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Accuracy in measurement of facial index on the basis of an android application
*Corresponding author: Dr. Prasad Chitra, Department of Orthodontics, Army College of Dental Science, Secunderabad, Telangana, India. prasadchitra@yahoo.co.uk
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Received: ,
Accepted: ,
How to cite this article: Pulgaonkar R, Thakur A, Chitra P. Accuracy in measurement of facial index on the basis of an android application. Ann Natl Acad Med Sci (India). 2026;62:77-81. doi: 10.25259/ANAMS-2022-3-1-(565)
Abstract
Objectives
The Prosopic index or Facial index is an important measurement, required to be taken during the orthodontic diagnosis phase. Classification of faces into broad, average, or narrow not only allows comprehensive orthodontic treatment planning based on aesthetics and facial form, but could also be used in other fields such as anthropology and forensic medicine. The authors have developed a photographic app for AndroidOS that will enable easy classification of the face into different facial types as given by Banister. The aim of this study is to compare the findings of the facial index of a sample of randomly selected subjects derived from the application with findings of the same subjects derived manually, to check for the accuracy of the new technology.
Material and Methods
The measurements of facial index derived from this app were compared to the facial index of a sample of Indians derived manually (using an independent t-test). Inter-examiner reliability of the measurements by three different investigators on the same sample was tested using Cronbach’s alpha. A Chi-square test was used to compare the Prosopic index based on the method of determination.
Results
There was no significant difference between the values of facial index generated from the app compared to those derived manually. The Cronbach’s Alpha value derived was 0.998, indicating the best agreement between examiners. Maximum subjects showed a hyperleptoprosopic facial phenotype in both the app and manual methods.
Conclusion
The app, utilizing a smartphone, enables quick and reliable measurement of the Facial Index without the need for equipment like calipers or calculations.
Keywords
Diagnosis
Facial index
Facial types
Orthodontic app
Prosopic index
INTRODUCTION
The developments in mobile phone technology have been a boon to orthodontists, who can now utilize applications and software for better practice management, diagnosis, treatment planning, and various other purposes. The authors have developed an app for AndroidOS, which can be used to easily calculate the prosopic index (facial index). The Prosopic Index, as described by Banister et al.1 (1995), helps classify faces as broad, average, or long, which helps orthodontists assess and plan treatment for their cases according to facial aesthetics and tentative growth patterns. Apart from orthodontics, the index is applied in craniofacial anthropometry, where morphologic characteristics of the head and face are used to study racial and demographic characteristics of populations.2 Forensic medicine also makes use of the facial index for the identification of the deceased in medicolegal cases.3 Furthermore, the Facial index has applications in the diagnosis of craniofacial deformities in various genetic syndromes.
The traditional method of recording facial measurements involves the use of a craniometer or sliding calipers to record the Facial index.4 However, the use of calipers comes with some inherent disadvantages. Most sliding calipers record up to a range of only 130 mm, require measurements and percentage calculations, and are cumbersome to clean, sterilize, and use. [Figure 1] These problems are overcome with the use of the FI-App, in which patient facial photos taken at the diagnosis stage can be uploaded to obtain the facial type without the need for manual measurements. Unlike linear metric measurements, proportional analyses such as the Prosopic index do not change with or depend on the resolution/magnification of the photographs recorded. Hence, at any magnification, the values given by this app will be accurate and reliable.
- Use of a digital caliper for facial measurements.
