Effect of extraction of first four premolars on smile aesthetics

SUMMARY The objective of the present study was to assess smile aesthetics after orthodontic treatment in subjects with and without the extraction of four first premolars. Post-treatment coloured frontal photographs (4 × 6 inches) of 60 Pakistani subjects, 21 males and 39 females, aged between 15 and 30 years were assessed by one examiner. For 50 per cent ( n = 30) of the patients, treatment included the extraction of four first premolars, whereas the other half were treated without extractions. Smile aesthetics were evaluated by a panel of 10 laypersons, five males and five females, aged between 20 and 30 years. Smile aesthetic parameters were measured using seven variables, and the ratios were calculated. An independent sample t -test was used to assess smile aesthetics. All seven variables namely smile width, smile height, visible dentition width, maxillary intercanine width, visible maxillary first molars, visible mandibular teeth, visible maxillary gingival margin, and the ratios between the above mentioned variables showed no detrimental effects following extraction of four first premolars on smile aesthetics.


Introduction
Aesthetics in orthodontic treatment have always been associated with profile enhancement (Sabri, 2005). Facial beauty and harmony were the primary treatment objectives of Angle (James, 1998). However, Angle's classification of malocclusion and cephalometric analyses focused more on the profile and less on the frontal view. The orthodontic literature to date has more studies on skeletal than on soft tissue relationships and smile aesthetics (Sabri, 2005).
An attractive balanced smile is a prime objective of modern orthodontic therapy (Roden-Johnson et al., 2005). The beauty of the smile lies in the orthodontists' ability to recognize the positive factors in smile aesthetics enhancement and to plan treatment accordingly (Sarver and Ackerman, 2003a,b). This topic has gained importance for clinicians as orthodontic patients evaluate the outcome of their treatment not only through occlusal harmony but also through their smiles and the enhancement in facial appearance at the end of treatment (Işiksal et al., 2006).
Extensive studies have resulted in the establishment of norms that orthodontists use as guidelines to evaluate facial form and to direct therapy. Research supporting these established norms has been directed to the lateral view of the face and most of the knowledge has been derived from it (Roden- Johnson et al., 2005). However, Mackley (1993) demonstrated that the profile is not a reliable predictor of the appearance of a person's smile.
Smile and facial attractiveness are strongly correlated. Smile aesthetics are influenced by features such as the amount of gingival display, the presence of a smile arc, and the shade of the teeth. A smile with minimal gingival display has been considered more aesthetically pleasing than one with excessive gingival display (Hulsey, 1970;Rigsbee et al., 1988;Kokich et al., 1999). In an attractive smile, the upper lip elevates to reveal approximately 10 mm of the maxillary incisors, the mouth increases to 130 per cent of its original width, and the lips are separated approximately 12 mm (Peck and Peck, 1995). The location of the gingival smile line is also dependent on the subject's gender. On average, the smile line in females is 1.5 mm higher than in males (Sarver, 2001), with a smile exhibiting the upper lip at the height of the gingival margin of the maxillary central incisors being more attractive (Hulsey, 1970). A smile showing the curvature of the maxillary incisal edges (smile arc) that parallels the curvature of the lower lip has been reported to be more aesthetic than one with a flat maxillary incisal edge (Hulsey, 1970;Sarver, 2001;Sarver and Ackerman, 2003a,b). Furthermore, coincidence of the maxillary midlines with the facial midline has been found to be important (Tjan et al., 1984), as has a light shade of the teeth (Dunn et al., 1996).
Another important smile feature is the presence or absence of buccal corridors. Frush and Fisher (1958) defined a buccal corridor as the space between the facial surfaces of the posterior teeth and the corners of the lips during smiling. A broad smile with minimal buccal corridors is considered to be more attractive than a narrow smile with large buccal corridors when judged by laypersons (Moore et al., 2005). The presence or absence of buccal corridors during smiling shows no significant difference in smile scores among dentists, orthodontists, and laypersons (Roden-Johnson et al., 2005). Ker et al. (2008), in a computer-based survey on aesthetics and smile characteristics from the layperson's perspective, concluded that raters preferred a broader smile. Smiles displaying the maxillary first permanent molars are considered to be more aesthetic (Yoon et al., 1992;Dong et al., 1999).
Smile aesthetics vary for different subjects according to their age, gender, and race. Females tend to present a softer appearance than males (Frush and Fisher, 1958) and have greater facial animation than males during smiling (Rigsbee et al., 1988). With age, the upper lip tends to conceal more of the maxillary incisors and a concomitant greater degree of mandibular incisor display (Zachrisson, 1998(Zachrisson, , 2007. Some investigators have noted a difference in how laypeople and dentists evaluate and rate the smile (Brisman, 1980;Kokich et al., 1999). However, according to Wylie (1955), the layman's opinion of the human profile is every bit as good as the orthodontists and perhaps even better since it is not conditioned by orthodontic propaganda.
Recent criticism concerning the detrimental effects of premolar extractions on smile aesthetics has added another dimension to the debate concerning extraction versus nonextraction treatment (Kim and Gianelly, 2003). It is thought that extraction leads to constricted dental arches, which in turn result in increased buccal corridors, thus making the smile less aesthetic (Spahl and Witzig, 1987). Studies on the detrimental effects of premolar extraction are still limited. The aim of this investigation was to assess smile aesthetics after orthodontic treatment in subjects with and without extraction of the four first premolars.

