The duration of orthodontic treatment with and without extractions: A pilot study of five selected practices Peter S. Vig, BDS, PhD, D. Orth., FDSRCS (Eng.)," Jane A. Weintraub, DDS, MPH, b Colleen Brown, DDS, MS, = and Charles J. Kowalski, PhD d
Ann Arbor, Mich., C/lapel Hill, N.C., and Berkeley, Calif. Contemporary orthodontic practice is diverse, both in the variety of clinical problems treated and in the methods used. Practices differ with respect to their patient composition as well as in many variables relative to treatment protocols. Such heterogeneity makes it difficult to make valid generalizations concerning the characteristics of orthodontic treatment procedures or outcomes; yet data and methods are required for assessment of issues of efficacy and utility. The frequency of orthodontic extractions is an objective criterion that distinguishes practices and may also be related to differences in treatment outcome variables, such as duration. Following a telephone survey to estimate extraction rates in the practices of 238 Michigan orthodontists, five practices with very high or low reported rates were chosen for this pilot study. Our primary aim was to determine whether a systematic relationship existed between the relative frequency of extraction treatments and the duration of active appliance therapy. Records of 438 patients from these practices were examined. The extraction rates of the practices ranged from a low of 25% to a high of 84%. Treatment duration was affected by several vadables, such as the number of arches treated, the number of treatment phases, and the practice selected. When the data for all five practices were pooled, and all of the extraction versus nonextraction treatments were compared, the mean durations of treatment were 31.2 and 31.3 months, respectively. Data from individual practices, however, indicated that extraction treatment in each of the practices was of longer duration than n0dextraction therapy. These differences in duration were 3.0, 6.6, 2.4, 3.0, and 7.3 months in the five practices. The pooling of all extraction and nonextraction cases evidently obscures the effect of other variables that contribute to treatment duration and indicates the need for a larger and more representative sample of practices, as well as the use of multivariate statistical analyses of appropriate "power," to derive unambiguous inferences with respect to the effect of the extraction decision on treatment dtiration specificially and efficacy in general. This pilot study thus provides initial estimates of ranges in treatment duration and variability in orthodontic practices with high and low rates of extraction, it also illustrates the use of Stepwise regression to clarify the effect that some frequent clinical decisions may have on the length of active orthodontic treatment and reveals the need for further studies. (AMJ ORTHOD DENTOFAC ORTHOP 1990;97:45-51.)
A l t h o u g h there is an extensive amount of research literature on the effects of orthodontic treatment, the efficacy of orthodontics has not yet been systematically addressed. Treatment effect studies deal with issues of changes that are attributable to some biomechanical intervention and address questions con• Professor. Department of Orthodontics and Pediatric Dentistry, and Research Scientist, Center for tluman Growth and Development, The University of Michigan. Ann Arbor. bAssistant Professor, Department of Dental Ecology, School or Dentistry, University of North Carolina, Chapel Hill. ¢In private orthodontic practice, Berkeley, Calif. JProfessor, Department of Dentistry, and Research Scientist, Dental Research Institute. The University of Michigan, Ann Arbor. 8/!/10455
cerning the response of the biologic system to a particular technique or protocol of prescribed force application. Such studies have added much tO our understanding and form the background for our clinical practice. Efficacy issues, by contrast, deal with questions that reflect on the relative utility of the outcome of clinical decisions and procedures, to both the provider and the consumer, and are thus also in the realms of socioeconomic and health services research rather than being entirely rooted in traditional craniofacial biology. In common with all forms of health service, orthodontics today is also subjected to scrutiny and occasionally to criticism that may be unjustified. Reliable 45
46
Vig et al.
information on benefits, costs, and risks associated with treatment is not yet available. Despite, or possibly because of, this lack of objective data, confliciing assertions concerning the standard of orthodontic care are being made find subjective Opinions, often strongly held, are expressed to promote one approach at the expense of oihets. While such rhetoric can and does have a temporary impact, the long-term resolution of how we can provide our patients with truly optimum care rests on our ability to demonstrate that, like other speciaities, We are able to provide valid, quantitative evidence of both effects ~ind efficacy of treatment: Justification o f any alternative clinical strategies on the basis of some demonstrably excellent results, derived from selected case reports and aitributable to one form of treatment oi: another, is no longer sufficient. Because most of the cUrrently popular modalities can and tlo produce excellent results some Of the time, but none do so all of the time, it is necessary to begin the generationof objective data on efficacy as well as treatment effects. TO make the point more explicit, it maY be stated that clinical research can be viewed as being of two types. The tJaditional type of Clinical research deals with establishment of how, and how well, a particular treatment works tiiader the most favorable conditions and when performed by the most expert operator. This is essential for determining the optimum potential of any form of treatment. The more recently emerging t~,pe of clinical research seeks to determine the probabilities of treatment outcomes under more global and hence more "real-world" conditions. Thus one can get estimates of the usual rather than the ideal. This pilot study is a step toward such a goal. The literature reveals n/.imerous statements concerning the duration of treatment for selected patients whose treatment was performed according to a specified technique. Examples include reports by FrankeP in 1974 and Pancherz~ in 1984. Both these and other articles with similar aims provide useful information on howiong the treatment of selected cases with excellent results took. Such information, however, is an incidental finding in studies aimed at elucidation of treatment effects and hence is not an appropriate.basis for the comparison of treatment duration between Or among alternative techniques. It certainly does not (nor is it intended to) permit extrapolation to orthodontic practice in general. The relationship between duration of treatment and serial extraction h~is been systematically investigated by Ringenberg 3 in a controlied clinical study. In his sample of children with Class I crowding, there were
Am. J. Orthod. Dentofac. Orthop. January 1990
significant differences in both total treatment duration (53 versus 37 months) and length of active appliance therapy (12.7 versus 19.0 months) between those patients who had serial exiraction before definitive treatment and those who did not. This excellent study is unique in the clinical orthodontic literature in that it does provide estimates on which clinical decisions may be based concerning the trade-off between total treatment time and length of ~tctive appliance treatment in a defined patient population with a clinical problem for which at least two alternative treatment options exist. Although the controversy regarding whether to, when, and how often it is desirable to extract teeth for 0flhodontic reasons has a long history and is relevant to malocclusions of many types, the issue has not been the direct focus of stildies specifically designed to clarify this question. The decision of whether to extract teeth as part of orthodontic treatment may affect the outcome of treatmerit, including/he soft tissue profile, esthetics, occlusion, function, patient satisfaction, and duration of treatment. If may also affect the process of treatment, such as the number of visits needed, costs to both the patient and the clinician associated with extra visits, and potential adverse side effects from a surgical procedure. Ideally, to make this decision ratiofially, one must have some knowledge of both the risks and the benefits. To date, no quantifiable information is available to test the consequences of this decision because rigorous clinical epidemioiogic studies have not been conducted in orthodontics. At present orthodontists opt to extract or not according to their education, past experiences, and generally stibjective considerations. This study was undertaken to begin the objective assessment of these ti'eatment options and also to consider some of the confounding factors that may influence the outcomes. The specific purpose of this study is to evaluate how the extraction versus the nonextraction approach affecis the duration of orthodontic treatment and to determine whether there are systematic differences in treatment duration between practices with high and low extraction rates. Since duration of treatment is only one aspect of the outcome of orthodontic treatment, the value or utility to a patient of a particular duration should be weighed against other possible consequences of the treatment decision. Neither the probabilities of such outcomes nor their relative utilities are yet known. Until they are, a formal analytic approach to the evaluation of extractions in orthodontic treatment ]s not possible. In focusing on duration, which is just one aspect of treatment outcome, this study is not comprehensive;
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Duration of orthodontic treatment with and without extractions
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Number 1
however, to the extent that it does yield data on a major and long-standing cause of disagreement among clinicians, it contributes information that is integral to studies o f the efficacy of orthodontic treatment.