This application is available on Google Play Store and can be downloaded using the following link- https://play.google.com/store/apps/details?id=com.rohan.user.imageratioapp
A tutorial video on how to use the app is also available on YouTube, which can be accessed using the following link- https://youtu.be/XNCkXHrKKSw
MATERIAL AND METHODS
Sample and population
The app was developed and tested for accuracy of measurements on a random sample of 50 adult Indians (no age/sex preference). The current research was undertaken as a pilot study for development and testing of the application before its launch on the world wide web as open source, hence, sample size was kept to a minimum, in accordance with a similar study done previously on ethnic Telangana population, with the aim to obtain test powers >95% for the comparisons.5 Institutional ethical clearance for the study was also obtained prior to beginning the study. Written informed consent was obtained from all subjects after thorough oral explanation of the procedures involved prior to beginning (explained in both English as well as the native tongue of the patients) in accordance with the guidelines prescribed by the American Cleft- Palate Craniofacial Association regarding the consent process for ethical utilization and publication of facial photographs in scientific research.6
Calculation of facial index- manual method
For the Facial index, the subjects were measured from the nasion to the mental tubercle and bizygomatic width as facial width. Subjects were seated in a relaxed, upright position with the head in the natural head position, Frankfurt horizontal (FH) plane kept parallel to the floor, eyes looking straight, and teeth in physiologic resting position. A digital caliper was used to measure the bi- zygomatic width or the distance between the prominent points on both cheekbones, for width of the face; and distance between Nasion (point in between the eyes) - Gnathion (prominent point on the chin) for length of the face; to calculate the Facial index manually [Figure 1].4
Calculation of facial index- using the App
Photographs of the subjects were taken in frontal view using a digital SLR camera (Canon 1500D) with a 100 mm f 2.8 1:1 magnification macro lens, mounted on a tripod. The head of the subject was oriented in the natural head position, FH plane parallel to the floor, eyes looking straight, and teeth in physiologic rest position.
These photographs were uploaded on the application in the following manner to calculate the Facial index-
The app initially opens a page with a ‘Load Image’ screen. The user has to upload the frontal photograph of the patient, or he/she can use the camera of a smartphone to click a new picture [Figure 2].
- Step 1.
After selecting the image, the app will prompt the user to rotate the image. The user must make sure to capture images in full frame to avoid any need for cropping.
For demonstration, we have used an image with black dots corresponding to the Na-Gn and Zy-Zy’ lines (as used in the measurement of facial index).4 After correcting the orientation of the image, select ‘OK’ [Figure 3].
- Step 2.
Next, the app will prompt the user to mark the points by dragging from the Green Box (top left side of the app screen). The app is programmed in such a way that the points should be placed in the following order: Nasion–Gnathion–Zygoma –Zygoma’.
In case the user misplaces the dragged marker, they can reposition the dragged point by pressing on the marker and dragging it to its desired position [Figure 4].
- Step 3.
Once all four markers are placed in their desired positions by the user, the app calculates the ratio of Na-Gn/Zy-Zy’ and displays it at the bottom of the screen [Figure 5]. When multiplied by 100, it will give the Facial index in percentage.
- Step 4.
Example- In this case, the ratio = 1.18. Thus, Facial index is 118%.
(The patient’s morphologic Facial index comes under the hypeleptoprosopic classification.)
Statistical analysis
The manually derived values of the Facial index (caliper method) were compared to those calculated from the application using the independent t-test. Following this, three different investigators (RP, AT, PC) calculated the Facial index of these subjects on the app independently, and inter-examiner reliability was tested using the Cronbach’s alpha test.
The sample was segregated into different facial types (mesoprosopic, leptoprosopic, hyperleptoprosopic) depending on the value of the Facial index generated using both methods (manually calculated and app-generated). The Chi-square test was used to compare the Prosopic index based on the method of determination.
RESULTS
It was observed that the mean values of the Facial index were similar for both the app method (94.5±4.76) and manual measurements (94.9±4.8). p-value was 0.734, which indicated an insignificant statistical difference [Table 1].
| Method | N | Mean | SD | p-value |
|---|---|---|---|---|
| App generated | 50 | 94.5881 | 4.76316 | 0.734 |
| Manual | 50 | 94.9146 | 4.82886 |
Independent t-test; p<0.05 considered statistically significant, SD: Standard deviation
Upon calculation of Inter-examiner reliability, Cronbach’s Alpha value was found to be 0.998, indicating the best agreement between examiners [Table 2].
| Intraclass correlation | 95% Confidence interval | F test with true value 0 | |||||
|---|---|---|---|---|---|---|---|
| Lower bound | Upper bound | Value | df1 | df2 | Sig | ||
| Single measures | 0.994 | 0.990 | 0.996 | 467.208 | 49 | 98 | 0.000 |
| Average measures | 0.998 | 0.997 | 0.999 | 467.208 | 49 | 98 | 0.000 |
Cronbach’s Alpha value=0.998 indicating best agreement between examiners; p<0.05 considered statistically significant, df: Degree of freedom, Sig: Significance.
