Maxillary Lateral Incisor Agenesis and Its Relationship to Overall Tooth Size Jane Wright, DDS, MS,A Jose A

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RESEARCH AND EDUCATION Maxillary lateral incisor agenesis and its relationship to overall tooth size Jane Wright, DDS, MS,a Jose A. Bosio, BDS, MS,b Jang-Ching Chou, DDS, MS,c and Shuying S. Jiang, MSd

Prosthodontists, orthodontists, ABSTRACT and general dentists frequently fi Statement of problem. Agenesis of the maxillary lateral incisor has been linked to differences in encounter dif culties when the size of the remaining teeth. Thus, the mesiodistal space required for definitive esthetic resto- attempting to restore the oc- ration in patients with missing maxillary lateral incisors may be reduced. clusion if unilateral or bilateral Purpose. The purpose of this study was to determine whether a tooth size discrepancy exists in maxillary lateral incisors are orthodontic patients with agenesis of one or both maxillary lateral incisors. congenitally missing. Restora- tion of the missing lateral Material and methods. Forty sets of dental casts from orthodontic patients (19 men and 21 women; mean 15.9 years of age; all of European origin) were collected. All casts had agenesis of one incisor using an implant- or both maxillary lateral incisors. Teeth were measured with a digital caliper at their greatest supported crown, a partial mesiodistal width and then compared with those of a control group matched for ethnicity, age, and fi xed dental prosthesis, or sex. Four-factor ANOVA with repeated measures of 2 factors was used for statistical analysis (a=.05). mesial movement of the Results. Orthodontic patients with agenesis of one or both maxillary lateral incisors exhibited canine are treatment options. smaller than normal tooth size compared with the control group. The maxillary arch had a larger In order to establish the tooth size difference between the control and test groups than the mandibular arch (there was a optimal amount of space significant Jaw×Group interaction [F=4.78, P=.032]). required for an ideal restora- Conclusions. Agenesis of one or both maxillary lateral incisors is significantly associated with tion, the contralateral lateral tooth size discrepancy, which may affect the space remaining for restoration of the remaining incisor size can be used as a teeth. (J Prosthet Dent 2016;115:209-214) guide for determining the size of the missing lateral incisor.1 However, the contralateral where midlines are coincident, canines are in a class I tooth is frequently peg shaped or also missing.2 In order relationship, and an ideal vertical and horizontal overlap to determine the appropriate size, 2 methods have been is present, the space required to restore the maxillary suggested: use of the golden proportion, which indicates lateral incisor may still be insufficient. A tooth size that the lateral incisor should be approximately 62% of discrepancy in the mandibular, maxillary, or both arches the width of the central incisor; and the Bolton analysis, could explain this clinical situation. which is used to determine the required space for a Numerous studies have evaluated tooth size and missing lateral incisor.3,4 malocclusion,4-11 but no firm conclusions have been A minimum of 6 mm, but ideally 7 mm of space, is drawn as to whether tooth size can determine or affect usually recommended for an implant in the area of the malocclusions. One study found the mesiodistal dimen- lateral incisor.5,6 However, in many clinical situations, sion of the maxillary lateral incisor to be the most

Supported, in part, by the American Association of Orthodontists and American Association of Orthodontists Foundation (to J.A.B.). This article is based on the Master’s thesis (J.W.) presented in 2011 in partial fulﬁlment of requirements for the degree of Master of Science, Marquette University School of Dentistry, 2011. aAdjunct Professor, Department of Developmental Sciences, Marquette University School of Dentistry, Milwaukee, Wis. bAssociate Professor, Department of Orthodontics, Rutgers School of Dental Medicine, Newark, N.J. cAssistant Professor, Department of Oral Health and Rehabilitation, University of Louisville School of Dentistry, Louisville, Ky. dResearch Associate I, Department of Institutional Assessment and Quality Improvement, Rutgers School of Dental Medicine, Newark, N.J.