MATERIALS AND METHODS Selection of practices A telephone survey of all Michigan orthodontists was conducted to determine the estimated extraction rate in their practices. The question posed was, "What is the estimate of the percentage o f extraction versus nonextraction treatment in your practice?" Responses from 238 orthodontists (90% response rate) indicated that the mean estimated extraction rate was 39%, with a range of 5% to 88%. The practices at the two extremes of estimated extraction rate were identified. There were 18 who reported an estimated rate of 70% or higher and 25 who reported an estimated rate of 15% or less. Details of this study are reported elsewhere. 4 Five practices in southeastern Michigan were selected~from these two extremes for comparison. Any case in which one or more extraction procedures were associated with orthodontic care, excluding the removal of third molars, qualified as an "extraction case." In four of the practices, recently debanded patients were selected for this survey. The patients in practice 5, however, were treated over a wide range of years, with debandings from 1947 to' 1984. This practice was selected because of the complete records available in addition to the fact that it satisfied the other criteria for selection. All the data were obtained by one of us from the records of the five practices. The following information was recorded for each patient: gender, age at first examination, birth date, Angle's classification of molar relationship, missing • teeth (except third molar), number of treatment pha~es, nature o f treatment phases, arch(es) treated, starting date(s) of treatment phase(s), debanding date(s), nonextraction or extraction and type of extraction pattern, pretreatment overbite and overjet, and posttreatment overbite and overjet. It was also noted whether or not the patients had orthognathic surgery, were being re-treated, had transferred into the practice, or had moved awaY during treatment. This information was obtained from the patients' written records. Radiographs and models were used to supplement and verify the accuracy of the written records. Overbite and overjet measurements were made on study models with a Boley gauge, calibrated to O. 1 mm. Orthodontists were asked to identify patients in their part of the sample who had asked to have their appliances removed "early" (i.e., before completion of
treatment), and these patients were not included in the sample. In addition, two patients who were identified as having missed "a lot" of appointments were not included. It was decided that if any adjunctive appliance, such as headgear, a Lewis plate (a removable appliance with springs to move canines), or a Bionator (a functional appliance intended to stimulate mandibular growth), was used before a regular phase of treatment, and there was no "lag" time between phases (i.e., a recall or retention period), it was classified as a separate phase o f treatment only if it was used for 6 months or more. The start of treatment was defined as the date of first arch wire placement and debanding as the date on which fixed appliances were completely removed.
Statistical design Since the frequency of treatment duration was normally distributed, parametric statistics were used. The null hypothesis that there is no significant difference between duration of treatment for patients who had extractions and those who did not was tested by the StudentAwo-tailed t test. This test was applied to the entire patient sample and also to the patient groups from each practice contributing to the sample. Stepwise regression analysis was used to determine what statistical relationship existed between the dependent variable, duration of treatment, and a number of independent variables. The following regression model was used: y = a + b, x , + b2 x 2 + b3 x 3 + bx + b4 x , + b~ x ~ + b6 x , + b7 x 7 + b8 x 8 + b9 x where y = length of treatment time a = equation constant x ' = gender x 2 = molar relationship x~-~= arches treated x4 = age at start of treatment x 5 = number of phases of trea!ment :. x 6 = nonextraction or extraction x 7 = change in overbite x 8 = change in overjet x 9 = practice and b, through b9 are the corresponding regression coefficients for the nine variables.
RESULTS The records o f 438 patients were included in the analysis after exclusion of 119 for the following reasons: missing teeth, transfer patients, re-treatment, pre-
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Am. J. Orthod. Dentofac. Orthop. January 1990
Table i. Distribution o f gender and age at start o f treatment o f patients in five orthodontic practices in southeastern Michigan Age at start (yr-mo) No. of patients
Practice(s)
Females (%)
Median
Age at debanding (yr-mo)
Range
Median
I
Range
I
I-5 1 2 3 4 5
438 80 93 87 89 89
59.6 63.8 52.7 63.2 57.3 61.8
13-3 13-11 12-2 13-9 12-3 13-3
7-0 to 53-1 7-3 to 53-1 7-0 to 35-1 11-I to 38-2 7-8 to 29-2 9-1 to 17-10
15-7 16-10 15-3 16-2 14-11 15-3
11-9 to 55-4 12-4 to 55-4 11-9 to 36-8 12-11 to 39-8 12-3 to 31-6 12-10 to 19-6
Table II, Mean pretreatment occlusal relationship variables in the patients o f five orthodontic practices surveyed Molar relationship Practice(s)
No. of patients
Clas r I (%
Class H, Div. 1 (%)
Class 11, Div. 2 (%)
Class HI (%)
Mean pretreatment overbite (mm)
Mean pretreatment overjet (ram)
1-5 I 2 3 4 5
438 80 93 87 89 89
43.7 27.5 62.0 54.0 41.6 31.5
43.5 42.5 19.6 41.4 48.3 66.3
7.3 17.5 6.4 4.6 9.0 0
5.5 12.5 12.0 0 l.l 2.2
3.2 n = 411 3.1 n = 73 3.1 n = 78 3.4 n = 87 2.9 n = 84 3.6 n = 89
4.9 n = 411 4.8 n = 73 4.0 n = 78 4.5 n = 87 4.6 n = 84 6.3 n = 89