Upon comparison of the Facial index based on the method of determination using the Chi-square test, it was observed that both the app and manual methods revealed a similar type of facial phenotype, i.e., hyperleptoprosopic [Table 3].
| Facial phenotype using the prosopic index | Manual | p-value | |||||
|---|---|---|---|---|---|---|---|
| Mesoprosopic | Leptoprosopic | Hyperleptoprosopic | Total | ||||
| App generated | Mesoprosopic | N | 9 | 2 | 0 | 11 | 0.000* |
| % | 18 | 4 | 0 | 22 | |||
| Leptoprosopic | N | 0 | 13 | 2 | 15 | ||
| % | 0 | 26 | 4 | 30 | |||
| Hyperleptoprosopic | N | 0 | 0 | 24 | 24 | ||
| % | 0 | 0 | 48 | 48 | |||
| Total | N | 9 | 15 | 26 | 50 | ||
| % | 18 | 30 | 52 | 100 | |||
Chi square test; *p<0.05 considered statistically significant.
DISCUSSION
The facial or prosopic index determination is one of the fundamental steps in orthodontic diagnosis and treatment planning. To a certain extent, the shape and type of face influence the treatment approach, including the appliances and biomechanical principles used. Since there is increased reliance on digital technology by professionals these days, the authors developed this Android application so that orthodontic practitioners, assistants, dental students, anthropologists, researchers, and forensic investigators can easily calculate the said index with just a photograph of the subject. For the same sample, when the mean Prosopic index derived from the app was compared to that calculated manually, there was no significant difference, indicating that the app generated accurate readings.
No age/sex criteria were included in our research for a couple of reasons. First, indices of the skull are dependent on facial dimensions like height of the face and bi-zygomatic width, which do not show significant changes after the growth period.7 Hence, our research included adults (mean age- 26.5 years). Second, while there are significant differences in the facial indices of males and females, our research involved a comparison of the indices of a subject with the manually derived index values of the same subject, and not with other subjects. Thirdly, to launch an authentic, well-researched piece of technology on the open-source platform, maximum inclusivity and minimum restrictions in the samples studied were necessary.
Since this application requires the operator to mark 4 points on the frontal photograph of subjects, a fair knowledge of the landmarks of the face is required. When three experienced investigators were asked to independently mark landmarks and derive facial index for the sample population, it was found that there was a high level of agreement between the measurements, indicating that the app generates approximately similar values of the facial index if the landmarks are marked correctly. Photographs showing the face at an appropriate magnification (up to the neck of the subject) will generate correct results, irrespective of the magnification of the photographs. This is because ratio-based analyses, such as the Facial index, do not change with or depend on the resolution/magnification of the photographs recorded.
Furthermore, comparisons drawn while developing our application found that the majority of our sample had a hyperleptoprosopic Facial index, which was in accordance with an anthropologic study of a similar Indian sample population, and on a population of Americans of Indian origin.8,9 Thus, the FI-App generates reliable and accurate results of the Facial index, which have been demonstrated scientifically and statistically in this study.
Some limitations of this study include the small sample size used and the sample being limited to a particular ethnic population. Since ours was a pilot study for the design and testing of the Android application, the samples involved were limited. Future studies may include various ethnic groups and a wider population base.
CONCLUSION
This user-friendly application is a modern, easy-to-use method for measuring the Prosopic index. It will benefit users by making diagnosis faster, reducing armamentarium and facilitating post- treatment comparisons for any subsequent changes in the patients’ face. Anthropologists and forensic professionals can also put this app to good use in their respective fields.
Authors’ contributions
RP: Conceptualization, data curation; AT: Writing original draft, data curation; PC: Project administration, supervision, final editing and revision.
Ethical approval
The research/study approved by the Institutional Review Board at Army College of Dental Sciences, number ACDS/IEC/85/2022, dated 20th February 2022.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms. In the form, the patients have given their consent for their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
Use of artificial intelligence (AI)-assisted technology for manuscript preparation
The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.
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