Predictors of variation in mandibular incisor enamel thickness
Nathan E. Hall, DDS, MS,
Steven J. Lindauer, DMD, MDSc,
Eser Tüfekçi, DDS, MS, PhD and
Bhavna Shroff, DDS, MDentSc
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ABSTRACT
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Background. Interproximal reduction of mandibular incisor enamel often is performed as an adjunct to orthodontic alignment. The authors conducted a study to determine factors contributing to variations in incisor enamel thickness.
Methods. The authors compared enamel thickness between mandibular central and lateral incisors, between mesial and distal surfaces, between male and female subjects (N = 40 each) and between African-American and white subjects (N = 40 each). The authors also evaluated correlations between overall tooth width and enamel thickness.
Results. The authors found significantly greater enamel thickness in lateral incisors, on distal tooth surfaces and in black subjects (P < .0001 for each); they found no differences between male and female subjects. They found that wider teeth were associated with greater enamel thickness (P < .01) but that the amount of thickness varied greatly among subjects (range: 0.44–1.28 millimeters).
Conclusions. Thicker enamel was found on the distal aspect of lateral incisors, in black subjects and in wider teeth. The authors observed, however, that the variations in thickness are not fully explained by these factors alone.
Clinical Implications. If substantial enamel reduction is planned as part of dental treatment, the authors recommend that clinicians use calibrated radiographs to measure the thickness of their patients’ enamel surfaces because of the extensive variation in enamel thickness among and within people.
Key Words: Enamel; incisor; interproximal reduction; malocclusion; mandible; orthodonticsAbbreviations: CEJ: Cementoenamel junction • DEJ: Dentinoenamel junction • IPR: Interproximal reduction
Interproximal reduction (IPR), also known as enamel stripping or reproximation, is the removal and reshaping of enamel from the contact points of adjacent anterior teeth, most commonly the mandibular incisors. Clinicians often perform IPR before, during or after orthodontic treatment involving fixed or removable appliances to create space to align crowded teeth, correct tooth size discrepancies or improve stability. Chenin and colleagues1 stated that when using a series of removable appliances such as the Invisalign system (Align Technology, Santa Clara, Calif.), clinicians can resolve crowding primarily with IPR or mandibular incisor extraction to avoid excessive advancement of the incisors. Peck and Peck2 and Aasen and Espeland3 suggested that clinicians could use IPR during and after orthodontic treatment to alter tooth shape and, thereby, enhance the stability of incisor alignment.
Before deciding how much enamel to remove safely, clinicians need to know the total thickness of enamel present on a tooth’s surface. Hudson4 estimated that up to one-half of the enamel thickness could be removed with minimal or no negative side effects. Subsequently, others5–13 have reported removing up to 50 percent of enamel thickness with no signs of problematic sequelae.
Studies have yielded little information that can be used to predict the thickness of enamel in individual teeth. Gillings and Buonocore14 sectioned 27 extracted mandibular incisors and described changes in the thickness of enamel from the cementoenamel junction (CEJ) to the incisal edge on the buccal, lingual, mesial and distal surfaces. They reported that enamel on the distal surface was slightly thicker than that on the mesial surface, that thickness of enamel varied depending on its distance from the CEJ and that enamel thickness differences among teeth were not related to crown height.
Peck and Peck15 measured enamel thickness on the combined mesial and distal surfaces in a sample of 116 extracted mandibular incisors by grinding the incisal edges until dentin was exposed evenly. They collected the extracted incisors from white subjects and did not categorize them according to sex of subject or type of tooth (central or lateral). They found a significant correlation between incisor tooth width and enamel thickness on the combined mesial and distal surfaces in their sample.
Similarly, Shillingburg and Grace16 did not differentiate between central and lateral incisors when they reported mean enamel thickness for their sample of 23 incisors. They took measurements at 1-millimeter intervals incisogingivally by grinding each tooth incrementally parallel to a line connecting the CEJ on the facial and lingual surfaces. Mean enamel thickness varied depending on the distance from the CEJ. The researchers explained that measures of enamel thickness made using the grinding technique are larger than the actual enamel thickness owing to the geometry of the cuts.
