Mesiodistal width and proximal enamel thickness of maxillary first bicuspids

This study aimed at evaluating measurements relative to the mesiodistal crown width and enamel thickness of maxillary first bicuspids. The sample consisted of 40 extracted sound bicuspids (20 right and 20 left), selected from white patients (mean age: 23.7 ± 4.2 years), who were treated orthodontically with tooth extraction at a private clinic in São Paulo, SP, Brazil. All teeth were embedded in acrylic resin and cut along their long axis through the proximal surfaces, parallel to the buccal side, to obtain 0.6-mm central sections. The mesiodistal crown width and proximal enamel thickness were measured using a stereoscopic microscope connected to a computer. Measurements for right and left teeth, as well as the mesial and distal enamel thicknesses in the total sample, were compared by the Wilcoxon test (α = 0.05). The mesiodistal crown width mean values found were 7.51 mm (± 0.54) on the right side and 7.53 mm (± 0.35) on the left side. The mean enamel thickness on the distal surfaces for both sides was 1.29 mm (right: s.d. = 0.12 and left: s.d. = 0.18). The mean values for the mesial surfaces were 1.08 mm (± 0.14) and 1.19 mm (± 0.25), on the right and the left sides, respectively. No significant differences were found between the crown measurements and enamel thicknesses on the left and right sides. However, enamel thickness was significantly greater on the distal surfaces. Reliable measurements of enamel thickness are useful to guide stripping, which may be an attractive alternative to tooth extraction because it allows the transverse arch dimension to be maintained. Descriptors: Bicuspid; Tooth crown; Dental enamel; Orthodontics. Introduction Enamel stripping on the proximal surfaces has routinely been used to correct tooth discrepancies and enhance arch stability.1-3 This procedure may be an attractive alternative to tooth extraction because it allows the transverse arch dimension and anterior inclinations to be maintained.4,5 Nevertheless, a critical issue when performing stripping is to estimate the amount of proximal enamel to be removed, so that dentin exposure may be prevented. Considering that previous studies6,7 have mentioned that about fifty percent of the whole proximal enamel thickness could be removed safely, reliable measurements of tooth crown dimensions and enamel thickness would constitute a useful guide to the orthodontist during the stripping procedure. Thus, the aim of the present study was to estimate the mesioMacha AC, Vellini-Ferreira F, Scavone-Junior H, Ferreira RI Braz Oral Res. 2010 Jan-Mar;24(1):58-63 59 distal crown width and the proximal enamel thicknesses of maxillary first bicuspids. Material and Methods This study is in agreement with Resolution 196/96 from the National Health Council/Health Department (Brazil). Sampling An in vitro model consisting of 20 standardized repetitions would be statistically suitable for estimating the mesiodistal crown width and enamel thickness of eligible teeth from patients born in the same geographical area. Based on this methodological premise, 40 erupted sound human maxillary first bicuspids (20 right and 20 left) were collected from white adult patients aged 19-31 years (mean age: 23.7 ± 4.2 years), submitted to corrective orthodontic treatment with tooth extraction at a private clinic in the city of São Paulo, SP, Brazil. After extraction, the teeth were washed in saline solution to remove blood residues and then cleaned thoroughly with water. Tooth preparation All teeth were embedded in self-curing acrylic resin (UceflexTM 2110, Elekeiroz S/A, São Paulo, SP, Brazil), using round plastic containers (Extec Corp., Enfield, CT, USA). The specimens (n = 40) were numbered and then separated into two groups according to the side: twenty from the left side and twenty from the right side. The teeth were cut along their long axis through the proximal surfaces, parallel to the buccal surface, to obtain 0.6-mm central sections. Each cut section corresponded to the central area of the proximal surfaces, in that it encompassed the greater crown dimension and the thicker enamel portion. The cut sections were obtained using a 4-inch diameter, high concentration diamond wafering blade (Extec Corp., Enfield, CT, USA) mounted onto a high precision IsometTM 1000 saw (Buehler, Lake Bluff, IL, USA), under water refrigeration to prevent organic component loss. To avoid damaging the specimens, the diamond disc speed was set at 200 rotations per minute. The specimens were glued with cyanoacrylate adhesive (Super BonderTM, Loctite Corp., Itapevi, SP, Brazil) on glass slides (Star FrostTM, Knittel Glaser, Braunschweig, Lower Saxony, Germany) and diaphanized in 100% xylol for ten minutes, three times consecutively. This procedure increased transparency, so that the cut sections could be better visualized under the microscope. Otherwise, the dentinoenamel junction (DEJ) could not be clearly depicted. Taking measurements The cut sections were placed under a stereoscopic microscope (Wild M3Z, LeicaTM, Wetzlar, Hessen, Germany) connected to a computer. Digital images were acquired by a coupled camera (EclipseTM E-6000, Nikon Corp., Tokyo, Japan) and imported into the Image Pro-PlusTM software (Media Cybernetics, Silver Spring, MD, USA) for taking tooth measurements. A calibrated operator used the software tools to trace the outer borders of the mesial and distal surfaces at the contact areas, designated L1 and L4, respectively. The line parallel to L1, corresponding to the DEJ on the mesial surface, was designated L2. On the distal surface, the above-mentioned inner border was designated L3. The sections were assessed at six and a half times magnification (Figure 1). Subsequently, the distances between L1-L4 (mesiodistal crown width), L1-L2 (mesial enamel thickness) and L3-L4 (distal enamel thickness) were measured using the software ruler. Figure 1 Proximal enamel extension limited by lines parallel to the tooth’s long axis (mesial: L1-L2 and distal: L3-L4). Mesiodistal width and proximal enamel thickness of maxillary first bicuspids Braz Oral Res. 2010 Jan-Mar;24(1):58-63 60 Statistical analyses Minimum and maximum values, medians, means and the respective standard deviations of the L1-L4, L1-L2 and L3-L4 distances were calculated and represented as box plots. The right and left mesiodistal crown widths and proximal enamel thicknesses, as well as the mesial and distal enamel thicknesses, were compared using the Wilcoxon test. The signifi cance level was set at 5%. Results Graph 1 summarizes the data distribution for the crown width dimensions. Although the right teeth presented a smaller minimum value (6.53 mm) and a greater maximum value (9.30 mm) in relation to the left teeth (6.88 mm and 8.44 mm, respectively), the medians were quite similar (approximately 7.5 mm). According to the box plots shown in Graph 2, the distal enamel thickness measurements were greater than those observed for the mesial surface. The mean values of the mesiodistal crown dimension as well as the enamel thickness observed for the maxillary fi rst bicuspids are shown in Table 1. No signifi cant differences were found between measurements taken on the right and the left teeth (Table 1). However, distal enamel thickness mean values (1.29 mm on both sides) were signifi cantly greater than the values observed for the mesial surfaces (1.08 mm for the right teeth and 1.19 mm for the left teeth), p = 0.0003. The average mesiodistal crown width for the right and left teeth corresponded to 7.51 mm and 7.53 mm, respectively. Discussion The main indication of enamel reduction, also called enamel reproximation, during orthodontic treatment planning is particularly related to the challenging borderline malocclusions with a mild or moderate degree of crowding.5,7 Some authors4 have advocated stripping in cases of Class l arch-length discrepancies with orthognathic profi les, minor Class ll dental malocclusions especially those in which patients have stopped growing and Bolton tooth-size discrepancies. Germeç and Taner5 (2008) reported that stripping combined with nonextraction therapy provided effective results for treating 6 7 8 9 10


