Maxillary Central Incisor Tooth Form, Facial Form and Gender
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Title: Correlation of the maxillary central incisor tooth form with face form and gender in a section of the North Indian population – An AutoCAD analysis
Aim: An esthetically pleasing restoration or prosthesis should not be identifiable as artificial, but should be in harmony with the natural tooth arrangement of an individual. This study aimed to determine whether a correlation exists between the maxillary central incisor tooth form and the facial form and gender of subjects in a selected sample of the North Indian population. Materials and Methods: The study was conducted in the Department of Prosthodontics and Crown and Bridge and Implants, Manav Rachna Dental College, Faridabad. Two standardized photographs each (portrait [closed lips] and teeth in occlusion [with cheek retractor]) of 200 subjects of North Indian origin (age range 18-25 years) studying at Manav Rachna International University were assessed for facial form and tooth form, using the AutoCAD© (Autodesk Inc. 2013, California, USA) software. Results and Conclusion: A statistically significant correlation of the maxillary central incisor tooth form with the inverted face form of the subject was found in a section of the North Indian population. However, the correlation between the maxillary central incisor tooth form and gender of an individual was found to be statistically insignificant.
Keywords: Maxillary central incisor, tooth form, face form, gender, AutoCAD
Esthetic dentistry is fast becoming a key consideration for dentists and their patients. Although, a smile is characterized by numerous factors involving both the teeth and surrounding soft tissues, the esthetic appearance of the anterior teeth contributes to a person's self confidence and how attractive he/she is perceived by others.1 For an esthetically pleasing smile, the position, form and colour of the maxillary central incisors are the most essential factors.2
Since the maxillary central incisors play a key role in the dental composition, various anthropometric and biometric references are used in their design and selection in esthetic accordance with the facial form of the patient.3 When only parts of the dentition have to be restored, the remaining natural dentition can serve as a guide, providing a relatively simple and direct procedure to design artificial teeth that blend with or complement the natural dentition. However, in cases where all the teeth are missing and the entire dentition has to be restored, and no information can be gained from the remaining natural teeth, old photographs, or cast models, other methods have to be applied to select and design the missing teeth.1 These methods are not only useful for complete dentures but also for any complex anterior restoration.2
Esthetic dentistry tries to produce a prosthesis that defies detection. This is possible only when, the tooth form, shade and tooth size of the patient are in optimum harmony with their facial form, dimensions and facial complexion. In 1914, Williams 4 was the first to suggest that a correlation existed between the upside down facial shape and the shape of the upper central incisors. Form shape came to be guided by comparison with the inverted shape of the face. According to this classification, the dental outlines of the upper incisors were in three shapes: tapered, ovoid and square, which is still considered by prosthodontists.5 In 1955, Frush and Fisher6 introduced the concept of dentogenics or the dentogenic theory which stated that the correct tooth form should be designed or selected with regard to the patient's sex/gender, personality and age (SPA factor).
The prosthodontics literature pertains primarily to the non-Indian population and there is a lack of information on the selection and design of artificial anterior tooth forms in the Indian population. The knowledge of racial norms for facial appearance might aid practitioners, since the treatment given would be in harmony with the facial appearance for patients of different races.
Most studies,7, 8, 9 on the subject of artificial teeth design and selection, with respect to facial form and gender, have been based on subjective evaluation and assessments of different individuals; with lack of standardized methods and techniques and unavailability of technological resources. Hence, their reliability has been questionable. This study attempts to increase the accuracy of the methodology by employing a software known as AutoCAD© (Autodesk Inc. 2013, California USA) - Auto Computer Aided Design and Drafting. AutoCAD is a 2-D and 3-D, vector based, computer-aided drafting software application used in architecture, construction and manufacturing to assist in the preparation of blueprints and other engineering plans. 10 This high end technology has been applied to make the process of evaluating the maxillary central incisor tooth form, easier, objective and more accurate.
The present study aims to assess whether a correlation exists between the maxillary central incisor tooth form and face form of the subject and also to assess if any gender differences could be seen in relation to the form of the maxillary central incisor. The null hypothesis of this study is that there are no gender specific differences in tooth form as well as no correlation between facial form and tooth form of a person.
