Bonded molar tubes: A retrospective evaluation of clinical performance

CONTINUING EDUCATION ARTICLE Bonded molar tubes: A retrospective evaluation of clinical performance Declan T. Millett, BDSc, DDS, FDS, MOrth,a Anders Hallgren, DDS, MS,b Anne-Charlott Fornell,c and Michele Robertson, BScd Glasgow, UK This study investigated time to first failure of stainless steel orthodontic first permanent molar tubes (Ormco Corp) bonded with a light-cured resin adhesive (Transbond) and assessed whether this was related to patient gender, age of the patient at the start of treatment, the presenting malocclusion, or the operator. All first molar tubes were bonded to intact buccal enamel, free of any restoration. Survival analysis was carried out on data from 483 patients with 1190 bonded first molar tubes. For each case, a single molar tube, either that which was first to fail or had the shortest follow-up time, was chosen for analysis. The median time until first bonded tube failure was 699 days with an overall failure rate of 21% recorded. There was no significant difference in time to first failure of molar tubes with respect to patient gender or presenting malocclusion but significant differences were recorded with respect to the patients’ age at the start of treatment and the operator. Age at the start of treatment and operator were identified as independently useful predictors of bonded molar tube survival. (Am J Orthod Dentofacial Orthop 1999;115:667-74)

After the introduction of acid etching of enamel by Buonocore in 1955, direct bonding of 1

orthodontic brackets to incisors, canines, and premolars is now carried out routinely as part of fixed appliance treatment.2-4 Bands, however, remain the most common means of attaching components to molars, and bonding of brackets to molar teeth is a less frequently adopted practice.5,6 The decision to band or bond a molar may be influenced by several factors including a history of congenital cardiac defect, rheumatic fever or prosthetic cardiac valve placement,7 the height of the clinical crown, or the need to use headgear.2 Bonding rather than banding molars, however, reduces chairside time and leads to less plaque accumulation and gingival inflammation,8 thereby reducing the risk of enamel demineralization.9 A bonded molar attachment should be capable of resisting tensile, shear, torque, and peel functional stresses if it is to remain attached to the tooth surface throughout treatment.10 The etch time11 and concentration,12 the bonding technique13 and bonding agent4,14-15 used as well as the characteristics of the bonding aSenior Lecturer/Honorary Consultant in Orthodontics, Orthodontic Unit, Glasgow Dental Hospital and School, Glasgow, UK. bSenior Orthodontist, Tandregleringen, Halmstad, Sweden. cSenior Dental Hygienist, Tandregleringen, Halmstad, Sweden. dResearch Assistant, Robertson Centre for Biostatistics, University of Glasgow, UK. Reprint requests to: Dr D. T. Millett, Orthodontic Unit, Glasgow Dental Hospital and School, 378 Sauchiehall Street, Glasgow G2 3JZ, UK; e-mail, [email protected] Copyright © 1999 by the American Association of Orthodontists. 0889-5406/99/$8.00 + 0 8/1/94288

base16,17 affect the clinical performance of any bonded attachment. In addition, specific patient factors4 such as age, gender, and presenting malocclusion together with operator variables4,18 relating to clinical technique impact on the survival of bonded fixed appliance components. The performance of attachments bonded to molar teeth has received very limited attention in the orthodontic literature. One in vitro study19 and one in vivo study2 found lower bond strengths and higher failure rates, respectively, for brackets bonded to the buccal aspect of permanent molar teeth compared with those bonded to anterior teeth. In addition, it appears that only one clinical study has evaluated the performance of tubes, rather than brackets, bonded to permanent molars with a chemically cured resin adhesive (Concise, 3M, St Paul, Minn).20 Furthermore, in the study by Lovius et al,18 only three molars were bonded, but it is not clear whether brackets or tubes were used, and the number of attachments bonded with a chemically cured or light-cured adhesive is not specified. In any case, the sample size is too small to allow any inference to be drawn with respect to the use of light-cured adhesives for molar bonding. The difficulty in maintaining a dry field until a chemically cured resin sets may be a contributory factor in failure of bonded attachments on molars. With the advent of visible light-cured adhesives,11,21,22 command setting of the resin is possible, minimizing the time required to maximize bond strength.23 No study, however, has examined comprehensively the effect of patient and operator variables on 667

