Background: Reverse panoramic radiography is a radiographic technique to view the lateral aspect of the condylar head and its neighboring structures more clearly and with less distortion. The technique is simple to perform with the patient in the reverse position in an orthopantomograph particularly in a unit that does not have a provision to take a TMJ tomogram.
Methods: The machine used was Rotograph 230 of 1992 model. The chin rest was removed so that the patient can be positioned posteriorly such that the condylar region is moved closer to the lateral centre of rotation within a fixed distance between the X ray source and the cassette.
Results: The technique demonstrated the normal anatomy as well as the osteoarthritic changes in the condyle with mouth opened and closed position. (fig 2 and 3)
Conclusion: Reverse panoramic radiograph is yet another radiographic technique to view the lateral aspect of the condyle
Panoramic radiography is a popular radiographic technique in dentistry. It is an essential element in the radiographic diagnosis. Panorama means an unobstructed region in any direction. A panoramic film shows mandible and maxilla from condyle to condyle in a single film.
Panoramic radiography was first described by Numata1,2 and Paatero3 independently and later by several others like Hudson4 and Blackman5,6. The first commercial orthopantomograph was available in 1961. Many panoramic units are available in the market today. They all primarily differ in the number and location of the centre of rotation, the choice of fixed or adjustable focal trough, type and shape of film transport mechanism7.
Panoramic radiography is an invaluable aid in orofacial diagnosis because of the broad coverage of the facial bones and teeth. It also serves as a diagnostic modality for hard tissue imaging of the temperomandibular joint. Some panoramic units have a provision to take TMJ Tomogram. To accomplish this, the unit must have an adjustable focal trough which is set for the position of the condyle.
There are several other radiographic techniques to show the lateral aspect of the condyle but each technique has its own limitations. The lateral skull and lateral oblique shows much of superimposition and distortions. Transcranial view shows the lateral joint contours. The ipsilateral petrous ridge may superimpose over the condylar neck. It also needs the use of a suitable craniostat. Transpharyngeal view shows the sagittal view of the medial pole of the condyle but needs the mouth to be opened widely and it delivers a high radiation dosage to the skin. Linear Tomography and Computed Tomography cannot be used for general screening purpose. Even a standard panoramic view results in some degree of distortion and magnification in the region of mandibular ramus. A reverse panoramic radiograph provide a clearer and less distorted view of the ascending mandibular ramus, condylar head and adjacent structures especially when the mouth opening is limited.
This technique was used on several patients who visited the OP of Meenakshi Ammal Dental College, to demonstrate the normal anatomy as well as the osteoarthitic changes of the condyle. The machine that was used for the study was Rotograph 230 of 1992 model. Despite the advent of new technological advancement and innovation this unit is still in use to take OPGs and reverse OPGs. Certain modifications were made in the machine. The chin rest was removed so that the patient can be positioned posteriorly such that the condylar region is moved closer to the lateral centre of rotation within a fixed distance between the X ray source and the cassette. The procedure was explained to the patients regarding the movement of the tube around the patient’s head. Patient was asked to remove the eyeglasses, earrings, dentures, hairpins, etc. patient can sit or stand erectly in the machine in the reverse position. The head was aligned with the mid saggital plane perpendicular to the floor. The chin was slightly tilted down so that the Frankfort line is parallel to the floor. Lateral head stabilizers are used to ensure that the patient’s head is central. Exposure is made with both mouth opened and closed position. It should be remembered that if the left and right markers are built-in the cassette, they will be incorrect. So the film marking has to be altered.
This technique can demonstrate the normal anatomy as well as the osteoarthritic changes in the condyle with mouth opened and closed position. (fig 2 and 3)
Panoramic radiograph is a curved surface tomogram. It is based on the principle of reciprocal movement of an x ray source and an image receptor around a central plane while the patient remains stationary. This central plane of the object that is not blurred on the radiograph is called as the plane of acceptable detail or image layer.
Patient positioning is critical and needs to be accurate such that condylar region is close to the lateral centre of rotation. The speed is reduced so that the image layer moves closer to the centre of rotation and becomes thinner resulting in a clearer and less distorted view.This view is used especially when the mouth opening is limited.
Errors in the technique mostly occur with improper positioning of the patient.
Shows the condyle and its neighboring structures with the ascending ramus that is usually overlapped by the soft tissue shadows in the OPG
The technique is simple to perform. Retakes are possible. And quality control is easier to maintain.
It practically eliminates problem in patient with trismus or uncooperative patient
Reverse OPG is not without problems. Positioning of the patient is more critical than for the standard view.
It is believed that this view increases the exposure to the eye. It is precautious to use lead eye shields. Direct measurement of the eye doses were made using thermoluminescent dosimeters mounted on a ‘Rando’ tissue equivalent skull phantom (Alderson Laboratories). Dosimetric studies shows that with correct positioning of the patient the dose delivered appears on an average to be very similar in both standard and reverse technique and that there is no increase when using the reverse method.(Table I)
Table I Comparison of dosages in Standard and reverse positions using ‘Rando’ phantom
Reverse panoramic radiograph is yet another radiographic technique to view the lateral aspect of the condyle especially in patients with limited mouth opening and it can be taken with orthopantomographic units that do not have the provision to take a TMJ tomogram.
Numata H. Consideratiions of the parabolic radiography of the dental arch. J. Shimizu Stud 1933, 10 : 13 ( in Japanese)
Numata H, A trial on the narrow beam radiography. J. Shimizu Stud 1934, 12 : 6 ( in Japanese)
Paatero Y. V. A New radiographic method in dentistry. Svom Hammaslaak Toimi; 1946; 87:37 (Finnish)
Hudson D.C. Kumpala J. W. Dickson G. A panoramic X ray dental machine. US Armed Forces Med J 1957; 13 : 46
Blackman S. Panoramic tomography. Dental Pract 1955; 5 : 368
Blackman S. Rotational tomography of the face. Br. J Radiol 1960: 33: 408