牙科超音波器械之頻率與幾何形狀對振動模態及切削效能之效應--有限元素分析及實驗驗證

Abstract

超音波器械已廣泛用於牙科領域，使用上以25至40 kHz為頻率範圍，其中共振頻率與振動模態是器械設計與使用的重要環節，有限元素分析雖然尚未在牙科超音波器械振動呈現具體成果，但已成功應用於各類研究，因此，本研究以有限元素分析探討牙科超音波器械振動，分析機械性質與幾何型態對器械共振頻率與振動模態的影響，並檢視臨床應用結果。 本研究共分五部分，第一部分建立有限元素分析模組探討密度、楊式模數及卜松比對共振頻率與振動模態的影響，超音波器械結合換能器的有限元素模組經過實驗量測驗證後可預測20至50 kHz的共振頻率與振動模態，選擇分析範圍內最高振幅模態比較後可發現密度增加共振頻率下降而楊式模數增加共振頻率則上升，波松比則不對共振頻率造成影響，另外密度增加同時也造成振幅下降。 第二部分以有限元素分析幾何型態對器械共振頻率與振動模態的影響，針對設計用於牙科矯正器移除的超音波器械，在長度、直徑、彎曲角度等幾何型態進行分析，比較共振頻率與振幅後選出表現最佳的幾何外型。 第三部分以有限元素分析選出幾何外型的器械進行陶瓷與金屬矯正器移除與齒質表面殘存樹脂清除實驗，結果顯示超音波移除矯正器在移除矯正器時間和傳統方式沒有顯著差異，但超音波移除可顯著降低陶瓷矯正器被破壞，同時減少清除殘存樹脂的花費時間也顯著少於傳統方式，因此超音波器械可成為去除矯正器與清除殘留樹脂的另一選擇。 第四部分以動物實驗探討不同共振頻率對牙髓組織的影響，在組織切片下35 kHz和83 kHz兩組超音波器械共振頻率結果相似且優於傳統高速手機，由於高頻振動可降低振動幅度提升穩定度，因此可做為日後應用上的考量。 最後以臨床實驗評估下顎第三大臼齒拔除過程中使用超音波器械和傳統高速手機對術後癒合的比較，觀察牙周囊袋深度與牙周附連喪失情形發現，使用超音波器械拔除下顎第三大臼齒有助於傷口初期復原，因此在此應用中超音波器械有取代傳統高速氣動式器械的潛力， 經過系列研究，我們建立牙科超音波器械共振頻率和振動模態的有限元素分析模組，對密度、楊式模數、波松比和幾何型態對共振頻率和振動模態的影響提出初步分析。並以體外實驗驗證有限元素分析之牙科超音波器械在牙科矯正器移除的效用，以動物實驗對不同超音波器械頻率對牙髓組織影響有了初步了解，並由人體實驗發現超音波器械可有利於下顎第三大臼齒拔除傷口的初期癒合。 綜合以上實驗，未來牙科超音波器械的共振頻率及振動模態可經由有限元素分析模擬，而後設計各項應用於臨床治療的器械。

Currently, ultrasonic instruments are widely used in the dental field. Evaluating the resonant frequency and vibration mode is important when designing an ultrasonic instrument. Presently, finite element analysis (FEA) is successfully and widely used in dental research. However, the FEA results of dental ultrasonic instruments are insufficient because of the variety observed among ultrasonic instruments. The aims of this study are to build an FEA model of the resonant frequency and vibration mode of a dental ultrasonic instrument, to evaluate the effect of mechanical properties and geometry on the dental ultrasonic instrument vibration, and to demonstrate the application of the ultrasonic instrument for use in a dental clinical setting. In the first part of this study, we generated an FEA model to evaluate the resonant frequency and vibration mode of a dental ultrasonic instrument. The effect of mechanical properties on the vibration of the instrument was observed. The simulation of a model with an instrument and transducer combined showed that the resonant frequency and vibration mode occurred between 20 and 50 kHz and could recapitulate the experimental measurements. The analysis of the vibration mode with the highest vibration amplitude indicated that an increase in density decreased the resonant frequency, whereas an increase in the Young’s modulus increased the resonant frequency. Changing the Poisson’s ratio did not affect the resonant frequency. In contrast, an increase in density decreased the vibration amplitude. The effect of mechanical properties should be noted when designing a dental ultrasonic instrument. In the second part of this study, we used FEA to simulate the resonant frequency and vibration mode of a specific designed ultrasonic orthodontic bracket removal instrument. The geometric parameters, including length, curvature, and diameter, were evaluated. The proper geometry for the instrument application was determined after comparison of the resonant frequency and vibration mode. The ultrasonic instrument design determined by the FEA evaluation was used for orthodontic ceramic and metal bracket debonding and residual resin removal in the third part of this study. The results indicated that there was no significant difference in bracket debonding time between the ultrasonic method and the conventional plier method. The ultrasonic method could significantly reduce the ceramic bracket breakage compared with the conventional method. Similarly, the ultrasonic method significantly decreased the procedure time to remove the residual resin compared with the conventional carbie low-speed air-turbine instrument method, and a smoother enamel surface after residual resin removal was observed. These results indicate that the ultrasonic method maybe a better choice for this orthodontic application. In the fourth part of this study, we designed an animal model to evaluate the effect of different resonant frequencies on the pulp-dentin reaction. The analyzed ultrasonic instruments had resonant frequencies at 35 and 83 kHz. The tooth chamber was perforated in the two resonant frequency experimental groups and in a high-speed air-turbine instrument group, which served as the control. The results indicate that there was no difference between the two resonant frequency ultrasonic preparations and that both ultrasonic resonant frequency groups had a better result compared with the control group. A higher resonant frequency could reduce the vibration amplitude and enhance the precision of the preparation. Therefore, increasing the resonant frequency has much potential when a precise and safe approach in cavity preparation is needed. In the final part of this study, we used an ultrasonic instrument in a clinical trial. The wound healing after mandibular third molar extraction with an ultrasonic instrument and a conventional high-speed air-turbine instrument was evaluated through the probing depth and examination of the loss of attachment. The mandibular third molar extraction with an ultrasonic instrument had better initial wound healing compared with the high-speed air-turbine instrument. Use of the ultrasonic instrument provides another treatment option for the mandibular molar extraction procedure. In this study, we provide an FEA method to evaluate the resonant frequency and vibration mode of the dental ultrasonic instrument and some suggestions and warnings on the influence of the mechanical properties and geometry on the ultrasonic instrument vibration. In addition, an ultrasonic orthodontic bracket removal method was introduced and demonstrated in this study. The pulp-dentin reaction in ultrasonic instruments with different resonant frequencies was evaluated and compared in an animal study, and the benefit of the ultrasonic instrument in mandibular third molar extraction was demonstrated.