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the clinical performance of tubes bonded to molar teeth with a light-cured adhesive. The aim of this study was to investigate the time to first failure of stainless steel orthodontic first molar tubes bonded with Transbond (3M/Unitek, Monrovia, Calif) and to assess whether this was related to patient gender, patient age at the start of treatment, the presenting malocclusion, or the operator. MATERIAL AND METHODS

Since 1988, patients undergoing fixed appliance therapy at the Orthodontic Department, County Hospital, Halmstad, Sweden, have either had a band with prewelded attachment cemented to a first permanent molar or a molar tube has been bonded to the buccal aspect of the molar tooth. The decision to band or bond a molar has been made by the clinician undertaking orthodontic treatment. Where significant occlusal trauma to a bonded attachment was likely or where anchorage reinforcement with a transpalatal arch or extraoral traction was indicated, the first permanent molars were banded rather than bonded. The potential need for intermaxillary traction did not, however, preclude the use of bonding tubes to the first permanent molars. The record files of patients completing fixed appliance orthodontic treatment during the period Jan 1, 1992 and Dec 31, 1996 inclusive were examined. Patients who had bands cemented to first permanent molars as part of their treatment were excluded. All patients included in the study had orthodontic treatment with an 0.018 inch preadjusted edgewise system (Ormco mini diamond brackets, Ormco Corp, Glendora, Calif) with bonded first permanent molar tubes (Ormco Corp, Glendora, Calif). All brackets and tubes were bonded with a light-cured resin adhesive, Transbond (3M/Unitek). Each first molar tube was bonded to intact buccal enamel free of any restoration. All bonding procedures were carried out by orthodontists with many years experience or by trained orthodontic auxiliaries with at least 2 years postqualification experience. Before bonding, teeth were cleaned with a nonabrasive liquid (Tubilicid, Dental Therapeutics, Nacka, Sweden), washed with water and dried with a stream of air. The midbuccal aspect of each first permanent molar was then etched for 60 seconds with 37% orthophosphoric acid gel, washed for 60 seconds and dried with compressed air. To prevent moisture contamination of the etched enamel surface, each molar was isolated with cotton wool rolls and a high volume saliva ejector. Bonding of each molar tube with Transbond was carried out according to manufacturer’s instructions. After removal of excess adhesive from around the attach-

American Journal of Orthodontics and Dentofacial Orthopedics June 1999

ment, the material was cured for 40 seconds (20 seconds from each of the incisal and gingival aspects of the bonded molar tube). On completion of bonding, 0.010 or 0.012 stainless steel wires (TP, Australia) or 0.0155 Dentaflex wires (Dentaurum, Pforzheim) were applied for initial alignment. Verbal and written instructions were given to each patient in relation to appliance care. In addition, each patient was specifically recommended to contact the department if any bonded attachment became dislodged. During treatment, patients attended at 4 to 6 week intervals. Arch wire sequence and treatment mechanics were similar for each case. Bond failures were recorded accurately in the patient’s record file with the date of bond failure identified as the date when bond failure was noticed. From the hospital record file of each treated case, details of which first permanent molars were bonded, including date of placement of each bonded molar tube and their fate up to Dec 1996, were recorded. The date of birth and gender of the patient, presenting malocclusion based on the incisor relationship, and the operator involved in treatment were recorded also. A code was allocated to each bonded tube, indicating whether it survived the course of treatment (censored, code 1), was lost to follow-up because of patient transfer (withdrawn, code 2) or had debonded (failed, code 3). Survival analysis was carried out on a single molar tube per patient using SAS for Windows, Version 6.12. As the analyses assume that the observations are all independent of each other, all formal analyses were carried out on a single molar tube per patient using time to first failure or censored time in days. Univariate Analysis

The impact of each of the factors, ie, patient age at the start of treatment, patient gender, presenting malocclusion, and operator, on the survival time of each bonded molar tube was assessed by producing KaplanMeier estimates of survival curves stratified by the factor and using the log-rank test to compare the various levels of the factor. Relative hazards were calculated based on a Cox proportional hazards model to allow comparison of failure rates for one subgroup relative to another. No difference in the failure rate between the two subgroups is indicated by a relative hazard of 1; a relative hazard of 2 indicates twice the failure rate in one group relative to the other. Multivariate Analysis

A forward stepping approach, based on a Cox proportional hazards model, was used to identify independently useful predictors of bonded molar tube survival.