Introduction
2][3] This procedure may be an attractive alternative to tooth extraction because it allows the transverse arch dimension and anterior inclinations to be maintained. 4,5evertheless, a critical issue when performing stripping is to estimate the amount of proximal enamel to be removed, so that dentin exposure may be prevented.
Considering that previous studies 6,7 have mentioned that about fifty percent of the whole proximal enamel thickness could be removed safely, reliable measurements of tooth crown dimensions and enamel thickness would constitute a useful guide to the orthodontist during the stripping procedure.Thus, the aim of the present study was to estimate the mesio-distal crown width and the proximal enamel thicknesses of maxillary first bicuspids.

Material and Methods
This study is in agreement with Resolution 196/96 from the National Health Council/Health Department (Brazil).

Sampling
An in vitro model consisting of 20 standardized repetitions would be statistically suitable for estimating the mesiodistal crown width and enamel thickness of eligible teeth from patients born in the same geographical area.Based on this methodological premise, 40 erupted sound human maxillary first bicuspids (20 right and 20 left) were collected from white adult patients aged 19-31 years (mean age: 23.7 ± 4.2 years), submitted to corrective orthodontic treatment with tooth extraction at a private clinic in the city of São Paulo, SP, Brazil.After extraction, the teeth were washed in saline solution to remove blood residues and then cleaned thoroughly with water.

