Introduction
Iatrogenic damage to the tooth surface in the form of enamel tearouts can occur during removal of fixed orthodontic appliances. The aim of this study was to assess debonded metal and ceramic brackets attached with a variety of bonding materials to determine how frequently this type of damage occurs.
Methods
Eighty-one patients close to finishing fixed orthodontic treatment were recruited. They had metal brackets bonded with composite resin and a 2-step etch-and-bond technique or ceramic brackets bonded with composite resin and a 2-step etch-and- bond technique, and composite resin with a self-etching primer or resin-modified glass ionomer cement. Debonded brackets were examined by backscattered scanning electron microscopy with energy dispersive x-ray spectroscopy to determine the presence and area of enamel on the base pad.
Results
Of the 486 brackets collected, 26.1% exhibited enamel on the bonding material on the bracket base pad. The incidences of enamel tearouts for each group were metal brackets, 13.3%; ceramic brackets, 30.2%; composite resin with self-etching primer, 38.2%; and resin-modified glass ionomer cement, 21.2%. The percentage of the bracket base pad covered in enamel was highly variable, ranging from 0% to 46.1%.
Conclusions
Enamel damage regularly occurred during the debonding process with the degree of damage being highly variable. Damage occurred more frequently when ceramic brackets were used (31.9%) compared with metal brackets (13.3%). Removal of ceramic brackets bonded with resin-modified glass ionomer cement resulted in less damage compared with the resin bonding systems.
Highlights
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Enamel damage was more frequent with ceramic bracket removal than metal brackets.
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Damage was less when ceramic brackets were attached with RMGIC than composite resin.
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Ceramic bracket fracture was greatest with 2-step etch-and-bond composite resin.
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The most common bond failure site was the bracket-bonding material interface.
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Maxillary lateral incisors had a greater risk of damage than central incisors and canines.
The aim of bracket debonding is to remove appliances and any bonding material from the teeth, restoring the original esthetics and contours while minimizing iatrogenic enamel loss during the procedure. Appliance and bracket removal does cause some damage to the tooth surface. This can occur at bracket removal if the enamel fails cohesively, resulting in cracks and tearouts, or it can occur during removal of adhesive remnants.
Many bonding materials and methods are available for the placement of orthodontic brackets. All of these have their own unique material properties that influence their mode of failure during orthodontic bracket removal. It is estimated that bonded orthodontic brackets need a bond strength of 5.9 to 7.8 MPa to be retained. Many bonding systems meet or exceed this requirement. For example, metal brackets bonded to enamel with conventional 2-step etch and bond with Transbond XT composite resin (3M Unitek, Monrovia, Calif) had a reported bond strength of 20.2 MPa, the strength of self-etching primer (SEP) with Transbond XT was 11.1 MPa, and the strength of Fuji Ortho LC resin-modified glass ionomer cement (RMGIC) (GC Corporation, Tokyo, Japan) was 13.6 MPa. Ceramic brackets may also have high bond strengths to enamel. Using a 2-step etch-and-bond technique, Heliosit composite resin had a reported bond strength of 24.25 MPa. Therefore, in some situations, the bond strength of the adhesive to the enamel may be higher than the cohesive strength within the enamel, resulting in cracks or tearouts at debonding.
Although enamel loss as tearouts during orthodontic bracket removal has frequently been reported in vitro, there are few clinical studies on this topic. Using backscattered scanning electron microscopy, Boyde reported that enamel tearouts were clearly observable on the bonding surfaces of clinically debonded ceramic orthodontic brackets attached with composite resin. However, the influence of bracket and adhesive type was not explored. A more recent, energy dispersive x-ray spectrometry evaluation of clinically debonded metal brackets attached with a 2-step etch-and-bond retained composite resin showed that as the amount of resin on the base pad increased, so did the amount of calcium (assumed to be enamel). This study did not report the overall frequency of damage or examine the extent of damage when ceramic brackets were used. Therefore, more clinical investigation into the frequency and extent of this problem is warranted to ensure that bracket-adhesive combinations are selected to minimize iatrogenic enamel loss as tearouts during bracket removal.