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Table I. Sample

characteristics for 1190 bonded first molar tubes

Number of patients (204 males; 279 females) Median age at start of treatment (years) (lower quartile 12.9 years; upper quartile 15.6 years) Number of patients per malocclusion type Class I Class II Division 1 Class II Division 2 Class III Number of patients bonded per operator Operator I Operator 2 Operator 3 Operator 4 Operator 5 Operator 6 Operator 9 (combination)

483 14.1 261 191 23 8 20 44 53 101 151 98 16

Table II. Effect

of age of patient at the start of treatment, patient gender, malocclusion, and operator on bonded molar tube survival Number of bonded molar tubes per group Age of patient at start of treatment q1-med >med - q3 >q3 Patient gender Male Female Malocclusion Class I Class II Division 1 Class II Division 2 Class III Operator 1 2 3 4 5 6 9

Number failed (%)

Number censored

120 121 122 120

47 (39) 44 (36) 54 (44) 25 (21)

73 77 68 95

204 279

69 (34) 101 (36)

135 178

261 23 8

86 (33) 191 13 (57) 3 (38)

175 68 (36) 10 5

20 44 53 l01 151 98 16

5 (25) 15 (34) 14 (26) 38 (38) 29 (19) 60 (61) 9 (56)

15 29 39 63 122 38 7

Level of significance of factor **

NS

NS 123

***

q1, lower quartile; med, median; q3, upper quartile; code 9, more than one operator noted per patient. **P < .01; ***P <.001; NS, not significant.

RESULTS Overall Analysis

Details of the study population are given in Table I. No bonded molar tubes failed in 313 patients whereas at least one molar tube failed in 170 patients. In five patients, two molar tubes failed; both were bonded to lower first permanent molars. None of the patients had more than two bonded molar tubes fail. The overall failure rate of bonded first molar tubes was 21% (246 tubes). The failure rate of tubes bonded to upper or

lower first permanent molars was 22% and 20%, respectively. Fig 1 shows the Kaplan Meier estimate of the overall survival curve. The median survival time of bonded first molar tubes was 699 days. Survival Analysis for the Various Factors

The effect of patient gender, age of the patient at the start of treatment, presenting malocclusion, and operator on the survival time of bonded first molar tubes are given in Table II.

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Fig 1. Graph of overall survival time of bonded molar tubes.

Fig 2. Survival time plots for bonded molar tubes stratified for gender.

Effect of patient gender (Fig 2). There was no significant difference in bonded molar tube survival between male and female patients (log rank test, P = .323, relative hazard for males compared with females: point estimate 0.86, 95% confidence interval 0.63 to 1.16). Effect of age of the patient at the start of treatment (Fig 3). To facilitate analysis, data were split arbitrarily at the quartiles, thereby giving four equally sized groups. Differences between subgroups were significant (log rank test, P = .005, relative hazard per 1 year advance in age at the start of treatment: point estimate

0.92, 95% confidence interval 0.86 to 0.98). Effect of presenting malocclusion (Fig 4). No evidence was found of a difference in bonded molar tube survival for the four categories of malocclusion (log rank test: P = .292). Effect of operator (Fig 5). Six operators were involved in the study, and a combination of operators carried out treatment for some patients. Therefore, an arbitrary code of 9 was given where more than one operator was noted per patient. There was a significant difference in molar tube survival between operators (log rank test, P < .0010). Comparing each pair of

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Fig 3. Survival time plots for bonded molar tubes stratified for age.

