Abstract
Orthodontic surveys of the adolescent and adult population of the United States have shown that the incidence of anterior open bite is three to four times higher in blacks than in whites. A cephalometric comparison of black subjects with and without an open bite was used to identify skeletal and dental differences between the two groups. Statistically significant differences were found in the vertical skeletal dimensions and incisor proclination. The open-bite group had a significantly longer anterior lower facial height and total facial height. The mandibular plane was rotated down relative to the cranial base and Frankfort plane and gonial angle was increased in the open-bite sample. There were small differences between the open bite and non–open-bite groups in the cranial base angle and the overbite depth indicator of Kim. No significant differences were found in the skeletal anteroposterior dimensions or dental vertical development. The vertical skeletal pattern and the greater degree of dental proclination differentiated black patients with an anterior open bite from those without.
INTRODUCTION
Much literature has been published about anterior open-bite malocclusion, attempting to describe both its characteristics and causes. Traditionally, open bite has been viewed as one of the more difficult malocclusions to treat to a satisfactory and stable result. Understanding the etiology is essential if one is to establish the proper diagnosis and treatment plan for an open-bite patient. Considerable evidence exists to indicate that there is a large racial variation in the incidence of open bite.1–3 In 1965, the United States Public Health Service performed dental examinations on 7400 adolescents aged 12–17 as part of a National Health Study.1 This examination included an assessment of the occlusion and the results of the study showed that both the prevalence and the severity of anterior open bite were associated with race. Open bite was defined as any degree of negative overbite (less than 0.00 mm). Significantly more of the black children had open bites than white children (16.3% as compared with only 4.0% in whites), indicating that the black population may, at times, have as much as four times as many open bites as whites.
A more recent national survey of malocclusion in the United States was conducted in 1988–1991 as part of the National Health and Nutrition Examination Survey (NHANES III).3 When assessing the vertical relationships, less than 5% of the 7000 US children and youths in the sampled racial-ethnic age groups had an open bite. The NHANES III data for all ages showed that 6.6% of blacks had an open bite of varying severity compared with 2.9% for whites and 2.1% for Mexican-Americans. Negative overbite was equivalent to open bite with the average being 1.1 mm. Non-Hispanic blacks had, on average, 1.6 mm open bite compared with 0.8 mm for non-Hispanic whites and 1.1 mm for Mexican-Americans. Open bite was both more severe and more prevalent in blacks.
There is little in the literature to explain the reasons why blacks have 2½–4 times as many open bites as whites and why their open bites are more severe. There are a number of studies that describe the skeletal and dental characteristics of the anterior open bite and discuss the various etiological factors that play a role in producing an open bite.4–9 But these studies generally include only white patients with anterior open bites, or if other races are included, the entire sample is compared with non–open bite white norms. Altemus,10 Drummond11 and Cotton et al12 have all completed studies that have helped to establish cephalometric standards for blacks, but these standards have rarely been used to compare black open bites with black normals. Jones,13 in 1989, compared cephalometric data from black open-bite patients with the black norms of Altemus and Drummond and the white norms of the Steiner14 and Tweed15 analyses. The skeletal and dental differences reported by Jones for black open-bite patients must be interpreted with caution because of the relatively small number of patients studied and the few parameters included in the analyses.
The purpose of this study was to compare black open-bite subjects with black non–open-bite subjects to identify skeletal or dental factors that may help categorize open bite in the black population.
MATERIALS AND METHODS
Pretreatment cephalometric films of 103 African-Americans were obtained from the Orthodontic Department at the University of North Carolina (n = 45) and from the former private office of the principal investigator (n = 58). No age restriction was placed, but the subjects had to have their permanent first molars and all permanent incisors fully erupted. Ethnicity was based on patient self-identification on a patient history form. All patients with anterior open bite were included, with no attempt to differentiate between skeletal or dental open bite or Angle's classification. An open bite was defined as a lack of contact between the upper and lower anterior teeth in the vertical plane. The open-bite sample consisted of 51 subjects, 39 females and 12 males. The age range of this group was from age 8 to 37 with a mean age of 19.4 years (Figure 1).
We used a black non–open-bite sample from the same geographic region for comparison with the open-bite subjects. The control group was selected from the same office sites and was similar in age and sex, consisting of 52 subjects, 46 females and 6 males, with an age range of 10–41 with a mean age of 22.7 years (Figure 1). These were all Angle Class I patients with a positive overbite. Because of the very small number of male subjects in both groups, no attempt was made to distinguish differences by gender.
The cephalograms were digitized by one operator for consistency of landmark identification. Landmarks used are illustrated in Figure 2 and the cephalometric measurements are presented in Table 1. The magnification factor was 7.5% for records taken at The University of North Carolina and 9.0% for records taken in the private office. No adjustments were made for these small differences in magnification.
ABpl-MP was included in this study, in addition to FH-PP, for calculation of Kim's overbite depth indicator (ODI),16 which is claimed to be a reasonably good indicator of open-bite tendency. The ODI is calculated by determining the angle formed by the mandibular plane and the A-B plane. The angle formed by the palatal plane to the Frankfort plane is either added to, or subtracted from, the MP-ABpl angle. The angle is negative (subtracted) if the palate is tipped down posteriorly or positive (added) if the palatal plane is tipped down in the front.