The aim of this clinical study was to determine the extent and frequency of iatrogenic damage to enamel when debonding metal and ceramic orthodontic brackets attached by various bonding materials in vivo.
Material and methods
Ethics approval for the study was obtained from the Human Research Ethics Committee (number 1136902) of the University of Melbourne in Australia. Patients close to finishing fixed orthodontic treatment were recruited from 5 private orthodontic practices. Practices were invited to participate if they met the following inclusion criteria: (1) the clinicians in the practice used either metal In-Ovation R brackets (GAC International, Bohemia, NY) or ceramic In-Ovation C brackets (GAC International); (2) they used 1 of the 3 following adhesive protocols: 37% phosphoric acid etch, Orthosolo bond (Ormco, Orange, Calif), and Transbond XT composite resin (3M Unitek); pumice with a cup, SEP (Transbond Plus Self Etching Primer, 3M Unitek), and Transbond XT composite resin; or pumice with a cup, Fuji Ortho Conditioner (10% polyacrylic acid), and RMGIC (Fuji Ortho LC encapsulated; GC Corporation); and (3) the clinicians needed to debond ceramic brackets using the sharp blades of the debonding pliers at the level of the base pad, placing a wedging force at the adhesive level, and debonding metal brackets using the debracketing instrument (444-761; 3M Unitek). This instrument has a wire hook that goes under a bracket wing and is stabilized against the adjacent enamel, enabling the bracket to be pulled off the adhesive and tooth in a safe and comfortable manner. It is a reproducible and standardized technique. The instrument cannot be used on ceramic brackets because of their brittleness.
Therefore, patients fell into 1 of the following 4 groups: MEC, metal bracket attached with Transbond XT composite resin bonded to the tooth with a 2-step etch-and-bond technique; CEC, ceramic bracket attached with Transbond XT composite resin bonded to the tooth with a 2-step etch-and-bond technique; CSEP, ceramic bracket attached with Transbond XT composite resin bonded to the tooth with SEP; or CGIC, ceramic bracket attached with RMGIC. The numbers of patients from each office are shown in Table I .
| Ceramic brackets ∗ | Metal brackets ∗ | |||
|---|---|---|---|---|
| CGIC † (n = 66 ‖ ) | CSEP ‡ (n = 144 ‖ ) | CEC § (n = 126 ‖ ) | MEC † (n = 150 ‖ ) | |
| Presence of enamel | ||||
| No | 80.3 | 61.8 | 69.8 | 86.7 |
| Yes | 19.7 | 38.2 | 30.2 | 13.3 |
| Percentage of bracket base covered with enamel | ||||
| 0% | 80.3 | 61.8 | 69.8 | 86.7 |
| >0%-<1% | 15.2 | 16.7 | 12.7 | 8.7 |
| 1%-<5% | 3.0 | 9.0 | 7.9 | 3.3 |
| 5%-<10% | 1.5 | 5.6 | 4.0 | 1.0 |
| ≥10% | 0.0 | 6.9 | 5.6 | 1.0 |
| BARI | ||||
| Score 0 | 53.0 | 36.8 | 49.2 | 39.3 |
| Score 1 | 37.9 | 29.9 | 30.2 | 49.3 |
| Score 2 | 9.1 | 9.0 | 4.8 | 6.0 |
| Score 3 | 0.0 | 11.8 | 5.6 | 2.7 |
| Score 4 | 0.0 | 10.4 | 7.9 | 2.7 |
| Score 5 | 0.0 | 2.1 | 2.4 | 0.0 |
| Bracket fracture | ||||
| No | 87.9 | 93.8 | 73.8 | 100.0 |
| Yes | 12.1 | 6.3 | 26.2 | 0.0 |
∗ Cell values are percentage of surfaces.
† Orthodontist A treated 11 patients with CGIC and 25 patients with MEC.
‡ Orthodontists B and C treated 15 and 9 patients with CSEP, respectively.
§ Orthodontists D and E treated 13 and 8 patients with CEC, respectively.