Fig 4. Survival time plots for bonded molar tubes stratified for malocclusion class.

operators gave 21 comparisons, but only the following comparisons were statistically significant: relative hazards of operator 3 versus operator 6, point estimate 1.35, 95% confidence interval 1.00 to 1.84; operator 4 versus operator 5, point estimate 0.43, 95% confidence interval 0.20 to 0.91; operator 5 versus operator 6, point estimate 4.21, 95% confidence interval 2.11 to 8.39. Multivariate Analysis

Patient age at the start of treatment and operator were identified as independently useful predictors of bonded molar tube survival. Once these variables were entered into the model, neither the effect of patient gen-

der or presenting malocclusion were statistically significant. DISCUSSION

The clinical performance of 1190 first molar tubes bonded with a light-cured resin adhesive in 483 patients has been examined over a 5-year period. Survival analysis was used for data analyses and has been used previously in the clinical profiling of bracket4,11,18,24 or molar band failure.25,26 It is a more sophisticated tool for analyzing data of this nature as it gives information not only on the failure rate of fixed appliance components but more importantly on their time to failure.27 In addition, the effect of several inde-

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Fig 5. Survival time plots for bonded molar tubes for each operator.

pendent factors on attachment survival can be assessed and predictors of survival can be identified. Overall Analysis

In the study reported here, the overall failure rate of bonded molar tubes was 21%, with 22% of upper and 20% of lower molar tubes failing during the observation period. These data compare favorably with the failure rates of 18.8% and 29.5% recorded by Zachrisson2 for brackets bonded to upper and lower first permanent molars respectively using Concise resin (3M, St Paul, Minn). Over a 3 to 5 month observation period, Geiger et al20 found a 21.1% failure rate of bonded maxillary molar tubes, but the failure rate over a 15 month period was 12.4%. The failure rate observed over the 3 to 5 month period in that study is similar to that recorded in the present study. The failure rate of bonded molar tubes was greater than that recorded for incisor teeth, which has been in the range of 5% to 10%.2,4,11,18 The failure rate of first molar bands cemented with glass ionomer cement has, however, varied from 0.56%28 to almost 20%.26,29 The difference in failure rate between attachments bonded to incisor or molar teeth and between bonded or banded molars is more likely explained by the generation of greater occlusal forces in the molar region.30 The possibility of moisture contamination during the bonding procedure having a bearing on the failure rate is possible also although a light-cured adhesive was used to allow rapid curing of the adhesive once the molar tube had been positioned firmly on the tooth surface. A 60 second etching time was used for enamel preparation

before bonding as this has been shown to give the optimal etch pattern on molars.31 The likelihood of the etch pattern having a major influence on tile failure rate of bonded molar tubes can, therefore, be eliminated. The median survival time of molar tubes observed in this study was 699 days or almost 23 months. Taking the failure rate alone, one might infer that routine bonding of first permanent molars is unreliable clinically but as the median survival time of molar tubes falls within the average duration of orthodontic treatment (18 to 24 months), this inference would be incorrect. Molar tubes can be expected to perform reliably throughout most fixed appliance orthodontic treatment, but the failure rate appears to increase as treatment time increases. This may be related to an increase in masticatory force in the molar area as the patient matures32 over a 2-year treatment period leading to fatigue failure of the resin adhesive bond. Impact of Each Factor on Bonded Molar Tube Survival

There was no significant difference between male and female patients in time to first failure of bonded molar tubes. The absence of a gender difference in survival of bonded attachments confirms the findings of Kinch et al11 who assessed bracket survival after different etch times and subsequent bonding with Concise orthodontic resin. In addition, other studies have found no evidence of a gender difference in band failure on first permanent molars.26,33,34 Although the evidence in relation to age of the patient and survival of brackets bonded to teeth anterior to the first permanent molars is equivocal,4,11 in