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the maxillary lateral incisor have the same sized teeth in the Clinical Implications maxilla and mandible as those in a matched control sample. Because maxillary and mandibular teeth are smaller The secondary hypothesis was that sex, arch, and tooth type than normal in patients missing one or both affect tooth size in the missing lateral incisor group. maxillary lateral incisors, the space created or MATERIAL AND METHODS remaining for the definitive restorations may be smaller than ideal. Thus, clinicians should plan This research was approved by the Institutional Review accordingly. Board. Forty sets of dental casts (21 women, 19 men, mean 15.9 years of age), with missing maxillary lateral incisors (22 unilateral, 18 bilateral) (Fig. 1), were collected significant variable affecting tooth size arch size from local orthodontic practices. For comparison, an discrepancy compared with the mesiodistal and bucco- equal number of dental casts were collected from the lingual dimensions of other teeth.10 Graduate Orthodontics program at the Marquette Uni- Research has demonstrated that genetic factors may versity School of Dentistry to form a control group contribute to agenesis and tooth size discrepancies.12-19 matched for ethnicity, age, and sex. Inclusion criteria for Specifically, MSX1 and PAX9 mutations have been asso- the test population were: white individuals with unilat- ciated with tooth agenesis.12,15 PAX9 gene mutation has eral or bilateral agenesis of the maxillary lateral incisors, also been associated with smaller than normal teeth.19 with the other permanent teeth (except for third molars) Although individuals of different ethnic origins in both arches fully erupted; and with no evidence of experience dental agenesis, those of European origin are extreme wear, breakdown, or interproximal reduction of more often missing maxillary lateral incisors, with a any teeth. Pretreatment and post-treatment dental casts higher incidence in women.16,20,21 Men generally have were used for measurement. Post-treatment dental casts larger teeth than women within any given ethnicity. To were examined only to confirm that no interproximal minimize ethnic variations within the sample population, enamel reduction or enameloplasty had been conducted this study focused on the agenesis of maxillary lateral during treatment. Dental casts with crowns or mesio- incisors of individuals of European origin. distal restored teeth were rejected because of the modi- Few studies have discussed the relationship between fication of tooth structure and size. tooth size and agenesis.22-26 However, some studies have The mean age for the test group was 15.9 years suggested that newer implants require smaller spaces for (SD=7.12), ranging from 11 to 47 years of age, and the implant placement, leading to size reduction in tooth mean age for the control group was 15.9 years (SD=6.7). replacement.27-30 Thus, the purpose of this study was to The orthodontic program’s computerized charting system evaluate whether tooth size discrepancy is observed in (axiUm; Exan Group) was used to search for the matching white orthodontic patients with agenesis of one or both sample. Mesiodistal widths of each tooth were measured maxillary lateral incisors. Thenullhypothesiswasthator- with a high-precision digital caliper (Digital Calipers; Masel, thodontic patients with unilateral or bilateral agenesis of Henry Schein Orthodontics), with measurements rounded

Figure 1. A, Patient cast showing unilateral agenesis of left maxillary lateral incisor, retained left primary canine, left permanent canine in position of lateral incisor, and peg-shaped right lateral incisor. B, Cast of patient with agenesis of both maxillary lateral incisors. Maxillary canines moved mesially into lateral incisor space.

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As expected, tooth size in the maxillary arch was larger than in the mandibular arch (F=680.84, P<.001), and the maxillary arch had larger tooth size differences between control and test groups than the mandibular arch (there was a significant arches×group interaction [F=4.818, P=.031]) (Table 2). Central incisors presented the largest tooth size differences between the maxillary and mandibular arches (there was a significant tooth number×jaw interaction [F=1879.67, P<.001]). The mean width for maxillary central incisors was 8.51 mm, and the mean for mandibular incisor was 5.24 mm (Table 1). First premolars had the smallest tooth size difference between maxillary and mandibular arches (Fig. 3C). The Shrout-Winer intrarater reliability test found a consistency of 0.994, confirming the excellent Figure 2. Digital caliper measuring mesiodistal widths of maxillary teeth. Teeth were measured at their widest points. reliability of the test.

