Evaluation of Tweed’s Facial Triangle among Students in Lagos, Nigeria

Introduction: The advent of cephalometric analysis has aided orthodontic diagnostic procedures, while the Tweed’s facial triangle which is a type of cephalometric analysis has complemented routine orthodontic treatment planning. This aim of this study was to evaluate Tweed’s facial triangle among a selected Nigerian population with normal occlusion. Materials & Method: A sample of 100 subjects, selected by multistage sampling from three schools in Ikeja local government, Lagos State had lateral cephalometric radiographs taken. The final sample analysis of the radiographs was 84. The error of the cephalometric method was assessed using the intra-class correlation co-efficient. The Tweed’s facial triangle was analyzed on the cephalometric radiographs and comparison between this Nigerian population and Tweed’s Caucasian cohort was carried out. Result: Out of the 84 subjects, 41(48.8%) were males and 43(51.2%) females. This study reported mean values of 23.26° ± 4.78° for FMA, 54.08° ± 5.11° for FMIA and 103.47° ± 6.26° for IMPA. Females had higher FMIA and IMPA values compared to males which was statistically significant for FMIA. The FMA was higher in males with a statistically significant difference between genders. The results suggest increased proclination of lower incisors in this Nigerian population compared to Tweed’s Caucasian population. Conclusion: The mean values for Tweed’s facial triangle in this Nigerian population were significantly different from that obtained by Tweed. Orthodontists should aim at achieving these values when analyzing cephalometric radiographs using Tweed’s facial triangle in Nigerians. This will further aid proper diagnosis, treatment planning with or without extractions and planning of retention after orthodontic treatment.


INTRODUCTION
Since its introduction in 1931 by Broadbent 1 and Hofrath, 2 cephalometric analysis has been used in diagnosis, treatment planning and evaluation of orthodontic cases. Lateral cephalographs are routine components of the diagnostic records taken in orthodontics. 3 They are frequently used as a research tool in evaluating the effects of orthodontic treatment and describing facial growth. 4 Other uses include orthodontic treatment planning, monitoring the progress of treatment and soft tissue analysis. Different types of cephalometric analysis have been described and used overtime. These include Steiner's, 5 Downs, 6 Jarabak's, 7 Rickett's 8 and McNamara's. 9 The most common include Steiner's, Downs' and Tweeds facial triangle. 10 Dr Olawande Adebisi. Ajisafe, 1 Dr Babatunde Ogunbiyi. Ogunbanjo, 2 Dr Kikelomo Olubukola. Adegbite, 3 Dr Afolabi Oyapero 4 The Tweed's analysis is easy and simple to carry out and can be done in addition to other analyses commonly used. Tweed 11 believed that upright mandibular incisors were related to post treatment facial balance and successful treatment. He made use of three planes that form a diagnostic triangle. The planes used were the Frankfort horizontal plane, the mandibular plane and the long axis of lower incisor. 11,12 The three planes meet to form a triangle: Tweed's facial triangle. The first angle is the Frankfort mandibular plane angle (FMA) which has a normal range of 22°-28° (average of 250) in Caucasians, 11,13 21.5 o ± 5 o in Pakistanis, 14 34.0° ± 5.1° in Kenyans 15 and 20.8°-26.1° in Nigerians. 16 The Frankfort plane has been linked to the slope of the mandibular incisor as an important angle in the Tweed's triangle. 10,17 An increased FMA value indicates excessive vertical growth while a reduced value indicates a reduced vertical growth pattern. The second angle is the Frankfort mandibular incisor angle (FMIA). The FMIA has a value of 65 o in Caucasians, 10 50.9 o in Brazilians 18 and 57 o in the Nepalese. 12 This angle tends to increase with increasing age to balance the corresponding decrease in the FMA. 19 It has a value of 68° when the FMA is between 22°-28° and 65° when FMA is 30° and above. 11 The third angle is the incisor mandibular plane angle (IMPA) which is usually 88° in Caucasians for the upright lower incisors. 10 If the lower facial profile is flat the lower incisors may be corrected into a labial position then the IMPA will be 94°.  16,20 The post treatment stability of orthodontic treatment and prevention of relapse after treatment is of great concern to many clinicians. 21 Relapse of orthodontically treated dentition may be influenced by apical base differences, the subject's age, the duration of retention, post-treatment growth, third molar development, habits, occlusal functioning, Bolton's discrepancies, continued decrease in arch length and incisor positions relative to basal bone. 21 To prevent relapse therefore, Tweed 10 proposed post-treatment stability in relation to the position of the lower incisors to basal bone in order to achieve a balanced and pleasing occlusion. The lower incisor position, facial profile, in combination with a tooth arch size analysis, provides clues that can aid in making a decision whether an extraction or nonextraction treatment protocol must be followed to achieve stability. 21 Few studies to find out the cephalometric norms using the Tweed's analysis have been reported (among the Caucasians, 10,17 Nepalese 12 and the Brazilians 18 ) but none has been carried out in Nigerians. The aim of this study was to evaluate the Tweed's facial triangle and to establish baseline norms in a population of 12-16 year old Nigerian students situated in Ikeja, Lagos State.

