Cephalometric Profile of Bangladeshis: Tweed’s Analysis

Introduction

The systematic cephalometric analysis developed by Downs,^[1] Steiner,^[2] Ricketts,^[3] and Tweed^[4] are widely accepted. Gradually, several cephalometric analyses specific to racial groups have been established by Cotton,^[5] Craven,^[6] Mieura,^[7] Kam,^[8] Drummond,^[9] Park’s^[10] Koreans, Mieura’s Japanese, Chan’s^[8] Chinese, Nanda’s^[11] North Indians. Garcia’s^[12] Mexican Americans, Drummond’s^[9] Negroes, Lim’s^[13] Filipinos and Bhattarai’s^[14] Nepalese study.

Tweed’s triangle provided the clinician with simple and basic definite guidelines in the treatment planning of malocclusion.^[15,16] Tweed’s analysis is popularly followed in our country; hence, the present study was undertaken.

Materials and Methods

The material for this study consisted of standardized lateral head roentgenograms of 89 Bangladeshi young adults (45 males and 44 females). The study was conducted in Dhaka Dental College and Hospital Department of Orthodontics and Dentofacial Orthopedics. The subjects were in the age range of 19–27 years. The following criteria were used for selection of subjects; healthy individual with normal growth and development, excellent facial harmony, full complement of fully erupted teeth (excluding third molars) in good occlusion, class I molar and canine relationship, acceptable overjet and overbite, no crowding, no rotations, no cross-bite and a negative history of orthodontic treatment. A lateral head cephalogram was taken for each individual on a standard universal counterbalancing cephalostat. Cephalometric variables were recorded on tracing paper according to Tweed’s description. Mean and standard deviation (SD) for all the three variables were computed separately and combined for males and females. The linear regression equation of incise mandibular plane angle (IMPA) on Frankfort-mandibular plane angle (FMA) was fitted, and the estimated value of IMPA was computed for a given FMA.

Written informed consent was obtained from each subject before taking X-ray. Ethical clearance for the study was reviewed and approved by Ethics Committee of Dhaka Dental College Ref: DDC/2016/1766 dated: October 16, 2016. This research has also been conducted in full accordance with the World Medical Association Declaration of Helsinki.

Results

The most relevant data of this study are presented in Tables 2 and 3. All three angular parameters FMA [Figure 1], Frankfort-mandibular incisal angle (FMIA) [Figure 1], IMPA [Figure 1] were measured and found to be 24.52°, 54.60°, and 100.88°, respectively. The difference in the mean values of IMPA, FMA, and FMIA between males and females was found to be statistically insignificant. The mean FMA has been found to be 24.52° (with a range of 14°–36°) which is quite close to Tweed’s norm and found to be statistically insignificant. However, IMPA and FMIA values of Bangladeshis found to be statistically significantly different from that of the Caucasians. The linear regression equation of IMPA on FMA was fitted, and the estimated value of IMPA was computed for a given FMA [Table 1 and Figure 2].

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Figure 1

Tweed’s analysis with reference points identified

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Figure 2

Prediction of incise mandibular plane angle according to Frankfort-mandibular plane angle of the individual as given by the present study based on cephalometric data of Bangladeshis. Normogram constructed using micro soft excel office 2007, XY scatter chart template for determining expected incise mandibular plane angle for a given Frankfort-mandibular plane angle. The correlation was obtained by measuring the angles from 89 normal cephalograms of Bangladeshi young adults. While the area between the red and light green colored lines provide the range of the incise mandibular plane angle for a stipulated Frankfort-mandibular plane angle, the central deep blue line provides the Ideal incise mandibular plane angle

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Observations and Discussion

Two kindred studies have been published by investigators in local journals of Bangladesh but instead of simply expressing the facts have given birth to perplexity.

Hasan MN et al.^[17] In their study, have stated that they have collected 112 radiographs (56 males and 56 females) on the basis of having a harmonious face with a convex facial profile (from their photographic record) from patients’ record of Orthodontics Department, Faculty of Dentistry, Bangabandhu Sheikh Mujib Medical University, and other two private orthodontic office of Dhaka, Bangladesh.

However, the question remains when you are collecting cephalograms from patients’ record needing orthodontic treatment in an orthodontic office having convex profile how can you wish to establish the cephalometric norms of that group of people?

