1.      Craniofacial symmetry is one of the elements of a balanced facial appearance. Orthodontic and orthognathic surgery treatment is an effective method to achieve good occlusion and create a balanced and harmonious facial appearance. However, every person has some asymmetry between the right and left sides of the face and it ranges from clinically imperceptible to a gross abnormality. The facial asymmetry might occur due to a combination of genetic and environmental influences. Other causes of facial asymmetry include trauma, infections, developmental abnormalities, myospasm, chronic muscle shortening, or muscle splinting; syndromes such as Treacher Collins; occlusal interferences; and joint pathologies such as rheumatoid arthritis, etc. (1,2)Various tools have been used for the conventional diagnosis of facial asymmetry. These might be clinical examination followed by photographs of various frontal and side views, in addition to radiographs such as lateral and posteroanterior cephalograms, oblique radiographs of the mandible taken at 45°, and panoramic radiographs. (3,4) Three-dimensional modalities are used for the diagnosis of facial asymmetry, but these have the disadvantages of radiation exposure and more cost. (5,6) Hence two-dimensional radiographs are commonly used for the diagnosis of facial asymmetry. In these two dimensional radiographs, facial symmetry is usually planned based on measurements from posteroanterior (PA) encephalography. The cephalometric posteroanterior projection is a valuable tool in the study of the right and left structures since they are located at equal distance from the film and x-ray source Facial asymmetry and treatment results are objectively assessed concerning various cephalometric indices. (7,8) Many morphologic studies have evaluated skeletal and soft tissue characteristics in the frontal aspects using PA cephalometry for diagnosis and treatment planning. (7,8)

These posteroanterior (PA) cephalograms are not the routine part of orthodontic diagnosis radiograph and are required only for diagnosis of facial asymmetry. Hence patient has an extra burden of cost and exposure to additional radiographs. Another option to diagnose facial asymmetry is the panoramic radiograph and considered the standard of care for orthodontic and orthognathic diagnosis and treatment planning. It provides a significant amount of information about the teeth and supporting bone and is used to screen for cysts, cancer, extra teeth, the congenital absence or premature loss of teeth, teeth fused to the bone or abnormally retained teeth, tooth eruption path, bone pathology, and facial asymmetry. (9,10)  It is also studied that conventional and digital panoramic films are plagued with magnification errors and disproportional enlargement. (11-14)

Various studies had been conducted to know the accuracy of asymmetry measurement with a panoramic radiograph. Tronje et al 15 mathematically calculated the accuracy of panoramic measurements using an object composed of steel wires in a cross-formation to compare actual measurements and calculated values to those taken from the radiograph. They concluded that the panoramic film can be used for vertical but not for horizontal measurements if the patient is properly positioned and has no marked anatomic deviations from the norm. The accuracy of vertical measurements for this panoramic unit was ±10% and therefore should not be relied on for high accuracy. Large and Svanaes 16 showed a magnification factor of 18% to 21% for the vertical variables particularly in the anterior region, but gonial angle measurements on the skulls were almost identical to those measured on the panoramic image.

Kjellberg et al 17 found that on average, the condylion to gonion measurements from the panoramic films were magnified by 1.16 to 1.21 mm. Turp et al42 radiographed concluded that panoramic radiographs are not appropriate for exact vertical measurements such as condylar and ramus heights, or a combination, to assess mandibular asymmetry. Batenburg et al 18 concluded that orthopantomography is not a reliable technique to evaluate alveolar bone height in an edentulous mandible. Laster et al 19 compared horizontal and vertical measurements of anatomic points on 30 skulls imaged with ideal, 7 mm laterally shifted, and 10° rotation around a vertical axis positioning in a Sirona Orthophos Plus panoramic unit with measurements obtained by software on digital images. It was concluded that the panoramic accuracies in detecting mandibular asymmetry were 67%, 70%, and 47% for ideal, rotated and shifted skull positions. Therefore, the assessment of posterior mandibular facial symmetry with a panoramic unit was considered unreliable.

Based on findings of these studies it can be concluded that orthopantomography is not a reliable radiograph to diagnose of asymmetry due to magnification. Most practitioners do not use panoramic images for asymmetry diagnosis. (11, 14)

Hence it is necessary to conduct a study for the computation of magnification factor of the OPG image and propose a software method to correct the magnification factor of the OPG image for the derivation of the measurements likely to measurements on PA image.