可生物降解的連結桿在腰椎後融合手術的適用性評估

Abstract

椎間融合及後固定手術是治療腰椎退化性疾病常見的治療方式。經椎弓根鏍釘固定可增加脊椎的穩定性及提高骨融合的機會，但椎體經後固定融合後剛性提高造成相鄰節段的脊椎不正常的受力，被認為是加速相鄰節段脊椎退化的主因。但是，其機轉尚未明瞭。目前生物可降解的植入物逐漸受到骨科及外科醫師的重視。 生物可降解的植入物在經過設計及規畫的時間後降解，有機會可以減少傳統植入物的剛性，進而減少相鄰節段的壓力。所以這種植入物提供了足夠的穩定性，但是剛性會相對地降低。 我們使用一個有效力而且受過文獻驗證的腰薦椎有限元素分析模型，分析三種不同的第四/五腰椎固定模式：1) 中度退化的椎間盤，2)經椎弓根鏍釘的後固定及使用椎間融合器，3)骨融合後移除椎弓根鏍釘、只剩椎間融合器。我們檢視椎體間活動的角度、椎間盤所受的壓力、及小關節所承受的負擔。我們也模擬了四種腰椎的運動模式：彎曲、伸展、側彎及扭轉。 此外，我們另外又建立了六個有限元素分析的模型來模擬脊椎在有或沒有固定物下的情形。我們使用傳統的鈦合金的連結桿或是可生物降解的連結桿來當做固定物。運動模式為受到縱軸向的壓力或是單純的運動模試。我們把相鄰節段的可運動的範圍、椎間盤受到的壓力、還有小關節所受到的壓力記錄下來。根據國際標準規範ASTM F1717-11a，我們使用三點彎曲測試及動態彎曲測式來檢驗使用可生物降解連結桿在用做脊椎固定物的表現。 臨床上，通常頭側比尾側的相鄰節段有更嚴重的退化情形。在穩定的骨融合及固定之後，相鄰節段的可運動的範圍、椎間盤受到的壓力、還有小關節所受到的壓力分別增加 57.6%、47.3%及59.6%。但是當我們移除固定物後，這些增加的幅度分別降低為30.1%、22.7%及27.0%。這種情形我們歸因於骨融合及固定之間不同的生物力學特性。 此外進一步的有限元素分析模擬告訴我們，在腰椎後固定融合手術中使用生物可降解連結桿在骨融合的節段及相鄰節段有類似的可活動範圍。當連結桿隨著時間被降解後，這個現象造成相鄰節段小關節及椎間盤受力的減少，而且會造成前面骨移植物受力的增加。力學測試的結果顯示，生物可降解連結桿起始的材料疲勞強度為145 N，但是浸潤在模擬體內環境的液體內，材料疲勞強度六個月後降低到115 N及十二個月後降低到55 N。傳統鈦合金及生物可降解的連結桿，都可以承受5,000,000次145 N縱軸向受力的動態受力測式。 脊椎手術的後固定物，會使得原本已經骨融合的節段有更高的剛性。目前的研究顯示如果骨融合後，移除掉後固定物可以減少相鄰節段所受的壓力。經由微創手術的技術，在我們移除後固定物的二次手術也可以減少脊椎旁肌肉的破壞。此外，我們的研究顯示，生物可降解的連結桿可能可以提供一個比傳統骨融合手術更佳的預後。這種多分子聚合物的可生物降解連結桿不只在一開始提供了足夠的穩定度，而且在骨融合節段，隨著時間連結桿降解造成剛性逐漸降少，也逐漸減少了相鄰節段的受力，進一步避免需要二次手術來移除連結桿。 此外，微創脊椎手術越來越廣泛地被使用，而且提供病人許多的好處如：減少術中的出血、更少的傷口疼痛、比較小的感染風險、而且也讓病人縮短了住院天數及讓病人提早能回到工作崗位上。但是，外科醫師在執行微創脊椎手術的時候，在術中使用X光定位時，特別是在植入骨釘的時候，會接觸到更多的游離輻射。更甚於此，基於設備問題，很多的醫院並不會備有脊椎導航設備。我們展示了一個只術中只需要照前後面向X光的植入骨釘的方法，並且同時有一個在2010至2015年回溯性670個病例4072支經椎弓根釘及2014至2016年前曕性88個病例413支經椎弓根釘植入的研究。特別是在前曕性的研究中，使用術後追蹤的電腦斷層來評估經椎弓根釘位置的準確性。顯示這種植入骨釘方法的準確性並不會較有導航系統為差。