phase (e.g., "functional") only completed, missed "a lot" o f appointments, asked for appliances off early, had surgery or, in the clinician's opinion, needed surgery, or patients moved. The distributions o f the five practices with respect to gender and median age o f patients at the start o f treatment and at the time o f debanding are shown in Table I. There were 177 male (40.4%) and 261 female (59.6%) patients. Each practice had a similar gender distribution. A g e at the start o f treatment ranged from 7 years to 53 years 1 month, and the median age was 13 years 3 months. The most striking differences between the practices were those o f the patients in practice 3, in which none started treatment before the age o f 11 years 1 month, and o f the patients in practice 5, in which none started treatment after the age o f 17 years I0 months. The age at debanding ranged from I 1 years 9 months to 55 years 4 months, and the median age was 15 years 7 months. The distribution o f molar relationships found among the five practices is presented in Table II. O f the 438
patients, 191 (43.7%) had an Angle Class I molar relationship, 190 (43.5%) had Class II, Division 1, 32 (7.3%) had Class II, Division 2, and 24 (5.5%) had Class III. There was, however, considerable variability among the practices. Practice 1 had a low prevalence o f Class I patients in the sample (27.5%), whereas practice 2 had a high prevalence o f Class I patients (62.0%). Also, both practices l and 2 had a high prevalence o f Class III patients (12.5% and 12.0%, respectively). There were no Class II, Division 2 patients in practice 5. The distributions o f mean pretreatment overbite and overjet measurements are presented in Table II. These pretreatment measures were available for 411 o f the 438 patients. The mean pretreatment overbite was 3.2 mm, with a range o f - 9 . 4 m m (open bite) to 100 m m (deep bite) and a standard deviation o f 2.1 mm. The mean pretreatment overjet was 4.9 mm, with a range o f - 1.3 m m to 13.2 m m and a standard deviation o f 2.4 mm. The posttreatment measures were available for only 141 patients (data not shown). For these patients,
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Duration of orthodontic treatment with and without extractions
49
Table III. Distribution of treatment variables and extraction rates in five orthodontic practices in southeastern Michigan Practice(s)
I~oo~lBotharcheslOnearchlOnephase patients (%) (%) (%)
1-5 1 2 3 4 5
438 80 93 87 89 89
88.6 86.2 92.5 69.8 97.8 95.5
11.4 13.8 7.5 30.2 2.2 4.5
TwophaseslThree+phases (%) (%)
76.4 57.7 75.3 94.3 65.2 87.6
21.2 37.2 21.5 5.7 30.3 12.4
Nonextraction Extraction (%) (%)
2.5 5.1 3.2 0 4.5 0
46.1 75.0 44.6 31.4 67.4 14.6
53.9 25.0 55.4 68.6 32.6 85.4
Table IV. Duration of treatment and extraction rates in five orthodontic practices in southeastern Michigan Mean duration of treatment (in months) Practice(s)
Extraction rate percent
No.
1-5 I 2 3 4 5
53.9 25.0 55.9 68.6 32.6 84.4
202 60 41 27 60 14
None.ttraction 31.3 34.8 32.0 26.7 32.3 17.5
(SD (SD (SD (SD (SD (SD
the mean posttreatment overbite was 2.6 mm, with a range of 0 to 7.8 mm and a standard deviation of 1.5 mm. The mean posttreatment overjet was 2.7 mm, with a range of 0.5 to 6.2 mm and a standard deviation of 0.8 mm. Table III shows the distributions in each practice of the percent of patients treated by number of arches treated, number of phases of treatment, and extraction rate. The majority of patients (88.6%) had treatment involving both upper and lower arches. Practice 3 was unusual in that 30.2% of its patients received treatment in only one arch. For the overall sample, 76.4% of the patients had one phase of treatment, 21.2% had two phases, and 2.5% had three or more phases. When we look at each practice separately, it can be seen that practices 1 and 4 had low percentages of patients with one phase of treatment and relatively high percentages of patients who had two or more phases of treatment. For these five practices, the mean extraction rate was 54%, with a range from 25% to 85%. There were also obvious differences between these practices in terms of their treatment strategies in addition to differences in extraction rate. Practice 3 had many more patients in whom only one arch was treated than the other practices. Differences in types of orthodontic appliance used were clearly present and could be associated with extraction rates (data not shown). Expansion techniques were used the most in practice 1, and functional appliances were used the most in
= = = = = =
14.58) 15.42) 15.84) 11.65) 13.57) 8.15)
No. 236 20 52 60 29 75
Extraction 31.2 37.8 38.6 29.1 35.3 24.8
(SD (SD (SD (SD (SD (SD
= = = = = =
13.18) 13.42) 16.42) 10.53) 10.54) 9.52)
p 0.98 0.45 0.06 0.36 0.29 0.01
practice 4. Second molar extractions were also more prevalent in these two practices, which had the lowest extraction rates in the sample. In practice 5, which had the highest extraction rate, certain techniques (notably wearing of headgear) were also used more extensively than in the other practices. As shown in Table IV, for the total sample of 438 patients, the results of Student t test showed no difference in duration of treatment between patients who were treated with extractions and those who were not. Mean duration of treatment for each group was 31.2 and 31.3 months, respectively. For the entire sample, there was a range in duration from 4 months to 7 years 6 months, with a standard deviation of 13.9 months. When each practice was analyzed separately, a statistically significant difference was found for practice 5, although the small number of nonextraction patients in this practice needs to be kept in mind. In this practice, extraction treatment was found to last about 7.4 months longer than nonextraction treatment (p = 0.01). In practices 1 to 4, extraction treatment was also found to be longer, by 3 months, 6.5 months, 2.3 months, and 3. I months, respectively, but these differences were not statistically significant. On the basis of these results, it is not possible to reject the null hypothesis that there is no significant difference between duration of treatment for patients who had extractions and those who did not. These results show, however, that when a difference does exist, extraction treatment usually takes Ion-
50
Vig et al.
