Research: Class of 2006 (March)

Almeida, M.A. "Analysis of aerodynamically respirable dust generated from quartz containing orthodontic and dental composites utilizing air rotary abrasion" Orthodontic Thesis for M.S. degree, Oregon Health & Science University, March 2006. 


Introduction: Quartz has been cited by the International Agency for Research on Cancer (IARC) as a human carcinogen. Exposure to quartz containing aerodynamically respirable aerosols (particle size of 0.5-5.0 μm) is possible during air rotary abrasion to remove the adhesive from teeth following orthodontic bracket debond.  

Purpose: 1) characterize the particle size distribution of composite adhesive dust within an aerosol generated by air-rotary abrasion of quartz filled adhesives, 2) determine the potential for quartz exposure to the respiratory system, and 3) determine the effectiveness of dental masks in removing aerodynamically respirable dust (0.5 – 5.0 μm).  

Methods: Quartz-containing composites, Transbond XT (3M Unitek) and Light Bond (Reliance), were abraded into an aerosol using high speed air-rotary abrasion, and separated into size fractions using an 8-stage Cascade Impactor (Tisch, Cleves OH). In order to breakdown the distribution of dust particles, the average mass percent was calculated for each stage. The average filler percent per stage was determined by collecting the fractioned dust into porcelain crucibles and placing them in a 600oC furnace to vaporize the resin component. Separate trials using a Technol dental mask (Fisher Scientific, Hampton NH) covering the intake port were conducted using Transbond XT. The mass percent per stage, estimated number of particles per stage, and average filler percent per stage were analyzed and compared to the unfiltered Transbond trails. The data was analyzed using two-way ANOVA with Tukey's test.  

Results: The results showed that approximately 5% of the total dust generated was potentially aerodynamically respirable (particle size of 0.5 µm – 5 µm). The dust contained a decreasing percentage of quartz filler on average from 70-40% as particle sizes decreased. Calculations based on particle size and mass collected showed that billions of dust particles were generated in the aerodynamically respirable range. Use of a dental mask removed over 99% of the dust mass that was collected, however it displayed little ability to filter dust particles in the 2-5 µm size range and offered no filtering protection from particles smaller than 2 µm. Conclusion: Exposure to aerodynamically respirable dust containing quartz particles appears likely during orthodontic adhesive removal with a high-speed handpiece. The results showed that use of a dental mask did not protect operators from exposure to smaller sized particles which are the greatest risk for inhalation to deeper regions of the lungs. Further studies are needed to determine the extent of this exposure in clinical settings.  

Borgen, K.K. "The effect of curing light source on polymerization contraction stress and degree of conversion over time in an orthodontic adhesive" Orthodontic Thesis for M.S. degree, Oregon Health & Science University, March 2006.


Objective: The purpose of this study was to determine the effect of two different curing light sources on contraction stress and degree of conversion (DC) over time in an orthodontic adhesive.   

Methods: Contraction stress was determined with a contraction stress measurement (CSM) device in which the orthodontic adhesive, Transbond XT (3M Unitek), was placed between a glass plate and steel rod. The steel rod passed through a washer load cell. A halogen (Ortholux XT, 3M Unitek) and an LED (Ortholux LED, 3M Unitek) were the curing light sources tested. Light was introduced from the bottom of the glass plate and loads developed within the adhesive were transferred to the load cell. Contraction force was recorded for four different time intervals: 5 minutes, 10 minutes, 1 hour, and 24 hours. Degree of conversion of each specimen at the four time points was measured with FTIR. Data were analyzed with two-way ANOVA and Bonferroni's post-hoc testing at the 0.05 level of significance. Regression analysis was also performed between contraction stress and DC.  

Results: With equal light energy density the LED led to significantly greater contraction stress within the adhesive than the halogen unit (p<0.05). Contraction stress and DC values were significantly (p<0.05) greater after 24 hours compared to all other time points after initial light exposure. Irradiation condition did not significantly affect degree of conversion. The relationship between stress and DC was non-linear (r2 = 0.91).  

Significance: Spectral irradiance may play a significant role in polymerization efficiency. Greater contraction stress obtained with the LED, without a significant increase in DC, may adversely affect bond strength. Furthermore, other developing properties of the composite contribute to stress build-up – such as cross-linking and modulus development – and further research is needed to determine their possible clinical effect on bond strength.  

Hobson,K.A. "Patterns of dental care utilization among patients with temporomandibular disorders" Orthodontic Thesis for M.S. degree, Oregon Health & Science University, March 2006.


Objective: A limited number of studies suggest that TMD subjects utilize two to three times more health-care services than non-TMD subjects. This study used data from a large health maintenance organization to determine whether this increased pattern of care extends to dental treatment among TMD subjects.   

Methods: Subjects were enrolled with Kaiser Permanente Northwest (KPNW) between 1998 and 2003. For each subject who had a TMD diagnosis during the study period, two age and gender matched controls were selected. Dental treatment was categorized using CDT codes. Dental utilization rates during periods before and after TMD diagnoses were calculated based on age, gender, categories of treatment, and overall utilization, and the rates were compared using t-tests.   

