Ya Shen


Relevant Degree Programs


Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - Nov 2020)
Dynamics of killing biofilm bacteria (2013)

Oral bacteria are the main cause of common oral diseases such as caries, periodontal infection and root canal infections. Bacteria in nature survive predominantly as biofilms, which are complex microbial communities composed of populations of microorganisms adhered to living or non-living surfaces and embedded in a self-produced matrix of extracellular polymeric substance (EPS). Various biofilm models have been developed to simulate the real situation in nature. In this thesis, I studied a new in vitro biofilm model, and examined the antimicrobial efficacy of current as well as newly developed endodontic irrigants/protocols against planktonic and biofilm bacteria. Single and multispecies biofilms were grown on sterile hydroxyapatite and dentin discs coated with bovine dermal collagen Type I . Transmission electron microscopy (TEM) was used to examine the biofilm microorganisms. The antimicrobial effect of sodium hypochlorite (NaOCl), iodine potassium iodide (IPI), chlorhexidine (CHX) and a new disinfecting agent (QMiX- a mixture of CHX, EDTA, and a detergent) was evaluated. The antimicrobial strategies included in the studies were photoactivated disinfection (PAD) and its experimental modifications. Biofilms at different stages of maturation were exposed to various antibacterial agents, and the killing of biofilm bacteria was observed using viability staining and confocal laser scanning microscopy (CLSM).The new in vitro biofilm model had similarities to in vivo biofilms, as described in the literature. This biofilm model reached maturation between two and three weeks. Mature biofilms were less sensitive to disinfecting agents than young biofilms. The time required for the biofilms to become resistant to disinfecting agents (maturation) was not dependent on the source of biofilm bacteria or the type of disinfectant used. Modified photoactivated disinfection was up to almost twenty two times more effective in killing biofilm bacteria than conventional PAD and up to almost eight times more effective than the commonly used endodontic irrigants. A new endodontic irrigant, QMiX was more effective in killing planktonic and biofilm bacteria than 2% CHX, BioPure MTAD (a mixture of tetracycline isomer, an acid, and a detergent), and 1% and 2% NaOCl.The new biofilm model seems promising for testing and developing efficient methods to eradicate oral biofilm bacteria.

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Master's Student Supervision (2010 - 2018)
Cyclic fatigue of ProTaper Gold in single and double curvature canals (2018)

Objectives: This study aimed to evaluate and compare the fatigue resistance of ProTaper Gold (PG) and ProTaper Universal (PU) in artificial single and double curvature canals in 5% sodium hypochlorite (NaOCl) at body temperature. Methods: PG and PU files (size F1) were subjected to fatigue tests inside two different custommade ceramic artificial canals. The first one was a single curvature canal (group 1: 60° curvature, 5 mm radius) and the second was a double curvature canal (group 2: first [coronal] curve of 60° curvature and 5-mm radius and the second one [apical] of 30° curvature and 2 mm radius). The artificial canals were milled in an InCoris ZI zirconium oxide disc (Dentsply Sirona, Bensheim, Germany) using the inLab MC X5 Digital CAD/CAM System (Dentsply Sirona). The first 19 mm of each file tip was introduced into the artificial canal which was immersed in either distilled water or 5% NaOCl at body temperature (37oC). The total number of cycles to failure (NCF) was recorded, and the length of the detached fragments was measured. Data were analyzed using t-test and multiple linear regression analyses. Results: The fatigue performance of PG is better than that of PU (P
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Effect of torsional and cyclic fatigue preloading on the fracture resistance of HyFlex EDM NiTi files (2017)