Am. J. Orthod. Dentofac. OrJhop. January 1990
ger than nonextraction treatment within a particular practice. Stepwise regression analysis was used to test the associations between duration of treatment and the nine variables defined previously for all practices and each of the individual practices. The number of treatment phases, arches treated, molar relationship, age at start of treatment, "and extraction or nonextraction status were significantly correlated with duration of treatment. The following equation with an r squared value of 0.33 and multiple r = 0.57 was generated for the entire group: y = 18.8 + (13.56)(vi) + (-7.78)(v2) + (4.50)(v3) + (2.95)(v4) + (-.03)(v5) where y = duration of treatment, in months Vl = number of phases
1 2 3 0 I 0
= 1 phase = 2 phase = 3 + phases V2 = arches treated = both = one only V3 = molar relationship, = MR = Class II, Class II, Division 1, or Class III Class II, Division 2 1 = MR = Class II, Division 2 V4 = extraction/non~ 0 = non-extration extraction 1 = extraction V4 = starting age, analytical variable recorded in months Gender and the other dental relationship variables did not contribute significantly to the equation. The variables included in the equation affect the duration o f treatment in the following ways: 13.56 months are added for each phase of treatment (p < 0.0001); 7.78 months are subtracted if only one arch is treated (p < 0.001); 4.50 mbnths are added if the malocclusion is Class II, Division 2 (p = 0.04); 2.9 months are added if extractions are used (p = 0.01), and the starting age in months multiplied by 0.03 is subtracted (p < 0.0001). For example, for a 12-year-old patient with a Class I molar relationship being treated in one phase and having both arches treated, without extractions, the predicted duration o f treatment, y, would be 28 months. DISCUSSION
The five practices examined were very different with respect to most o f the variables studied. Some variability was virtually guaranteed by the initial selection of practices at the two extremes o f extraction frequency rates. Together, however, the patient popu-
lations in these practices are similar to those found in larger surveys of orthodontic practices in terms of gender distribution ~7 and prevalence of molar relationships2 .7~° Gender distribution continued to be representative when each practice was considered separately, but, as described earlier, this was not true for prevalence of each type of molar relationship. Average duration of treatment for the entire sample was found,to be about 31 months, which is quite long when one consideres that, typically, an orthodontist predicts duration of treatment to be "18 to 24 months." The practices were also different in this respect, with practice 5, the practice with the highest extraction rate, having the shortest mean duration of treatment (2 years), and practices 1 and 2 having the longest (both at 2 years 11.5 months). The weighted mean duration of treatment for the extraction and nonextraction cases in the five practices is influenced by practice 5. This practice is an outlier with its short mean duration of treatment and small proportion of nonextraction patients. The extraction of teeth is one of the more obvious and less desirable aspects of orthodontic treatment. To date, no cost-benefit or risk-benefit analyses have been applied to the extraction/nonextraction controversy. It is not clear whether the process of orthodontic therapy and its long-term treatment results are significantly or predictably affected by the extraction o f teeth. Clearly, some clinicians and some patients are more averse to extraction therapy than others. The extent to which such treatment decisions are based on the biologic attributes of the patients, such as the cause or severity of malocclusion, or the treatment method preferences of clinicians is not known. Studies that demonstrate the associations between different procedures and their outcomes are needed. Comparisons of both the process and the outcome o f alternative treatment are necessary if clinicians are to choose the most efficacious procedures. As Johnston t~ points out, " . . . the decison to treat in a certain way is, in and of itself, a predictor that can account for more variance in the outcome o f treatment than that produced by random variation in the pattern of growth." It is quite likely that this is not only true of the morphologic results or treatment effects but also holds for some aspects of the efficacy o f orthodontic treatment in general. Whether or not this is true can and should be determined. The increasing press.ures for cost containment in health care concurrent with the need and desire to provide treatment of optimum quality dictates that research endeavors address the cost-effectiveness of alternative clinical policies and decisions and their outcomes.
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Duration o f orthodontic treatment with attd without extractions
It is evident from this preliminary study that the duration of treatment is dependent on a number of variables that include attributes of the patient, the objectives of treatment, and the methods used to accomplish the goals of the clinician. The extraction decision is merely one of the clinical variables and should therefore be considered in that light. it also is clear that, for any given practice, it is possible to develop/i regression model that can aid the clinician in making more accurate estimates of treatment duration for iridividual patients, given the demographic and clinical variables that reflect that orthodontist's pa~em of clinical activity. As fo r characterizing attributes of the efficacy of orthodontics in a general or global sense, future studies of this kind need to be performed on a larger sample or" practices, selected to be representative of the broad spectrum of practice variation. If data of the type presented here are t6 be pooled, then it may be desirable to stratify practices by type and to analyze subgroups derived from a consideration of the frequency distributions of those variables that, a priori, are deemed to have a significant effect on aspects of efficacy. SUMMARY AND CONCLUSIONS
On the basis of the results of this study of 438 patient records from five orthodontic practices, the following conclusions may be drawn: 1. Differences in duiation of treatment were apparen~ ,~,hen extraction and nonextraction patients within e/ach individual practice were compared. Treatment for those who had extractions was more likely to take longer. 2. The duration of orthodontic treatment may be affected not only by the extraction decision but by such variables as whether one or both arches are treated and the ntimber of phases of treatment. The duration of treatment was also affected by which practice was chosen. 3. Considerable differences exist in treatment strategies between the practices with low extraction rates
51
and those with high extraction rates, but clearly a larger number of practices needs to be evaluated to establish associations between treatment variables and outcomes. 4. The sampling method used to identify practices with high and low extraction rates and the pooling of data from all five practices tends to obscure differences in duration between extraction and nonextraction treatments by virtue of other important confounding Variables that are not accounted for when patients are compared merely on the singular criterion of extraction. REFERENCES 1. Frankel R. Decrowding during eruption under the screening influence of vestibular shields. AM J OR'nIOD 1974;65:372-405. 2. Pancherz H. A cephalometric analysis of skeletal and dental changes contributing to Class II correction in activator treatment. AM J ORTtlOD 1984;85:125-33. 3. Ringenberg QM. Influence of serial extraction on growth and development of the maxilla and mandible. AM J ORTtIOD 1967;53:19-25. 4. Brown C. Duration of orthodontic treatment and the extractiontnonextraction treatment decision [Master's thesis]. Ann Arbor, Michigan: University of Michigan, 1987. 5. Hooper JD. Orthodontics as a public service: the Wessex survey. Dent Pract 1967; 17:259-68. 6. Peck S, Peck H. Frequency of tooth extraction in orthodontic treatment. AM J OR'rHoD 1979;76:491-6. 7. Rose JS. A thousand cases: a survey. Br J Orthod 1974;1:4554. 8. Beresford JS. Tooth size and class distinctioa. Trans Br Soe Study Orthod 1968-9;5:111-8. 9. Berg R. Post-retention analysis of treatment problems and failures in 264 consecutively treated cases. Eur J Orthod 1979;1:5568. 10. Sheiham A, Hobdell MW, Vig P, Griffiths PJ. Orthodontic treatment in the General Dental Service in England and Wales; a study. Br Dent J 1971;131:535-8. 11. Johnston LE. A comparative analysis of Class II treatments. In: Vig P, Ribbens K, eds. Science and clinical judgment in orthodontics. Monograph 19. Ann Arbor, Michigan: Center for Human Growth and Development, University of Michigan, 1986. Reprint requests to: Dr. Peter Vig Department of Orthodontics and Pediatric Dentistry Center for Human Growth and Development University of Michigan Ann Arbor, MI 48109
