Unilateral posterior crossbite with functional mandibular shift in the mixed dentition
An 8-year-4-month-old girl attends with her mother, who reports that the upper right back teeth 'bite inside' the lower ones and that her chin 'slides to one side' when she closes. Digit sucking stopped at age 6.5 years. She snores at night and mouth-breathes during sleep; an ENT review found no obstructive adenoids. Clinical findings: overjet 2 mm, overbite 20%; the lower midline deviates 3 mm to the right in centric occlusion but coincides with the upper midline at first contact in centric relation. The right posterior teeth (16, 55, 54) are in crossbite; the left posterior segment has normal buccal overjet. Maxillary intercanine width 30 mm and intermolar width 44 mm; mandibular intermolar width 47 mm. There is mild lower anterior crowding, and the leeway space is largely intact. The mother asks whether treatment should start now or wait until all permanent teeth have erupted, and whether correcting the bite will cure the night-time mouth breathing.
1. Classify this crossbite and justify whether to treat now or defer until the permanent dentition.
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The first step is to separate dental, skeletal and functional components. A single tooth tipped lingually would be a dental crossbite; a narrow maxilla with a mandibular shift is a skeletal/functional problem. Here the mandibular midline moves 3 mm to the right between centric relation and centric occlusion, the crossbite is unilateral, and the maxillary intermolar width is 44 mm against 47 mm in the mandible. That pattern fits maxillary transverse constriction with a functional mandibular shift, not simple lingual inclination of 16.
Because the shift is still reversible, treatment now is justified. If left, the interdigitation maintains the crossbite, the mandible continues to function asymmetrically, and the patient develops unilateral attrition and a more difficult expansion later. A 3 mm shift plus 1–2 mm of overcorrection means about 5 mm of transverse expansion is needed. The digit sucking and mouth breathing are contributing factors, not the primary cause, and both should be addressed separately. Deferring to 12 years would allow the asymmetry to consolidate and is not my choice.
2. Select an appliance, give the activation protocol and expected response, and state how you would retain the correction.
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The target is roughly 5 mm of intermolar expansion (3 mm discrepancy plus overcorrection) with buccal overjet of 1–2 mm at the end. At 8 years the midpalatal suture is open, so either a bonded rapid maxillary expander (RME) or a quad helix can work.
- RME: 0.25 mm per turn, one turn per day (0.25 mm/day), about 20–24 turns over 3–4 weeks. More skeletal effect, faster split, needs good parent cooperation and hygiene; expect a midline diastema and possible nasal width change.
- Quad helix: 4–6 mm capacity, activation of 1–1.5 mm per visit every 6–8 weeks; slower and more dentoalveolar, but robust and can later serve as a transpalatal retainer.
I would use a bonded RME if the constriction is clearly skeletal and the family can comply; otherwise a quad helix. Stop when the crossbite is overcorrected, then retain 3–6 months with the same appliance or a transpalatal arch, and confirm that the midlines coincide in centric relation at each visit.
3. What is the prognosis, what phase 2 monitoring is needed, and what do you tell the mother about the breathing?
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Transverse expansion in the mixed dentition is stable in most children but partial relapse of 20–30% is common if retention is stopped early. I would retain until the permanent buccal segments are established, then review at 11–12 years with study models and a panoramic radiograph. Phase 2 need depends on arch length: leeway space is about 3–4 mm, so if lower crowding exceeds 4 mm, plan for either expansion or later extraction; if it is less, simple observation may suffice.
On breathing, be honest: expansion can modestly reduce nasal resistance, but the evidence that it cures mouth breathing or snoring is weak, and adenoid status can change with age. Persistent snoring or witnessed apnoea warrants repeat ENT and possibly sleep assessment. Myofunctional therapy has limited evidence and should not be sold as a substitute for correcting the occlusion.
Document midline coincidence and buccal overjet at every visit, and warn the mother that a second phase is likely and will be planned on the records at 11–12 years.