牙齒復形後之阻尼性質對受力反應之影響 －利用動態力分析黏彈性有限元素模

1. 牙齒復形後之阻尼性質對受力反應之影響－利用動態力分析黏彈性有限元素模牙齒復形後之阻尼性質對受力反應之影響－利用動態力分析黏彈性有限元素模 • 牙齒復形後之受力反應可表現在牙齒之抗斷裂性上，而牙齒抗斷裂性和復形材與牙齒間的鍵結強度，及剩餘牙齒結構多寡有關。然而，牙科復形材影響牙齒復形後受力反應的確切因素尚未被完整討論。物體阻尼性質與物體受到衝擊力時所造成的破壞程度有關。在本研究體外實驗中，進行測量並比較牙齒復形前後之阻尼比，在方法上選擇上顎小臼齒，依布雷克氏第一級（Black’s Class I）窩洞修形成長寬高各為4公釐之窩洞後，依次填入銀粉、光聚合玻璃離子樹脂及光聚合複合樹脂。填入後以模態測試（modal testing）測量牙齒復形前後之阻尼比，並據此量測復形材之阻尼性質，作為建立三維有限元素模型之輸入條件。接著以此有限元素模型分析牙齒受動態力作用後之應力大小及分布。結果發現牙齒復形前之阻尼比為0.120±0.020；以銀粉復形牙齒之阻尼比（0.081±0.022）較以玻璃離子樹脂（0.106±0.024）及複合樹脂（0.111±0.020）於統計上為低（p < 0.05）。在有限元素分析中，比較有無阻尼性質之最大應力變化率發現，牙齒修形前之最大應力變化率為0.11%，而以銀粉復形後有最高應力變化率（0.20%），以玻璃離子樹脂復形（0.12%）次之，以複合樹脂復形（0.10%）最低。比較牙齒復形前後於頰側咬頭內部接近遠心齒頸部處之最大等效應力值發現，牙齒未修形前為9.58 MPa；以銀粉復形後為8.15 MPa；以玻璃離子樹脂復形後為11.59 MPa；以複合樹脂復形後為9.77 MPa。結果顯示不同復形材因為有不同阻尼性質，的確會影響牙齒受動態力作用時之反應，因此在選擇牙科復形材時應考量其阻尼效應。

2. Effects of Damping Properties of Restored Teeth on Loading Response－Analysis of a Viscoelastic Finite Element Model under Dynamic Force • Loading response of restored teeth can be expressed by fracture resistance. Fracture resistance of restored teeth was found to correlate to bonding strength of restorative materials and the amount of remained tooth structure. However, the exact effect of the restorative materials on the loading response of restored teeth was still not discussed. Damping effect of a structure is associated with the degree of damage when the structure was sustained to a sudden impact. In our in vitro study, the damping ratios of teeth with and without restoration were detected and compared. Human maxillary premolars were chosen and prepared as Black’s Class I cavity (with geometry of 4x4x4 mm). The tested teeth were divided into three catalogues which restored with amalgam, glass ionomer, and composite resin, respectively. Modal testing was used to measure the damping ratios of the teeth before and after restorations. The mean damping ratio of the teeth before restorations is 0.120±0.020. The damping ratio of teeth restored with amalgam demonstrated a statistically lower value (0.081±0.022) than those of teeth restored with glass ionomer (0.106±0.024, p<0.05) and composite resin (0.111±0.020, p<0.05). These values were then input into a 3-D finite element model to calculate its viscoelastic response when subjected to a dynamic load. In the finite element analysis of this study, the change of maximum stress of models with and without damping properties was compared. The change ratio of the model with amalgam restoration (0.20%) was higher than those of teeth without restoration (0.11%), with glass ionomer restoration (0.12%) and with composite resin restoration (0.10%). The maximal equivalent stress of the point in buccal cusp near distal cervical area before and after restoration was compared. The values of teeth were 9.58 MPa (original), 8.15 MPa (amalgam restoration), 11.59 MPa (glass ionomer restoration), and 9.77 MPa (glass ionomer restoration). Our results demonstrated that different restorative materials have different damping properties. In addition, the damping properties really affect the loading response of the restored teeth. Accordingly, we suggest that the damping effects should be took into account when choosing restorative materials.