Ann Arbor, Mich., C/lapel Hill, N.C., and Berkeley, Calif. Contemporary orthodontic practice is diverse, both in the variety of clinical problems treated and in the methods used. Practices differ with respect to their patient composition as well as in many variables relative to treatment protocols. Such heterogeneity makes it difficult to make valid generalizations concerning the characteristics of orthodontic treatment procedures or outcomes; yet data and methods are required for assessment of issues of efficacy and utility. The frequency of orthodontic extractions is an objective criterion that distinguishes practices and may also be related to differences in treatment outcome variables, such as duration. Following a telephone survey to estimate extraction rates in the practices of 238 Michigan orthodontists, five practices with very high or low reported rates were chosen for this pilot study. Our primary aim was to determine whether a systematic relationship existed between the relative frequency of extraction treatments and the duration of active appliance therapy. Records of 438 patients from these practices were examined. The extraction rates of the practices ranged from a low of 25% to a high of 84%. Treatment duration was affected by several vadables, such as the number of arches treated, the number of treatment phases, and the practice selected. When the data for all five practices were pooled, and all of the extraction versus nonextraction treatments were compared, the mean durations of treatment were 31.2 and 31.3 months, respectively. Data from individual practices, however, indicated that extraction treatment in each of the practices was of longer duration than n0dextraction therapy. These differences in duration were 3.0, 6.6, 2.4, 3.0, and 7.3 months in the five practices. The pooling of all extraction and nonextraction cases evidently obscures the effect of other variables that contribute to treatment duration and indicates the need for a larger and more representative sample of practices, as well as the use of multivariate statistical analyses of appropriate "power," to derive unambiguous inferences with respect to the effect of the extraction decision on treatment dtiration specificially and efficacy in general. This pilot study thus provides initial estimates of ranges in treatment duration and variability in orthodontic practices with high and low rates of extraction, it also illustrates the use of Stepwise regression to clarify the effect that some frequent clinical decisions may have on the length of active orthodontic treatment and reveals the need for further studies. (AMJ ORTHOD DENTOFAC ORTHOP 1990;97:45-51.)
A l t h o u g h there is an extensive amount of research literature on the effects of orthodontic treatment, the efficacy of orthodontics has not yet been systematically addressed. Treatment effect studies deal with issues of changes that are attributable to some biomechanical intervention and address questions con• Professor. Department of Orthodontics and Pediatric Dentistry, and Research Scientist, Center for tluman Growth and Development, The University of Michigan. Ann Arbor. bAssistant Professor, Department of Dental Ecology, School or Dentistry, University of North Carolina, Chapel Hill. ¢In private orthodontic practice, Berkeley, Calif. JProfessor, Department of Dentistry, and Research Scientist, Dental Research Institute. The University of Michigan, Ann Arbor. 8/!/10455
cerning the response of the biologic system to a particular technique or protocol of prescribed force application. Such studies have added much tO our understanding and form the background for our clinical practice. Efficacy issues, by contrast, deal with questions that reflect on the relative utility of the outcome of clinical decisions and procedures, to both the provider and the consumer, and are thus also in the realms of socioeconomic and health services research rather than being entirely rooted in traditional craniofacial biology. In common with all forms of health service, orthodontics today is also subjected to scrutiny and occasionally to criticism that may be unjustified. Reliable 45
46
Vig et al.
information on benefits, costs, and risks associated with treatment is not yet available. Despite, or possibly because of, this lack of objective data, confliciing assertions concerning the standard of orthodontic care are being made find subjective Opinions, often strongly held, are expressed to promote one approach at the expense of oihets. While such rhetoric can and does have a temporary impact, the long-term resolution of how we can provide our patients with truly optimum care rests on our ability to demonstrate that, like other speciaities, We are able to provide valid, quantitative evidence of both effects ~ind efficacy of treatment: Justification o f any alternative clinical strategies on the basis of some demonstrably excellent results, derived from selected case reports and aitributable to one form of treatment oi: another, is no longer sufficient. Because most of the cUrrently popular modalities can and tlo produce excellent results some Of the time, but none do so all of the time, it is necessary to begin the generationof objective data on efficacy as well as treatment effects. TO make the point more explicit, it maY be stated that clinical research can be viewed as being of two types. The tJaditional type of Clinical research deals with establishment of how, and how well, a particular treatment works tiiader the most favorable conditions and when performed by the most expert operator. This is essential for determining the optimum potential of any form of treatment. The more recently emerging t~,pe of clinical research seeks to determine the probabilities of treatment outcomes under more global and hence more "real-world" conditions. Thus one can get estimates of the usual rather than the ideal. This pilot study is a step toward such a goal. The literature reveals n/.imerous statements concerning the duration of treatment for selected patients whose treatment was performed according to a specified technique. Examples include reports by FrankeP in 1974 and Pancherz~ in 1984. Both these and other articles with similar aims provide useful information on howiong the treatment of selected cases with excellent results took. Such information, however, is an incidental finding in studies aimed at elucidation of treatment effects and hence is not an appropriate.basis for the comparison of treatment duration between Or among alternative techniques. It certainly does not (nor is it intended to) permit extrapolation to orthodontic practice in general. The relationship between duration of treatment and serial extraction h~is been systematically investigated by Ringenberg 3 in a controlied clinical study. In his sample of children with Class I crowding, there were
Am. J. Orthod. Dentofac. Orthop. January 1990
significant differences in both total treatment duration (53 versus 37 months) and length of active appliance therapy (12.7 versus 19.0 months) between those patients who had serial exiraction before definitive treatment and those who did not. This excellent study is unique in the clinical orthodontic literature in that it does provide estimates on which clinical decisions may be based concerning the trade-off between total treatment time and length of ~tctive appliance treatment in a defined patient population with a clinical problem for which at least two alternative treatment options exist. Although the controversy regarding whether to, when, and how often it is desirable to extract teeth for 0flhodontic reasons has a long history and is relevant to malocclusions of many types, the issue has not been the direct focus of stildies specifically designed to clarify this question. The decision of whether to extract teeth as part of orthodontic treatment may affect the outcome of treatmerit, including/he soft tissue profile, esthetics, occlusion, function, patient satisfaction, and duration of treatment. If may also affect the process of treatment, such as the number of visits needed, costs to both the patient and the clinician associated with extra visits, and potential adverse side effects from a surgical procedure. Ideally, to make this decision ratiofially, one must have some knowledge of both the risks and the benefits. To date, no quantifiable information is available to test the consequences of this decision because rigorous clinical epidemioiogic studies have not been conducted in orthodontics. At present orthodontists opt to extract or not according to their education, past experiences, and generally stibjective considerations. This study was undertaken to begin the objective assessment of these ti'eatment options and also to consider some of the confounding factors that may influence the outcomes. The specific purpose of this study is to evaluate how the extraction versus the nonextraction approach affecis the duration of orthodontic treatment and to determine whether there are systematic differences in treatment duration between practices with high and low extraction rates. Since duration of treatment is only one aspect of the outcome of orthodontic treatment, the value or utility to a patient of a particular duration should be weighed against other possible consequences of the treatment decision. Neither the probabilities of such outcomes nor their relative utilities are yet known. Until they are, a formal analytic approach to the evaluation of extractions in orthodontic treatment ]s not possible. In focusing on duration, which is just one aspect of treatment outcome, this study is not comprehensive;
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Duration of orthodontic treatment with and without extractions
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Number 1
however, to the extent that it does yield data on a major and long-standing cause of disagreement among clinicians, it contributes information that is integral to studies o f the efficacy of orthodontic treatment.
