The SPA Factor or Not? Distinguishing Sex on the Basis of Stereotyped Tooth Characteristics

History

During the ivory age, teeth were historically selected mostly by dimensional measurements. Once Madame Schimmelpeinik classified geometric face form and profiles in 1815, dentists began to more seriously consider face form and artistic qualities for more esthetic teeth selection. The “correspondence and harmony” concept proposed by J.W. White in 1872 argued that patient temperaments should be characterized by a corresponding tooth form and color, harmony required that the size of the tooth be proportionate to the face, and the shape and color of the tooth be in harmony with the patient’s facial complexion, sex, and age. The challenge to dentists was then to correlate and supplement these values into a dental temperamental classification useful for selecting the tooth mold.¹ Since then, dentists have fervently tried to identify smile characteristics that both enhance and detract from a smile. However, acceptable deviations for smile characteristics are wide ranging, making it difficult to quantify ideal and maximum acceptable smile deviations for smile characteristics.³

Nevertheless, certain objective tooth characteristics related to shape can be and have been quantified. In 1905, Berry discovered that the upper central incisors had a definite proportional ratio to face proportions, equal to one-sixteenth of the face width and one-twentieth of the face length.⁴ Berry’s “biometric method” eventually evolved into the “typal form method,” in which Williams in 1914 subsequently classified human facial forms as square, tapering, or ovoid.⁵ Williams believed that a definitive relationship exists between the inverted face form and the form of maxillary central incisor. In turn, he proposed the “law of harmony” and described three corresponding square, tapering, and ovoid tooth forms that would be harmonized with the patient’s facial form.^6,7

Consequently, Wright developed a “photometric method” that used photographs of the patient with natural teeth to establish a ratio by comparative computation of measurements of like areas of the face and photograph.^1,8 In 1935, Myerson went one step further with the “multiple choice method” to address the need for a selective range in labial surface characteristics with transparent labial and mesial surfaces, varying surface color tone and characterizations of teeth to mimic the effects of time and wear.^1,9

Finally, the dawn of the “dentogenic restoration” arrived, and in 1955 Frush and Fischer proposed selecting teeth on the basis of the sex, personality, and age of the patient.¹⁰ Because all patients possessed these three essential factors—sex, personality, and age—they reasoned that the “SPA” factor ultimately needed to be interpreted prosthodontically to optimize the esthetic appearance of the prosthesis.^11,12

According to Frush, constructing a dentogenic restoration effectively—that is, one that would create psychologic as well as physiologic comfort for the patient—was a matter of properly interpreting the sex, personality, and age of the patient in the denture. This is done through detailed consideration of the denture tooth, its position, and the matrix, which, if treated properly by the dentist, will reveal the qualities of femininity, masculinity, personality, and the various physiologic age in the smile. This, in turn, will afford the edentulous patient with teeth that have a natural and authentic appearance.¹¹

In personality mold selection, a “delicate” mold is feminine and a “vigorous” mold is masculine. A medium-pleasing personality mold is made more masculine by squaring the incisal edges. Each dentist should arrive at his or her own choice of these molds through trial and follow an independent course of creative effort.

The SPA factor—the evaluation of sex, personality, and age—was first summarized by Sears in his concept of esthetic dental prosthetics.^13,14 When the laterals are nearly as broad as the other front teeth, the set is presumed to have a strong or masculine appearance. When the laterals are narrower than average, the set is considered feminine or delicate. According to Sears, “strong” generally meant “large” or “square,” and “delicate” meant “rounded” or “smaller.”

By “keeping a woman a woman and a man a man, after each loses the natural teeth,” Frush believed that the dentogenic restoration would bring mental comfort and well-being to denture-wearers.¹² In general, femininity is expressed in the female form with roundness, smoothness, and softness that is typical of women. Masculinity is expressed in the masculine form, ie, the cuboidal, hard, muscular, vigorous appearance that is typical of men. Masculinity expresses aggressiveness, boldness, hardness, strength, action, and forcefulness. A tooth that expresses masculine characteristics shows vigor, boldness, and hardness. A hardened smile for the vigorous male can be achieved by rotating the lateral incisor mesially. By sculpting teeth to properly express vigor and delicacy, the effect of sex identity can be achieved in a denture.

The degree of hardness or softeness will depend on the characteristics of hardness or softeness expressed by the individual patient. In the spectrum of human personalities: “The rugged male extrovert could only fit into the red end of the spectrum; the shrinking violet type female could only belong at the right end of the scale, and the normal type, male or female, would fit somewhere in between.”¹⁰

Because the term “law of harmony” and the concept of “dentogenic restorations” were coined in the 1950s, many textbooks continue to elaborate that the shape of the upper anterior teeth is determined by, among other things, the sex of the patient. Although this theory has never been proven, contemporary methods to select tooth molds are based on these principles. The trubyte tooth indicator, for example, uses the forehead, base of the nose, and prominent point of the chin to determine the patient’s facial profile. Based on these three points, the profile can be straight, convex, or concave.^1,15 The labial surface of the tooth viewed from mesial should show a contour similar to that when viewed in profile. The labial surface of the tooth when viewed from the incisal should show a convexity or flatness similar to that seen when the face is viewed from under the chin or from the top of the head. Curved facial features are associated with femininity, and square features are associated with masculinity. Because it is assumed harmony exists between tooth form and face form, it follows that teeth of females are ovoid or tapering rather than square.

