Ankylosis and Infraocclusion of Primary Molars
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Direct answer
A primary molar whose periodontal ligament has been locally replaced by bone becomes ankylosed — fused to the socket — and, while the alveolar ridge and neighbouring teeth keep growing, it stays put and appears progressively submerged below the occlusal plane, the state called infraocclusion. Reported in a few per cent of children, it favours the mandibular second primary molar, runs in families, and is markedly more common when the permanent successor is congenitally missing. Diagnosis is clinical: the tooth sits low with a surrounding gingival collar, shows no mobility, and gives a characteristically high, solid, metallic percussion note, with radiographs showing an interrupted periodontal ligament space. Management is watchful by default — many ankylosed primary molars still shed spontaneously, because ankylosis is often intermittent — but extraction with space management is indicated for progressive submergence, tipping of adjacent teeth into the defect, overeruption of the opponent, or an absent successor that compromises the eruptive plan.
What you must remember
- Definition and mechanism: union of the root cementum or dentine with alveolar bone across a lost periodontal ligament; the tooth then fails to keep pace with vertical alveolar growth, hence infraocclusion (submergence).
- Epidemiology: a few per cent of children; mandibular second primary molar the classic tooth, then first primary molar and primary canine; familial tendency recognised.
- The successor link: ankylosis is strongly associated with congenital absence of the succedaneous premolar — a missing second premolar with a submerged second primary molar is a textbook pairing.
- Diagnosis triad: infraocclusion below the occlusal plane; absent physiological mobility; and a high, sharp, solid percussion note, crisper than the muted note of a normally suspended tooth; a radiograph confirms the interrupted ligament space, though fusion is rarely visible all around.
- Grading: mild, moderate and severe by millimetres below the occlusal plane — grading drives the decision, not the diagnosis itself.
- Natural history: ankylosis is frequently intermittent, so many teeth continue to resorb and exfoliate on their own; spontaneous re-eruption is also described when the ankylosed area resorbs.
- Indications to extract: progressive submergence with adjacent tipping and space loss, overeruption of the opposing tooth, vertical bone defect formation, absence of the successor, or interference with orthodontic plans; timing is often guided by the successor's root development (about two-thirds) or the planned orthodontic sequence.
- Extraction technique: ankylosed teeth cannot be luxated normally — expect surgical removal, often with sectioning of the tooth and sometimes limited bone removal, so consent and theatre planning differ from routine exodontia.
Deciding between watch and extract
An eight-year-old presents with the lower right second primary molar sitting 2 mm below its neighbours, sharp-sounding on percussion and rock-solid; radiographs show the second premolar present with half its root formed. The reasoning balances three clocks. The child's vertical growth will sink the tooth further, inviting the first permanent molar and second premolar neighbours to tip over its sunken contour and the upper opponent to overerupt — costs that grow with time. The premolar's clock says its eruption is still a year or two away (root half-formed, Nolla 7), so it cannot rescue the situation soon. The ankylosis clock says the tooth may still resorb and shed, but with submergence already moderate and the contralateral side erupted flat, waiting is the riskier choice — a judgement worth explaining to the family. Plan: surgical extraction now, accepting that sectioning may be needed, with a review in three months; if the first permanent molar tips mesially into the healed site, a simple band-and-loop space maintainer on the first permanent molar preserves the premolar space while the premolar is monitored to eruption. Had the premolar been absent, the plan tilts earlier toward extraction plus definitive space planning with the orthodontist.
Where students slip
Three misconceptions recur. First, that ankylosis forbids exfoliation — many ankylosed primary teeth do resorb and shed, because the fusion is often patchy; the examinable corollary is that observation is legitimate management. Second, confusion between ankylosis and primary failure of eruption: ankylosis affects a submerged primary tooth with an interrupted ligament, PFE affects permanent posterior teeth failing to erupt despite clear paths. Third, the extraction answer is written as a simple forceps removal — the honest answer includes sectioning and surgical technique. Finally, candidates forget the missing-successor association, which changes both risk assessment and planning.
Frequently asked questions
Which primary tooth is most often ankylosed?
The mandibular second primary molar, followed by the mandibular first primary molar and primary canines.
Why does an ankylosed tooth appear submerged?
The tooth is fixed to bone and stops erupting while the surrounding alveolus and neighbouring teeth continue vertical growth, so it sits progressively below the occlusal plane — infraocclusion.
What percussion finding suggests ankylosis?
A high, sharp, solid or metallic percussion note, together with absent mobility, compared with the duller note of a normally suspended tooth.
What is the association with congenitally missing teeth?
Ankylosis of a primary molar is significantly more likely when its succedaneous premolar is agenetic, which is why every submerged molar earns a radiograph of the successor.
When is extraction preferred over observation?
Progressive submergence, adjacent tooth tipping with space loss, overeruption of the opposing tooth, bone defects, an absent successor, or conflict with orthodontic treatment plans — accepting that removal is surgical and may require sectioning.