Jason L.Bird, D.D.S.
Patients exhibiting a unilateral or bilateral absence of maxillary lateral incisors present difficult problems with respect to treatment planning and mechanotherapy. The challenge in determining the correct treatment plan hinges on whether to develop a wide enough space to replace the missing lateral(s) with a prosthetic solution or to attempt to close the space using orthodontics. Furthermore, the orthodontist must consider the patient’s profile as well as future growth. The position of the maxillary canines is critical in making a correct diagnosis and treatment plan. The orthodontist is required to examine the position, size, shade, and inclination of the maxillary canines. Among other factors to be contemplated are: other aplastic teeth, skeletal and tooth size discrepancies.
The incidence of absence of maxillary lateral incisors has been found to be 5%.  There are presently two theories on the etiology of aplasia of the maxillary lateral incisor. The absence of maxillary lateral incisors may be an expression of an evolutionary trend of relaxed selection leading to the simplification of man’s dentition through a reduction in tooth number.  Alternatively, a disturbance in the fusion of the embryonic facial processes may result in the incomplete expression of a primary cleft, which is manifest as the absence of the maxillary lateral incisor(s).  Currently, there is insufficient documentation to support either or both theories. Moreover, these studies have proposed that the absence of maxillary lateral incisors may only be one representation of a complex, multifactorial, craniofacial anomaly.  Features that have been reported in association with the absence of lateral incisors include a higher incidence of absence of other teeth, more frequent impactions, and tooth size discrepancies in both arches. 
In developing an harmonious relationship between the maxillary and mandibular arches, it is imperative to examine the mesiodistal width of the teeth. In 1952 Dr. W.A. Bolton developed a method of tooth size comparison, which would be an aid in diagnosis and treatment planning of orthodontic cases and in determining the functional and aesthetic outcome of the orthodontic case.  In developing the Bolton Index, Dr. Bolton compared his figures for tooth size with Wheeler’s text in tooth size dimensions. If there are no discrepancies in tooth size and an ideal overjet and overbite can be established, then by definition of Bolton’s Index, the orthodontist should be able to establish an ideal class I, canine protected occlusion.
Dr. Ricketts also conducted a study comparing tooth sizes in the mesiodistal dimension. His study researched the biological significance of the divine proportion and the Fibonacci series in the craniofacial complex. This article describes the application of basic mathematical and geometrical principles to the normal morphology of structures regularly involved in orthodontics and dentistry. For an appreciation of beauty, it has been suggested that the human mind functions at the limbic level. This is due to attractions to the proportions in harmony with the divine proportion.  This is a proportion of 1.618 and its reciprocal of 0.628 in geometry. Fibonacci numbers express precisely the same ratio and are mathematically unique. Measurements of arch length and tooth size were taken on 104 stable, long term, treated patients. The lower incisor became the basic unit. The upper incisor was divine to the width of the lower incisor. Taking both the upper and the lower central incisor as a ratio, the 1.618 relationship is a mark of dynamic symmetry.
The purpose of this study is to compare the anterior tooth size width in patients with aplastic maxillary lateral incisors using Bolton’s Index, as well as the divine proportion. This will enable the orthodontist to better determine the correct diagnosis in the treatment of patients exhibiting aplastic maxillary lateral incisors.
Material and Methods
The sample comprised twenty-one patients (14 males, 7 females) of diverse cultural origins exhibiting bilateral absence of maxillary lateral incisors. Another sample comprised of fifteen patients (6 males, 9 females), also of diverse racial backgrounds, exhibiting a unilateral absence of maxillary lateral incisors. The cases were selected from New York University College of Dentistry’s Postgraduate Orthodontic program and a local private practice (M.A.L.). Sample selection was based solely on the availability of dental cast records. The mean ages of the bilateral and unilateral cases was 18.5 and 15.4 years respectively.
The greatest mesiodistal width of each anterior tooth was measured to the nearest 0.1mm at the interproximal contact using a Boley gauge. If a gap existed, it was not accounted for in the total anterior mesiodistal dimension, since gaps are either opened or closed based upon the treatment decision. Each measurement was repeated three times by the same observer. If the values were not repeated within 0.1mm, at least twice, all three measurements were repeated.