Objective: Endodontic nickel-titanium (NiTi) files are submitted to a combination of cyclic fatigue and torsional stresses when used in a root canal, which could lead to their fracture. The purpose of this study was to evaluate the effect of various degrees of cyclic fatigue preloading on the torsional failure and of torsional preloading on the cyclic fatigue life of heat-treated HyFlex EDM NiTi files (Coltene/Whaledent, Altstätten, Switzerland).Methods: The mean number of cycles until failure (Nf) of HyFlex EDM and HyFlex CM NiTi (Coltene/Whaledent) files (size #40, taper 0.04 for both files) was examined in a 5mm radius and 60° single curve. Torque and distortion angle at the failure of new instruments and instruments stressed to 50% and 75% of the Nf were measured according to ISO 3630-1. Other new files were preloaded at 5%, 15%, 25% and 50% of the mean distortion angle before the fatigue test. After torsional preloading, the Nf was examined. The fracture surface of each fragment was examined with a scanning electron microscope.Results: The fatigue resistance of HyFlex EDM files was higher than that of HyFlex CM files (P
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The effectiveness of heated sodium hypochlorite on Enterococcus faecalis in infected dentinal tubules (2017)

Introduction: The goal of chemical irrigation in endodontics is to maximize the reduction of microbes and necrotic tissue remnants in the root canal system. Enterococcus faecalis is most frequently associated with persistent endodontic infections. Sodium hypochlorite (NaOCl) is the most commonly used root canal irrigant. Heating NaOCl has a positive effect on the tissue dissolving abilities of the solution. Objective: The aim of this study was to compare the killing effectiveness of two different sodium hypochlorite concentrations (2 % vs. 5.25 %) at two different temperatures (20 °C vs. 60 °C) against two strains of E. faecalis biofilm at different ages of maturation (3 days vs. 3 weeks), in a previously described dentin block model. Hypotheses: Temperature, concentration, the exposure time, the age of the biofilm nor the strain of E. faecalis have no effect on the killing efficacy of NaOCl on E. faecalis.Methods: Dentin blocks were prepared from human root dentin. Two E. faecalis strains were introduced into dentinal tubules by centrifugation and incubated to form biofilms. After 3 days and 3 weeks of growth, the E. faecalis dentin biofilm samples were exposed to sterile water, 2 %, and 5.25 % NaOCl for 3 and 10 minutes at room temperature (20 °C) and at 60 °C. After the exposure, the proportions of killed bacteria in dentin canals were assessed by viability staining and confocal laser scanning microscopy.Results: The killing efficacy of E. faecalis in dentin tubules was affected by an increase in temperature of the NaOCl solution. The concentration of the NaOCl solution and exposure time to the irrigant played a role in the killing efficacy of NaOCl. Overall, 5.25 % NaOCl demonstrated a greater effect on the killing efficacy on E. faecalis, except when NaOCl is used at 60 °C with an exposure time of 10 minutes. There was no significant difference (p>0.5) between the two different strains of E. faecalis or between the 3-day and 3-week old biofilms in their sensitivity to the bactericidal effect of NaOCl.Conclusion: Bacterial killing by NaOCl is enhanced by an increase in temperature and concentration.

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The effect of cyclic fatigue on the hardness of new NiTi endodontic files : a nanoindentation study (2016)