Results: Overall and for most dental categories, TMD subjects utilized significantly more dental services than comparison subjects. This difference was between 10% to 20%, or about one additional dental procedure per year. A positive linear relationship existed between dental utilization rates and age, with a significant increase in utilization for each ascending age cohort. Females had higher dental utilization rates than males in TMD and non-TMD groups and in all categories of dentistry. After a TMD diagnosis, major differences in utilization were not found when compared to patterns of treatment prior to TMD.   

Conclusion: TMD subjects utilized 10% to 20% more dental services than non-TMD subjects. Gender and age were important factors, with increased utilization in females and older patients. In addition, TMD diagnosis did not affect dental utilization patterns, indicating that most TMD patients continue to seek routine dental care.   

Kang, D. "Evaluation of the Load to Failure of Fiber Reinforced Composite Orthodontic Bracket Tie Wings" Orthodontic Thesis for M.S. degree, Oregon Health & Science University, March 2006.


Introduction: Esthetic brackets may be provided to patients that desire a less noticeable orthodontic treatment. While the brackets are designed to be clear or tooth colored in appearance, they are not as dimensionally stable as traditional stainless steel brackets during clinical use and thus tooth movement would be less predictable. Frequent failures of esthetic brackets have been reported particularly within the tie wing complex.   

Objectives of this study were to: 1) design and fabricate a potentially esthetic prototype bracket made of a long glass fiber-reinforced polymer matrix composite, 2) evaluate the static load to initial failure of the tie wings in experimental and conventional orthodontic esthetic brackets (a polycarbonate and ceramic bracket), and 3) determine if the tie wing load to initial failure increased with an increase in fiber content.  

Methods: Brackets (n=6) classified in13 groups based on fiber fill and orientation were fabricated in molds made using a vinyl polysiloxane impression of a modified commercially available polycrystalline bracket (Mystique-GAC Intl, Islandia, NY). E-glass fibers (everStick®-ORTHO, Stick Tech Ltd. Turku, Finland) were incorporated in four different configurations within a di-methylmethacrylate polymer matrix. The amount of E-glass fiber used was doubled for each different configuration. The weight fraction (Wf) of 2 and 4 mm E-glass fiber was calculated to be 13.75% and 25.82%, respectively. The test groups consisted of: Group A- a commercial polycrystalline bracket (Mystique-GAC Intl, Islandia, NY), Group B- a commercial polycarbonate bracket (Vogue-GAC Intl, Islandia, NY), Group C- a commercial posterior composite (3M ESPE, Z100T) formed into a bracket, Group D- the pure resin (1:1:1 BisGMA, TEDGMA, UDMA) bracket, and Group E- a particulate (80% strontium) filled resin bracket. Groups F and J consisted of straight E-glass fibers placed perpendicular to the bracket base extending out into the tie wing. Groups G and K consisted of curved E-glass fibers running from the center of the bracket extending out into the tie wing. Groups H and L consisted of chopped E-glass fibers assorted in a random fashion throughout the base and tie wing. Groups I and M consisted of straight E-glass fibers placed in a mesial-distal direction within the tie wing. The brackets were tested to failure with a static load placed directly upon the incisal tie wing complex. The failed brackets were examined using a scanning electron microscope (SEM) where the accuracy of glass fiber placement and the failure mechanisms was assessed.   

Results: SEM images revealed that the E-glass fibers were able to be manipulated into various shapes for incorporation within the tie wing complex. The fracture line of the FRC prototype brackets progressed in a linear fashion until reaching the area enriched with E-glass fiber. Bracket failure results ranged from a maximum mean initial load to failure of 101.01±19.99 N with Group A (commercial polycrystalline bracket; Mystique-GAC Intl, Islandia, NY) to a minimum mean initial load to failure of 28.17±5.20 N with Group-B (commercial polycarbonate filled with 35% alumina bracket; Vogue-GAC Intl, Islandia, NY). Of the different prototype design configurations, Group-J (straight E-glass fibers (Wf = 25.82%) placed perpendicular to the bracket base extending into the tie wing) had the highest mean initial failure value (63.39 ±16.79 N). There was no statistically significant difference between the mean initial failures of the FRC groups at p<0.05. All of the FRC groups, except for groups H and L, had load to initial failure values that were greater than the pure resin control group D. The Group B brackets (commercial polycarbonate filled with 35% alumina) were found to fail with loads significantly less than all experimental groups with fiber incorporated except for utilizing random fiber orientation.   

Further studies on the amount and orientation of glass fiber are needed in order to investigate if fracture toughness can be improved , for example, placing fibers closer to site where initial fracture was found in this study. Additional research may lead to development of esthetic brackets made with long glass fiber-reinforced polymer matrix composite that have improved fracture testing properties.