MATERIALS AND METHODS Selection of practices A telephone survey of all Michigan orthodontists was conducted to determine the estimated extraction rate in their practices. The question posed was, "What is the estimate of the percentage o f extraction versus nonextraction treatment in your practice?" Responses from 238 orthodontists (90% response rate) indicated that the mean estimated extraction rate was 39%, with a range of 5% to 88%. The practices at the two extremes of estimated extraction rate were identified. There were 18 who reported an estimated rate of 70% or higher and 25 who reported an estimated rate of 15% or less. Details of this study are reported elsewhere. 4 Five practices in southeastern Michigan were selected~from these two extremes for comparison. Any case in which one or more extraction procedures were associated with orthodontic care, excluding the removal of third molars, qualified as an "extraction case." In four of the practices, recently debanded patients were selected for this survey. The patients in practice 5, however, were treated over a wide range of years, with debandings from 1947 to' 1984. This practice was selected because of the complete records available in addition to the fact that it satisfied the other criteria for selection. All the data were obtained by one of us from the records of the five practices. The following information was recorded for each patient: gender, age at first examination, birth date, Angle's classification of molar relationship, missing • teeth (except third molar), number of treatment pha~es, nature o f treatment phases, arch(es) treated, starting date(s) of treatment phase(s), debanding date(s), nonextraction or extraction and type of extraction pattern, pretreatment overbite and overjet, and posttreatment overbite and overjet. It was also noted whether or not the patients had orthognathic surgery, were being re-treated, had transferred into the practice, or had moved awaY during treatment. This information was obtained from the patients' written records. Radiographs and models were used to supplement and verify the accuracy of the written records. Overbite and overjet measurements were made on study models with a Boley gauge, calibrated to O. 1 mm. Orthodontists were asked to identify patients in their part of the sample who had asked to have their appliances removed "early" (i.e., before completion of
treatment), and these patients were not included in the sample. In addition, two patients who were identified as having missed "a lot" of appointments were not included. It was decided that if any adjunctive appliance, such as headgear, a Lewis plate (a removable appliance with springs to move canines), or a Bionator (a functional appliance intended to stimulate mandibular growth), was used before a regular phase of treatment, and there was no "lag" time between phases (i.e., a recall or retention period), it was classified as a separate phase o f treatment only if it was used for 6 months or more. The start of treatment was defined as the date of first arch wire placement and debanding as the date on which fixed appliances were completely removed.
Statistical design Since the frequency of treatment duration was normally distributed, parametric statistics were used. The null hypothesis that there is no significant difference between duration of treatment for patients who had extractions and those who did not was tested by the StudentAwo-tailed t test. This test was applied to the entire patient sample and also to the patient groups from each practice contributing to the sample. Stepwise regression analysis was used to determine what statistical relationship existed between the dependent variable, duration of treatment, and a number of independent variables. The following regression model was used: y = a + b, x , + b2 x 2 + b3 x 3 + bx + b4 x , + b~ x ~ + b6 x , + b7 x 7 + b8 x 8 + b9 x where y = length of treatment time a = equation constant x ' = gender x 2 = molar relationship x~-~= arches treated x4 = age at start of treatment x 5 = number of phases of trea!ment :. x 6 = nonextraction or extraction x 7 = change in overbite x 8 = change in overjet x 9 = practice and b, through b9 are the corresponding regression coefficients for the nine variables.
RESULTS The records o f 438 patients were included in the analysis after exclusion of 119 for the following reasons: missing teeth, transfer patients, re-treatment, pre-
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Am. J. Orthod. Dentofac. Orthop. January 1990
Table i. Distribution o f gender and age at start o f treatment o f patients in five orthodontic practices in southeastern Michigan Age at start (yr-mo) No. of patients
Practice(s)
Females (%)
Median
Age at debanding (yr-mo)
Range
Median
I
Range
I
I-5 1 2 3 4 5
438 80 93 87 89 89
59.6 63.8 52.7 63.2 57.3 61.8
13-3 13-11 12-2 13-9 12-3 13-3
7-0 to 53-1 7-3 to 53-1 7-0 to 35-1 11-I to 38-2 7-8 to 29-2 9-1 to 17-10
15-7 16-10 15-3 16-2 14-11 15-3
11-9 to 55-4 12-4 to 55-4 11-9 to 36-8 12-11 to 39-8 12-3 to 31-6 12-10 to 19-6
Table II, Mean pretreatment occlusal relationship variables in the patients o f five orthodontic practices surveyed Molar relationship Practice(s)
No. of patients
Clas r I (%
Class H, Div. 1 (%)
Class 11, Div. 2 (%)
Class HI (%)
Mean pretreatment overbite (mm)
Mean pretreatment overjet (ram)
1-5 I 2 3 4 5
438 80 93 87 89 89
43.7 27.5 62.0 54.0 41.6 31.5
43.5 42.5 19.6 41.4 48.3 66.3
7.3 17.5 6.4 4.6 9.0 0
5.5 12.5 12.0 0 l.l 2.2
3.2 n = 411 3.1 n = 73 3.1 n = 78 3.4 n = 87 2.9 n = 84 3.6 n = 89
4.9 n = 411 4.8 n = 73 4.0 n = 78 4.5 n = 87 4.6 n = 84 6.3 n = 89
