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THESIS ABSTRACTS

Differential scanning calorimetricdetermination of glass transition tem-peratures in orthodontic elastomericchains, in vitro and in vivoMichele Robin Renick, DMDPresented in partial fulfillment of the requirements for the Master of Sci-ence degree in the Graduate School of the Ohio State University, 1999

Plastic modules are widely used in clinical orthodontics,yet there is little fundamental information available on thestructure of these products. While it is reported that the plas-tic modules are polyurethanes, further details remain propri-etary. Understanding the structure of these polymers affordsinsights into their physical behavior, which then becomes aclinically relevant issue to the orthodontist. The practitionerrequires an elastic material that can provide sufficient forcedelivery for tooth movements such as space closure and cor-rection of rotations. The purpose of this study was to investi-gate the glass transition temperatures of the elastomericchains, in vitro and in vivo. The glass transition temperature(Tg) was determined with differential scanning calorimetry(DSC). Plastic modules in spool chain form were obtainedfrom 3 manufacturers: Rocky Mountain Orthodontics,Ormco, and G&H. Three different colored products (gray,red, and purple) of each brand were selected (N = 7) for eachpigment-brand combination. Each pigment-brand combina-tion was subjected to one of 3 states: as-received, in vitro as4 links stretched in the air to a length of 2.5 cm on a block for4 weeks, and in vivo stretched for 4 weeks. The length of 2.5cm was chosen for the block specimens to simulate the dis-tance one of these modules might be stretched from molar tocanine in the oral environment.

DSC thermograms (TA 910) were produced over a tem-perature range from approximately –60°C to 150°C, with aheating rate of 10°C/min. The glass transition temperatures ofthe specimens were determined by using the computer soft-ware associated with the DSC apparatus.

Results of the glass transition temperature determinationswere analyzed statistically with multiple nonparametricMann-Whitney-Wilcoxon tests because the data were not nor-mally distributed and exhibited wide variations in SDs, rang-ing from 0.5°C in some cases to 10.0°C in others. A post hocstepdown Bonferroni analysis followed, examining differ-

ences due to pigmentation within brands and differences dueto brands within each pigmentation category for the as-received samples. In addition, a similar post hoc stepdownBonferroni analysis was completed for the in vivo state. Therewere significant differences due to pigmentation found withinthe G&H brand but not within either the Ormco or RockyMountain Orthodontics brands in both the in vitro and in vivostates. For the as-received specimens, the Rocky MountainOrthodontics products had mean values of Tg ranging from–23.95°C to –21.07°C, whereas the Ormco and G&H prod-ucts had mean values of Tg ranging from –45.96°C to–38.67°C, suggesting a brand difference. In addition to differ-ences noted with the post hoc tests, all of the in vivo speci-mens demonstrated a second glass transition temperature notpresent in the in vitro states. Because the second glass transi-tion temperature was not present in the specimens stretched inthe air, this finding suggests that the second glass transition isnot due to stretching alone but to some factor in the oral envi-ronment.

The observed values of Tg are in good agreement withpublished values for polyurethanes. Results suggest thatthe Ormco and G&H products should have greater flexibil-ity than the Rocky Mountain Orthodontics productsbecause of their lower glass transition temperatures, withthe Rocky Mountain Orthodontics products being morerigid polymers. Significant differences in force-degrada-tion behavior would be expected for the different productsof each brand. Furthermore, pigmentation did not seem tomatter clinically within the Ormco or Rocky MountainOrthodontics brands, yet it may be a factor in the propertiesof the G&H products.

0889-5406/2001/$35.00 + 0 8/3/112993doi:10.1067/mod.2001.112993

Fracture characteristics, hardness, andgrain size of five polycrystalline aluminaorthodontic bracketsGiao (Robert) Ngoc Pham, DDSPresented in partial fulfillment of the requirements for the Master of Sci-ence degree in the Graduate School of the Ohio State University, 1999

While polycrystalline alumina ceramic brackets havesuperior esthetic qualities compared with stainless steelbrackets, there is concern about fracture of these ceramicbrackets during clinical manipulation and debonding. The

REVIEWS AND ABSTRACTS

Book reviews and article abstractsAlex Jacobson, DMD, MS, PhDBirmingham, Ala

American Journal of Orthodontics and Dentofacial Orthopedics Reviews and Abstracts 93Volume 120, Number 1

purpose of this study was to investigate the fracture charac-teristics and related properties of 5 currently marketed alu-mina maxillary central incisor brackets (.018 × .022-in slotwidth). The following products were selected: Clarity(Unitek/3M), Allure NSB (GAC), Intrigue (Lancer), Con-tour (Class One) and MXi (TP). Five samples of eachbracket were resin-mounted, and the bases were polishedwith diamond abrasives. Vickers indentations were made onthe bases, using a 1-kg load, to measure the hardness anddetermine the fracture toughness. Three sample brackets ofeach brand were notched with a diamond disk and fracturedto reveal the bulk microstructure. In addition, another sam-ple of each brand was left in the as-received condition forexamination of the bracket design. All specimens wereobserved under the optical microscope and then coated witha gold-palladium film and observed with a scanning elec-tron microscope. Values of Vickers hardness and grain sizewere compared with the use of analysis of variance(ANOVA) and the Ryan-Einot-Gabriel-Welsch (REGW)multiple range test.

