4.5 Impacted Teeth & Radiographic Localisation
Key Takeaways
- Parallax localisation follows the SLOB rule — Same Lingual, Opposite Buccal: an object that appears to move in the same direction as the X-ray tube shift lies palatally or lingually
- Horizontal parallax uses two periapicals with a horizontal tube shift; vertical parallax typically compares a panoramic radiograph with an upper standard occlusal, the tube shift being vertical
- About 85 per cent of impacted maxillary canines are palatal, and the main risk of leaving one untreated is resorption of the adjacent lateral incisor root
- Third molar impaction is described by Winter's angulation classification and Pell and Gregory's relation to the ramus and depth, both of which predict surgical difficulty
- The Rood and Shehab radiographic signs of an intimate relationship to the inferior alveolar canal include darkening of the root, deflection of the root, narrowing of the root, a dark and bifid apex, interruption of the white lines of the canal, diversion of the canal and narrowing of the canal
Radiographic Localisation: Parallax
Outcome C2.17 asks candidates to interpret radiographs to accurately locate impacted teeth, using parallax techniques. Parallax exploits the fact that when the X-ray tube moves, objects at different depths appear to move by different amounts relative to one another.
The SLOB rule
Same Lingual, Opposite Buccal. An object that appears to move in the same direction as the tube shift lies lingually or palatally. An object that appears to move in the opposite direction lies buccally.
| Technique | Films used | Direction of tube shift |
|---|---|---|
| Horizontal parallax | Two periapicals of the same region taken at different horizontal angulations, or a periapical plus an upper standard occlusal | Horizontal |
| Vertical parallax | A panoramic radiograph (tube directed slightly upwards) plus an upper standard occlusal (tube directed downwards) | Vertical |
For maxillary canines, vertical parallax with a panoramic film plus an upper standard occlusal is the everyday combination, because both are commonly taken anyway and the vertical tube-shift is large. If the canine moves up relative to the incisor roots as the tube moves up (from occlusal to panoramic), it lies palatally.
Cone beam CT is reserved for cases where two-dimensional parallax cannot answer the question, where root resorption of the adjacent teeth is suspected, or where the surgical approach is unclear — it is not a routine substitute for parallax.
Impacted Maxillary Canines
The maxillary canine is the second most commonly impacted tooth after the third molar, affecting about 1–3% of the population; roughly 85% of impactions are palatal. Female predominance is recognised, and there is an association with a small or absent lateral incisor.
Assessment (outcome C2.18)
- Palpation of the buccal sulcus from age 9–10; a canine not palpable by about age 11, or asymmetry between sides, prompts imaging.
- Clinical signs: a retained primary canine, a distally tipped lateral incisor, a palatal bulge, absence of a labial canine bulge.
- Radiographic assessment: vertical position, angulation to the midline, mesiodistal position relative to the lateral incisor root, and — critically — resorption of the lateral incisor root, the complication that converts observation into urgent treatment.
Management options
| Option | When it applies |
|---|---|
| Interceptive extraction of the primary canine | Age roughly 10–13 with a favourably positioned permanent canine; improves the chance of spontaneous correction, reviewed radiographically at about 12 months |
| Surgical exposure and orthodontic alignment | The usual definitive option; an open exposure with a pack, or a closed exposure with a bonded gold chain and traction under a fixed appliance |
| Surgical removal of the canine | Where it is severely ectopic or ankylosed, where the patient declines lengthy treatment, or where the lateral incisor is being resorbed and cannot be protected; the space is then closed or restored |
| Autotransplantation | Occasionally, where alignment is impossible but the tooth is otherwise sound |
| Observation | Only where the patient declines treatment and there is no resorption; requires periodic radiographic review because a retained impacted canine can develop a dentigerous cyst |
Impacted Third Molars
Outcome C3.11 requires assessment of impacted third molars. NICE Technology Appraisal 1 remains the governing UK guidance: prophylactic removal of pathology-free impacted third molars should be discontinued in the NHS.
Classifying the impaction
Winter's classification describes the angulation of the third molar to the long axis of the second molar: mesioangular (commonest), vertical, distoangular (often the most difficult, because the tooth is delivered against the ramus), horizontal, transverse and inverted.
Pell and Gregory's classification describes two further dimensions:
- Relation to the anterior border of the ramus — Class I (sufficient space between the second molar and the ramus), Class II (about half the crown covered), Class III (entirely within the ramus).
- Depth relative to the occlusal plane of the second molar — Level A (at or above the occlusal plane), Level B (between the occlusal plane and the cervical line), Level C (below the cervical line).
Other predictors of difficulty include root number and curvature, a wide or dense follicle, bone density and the patient's age and mouth opening.
Radiographic signs of proximity to the inferior alveolar canal
The classic Rood and Shehab signs on a panoramic radiograph are:
- Darkening of the root where it crosses the canal
- Deflection of the root as it approaches the canal
- Narrowing of the root
- A dark and bifid apex
- Interruption of the white lines (cortical borders) of the canal
- Diversion of the canal
- Narrowing of the canal
The three most predictive are darkening of the root, interruption of the white lines and diversion of the canal. Where these are present, CBCT may be justified to establish the true three-dimensional relationship, and coronectomy — removing the crown and leaving the roots undisturbed — becomes a recognised option in a vital, non-infected tooth as an alternative to a high-risk extraction. Coronectomy is contraindicated where the tooth is mobile, where the roots are horizontally impacted along the canal, and in the presence of active infection involving the roots.
A panoramic radiograph and an upper standard occlusal are taken to localise an unerupted maxillary canine. Between the occlusal view and the panoramic view the canine appears to move upwards relative to the incisor roots, in the same direction as the vertical tube shift. Where does the canine lie?
A 10-year-old has a maxillary canine that is not palpable buccally; radiographs show it palatally positioned but with a favourable angulation and no resorption of the lateral incisor. What is the most appropriate initial management?
Which set of panoramic findings most strongly suggests an intimate relationship between a lower third molar and the inferior alveolar canal?
For which of the following is coronectomy of a lower third molar contraindicated?