For each measurement, an analysis of covariance was used to assess whether the mean values of the open bite and non–open-bite groups differed. Because many of these measures do show age-related changes, age was entered in the analysis as a covariate and the cephalometric means of the two groups compared at the mean overall age (21 years old). Without this adjustment, it would not be possible to determine whether a significant difference between the means was solely due to the effect of age. The open-bite sample contained more older patients than the comparison group of non–open-bite subjects. The test for parallelism, the interaction between age and open bite, was performed first for each cephalometric variable and no overall statistically significant lack of parallelism was indicated. Therefore, only the main effect model results are reported. If the overall model for a given variable was significant (P < .05) then the comparison of mean values of the two groups was made using the type III sum of squares, which gives the variability attributable to the patient's group classification beyond that already explained by the relationship between age and the measure. The level of significance was set at .05. No multiple comparisons adjustment was made to the level of significance because this was considered an exploratory study.
RESULTS
The results are reported for anteroposterior and vertical skeletal and dental measures. In Table 2–5 the means and standard deviations are given for both the groups. The tables show the mean value and standard deviation for each dependent variable without any adjustments made for the differences in age of the two groups. The cephalometric means for both open-bite and control group at the mean age of the combined sample (21 years) are also presented. The P value (age), which gives the probability level of the linear relationship between age and each dependent variable, shows that as expected, many of these measures are linearly related to the age of the patient, emphasizing the importance of adjusting for age in comparisons of these types of samples. The P value (group) gives the probability level for the difference between the open-bite and control group beyond the value already explained by the relationship between age and the particular measure.
Descriptive Statistics for Skeletal Anterior-Posterior Cephalometric Measurements Before and After Adjusting for Agea
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None of the skeletal anterior-posterior measures (Table 2) were statistically different between the open-bite and non–open-bite groups. The average unit difference, a comparison of the size of the mandible and maxilla, indicated a slightly larger skeletal discrepancy in the open-bite sample, but this difference in average discrepancy was not statistically significant.
The skeletal vertical comparative data are provided in Table 3. There were a number of significant differences observed in this category. The lower facial height (LFH; P = .01) and anterior facial height (AFH; P = .03) were statistically different, with the open-bite group, on average, having longer face heights because of the lower not the upper face height. The ratio of upper facial height to lower facial height (UFH:LFH) was smaller in the open-bite group (0.68 vs 0.71), but the difference did not reach the .05 level of significance set for this study. Posterior facial height is obviously strongly correlated with a patient's age, but after age adjustment the difference between the open-bite and control group did not differ significantly. The ratio of posterior facial height to anterior facial height (PFH:AFH) was significantly different (P = .01) in the two groups, with the open-bite group having a normal posterior facial height and a longer anterior facial height.
The angulation of the mandibular plane to the cranial base (SN-GoGn) was found, on average, to be greater in the open-bite sample as was that of the mandibular plane to the palatal plane. Gonial angle was also significantly increased in the open-bite group (134.55° vs 131.73°). On average, the PP-MP angle was greater in open-bite patients, but the palatal plane bore the same relationship with the Frankfort plane as the non–open-bite group, suggesting that this is more a function of mandibular plane change rather than tipping of the palatal plane.
Table 4 shows the comparisons between the open-bite and control groups when examining the anteroposterior position and angulation of the upper and lower dentitions. The upper incisor was significantly more proclined, on average, to the palatal plane (U1-PP) in the open-bite sample (P < .01). The lower incisor mean values also suggested more proclination of the lower incisors in the open-bite group. The large difference in mean values for the two groups was observed in the relative angulation of the upper to lower incisors (interincisal angle). The overjet was similar between the two groups.
The vertical height of the dentoalveolar segments in the open-bite subjects and the normal overbite group are given in Table 5. With the exception of the overbite (the criterion on which the groups were selected), none of the dental vertical values were statistically significantly different between the open-bite and non–open-bite groups.
DISCUSSION
The purpose of this study was to compare two groups of racially similar black patients to determine whether there were skeletal or dental characteristics of open bite. At this point in history, it is impossible to be sure of the racial background of any patient. The patients used in this study had all identified themselves as black, lived in the same geographic region, and attended the same two practice locations. Although they are likely to be of similar racial background, we can provide no assurance that this is indeed so.
The anterior posterior skeletal measurements had no apparent association with the presence or absence of an open bite in these black patients. Although the size of the maxilla and mandible appeared similar in the two groups, the unit difference was found to be slightly smaller in the open-bite group. Although the difference was not statistically significant, this could have contributed to a more Class II skeletal type, which is frequently seen in the open-bite case.