Materials and methods
Data for this study were obtained from post-treatment frontal photographs of Pakistani subjects at the Orthodontic Clinic, Section of Dentistry, Aga Khan University Hospital, Karachi, Pakistan, treated consecutively over a period of 5 years (2002)(2003)(2004)(2005)(2006)(2007) with fixed mechanotherapy. The patients were aged between 15 and 30 years. Exclusion criteria were no previous orthodontic treatment, congenitally missing teeth other than third molars, or craniofacial anomalies and syndromes.
Following assessment of the post-treatment frontal photographs by one author (FG), two groups of 30 patients were formed; an extraction group (10 males and 20 females) in whom the four first premolars were extracted and a nonextraction group (11 males and 19 females). The photographs of the patients had been taken with a single Nikon-5700 camera (maximum resolution 2560 × 1920, pixels 8 MP/ cm, digital zoom; Nikon Corporation, Tokyo, Japan). Coloured glossy printed photographs (4 × 6 inches) were produced for each exposure. The selected photographs were then covered with black construction paper leaving only a proportionate rectangular area around the mouth (Figure 1). Smile aesthetics were evaluated by a panel of 10 laypersons, five males and five females, aged between 20 and 30 years; the only exclusion criteria was a professional background in any aspect of dentistry. They were asked to score each photograph using the following scale: 1 = poor, 2 = fair, 3 = good, 4 = very good, and 5 = excellent (Johnson and Smith, 1995).
Measurements obtained for each photograph, with landmarks as defined by Philips et al. (1984), were carried out by one author (FG; Table 1, Figure 2). Data analysis was undertaken using the Statistical Package for Social Sciences for Windows version 16.0 (SPSS Inc., Chicago, Illinois, USA). The mean and standard deviation of all the measurements for both the extraction and the nonextraction groups were computed. Comparison between both groups was undertaken using an independent sample t-test and P-value of 0.05 or less was considered to be significant.
As measurements on photographs are not as precise as on cephalometric radiographs, the ratios were calculated for measurements 1-4 (Table 1). To determine measurement error, 10 photographs were randomly selected by same examiner and remeasured after a period of 1 month using a paired sample t-test (Table 2).