phase (e.g., "functional") only completed, missed "a lot" o f appointments, asked for appliances off early, had surgery or, in the clinician's opinion, needed surgery, or patients moved. The distributions o f the five practices with respect to gender and median age o f patients at the start o f treatment and at the time o f debanding are shown in Table I. There were 177 male (40.4%) and 261 female (59.6%) patients. Each practice had a similar gender distribution. A g e at the start o f treatment ranged from 7 years to 53 years 1 month, and the median age was 13 years 3 months. The most striking differences between the practices were those o f the patients in practice 3, in which none started treatment before the age o f 11 years 1 month, and o f the patients in practice 5, in which none started treatment after the age o f 17 years I0 months. The age at debanding ranged from I 1 years 9 months to 55 years 4 months, and the median age was 15 years 7 months. The distribution o f molar relationships found among the five practices is presented in Table II. O f the 438
patients, 191 (43.7%) had an Angle Class I molar relationship, 190 (43.5%) had Class II, Division 1, 32 (7.3%) had Class II, Division 2, and 24 (5.5%) had Class III. There was, however, considerable variability among the practices. Practice 1 had a low prevalence o f Class I patients in the sample (27.5%), whereas practice 2 had a high prevalence o f Class I patients (62.0%). Also, both practices l and 2 had a high prevalence o f Class III patients (12.5% and 12.0%, respectively). There were no Class II, Division 2 patients in practice 5. The distributions o f mean pretreatment overbite and overjet measurements are presented in Table II. These pretreatment measures were available for 411 o f the 438 patients. The mean pretreatment overbite was 3.2 mm, with a range o f - 9 . 4 m m (open bite) to 100 m m (deep bite) and a standard deviation o f 2.1 mm. The mean pretreatment overjet was 4.9 mm, with a range o f - 1.3 m m to 13.2 m m and a standard deviation o f 2.4 mm. The posttreatment measures were available for only 141 patients (data not shown). For these patients,
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Duration of orthodontic treatment with and without extractions
49
Table III. Distribution of treatment variables and extraction rates in five orthodontic practices in southeastern Michigan Practice(s)
I~oo~lBotharcheslOnearchlOnephase patients (%) (%) (%)
1-5 1 2 3 4 5
438 80 93 87 89 89
88.6 86.2 92.5 69.8 97.8 95.5
11.4 13.8 7.5 30.2 2.2 4.5
TwophaseslThree+phases (%) (%)
76.4 57.7 75.3 94.3 65.2 87.6
21.2 37.2 21.5 5.7 30.3 12.4
Nonextraction Extraction (%) (%)
2.5 5.1 3.2 0 4.5 0
46.1 75.0 44.6 31.4 67.4 14.6
53.9 25.0 55.4 68.6 32.6 85.4
Table IV. Duration of treatment and extraction rates in five orthodontic practices in southeastern Michigan Mean duration of treatment (in months) Practice(s)
Extraction rate percent
No.
1-5 I 2 3 4 5
53.9 25.0 55.9 68.6 32.6 84.4
202 60 41 27 60 14
None.ttraction 31.3 34.8 32.0 26.7 32.3 17.5
(SD (SD (SD (SD (SD (SD
the mean posttreatment overbite was 2.6 mm, with a range of 0 to 7.8 mm and a standard deviation of 1.5 mm. The mean posttreatment overjet was 2.7 mm, with a range of 0.5 to 6.2 mm and a standard deviation of 0.8 mm. Table III shows the distributions in each practice of the percent of patients treated by number of arches treated, number of phases of treatment, and extraction rate. The majority of patients (88.6%) had treatment involving both upper and lower arches. Practice 3 was unusual in that 30.2% of its patients received treatment in only one arch. For the overall sample, 76.4% of the patients had one phase of treatment, 21.2% had two phases, and 2.5% had three or more phases. When we look at each practice separately, it can be seen that practices 1 and 4 had low percentages of patients with one phase of treatment and relatively high percentages of patients who had two or more phases of treatment. For these five practices, the mean extraction rate was 54%, with a range from 25% to 85%. There were also obvious differences between these practices in terms of their treatment strategies in addition to differences in extraction rate. Practice 3 had many more patients in whom only one arch was treated than the other practices. Differences in types of orthodontic appliance used were clearly present and could be associated with extraction rates (data not shown). Expansion techniques were used the most in practice 1, and functional appliances were used the most in
= = = = = =
14.58) 15.42) 15.84) 11.65) 13.57) 8.15)
No. 236 20 52 60 29 75
Extraction 31.2 37.8 38.6 29.1 35.3 24.8
(SD (SD (SD (SD (SD (SD
= = = = = =
13.18) 13.42) 16.42) 10.53) 10.54) 9.52)
p 0.98 0.45 0.06 0.36 0.29 0.01
practice 4. Second molar extractions were also more prevalent in these two practices, which had the lowest extraction rates in the sample. In practice 5, which had the highest extraction rate, certain techniques (notably wearing of headgear) were also used more extensively than in the other practices. As shown in Table IV, for the total sample of 438 patients, the results of Student t test showed no difference in duration of treatment between patients who were treated with extractions and those who were not. Mean duration of treatment for each group was 31.2 and 31.3 months, respectively. For the entire sample, there was a range in duration from 4 months to 7 years 6 months, with a standard deviation of 13.9 months. When each practice was analyzed separately, a statistically significant difference was found for practice 5, although the small number of nonextraction patients in this practice needs to be kept in mind. In this practice, extraction treatment was found to last about 7.4 months longer than nonextraction treatment (p = 0.01). In practices 1 to 4, extraction treatment was also found to be longer, by 3 months, 6.5 months, 2.3 months, and 3. I months, respectively, but these differences were not statistically significant. On the basis of these results, it is not possible to reject the null hypothesis that there is no significant difference between duration of treatment for patients who had extractions and those who did not. These results show, however, that when a difference does exist, extraction treatment usually takes Ion-
50
Vig et al.