Purpose

Harmonizing the face with teeth or the teeth with a patient’s sex, personality, and age (SPA factor) has never been proven to result in an ideal esthetic appearance. Therefore, the current study attempts to answer the following questions: Are dentists’ and dental students’ concepts of esthetic appearance similar to those of patients’? Are square teeth considered masculine and tapered ovoid teeth considered feminine, and do women prefer a different type of teeth from men, presumably a feminine type of teeth? Can faculty, dental students, and laypeople truly determine the sex of a patient solely by looking at pictures of the anterior teeth?

Methods

This study was approved by the Nova Southeastern University Institutional Review Board. The study was eligible for exempt review under 45CFR46.101 (b)(2) because the study/project involves eligible research using educational tests, surveys, interview procedures, or observation of public behavior. Fifty faculty, 100 students, and 50 patients evaluated 40 (1 inch by 2 inches) photographed smiles for 8,000 observations. An equal number of male and female smiles were presented (20 male and 20 female). Each participant was asked to indicate whether the “smile” was from a male or female based on seeing only the teeth (to determine if gender can be predicted from the shape of the teeth). All faculty, students, and patients were chosen at random without regard for their level of expertise. The allotted time to take the survey was 5 minutes.

A focus group, comprised of 2 educators and 2 students, was used for questionnaire development. The group, led by a moderator, was first brought together to discuss questionnaire objectives. Two adjudicators then decided whether the questionnaire was ready for pretesting. A pilot study was conducted among a group of restorative dentistry faculty attending a monthly faculty dinner to pretest procedures and materials involved in data collection. Respondent debriefings (respondents’ comments on specific questions or the survey as a whole) ensured that the questionnaire adequately conveyed the intended research questions; measured the intended attitudes, values, and reported facts; and that the collections of data were conducted according to specified study protocols.¹⁶

Examples of photographs of the teeth are presented in Figure 1 through Figure 5. The photographs were cropped to ensure that all facial hair, lipstick, lips, and other factors that might influence gender selection were removed. Contingency table analysis was used to determine whether all study participants labeled the smiles similarly and to look for differences within each group. Care was taken to model the effect of individual variation. To ascertain differences between faculty, students, and the public, a nested logistic regression was employed to account for the correlation within subjects’ responses. The study was conducted in a HIPAA-compliant manner. Absolutely no identifiers were collected or retained through the data-collection process.

Results

One would expect to see 50% of the smiles to be labeled as male and 50% as female. Statistical differences were found for the total group, as all participants were more likely to rate a smile as female (χ2 = 38.19, P < .0001). Using the odds ratio, study participants were 1.32 times more likely to view a smile as female. Contingency table analysis was utilized to look for differences within groups. No statistical differences were found within the patients’ and students’ sample, but differences were found within the faculty group (χ2 = 4.80, P < .02). The faculty group was more likely to consider a smile as female. The nested logistic regression then tested for differences between the 3 groups—patients, dental students, and dental faculty. The results show statistical differences between the groups (χ2 = 706.93, P < .0001). Specifically, the faculty group was 1.14 times more likely to choose a female response over a male than the patient and student groups. Clearly, the faculty group was less likely to identify the correct smile than the patient and student group. Table 1 through Table 4 provides overall, faculty, student, and patient response information, respectively.

Discussion

According to current principles and practices of complete dentures, the facial form of the patient should be classified into square, ovoid, or tapering. The form of the teeth should be in harmony with the form of the face—square teeth are used for those with a square face, etc. In addition, the patient’s age, gender, and personality should be considered when trying to improve the appearance. The color is also important; dark and opaque teeth should be selected for elderly patients, while light and translucent teeth should be chosen for young patients.¹⁷ These principles, which are based on landmark studies in the 1950s, are taught in dental schools today.

In 1914, Williams concluded that human teeth could be classified into three principal shapes: rectangular, triangular, and ovoid. He claimed that the most pleasing appearance is one in which the outline form of the individual’s face turned upside down and the outline form of the individual’s maxillary central incisors are identical.¹ Williams’ method of harmonizing the face with teeth, however, has never been proven to result in an ideal esthetic appearance, nor has it been proven that harmonizing the teeth with a patient’s sex, personality, and age (SPA factor)—principles upon which current practices of complete dentures are based—leads to the best esthetic outcome.