Tooth preparation
All teeth were embedded in self-curing acrylic resin (Uceflex TM 2110, Elekeiroz S/A, São Paulo, SP, Brazil), using round plastic containers (Extec Corp., Enfield, CT, USA).The specimens (n = 40) were numbered and then separated into two groups according to the side: twenty from the left side and twenty from the right side.
The teeth were cut along their long axis through the proximal surfaces, parallel to the buccal surface, to obtain 0.6-mm central sections.Each cut section corresponded to the central area of the proximal surfaces, in that it encompassed the greater crown dimension and the thicker enamel portion.The cut sections were obtained using a 4-inch diameter, high concentration diamond wafering blade (Extec Corp., Enfield, CT, USA) mounted onto a high precision Isomet TM 1000 saw (Buehler, Lake Bluff, IL, USA), under water refrigeration to prevent organic component loss.To avoid damaging the specimens, the diamond disc speed was set at 200 rotations per minute.
The specimens were glued with cyanoacrylate adhesive (Super Bonder TM , Loctite Corp., Itapevi, SP, Brazil) on glass slides (Star Frost TM , Knittel Glaser, Braunschweig, Lower Saxony, Germany) and diaphanized in 100% xylol for ten minutes, three times consecutively.This procedure increased transparency, so that the cut sections could be better visualized under the microscope.Otherwise, the dentinoenamel junction (DEJ) could not be clearly depicted.

Taking measurements
The cut sections were placed under a stereoscopic microscope (Wild M3Z, Leica TM , Wetzlar, Hessen, Germany) connected to a computer.Digital images were acquired by a coupled camera (Eclipse TM E-6000, Nikon Corp., Tokyo, Japan) and imported into the Image Pro-Plus TM software (Media Cybernetics, Silver Spring, MD, USA) for taking tooth measurements.
A calibrated operator used the software tools to trace the outer borders of the mesial and distal surfaces at the contact areas, designated L1 and L4, respectively.The line parallel to L1, corresponding to the DEJ on the mesial surface, was designated L2.On the distal surface, the above-mentioned inner border was designated L3.The sections were assessed at six and a half times magnification (Figure 1).Subsequently, the distances between L1-L4 (mesiodistal crown width), L1-L2 (mesial enamel thickness) and L3-L4 (distal enamel thickness) were measured using the software ruler.

Statistical analyses
Minimum and maximum values, medians, means and the respective standard deviations of the L1-L4, L1-L2 and L3-L4 distances were calculated and represented as box plots.The right and left mesiodistal crown widths and proximal enamel thicknesses, as well as the mesial and distal enamel thicknesses, were compared using the Wilcoxon test.The significance level was set at 5%.

Results
Graph 1 summarizes the data distribution for the crown width dimensions.Although the right teeth presented a smaller minimum value (6.53 mm) and a greater maximum value (9.30 mm) in relation to the left teeth (6.88 mm and 8.44 mm, respectively), the medians were quite similar (approximately 7.5 mm).According to the box plots shown in Graph 2, the distal enamel thickness measurements were greater than those observed for the mesial surface.The mean values of the mesiodistal crown dimension as well as the enamel thickness observed for the maxillary fi rst bicuspids are shown in Table 1.
No signifi cant differences were found between measurements taken on the right and the left teeth (Table 1).However, distal enamel thickness mean values (1.29 mm on both sides) were signifi cantly greater than the values observed for the mesial surfaces (1.08 mm for the right teeth and 1.19 mm for the left teeth), p = 0.0003.The average mesiodistal crown width for the right and left teeth corresponded to 7.51 mm and 7.53 mm, respectively.