Sample Size Determination
The Fisher's formula 22 used for determining the study sample size was :n =Z 2 pq/d 2 .
Using a prevalence value of 24.7% based on a study describing prevalence of normal occlusion in Nigeria 23 and making provision for 30% attrition, a sample size of 100 was determined.

Inclusion Criteria
Subjects of Nigerian origin aged 12-16 years, with full complement of permanent dentition excluding the third molars, having normal occlusion and with no history of previous orthodontic treatment who gave their assent and obtained parental consent.

Exclusion Criteria
Non-Nigerian origin or of mixed race subjects with history of previous extractions of permanent teeth or previous orthodontic treatment; Overjet less than 2mm or greater than 4mm; Overbite less than one-third or greater than half of lower incisors and Subjects not willing to participate in the study were excluded.

Diagnostic Criteria for normal occlusion 24
Subjects with skeletal pattern 1; Anteroposterior relationship of Class I; Normal overjet of 2-4mm; Normal inclination and angulation of teeth; Normal overbite of one-third to half vertical overlap of lower incisors; Absence of rotations, cross bites and arch length discrepancy (no spacing or crowding).

Sample Selection
A multistage sampling method was adopted to select the subjects. The first stage made use of simple random sampling (balloting) to select 3 schools in Ikeja local government area (LGA) using the list of schools in the LGA obtained from the Lagos State Ministry of Education as the sampling frame. The 100 subjects were distributed among the three public schools by stratified random sampling. The second stage involved simple random sampling (balloting) for one class in each arm with most children aged between 12-16 years (JSS 2, JSS 3 and SS1). At this stage questionnaires were administered by the researcher to all the students of the classes and oral examination was carried out to determine those who met the inclusion criteria. Subjects that fell within 12-16 years of age and that met the other inclusion criteria outlined below were enrolled into the study and allowed to proceed to the third stage. The third stage involved balloting to select the number of subjects in each gender required per class to make up the sample size. Prior to balloting, eligible students were grouped by their sex in each arm.

Ethical Considerations
Approval for the study was obtained from the Health Research and Ethics Committee of the Lagos State University Teaching Hospital and from the Lagos State Education Review Board of the Ministry of Education. Informed consent and assent were obtained from the parents of the subjects and the subjects consecutively before proceeding with the study.

Demographics
The subjects' demographic data was obtained in the first section of the questionnaire. This consisted of age, gender, tribe, nationality and student's class. The dental and orthodontic histories were obtained from the second section of the questionnaire. These included the history of previous extractions and orthodontic treatment

Clinical examination
Clinical examination was performed by the principal investigator (A.O) under natural light. Values for normal overjet ranged between 2-4mm and any subject with value less or greater than this was excluded. The anteroposterior molar and canine relationships were determined by the Angle's classification as Class I, II or III. The subjects that had Class I molar relationships were included while those with Class II or III relationships were excluded. The midline was assessed using the tip of the nose and philtrum as a guide comparing the midline between the upper and lower central incisors. Any crossbite seen was documented as anterior or posterior crossbite. Tooth bone ratio was recorded as crowding or spacing; mild (0-3mm), moderate (4-7mm) and severe (>8mm), while lip competence was assessed and recorded as competent or incompetent using the Jackson's lip classification. Strict infection control procedures were adhered to during the examinations.

Cephalometric Radiographs
Appointments were given to subjects who met the inclusion criteria and they were conveyed to the Cephalometry Radiology Unit of the Lagos State University College of Medicine, to take the lateral cephalometric radiographs with a digital orthopantomograph /cephalostat, (Vatech PaX-400C IEC60601-2-7 by Vatech Ltd Gyeonggi, Korea). The subject's head was placed in the natural head position and with the teeth in maximum intercuspation (as shown in figure 1). Prior to taking the radiograph, each subject was provided with a lead apron to protect vital organs and the number of images taken were limited to the minimum (1 or 2 exposures) to minimize the risks associated with radiation exposure. Each subject was positioned at 150cm from the source of radiation and 15cm from the film 25 and exposed to the radiation for 5 seconds at a setting of 70 kilovoltage and 4 milliamperes. The above precautions were taken to achieve the ALARA principle. 26

Data Analysis
The

RESULT
The assessment of intra-examiner error in tracing the cephalometric radiographs revealed a high degree of reliability of the angles measured (  6.26° with a slightly higher value in females which was not statistically significant ( Table 3).

The comparison between the components of this
Nigerian sample and the Tweed population is shown in      There was no statistically significant difference in ANB between males and females.

DISCUSSION
The use of Tweed values in orthodontic treatment evaluation and planning became popular with the advent of extractions to create a balanced profile. 10 In the present study, all participants were from secondary schools in Lagos state and they were aged between 12 to 16 years. The age range was representative of that at which majority of adolescent patients present to the orthodontic clinic. Some studies are in agreement with this age group 28,29 while others are in support of an older age group. 30

Conflict of interest
The author declares that there is no conflict of interest regarding the publication of this paper.