Investigators have examined the differences in tooth size and composition between males and females. Moss and Moss-Salentijn17 evaluated sexual dimorphism in human teeth, particularly permanent and primary canines, and they found that males have larger canine crowns. They felt that the difference was due to greater enamel thickness; however, they based their conclusion on indirect evidence presented in the literature. Stroud and colleagues18,19 examined bitewing radiographs and found that molars and premolars were larger in males than females. The difference in tooth size, however, was due to increased dentin thickness in males, while enamel thickness was not significantly different. Similarly, Harris and Hicks20 found that maxillary incisors were wider in males than in females owing to increased dentin thickness, but not to enamel thickness.
Other studies have focused on differences in tooth size between racial groups. Merz and colleagues21 reported that African-American subjects had significantly wider mandibular canines, premolars and first molars than did white subjects, but they found no significant size differences in mandibular incisors. Richardson and Malhotra22 found that all teeth were wider in black male subjects than in black female subjects. Harris and colleagues23 reported that black subjects’ primary molars were larger than those of white subjects and that they had disproportionately greater enamel thickness. They also found that males had wider primary teeth than did females, but they concluded that that difference was not due to a difference in enamel thickness. Otuyemi and Noar24 compared permanent tooth crown dimensions between a Nigerian population and a British population and reported that mesiodistal crown diameters were consistently larger in the Nigerian population.
We conducted this study to gather information that could be used to help predict variation in the enamel thickness of mandibular incisors. Specifically, we compared enamel thickness between mandibular lateral and central incisors, between mesial and distal surfaces, between male and female subjects, and between black and white subjects. We also determined the relationship between tooth width and enamel thickness. Our goal was to aid practitioners who are considering using IPR to reduce mandibular incisor tooth width to resolve crowding, correct anterior tooth size discrepancies or to improve alignment stability.
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SUBJECTS, MATERIALS AND METHODS
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We evaluated 80 subjects at Virginia Commonwealth University School of Dentistry who agreed to participate in the study. We obtained approval from the university’s institutional review board and received informed consent from each subject or his or her parent or guardian. Our eligibility criteria were that subjects have no history of IPR and that they have well-aligned permanent mandibular incisors; we required the latter so that we could obtain radiographs with no overlap of adjacent teeth. We considered well-aligned incisors with spacing present to be acceptable.
We excluded from the study people who had mandibular incisors with interproximal restorations or noticeable signs of wear. We also excluded those who were pregnant or believed they might be pregnant; those who were Asian, Hispanic or American Indian, owing to the small numbers of patients from these ethnic groups who were available; people with special needs or cognitive impairments; and people who were not in good general health.
A total of 40 black subjects and 40 white subjects (20 male and 20 female subjects from each racial group) participated in the study.
We made a dental stone study model of each subject’s mandibular arch using an alginate impression. We took one digital periapical radiograph of each subject’s right mandibular incisors and one of each subject’s left mandibular incisors. We took additional periapical radiographs if any of the enamel surfaces we were evaluating were not clearly visible owing to overlap. One operator (N.E.H.) took all of the radiographs to ensure that radiographic quality and paralleling techniques were consistent for all subjects.
We measured the width of each mandibular incisor from the mesial contact point to the distal contact point. We measured mesiodistal tooth width on the dental casts to 0.001 mm using a sharpened digital Boley gauge (Ultra-Cal II, Fred V. Fowler, Newton, Mass.). We matched the dental cast measurements to the corresponding periapical radiographs and assigned them a random number. The examiner (N.E.H.) was blinded to which groups (male or female and black or white) the subject of each record belonged.
We took measurements from each radiograph using computer-aided design software (DesignCAD Pro 3000, Upperspace, Pryor, Okla.) precise to 0.001 mm. Once each radiograph was loaded in the program, we enlarged it to fill a flat-screen, 19-inch computer monitor. We calibrated the mesiodistal width of each incisor on the screen to the corresponding mesiodistal measurement made from the dental casts to correct for radiographic magnification. We then measured the thickness of the enamel layers on the mesial and distal surfaces from the mesial and distal contact points on the shortest line possible to the dentinoenamel junction (DEJ). When we enlarged the periapical radiographs to fill the computer monitor’s screen, we noted that the DEJ appeared as a zone rather than a distinct line. We measured the area of this zone closest to the enamel surface to determine the thinnest layer of enamel between the contact point and the dentin. To evaluate reliability of our measurement method, we repeated the measurements using the casts and radiographs of 10 subjects.
