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tainties that affect the interpretation of the results andthe validity of the conclusions.

Bias has been defined as “any trend in the choice ofa sample, the making of measurements on it, the analy-sis or publication of findings, that tend to give orcommunicate an answer that differs systematicallyfrom the true answer.”1 Bias can occur in differentphases of the investigation, including selection of thestudy population, observation of the sample, measure-ment of the outcome, and analysis, interpretation, andreporting of the results. Sackett et al identified 56types of bias2; the control of these is an importantmeasure of the validity of the results of a clinical study.The methodological quality of a clinical trial dependson several features of the study design, including themethod of randomization, criteria for subject selec-tion, description of intervention, blinding procedures,

Control of bias in randomized controlled trials published in prosthodontic journals

Herman B. Dumbrigue, DDM,a Jack S. Jones, DMD,b and Josephine F. Esquivel, DDM, MSc

Baylor College of Dentistry, Texas A & M University Health Science Center, Dallas, Texas, and Collegeof Dentistry, University of Florida, Gainesville, Fla.

Statement of problem. Randomized controlled trials (RCTs) have become the gold standard for eval-uating the effectiveness of treatment interventions. If not properly controlled, bias in the design of trialmethodology can affect the validity of the study results. Purpose. The purpose of this investigation was to assess the methodological quality of RCTs publishedin 3 prosthodontic journals over a 10-year period. Material and methods. Issues of The International Journal of Prosthodontics, The Journal of ProstheticDentistry, and The Journal of Prosthodontics published between 1988 and 1997 were searched manually toidentify RCTs. Specific inclusion and exclusion criteria were established to identify articles about studiesthat qualified as RCTs. Two independent reviewers evaluated all qualified RCTs on the basis of howpotential sources of bias in the trial methodology were controlled. Three areas—control of bias at entry,control of bias in assessment of outcome, and control of bias after entry—were evaluated with a schemedeveloped through the Cochrane Collaboration. A score of 1 or 0 was assigned for each of the 3 potentialsources of bias, with the maximum quality score for an RCT being 3 (good bias control) and the mini-mum 0 (poor control). Frequencies were calculated for each dimension of trial methodology and overallquality scores of the RCTs.Results. Sixty-two RCTs were identified from 3631 articles screened. The method of randomization wasexplicit in only 47% of the RCTs. Forty percent of RCTs incorporated blinding in the assessment of out-come, and 76% accounted for all subjects at the end of the study. Overall quality scores revealed that only16% of RCTs attempted to control bias in all 3 areas examined. Forty percent were deficient in 1 area,34% were deficient in 2 areas, and 10% were deficient in all areas examined. Conclusion. The quality of RCTs published in prosthodontic journals may be improved by minimizingpotential sources of bias and adequately reporting trial methodology. (J Prosthet Dent 2001;86:592-6.)

It is assumed that most health care professionalsread professional journals to keep informed about cur-rent concepts on the diagnosis and treatment ofpatients. Unfortunately, it is also probably assumedthat, once a study is published in a reputable peer-reviewed journal, the conclusions presented are valid.A review of abstracts will disclose that the abstract orsummary often do not contain enough detail for anassessment of the study’s validity. Careful scrutiny ofthe methodology section may reveal biases or uncer-

CLINICAL IMPLICATIONS

This study revealed inadequacies in the rigor with which randomized controlled trialshave been conducted and/or reported in prosthodontic journals.

aAssistant Professor and Assistant Program Director, AdvancedEducation in General Dentistry, Baylor College of Dentistry.

bAssistant Professor, Department of Prosthodontics, University ofFlorida College of Dentistry.

cAssistant Professor, Department of Prosthodontics, University ofFlorida College of Dentistry.

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DECEMBER 2001 593

treatment of data, and use of appropriate statisticalanalyses.3

The randomized controlled trial (RCT) is the mostreliable basis for evaluating treatment interventions.4An RCT is any planned experiment or investigation inwhich the assignment of subjects to treatment groupsis by random allocation.4 Randomization establishesthe basis for testing statistical significance by ensuringthat baseline subject characteristics that might con-found an observed association are distributed equally,except for chance variation, among the randomizedgroups. It does not affect confounding variables thatmay develop during the study or follow-up periods.5In medicine, confirmation of the efficacy of manytreatments and the uselessness or harmfulness of oth-ers has been made through RCTs.6

The purpose of this study was to evaluate themethodological features of RCTs published in 3 peer-reviewed prosthodontic journals. Randomized controlledtrials were assessed on the basis of how 3 potentialsources of bias in the trial methodology were con-trolled.