to the nearest hundredth of a millimeter (Fig. 2). One DISCUSSION investigator (J.W.) collected all dental cast measurements. Although clinicians discuss the minimum necessary For each combination of arches and teeth number, space for an implant when one or both maxillary lateral except for maxillary lateral incisor teeth, the left and right incisors are missing, they should focus on determining tooth sizes were averaged and used as an outcome var- the appropriate space for the implant/restoration. The iable for the tooth size in the data analysis. Because the golden proportion for the anterior teeth can be consid- maxillary lateral incisor was missing in one or both sides ered when they determine the size for the missing lateral of the arch, all lateral incisors were excluded from data incisor.3 Patients with smaller than normal maxillary and analysis. Tooth sizes were compared between the test mandibular teeth, as shown in the test sample of this and control groups and also compared for tooth study, may require a smaller than necessary 6.5-mm or numbers, both jaws, and sex. A 4-factor ANOVA with 7-mm space for an implant-supported replacement.5,6 repeated measures of 2 factors was used for data analysis. Although the size of the lateral incisors was elimi- Tooth number and jaw were within-subjects factors, nated from Table 1, the average size for maxillary lateral whereas group and sex were between-subjects factors. incisors in the test group was 5.39 mm. This size is Software (SAS v9.4; SAS Institute Inc) was used for data appreciably less than that of the minimal 7-mm tooth-to- analysis ( =.05). Descriptive statistics are shown in a tooth distance traditionally advocated for implant place- Table 1, and test results from repeated measures ANOVA ment.6 The 7-mm tooth-to-tooth distance allows for are shown in Table 2. In order to calculate the reliability placement of an implant approximately 4 mm in diam- of measurements, 2 casts from the test group were eter, with 1.5 mm between the implant and adjacent measured at 3 different time points, 2 months apart. The teeth.26 With the advent of platform switching and nar- intrarater version of the Shrout-Fleiss statistic test was row diameter implants, there is evidence that less space used to evaluate the reliability of the investigator’s tooth may be needed for implant placement.27,28 A minimal width measurements. tooth-to-tooth space of 5.5 mm may be acceptable if a 3.5-mm diameter platform-switched implant is placed RESULTS with 1 mm between the implant and adjacent teeth.29,30 The results of this study reject the null hypothesis that no This procedure would allow for an appropriate maxillary differences exist in tooth size in patients with agenesis. lateral incisor replacement. Orthodontic patients with agenesis of one or both Even though the present study evaluated the mean maxillary lateral incisors exhibited smaller than normal mesiodistal widths of each tooth within a group, the large tooth size than the control group. (F=4.01, P=.049) range of the size of the maxillary lateral incisor size in (Table 2; Fig. 3A). patients with unilateral agenesis (2.9 mm to 6.95 mm) is Men were found to have larger teeth than women. of note. This demonstrates a wide spectrum in tooth size (Table 2; Fig. 3B). Canine teeth presented the largest of the maxillary lateral incisor when the contralateral tooth size differences between men and women (there incisor is congenitally missing. was a signiﬁcant tooth type×sex interaction [F=5.51, Yaqoob et al25 found an association between P=.003]) (Table 2). First premolars presented the smallest the agenesis of maxillary lateral incisors and tooth size. tooth size differences between men and women. Mirabella et al26 also found that agenesis of a maxillary

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Table 1. Descriptive statistics of populations studied Maxillary Arch Mandibular Arch Total Tooth Factor n Mean SD Mean SD Mean SD Central incisor Group Control 40 8.62 0.64 5.34 0.34 6.98 0.45 Test 40 8.39 0.67 5.13 0.41 6.76 0.52 Sex Female 42 8.42 0.62 5.19 0.37 6.81 0.46 Male 38 8.60 0.70 5.28 0.42 6.94 0.53 Total 80 8.51 0.66 5.24 0.39 6.87 0.50 Canine Group Control 40 7.77 0.50 6.74 0.44 7.25 0.45 Test 40 7.53 0.47 6.52 0.48 7.03 0.45 Sex Female 42 7.47 0.37 6.47 0.32 6.97 0.31 Male 38 7.84 0.55 6.81 0.54 7.33 0.52 Total 80 7.65 0.49 6.63 0.47 7.14 0.46 First premolar Group Control 40 7.02 0.42 7.12 0.48 7.07 0.43 Test 40 6.80 0.48 6.93 0.45 6.86 0.45 Sex Female 42 6.88 0.43 7.01 0.44 6.95 0.42 Male 38 6.94 0.50 7.04 0.51 6.99 0.49 Total 80 6.91 0.46 7.02 0.47 6.97 0.45 Second premolar Group Control 40 6.65 0.41 7.14 0.39 6.90 0.38 Test 40 6.48 0.40 7.10 0.45 6.79 0.40 Sex Female 42 6.52 0.41 7.04 0.37 6.78 0.37 Male 38 6.62 0.41 7.21 0.46 6.91 0.41 Total 80 6.57 0.41 7.12 0.42 6.84 0.39 First molar Group Control 40 10.19 0.60 10.92 0.74 10.56 0.62 Test 40 9.93 0.55 10.93 0.59 10.43 0.51 Sex Female 42 9.95 0.47 10.72 0.50 10.34 0.44 Male 38 10.18 0.68 11.15 0.76 10.67 0.64 Total 80 10.06 0.59 10.93 0.67 10.49 0.57 All teeth Group Control 40 8.05 0.43 7.45 0.40 7.75 0.40 Test 40 7.83 0.43 7.32 0.40 7.58 0.40 Sex Female 42 7.85 0.38 7.29 0.33 7.57 0.34 Male 38 8.04 0.48 7.50 0.45 7.77 0.46 Total Total 80 7.94 0.44 7.39 0.40 7.66 0.41 lateral incisor was a strong predictor for reduced overall would indicate the discrepancy, nor has the study found tooth size. On average, that study showed that the dif- significant differences between specific teeth (central ference in mesiodistal width of the maxillary central incisor incisor, canines, first and second premolar or molars) in was 0.47 mm and that of the mandibular incisors was 0.43 the control and sample test. mm. However, they had no control for race and sex.26 Ballard8 discovered that 90% of teeth in his sample The results of the present study, where race, sex, were not symmetrically sized between right and left sides, and age were controlled, also demonstrated that the with differences as much as 0.25 mm. In the present study, mandibular and maxillary teeth of patients with agenesis no statistical differences were found between teeth on the of one of both maxillary lateral incisors were smaller than right and left sides. Therefore, the teeth were grouped those who had all permanent teeth. However, this study together, and an average size was created for each tooth. could not find a specific tooth or group of teeth that Because one or both maxillary lateral incisors were missing