Materials and Methods
A total of 200 dentulous subjects of North Indian origin (age range 18-25 years) studying at Manav Rachna International University, with well aligned natural maxillary anterior teeth were randomly selected. The subjects being young adults belonging to different parts of North India formed an appropriate representative sample.
Each subject was interviewed, to confirm the inclusion criteria and to rule out the exclusion criteria.
The exclusion criteria were: subjects with anterior teeth fractures, subjects having maxillary anterior teeth with extensive carious lesions, subjects with incisal wear of maxillary anterior teeth, subjects with gingival hyperplasia in the maxillary anterior teeth, subjects who have undergone orthognathic surgery, orthodontic treatment, and those with congenital or surgical facial defects, those with microdontia or macrodontia and subjects with any type of prosthetic restoration on their anterior teeth. A participant informed consent form (PICF) was provided in both english and hindi languages. The informed consent form was explained to each participant and was signed by them before starting the procedure.
A standardized photographic procedure was used to obtain images of the face and the maxillary central incisors. Each subject was made to sit upright on a chair with the occlusal plane of the maxillary teeth parallel to the floor. Two standardized photographs were taken for each subject: portrait (closed lips) and the maxillary incisors (without lips). For each photograph, standardized distances (portrait – 100cms, teeth- 12cms) were used (from the tip of the subject’s nose to the centre of the camera lens). A fixed focus of 1:1 was used for each subject, with theEF 100 mmf/2.8 Macro USM lens. The height of the Canon EOS 1100D DSLR camera mounted on a tripod (Traveller Mini Pro Tripod For Canon EOS 1100D) was adjusted individually according to the position of the subject’s face and teeth. A full face photograph with closed lips was obtained, with lens positioned parallel to subject face. The subject’s hair did not cover any part of the face and the teeth were in contact [Figure 1]. The standardized photographic procedure was followed as per the earlier method followed by Wolfart S et al.2 Intraoral photographs of maxillary central incisors were obtained until the lens was parallel to the labial surface of the teeth. Cheek retractors were used to obtain full exposure of the maxillary central incisors. The maxillary incisors were centred in the photograph, with the midline centred and perpendicular to the incisal plane [Figure 2]. Using the image editing software (Adobe Photoshop 8.0.1, Adobe systems, San Jose, California), the images were adjusted so that the maxillary incisors remain centred in the image, one at a time, with the long axis set vertically. [Figure 3].
The mathematical assessment of the photographs was done according to the methodology proposed by Wolfart et al 2.
The outline of the upper right incisor was traced and in the next step, the median of the tooth was drawn in the sketch. (X) was the most apical point of intersection between the median and the outline. Mesial and distal tangents were constructed to the outline – parallel to the median. Two perpendiculars were drawn on the median starting from the most apical (O1) and most incisal (O2) point of intersection between the outline and the tangent. They define two points of intersection (S1 and S2). The line S1S2 was bisected (S). By taking the width of the tooth through (S) the widest stretch across the tooth was defined (TB). Dividing the line SX into five parts, the line TA could be constructed parallel to TB on 4/5 of the length of XS. The outline sketch was turned upside down for the evaluation of the face shape. The lines FB (the widest stretch across the face) and FA (a defined parallel stretch to line FB) were constructed in an analogous manner to the lines TA and TB. [Figure 4]. For the evaluation of the face shape, the outline sketch of the portraits was turned upside down. The facial outline form was determined by the outline of the temporal bone at the height of the hairline, temporal process of the zygomatic arch and the gonion. The facial quotient (FQ) was calculated in an analogous manner to that of the tooth quotient (TQ) [Figure 5]. According to the numerical values that were obtained, dental and facial forms were classified as: 
- Tapered (≤ 0.61)
- Oval (≥0.61 and ≤ 0.70)
- Square (≥ 0.70)
The data, for each subject were recorded in a standardized proforma, along with the photographs of the subject before and after analysis.