MATERIAL AND METHODS

Issues of The International Journal of Prosthodontics(IJP), The Journal of Prosthetic Dentistry (JPD), andThe Journal of Prosthodontics (JP) published between1988 and 1997 were searched manually to identifyRCTs.7,8 Trials were considered RCTs if they met thefollowing criteria: (1) involvement of human subjects,(2) inclusion of at least 2 treatment groups in abetween-group or within-group comparison, and (3)random allocation of subjects to treatment groups.

Two calibrated reviewers independently evaluatedthe methodology of each RCT in 3 areas with the useof a scheme developed through the CochraneCollaboration.9 The 3 dimensions of trial methodolo-gy assessed were control of bias at entry, control of biasin the assessment of outcome, and control of bias afterentry. A score of 1 or 0 was assigned to each of the 3potential sources of bias. Thus, the maximum qualityscore for an RCT was 3 (good bias control), and theminimum was 0 (poor bias control). If there was dis-agreement in the scoring between 2 reviewers, a thirdcalibrated reviewer scored the trial methodology.Agreement between at least 2 reviewers established thefinal score. Frequencies were calculated for eachdimension of trial methodology and overall qualityscores of the RCTs.

The 3 dimensions of trial methodology weredefined and scored as follows. Control of bias at entry:The randomization procedure was evaluated to ensurethat the investigators were not able to predict or influ-ence which treatments patients would receive in thestudy. A score of 1 was given if the method of ran-domization was explicitly reported. Control of bias in

the assessment of outcome: The assessment procedurewas evaluated to ensure that neither the investigatornor the subjects had knowledge of the treatment allo-cation. A score of 1 was given when blinding of theinvestigators and/or subjects was performed andreported. Control of bias after entry: Dropouts andwithdrawals of subjects were evaluated to determinewhether they were study related. A score of 1 wasgiven if the study reported that there were no subjectdropouts or withdrawals, or when reasons for subjectdropouts or withdrawals were reported. If the studydid not meet the stated criteria for any category, it wasgiven an overall score of 0.

RESULTS

A total of 3631 articles in 196 journal issues werescreened manually. Of those, 62 articles (1.7%) wereidentified as reports of RCTs: 9 in IJP, 52 in JPD, and1 in JP.10-71 Tables I through III summarize theresults of the quality assessment.

Forty-seven percent of the RCTs explicitly reportedthe method of randomization. Within this group, themost common method was the use of a table of randomnumbers (28%), followed by stratification-randomiza-tion (24%), Latin squares (10%), random codes (10%),and coin toss (7%). Six additional methods were report-ed once (3.5% each). Forty percent of the RCTsincorporated blinding in the assessment of outcome.Within this group, 64% reported blinding of both inves-tigators and subjects to treatment allocation, whereas36% reported blinding of either investigators or sub-jects. All subjects were accounted for at the end of 76%of the RCTs evaluated.

Overall quality scores revealed that only 16% of RCTsattempted to control bias in all 3 areas examined. Fortypercent were deficient in 1 area, 34% were deficient in 2areas, and 10% were deficient in all areas examined.

DISCUSSION

Although all prosthodontic RCTs evaluated in thisstudy stated that allocation to treatment group was byrandomization, only 47% explicitly described themethod used for randomization. The remaining 53%merely stated that patients were randomly assigned to

Table I. Control of bias in RCTs

Potential sources of bias RCTs (n = 62)

At entry: Method of randomization was reported 29 (47%)

Assessment of outcome: Blinding was performed and reported 25 (40%)

After entry: All subjects accounted for at the end of the study 47 (76%)

RCTs = Randomized control trials.

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treatment groups. Without information on themethod of randomization used, the reader is unable tojudge the rigor with which assignment bias wasaddressed.