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Table 2. Repeated measures ANOVA 8.2 Group Factor F P Control Group 4.01 .049* 8.1 Test Sex 5.13 .026* 8.0 Tooth number 2908.66 <.001* 7.9 Arch 680.84 <.001* 7.8 Group ×sex 0.7 .406 Tooth number ×group 1.04 .384 7.7 Tooth number ×sex 5.51 <.001* 7.6 Arch ×group 4.78 .032* 7.5 Arch ×sex 0.30 .583

Tooth Size, Mean (mm) Size, Tooth 7.4 Tooth number ×arch 1879.67 <.001* Tooth number ×group ×sex 2.24 .065 7.3 Arch ×group ×sex 0.38 .542 7.2 Tooth number ×arch ×group 2.00 .094 Maxillary Mandibular Tooth number ×arch ×sex 2.01 .093 Tooth number ×arch ×group ×sex 1.72 .145 Arch A *Statistically significant at a level of .05. 11 Sex Female from the test sample group, all lateral incisors were Male excluded from the study to fit the statistical model. 10 Mandibular teeth were also affected by maxillary agenesis. However, the trend found in the mandibular 9 arch was not as large as that found in the maxillary arch. If the discrepancy in the maxillary arch is larger, then a 8 replacement space for the missing lateral incisor may need to be smaller in the maxilla. 7 The differences found for sex are contradictory in that Mean (mm) Size, Tooth some authors found a sex-linked genetic association between agenesis and tooth size19 and others25 found 6 similar results for both sexes. The present study found Central Canine 1st 2nd 1st that men had larger teeth than women in both test and Incisor Premolar Premolar Molar control groups. The canine teeth of men had the largest Tooth B difference in size, whereas first premolars presented the smallest difference (Fig. 3B); however, this study did not 11.0 Arches find a statistical correlation in tooth size between sex and Mandibular Maxillary group samples. 10.0 Although Kinzer and Kokich1 advocated using the Bolton analysis to determine the appropriate size for 9.0 the replacement of the missing maxillary lateral incisor, the Bolton analysis was not conducted in this study test group, 8.0 mainly because of the unilateral or bilateral maxillary lateral incisor agenesis. In retrospect, if the guidelines described by Kinzer and Kokich1 had been followed, then the sum of the 7.0

widths of the mandibular anterior teeth should have been Mean (mm) Size, Tooth divided by the sum of the available maxillary anterior teeth 6.0 plus x (the missing tooth), making this ratio equal to the ideal anterior Bolton ratio (0.78). This could then have been 5.0 solved for x. This procedure may work well in patients with Central Canine 1st 2nd 1st appropriately sized anterior teeth. However, in a popula- Incisor Premolar Premolar Molar tion with a suspected tooth size discrepancy, using the ideal Tooth C anterior Bolton ratio of 0.78, would not characterize such a Figure 3. A, Differences are shown for test versus control groups with discrepancy and could suggest that the missing lateral maxillary and mandibular arches as factors. B, Differences in tooth size incisor was larger or smaller than normal.7 Furthermore, are shown with sex and tooth types as factors. C, Differences in tooth with this equation, x (the size of the missing lateral incisor) size are shown with maxillary and mandibular arches and tooth type as may not match the existing contralateral maxillary lateral factors.

