Fracture Strength of Ceramic Bracket Tie Wings Subjected to Tension - 1
Original Article
Fracture Strength of Ceramic Bracket Tie Wings
Subjected to Tension
Gerald Johnson, DDSa; Mary P. Walker, DDS, PhDb; Katherine Kula, DDS, MSc
Abstract: One of the most common areas of ceramic bracket fracture is within the tie-wing complex.
When an archwire is ligated into position, tensile forces are placed under the tie wing. However, no study,
to date, has focused specifically on the fracture resistance of the tie-wing complex. The aim of this study
is to compare the tensile fracture strength of seven currently available ceramic brackets (Inspire, Fascination, Mystique, InVu, Clarity, Virage, and Luxi) as a function of bracket brand and bracket configuration,
semitwin vs true-twin. Based on a power analysis of pilot data, 10 maxillary central incisor brackets per
group were tested to failure with a tensile load placed directly under the distoincisal tie wing. The results
ranged from a maximum mean fracture strength of 147.71 (5.87) MPa with Fascination brackets to a
minimum mean fracture strength of 84.28 (7.01) MPa with Luxi brackets. The statistical analysis indicated
a significant effect on fracture strength as a function of bracket brand (P Ͻ .05) and that semitwin brackets,
Fascination, Mystique, and Virage, had significantly higher fracture strength than true-twin brackets, Clarity, InVu, and Luxi (P Ͻ .05). Interestingly, the only monocrystalline bracket in the study, Inspire, could
not be fractured using the investigation protocol. In fact, the steel ligature fixture wire would break before
tie-wing fracture at a mean fixture failure of 198.65 MPa. (Angle Orthod 2004;75:95-100.)
Key Words: Fracture resistance, Ceramic brackets
INTRODUCTION
Although esthetics is an obvious, inherent advantage of
ceramic brackets, they have do have several disadvantages,
such as high brittleness and increased susceptibility to fracture.1-3 Therefore, important mechanical properties related
to clinical function are tensile strength and fracture resistance.3 Tensile strength is defined as the ratio of the maximum load a material can support without fracture when
being stretched or elongated.5 The elongation of a ceramic
at failure is less than 1%, whereas that of stainless steel is
roughly 20%.3,4,6 The metallic bonding structure of metals
permits them to be significantly distorted without fracturing, even when compositional impurities exist or when
stress is concentrated at geometric interfaces.3,5 When stress
is placed on a metal, grain boundaries shift, which redistributes and relieves the stress. Shifting of atomic bonds
and redistribution of stresses does not occur within the ionic
bonding atomic arrangement of ceramics. Thus, ceramics
are much more brittle and have much lower tensile strength
than metals.3,5,6 In addition, ceramic bulk tensile strength is
also dependent on both the thickness of the material7 and
the material surface condition with associated defects.3,5,6,8
Thus, the manufacturing process itself plays a very important role in the strength of a ceramic.1-4,6,8-11 The presence
of pores, machining damage from milling, and cracks all
contribute to the reduction in fracture strength of a ceramic
bracket.4 Therefore, it is necessary to test and compare actual ceramic brackets, rather than bulk bracket materials.3,6
The ceramic orthodontic bracket was introduced in 1986,
and currently, many types are available from many orthodontic manufacturers.1-3 Ceramic brackets have been well
accepted by patients, clinicians, and the media for their
ability to mask the appearance of fixed orthodontic appliances. These new appliances also spurred an increased interest in orthodontics by the adult patient who found these
''white,'' ''clear,'' or ''invisible'' braces much more appealing than the traditional, more highly visible metal
brackets.
All ceramic brackets currently in production are made of
aluminum oxide (Al2O3), also called alumina.1-4 There are
2 types of ceramic brackets on the market, polycrystalline
alumina brackets, the most common type, and single-crystal
alumina or sapphire brackets.
Department Head, Orthodontics, US Navy, Naples, Italy.
Associate Professor, Director of Dental Biomaterials, University
of Missouri-Kansas City School of Dentistry, Kansas City, Mo.
c
Associate Professor, Chair, Department of Orthodontics and Dentofacial Orthopedics, University of Missouri-Kansas City School of
Dentistry, Kansas City, Mo.
Corresponding author: Mary P. Walker, DDS, PhD, Director of
Dental Biomaterials, UMKC School of Dentistry, 650 E 25th Street,
Kansas City, MO 64108 (e-mail: walkermp@umkc.edu).
a
b
Accepted: March 2004. Submitted: January 2004.
᭧ 2004 by The EH Angle Education and Research Foundation, Inc.
95
Angle Orthodontist, Vol 75, No 1, 2005
Table of Contents for the Digital Edition of Fracture Strength of Ceramic Bracket Tie Wings Subjected to Tension
Fracture Strength of Ceramic Bracket Tie Wings Subjected to Tension - 1
Fracture Strength of Ceramic Bracket Tie Wings Subjected to Tension - 2
Fracture Strength of Ceramic Bracket Tie Wings Subjected to Tension - 3
Fracture Strength of Ceramic Bracket Tie Wings Subjected to Tension - 4
Fracture Strength of Ceramic Bracket Tie Wings Subjected to Tension - 5
Fracture Strength of Ceramic Bracket Tie Wings Subjected to Tension - 6
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