Dental casts were also evaluated using Angle’s classification standards for canines and molars.
A Bolton Index was determined, for each model, from the recorded mesiodistal widths of the maxillary and mandibular anterior teeth. In the absence of maxillary lateral incisors, the maxillary central incisors were used as a reference to determine the ideal mesiodistal width. This was a ratio based upon Wheeler’s Index.  According to the Wheeler Index the average size of a maxillary central incisor is 8.5mm and a maxillary lateral incisor is 6.5mm. This ratio was determined to be 1.31. After all anterior teeth were measured; an anterior Bolton Index was then calculated using as a standard mean value of 77.2 with a standard deviation of 1.65.
In determining the divine proportion, only the mesiodistal widths of the central incisors were measured and calculated. The total mesiodistal width of the maxillary central incisors was divided by the total mesiodistal width of the two mandibular central incisors. Based upon this ratio, the mandibular central incisors were then multiplied by 1.618 to determine the ideal size of the maxillary central incisors based upon the size of the mandibular central incisors.
Table 1. Bilateral Aplasia Results
|Age||Canine||Molar||Rt max CI||Lft max CI||Rt + Lft||Rt mand 1||Lft mand 1||Rt + Lft||Divine pro||Adjust CI||Bolton In|
Bilateral Absence of Maxillary Lateral Incisors
There was a substantial variation in maxillary central incisor width, with a mean of 9.1mm, standard deviation of 0.65 and a range of 7.8mm to 20.0mm. This was also apparent when the Bolton Index was measured. The mean Bolton Index for bilateral absence of maxillary lateral incisors was 74.3 + 3.3 (range of 68.1 to 79.7). The mean measured Bolton ratio in bilaterally absent maxillary lateral incisors indicated a significant maxillary excess falling just outside the Bolton standard range of 74.5 to 80.4. In regards to the maxillary central incisor, it was an average of 0.6mm larger than the standard set by Wheeler in his text. 
In analyzing the divine proportion, the results were similar. The widths of the maxillary central incisors were compared to the widths of the mandibular central incisors. This was done in order to determine whether the width’s ratio was comparable to the divine proportion of 1.618. The mean was 1.69 with a standard deviation of 0.10 and a range of 1.55 to 1.90. The calculation of the divine proportion verified the accuracy of the Bolton Index. In patients exhibiting a bilateral absence of maxillary lateral incisors, there is an excessive amount of maxillary anterior tooth substance. This discrepancy is indicated to be in the width differences of the maxillary to mandibular central incisors.
Of the twenty-three models considered in the study, three exhibited aplasia of other teeth. Two models had aplastic mandibular second premolars and one had an aplastic mandibular lateral incisor. Two models were not used in the study due to anomalies that would have jeopardized the accuracy of the measurements. One study model exhibited a maxillary right canine transposed with the maxillary right first premolar. The other study model had a fusion of the lower left canine with the lower left lateral incisor.
Table 2. Unilateral Aplasia Results (mm.)
|Age||Canine||Molar||Rt max CI||Lft max CI||Rt + Lft||Rt mand 1||Lft mand 1||Rt + Lft||Divine pro||Adjust CI||Bolton In||max 2|
Unilateral Absence of the Lateral Incisor
The results of the measurements of patients exhibiting an absence of only one lateral incisor were closer to the standards of the Bolton Index. The average size of the maxillary central incisors measured 8.8mm with a standard deviation of 1.7mm and a range of 7.1mm to 10.3mm. The Bolton Index calculated for the unilateral absence was 78.2 with a standard deviation of 4.3mm and a range of 71.8 to 84.8. A Bolton Index of 78.2 is within the range of normal and borders on an index of maxillary insufficiency. The maxillary insufficiency and large standard deviation can be attributed to the unilateral maxillary lateral incisor. Referring to Wheeler’s work, the average width of a maxillary lateral incisor is 6.5mm.  In this study the average width was 5.9mm with a standard deviation of 1.3mm and a range of 4.1 to 8.0mm. Patients that exhibit a unilateral absence of a maxillary lateral incisor will have an undersized remaining lateral incisor. Often this lateral incisor will not only be undersized, but often odd in shape, color, and position. 
