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Scientific Section |
1 Orthodontic Department, Middlesbrough General Hospital, Ayresome Green Lane, Middlesbrough, TS5 5AZ, UK
2 Department of Child Dental Health, Newcastle Dental School, Framlington Place, Newcastle upon Tyne, NE2 4BW, UK
Abstract
This study aimed to identify the presence and pattern of differences in ex vivo shear bond strength between tooth types when bonding orthodontic brackets using Right-On®, and took the form of a prospective laboratory study of bond strength on different tooth types, at the Newcastle University Dental School Materials Science Laboratory, 1997–1999.
Ex vivo bond strength testing was undertaken using the technique described by Fox et al. (BJO 18, 125–130, 1991) on a total of 120 extracted incisor, canine, and premolar teeth of each dental arch. Analysis was by one-way ANOVA with Tukey's pairwise comparisons, and by Weibull Analysis. Shear stress to failure (measured in MPa) was recorded on Instron® 5567 universal testing machine.
Significant differences in mean bond strength existed between different tooth-type series. Canine (upper 12•3, lower 12•1) and premolar (upper 11•9, lower 10•9) teeth exhibited higher strengths than incisors (upper 6•9, lower 9•0).
The results of this study confirm that ex vivo bond strength is not uniform across all teeth.
Key words: Orthodontic Bonding, Shear Bond Strength.
Introduction
The materials involved in orthodontic bonding have been a field of constant change since the acid etch technique (Buonocore, 1955
) permitted adhesion of acrylic filling materials to enamel surfaces. New bonding materials are tested to ascertain whether they represent an improvement on those previously used. The ideal system would be quick and easy to place, remain in situ with sufficient bond strength to resist dislodging forces during the active treatment period, and be debonded at the end of treatment without complication. A number of means of assessing the success of a bond are available, including measurement of bond strength ex vivo or the recording of bond failure in vivo. The simplest of these is ex vivo bond strength testing.
Recording of bond strength can be achieved with a considerable degree of accuracy using modern testing systems and enables comparison of new materials with other materials already in use at the time of testing. The use of a universal-testing machine (e.g. Instron®; Zwick®) in orthodontic bond strength testing is widespread. These machines are capable of delivering a controlled and measured force to the bonded bracket via a moving crosshead (Figure 1). International Standards Organization (ISO, 1991) suggest testing to failure in shear, with the values of stress quoted in mega pascals (MPa).
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Since Newman (1965) first reported ex vivo bond strengths of epoxy adhesives in orthodontics, there has been a consistent tendency for research workers to report increased ex vivo bond strengths of new materials as indicating improved clinical performance. The validity of the long-held assumption that a high ex vivo bond strength is synonymous with low failure rates or enhanced clinical survival has been questioned. Sunna and Rock (1998) cast doubt over the applicability of this assumption when comparing ex vivo bond strength with in vivo clinical bond failure rates. Significant differences in ex vivo bond strength between various bracket/adhesive combinations did not correlate with clinical failure rates, which demonstrated no significant differences.
Extracted premolar teeth have formed the basis of most bond strength tests to date (Fox et al., 1994), possibly due to the relative ease of obtaining test specimens following orthodontic extractions. Results obtained from premolar testing have been interpreted as being applicable to all teeth in both dental arches. The validity of this interpretation has, however, not been proven. To date, only one study has examined whether premolar bond strengths are representative of all tooth types. Hobson et al. (1999) examined variations in shear bond strength between different tooth types. Their findings suggest that significant differences in shear bond strength exist between different tooth types and opposing dental arches. Upper anterior teeth demonstrated higher shear bond strengths than upper posterior teeth and lower posterior teeth demonstrated higher shear bond strengths than lower anterior teeth.
Recent evidence has suggested that different tooth types exhibit biological variation in their etch pattern after acid priming (Hobson and Mattick, 1997, 1998; Mattick and Hobson, 2000) and it has been proposed that these differences between tooth types may influence the bond strength that is achieved. This study set out to explore certain shortcomings in the current knowledge of orthodontic bonding. The following aims have been addressed:
Materials and Methods
One-hundred-and-twenty healthy human teeth extracted within the Oral and Maxillofacial Surgery Department of Middlesbrough General Hospital and peripheral units were collected over a 6-month period, and stored immediately after removal and continuously until testing in a solution of 0•5 per cent chloramine T disinfectant at room temperature. The teeth collected included all tooth types with the exception of molars (i.e. incisor, canine, premolar) from both dental arches.