The data obtained were arranged systematically and transferred to Statisticals Package for the Social Sciences (SPSS) version 20, IBM Inc. for analysis. The following statistical methods were employed: descriptive statistical tests, chi square tests, independent t-tests and Spearman correlation. The level of statistical significance was set at p-value ≤ 0.05.
Chi-square test was used for comparison between categorical variables. Student’s ‘t’ test was used for comparing two groups with respect to continuous and normally distributed variables.
The design and selection of maxillary anterior teeth for esthetically pleasing prosthetic rehabilitation has long posed a challenge in clinical practice. A reliable and reproducible method is required for prosthetically restoring anterior teeth in terms of their size and form, which are in proportion to the face and gender of the patient.
Our first objective was to determine the maxillary incisor tooth form of each subject using the AutoCAD software. The study was then classified into three forms - ovoid, square and tapering. The oval category was the most prevalent tooth form among the study population while the square tooth form was the least prevalent. These findings were in accordance with the study by Brunetto et al 11 on a Brazilian population. However, in a study conducted by Wolfart et al 2 on a German population, although the oval category of tooth form was most prevalent, the least prevalent tooth form seen was the tapered form. These differences may be due to ethnic and genetic population variability and greater software accuracy, adopted in the present study.
Our second objective of the study was to determine whether a correlation existed between the maxillary central incisor tooth form and the face form. Since this finding is statistically significant in the present study sample, therefore, the null hypothesis of the study, was rejected. This was in accordance with the results of studies by Selg 12 and Sellen et al 3 which postulated that the face form of a patient was helpful in selecting the artificial tooth form for their prosthesis. Our results also confirmed William’s “law of harmony” which states that a correlation exists between the inverted facial shape and the shape of the maxillary central incisors.4 Some studies 5 and literature reviews 3,13 confirm that even despite controversies surrounding the authenticity of William’s law of harmony, it is still the simplest, the most useful and the most predictable method of artificial tooth selection for anterior teeth. On the other hand, the validity of William’s classification has been questioned by various authors. 14,2,15,16,8,17,18 The positive correlation seen between tooth form and face form in this study as compared to statistically insignificant results obtained in various previous studies 2 can be attributed to the use of a more practical, technically advanced and precise AutoCAD© (Autodesk Inc. 2013, California USA) software which enhanced the accuracy and reliability of the analysis. Previous studies were mostly based on visual and subjective determination of the tooth and face forms and hence did not provide valid results. The genetic variability of different populations, on which past studies have been conducted, may also have some influence on the final results.
Our third objective was to determine whether a correlation existed between the tooth size and the gender of a person. This correlation was found to be statistically significant. The size of the maxillary central incisor in men was found to be larger than that of women. This is in accordance with studies which concluded that tooth size is known to correlate with gender.1,19,20,21
Our fourth objective was to determine whether there was a correlation between the tooth form and the gender of the person. This correlation was found to be statistically insignificant. This result was in agreement with that of Wolfart et al 2 and Berksun et al 16 who noted no significant differences in tooth form between the genders. However, Horvath et al 1 and Brunetto et al 11, reported that maxillary anterior teeth do have gender specific differences and that tooth shapes are gender specific. In our study, the oval tooth form was found to be the most prevalent in both the genders. This was similar to the results obtained by Brodbelt et al 22 and Wolfart et al 2. Hence, choosing the oval form has a better chance of matching the original tooth, irrespective of the gender. A higher percentage of the square tooth form was seen in women as compared to men. This result was also similar to that of Wolfart et al 4 and Brunetto et al. 11 This finding suggests that opting for a square tooth form for a male patient is statistically more likely to result in a mismatch. Hence, the dentogenic concept that suggests that the female teeth should be more rounded and the male teeth should be more squarish is not supported by the results of this study.
Within the limitations of this study, it was found that in a section of the North Indian population that:
- The oval tooth shape was most common while the square tooth shape was least common.
- The tooth size was generally found to be larger in males than in females.
- A significant correlation could be found between the face form and tooth form of a person, thus validating William’s law of harmony.2
- No correlation or gender- specific differences could be found between the tooth shape and gender of a person. Hence, the dentogenic (SPA) concept was which states that tooth form is related to the SPA of the patient is not validated and supported by this study.
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