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the study reported here, significant differences in bonded molar tube survival were found when patients were stratified into four groups according to age at the start of treatment. Differences in patient compliance with instructions regarding avoidance of hard, sticky foods may account for this variation between groups with a lower failure rate recorded in the most senior group (Table II). No significant difference in bonded molar tube survival was found between each class of presenting malocclusion. As occlusal forces vary between malocclusion groups,30 one may have expected to observe a difference in time to first molar tube failure between malocclusion classes. The lack of association, however, between these two factors may be explained by the large variation in sample size between the malocclusion subgroups. Groups of similar size would be required to clarify conclusively the existence of a link between malocclusion class and bonded molar tube survival. Some previous studies have noted a significant difference between operators in the survival times of bonded brackets,4-18 and a similar effect with respect to bonded molar tubes was observed in the present study. There was a two-fold increase in the failure rate of molar tubes between operator 4 and operator 5. Variation in bond failure rate of this magnitude has been recorded between operators in other studies.4,18 All operators assessed in this study had considerable experience in orthodontic bonding procedures but differences between operators in tube location and nonuniformity of the resin thickness between the enamel and bonding base19 may account for the differences in failure rate observed. No previous study on bond failure has attempted to identify predictors of bond survival. In this study, both the age of the patient before treatment and the operator were identified as independently useful predictors of bonded molar tube survival. As the operator is usually chosen or allocated in most instances, the only variable that may be controlled reliably is the patient’s age before treatment. From the results of the present study, it seems prudent to advise that bonding of molar tubes should be avoided in young patients and is likely to be more successful in those in their mid to late teens. This study analyzed time to first failure (median survival time) of molar tubes bonded with a lightcured adhesive (Transbond) and no comparison was made to molar tubes bonded with a chemically cured adhesive over a similar time period. The 21% failure rate recorded, however, in the present study is similar to that reported by Zachrisson2 and Geiger et al20 using a chemically cured resin adhesive, but those

studies only report data on 76 and 129 bonded tubes, respectively. The study reported here provides information on the largest sample of first molar tubes bonded with a single adhesive system and is the first to consider predictors of survival for these attachments. As such it provides useful data for all clinicians who consider bonding first permanent molars. CONCLUSIONS 1. The median survival time of 1190 tubes bonded to first permanent molars in 483 patients was 699 days with an overall failure rate of 21%. 2. The patients’ gender or presenting malocclusion had no significant effect on bonded molar tube survival, but the age of the patient at the start of treatment and the operator were identified as independently useful predictors of bonded molar tube survival. References 1. Buonocore MG. A simple method of increasing the adhesion of acrylic filling materials to enamel surfaces. J Dent Res 1955;34:849-53. 2. Zachrisson BU. A posttreatment evaluation of direct bonding in orthodontics. Am J Orthod 1977;71:173-89. 3. Mizrahi E. Orthodontic bands and directly bonded brackets: a review of clinical failure rate. J Dent 1983;11:231-6. 4. Millett DT, Gordon PH. A 5-year clinical review of bond failure with a no-mix adhesive (Right-On). Eur J Orthod 1994;16:203-11. 5. Gorelick L. Bonding: the state of the art, a national survey. J Clin Orthod 1979;13: 39-53. 6. Gottlieb EL, Nelson AH, Vogels DS. 1996 JCO study of orthodontic diagnosis and treatment procedures. Part I results and trends. J Clin Orthod 1996;30:615-29. 7. Hobson RS, Clark JD. Management of the orthodontic patient ‘at risk’ from infective endocarditis. Br Dent J 1995;178:289-95. 8. Boyd RL, Baurnrind S. Periodontal considerations in the use of bonds or bands on molars in adolescents and adults. Angle Orthod 1992;62:117-26. 9. Retief DH, Sadowsky PL. Clinical experience with the acid-etch technique in orthodontics. Am J Orthod 1975;68:645-54. 10. Maijer R, Smith DC. Variables influencing the bond strength of metal orthodontic bracket bases. Am J Orthod 1981;79:20-34. 11. Kinch AP, Taylor H, Warltier R, Oliver RG, Newcombe RG. A clinical trial comparing the failure rates of directly bonded brackets using etch times of 15 or 60 seconds. Am J Orthod Dentofacial Orthop 1988;94:476-83. 12. Oliver RG. The effects of differing acid concentrations, techniques, and etch time on the etch pattern of enamel of erupted and unerupted human teeth examined using the scanning electron microscope. Br J Orthod 1988;15:45-9. 13. Zachrisson BU, Brobakken BO. Clinical comparison of direct versus indirect bonding with different bracket types and adhesives. Am J Orthod 1978;74:62-78. 14. Sonis AL, Snell W. An evaluation of a fluoride-releasing, visible light-activated bonding system for orthodontic bracket placement. Am J Orthod Dentofacial Orthop 1989;95:306-1 15. Trimpeneers LM, Dermaut LR. A clinical trial comparing the failure rates of two orthodontic bonding systems. Am J Orthod Dentofacial Orthop 1996;110:547-50. 16. Gorelick L. Bonding metal brackets with a self-polymerizing sealant-composite: a 12month assessment. Am J Orthod 1977;71:542-53. 17. Buckwald A. A three-cycle in vivo evaluation of reconditioned direct-bonding brackets. Am J Orthod Dentofacial Orthop 1989;95:352-4. 18. Lovius BB, Pender N, Hewage S, O’Dowling I, Tomkins A. A clinical trial of a light activated bonding material over an 18 month period. Br J Orthod 1987;14:11-20. 19. Knoll M, Gwinnett AJ, Wolff MS. Shear strength of brackets bonded to anterior and posterior teeth. Am J Orthod 1986;89:476-9. 20. Geiger AM, Gorelick L, Swinnett AJ. Bond failure rates of facial and lingual attachments. J Clin Orthod 1983;17:165-9. 21. Tavas MA, Watts DC. Bonding of orthodontic brackets by transillumination of a light activated composite: an in vitro study. Br J Orthod 1979;6:207-8. 22. Joseph VP, Rossouw E. The shear bond strengths of stainless steel and ceramic brackets used with chemically and light-activated composite resins. Am J Orthod Dentofacial Orthop 1990;97:121-5. 23. Chamda RA, Stein E. Time-related bond strengths of light-cured and chemically cured bonding systems: an in vivo study. Am J Orthod Dentofacial Orthop 1996;110:378-82.