Accurate fracture toughness values could not be deter-mined because intergranular fracture yielded cracks that werenot the required straight lines for the injection-molded Con-tour and MXi brackets, and gross chipping-out of grains (alsointergranular fracture) occurred for the conventionally moldedClarity, Allure NSB, and Intrigue brackets. The approximatemean fracture toughness value of the Contour brackets (4.6MPa·m

1⁄2) was significantly greater than of the MXi brackets(3.6 MPa·m

1⁄2). The approximate Vickers hardness of the Con-tour brackets (3230 kg/mm2) was also significantly greaterthan that of the MXi brackets (2970 kg/mm2), which was sig-nificantly greater than that of the Intrigue, Clarity, and AllureNSB brackets (ranging from 2500-2360 kg/mm2). The grainsize of the Intrigue brackets (11 µm) was significantly greaterthan that of the Allure NSB and Clarity brackets (both 8 µm),which was significantly greater than that of the MXi and Con-tour brackets (both 0.6 µm). Results suggest that fractureresistance varies for commercially available alumina brackets.

0889-5406/2001/$35.00 + 0 8/3/112991doi:10.1067/mod.2001.112991

A comparison of parental expectationsand values when seeking orthodontic careCeceilia Markham, DMDPresented in partial fulfillment of the requirements for the Master of Sci-ence degree in the Graduate School of the Ohio State University, 1999

When choosing orthodontic therapy, parents make thedecisions about treatment for their children. A knowledge ofparental concerns and expectations assists the orthodontist incommunicating with the parent, resulting in a more positiveorthodontic experience for the parent and child. The purpose

of this research was to assess parental expectations beforetheir child started orthodontic treatment and to determine ifparental expectations in an institutional setting, such as adental school, differ from parental expectations in a privatepractice setting. A research questionnaire posed specificquestions to parents in order to identify the specific expecta-tions and the value they placed on certain expected out-comes.

A valid and reliable questionnaire that assessed perceptionof orthodontic treatment need, dental severity, orthodontic treat-ment expectations, and the value placed on certain outcomes,was administered to 2 consumer groups. The institutional sam-ple consisted of 176 parents of children screened in the gradu-ate orthodontic clinic. The private practice sample consisted of153 parents of children screened in 10 private practices. Severalprivate practices were included to enhance the sample size.Demographic information, such as the parents’age, gender, eth-nicity, and annual income, was also included.

Results of the questionnaire indicated that parents ofinstitutional patients had a lower income ($20,000 to $39,000vs over $60,000), more children (2.6 vs 2.3), and traveled agreater distance (26.5 vs 11.6 miles). The institutional groupfathers achieved a lower level of education than the privatepractice group fathers (“some college” vs “post bachelor’s”).Although the mother was most likely to bring the child in fortreatment in both settings, the institutional group fathersaccompanied the child more (23% vs 13%) than the privatepractice group fathers. There were also more single parents(26% vs 10%) in the institutional group.

Four major subscales were evaluated. Each subscale hada likelihood section and an estimation of the value of anoutcome section. The results of the benefits subscale evalu-ation indicated that all of the parents expected many bene-fits from orthodontic treatment. However, the private prac-tice group parents expected more (6.06 vs 5.95). Althoughthis was statistically significant (P = .0023), the interval(0.11) between the 2 groups was too small to be consideredclinically significant. The results of the short-term riskssubscale evaluation indicated that the private practice groupparents believed that the likelihood of short-term risks weremore likely than the institutional group parents (4.6 vs4.32). This result was statistically significant (P = .0036)but not clinically significant. The private practice groupparents believed that the long-term risks of orthodonticswere worse than did the institutional group parents (6.29 vs5.73). This result was both statistically (P = .0001) and clin-ically significant. The inconvenience subscale evaluationwas also statistically (P = .0006) and clinically significant.The private practice group parents believed inconveniencesassociated with orthodontics were more bothersome thandid the institutional group parents (3.34 vs 2.81).

0889-5406/2001/$35.00 + 0 8/3/112995doi:10.1067/mod.2001.112995