Blinding of the investigators and/or subjects totreatment allocation was performed or reported inonly 40% of the studies, making this area the mostpoorly controlled among the 3 potential sources ofbias. However, the low percentage reflects practicalproblems in the design of clinical studies in dentistry,where blinding frequently is not possible. After review-ing the nature of treatment in the RCTs that did notperform or report blinding, it was judged that only 12other studies (19%) could have incorporated blindinginto the assessment of outcome. Blinding of eitherinvestigators or subjects was not appropriate in 40% ofthe RCTs because of the nature of the treatment inter-vention.

A higher percentage of studies (76%) controlledbias after entry. Describing subject dropouts and with-drawals is crucial: adequate reporting allows the readerto judge whether the reasons for dropout or with-drawal were study related. Ideally, statistical resultsshould be presented with and without dropoutsaccounted for in the analysis.

This article is the third in a series of articles pub-lished on the subject of RCTs in prosthodonticjournals.7,8 A Register for Prosthodontic RCTs hasbeen initiated, and different search strategies foridentifying RCTs through MEDLINE have been

evaluated. In the hierarchy of evidence, informationfrom RCTs is considered the most reliable. This pre-supposes that the RCT trial methodology wasplanned and executed to eliminate bias inherent inother study designs. The quality scores for RCTsevaluated in this study reveal inadequacies in therigor with which RCTs have been conducted and/orreported in prosthodontic journals. There is roomfor improvement in the design and reporting of clin-ical trials.

Adequate reporting of trial methodology facilitatesinterpretation of the trial design and results as well ascomparison with other trials. It is hoped that assess-ments of the quality of RCTs will contribute to bettertrial design, implementation, and reporting.

CONCLUSIONS

Minimizing potential sources of bias and adequate-ly reporting trial methodology may enhance thequality of RCTs published in prosthodontic journals.

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Table II. Quality assessment of RCTs

Overall score Int J Prosthodont J Prosthet Dent J Prosthodont* RCTs

0 points 0 6 0 6 (10%)1 point 2 18 1 21 (34%)2 points 6 19 0 25 (40%)3 points 1 9 0 10 (16%)Total RCTs 9 52 1 62

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Table III. Frequency of studies scored according to source of bias

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10 1 1 1 3

*Minimum score 0 = bias poorly controlled in all areas; maximum score 3 = bias adequately controlled in all areas.

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BAYLOR COLLEGE OF DENTISTRY

3302 GASTON AVE, ROOM 601DALLAS, TX 75246FAX: (214)828-8952E-MAIL: hdumbrigue@tambcd.edu

Copyright © 2001 by The Editorial Council of The Journal of ProstheticDentistry.

0022-3913/2001/$35.00 + 0. 10/1/119980

doi:10.1067/mpr.2001.119980

Wettability of visible light-curing denture lining materialsZissis AJ, Polyzois GL, Jagger RG, Waters MG. Int JProsthodont 2001;14:250-4.

Purpose. This in vitro study examined the wetting properties of 10 visible light-polymerizing(VLP) denture reline materials.Material and methods. Ten VLP denture reline materials (Perform Soft, Light Liner Soft &Hard, Triad Reliner Hard, Resiline, Lightdon-U, DuaLine, Rebaron LC, and Astron Soft &Hard), 1 autopolymerizing denture reline material, and 1 autopolymerizing denture base mater-ial were studied. Five 30 � 10 � 1-mm specimens were prepared from each material. Thespecimens were stored for 1 month in distilled water at room temperature. Using a Cahn dynam-ic contact angle analyzer linked to a computer, the wettability of each specimen was estimated bymeasuring the equilibrium and hysteresis contact angles. Collected data were analyzed with a 1-way analysis of variance and a Student-Newman-Keuls test (α=.05).Results. Perform Soft exhibited statistically better wettability than all other materials tested. The2 autopolymerized materials demonstrated similar equilibrium contact angles. The soft VLPmaterials showed similar hysteresis. The hard VLP materials demonstrated similar hysteresis,except for Triad Reline. Triad was significantly different from Rebaron LC and Astron Hard.Conclusions. Perform Soft VLP material demonstrated better wettability than the other materi-als evaluated. The wetting properties of the VLP materials tested in this study were similar tothose of the conventional autopolymerizing hard lining and denture base materials evaluated. 14References. —DL Dixon

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