流體動力學理論(Hydrodynamic theory)是目前牙科中最為廣泛接受的牙齒疼痛理論機制。日常生活與牙科治療中，當牙齒受到外界的刺激，會導致牙髓腔內液體產生流動，引起牙齒疼痛。本研究將利用流固耦合(Fluid-Structure Interaction)的數值模擬分析方法探討牙齒受外界刺激後牙髓腔內液體流動的情況。本研究主要分成四部分:(1)利用體外實驗的方式來評估當牙齒受到外界刺激時，牙髓腔內的液體流出體積，並與流固耦合分析結果作比對。此實驗方法為將大臼齒根部移除，並將牙髓腔連接到毛細的小管並充滿水。給予大臼齒表面三個不同力量大小的垂直力(50N, 100N和150N)，量測毛細管中液體的移動的情形。建立與實驗相同的三維數值模型，並使用流固耦合分析驗證當牙齒受到不同大小的外力負載後，模擬結果與實驗的正確性。(2)建立完整單牙根小臼齒的流固耦合模型，且給予牙齒咬合面上，垂直向下不同負載速度的外力(負載大小皆從0N逐漸增加到100N)，並評估對於牙髓腔內液體的影響。(3)使用相同完整單牙根小臼齒的流固耦合模型，分別給予牙齒不同方向負載，並評對於牙齒牙髓腔內液體的影響。(4)探討牙齒咀嚼不同材料性質的食物，與咀嚼時不同的咬合速度時，對於牙齒牙髓腔內液體的影響。
研究結果顯示，流固耦合模擬後的結果與體外實驗結果趨勢相似，當給予牙齒的外力後，會導致牙髓腔內液體流出，當給予的力量越大，從牙髓腔流出的液體體積越多。當施以不同負載速度的外力後，也對於牙髓腔內液體速度有影響。當牙齒受到不同方向的外力負載後，水平力不僅會使牙齒結構容易產生變形而斷裂，也會導致腔內液體流動速度較快而使牙齒較容易產生疼痛，這應該是水平力產生較大的彎曲效應所造成的。當牙齒咬到高楊氏係數的食物時，會產生牙齒高應力與大變形，在牙齒骨頭上有較高的反作用力，而且在牙髓腔內，液體的流動速度也較快，另外快速地咀嚼食物時也會導致牙髓腔內液體的流動速度較快，因此導致牙齒的疼痛。本研究，為首次利用流固耦合模擬提供的完整資訊，來評估牙科生物力學。

Hydrodynamic theory is the most widely accepted theory explaining dental pain. An external stimulus on teeth during daily activities or clinical dental treatments may cause fluid flow in the dental pulp and induce dental pain. This study used the fluid-structure interaction (FSI) to simulate the fluid flow in dental pulp when teeth are subjected to external stimuli. This study was divided into four parts. Firstly the fluid flow behavior in the pulp chamber resulting from external mechanical stimulus was evaluated through in-vitro, and compared with the corresponding FSI simulation. In the experiment the root of one molar tooth was removed. The pulp chamber was filled with water and connected to a capillary. Vertical forces, 5N, 100N and 150N respectively were given on tooth crown. The fluid movement in the capillary was observed. Corresponding 3D model was created in FSI simulation to compare with experimental results. The second part of this study created an intact premolar tooth FSI model. Various loading rates of vertical, along occlusion direction, transient forces, from 0 to 100 N, were applied on the occlusal surface to simulate the fluid flow responses. The third part, using the same premolar model simulated dental intrapulpal responses under transient force, from 0 to 100 N, with various directions. The fourth part investigated the effects of food property, elastic modulus, and chewing speed on the dentinal fluid flow during mastication.
The results showed that FSI simulation results are similar as that of the experimental results. The external compression loading resulted in pulp fluid outflow. The force magnitude influences the fluid outflow volume, while the loading rate affects the fluid flow velocity, at both coronal pulp wall and radicular pulp. For the effect of force direction, the horizontal force not only increases the risk of tooth structure failure but also enhances the fluid flow velocity at the coronal pulp, which increasing the possibility of tooth pain. This should due to the large bending effect generated by this horizontal force. Masticating hard food, high elastic modulus, would induce high stress and large deformation on the tooth structure which transferred a high reaction force on the bone. This would cause faster dentinal fluid flow within the pulp. Combining with the fast chewing speed, the hard food particles can easily cause the fluid flow velocity in the radicular pulp reaching the tooth pain threshold, triggering dental pain. In conclusion, it is demonstrated, for the first time, that the FSI simulation can provide more complete information for the evaluation of dental biomechanics.

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