Periapical radiography describes intraoral techniques designed to show individual teeth and the tissues around the apices. Each film usually shows two to four teeth and provides detailed information about the teeth and the surrounding alveolar bone.
Main indications The main clinical indications for periapical radiography include: Detection of apical infection/inflammation Assessment of the periodontal status After trauma to the teeth and associated alveolar bone Assessment of the presence and position of unerupted teeth Assessment of root morphology before extractions During endodontics Preoperative assessment and postoperative appraisal of apical surgery Detailed evaluation of apical cysts and other lesions within the alveolar bone Evaluation of implants postoperatively.
Ideal positioning requirements The ideal requirements for the position of the film packet and the X-ray beam, relative to a tooth, are shown in the following Figure. They include: The tooth under investigation and the film packet should be in contact or, if not feasible, as close together as possible The tooth and the film packet should be parallel to one another
Ideal positioning requirements The film packet should be positioned with its long axis vertically for incisors and canines, and horizontally for premolars and molars with sufficient film beyond the apices to record the apical tissues The X-ray tubehead should be positioned so that the beam meets the tooth and the film at right angles in both the vertical and the horizontal planes The positioning should be reproducible.
Radiographic techniques The anatomy of the oral cavity does not always allow all these ideal positioning requirements to be satisfied. In an attempt to overcome the problems, two techniques for periapical radiography have been developed: The paralleling technique The bisected angle technique.
The long axis of the film is parallel with the long axis of the tooth (no distortion). The film close to the object
Paralleling technique Paralleling technique Theory 1. The film packet is placed in a holder and positioned in the mouth parallel to the long axis of the tooth under investigation. 2. The X-ray tubehead is then aimed at right angles (vertically and horizontally) to both the tooth and the film packet. 3. By using a film holder with fixed film packet and X-ray tubehead positions, the technique is reproducible.
1. The appropriate holder and size of film packet are selected. For incisors and canines (maxillary and mandibular) an anterior holder should be used and a small film packet (22 x 35 mm) with its long axis vertical. For premolars and molars (maxillary and mandibular) use a posterior holder (right or left as required) and a large film packet (31 x 41 mm) with its long axis horizontal, in addition: a. The smooth, white surface of the film packet must face towards the X-ray tubehead. b. The end of the film packet with the embossed orientation dot is placed opposite the crowns of the teeth (to avoid subsequent superimposition of the dot over an apex). 2. The patient is positioned with the head supported and with the occlusal plane horizontal.
3. The holder and film packet are placed in the mouth as follows: a. Maxillary incisors and canines — the film packet is positioned sufficiently posteriorly to enable its height to be accommodated in the vault of the palate b. Mandibular incisors and canines — the film packet is positioned in the floor of the mouth, approximately in line with the lower canines or first premolars c. Maxillary premolars and molars — the film packet is placed in the midline of the palate, again to accommodate its height in the vault of the palate d. Mandibular premolars and molars — the film packet is placed in the lingual sulcus next to the appropriate teeth. 4. The holder is rotated so that the teeth under investigation are touching the bite block.
5. A cottonwool roll is placed on the reverse side of the bite block. This often helps to keep the tooth and film packet parallel and may make the holder less uncomfortable. 6. The patient is requested to bite gently together, to stabilize the holder in position. 7. The locator ring is moved down the indicator rod until it is just in contact with the patient's face. This ensures the correct focal spot to film distance. 8. The spacer cone or BID is aligned with the locator ring. This automatically sets the vertical and horizontal angles and centres the X-ray beam on the film packet. 9. The exposure is made.
Bisected angle technique Bisected angle technique Theory The theoretical basis of the bisected angle technique is shown in the following Figure and can be summarized as follows: 1. The film packet is placed as close to the tooth under investigation as possible without bending the packet. 2. The angle formed between the long axis of the tooth and the long axis of the film packet is assessed and mentally bisected. 3. The X-ray tubehead is positioned at right angles to this bisecting line with the central ray of the X-ray beam aimed through the tooth apex. 4. Using the geometrical principle of similar triangles, the actual length of the tooth in the mouth will be equal to the length of the image of the tooth on the film.
Vertical angulation of the X-ray tubehead The angle formed by continuing the line of the central ray until it meets the occlusal plane determines the vertical angulation of the X-ray beam to the occlusal plane. Note: These vertical angles are often quoted but inevitably they are only approximate. Patient differences including head position, and individual tooth position and inclination mean that each positioning should be assessed independently. The vertical angulations suggested should be taken only as a general guide.
Positioning techniques The bisected angle technique can be performed either by using a film holder to support the film packet in the patient's mouth or by asking the patient to support the film packet gently using either an index finger or thumb. Using a film holder is the recommended technique to avoid irradiating the patient's fingers. However, using the finger is still widely used.
Full-mouth survey This terminology is used to describe a collection of periapical radiographs showing the full dentition. Not every tooth is radiographed individually, but enough films are taken to include all the teeth.
Advantages of the paralleling technique Geometrically accurate images are produced with little magnification. The shadow of the zygomatic buttress appears above the apices of the molar teeth. The periodontal bone levels are well represented. The periapical tissues are accurately shown with minimal foreshortening or elongation. The crowns of the teeth are well shown enabling the detection of approximal caries. The horizontal and vertical angulations of the X-ray tubehead are automatically determined by the positioning devices if placed correctly. The X-ray beam is aimed accurately at the centre of the film — all areas of the film are irradiated and there is no coning off or cone cutting. Reproducible radiographs are possible at different visits and with different operators. The relative positions of the film packet, teeth and X-ray beam are always maintained, irrespective of the position of the patient's head. This is useful for some patients with disabilities.
Disadvantages of the paralleling technique Positioning of the film packet can be very uncomfortable for the patient, particularly for posterior teeth, often causing gagging. Positioning the holders within the mouth can be difficult for inexperienced operators. The anatomy of the mouth sometimes makes the technique impossible, e.g. a shallow, flat palate. The apices of the teeth can sometimes appear very near the edge of the film. Positioning the holders in the lower third molar regions can be very difficult. The technique cannot be performed satisfactorily using a short focal spot to skin distance (i.e. a short spacer cone) because of the resultant magnification. The holders need to be autoclavable or disposable.
Advantages of the bisected angle technique Positioning of the film packet is reasonably comfortable for the patient in all areas of the mouth. Positioning is relatively simple and quick. If all angulations are assessed correctly, the image of the tooth will be the same length as the tooth itself and should be adequate (but not ideal) for most diagnostic purposes.
Disadvantages of the bisected angle technique The many variables involved in the technique often result in the image being badly distorted. Incorrect vertical angulation will result in foreshortening or elongation of the image. The periodontal bone levels are poorly shown. The shadow of the zygomatic buttress frequently overlies the roots of the upper molars. The horizontal and vertical angles have to be assessed for every patient and considerable skill is required. It is not possible to obtain reproducible views. Coning off or cone cutting may result if the central ray is not aimed at the centre of the film, particularly if using rectangular collimation. Incorrect horizontal angulation will result in overlapping of the crowns and roots. The crowns of the teeth are often distorted, thus preventing the detection of approximal caries. The buccal roots of the maxillary premolars and molars are foreshortened.