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24. Miller JR, Mancl L, Arbuckle G, Baldwin J, Phillips RW. A three-year clinical trial using a glass ionomer cement for the bonding of orthodontic brackets. Angle Orthod 1996;64:309-12. 25. Stirrups DR. A comparative clinical trial of a glass ionomer and a zinc phosphate cement for securing orthodontic bands. Br J Orthod 1991;18:15-20. 26. Millett DT, Gordon PH. The performance of first molar orthodontic bands cemented with glass ionomer cement: a retrospective analysis. Br J Orthod 1992; 19:215-20. 27. Miller JR. Commentary: basic concepts concerning bracket failure research. Angle Orthod 1997;67:167-8. 28. Fricker JP, McLachlan MD. Clinical studies of glass ionomer cements. Part I: a twelve month clinical study comparing zinc phosphate cement to glass ionomer. Aust J Orthod 1985;9:179-80.

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29. Maijer R, Smith DC. A comparison between zinc phosphate and glass ionomer cement in orthodontics. Am J Orthod 1988;93:2739. 30. Proffit WR, Fields HW, Nixon WL. Occlusal forces in normal- and long-face children. J Dent Res 1983:62:571-4. 31. Johnston CD, Hussey DL, Burden DJ. The effect of etch duration on the microstructure of molar enamel: an in vitro study. Am J Orthod Dentofacial Orthop 1996;109:531-4. 32. Bakke M, Holm B, Jensen BL, Michler L, Möler E. Unilateral isometric bite force in 8 68-year-old women and men related to occlusal factors. Scand J Dent Res 1990;98:149-58. 33. Mizrahi E. Further studies in retention of the orthodontic band. Angle Orthod 1977;47:231-8. 34. Mizrahi E. Success and failure of banding and bonding: a clinical study. Angle Orthod 1982;52:113-7.