Objective: The purpose of this study was to compare the flexibility and cyclic fatigue of ProTaper Universal (PTU; Dentsply Tulsa Dental Specialties, Tulsa, OK) and ProTaper Gold (PTG; Dentsply Tulsa Dental Specialties, Tulsa, OK) in relation to their phase transformation behavior, as well as to determine the effect of cyclic fatigue on the nanohardness with a nanoindentation method. Hypotheses: PTG and PTU have similar flexibility and fatigue resistance. Cyclic fatigue has no effect on the hardness of both PTG and PTU NiTi rotary endodontic instruments.Methodology: PTU and PTG instruments were subjected to rotational bending at a curvature of 40° and a radius of 6mm. The number of cycles to fracture (NCF) was recorded. According to the ISO 3630-1 specification, 45° bending tests was used to determine the flexibility. Unused and fractured instruments were studied by differential scanning calorimetry. The hardness and modulus of elasticity of new files, fractured instruments and instruments stressed to 50% of the NCF for sizes S1, F1 and F2 were measured with the use of a nanoindenter. Results: PTG had a cyclic fatigue resistance superior to PTU in all sizes (P <.001 the="" fractured="" files="" of="" both="" ptu="" and="" ptg="" showed="" typical="" fracture="" pattern="" fatigue="" failure.="" bending="" load="" results="" for="" were="" significantly="" lower="" than="" that="" differential="" scanning="" calorimetry="" analyses="" instruments="" had="" a="" higher="" austenite="" finish="" temperature="" there="" no="" significant="" differences="" in="" between="" unused="" subjected="" to="" process.="" statistically="" nanohardness="" elastic="" modulus="" groups="" conclusions:="" within="" limitation="" this="" study="" more="" flexible="" resistant="" files.="" exhibited="" different="" phase="" transformation="" behavior="" which="" may="" be="" attributed="" special="" heat="" treatment="" history="" instruments.="" process="" effect="" on="" hardness="" niti="" instrument.="" suitable="" preparing="" canals="" with="" sudden="" curvature.="">
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Influence of cyclic torsional loading on the fatigue resistance of K3XF instruments. (2015)

Objective: To evaluate the torsional and cyclic fatigue behavior of post-machining heat-treated K3XF and conventional K3 nickel titanium (NiTi) instruments. Methodology: New K3XF and K3 files size 25/0.04 (n = 15) were tested in torsion and fatigue tests until fracture to determine the mean number of cycles to failure (NCF) and torque to failure. The cyclic torsional loading experiment was conducted; K3XF and K3 files (n = 30 in each group) were programmed to repeatedly rotate from zero angular deflection to 180° and then return to zero torque. Each rotation was defined as one cycle. Each file was subjected to 10 cycles of torsional loading. Fifteen files from each group were subsequently tested in torsion until fracture. Also, fifteen files subjected to cyclic torsional loading were examined using a three-point bending apparatus to obtain the mean number of cycles to failure. The fracture surface was examined with a scanning electron microscope. The crack-initiation sites and the percentage of dimple area of the whole fracture cross-sectional area were recorded.Results: There was no statistically significant difference between the new K3 and K3XF instruments in the maximum torque or maximum angular deflection. For the previously cycled files, K3XF demonstrated higher torque at fracture values than K3 Instruments (P
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Low-cycle cyclic fatigue properties of novel nickel-titanium rotary endodontic instruments in the single- and double-curvatures. (2015)

Introduction: This in vitro study aimed to evaluate and compare the fatigue resistance of ProFile Vortex® (VX) and Vortex Blue® (VB) files in two different artificial double curvature canals (DC1 and DC2) and in an artificial single curvature canal (SC). The bending moment of VX and VB was assessed.Methods: The bending moment (g·cm) was used to measure flexibility of VX and VB (size 25/.04, length: 25mm) according to ISO 3630-1 specifications. Both files types were tested for cyclic fatigue failure inside canals containing: a single curvature (SC: 60⁰ curvature, 5 mm radius) and two canals with different double curvature (DC); [DC1: coronal curvature of 60⁰ and 5 mm radius, and apical curvature of 30⁰ and 2 mm radius; DC2: coronal curvature of 60⁰ and 5 mm radius, and apical curvature of 60⁰ and 2 mm radius]. The number of cycles to failure (NCF) was recorded and the fracture surface of all fragments was examined with a scanning electron microscope (SEM) to confirm cyclic fatigue failure and for qualitative analysis of pattern of fracture.Results: VX and VB followed slightly different trajectories in the identical canals, especially in double curvature canals. The mean bending moment value was significantly lower for VB than for VX (p .05). In the SC group, VB had NCF superior to VX (p .05). Multiple crack origins were observed for the majority of files fractured in DC1 and DC2 canals. Conclusions: DC1 and DC2 canals demonstrated a more stressful and challenging anatomy than the SC canal for VX and VB. In double curvature canals, degree of curvature and radius, and the file’s flexibility may affect the mean NCF.

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