The purpose of this study was to determine the effect of different surface conditions and different ligation methods on the fracture strength of ceramic brackets and the yield strength of metal brackets.

Ceramic brackets followed the Griffith model, which was used to interpret their fracture strength with respect to their surface condition. With surface damage (scratching), their fracture strength decreased, while metal brackets experienced work hardening and an increased yield strength. Undamaged single-crystal brackets had a higher fracture strength than undamaged polycrystalline brackets, but after scratching, their strength decreased to value near to that of polycrystalline brackets. The fracture strength of polycrystalline brackets was not affected by scratching. Thus, single-crystal brackets were more susceptible to surface damage than polycrystalline brackets.

Using an analysis of variance (ANOVA), a statistically significant difference was found between the strength of different bracket types. No significant difference was found between the strength of elastic ligated and wire ligated brackets. A significant difference was found between the strength of non-scratched and scratched brackets, with the non-scratched brackets having a higher strength.

Five different types of brackets (two polycrystalline, two single-crystal, and one metal) were tested under four categories. The four categories were: elastic ligation without scratch, elastic ligation with scratch, wire ligation without scratch, and wire ligation with scratch. A total of 200 brackets were tested, with each category containing 10 brackets from each type.

An acceptable testing method, which allowed the brackets to be tested in an accurate and reproducible manner, was developed. A "hard" bracket holding fixture was designed and attached directly to an Instron machine.

A torsional wire bending force, similar to the clinical torquing force placed on brackets, was used to test the failure strength of the brackets.

LLU Discipline





Graduate School

First Advisor

Joseph M. Caruso

Second Advisor

Garland E. Scott

Third Advisor

M. Toufic Jeiroudi

Degree Name

Master of Science (MS)

Degree Level


Year Degree Awarded


Date (Title Page)




Library of Congress/MESH Subject Headings

Orthodontic Appliances



Page Count

vii; 67

Digital Format


Digital Publisher

Loma Linda University Libraries

Usage Rights

This title appears here courtesy of the author, who has granted Loma Linda University a limited, non-exclusive right to make this publication available to the public. The author retains all other copyrights.


Loma Linda University Electronic Theses and Dissertations

Collection Website



Loma Linda University. Del E. Webb Memorial Library. University Archives