Wright et al THE JOURNAL OF PROSTHETIC DENTISTRY 214 Volume 115 Issue 2 incisor. This could lead the orthodontic clinician to prepare 4. Bolton WA. Disharmony in tooth size and its relation to the analysis and treatment of malocclusion. Angle Orthod 1958;28:113-30. an incorrect amount of space for the implant and crown 5. Kokich VG. Maxillary lateral incisor implants: planning with the aid of or- restoration. thodontics. J Oral Maxillofac Surg 2004;62(suppl 2):48-56. fi 6. Kinzer GA, Kokich VO Jr. Managing congenitally missing lateral incisors. Results that are statistically signi cant may not be Part III: Single-tooth implants. J Esthet Restor Dent 2005;17:202-10. clinically significant. A tooth size discrepancy of 1.5 to 2 7. Bolton WA. The clinical application of a tooth-size analysis. Am J Orthod 1962;48:504-29. mm within an arch (0.75 to 1 mm/side) is deemed clin- 8. Ballard ML. Asymmetry in tooth size: a factor in the etiology, diagnosis and ically significant. Othman and Harradine30 used the treatment to malocclusion. Angle Orthod 1944;14:67-70. fi 9. Uysal T, Sari Z, Basciftci FA, Memili B. Intermaxillary tooth size discrepancy Bolton analysis to determine this clinical signi cance. The and malocclusion: is there a relation? Angle Orthod 2005;75:208-13. study was conducted with a sample of orthodontic pa- 10. Agenter MK, Harris EF, Blair RN. Influence of tooth crown size on malocclusion. Am J Orthod Dentofacial Orthop 2009;136:795-804. tients with a full complement of teeth, making it harder 11. Basdra EK, Kiokpasoglou MN, Komposch G. Congenital tooth anomalies and to compare it with the present study of participants with malocclusions: a genetic link? Europ J Orthod 2001;23:145-51. 12. Vastardis H, Karimbux N, Guthua SW, Seidman JG, Seidman CE. A human agenesis. Because the present study did not calculate the MSX1 homeodomain missense mutation causes selective tooth agenesis. Nat tooth size discrepancy per participant (as with the Bolton Genet 1996;13:417-21. fi 13. Garib DG, Alencar BM, Lauris JR, Baccetti T. Agenesis of maxillary lateral analysis), it is dif cult to determine whether the results incisors and associated dental anomalies. Am J Orthod Dentofacial Orthop from this sample are clinically significant. These tooth 2010;137:732. e1-6. fi 14. McKeown HF, Robinson DL, Elcock C, al-Sharood M, Brook AH. Tooth di- size reductions may be clinically signi cant for some mensions in hypodontia patients, their unaffected relatives and a control group patients but not for others. measured by a new image analysis system. Eur J Orthod 2002;24:131-41. fi 15. Stockton DW, Das P, Goldberg M, D’Souza RN, Patel PI. Mutation of PAX9 The present study found signi cantly smaller teeth in is associated with oligodontia. Nat Genet 2000;24:18-9. patients with agenesis of one or both maxillary lateral 16. Bailit HL. Dental variation among populations. An anthropologic view. Dent fi fi Clin North Am 1975;19:125-39. incisor. However, these ndings were not speci c and may 17. Dempsey PG, Townsend GC. Genetic and environmental contributions to have varied among individuals as well as being dependent variation in human tooth size. Heredity 2001;86:685-93. fi 18. Brook AH, Elcock C, al-Sharood MH, McKeown HF, Khalaf K, Smith RN. on speci c clinical situations. It is thus advisable to Further studies of a model for the etiology of anomalies of tooth number and approach patients with maxillary lateral incisor agenesis on size in humans. Connect Tissue Res 2002;43:289-95. 19. Brook AH, Elcock C, Aggarwal M, Lath DL, Russell JM, Patel PI, Smith RN. a case-by-case basis, keeping in mind that these patients Tooth dimensions in hypodontia with a known PAX9 mutation. Arch Oral can have varying patterns of smaller than normal teeth. Biol 2009;54(suppl 1):S57-62. 