髓腔固位冠已成為根管治療後臼齒復形的可靠替代方案，然而，其邊緣設計至今尚無統一標準可循。根據壓縮穹頂概念（compression dome concept），當復形體邊緣位於轉折平面（inflection plane）之下時，可能承受張應力，導致黏著界面產生滲漏（leakage）。本研究旨在探討髓腔固位冠不同邊緣設計對應壓縮穹頂觀念之應力分佈，對其受力時的生物力學表現與黏著界面滲漏程度的影響。本研究採用光學同調斷層掃描（optical coherence tomography, OCT）、數位影像相關法（digital image correlation, DIC）與掃描式電子顯微鏡（scanning electron microscope, SEM）進行分析。
本實驗使用32顆人類下顎大臼齒，所有樣本均完成根管治療，並嵌入圓柱形環氧樹脂中。根據髓腔固位冠邊緣與轉折平面關係位置及型態分為四組：1. AF組：邊緣位於轉折平面上方，採環箍設計（ferrule margin）；2. AB組：邊緣位於轉折平面上方，採對接設計（butt joint margin）；3. BF組：邊緣位於轉折平面下方，採環箍設計；4. BB組：邊緣位於轉折平面下方，採對接設計。髓腔固位冠以二矽酸鋰陶瓷（IPS e.max CAD HT A2）製作，並以樹脂黏著劑黏著，儲存於37°C蒸餾水中24小時後，進行OCT掃描黏著界面，並以SEM觀察邊緣完整性。之後樣本以 15°舌側傾斜方式固定於萬能測試機上，於頰側施加300 N負載同時拍攝影像，並應用DIC方法分析應變。再經10,000次、300 N動態受力測試後，再次進行OCT、SEM與DIC觀察，最終以 1 mm/min 速率加載至斷裂，記錄其斷裂強度與模式。
OCT結果顯示，受力前AF與AB組的黏著界面滲漏顯著高於BF與BB組；但就邊緣設計（ferrule與butt joint）則未有顯著差異。受力後，各組滲漏皆增加，且AF與AB組仍高於BF與BB組，但BF組頰側面受力後滲漏增加最為顯著。SEM的邊緣分析方面，受力前BF與BB組表現優於AF與AB組；受力後則以BF組破壞最為明顯。應變觀察結果顯示，轉折平面上主要為壓力應變區，張力應變主要在舌側齒頸部，符合壓縮穹頂模型。各組分析顯示，AF與AB組邊緣以壓力應變為主，而BF與BB組邊緣僅頰側20%為壓力應變區，其餘為張力應變（正值）。邊緣設計則無顯著影響。受力前後比較顯示，AF與AB組壓應變值於受力後下降，BF與BB組則無顯著變化。斷裂測試結果顯示，BF與BB組的斷裂強度顯著高於AF與AB組，而邊緣設計亦無統計差異。所有組別斷裂模式多為可修復性斷裂，且無顯著差異。
綜合分析結果，髓腔固位冠邊緣與轉折平面之相對位置對修復體的生物力學行為與邊緣穩定性具重要影響。當邊緣位於轉折平面之上（AF與AB組），因處於壓力應變區，受力後黏著介面與邊緣破壞較少；但由於復形體厚度較薄，斷裂強度亦較低。相對地，BF組受力後介面與邊緣破壞較高，但BB組之張力應變破壞較小。整體而言，本研究結果顯示髓腔固位冠生物力學表現，也驗證壓縮穹頂觀念與邊緣設計之相關性，可協助臨床醫師依據剩餘齒質狀況，選擇最合適的修復策略以提升療效與長期預後。

Endocrowns have become a reliable alternative for the restoration of endodontically treated molars; however, their margin design lacks consistent standard. According to the compression dome concept, margins located below the inflection plane may be subjected to tensile stress, potentially leading to interfacial leakage. This study aimed to investigate the influence of margin designs on the biomechanical behaviors and interfacial debonding of endocrowns after dynamic loading, using optical coherence tomography (OCT), digital image correlation (DIC), and scanning electron microscopy (SEM).
Thirty-two human mandibular molars received root canal treatment and were embedded in cylinder epoxy resin. They were divided into four groups to receive endocrowns of different types and locations of margin design: (1) AF: above the inflection plane, ferrule margin; (2) AB: above the inflection plane, butt joint margin; (3) BF: below the inflection plane, ferrule margin; (4) BB: below the inflection plane, butt joint margin. Endocrowns were fabricated from lithium disilicate ceramic (IPS e.max CAD HT A2) and cemented with resin cement. After 24-hour storage in 37°C distilled water, pre-loading analyses of interfacial debonding and marginal integrity were performed using OCT and SEM, respectively. Strain field pattens were analyzed using DIC under a 300 N static loading on the buccal cusp at a 15-degree lingual tilting. Subsequently, specimens underwent 10,000 cycles of dynamic loading (300 N), followed by a load test (300 N, 1 mm/min) to examine the failure loads and modes.
Before loading, interfacial debonding in Groups AF and AB was significantly higher than in Groups BF and BB. There was no significant difference between the ferrule and butt joint designs. After loading, leakage increased in all groups. The most significant increased leakage occurred in the buccal sides of Group BF. Regarding marginal integrity, Groups BF and BB showed good sealing before loading, whereas Group BF showed significantly increased decay after loading. The DIC analysis confirmed strain distributions consistent with the compression dome model, with a compressive zone above the inflection plane and a tensile zone at the contralateral cervical area. Quantitatively, AF and AB margins were primarily under compression (negative strain), while BF and BB margins were mostly under tension (positive strain). No significant effects from margin designs were observed. Strain values in Groups AF and AB significantly decreased after loading. The failure load of Groups BF and BB was significantly higher than that of Groups AF and AB, with no statistical difference between margin designs. All groups showed restorable failures after the fracture tests.
The location of the endocrown margin relative to the inflection plane significantly impacts the biomechanical behavior and interfacial bonding of the restoration. Coronally placed margins in AF and AB exhibited less debonding due to their sustained compression. However, they showed lower fracture strength due to reduced restoration thickness. Conversely, margins placed apically in BF and BB are at a greater risk of marginal degradation under functional loading. Group BB showed less adverse effects of tensile strain. Overall, these findings reveal the biomechanical behaviors of endocrowns and validate the role of the compression dome concept. This study may provide evidence for selecting optimal restorative strategies to improve clinical outcomes and long-term prognosis.

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