In total, 81 patients were recruited into this study. Brackets from the maxillary right to left canine were collected and sterilized by 4.1 kGy gamma irradiation. Of the 486 brackets collected, 49 fractured at removal, so only 437 brackets were visualized by backscattered scanning electron microscopy under 60 times magnification (low vacuum; no coating; backscatter mode; spot size 4; 15 kV; resolution 2048 × 1768; Quanta 200F scanning electron microscope; FEI, Hillsboro, Ore). An elemental map of calcium, phosphorus, aluminum, and silicon on the bracket base was also made using energy dispersive x-ray spectrometry (resolution setting, 1024; acquisition time, frame count 3; pixel dwell time, 50 μS; process time, 5; energy range (KeV); auto; INCA SDD x-ray microanalysis system for elemental analysis; Oxford Instruments, Abingdon, United Kingdom).
One investigator (T.W.G.L.) used these images to determine the absence or presence of enamel on the bracket base pad by assuming that a pixel that contained both calcium and phosphorus was enamel ( Fig 1 ). None of the bonding materials used in this study contained detectable calcium or phosphorus, and any calcium or phosphorus that was not on the bonding materials was assumed to be calculus and excluded from the analysis. Calcium and phosphorus elemental maps images were opened in Image J (U.S. National Institutes of Health, Bethesda, Md), multiplied together to reduce detector noise, and converted into a binary mask. This mask indicated the pixels that were assumed to be enamel (calcium multiplied by phosphorus) on the bonding surface of the bracket. Ten masks and their respective backscattered scanning electron microscopy images were then used to determine the minimum grey value for enamel that was subsequently used for the analysis of all other backscattered scanning electron microscopy images. The mean and standard deviation of this threshold were 114 ± 30 grey values. Backscattered scanning electron microscopy images of each bracket were viewed through Image J as 8-bit images and calibrated using the scale bar in each image. The area of the image where bonding material was not present was set to black (grey value = 0) using the paintbrush tool (this removed any areas of presumed calculus). The histogram of the image was then acquired, and all pixels above the minimum threshold of 114 were counted to quantify the area of enamel in square millimeters. The bracket area for each tooth type was determined by measuring the mean area of 10 brackets from the same tooth type in Image J after the scale had been set as described earlier. The percentage area of the bracket base pad that was covered in enamel was calculated to account for the variations in the base pad area.
The amount of adhesive remnant on the bracket was categorized using a modified bracket adhesive remnant index (BARI): 0, no adhesive present; 1, less than a quarter of the adhesive left covering the bracket base; 2, more than a quarter but less than half the adhesive left covering the bracket base; 3, more than half but less than three quarters of the adhesive left covering the bracket base; 4, more than three quarters but less than all of the adhesive left covering the bracket base; and 5, the whole bracket base is covered with adhesive. Bracket fractures were also coded. When they occurred, it was assumed that all of the adhesive remained on the tooth surface, and therefore these samples were given a BARI score of 0.
To assess the reproducibility of the outcome measures, the elemental map images from 18 patients were rescored on a separate occasion by the same examiner without reference to the initial scores. The images selected for the reproducibility study by our statistician (G.G.A.) covered the entire range of values of the outcome measures.
Statistical analysis
All statistical analyses were carried out using a software package (Stata version 12; StataCorp, College Station, Tex). Cross-tabulations and chi-square tests were used to evaluate whether there was a difference in the presence of enamel, degree of enamel damage, and BARI when different bracket and bonding material combinations were used. Logistic regression analysis was used to evaluate whether different tooth types were more susceptible to damage for the various bracket and bonding material combinations. For this analysis, enamel damage was dichotomized as damage or no damage. Ordered logistic regression was used to evaluate whether there is a difference in BARI scores when various bracket and bonding material combinations are removed from different tooth types. For the reproducibility study, the BARI scores from the 2 sets of measurements were compared using the kappa statistic, and the percentage area of base pad covered by enamel values were compared using a generalized linear model, and an intraclass correlation coefficient was calculated. In all analyses, significance levels were adjusted for the clustered study design (brackets from the same patient) using the svy option. P values less than 0.05 were considered statistically significant.
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