Therefore, the current study attempted to answer the questions: Are dentists’ and dental students’ concepts of esthetic appearance similar to those of patients’? Are square teeth considered masculine and tapered ovoid teeth considered feminine? Do women prefer a different type of teeth from men, presumably a feminine type of teeth?

Brisman18 investigated preferences of patients, dentists, and dental students with respect to shape, proportion, and symmetry of maxillary incisors, analyzing their choices of variously shaped maxillary teeth in photographs with setups of so-called masculine or feminine teeth. These studies investigated whether the participants’ preferences carried over to photographs of maxillary central incisors in which the triangular shape was softened to make a tapered ovoid and the square shape was softened to make a square ovoid. Ovoid was the first choice in the majority of the groups and virtually tied for first choice of female patients. When Brisman investigated the concept that square central incisors are masculine and tapered ovoid central incisors are feminine, all groups preferred masculine teeth (the square ovoid shape) to the feminine teeth (the tapered ovoid shape) and the differences were not statistically significant. Twenty-four of 25 dentists termed the square ovoid masculine. Although most of the patients considered the square ovoid more masculine, many did not, and some could not decide. Differences in percentages were not statistically significant.¹⁸

Other investigations have also showed that dentists’ and patients’ concepts of esthetic appearance may differ. Although dentists and patients have the same preferences for the shape of maxillary antherior teeth, the preferences have differed for proportions of length and width, with a particularly large difference in the preferences with respect to symmetry of maxillary anterior teeth. Dental students seem to have preferences that are between those of patients and dentists; and when patients and dentists observe individual teeth, they prefer an elongated tooth that approximates the ideal, 3:5 proportion.¹⁹

What one may conclude from these studies is that dental esthetics is not so much empirical as it is a “science” based on the general tastes of society.¹⁸ Certain shapes and arrangements of teeth may be considered esthetically pleasing when many patients agree that they are, and not necessarily anatomically quantifiable. When considering which forms, shapes, colors, and other sensory impressions are generally preferred, the dentist is ultimately creating an illusion—one that is based on what the dentist learns in concepts and perceptions and tries to use to achieve the patient’s desire for an esthetic appearance. The studies may also be indicating that dentists may develop concepts of esthetic appearances that differ from patients. This can create unanticipated difficulties and communication problems.¹⁸

This was corroborated by the results of the present study in that neither faculty, students, nor patients could predictably relate stereotyped “feminine” and “masculine” tooth anatomy characteristics to the sample smiles. The objective of this exercise was to see if the traditional thought that shape is predicted by gender and, in turn, the practice of making dental decisions on tooth form based on the gender of patients, is correct. The results showed this not to be the case. Overall, the groups had similar results and the “professionally trained” faculty and students were no better than laypeople in correctly identifying the sex of the patient. In fact, the faculty performed the worst, and, although not statistically significant, the untrained patients were the best.

In summary, the study showed that stereotyped “feminine” and “masculine” tooth anatomy characteristics could not predictably be related to the sample by either faculty, students, or public to accurately identify the sex of the subject. Faculty, students, and laypeople were equally unlikely to accurately relate stereotyped feminine and masculine tooth anatomy characteristics to the samples shown.

The common denominator in available studies, however, is that none of them offers any conclusive evidence that the “law of harmony” or the principles of a dentogenic restoration predictably create an ideal smile that can be characterized as either male or female and predictably identified by either faculty, dentists, or laypeople. Dentists and laypeople may identify characteristics that both enhance and detract from a smile. However, the ranges of acceptable deviations for smile characteristics are vast, making it difficult to quantify ideal and maximum acceptable smile deviations for smile characteristics.³

Conclusion

The study showed that neither faculty, students, nor the public could predictably relate stereotyped “feminine” and “masculine” tooth anatomy characteristics to the sample to accurately identify the sex of the subject. Faculty, students, and laypeople were equally unlikely to accurately relate stereotyped feminine and masculine tooth anatomy characteristics to the samples shown. Faculty, dental students, and laypeople were equally unlikely to correctly identify the sex of the sample shown.

About the Authors

Timothy L. Hottel, DDS, MS, MBA
Professor
Department of Prosthodontics
College of Dentistry
University of Tennessee Health Science Center
Memphis, Tennessee

Chris S. Ivanoff, DDS
Associate Professor
Department of Bioscience Research
College of Dentistry
University of Tennessee Health Science Center
Memphis, Tennessee

John Antonelli, DDS, MS
Professor
Section of Prosthodontics
Nova Southeastern University
Health Professions Division
Fort Lauderdale, Florida

William Balanoff, DDS, MS
Former Adjunct Professor
Section of Prosthodontics
Nova Southeastern University
Health Professions Division
Fort Lauderdale, Florida

Rose Ann Habib-Chiang, DMD
Private Practice specializing in pediatrics
Englishtown, New Jersey

Stefan A. Hottel
D-2 Dental Student
College of Dentistry
University of Tennessee Health Science Center
Memphis, Tennessee