Although we did not divide the group into skeletal and dental open bites, the skeletal vertical dimension is a major factor in the increased tendency toward open bite.4,6–9 This report confirms these previous findings. The open-bite group, on average, had longer faces than the normal overbite group as indicated by the difference in both total lower and anterior facial heights. Because the mean difference in upper facial height was less than 0.5 mm, LFH may be the important contributing factor in causing an increased anterior facial height. The average ratio of PFH:AFH was statistically different and reflects the increase in anterior facial height. Previous studies have reported that open-bite patients also tend to have shorter posterior face heights,4,5,8 but this was not found in the present study.
Growth studies have generally shown that increased angulation of the cranial base is associated with increased vertical dimension.17,18 There is an assumption that this angle will be reflected in the position of the glenoid fossa and the temporomandibular joints. The theory is that the more acute the cranial base angle the more forward the position of the mandible and as the angle becomes more obtuse the mandibular position retrudes, suggesting a possible association between a large cranial base angle and open bite.19,20 Faustini et al21 and Richardson22 also reported that the cranial base flexure in African-Americans is more obtuse in both males and females. When comparing the cranial base of black and white adult subjects, D'Alosio and Pangrazio-Kulbersh23 found that the cranial base in blacks was shorter, steeper, and flatter. Our findings do tend to support the idea that the angulation of the cranial base is greater in open-bite patients. But the differences here were very small (132.40° vs 130.83°) and only just significant at the .05 level.
The mean mandibular plane angle (MPA) was significantly higher in the open-bite patients. This was evidenced by the angulation of MP to the cranial base, palatal plane, and AB plane. All these measurements indicated a downward rotation of the mandible in the open-bite group. This study also confirms the presence of an obtuse gonial angle as an indicator of open-bite tendency.4,5,8
The ODI analysis of Kim16 is purported to be of value in determining which patients may have an open-bite tendency. The ODI is a combined measurement of two facial angles—ABpl-MP and PP to FH. The incidence of open bite increases with a value below the mean of 75°. Wardlaw et al24 and Dung et al25 determined that the ODI proved to be a better diagnostic tool for identifying patients with a tendency toward open bite than any of the commonly used cephalometric measures or ratios. Our open-bite sample gave a mean value for ODI of 69.18°, well below the normative value of 75° found by Kim. Although our analysis did not suggest a significant linear relationship between ODI and age, Kim's original sample of 119 Northeastern Caucasians had a mean age of 10.8 years (range 7–14). This sample had a mean age of 21 years (range 8–42). The non–open-bite group in our study had an ODI of 72.09°. Although being higher than the open-bite group, this value is still lower than Kim's normative value, perhaps indicating that this sample of normal black subjects had a tendency toward reduced overbite. The principal angle measured by the ODI (ABpl-MP) was more acute in the open-bite group and points to a tendency for this group to be more retrognathic in addition to having an increased mandibular plane angle.
The angulation of the palatal plane in the open-bite group was somewhat more negative (tipped up anteriorly) than in the control group, but the difference was not significant.
There have been many studies of different racial groups showing that skeletal and dental patterns can, and do, vary.26,27 Examples can be cited where specific physical characteristics or attributes that are commonly found in a racial group come to be described as typical for that race or ethnic group. For example, the maxillary deficiency that is characteristic of some Asian populations can be identified as, perhaps, the primary factor resulting in the relatively high proportion of Class III malocclusions seen in these groups. Additionally, the Class II malocclusion, primarily a skeletal discrepancy, is much more common in whites than in blacks.1,22,23 Trouten et al,9 citing Enlow's counterpart analysis, proposed that a combination of craniofacial components work together to produce a vertical dysplasia. In the case of the open bite, a high mandibular plane angle, obtuse gonial angle, long lower face, increased cranial base angle and dental proclination have consistently been identified as contributors to this vertical problem. In this study, all these factors were present to a greater degree in black open-bite subjects than in the normal black sample. It may be that open bite in blacks is associated with the common finding of bimaxillary protrusions, relatively high mandibular plane angles, and longer faces.
The findings in this study may have some important clinical implications. When treating a black patient with an open bite, it is clear that the clinician should avoid increasing the anterior facial height. Extrusion of the posterior teeth should be prevented, but we found only a small difference in posterior height of the molars between the two groups. The presence of a bimaxillary protrusion in an open-bite patient presents an opportunity to improve or correct the open bite by reducing the angulation of the anterior teeth. As such, extraction therapy may well be considered in the treatment in many of these patients.
CONCLUSIONS
This study confirms the findings of past studies that the skeletal pattern plays a significant role in producing an open-bite malocclusion. There was an association between open bite and increased lower facial height, high mandibular plane angle, and increased gonial angle. The mean differences in the posterior facial height and angulation of the palate do not play such an important role. Angulation of the cranial base tended to be in the direction expected on the basis of previous reports on open bite in white patients.
Two summary findings appear from this study: (1) black open-bite subjects differ from black non–open-bite patients in the extent of the vertical developmental pattern in the anterior face and mandibular rotation and (2) a protrusive dentoalveolar inclination is a significant component of the open-bite malocclusion.
Acknowledgments
We would like to acknowledge Ms. Lyna Rogers and Ms. Debora Price for their assistance with the accumulation and analysis of the data for this research project.