The teeth were sorted into six groups (A to F) according to tooth type:
). The orientation of the teeth in acrylic was such that the tangent to LA (Andrews, 1976) point lay perpendicular to the plane of the slab. This was undertaken in an effort to minimize peel and maximize shear during testing. Each tooth was ascribed a number prefixed by the letter relating to which tooth type it was (e.g. A8). These numbers were inscribed onto each acrylic slab using a steel bur in a slow speed dental handpiece.
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Measurement of projected base surface area for each bracket type was performed using an Optomax Image Analyser® (Micro Measurements Ltd, Shire Hill, Saffron Walden, Essex, UK). This recorded the area of five brackets of each type on five successive occasions to derive a mean surface area per bracket, thereby allowing approximate conversion of force values into stress values (Newtons into mega pascals). The image analyser provided nominal cross-sectional areas of the bracket bases, but was unable to account for the curvature of the bracket base, nor for the increase in base surface area due to the mesh backing. The conversion into stress values is therefore a best estimate. True determination of bracket base area taking account of these additional factors was not possible. These errors existed to a similar degree for all brackets, so it was considered reasonable to use these measurements to compare brackets with each other.
The teeth were bonded and tested in batches corresponding to their tooth type. The protocol for testing the effect of different tooth types on bond strength was similar to that described by Hobson et al. (1999). Tooth preparation involved prophylaxis using pumice/water slurry for 15 seconds then rinsing under tap water. Buccal surface enamel was etched using 36 per cent phosphoric acid gel (DeTrey Conditioner 36®, Dentsply De Trey, D-78467 Konstanz, Germany) for 30 seconds then rinsed with water for 15 seconds and a water/air mixture for a further 15 seconds prior to air drying using oil-free compressed air until frosted. All attachments were bonded using Right-On®. The etched tooth surface and the bracket base had Right-On® liquid primer applied to them. Right-On® adhesive paste was then applied to the bracket base prior to seating on the tooth surface. All brackets were positioned on LA point (Andrews, 1976). Brackets were pressed against the tooth surface in their final position to ensure good approximation of constituents and to express excess adhesive. Peri-bracket flash was removed with a probe.
Once bonded, the teeth were stored in distilled water at 37°C (Raven Oven®, LTE Scientific, Greenfield, Oldham, UK) in darkness for 24 hours prior to testing. Bond strength testing was undertaken using the technique described by Fox et al. (1991). Orthodontic bond strength may be tested either in tension or shear, although Fox et al. (1994) suggested that control of the force vector can be difficult and resolution of the forces may reveal the force to be neither purely tensile nor purely in shear. They suggested use of a universal joint and a wire loop engaging the tie-wing slot fully in order to reduce directional error and enhance standardization. Shear bond strength to failure was tested on an Instron® 5567 Universal Testing Machine (Instron Corporation, 100 Royall Street, Canton, Mass. 02021, USA) with a crosshead speed of 1•0 mm per minute and a wire loop engaging the gingival tie-wings (Figure 1
). Specimens were mounted on a universal joint to ensure the direction of shear loading was gingivo-occlusal.
The laboratory bond strength data was subjected to one-way analysis of variance (ANOVA) and Weibull analyses. The ANOVA provided group means with 95 per cent confidence intervals to enable comparison of within group variance and between group variance. Tukey's pairwise comparisons were undertaken in preference to multiple t-tests as a means of determining which group means were significantly different from which others. This was done in order to reduce the multiplication of type one errors (false positives) that can be generated by multiple t-tests (Bulman and Osbourne, 1989).
The Weibull analysis (Weibull, 1951) was applied to generate probabilities of failure at given levels of stress for each tooth type. The Weibull data allowed generation of characteristic stresses at which 5 and 10 per cent of brackets would fail for each tooth type tested, together with their 95 per cent confidence intervals. Lack of overlap of these 95 per cent confidence intervals enabled a quick assessment of whether two groups were significantly different.
Results
Table 1 illustrates descriptive statistics of the bond strength characteristics relating to the six groups of teeth (A–F) sorted by tooth type. The mean bond strengths in mega pascals for brackets bonded to each tooth type are presented together with the median values for each group, the group ranges, standard deviations, and standard errors of the mean.