20. Polder BJ, Van’t Hof MA, Van der Linden FP, Kuijpers-Jagtman AM. A meta- This study was limited to white orthodontic patients, analysis of the prevalence of dental agenesis of permanent teeth. Community so generalizing tooth size discrepancy patterns across Dent Oral Epidemiol 2004;32:217-26. fi 21. Bishara SE, Fernandez Garcia A, Jakobsen JR, Fahl JA. Mesiodistal crown races may not be prudent. The main nding of the pre- dimensions in Mexico and the United States. Angle Orthod 1986;56:315-23. sent sample from a white population showed patients 22. Salmon D, Le Bot P. Congenital defects of the upper lateral incisors: multi- variate analysis of measurements of the other teeth, the superior arch, head missing one or both maxillary lateral incisors to have and face. Am J Phys Anthropol 1977;46:245-51. smaller tooth size than patients with all teeth present. 23. Sofaer JA, Chung CS, Niswander JD, Runck DW. Developmental interaction, size and agenesis among permanent maxillary incisors. Hum Biol 1971;43:36-45. However, possibly because the reduced sample size, this 24. Baidas L, Hashim H. An anterior tooth size comparison in unilateral and study could not find any specific tooth or group of teeth bilateral congenitally absent maxillary lateral incisors. J Contemp Dent Pract 2005;6:56-63. responsible for the difference. The sample size should be 25. Yaqoob O, DiBiase AT, Garvey T, Fleming PS. Relationship between bilateral increased in future studies. Furthermore, people of Afri- congenital absence of maxillary lateral incisors and anterior tooth width. Am J Orthod Dentofacial Orthop 2011;139:e229-33. can, Hispanic, or Asian descent with agenesis of the 26. Mirabella AD, Kokich VG, Rosa M. Analysis of crown widths in subjects maxillary lateral incisor(s) may show different tooth size with congenitally missing maxillary lateral incisors. Eur J Orthod 2012;34: 783-7. discrepancies or none at all. Future studies are recom- 27. Elian N, Bloom M, Dard M, Cho SC, Trushkowsky RD, Tarnow D. Effect of mended to evaluate these populations. interimplant distance (2 and 3 mm) on the height of interimplant bone crest: a histomorphometric evaluation. J Periodontol 2011;82:1749-56. 28. Jung RE, Jones AA, Higginbottom FL, Wilson TG, Schoolfield J, Buser D, CONCLUSIONS et al. The influence of non-matching implant and abutment diameters on radiographic crestal bone levels in dogs. J Periodontol 2008;79:260-70. Orthodontic patients with unilateral or bilateral agenesis of 29. Vela X, Méndez V, Rodríguez X, Segalá M, Tarnow DP. Crestal bone changes on platform-switched implants and adjacent teeth when the the maxillary lateral incisor have smaller than average teeth tooth-implant distance is less than 1.5 mm. Int J Periodontics Restor Dent when compared with a control-matched group. The 2012;32:149-55. 30. Othman S, Harradine N. Tooth size discrepancies in an orthodontic popu- maxillary arch has larger tooth size differences between the lation. Angle Orthod 2007;77:668-74. control and test groups than the mandibular arch. Clini- fi Corresponding author: cians should consider the ndings of this study when Dr Jose A. Bosio preparing maxillary lateral incisor sites for restorations. Rutgers School of Dental Medicine 110 Bergen Street, Rm C780 Newark, NJ 07103 REFERENCES Email: [email protected]

1. Kinzer GA, Kokich VO Jr. Managing congenitally missing lateral incisors. Acknowledgments Part II: Tooth-supported restorations. J Esthet Restor Dent 2005;17:76-84. The authors thank Drs William Lobb, Gerard T. Bradley, and Dawei Liu, Mar- 2. Woolf CM. Missing maxillary lateral incisors: a genetic study. Am J Human quette University School of Dentistry, for advice and suggestions; and Jessica Genetics 1971;23:289-96. Pruszynski and Dr Raphael Benoliel for statistical analysis. 3. Lombardi RE. The principles of visual perception and their application to dental esthetics. J Prosthet Dent 1973;29:358-82. Copyright © 2016 by the Editorial Council for The Journal of Prosthetic Dentistry.

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