以三維有限元素法分析磁性附連體與球狀附連體於下顎覆蓋式義齒受力時之應力分佈

Abstract

摘要 下顎留下犬齒牙根以建構覆蓋式義齒是補綴牙科臨床上常見的術式，犬齒牙根加上附連體，則更可提供較佳的固持力。唯此時牙根是否受到合理的應力以避免牙根及牙周組織之受傷，則尚待釐清。為了解磁性附連體支撐覆蓋式義齒在支柱牙應力方面是否優於機械式附連體，宜以適當之方法客觀評估。 本研究的目的即在建立較接近真實情況之磁性附連體下顎覆蓋式義齒系統與球狀附連體下顎覆蓋式義齒系統三維有限元素模型，並藉由有限元素的數值方法，探討並比較兩種附連體支撐之覆蓋式義齒於犬齒、小臼齒、大臼齒等不同部位接受垂直及斜向45度50牛頓之咬合力下的應力分佈情形。 結果顯示磁性附連體可有效減少支柱牙之力矩，避免支柱牙受到不當的側方應力而損傷。反之，球狀附連體無法有效化減義齒的槓桿效應，容易造成支台齒的應力集中與破壞。就保護牙根與周圍組織而言，磁性附連體義齒優於球狀附連體義齒。此外也發現斜向咬合力會造成彎曲力矩而增加支柱牙與牙周韌帶、皮質骨的應力，如此可能造成支柱牙斷裂、皮質骨吸收或牙周撕裂等傷害。但是我們也發現，覆蓋式義齒在受不同方向的咬合力時磁性附連體義齒的移動量略高於球狀附連體。 基於上述結果可以推論：1.增加固持力是覆蓋式義齒的治療目標，但是固持力的增加相對會帶來牙根應力的增高。2.磁性附連體在減少牙根應力上有較佳的利益。磁性附連體義齒能合理的提供義齒的固持力並有效分散應力於殘存組織，是覆蓋式義齒治療一項很好的選擇。

Abstract Keeping lower canine roots is clinically common to construct overdentures; the canine roots connected with attachments can provide even larger retention. However, it needs further verifying whether stress is reasonably loaded on roots so that damages can be avoided to the roots and the surrounding tissues. In order to realize whether magnetic attachment-retained overdentures outperform mechanical attachment-retained overdentures in the stress on abutment teeth, we need to adopt an appropriate approach to make objective evaluation. This study is to investigate the stress distributions under occlusal loading of a ball attachment-retained overdenture and a magnetic attachment-retained overdenture. We built up a more reliable 3-dimensional finite element structural model for overdentures supported by two types of attachments, and to compare the differences in stress distribution on roots and surrounding tissues. The analytical approach of finite element is utilized to study and compare stress distributions of the 2 types of attachments; studies are designed to exert occlusal loading on the overdentures on canine、premolars、and molars, by 50 NT vertically and oblique 45 degree, respectively. The result reveals that a magnetic attachment can effectively reduce the moment of an abutment tooth when a denture is enforced with occlusal loading, and can accordingly avoid the damage to an abutment tooth from inappropriate lateral stress. On the contrary, a ball attachment cannot effectively alleviate the leverage effect, and will possibly result in stress concentration and therefore damage an abutment tooth. Besides, we found that oblique occlusal loading incurs bending moment, and consequently increases the stress among the abutment tooth, periodontal ligament and cortical bone, as might result in the fracture of abutment teeth, the resorption of cortical bone, PDL laceration, and etc. Nevertheless, we also find that the displacement of a magnetic attachment-retained overdenture is slightly higher than that of a ball attachment-retained overdenture. Based on the findings mentioned above, we can infer the following. First, retention enhancement is an overdenture treatment target; however, enhancing retention increases the stress on the roots. Secondly, magnetic attachments perform better in reducing stress on the roots. To sum up, a magnetic attachment-retained overdenture provides sufficient retention and can effectively disperse stress on the residue tissues; it is therefore a very good choice for clinical overdenture treatments.