在下顎骨切除後使用固定植體贋復物重建之應力分析

Abstract

研究背景與動機 臨床上，部分下顎骨切除術(partial mandibulectomy)中的邊緣下顎骨切除術(marginal mandibulectomy)與整段式下顎骨切除術(segmental mandibulectomy)一直是以10公釐 (mm)的剩餘骨脊高度為選擇術式的判斷標準。在下顎骨進行邊緣切除手術後，會造成應力集中在缺損處造成發生骨折風險提高；在整段式下顎骨切除後，理想上也會為了美觀及功能使用腓骨皮瓣重建下顎骨；而為了患者生活品質，常利用牙科植體贋復物重建上述缺損區域，由於術後下顎骨剩餘骨脊高度減少植體固定贋復物增加，形成不良的植體長度/贋復物高度比例，可能會加劇應力集中，也有可能進而使下顎骨骨體應力、應變分布產生改變，影響我們對於下顎骨切除手術後風險的評估。

研究目的 以三維有限元素分析探討不同殘餘骨脊高度以及不同腓骨皮瓣重建時對於重建區域最大應變值、最大應力值的影響程度、相關性，藉以探討邊緣下顎骨切除手術以及腓骨皮瓣重建整段式下顎骨切除時以植體固定贋復物重建之後影響術後骨折風險的評估標準。

研究方法 本研究分為兩個部分，第一個部分利用三維有限元素模型分析下顎骨大範圍邊緣切除術後，模擬不同的剩餘骨脊高度加上固定植體贋復物，觀察經過邊緣切除術下顎骨在植體贋復物受力下，應變及應力分佈的情形，並用以評估手術後的骨折風險。而第二部分是模擬整段式下顎骨切除術後以單層腓骨皮瓣或雙層腓骨皮瓣重建，並放上固定植體贋復物，測試在植體贋復物受力下，下顎骨重建區應變及應力分佈的情形。實驗將電腦斷層掃描影像輸入ABAQUS 6.14-1建立下顎骨模型，模擬右側小臼齒至大臼齒區域(48 mm)被切除區，設定三種殘餘下顎骨脊高度(12mm、10mm、8mm)並在其上放置3隻直徑為4公釐 (mm)的植體，設定不同的骨內植體長度(6mm、8mm、10mm)和植體贋復物高度(20mm、18mm、16mm)，並以長48 mm、寬度及高度皆為5 mm的金屬桿相連。將下顎骨模型的海綿骨以及植體贋復物以十節點之四面體元素(C3D10)、皮質骨以三節點之三角形殼元素(S3R)網格化之後，探討在右側大臼齒咬合時，右側邊緣切除重建區，所受之最大拉應變(MTS)與最大壓應變(MCS)的位置及數值大小，並分別以3000 microstrain(MTS)與4000 microstrain(MCS)的應變門檻探討其發生骨折的風險。同時探討最大等效應力值(von Mises stress)的位置及數值，以120 MPa為突發性骨折(sudden mandible fracture)門檻探討其骨折風險。第二部分同樣是利用三維有限元素模型分析下顎骨邊緣切除術後以單層腓骨皮瓣和雙層腓骨皮瓣重建，並以相同大小的植體(直徑4mm)及植體金屬桿(5*5*48mm)作為贋復物，探討最大拉應變(MTS)、最大壓應變(MCS)以及最大等效應力值(von Mises stress)的位置及數值。

研究結果 第一部分的結果顯示殘餘骨脊越高時，拉應變、壓應變和應力會較小，最大拉應變以及壓應變都超過發生骨折風險的閾值，顯示雖然應力集中的位置雖然力量不足以發生下顎骨折，但是發生的應變仍有產生骨折的風險；而植體周圍骨的應變顯示有骨吸收風險。 第二部分的實驗結果顯示，在雙層腓骨皮瓣的重建中，應變及應力的大小較單層腓骨皮瓣重建低，而應力最大值亦低於發生骨折風險的閾值，但是發生的應變數值來看仍是有發生骨折的風險。在雙層腓骨皮瓣重建組別植體周圍骨應力和應變則皆為安全範圍。

研究結論 不論是在殘餘骨脊8公釐、10公釐、12公釐組別中的下顎骨重建，皆有發生骨頭微損傷的可能，並且有發生骨折的風險。其中10公釐和12公釐組別和8公釐組別相比應變及應力皆較低。植體周圍骨的應變皆大於骨頭屈服點的閾值，顯示未來有發生骨吸收的風險。總結以上結果顯示不能低估在邊緣性下顎骨切除後以植體贋復物重建的骨折以及植體周圍骨吸收的風險。單層腓骨皮瓣組別微應變的觀察中可以發現拉應變及壓應變都超過發生骨折風險的閾值，有引發骨頭機械性疲勞及骨頭微損傷的可能。雙層腓骨皮瓣重建組別中觀察到的應力大小皆小於骨頭的屈服點但是有一處應變仍大於屈服點。兩者組別中植體周圍骨應力及應變皆在骨頭屈服點的閾值之下因此植體重建是安全可行的重建方式。總結以上結果顯示，單層腓骨皮瓣重建仍有發生骨折風險，雙層腓骨皮瓣重建下顎骨以及其上的植體周圍骨內的應力及應變分布較單層腓骨皮瓣重建理想。

