Steven Jefferies MS, DDS, PhD
While laboratory orthodontic bond strength studies cannot be regarded as a substitute for in-vivo testing; variability of outcomes reported in these laboratory bond strength studies could be reduced with better standardization of test conditions, and thus provide a more reliable methodology for initial screening and selection of materials.
Three specific experimental conditions (water storage, polymerization time, and testing strain-rate) significantly affected in-vitro strength testing, and should be examined carefully in assessing these reports. Overall, the strength of evidence is weak, the bias is high; hence, clinicians should be cautious in the interpretation and utilization of current laboratory bond strength data in clinical decision-making in this area of orthodontic treatment.
Do the in-vitro characteristics of enamel bond strength testing provide consistent data to reliably translate to in-vivo clinical orthodontic practice?
The authors conducted a comprehensive search of two databases and identified 121 relevant studies. Criteria for study selection and detailed assessment of experimental conditions were 1) an in-vitro investigation; 2) shear bond strengths of metal brackets, expressed in megapascals (MPa); and 3) use of sound buccal enamel of human premolars. The authors excluded case reports, abstracts, letters, and narrative reviews. They sought consensus on 27 items to review the selected papers and established consensus on 12 required experimental conditions (14 specific data items) included in a meta-analysis of 24 studies. The authors related exclusion of studies to inadequate reporting of test conditions and specimen storage. To identify testing conditions (independent variables) that truly impacted bond strength (dependent variable), the authors considered effects with p < 0.05 as significant.
On average, from the 121 studies, the review reported a mean of 20.4 (SD 2.8) experimental testing conditions listed in testing methodology (minimum of 12/maximum 26 test conditions). Meta-analysis of 24 studies demonstrated that three experimental conditions significantly affected in-vitro bond strength testing. These included statistically significant effects exerted by water-storage (negative effect), polymerization time (positive effect), and crosshead speed of the testing device (positive effect). Water storage, on average, decreased bond strength by 10.7 MPa (assuming other predictors remain constant). Each additional second of photo-polymerization time increased bond strength by 0.077 MPa; while increases of crosshead speed of 1 mm/minute increased bond strength by 1.3 MPa.
Variability in testing parameters (water storage of the bonded specimens, photopolymerization time, and crosshead speed) appear to create a high degree of variability in laboratory results. This finding strongly suggests that in-vitro enamel bond strength testing, currently, provides limited guidance in the clinical orthodontic practice regarding selection of new materials and techniques for use with enamel-bonded brackets and appliances.
Source of Funding:
Not stated or indicated.
Importance and Context:
Orthodontic adhesive bonding of brackets to teeth is a standard, common-place procedure in comprehensive orthodontic treatment using fixed appliances to control tooth movement in all dimensions. Due to the limited number of randomized controlled clinical studies evaluating the clinical performance of orthodontic adhesives, in-vitro bond strength testing of these adhesives assumes added significance in assessing potential clinical efficiency and effectiveness. Review of the literature and assessment of testing variables for this procedure is essential to establish test validity and reliability, as well as provide a basis for improving and standardizing test methods.
Strengths and Weaknesses of the Systematic Review:
This comprehensive, systematic review included a search of only two databases, and inclusion and exclusion criteria were not always clearly explained or defined. The authors did establish criteria for quality of studies evaluated. Interobserver agreement (Cohen kappa) for reporting the 27 experimental conditions of the included studies was 0.86. The authors did not analyze publication bias, but tests for heterogeneity among studies concluded that there was significant "clinical" heterogeneity. This systematic review did not state that a sensitivity analysis to determine robustness of the data was conducted. Exclusion of languages other than English may have eliminated potentially useful or important studies. Assuming that such efforts would have produced fruitful information, failure to perform hand searching for recent articles, to extend the search to include additional data bases, and to contact authors were limitations.
Strengths and Weaknesses of the Evidence:
As this systematic review included only in-vitro observational studies, with a lack of consistency in the data; the strength of evidence is weak, and the bias is high. The predominant use of shear bond strength testing on buccal enamel of pre-molars contributed to the strength of the data in the study literature. The lack of standardization or consistency in test methods for in-vitro orthodontic bond strength testing, revealed by this systematic review and meta-analysis,limited the strength of the evidence analyzed.
Implications for Dental Practice:
Bond strength of orthodontic adhesives must provide durability and reliability during the treatment phase to optimize treatment time and minimize the additional cost and burden of care if additional appointments are required for re-bonding. The adhesive bonding strength should not interfere with bracket debonding at the end of treatment, which would be an important future in-vitro study. Standardized of in-vitro test protocols for orthodontic adhesives could provide more consistent data to guide clinicians in evaluation and selection of adhesives. Hence, clinicians should be cautious in the interpretation and utilization of current laboratory bond strength data in clinical decision-making.