Extra oral Radiographs
Extra-oral radiographs may be taken with a standard dental X-ray
machine and are especially useful for young children because:
(i) They require less co-operation as they avoid the discomfort of
holding an intra-oral film in the lower lingual sulcus.
(ii) They require less exposure, i.e. two rotated oblique lateral views
and two anterior occlusal views give a complete picture of the
child’s dentition; the equivalent in twelve smaller intra-oral films
would involve three times the exposure.
(iii) A wide field is visible from the antral and nasal floor down to the
lower border of the mandible showing any unerupted extra teeth
or other anomaly outside the field of the smaller intrasoral film.
When taking a rotated oblique lateral view, for economy, a ½ plate
cassette (or its modern metric equivalent 13 cm x 18 cm) containing
intensifying screens may be used with a lead-rubber flap protecting one
half of the film whilst the other half is being exposed
As these extra-oral films are not provided with an embossed ‘pip’, it
is necessary to use a metal marker to indicate the side of the patient
being radiographed. A head-positioning board and a wire antenna may
be used to simplify the technique .
Alternatively a panoramic view of the whole dentition extending from
the nasal floor to the lower border of the mandible may be obtained by
ma eans of a rotating panoramic X-ray machine such as an
orthopantomograph or a Panorex .
EXAMINATION OF THE PATIENT
When an unerupted tooth is situated beyond the apices of erupted teeth, its exact position may also be localized by using an antero posterior and a lateral skull film. It is common practice to use for this ‘purpose the machine for taking cephalometric radiographs.
– Cephalometric Radiographs in order to study the growth changes in the human skull, Broadbent in 1931, introduced a radiographic technique whereby the head was positioned in a specially designed headholder (cephalostat) by means of ear rods, so that, at regular intervals, serial frontal and lateral radiographs of the same individual were obtained with the minimum of inaccuracy . Information gathered from these growth studies has encouraged its employment as an aid to diagnosis.
– In order to compare cephalometric radiographs with one another, it has been found necessary to trace the outline of the relevant structures on acetate tracing paper. The traced outlines can then be superimposed. This is made meaningful by locating each tracing by means of a fixed point, or registration point. This point should be capable of precise identification and should be as free as possible from the influence of growth.
– Although a number of registration points have been used in the past it is now common practice to use the centre of SellaTurcica. Further location is required in order to orientate the tracing around the registration point, and the line S.N. (Sella Turcica to the ‘front-most’ point of the frontosnasal suture) is now frequently used for this purpose.
– It has been shown by De Coster that the small amount of growth change to be expected here after the age of seven years makes this area of the cranial base an acceptably .stable site for the registration of lateral skull radiographs.
– Where quantitative analysis and comparison is to be made it is necessary to make measurements. In order to overcome difficulties arising from enlargement and the projection of a three-dimensional structure on to a two-dimensional image the measurements are not linear but angular only. It has been found that the measurement of an angle Which subtends a dimension on the radiograph is less deceptive than the use of linear measurement itself, For this purpose several planes are used, some of which have been derived from anthropology .
– The technique for using these radiographs is described in detail elsewhere, and the reader who wishes to Pursue the subject further is advised to refer to the bibliography at the end of the book.
Points of reference used in cephalometry
The highest bony point on the upper margin
of the external auditory meatus.
The most anterior point midway between
frontal and nasal hones on the fronto-nasal
The lowest point on the inferior bony margin of the orbit.
Point A (A)
The deepest point in the mid-line between the anterior nasal spine and the alveolar crest of the maxilla. It is usually found by drawing a tangent to this contour from nasion.
Point B (B)
The deepest point in the mid-line between the alveolar crest of the mandible and the mental process. It is usually found by drawing a tangent to this contour from nasion.
The lowest point on the cross-section of the mandibular symphysis.
The point on the angle of the mandible crossed by the bisector of that angle formed by the mandibular plane .and plane tangent to the posterior border of the vertical ramus.
Sella Point (S)
The mid-point in the outline outline of sella Turcica determined by inspection.
The following planes arc used in cephalometric analysis:
Sella-Nasion plane (S-N)
This plane is now usually used for registration . It has also been used as a baseline for growth studies.
Frankfort Horizontal Plane (PM)
This plane passes through both poria and the left orbitale. it has the advantage of being identifiable clinically as well as on a radiograph.
This extends from the anterior nasal spine (ANS) to the posterior nasal (PNS) and is possibly easier to identify cm a cephalograph than the Frankfort horizontal Plane.
Mandibular Plane (MP)
This plane passes through menton and both gonia. It is usual to take the mid-point between the two gonia when drawing it upon a tracing.
Axial inclination of upper central incisor (UIA)
A line drawn through the incisal edge and root apex of an upper central incisor. It’s usually extended to intersect the Frankfort plane, the average angle being 109°.
Axial inclination of lowe central incisor
A line drawn through the incisal edge and root apex of a lower central incisor. It is usually extended to intersect the mandibular plane, the average angle being 89°. It has been found to be inversely proportional to the Frankfort mandibular plane angle when the incisors are in normal contact.
Frankfort -Mandibular plane Angle (FMA)
The angle formed by backward continuations of the mandibular and Frankfort planes is used as a measure of vertical development of the anterior part of the face. The infra-nasal height is proportional to the size of this angle which averages 28°.
Maxillary-Mandibular Plane Angle (MMA)
The angle formed by the backward continuation of the maxillary and mandibular planes. Its use is similar to that of the Frankfort-Mandibular Plane Angle. It also averages 28°.
Sella-nasion to Point A Angle (SNA)
This angle is a measure of the degree of prominence of the upper dental base •(maxillary prognathism). Its average is 82°.
Sella-nasion to Point B Angle (SNB)
This angle is a measure of the degree of prominence of the lower dental base (mandibular prognathism) its average being 770• Comparison of these two angles will show the relative prominence of the upper dental base to the lower. A difference of 2-5° will occur in Skeletal Class I cases. Difference of more than 5′ or less than 2′ will occur in Skeletal Class 2 and Class 3 cases respectively.