Most patients undergoing routine orthodontic screening will have a dental pantomogram. Location of tooth position requires two radiographs in different positions. In the interests of radiation hygiene it is sensible to use this as a base x-ray and to take further location radiographs in relation to the dental pantomogram. An anterior occlusal radiograph allows this and the principle of vertical parallax can be used to locate the position of the canine. The tube has to shift in order to take an anterior occlusal and it moves in a vertical direction. If the tooth crown appears to move in the direction of the tube shift (ie vertically) then the tooth will be positioned on the palatal aspect. If the crown appears to move in the opposite direction it is buccal and if it shows no movement at all it is in the line of the arch (Figures 5 to 7). This also provides a reasonably detailed intra-oral view in cases of root resorption.
Although there are other radiographic techniques which can be used to locate canines, this method works well. If there are difficulties in being sure of the exact location then two peri-apicals taken of the region with a horizontal shift of the tube may give slightly better precision. The periapical radiographs will also give good detail of the roots of adjacent teeth, particularly the lateral incisors. The proximity of the canine crown to incisor roots does make them vulnerable to root resorption. The percentage of teeth adjacent to the crown of the canine which undergo some form of resorption is probably quite high at the microscopic level. No extractions should be contemplated until the canines have been located. Figure 8 shows a dental pan-tomogram of a patient who had both upper and lower first premolars removed as part of a treatment plan, but where the canines had not been located beforehand. The dental pantomogram clearly shows the poor position of the canines and subsequent treatment involved the removal of the permanent canines since their position was deemed hopeless and movement of the canines may well have resulted in root resorp-
tion of the incisors. The canines had to be replaced prosthetically and it would be difficult to see how a legal defence of this situation could be raised.
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