In determining the divine proportion between the maxillary and mandibular central incisors, there was a slight variance from the standard of 1.618. The mean was 1.64 with a standard deviation of 1.1 and a range of 1.57 to 1.69. The divine proportion of unilateral cases demonstrated maxillary excess. This is a more accurate measurement in determining an anterior interarch discrepancy because the remaining lateral is not taken into consideration.
This study found a maxillary excess in cases with congenitally missing maxillary lateral incisors. This is consistent with the work of Woodworth. 
In verifying the discrepancies between anterior teeth, it was important to incorporate more than one analysis. This was evident in the cases involving the absence of only one lateral incisor. In conducting the Bolton Index in unilateral cases, this study showed a maxillary insufficiency. Whereas, when the divine proportion was determined, the maxillary central incisors exhibited excess to the mandibular central incisors. In re-evaluating the measurements in the unilateral cases, it was found that the error in the Bolton Index was primarily due to the undersized maxillary lateral incisor. In a future study, the lateral incisor could be adjusted to ideal width and then a Bolton Index calculated. This refined measurement would most likely demonstrate that unilateral cases have slight maxillary excess, but not to the extent of bilateral cases.
Cases involving the absence of both lateral incisors confirm a Bolton Index of maxillary excess. In evaluating the mesiodistal widths of maxillary and mandibular central incisors, the maxillary central incisors were more likely to show excess than the mandibular central incisors to demonstrate a deficiency. This data contradicts the research done by Woodworth, which described the mandibular central incisors to be undersized and the maxillary central incisors to be of normal width.  This could be due to the population surveyed. Woodworth’s sample was done on a homogenous population of northern european descent. This study did not consider a homogenous sample. A large enough homogenous sample size was not available to conduct this study. Further research could be conducted to evaluate tooth size discrepancies of various races.
The value of this study can be seen in treatment planning patients with aplastic lateral incisors. If implants are to be considered, the implant site will need to be developed in order to allow the placement of a fixture. Most oral surgeons will need anywhere from 5.5 –7.0mm of space development to place the implant. If more space is needed than can be allotted by orthodontic movement, the orthodontist may elect to consider the mesiodistal width of the central incisors. And reproximating the width. Not only will this create more space for the placement of the implant, It will lead to a more esthetic and balanced appearance of the maxillary dentition.
In considering space closure in-patient’s exhibiting an absence of lateral incisors, the orthodontist will need to evaluate the size of the central incisors, especially in cases where the space cannot be entirely closed. Rather than increasing the width of the maxillary central incisors, the orthodontist may suggest that the restorative dentist increase the widths of the maxillary canines and first premolars.
- A wide variation of mesiodistal tooth sizes exists with patients exhibiting an absence of both maxillary lateral incisors.
- The maxillary central incisors are larger than Wheeler’s standards by a mean of 0.6mm. The mandibular central incisors are within 0.1mm of the standard.
- The Bolton Index demonstrated maxillary excess, and this was further confirmed by evaluating the divine proportion of the maxillary and mandibular incisors.
- Patients with aplastic lateral incisors are also prone to other aplastic teeth and other anomalies.
- The results in Patients exhibiting an absence of only one lateral incisor were closer to the mean than those with two absent lateral incisors.
- The existing lateral incisor was undersized by an average of 0.6mm
- In measuring the divine proportion, the maxillary central incisors were slightly larger in mesiodistal width than the mandibular central incisors.
- Further research is needed in establishing ideal widths of both maxillary lateral incisors before applying the Bolton Index.
4. LeBot P, Gueguen A, Salmon D: Congenital defects of the upper lateral incisors (ULI): Condition and measurements of the other teeth, measurements of the superior arch, head, and face. Am J Phys Anthropol 46:213-243, 1977.
Jason L. Bird
1998 Orthodontic Graduate, New York University. Currently in private practice in Cheyenne,WY.