22.3 Etiology of Malocclusion, Orthodontic Diagnosis and Treatment Planning
Key Takeaways
Prolonged digit sucking typically produces an anterior open bite, proclined maxillary incisors, retroclined mandibular incisors and a posterior crossbite from a narrow maxillary arch.
The Tanaka-Johnston method predicts the combined width of the unerupted canine and premolars in one quadrant as half the width of the four mandibular incisors plus 10.5 mm for the mandible or 11.0 mm for the maxilla.
The Bolton anterior ratio (six mandibular to six maxillary anterior teeth) averages about 77.2% and the overall ratio (twelve to twelve) about 91.3%; small maxillary lateral incisors cause a maxillary tooth-size deficiency.
Space can be created by extraction, arch expansion, distal movement of molars, incisor proclination or interproximal enamel reduction, and severe crowding with protrusive lips usually favors extraction.
A functional anterior or posterior crossbite with a mandibular shift from retruded contact to intercuspal position should be identified because it is corrected early.
Orthodontic diagnosis turns a collection of findings into a problem list. The SDLE tests whether you can identify the cause, measure the space problem and choose a sensible plan or referral.
Etiology of Malocclusion
| Factor | Examples and effects |
|---|---|
| Genetic / skeletal pattern | Inherited jaw size and position (familial mandibular prognathism), tooth size, hypodontia |
| Digit or dummy sucking | Anterior open bite (often asymmetric), proclined maxillary incisors, retroclined mandibular incisors, increased overjet, posterior crossbite from maxillary constriction |
| Mouth breathing / enlarged adenoids | Associated with a long lower face, narrow maxilla and open bite (the evidence is debated) |
| Tongue thrust | Usually adaptive to an open bite rather than its cause |
| Lip trap | Lower lip behind the upper incisors worsens overjet |
| Local dental factors | Early loss of primary teeth (space loss), retained primary teeth, supernumerary teeth, ankylosed primary molars, dilaceration, prominent labial frenum (midline diastema) |
| Trauma and pathology | Condylar fracture in childhood, cysts, tumors |
| Syndromes | Cleft lip and palate, Down syndrome, Crouzon syndrome, Pierre Robin sequence |
Orthodontic Examination and Records
- History: chief complaint, medical and dental history, habits, growth stage, family history of jaw patterns, motivation.
- Extraoral: facial profile (convex, straight, concave), facial symmetry, vertical proportions (lower face height), lip competence and nasolabial angle, smile line and incisor show, TMJ function.
- Intraoral: oral hygiene, caries and periodontal status; molar, canine and incisor relationships; overjet (normal about 2-4 mm) and overbite (normal about one-third of lower incisor crown height); crowding or spacing; centrelines; crossbites; mandibular displacement on closure.
- Records: study models or intraoral scans, photographs, panoramic radiograph, lateral cephalogram, other radiographs or CBCT for impacted teeth.
Index of Orthodontic Treatment Need (IOTN): the Dental Health Component grades need from 1 (none) to 5 (very great) using the single worst feature, for example an overjet over 9 mm or impeded eruption (grade 5) and an overjet of 6.1-9 mm or a crossbite with more than 2 mm displacement (grade 4). The Aesthetic Component uses a 10-photograph scale.
Space Analysis
Space required (sum of tooth widths) minus space available (arch perimeter) gives crowding or spacing. Crowding is commonly described as mild (under about 4 mm), moderate (about 4-8 mm) and severe (over about 8 mm).
Mixed dentition analysis predicts the size of unerupted canines and premolars:
| Method | Principle |
|---|---|
| Moyers | Probability tables using the sum of the four mandibular incisor widths (the 75th percentile is commonly used) |
| Tanaka-Johnston | Half the sum of the four mandibular incisors + 10.5 mm (mandibular quadrant) or + 11.0 mm (maxillary quadrant) |
| Radiographic (Hixon-Oldfather and others) | Measure unerupted teeth on radiographs, corrected for magnification |
Example: mandibular incisors total 22 mm. Predicted mandibular canine and premolars per quadrant = 11 + 10.5 = 21.5 mm; maxillary = 11 + 11.0 = 22.0 mm.
Additional space is needed to level a deep curve of Spee and to retract proclined incisors, while proclining incisors or expanding arches gains space.
Bolton tooth-size analysis
- Anterior ratio = sum of 6 mandibular anterior teeth / sum of 6 maxillary anterior teeth x 100 = about 77.2%.
- Overall ratio = sum of 12 mandibular teeth / sum of 12 maxillary teeth x 100 = about 91.3%.
- A larger ratio means mandibular tooth excess (or maxillary deficiency, such as peg-shaped lateral incisors); the finish then shows spacing, increased overbite and overjet problems, or a compromised canine relationship. Discrepancies are managed with interproximal reduction, composite build-ups or accepting a compromise.
Creating Space and Extraction Decisions
| Method | Notes |
|---|---|
| Extraction | Commonly premolars; for severe crowding, bimaxillary protrusion, camouflage of skeletal discrepancies |
| Arch expansion | Rapid maxillary expansion for transverse deficiency; limited stability of mandibular expansion |
| Distal movement of molars | Headgear, TAD-supported mechanics, aligners |
| Incisor proclination | Limited by lip support and periodontal tissues |
| Interproximal reduction | Small amounts of enamel removed from contacts; useful for mild crowding or Bolton excess |
Factors favoring extraction: severe crowding, protrusive incisors and lips, need to camouflage a skeletal discrepancy, a high-angle vertical pattern, and teeth of poor prognosis (which may be chosen for extraction instead of premolars).
Primary tooth extractions in the mixed dentition:
- Balancing extraction: the same tooth on the opposite side of the same arch (commonly the primary canine) to prevent a centreline shift.
- Compensating extraction: the same tooth in the opposing arch on the same side to maintain the buccal occlusion.
The Treatment Plan
- List problems in each plane (anteroposterior, vertical, transverse), plus crowding, soft tissue and dental health.
- Set objectives and decide whether the problem is dental or skeletal.
- For skeletal problems choose growth modification (growing patient), camouflage (mild to moderate problems) or orthognathic surgery (severe problems after growth).
- Plan anchorage, sequence, retention and the role of other specialists.
- Obtain informed consent, including risks such as decalcification, root resorption and relapse.
Exam Traps
- Do not plan treatment before dental disease is controlled.
- Treat a functional crossbite early; it causes asymmetric adaptation of the mandible if left.
- A positive Bolton excess in the mandible can stop the canines reaching Class I without spacing.
A 7-year-old has a thumb-sucking habit. Which combination of features is most likely?
Generalized crowding with no change in overjet or overbite whatsoever
Class III incisors with spacing in the mandibular labial segment only
Deep bite, retroclined maxillary incisors and a mandibular prognathism
Anterior open bite, proclined maxillary incisors and a posterior crossbite
In a mixed dentition, the four mandibular permanent incisors measure 23 mm in total. Using the Tanaka-Johnston method, what is the predicted total width of the unerupted mandibular canine and premolars in one quadrant?
22.5 mm
22.0 mm
23.0 mm
21.5 mm
A patient has peg-shaped maxillary lateral incisors and otherwise normal tooth sizes. How will this affect the Bolton anterior ratio?
The ratio rises above about 77.2%, indicating a relative maxillary tooth-size deficiency
The overall ratio alone changes, while the anterior ratio stays unchanged for this case
Bolton ratios are unaffected because lateral incisors are excluded from the calculation
The ratio falls below about 77.2%, indicating a relative mandibular tooth-size deficiency
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