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. Significant differences between groups are identified by examining the values at the intersection of groups within the grid in Table 3. If the two values at a given intersection span zero, it is valid to conclude that there is not a significant difference between the means of the two groups under examination.
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Weibull analysis was undertaken on the ex vivo bond strength data. This generated probabilities of failure at given levels of applied stress (Weibull, 1951; McCabe and Carrick, 1986) for each tooth type. It was thus possible to generate the characteristic stresses associated with failure in 5 and 10 per cent of brackets for each tooth type, as these are levels of failure deemed to be clinically acceptable. These stresses and their 95 per cent confidence intervals are presented in Tables 4 and 5, together with the Weibull modulus for each tooth type.
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. The graphs given for each dental arch plot the cumulative probability of bond failure within any tooth type against applied stress.
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Discussion
Human teeth were used in this study in order to replicate as far as possible the in vivo clinical situation of bond failure. The teeth were extracted over a 6-month period, with their storage from extraction to testing being in 0•5 per cent chloramine T solution at room temperature. This differed from certain studies in which storage was either in chloramine refrigerated at 5°C (Hobson et al., 1999) or in distilled water refrigerated at 5°C (Larmour et al., 1998a; 1998b). The origin of the teeth was non-specific; previous studies have reported exclusive use of teeth removed for orthodontic purposes or of teeth from patients of a specific age range (Sunna and Rock, 1998; Larmour et al., 1998a,b; Hobson et al., 1999). It was necessary to be less specific about the origin of the teeth in this study in order to obtain sufficient numbers of teeth to provide adequate sample sizes for each tooth type, which McCabe and Walls (1986) and Fox et al. (1994) suggest as being no fewer than 20 teeth. Even so, it was not possible to obtain sufficient numbers of extracted lower canine teeth to match that sample to the others.
It is important to exercise caution in interpreting results if the origin of the teeth used is not fully known. Incisors are seldom removed for orthodontic purposes, so it would seem likely that those used in this ex vivo study came from a population of patients older than that typically undergoing orthodontic treatment. The enamel on these teeth may therefore have been qualitatively different from that present in orthodontic patients in vivo, with a higher surface fluoride content (Weatherell et al., 1972). This feature may be responsible for the significant differences in bond strength recorded between incisors and other tooth types.
Testing was performed in a widely accepted manner (Fox et al., 1991). Testing at 24 hours was chosen as it has been widely reported previously, and permitted comparison with other ex vivo bond strength studies. The predominant vector of force was in shear. This was achieved by means of careful embedding and bracket bonding together with use of a wire ligature to engage the bracket tie-wings fully. Jigs were not used to standardise mounting and testing, and this may have contributed to the relatively wide spread of data within each study sample (Littlewood and Redhead, 1998). The direction of shear force was gingivo-occlusal. This differs from that used in certain studies (Sunna and Rock, 1998) and it is unlikely that this mimics the loading a bracket receives during mastication. Comparison of results between this study and other ex vivo studies with a different direction of debond force may reveal differences purely related to the force direction. In addition, the fact that the ex vivo testing was performed in a manner dissimilar to in vivo loading may limit the value of comparisons made between the two.
The results of this study suggest that shear bond strength of orthodontic brackets varies between tooth types. This finding supports the work of Hobson et al. (1999) in which tooth type was found to have a significant effect on the bond strength achieved with Transbond®. The patterns of bond strength recorded in this study differed from those recorded by Hobson et al. (1999). In this study upper incisors demonstrated significantly lower mean shear bond strength than all other tooth types except lower incisors, whereas Hobson et al. (1999) found upper incisors to obtain the highest shear bond strength of all teeth. Lower canines in this study demonstrated significantly higher mean shear bond strength than lower incisors, whereas Hobson et al. (1999) found no statistical difference between these tooth types.
The protocol for tooth preparation, subsequent storage and testing in this study matched that used by Hobson et al. (1999). The most significant differences in technique were the use of a different adhesive and of slightly different brackets. Base surface areas between the two types of brackets were closely matched (<10 per cent), and always above the 6•82 mm2 threshold found to be critical by MacColl et al. (1998). The role played by non-identical brackets in generating differences should therefore be minimal.
