要發現為：一、大螺釘於植入當下時，固定強度較強；二、經疲勞負載後，其固定強度下降至與小螺釘相同。因此未來臨床手術醫師對椎弓窄小之椎節進行椎弓根螺釘的植入時，可選擇直徑較小的螺釘進行植入，而不必擔心固定強度會較大螺釘來得弱，此外使用較小直徑的螺釘還能更正確、安全地植入椎弓，降低破壞椎弓內側進而傷到脊髓的風險。 此外，本實驗另外發現若螺釘進入點較偏外側(如橫脊突上)，需特別注意勿植入太深，使得螺頭撐裂側後方皮質骨，使得固定強度下降。

Objective: This study aimed to investigate the effect of oversized pedicle screw on the fixation strength after fatigue loading. Introduction: Screw-rod type spinal implantation is widely used in the treatment of spinal disorders. Despite the advances in medical technology, screw looseness remains one of the most frequent failures for this type of implantation system. The selection of appropriate screw size plays a crucial role in the success of spinal implantation, as larger screws increase the risk of pedicle failure during insertion, but smaller screws are thought to compromise the stability of the implantation. By investigating the relationship between screw diameter and the pullout strength of pedicle screw after fatigue loading, this study seeks to find quantitative biomechanical data to assist surgeon in the selection of the appropriate screw. There are two hypotheses for this research: 1) the fixation strength of larger screw will be higher than that of smaller screws immediately after implantation. 2) After fatigue loading, while the fixation strength will decrease for all screw sizes, larger screws will more effectively retain their fixation strength. Materials and methods: Twenty-seven human cadaveric thoracic spine vertebrae (T3-T8) were harvested from 5 human cadavers (2 males and 3 females, mean age: 67.4, ranged: 52-83). The mean bone mineral density (BMD) of the specimens was 0.645 g/cm2 (ranged: 0.353-0.848 g/cm2), which is indicative of severe osteoporosis. Two sizes of poly-axial screws (4.35mm x35 and 5.0mm x35) were randomly chosen and implanted into each of the two pedicles of each vertebrae by an experienced surgeon, and specimens were randomly distributed into a control group and 2 fatigue groups (5,000 and 10,000 cycles). The peak-to-peak force and loading frequency of fatigue loading were 10-100 N and 1 Hz. Specimens of the control groups were evaluated with a pullout test after natural frequency measurement. For the fatigue groups, natural frequency was measured both before and after fatigue loading. Three specimens of each group were randomly selected to be scanned with micro-CT and the other specimens were imaged using radiography. For statistical analysis, a paired t-test was conducted to determine difference in natural frequency between before and after fatigue loading. Differences in natural frequency, fatigue damage, pullout strength, and pullout stiffness between the different screw sizes were also considered. One-way ANOVA was conducted to determine difference of BMD, fatigue damage, pullout strength, and pullout stiffness between the control group and both fatigue groups. If significant differences were observed, the post-hoc tests were conducted to determine the relationship between each group. Pearson correlation was used to determine the relationship between BMD, natural frequency, fatigue damage, pullout strength, and pullout stiffness. A p-value of < 0.05 was considered to be statistically significant. Results From radiography and micro-CT images, we found that both sizes of screws cause lateral breaches at pedicle. BMD had no significant difference between each group (p=0.492). The natural frequency of larger screws was higher than that of smaller screws, indicating that the binding power of larger screws seemed to be higher than that of smaller screws. After fatigue loading, the natural frequency of the screws decreased, indicating that fixation strength decreased. For fatigue damage analysis, we discovered that some specimens had micro fracture (MF) after fatigue loading in the 10,000 cycle fatigue group. Therefore, we separated the 10,000 cycle fatigue group into two sub-groups, one with MF and the other without MF. The fatigue damage of the MF group was significantly higher than that of the non-MF group and the 5,000 cycle fatigue group (p<0.032). The pullout strength of the larger screws was higher than that of the smaller screws in the control group but in the fatigue groups, pullout strength were closer to each other. However, there was no significant difference in pullout strength between each group. Finally, according to the regression model of pullout strength against total damage, we found that pullout strength dropped sharply at first but slowly decreased as total damage increased. Conclusion: In this study, we proved the first hypothesis, but failed the second one. Two main findings were concluded in this research: 1) The pullout strength of the larger screw was higher than that of the smaller screw just after implantation. 2) After fatigue loading, both sizes exhibited similar pullout strengths. The pullout strength of larger screws is not higher than the smaller screws. This indicates that the smaller size of screws may be chosen for less risk of pedicle breakage without sacrificing fixation strength. In addition, we found if the entry point was located between the articular process and transverse process, an over insertion of pedicle screw may damage the cortex of transverse process; hence decreased the fixation strength of pedicle screw after fatigue loading.