Results
A total of 60 photographs were assessed by the laypersons and the principal examiner. Table 3 shows the comparison of the smile aesthetic variables between the extraction and non-extraction groups as judged by the principal examiner. There was no statistically significant difference in any smile aesthetic parameters between the two groups.
The distribution related to non-parametric variables such as visible maxillary first molar, visible mandibular teeth, and visible maxillary gingival margin in the extraction and non-extraction groups are shown in Table 4 as judged by the principal examiner. No significant differences were observed between the groups. The mean aesthetic scores of subjects in the extraction and non-extraction groups as judged by laypersons showed no significant difference between the groups.    corridor area during posed smiling concluded that there was no significant difference in the buccal corridor area ratio between extraction and non-extraction groups. McNamara et al. (2008) studied various skeletal, dental, and soft tissue relationships related to the aesthetics of smile and found no significant correlations between smile arc, amount of incisogingival display, size of the buccal corridors, and smile aesthetics in malocclusion patients. The results of the present research are in agreement with those studies; variables such as smile width, smile height, visible dentition width, and maxillary intercanine width showed no significant difference on smile aesthetics between subjects treated with or without four first premolar extractions. Gianelly (2003) and Kim and Gianelly (2003) on examining the arch width of patients treated with extractions or non-extraction found no differences in arch width between the two treatment protocols and noted that extraction does not produce buccal corridor spaces. According to Luppanapornlarp and Johnston (1993), the post-treatment intercanine widths of the maxillary and mandibular arches were again the same in the extraction and non-extraction groups.
Prahl-Andersen et al. (1979) and Peck et al. (1992a,b) indicated that dentists and laypersons judge facial aestheics differently as dentists have been trained to observe features that do not seem to influence the general public. In the current study, the smiles evaluated by laypersons did not show any significant difference (P = 0.8) between the extraction and non-extraction groups.
It is assumed that a smile displaying the maximum number of teeth is considered to be the most aesthetic. According to Tjan et al. (1984), a typical or average smile displays the six maxillary anterior teeth and first or second premolars in young adults. In the present study, the subjects treated with and without extraction of four first premolars displayed an equal number of teeth during smiling. Similarly, in the study of Kim and Gianelly (2003), the number of teeth visible during smiling in both the extraction and non-extraction groups was similar with half of the subjects displaying 10 teeth. The results of the present research showed that in both groups, only a few subjects exhibited the maxillary first molars during smiling. This is in agreement with the study of Tjan et al. (1984) who reported that only a small percentage of the population will show the maxillary first molars during smiling. However, Johnson and Smith (1995) and Mackley (1993) emphasized that patients with better aesthetic scores had a significantly greater frequency of visible maxillary first molars.
The most important feature of the smile that affects aesthetics is the amount of maxillary gingival display (Mack, 1991;Peck et al., 1992a,b). There is no evidence that extraction treatment has an effect on this feature. The results of the present study also demonstrated no deleterious effects of four first premolar extractions on the amount of maxillary gingival margin display during smiling.

Discussion
Only a few studies (Spahl and Witzig, 1987;Johnson and Smith, 1995;Kim and Gianelly, 2003) have been carried out to assess and compare the effects of premolar extractions on smile aesthetics, and documentation on the adverse effects of extraction treatment is still limited. This study was undertaken to assess smile aesthetics after orthodontic treatment in subjects treated with or without extraction of four first premolars. The findings showed no significant difference in smile aesthetics when the extraction and non-extraction groups were compared. Buccal corridor spaces have been described by several investigators as undesirable (Lombardi, 1973;Blitz, 1997;Gianelly, 2003;Sarver and Ackerman, 2003a,b). Orthodontists and laypeople rate smiles with small buccal corridors as significantly more attractive than those with large buccal corridors. In fact, orthodontists rate first molar to first molar smiles as more attractive, whereas laypeople prefer second premolar to second premolar smiles (Martin et al., 2007).
Some orthodontists consider that extraction causes an arch width reduction that could lead to a decrease in the buccal corridor ratio and poor smile aesthetics (Spahl and Witzig, 1987). Hulsey (1970) found that the mean rated smile scores of orthodontically treated subjects were significantly lower than those of subjects with normal occlusions. In contrast, Johnson and Smith (1995) and Mackley (1993) concluded that variables related to the buccal corridor or other measurements of the relationship between the width of the dentition and mouth during smiling showed no relationship with smile aesthetics. They determined that smile aesthetics, aesthetic scores, and visible dentition during smiling were the same in both the extraction and the non-extraction groups.
Similarly, a study by Yang et al. (2008) that investigated hard and soft tissue factors related to the amount of buccal The findings of the present investigation also showed that there was no difference between smile aesthetics in the extraction and non-extraction groups, thus the decision regarding extraction of the four first premolars in treatment planning should not be solely based on smile aesthetics but factors such as overjet, overbite, crowding, and soft tissue characteristics should be taken into consideration.

Conclusions
1. No significant differences were seen in smile aesthetic parameters of orthodontic patients treated with and without extraction of the four first premolars. 2. The smile aesthetic scores of subjects treated with and without extraction of the four first premolars did not show any statistically significant difference.