Hasan MN et al.^[17] have further concluded that increased FMA value of Bangladeshi population than that of Caucasian’s and Nepalese population indicate Bangladeshis have a bit larger mid-facial height than that of Caucasian’s and Nepalese. Furthermore, the Bangladeshis according to them have a significantly low value of IMPA, than that of Caucasian’s and Nepalese hence Bangladeshis have incisor teeth that are a bit retroclined than that of Caucasian’s and Nepalese. On the other hand, Bari and Hossain^[18] in their study, have found that the difference of values of FMA, FMIA, IMPA of Bangladeshis with North Indians were statistically insignificant. The difference of values of FMA, FMIA, IMPA of Bangladeshis with Caucasians was found statistically significant. Yet Kharbanda et al.^[19] did not find any significant differences of values of FMA of North Indians with Caucasians.

Bari and Hossain^[18] concluded that FMIA of Bangladeshis (54.22) was compared with Tweed’s value (68.2) is lower for Bangladeshi population suggesting that proclination of lower incisor is more for Bangladeshi subjects than Caucasians.

These contradictory findings of two different groups of investigators of Bangladeshi puerile adults with mean age 19 and 22 with Tweed’s analysis surely demand elucidation. Hence, the present study was undertaken.

The Bangladeshi male samples presented a mean FMIA (53.34°) which is slightly smaller than that of the Bangladeshi female sample (55.89°) [Table 2]. The mean difference of 2.55° which is marginally significant (P = 0.10) [Table 2] and suggests that Bangladeshi male tend to have more proclined lower incisors than that of the Bangladeshi female subjects. However, no statistical difference was found in the FMIA between the Bangladeshi male and female samples. The difference in the mean values of IMPA and FMA between males and females was also found to be statistically insignificant.

The mean FMA has been found to be 24.52° (with a range of 14°–36°) which is quite close to Tweed’s norm, and it was found that there is no statistically significant difference in the mean values between Bangladeshis and Caucasians [Table 3]. However, the FMIA has been found to be only 54.60° (with a range of 39°–71°) [Table 3] which is quite low compared to Tweed’s norm of 65°. The IMPA values in the present study ranged from 87° to 119°, [Table 3] with a mean of 100.88° [Table 3]. This is considerably more than Tweed’s mean of 90° suggesting that Bangladeshis have more protrusive mandibular incisors compared to Caucasians. This is an important aspect to be kept in mind when defining the treatment goals for Bangladeshis.

A review of other available studies on various Bangladeshi population groups was made [Table 4]^[18] and the mean IMPA values were found to range from 83° to 119° [Table 4]. Although there is some variation in the skeletal pattern of different studies of population groups in Bangladesh (the difference in the FMA values in the two races Bangladeshis and Caucasians being very little)., the over-all picture was suggestive of an IMPA value close to 100°, approximately, 10° more than that observed by Tweed, in Caucasians.

Tweed also observed that those subjects whose FMA ranged upward from 25°, demonstrated nature’s compensation of a reduced IMPA whereas in those with low or flat FMA (<25°) nature compensated by a higher IMPA, keeping the FMIA relatively constant in all cases.^[19] His norms of 25° FMA, 90° IMPA, and 65° FMIA were widely accepted and followed. He also calculated from his studies that for each degree of increase or decrease in FMA, if the IMPA is compensated to maintain the FMIA relatively constant at 65° best results were obtained.

He, therefore, postulated the FMIA is critical for optimal esthetics and suggested that for every degree increase of FMA, the IMPA should be similarly compensated to a minimum of 77°. Likewise, for every decrease of FMA, there should be a compensatory increase of the IMPA, to a maximum of 105°. The findings of the present study also corroborate Tweed’s observations. It was observed that the correlation of IMPA with FMA and FMIA was negative and highly significant (P = 0.001) indicating that any increase or decrease in the FMA was compensated by an inverse change in the IMPA to maintain good facial harmony.

Conclusion

Tweed’s analysis is simple and clinically useful analysis. His norms should be considered only as a guide and not absolute achievable objectives. The treatment objectives of IMPA should be considered according to the facial pattern, i.e., FMA. Racial/ethnic variations of norms cannot be overlooked while outlining goals and planning the treatment.