Impacted Teeth in Orthodontics
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Direct answer
After third molars, the maxillary canine is the tooth most often impacted — roughly 1–3 per cent of patients, palatal impactions outnumbering labial by about two to one — with mandibular canines, premolars and mesiodens-associated centrals trailing. The screening rule is tactile: a canine bulge should be palpable buccally by age ten to eleven; a non-palpable canine at eleven to twelve earns radiographs. Localisation uses the tube-shift (SLOB) rule with two periapical films or an occlusal view — an object that moves with the tube sits lingual or palatal — with cone-beam CT added when adjacent root resorption or three-dimensional position is in question, and CBCT series report incisor root resorption in a substantial share of palatal canine cases. Interception is powerful: extracting the retained primary canine before the early teens lets a large majority of palatally displaced canines erupt on their own (the Ericson and Kurol finding); established impactions need space creation, surgical exposure and light traction.
What you must remember
- Prevalence order: third molars first, maxillary canines next at about 1–3 per cent (females slightly more than males; palatal roughly twice labial), then mandibular canines and premolars; mesiodens is the classic cause of impacted centrals.
- Aetiology in two theories: guidance failure (labial — crowding and lack of guidance from the lateral incisor root) versus genetic (palatal — associated with agenesis and anomalous laterals); mechanical obstruction by supernumeraries, cysts and odontomas; primary failure of eruption (PTH1R-related) as a separate entity where teeth fail despite cleared paths.
- Localisation toolkit: SLOB/tube shift — same direction as tube means lingual/palatal; vertical parallax with occlusal film; two periapicals at different angles; CBCT for resorption, exact 3D position and treatment planning.
- Interceptive evidence: extraction of the primary canine in mixed dentition (roughly 10–13 years) normalises eruption in about three-quarters of palatal displacement cases when selected by position — Ericson and Kurol's series.
- Surgical choices: open exposure with an apically positioned flap for labially positioned teeth needing attached gingiva; closed exposure with a bonded attachment and full flap replacement, traction drawn through attached gingiva, for palatal teeth — generally kinder to the periodontium.
- Traction mechanics: create space first (arch length or extraction), then 25–50 g of light force on a stiff supporting archwire, direction chosen to avoid the lateral incisor root and the antrum; expect months, not weeks.
- Exit options: extraction with space closure (when the canine is hopeless or the patient declines surgery), autotransplantation in the young with open apices, and acceptance or prosthetic replacement of the primary-canine space.
- Related exam entities: mandibular second molars uprighted mechanically; ankylosis (percussion ring, arrested traction) demanding re-surgery or extraction.
A canine case from screening to bracket
An 11-year-old referred for a "missing" upper left canine: no bulge palpable, primary canine firm. The orthopantomogram shows the crown overlapping the lateral root; a second periapical with a mesial tube shift moves the crown with the tube — palatal. CBCT adds the decisive detail: 1.5 mm of lateral incisor root resorption. Because she is within the interceptive window, the primary canine is extracted and the arch developed with expansion; six months later the permanent canine has erupted unassisted — the Ericson-Kurol outcome. Had she presented at 15, the path would be surgical: closed exposure, a bonded button with gold chain, space opened to width, traction at 30 g toward the line of the arch avoiding the lateral root, over perhaps a year — with an ankylosis contingency (no movement after months of honest force) that converts to extraction and space closure.
Where viva examiners probe
The SLOB rule is recited wrongly under pressure; the safe formula: the object that moves with the tube shift lies on the same side as the tube's movement — lingual or palatal. The second probe pairs exposure techniques with indications: apically positioned flap for labial impactions to preserve keratinised gingiva, closed-eruption technique for palatal teeth. Third, the interceptive numbers: primary canine extraction before roughly 12 years succeeds in about three-quarters of selected palatal cases; crowding and horizontal position cut the rate. Finally, traction force (25–50 g) and the ankylosis escape plan separate candidates who have read from those who have treated.
Frequently asked questions
Which is the most commonly impacted tooth after the third molar?
The maxillary canine, at roughly 1–3 per cent prevalence, with palatal impactions about twice as frequent as labial ones.
State the tube-shift (SLOB) rule for localisation.
With two films taken at different horizontal angles, an object that appears to move in the same direction as the tube shift lies lingual or palatal; opposite movement indicates buccal position.
What did Ericson and Kurol show about primary canine extraction?
In mixed-dentition patients with palatally displaced canines, timely extraction of the primary canine allowed normal eruption in roughly three-quarters of appropriately selected cases.
How do open and closed exposure differ?
Open exposure (apically positioned flap) leaves the tooth uncovered, suiting labial impactions needing attached gingiva; closed exposure bonds an attachment under a replaced flap and tractions the tooth through it.
What force is used for orthodontic traction of an impacted canine?
Light force of about 25–50 g on a rigid supporting archwire, directed to clear adjacent roots and the maxillary antrum, sustained over months.