Discussion
The main indication of enamel reduction, also called enamel reproximation, during orthodontic treatment planning is particularly related to the challenging borderline malocclusions with a mild or moderate degree of crowding. 5,7Some authors 4 have advocated stripping in cases of Class l arch-length discrepancies with orthognathic profi les, minor Class ll dental malocclusions -especially those in which patients have stopped growing -and Bolton tooth-size discrepancies.Germeç and Taner 5 (2008) reported that stripping combined with nonextraction therapy provided effective results for treating  Class I borderline patients.This protocol decreased treatment time by 8 months and, at the end of treatment, patients had well-balanced and desirable facial esthetics.A certain degree of mandibular incisors crowding relapse may occur, but appears to be acceptable. 3 gain of at least 7 mm might be expected with 50% of proximal enamel reduction. 1,4In selected patients, stripping has the advantage -over extraction therapy -of a significant decrease in treatment time because the amount of tooth structure removed corresponds approximately to the amount of crowding. 7rom a clinical point of view, orthodontists should also take the patient's opinion into account, since many people may be unwilling to undergo tooth extraction. 4By enamel reduction, space may be created and teeth can be reshaped to a more ideal form. 8ower incisor reproximation can improve stability by slightly narrowing the teeth and broadening their proximal surfaces to resist contact slippage. 9urthermore, with enamel reduction and cosmetic recontouring, the long-term maintenance of mandibular incisor alignment is possible 2 , and the transverse arch dimension as well as anterior inclinations may be sustained. 4The preserved intercanine width and the broadened contact surfaces can help prevent posttreatment relapse. 7herefore, clinicians should be aware of the applicable methods for estimating the amount of tooth structure that can be safely removed.To avoid any damage to the tissues under the enamel, it is highly recommended that orthodontists willing to perform enamel reduction use accurate radiographs for measuring the amount of enamel to be grinded, since proximal enamel thickness may vary among different people and in the same individual. 10,11The use of computed tomography has also been registered for measuring enamel thickness. 12More recently, the use of cone beam computed tomography has been suggested for measuring tooth crown dimensions. 13rthodontists need good quality radiographic images and may also use commercially available gauges accurate to within one-tenth of a millimeter to measure the interproximal site after stripping. 14evertheless, it is important to have some idea about the actual tooth crown dimensions.In the present study, histological data are provided pertaining to the proximal enamel thickness, and to the mesiodistal crown width of maxillary first bicuspids.As shown in Table 1, proximal enamel thickness mean values ranged from 1.08 mm to 1.29 mm.Proximal enamel was significantly thicker on the distal surface, in comparison with the mesial surface.Actually, the reference mean values presented in this paper may offer a valid contribution to clinicians for stripping procedures.However, it must be noted that genetic background plays a prominent role in determining many phenotypic traits, such as tooth size.The marked influence of intrinsic factors on tooth formation justifies the conducting of future studies involving tooth crown measurements in different ethnic groups.
To date, there are few studies reporting mean values of proximal enamel thickness for maxillary first bicuspids.Radiographic assessment of mandibular first bicuspids estimated enamel thickness for mesial and distal surfaces at 0.99 mm and 1.07 mm, respectively. 4In another study 15 using tooth sectioning, the proximal enamel thickness values calculated for maxillary first bicuspids were 1.48 mm on mesial surfaces and 1.54 mm on distal surfaces.In fact, these differences may be attributed to methodological and ethnic variations.
According to the findings of this study, during treatment planning, orthodontists should consider that the average mesiodistal crown width of maxillary first bicuspids is about 7.5 mm.Moreover, in agreement with previous investigations, 1,4,5,14 the suggested thresholds for proximal enamel reduction may be in the interval of 0.4-0.5 mm, irrespective of the side.Based on Table 1, the enamel reduction could be slightly more pronounced on the distal surfaces.
In clinical practice, the most commonly used techniques for enamel reduction require hand-held abrasive strips, handpiece-mounted diamond-coated disks and tungsten-carbide or diamond burs. 2 It should be borne in mind that all stripping methods may roughen the enamel surfaces. 2Hence, polishing the stripped enamel is highly advisable.On the other hand, air-rotor stripping has not been related to an increase in the incidence of proximal enamel caries lesions. 7rictional heat is a registered side effect of stripping procedures, and appropriate measures should be taken particularly for high-speed handpiece stripping. 8Air cooling should be preferred to air-water sprays because it provides greater visibility. 8Thermal changes may lead to pulpal reactions.However, periodontal diseases do not seem to increase after enamel reduction. 3ome authors have focused their studies on tooth size discrepancies and/or enamel-dentin differences based on ethnicity, 10,16-21 gender 10,22,23 and dentition phase. 24Since Brazilians may be considered one of the most ethnically varied populations in the world as regards their origin, mostly of European and African descent, it would seem rather difficult to classify them according to specific ethnic groups.Ad-ditionally, it has been suggested that the amount of enamel reduction would not be influenced by gender, since tooth crown dimensions are mainly related to dentin thickness variability rather than to enamel thickness variability. 23

Conclusions
According to the results of the present study, the mean mesiodistal crown width observed for the maxillary first bicuspids examined was approximately 7.5 mm.The mean values obtained for proximal enamel thickness were significantly greater on the distal surfaces (1.29 mm for both sides) than on the mesial surfaces (1.08 mm for the right side and 1.19 mm for the left side).

Graph 1 -Graph 2 -
Box plots representing the distribution pattern for measurements (mm) of mesiodistal crown width of right and left maxillary first bicuspids.Box plot diagrams illustrating the distribution pattern for the measurements (mm) of proximal enamel thickness of the mesial and distal surfaces of maxillary first bicuspids.

Table 1 -
Comparative assessment of the measurements taken for the right and left maxillary first bicuspids.
* Mean values are expressed in millimeters.