The origin of every tooth in the study by Hobson et al. (1999) was recorded as being from patients aged 10–22 years of age living in NE England. Although the teeth in this study were similar in that they were from the same broad geographical location, it was not possible to be specific in ascribing an age range to the patients from whom the teeth had been extracted. It is possible that this may have led to the differences in bond strength pattern by tooth type, particularly that which affected incisors in this study.
The other variable between this study and Hobson et al. likely to have contributed to the different bond strength patterns was the type of adhesive used. It is known that the light-cured composite adhesive Transbond® behaves differently to the no-mix chemically cured composite adhesive Right-On®. Although Wang and Meng (1992) found light-cured composite bond strengths at least matched those of chemically cured composites and Sargison et al. (1995) found no difference in shear bond strength between Transbond® and Right-On®, the profile by which the final bond strength is attained for each material differs. Chamda and Stein (1996) found that both set materials increased in bond strength over the first 24 hours, but the chemically-cured composites started at a strength below that deemed to be clinically adequate whereas light cured composites stared with strength above that threshold. Sunna and Rock (1999), however, did find a significantly higher shear bond strength with Transbond® than with Right-On® in tests on premolars ex vivo. This finding is not borne out when premolar bond strengths are compared between this study and those obtained by Hobson et al. (1999) using Transbond®. In their study, mean upper premolar bond strength (9•2 MPa) was less than in this study (11•9 MPa) and their mean lower premolar bond strength (8•9 MPa) was also less than in this study (10•9 MPa).
Right-On® is dependent upon adequate integration of its two phases to ensure full polymerization. If incomplete polymerization occurs, the potential exists for the resulting bond to be weak and prone to cohesive failure. Similarly, Transbond® may also polymerize incompletely if not exposed to sufficient light beneath the bracket base. This may also adversely affect bond performance. Both materials, although manipulated differently, are therefore technique sensitive.
Survival of both materials was found by Armas Galindo et al. (1998) to be matched at 11 months, but when each material does fail the locus of failure is likely to differ. Egan et al. (1996) found that with chemically cured composites the majority failed adhesively at the enamel/adhesive interface. Bishara et al. (1999) found that light-cured composite predominantly failed adhesively at the adhesive/bracket interface. It seems likely that the difference in bonding material used in this study and that of Hobson et al. (1999) is the most likely reason for the different patterns of bond strength observed.
The Weibull analysis allowed an estimation of the characteristic strength required for 5 and 10 per cent probabilities of failure under loading—a range of failure rate widely held to be clinically acceptable. Upper incisors required less than half the stress of any other tooth type for failure to occur either at 5 or 10 per cent probability. This difference was statistically significant.
The reason that different tooth types should exhibit different shear bond strengths and probabilities of failure at a given stress is not fully known. It seems likely that differing enamel anatomy is a contributory factor. Whittaker (1982) found premolar teeth contained relatively greater proportions of aprismatic enamel, which may adversely affect bond strength and survival. Mattick (1996) noted that etch pattern differed between tooth types. Hobson and Mattick (1997, 1998) and Mattick and Hobson (1997, 2000) further confirmed that in vivo and ex vivo etch patterns differ significantly between different tooth types and between opposing dental arches, although this latter difference was not significant.
Alternatively, the differences in shear bond strength found between different tooth types may relate to gross anatomical variability. Teeth with a highly variable morphology will demonstrate inconsistent adhesive film thickness. It may be that certain tooth types have greater morphological variation than others thereby generating a more variable adhesive film thickness and altered bond strength characteristics.
Conclusions
The results of this study confirm that significant differences in bond strength exist between teeth of different tooth-type series. Canine and premolar teeth exhibited significantly higher shear bond strengths and significantly lower probabilities of failure at given levels of applied stress than incisors. Comparison of this study's findings with those from a previous study using a similar methodology, but different bonding materials, confirm that significant differences in magnitude and pattern of bond strength exist between differing orthodontic adhesive/bracket combinations.
Acknowledgments
The authors wish to thank Professor J. F. McCabe and his staff for allowing access to the dental materials science laboratory at Newcastle Dental School, and for their help with the Weibull statistics. We also wish to thank the staff of the Oral and Maxillofacial Surgery Department at Middlesbrough General Hospital for collecting the extracted teeth used in this study.
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