Am. J. Orthod. Dentofac. OrJhop. January 1990
ger than nonextraction treatment within a particular practice. Stepwise regression analysis was used to test the associations between duration of treatment and the nine variables defined previously for all practices and each of the individual practices. The number of treatment phases, arches treated, molar relationship, age at start of treatment, "and extraction or nonextraction status were significantly correlated with duration of treatment. The following equation with an r squared value of 0.33 and multiple r = 0.57 was generated for the entire group: y = 18.8 + (13.56)(vi) + (-7.78)(v2) + (4.50)(v3) + (2.95)(v4) + (-.03)(v5) where y = duration of treatment, in months Vl = number of phases
1 2 3 0 I 0
= 1 phase = 2 phase = 3 + phases V2 = arches treated = both = one only V3 = molar relationship, = MR = Class II, Class II, Division 1, or Class III Class II, Division 2 1 = MR = Class II, Division 2 V4 = extraction/non~ 0 = non-extration extraction 1 = extraction V4 = starting age, analytical variable recorded in months Gender and the other dental relationship variables did not contribute significantly to the equation. The variables included in the equation affect the duration o f treatment in the following ways: 13.56 months are added for each phase of treatment (p < 0.0001); 7.78 months are subtracted if only one arch is treated (p < 0.001); 4.50 mbnths are added if the malocclusion is Class II, Division 2 (p = 0.04); 2.9 months are added if extractions are used (p = 0.01), and the starting age in months multiplied by 0.03 is subtracted (p < 0.0001). For example, for a 12-year-old patient with a Class I molar relationship being treated in one phase and having both arches treated, without extractions, the predicted duration o f treatment, y, would be 28 months. DISCUSSION
The five practices examined were very different with respect to most o f the variables studied. Some variability was virtually guaranteed by the initial selection of practices at the two extremes o f extraction frequency rates. Together, however, the patient popu-
lations in these practices are similar to those found in larger surveys of orthodontic practices in terms of gender distribution ~7 and prevalence of molar relationships2 .7~° Gender distribution continued to be representative when each practice was considered separately, but, as described earlier, this was not true for prevalence of each type of molar relationship. Average duration of treatment for the entire sample was found,to be about 31 months, which is quite long when one consideres that, typically, an orthodontist predicts duration of treatment to be "18 to 24 months." The practices were also different in this respect, with practice 5, the practice with the highest extraction rate, having the shortest mean duration of treatment (2 years), and practices 1 and 2 having the longest (both at 2 years 11.5 months). The weighted mean duration of treatment for the extraction and nonextraction cases in the five practices is influenced by practice 5. This practice is an outlier with its short mean duration of treatment and small proportion of nonextraction patients. The extraction of teeth is one of the more obvious and less desirable aspects of orthodontic treatment. To date, no cost-benefit or risk-benefit analyses have been applied to the extraction/nonextraction controversy. It is not clear whether the process of orthodontic therapy and its long-term treatment results are significantly or predictably affected by the extraction o f teeth. Clearly, some clinicians and some patients are more averse to extraction therapy than others. The extent to which such treatment decisions are based on the biologic attributes of the patients, such as the cause or severity of malocclusion, or the treatment method preferences of clinicians is not known. Studies that demonstrate the associations between different procedures and their outcomes are needed. Comparisons of both the process and the outcome o f alternative treatment are necessary if clinicians are to choose the most efficacious procedures. As Johnston t~ points out, " . . . the decison to treat in a certain way is, in and of itself, a predictor that can account for more variance in the outcome o f treatment than that produced by random variation in the pattern of growth." It is quite likely that this is not only true of the morphologic results or treatment effects but also holds for some aspects of the efficacy o f orthodontic treatment in general. Whether or not this is true can and should be determined. The increasing press.ures for cost containment in health care concurrent with the need and desire to provide treatment of optimum quality dictates that research endeavors address the cost-effectiveness of alternative clinical policies and decisions and their outcomes.
Volume 97 Number 1
Duration o f orthodontic treatment with attd without extractions
It is evident from this preliminary study that the duration of treatment is dependent on a number of variables that include attributes of the patient, the objectives of treatment, and the methods used to accomplish the goals of the clinician. The extraction decision is merely one of the clinical variables and should therefore be considered in that light. it also is clear that, for any given practice, it is possible to develop/i regression model that can aid the clinician in making more accurate estimates of treatment duration for iridividual patients, given the demographic and clinical variables that reflect that orthodontist's pa~em of clinical activity. As fo r characterizing attributes of the efficacy of orthodontics in a general or global sense, future studies of this kind need to be performed on a larger sample or" practices, selected to be representative of the broad spectrum of practice variation. If data of the type presented here are t6 be pooled, then it may be desirable to stratify practices by type and to analyze subgroups derived from a consideration of the frequency distributions of those variables that, a priori, are deemed to have a significant effect on aspects of efficacy. SUMMARY AND CONCLUSIONS
On the basis of the results of this study of 438 patient records from five orthodontic practices, the following conclusions may be drawn: 1. Differences in duiation of treatment were apparen~ ,~,hen extraction and nonextraction patients within e/ach individual practice were compared. Treatment for those who had extractions was more likely to take longer. 2. The duration of orthodontic treatment may be affected not only by the extraction decision but by such variables as whether one or both arches are treated and the ntimber of phases of treatment. The duration of treatment was also affected by which practice was chosen. 3. Considerable differences exist in treatment strategies between the practices with low extraction rates
51
and those with high extraction rates, but clearly a larger number of practices needs to be evaluated to establish associations between treatment variables and outcomes. 4. The sampling method used to identify practices with high and low extraction rates and the pooling of data from all five practices tends to obscure differences in duration between extraction and nonextraction treatments by virtue of other important confounding Variables that are not accounted for when patients are compared merely on the singular criterion of extraction. REFERENCES 1. Frankel R. Decrowding during eruption under the screening influence of vestibular shields. AM J OR'nIOD 1974;65:372-405. 2. Pancherz H. A cephalometric analysis of skeletal and dental changes contributing to Class II correction in activator treatment. AM J ORTtlOD 1984;85:125-33. 3. Ringenberg QM. Influence of serial extraction on growth and development of the maxilla and mandible. AM J ORTtIOD 1967;53:19-25. 4. Brown C. Duration of orthodontic treatment and the extractiontnonextraction treatment decision [Master's thesis]. Ann Arbor, Michigan: University of Michigan, 1987. 5. Hooper JD. Orthodontics as a public service: the Wessex survey. Dent Pract 1967; 17:259-68. 6. Peck S, Peck H. Frequency of tooth extraction in orthodontic treatment. AM J OR'rHoD 1979;76:491-6. 7. Rose JS. A thousand cases: a survey. Br J Orthod 1974;1:4554. 8. Beresford JS. Tooth size and class distinctioa. Trans Br Soe Study Orthod 1968-9;5:111-8. 9. Berg R. Post-retention analysis of treatment problems and failures in 264 consecutively treated cases. Eur J Orthod 1979;1:5568. 10. Sheiham A, Hobdell MW, Vig P, Griffiths PJ. Orthodontic treatment in the General Dental Service in England and Wales; a study. Br Dent J 1971;131:535-8. 11. Johnston LE. A comparative analysis of Class II treatments. In: Vig P, Ribbens K, eds. Science and clinical judgment in orthodontics. Monograph 19. Ann Arbor, Michigan: Center for Human Growth and Development, University of Michigan, 1986. Reprint requests to: Dr. Peter Vig Department of Orthodontics and Pediatric Dentistry Center for Human Growth and Development University of Michigan Ann Arbor, MI 48109