We used paired t tests to determine differences in tooth width and enamel thickness on the mesial and distal surfaces between right and left incisors and between central and lateral incisors. We used two-way multivariate analyses of variance to detect differences in tooth width and enamel thickness on the mesial and distal surfaces between male and female subjects and between black and white subjects. We used correlation analyses to determine significant relationships between tooth width and enamel thickness on the mesial and distal surfaces. We set the level of significance for all tests at P 
.05.
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RESULTS
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We detected no statistically significant differences between the repeated sets of measurements for 10 subjects. The correlation between the sets was high for tooth thickness (r = .99), enamel thickness on the mesial surface (r = .95) and enamel thickness on the distal surface (r = .98). Average differences between repeated measures were less than 0.01 mm.
Table 1
shows the mean values for incisor width and enamel thickness for all subjects. We detected no significant differences between the right and left central incisor widths (P = .63) or between the right and left lateral incisors widths (P = .46). We also detected no statistically significant differences between the enamel thicknesses of contralateral teeth except for the enamel thickness on the mesial surface between the right and left lateral incisors. The mean enamel thickness on the mesial surface of the right lateral incisors ± standard deviation was 0.79 ± 0.11 mm compared with 0.81 ± 0.11 mm for the left lateral incisors. We found that this 0.02-mm mean difference was statistically significant (P = .01), but we judged it to be clinically insignificant.
On average, lateral incisors were significantly wider than central incisors by 0.52 ± 0.27 mm (P < .0001). The enamel thickness on the distal surface was significantly thicker than that on the mesial surface for central and lateral incisors (P < .0001 for each incisor). The enamel on the mesial and distal surfaces of the lateral incisors was significantly thicker than the enamel on the mesial and distal surfaces of the central incisors (P < .0001). When we compared all of the incisors, we found that the enamel on the distal surface was 0.10 ± 0.09 mm thicker than that on the mesial surface on average (P < .0001).
Table 2 compares incisor width and enamel thickness between male and female subjects. We detected no significant differences between male and female subjects when we evaluated tooth width, enamel thickness on the mesial surface or enamel thickness on the distal surface.
Table 3 compares incisor width and enamel thickness between black and white subjects. We found that black subjects had significantly wider central and lateral incisors (P < .0005), and we found that all enamel surfaces for this group were significantly thicker (P < .0001) than the corresponding enamel thickness in white subjects.
Table 4 shows the correlations between incisor width and enamel thickness for black and white subjects. We found that enamel thickness was related significantly to tooth width for all incisors (P < .01). We found that the enamel on the mesial surface of the right lateral incisors in the black subjects was the only surface that showed no significant relationship to tooth width (P = .11).
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DISCUSSION
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IPR is a technique that many practitioners use to reduce incisor crowding, resolve anterior tooth size discrepancies and improve the stability of tooth alignment. It is believed that the thickness of an incisor’s enamel at the contact point is related to the amount of enamel that can be removed safely without deleterious sequelae.4–13 We conducted this study to determine factors that can be used to help predict the amount of enamel at the contact points on the proximal surfaces of mandibular incisors.
A 2001 study questioned whether dentists can determine the enamel thickness of teeth accurately from radiographs.25 Grine and colleagues25 measured the enamel thickness of posterior teeth on periapical radiographs and compared these measurements to those of the same teeth once they sectioned them. They detected a general overestimation of measurements made from the radiographs and a large variability in error. They stated that enamel thickness studies that employ radiographs need to be viewed with circumspection. However, mandibular incisor crowns have a thinner labiolingual thickness than do posterior teeth, and incisors’ DEJs may be easier to identify. In our study, we controlled magnification error by calibrating the radiographs using tooth width measurements made on dental casts. Also, clinicians can use actual crown dimensions to scale periapical radiographic measures for individual patients.
We detected no differences when we compared the right and left incisor widths and enamel thicknesses. The only exception was that the enamel on the mesial surface of the left lateral incisor was 0.02 mm thicker than that on the mesial surface of the right lateral incisor. This small difference was statistically significant, but it probably is not clinically important. Ballard26 found that 90 percent of people have at least one set of contralateral teeth with a width discrepancy of at least 0.25 mm.
Lateral incisors were about 0.5 mm wider on average than were central incisors, and this difference was statistically significant (P < .0001). Lateral incisors also had thicker enamel surfaces on average than did central incisors. The enamel surfaces on the distal surfaces of the central incisors were about 0.05 mm thicker than those on the mesial surfaces (P < .0001), and the enamel surfaces on the distal surfaces of the lateral incisors were about 0.15 mm thicker than those on the mesial surfaces (P < .0001). Though the difference between enamel thickness on the mesial and distal surfaces was small for the central incisors, the difference for the lateral incisors was large enough to affect the planned amount of enamel reduction.