Background Clinically, partial mandibulectomy, including marginal mandibulectomy and segmental mandibulectomy, have used a residual bone ridge height of 10 mm as the criterion for selecting the surgical method. After marginal mandibulectomy, stress concentration at the defect site can increase the risk of fractures. In segmental mandibulectomy, fibula flaps are ideally used for mandibular reconstruction to restore aesthetics and function. For better quality of life, dental implant prostheses are commonly used to reconstruct the defect areas mentioned above. However, the decreased residual bone ridge height combined with the increased implant-supported prostheses, creates a poor crown/ implant ratio. This can exacerbate stress concentration and alter the stress and strain distribution in the mandible, affecting the risk of mandible fracture.

Objective The objective of this study is to use three-dimensional finite element analysis (FEA) to explore the impact and correlation of different residual bone ridge heights and various fibula flap reconstructions on the maximum strain and stress values in the reconstructed area. The study aims to establish risk assessment criteria for post-operative fractures following marginal mandibulectomy and segmental mandibulectomy reconstructed with implant-supported prostheses.

Materials and methods The first part utilizes three-dimensional finite element models to analyze the stress and strain distribution in the mandible after marginal resection, with different heights of residual bone and fixed implant-supported prostheses. This is used to evaluate the risk of mandibular fractures. The second part simulates segmental mandibulectomy followed by reconstruction with single-barreled or double-barreled fibula flaps, with fixed implant-supported prostheses, to investigate the strain and stress distribution in the reconstructed mandible. The models were established using CT scan images input into ABAQUS 6.14-1 to create the mandibular models, simulating the resected area from the premolars to the molars (48 mm). Three residual mandibular bone ridge heights (12 mm, 10 mm, 8 mm) were set, with three implants of 4 mm diameter placed on each, varying the intraosseous implant lengths (6 mm, 8 mm, 10 mm) and the implant prosthesis heights (20 mm, 18 mm, 16 mm). A metal bar (48 mm long, 5 mm wide, and 5 mm high) connected these components. Under a biting force applied to the right molars, the study investigated the location and magnitude of the maximum tensile strain (MTS) and maximum compressive strain (MCS), using thresholds of 3000 microstrain (MTS) and 4000 microstrain (MCS) to assess fracture risk. The study also explored the location and magnitude of the maximum von Mises stress, using 120 MPa as the threshold for sudden mandibular fracture risk. The second part of the study similarly used three-dimensional finite element models to analyze mandible reconstructed with single-barreled and double-barreled fibula flaps, using identical implants (4 mm in diameter) and implant metal rods (5*5*48 mm) for prostheses. The study examined the location and magnitude of maximum tensile strain (MTS), maximum compressive strain (MCS), and maximum von Mises stress.

Results The first part of the study indicated that higher residual bone ridges resulted in lower tensile strain, compressive strain, and stress. The maximum tensile and compressive strains exceeded the fracture risk thresholds, indicating a risk of fractures despite stress concentration being insufficient to cause mandibular fractures directly. The strain around the implants suggested a risk of bone resorption. The second part showed that the strain and stress levels in double-barreled fibula flap reconstructions were lower than those in single-barreled reconstructions, with stress values below the fracture risk thresholds. However, strain values still indicated a risk of fractures. The stress and strain around the implants in double-barreled fibula reconstructions were within safe limits.

Conclusion In mandible with residual bone ridges of 8 mm, 10 mm, and 12 mm, there is a risk of microdamage and fractures. The 10 mm and 12 mm groups had lower strain and stress compared to the 8 mm group. The strain around the implants exceeded the bone yield point, indicating a future risk of bone resorption. These findings underscore the potential risk of fractures and bone resorption around implants in mandible status post marginal mandibulectomy. In single-barreled fibula flap reconstructions, MTS and MCS exceeded the fracture risk thresholds, suggesting potential mechanical fatigue and microdamage to the bone. In double- barreled fibula flap reconstructions, stress levels were below the bone yield point, though one strain location exceeded the threshold. Stress and strain around the implants in both groups were within the bone yield point, making implant reconstructions a viable and safe option. Overall, double- barreled fibula flap reconstructions provided better stress and strain distribution in the mandible and surrounding implant bone compared to single-barreled reconstructions.