Interbody fusion with posterior instrumentation is a common method for treating lumbar degenerative disc diseases. The transpedicular fixator aims to increase stability and enhance the fusion rate. However, the high rigidity of the fusion construct may produce abnormal stresses at the adjacent segment and lead to adjacent segment degeneration (ASD). However, how the fused disc and bridged vertebrae respectively affect ASD progression is not yet clear. As such, biodegradable implants are becoming more popular for use in orthopaedic surgery. These implants offer sufficient stability for fusion but at a reduced stiffness. Tailored to degrade over a specific timeframe, biodegradable implants could potentially mitigate the drawbacks of conventional stiff constructs and reduce the loading on adjacent segments. Using a validated lumbar sacral finite-element model, three variations at the L4-5 segment were analyzed: 1) moderate disc degeneration, 2) instrumented with a stand-alone cage and pedicle screw fixators, and 3) with the cage only after fusion. The intersegmental angles, disc stresses, and facet loads were examined Four motion tests, flexion, extension, bending, and twisting, were also simulated. Furthermore, six finite element models were developed in this study to simulate a spine with and without fixators. The spinal fixators used both titanium rods and biodegradable rods. The models were subjected to axial loading and pure moments. The range of motion (ROM), disc stresses, and contact forces of facet joints at adjacent segments were recorded. A 3-point bending test was performed on the biodegradable rods and a dynamic bending test was performed on the spinal fixators according to ASTM F1717-11a. The adjacent-segment disease was more severe at the cephalic segment than the caudal segment. After solid fusion and fixation, the increase in intersegmental angles, disc stresses and facet loads of the adjacent segments were about 57.6%, 47.3%, and 59.6%, respectively. However, these changes were reduced to 30.1%, 22.7%, and 27.0% after removal of the fixators. This was attributed to the differences between the biomechanical characteristics of the fusion and fixation mechanisms. The finite element simulation showed that lumbar spinal fusion using biodegradable implants had a similar ROM at the fusion level as at adjacent levels. As the rods degraded over time, this produced a decrease in the contact force at adjacent facet joints, less stress in the adjacent disc and greater loading on the anterior bone graft region. The mechanical tests showed the initial average fatigue strength of the biodegradable rods was 145 N, but this decreased to 115N and 55N after 6 months and 12 months of soaking in solution. Also, both the spinal fixator with biodegradable rods and with titanium rods was strong enough to withstand 5,000,000 dynamic compression cycles under a 145 N axial load. Fixation superimposes a stiffer constraint on the mobility of the bridged segment than fusion. The current study suggested that the removal of spinal fixators after complete fusion could decrease the stress at adjacent segments. Through a minimally invasive procedure, we could reduce secondary damage to the paraspinal structures while removing the fixators, which is of upmost concern to surgeons. In addition, the results of this study demonstrated that biodegradable rods may present more favourable clinical outcomes for lumbar fusion. These polymer rods could not only provide sufficient initial stability, but the loss in rigidity of the fixation construct over time gradually transfers loading to adjacent segments, avoiding the second surgery, which to remove the fixators. In addition, minimally invasive spine surgery has become increasingly popular in clinical practice, and it offers patients the potential benefits of reduced blood loss, wound pain, and infection risk and also diminishes the loss of working time and length of hospital stay. However, surgeons require more intraoperative fluoroscopy and ionizing radiation exposure during minimally invasive spine surgery for localization, especially for guidance in instrumentation placement. In addition, computer navigation is not accessible in some facility-limited institutions. We demonstrated a method for percutaneous screws placement using only the anterior-posterior trajectory of intraoperative fluoroscopy in patients who received posterior fixation with percutaneous pedicle screws for thoraco-lumbar degenerative disease or trauma. We retrospectively reviewed the charts of consecutive 670 patients who received 4072 pedicle screws between December 2010 and August 2015. Another case series study was conducted prospectively in three additional hospitals, and 88 consecutive patients with 413 pedicle screws were enrolled from February 2014 to July 2016. The fluoroscopy shot number and radiation dose were recorded. In the prospective study, 78 patients with 371 screws received computed tomography at 3 months post-operatively to evaluate the fusion condition and screw positions.