We detected no statistically significant differences between male and female subjects’ incisors. Stroud and colleagues18,19 examined the width and enamel thickness of posterior teeth between males and females and found no differences in enamel thickness. They did, however, find that the overall width of posterior teeth was greater in males than in females. They stated that this difference in tooth width was attributable to wider dentin components. The findings from our study suggest that there is no reason to expect mandibular incisor enamel thickness to differ between males and females.
On average, we found that central incisors in black subjects were about 0.23 mm wider than in white subjects, and that lateral incisors in black subjects were about 0.21 mm wider than in white subjects. The enamel thickness on the mesial surface of incisors in black subjects was about 0.10 mm greater than that in white subjects, and the enamel thickness on the distal surface of incisors in black subjects was about 0.15 mm greater than that in white subjects. This suggests that, in general, dentists can consider slightly more enamel reduction in black patients than in white patients.
We found significant correlations between tooth width and enamel thickness for nearly all enamel surfaces, indicating that wider incisors have thicker enamel surfaces than do narrower incisors. Our findings agree with that of Peck and Peck,15 who reported a significant correlation (r = 0.68) between enamel thickness on the combined mesial and distal surfaces and the mandibular incisor crown width. However, in our study there still was substantial variation in enamel thickness that was not explained by tooth width differences alone.
In our study, there were large ranges of enamel thickness between groups, within groups and even within subjects. Enamel surface thickness ranged from 0.44 to 1.12 mm among white subjects and from 0.58 to 1.28 mm among black subjects, illustrating that the thicknesses for some subjects were more than twice those of other subjects. We often observed a variation of more than 0.33 mm in enamel thickness between proximal surfaces of incisors within the same subject. However, even the thinnest enamel surface we observed (0.44 mm) could undergo reduction of 0.2 mm and still maintain more than one-half its original thickness, complying with accepted safety recommendations.4
The decision to perform IPR always should be made carefully and take into account that the procedure removes the outermost, fluoride-rich layer of enamel.27 While many studies report no deleterious effects when less than 50 percent of enamel is removed,5–13 IPR may not be appropriate for people who are prone to caries. Clinicians also must take care to avoid the creation of interarch tooth-size relationship (known as the "Bolton relationship"28) discrepancies, especially when IPR is performed only in one arch.
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CONCLUSIONS
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Our study examined the relationship between mandibular incisor proximal enamel thickness and tooth type, tooth surface, race, sex and overall tooth width. We found significant differences in enamel thickness related to tooth type and surface, as well as race. Lateral incisors had a greater proximal enamel thickness than did central incisors. Enamel thickness on distal surfaces was greater than that on mesial surfaces. Black subjects had thicker enamel on average than did white subjects. Overall tooth width correlated positively with the enamel thickness of mesial and distal surfaces. We detected no statistically significant differences, however, between male and female subjects. We observed substantial variability in enamel thickness both among and within subjects.
Based on the range of enamel thickness we observed, we feel that enamel reduction of 0.2 mm or less on any mandibular incisor surface would be safely within the enamel removal guidelines of up to one-half the total thickness, as suggested previously.4 We recommend, however, that practitioners who are planning to perform a substantial amount of IPR as part of dental treatment use radiographs to measure the thickness of enamel surfaces in their patients. Periapical radiographs can be calibrated using measurements of tooth width made clinically or from dental casts to correct for magnification.
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FOOTNOTES
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Dr. Hall is in private practice in orthodontics, Iowa City, Iowa.
Dr. Lindauer is the Norborne Muir professor and the chair, Department of Orthodontics, Virginia Commonwealth University, School of Dentistry, 520 N. 12th St., Richmond, Va. 23298-0566, e-mail "sjlindau{at}vcu.edu". Address reprint requests to Dr. Lindauer.
Dr. Tüfekçi is an assistant professor, Department of Orthodontics, Virginia Commonwealth University, School of Dentistry, Richmond.
Dr. Shroff is a professor, Department of Orthodontics, Virginia Commonwealth University, School of Dentistry, Richmond.
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REFERENCES
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ABSTRACT
SUBJECTS, MATERIALS AND METHODS
