Orthodontics

I'll cover the basic orthodontics concepts that commonly appear in dental exams, with special emphasis on numerical values, space maintainers, mixed-dentition development, terminal planes, eruption shifts and prediction formulas.

One question I remember what something along the lines: the exam gave lower and upper incisor widths of approximately 23 mm and 26 mm, respectively, it was likely testing mixed-dentition space analysis. So they are probably testing space analysis etc.

💡Forces used in orthodontics

Orthodontic tooth movement occurs when an applied force produces changes in the periodontal ligament and surrounding alveolar bone.

Types of orthodontic tooth movement

💡Approximate optimal forces

Force values vary with tooth type, movement, appliance, periodontal condition and textbook. These are commonly taught approximate ranges, not universal clinical prescriptions.

Exam pearl: Intrusion requires very light forces because excessive force can increase the risk of root resorption and periodontal damage.

💡Continuous versus interrupted forces

• Continuous force: Remains active between appointments, although its magnitude may decline.
• Interrupted force: Falls to zero when the appliance is inactive.
• Intermittent force: Alternates between active and inactive periods, as with some removable appliances.

Excessive force can cause hyalinization of the periodontal ligament, delayed tooth movement and increased risk of root resorption.

💡Mixed-dentition space analysis

Tanaka–Johnston analysis

Used to predict the combined mesiodistal width of the unerupted permanent canine, first premolar and second premolar in one quadrant, using the sum of the four erupted mandibular permanent incisors.

Let \(X\) = sum of the mesiodistal widths of the four lower permanent incisors.

• Predicted upper canine + premolars, per side = \(X/2 + 11.0\text{ mm}\)
• Predicted lower canine + premolars, per side = \(X/2 + 10.5\text{ mm}\)

Example: if the four lower incisors total 23 mm:

These values are estimates, not direct measurements of individual teeth. Also i dont exactly remember the question and exact values (sorry).

Important: This formula does not predict the width of each canine and premolar separately. It predicts their combined width on one side.

Moyers mixed-dentition analysis

Moyers also uses the sum of the four mandibular permanent incisors, but the predicted widths are obtained from probability tables.

• The tables provide estimates for the unerupted canine and premolars.
• Separate predictions are available for the maxilla and mandible.
• Tables may differ by sex and selected probability level.
• A commonly taught level is the 75th percentile, but the correct table and probability level depend on the reference being used.

What if the upper incisors total 26 mm?

The standard Tanaka–Johnston and Moyers methods use the sum of the four mandibular permanent incisors, not the upper incisor sum.

So if the question gave:

• Lower incisors = 23 mm
• Upper incisors = 26 mm

Use the lower incisor sum of 23 mm for Tanaka–Johnston or Moyers prediction. The 26 mm upper sum is not substituted into the standard formula.

If the question instead asks for incisor liability, that is a different calculation, explained below.

💡Incisor liability

Incisor liability is the difference between the combined mesiodistal widths of the permanent incisors and those of the primary incisors.

Permanent incisors are wider than primary incisors, so extra space is required when they erupt.

These are traditional approximate textbook values; individual variation occurs.

💡How is the extra space obtained?

• Physiological spacing in the primary dentition.
• Increase in intercanine width.
• Labial positioning of the permanent incisors relative to the primary incisors.
• Use of primate spaces and developmental spacing.

Exam pearl: Incisor liability is not the same as the combined width of the permanent canine and premolars. Don't confuse it with mixed-dentition space analysis.

💡Leeway space of Nance

Leeway space is the difference between the combined mesiodistal width of the primary canine and primary molars (C + D + E) and that of the permanent canine and premolars (3 + 4 + 5) that replace them.

Primary canine and molars are usually wider than their permanent successors.

The mandibular leeway space is greater than the maxillary leeway space.

Clinical importance

• Provides space for relief of mild crowding.
• Can be used during late mesial shift of the permanent first molars.
• May help improve the molar relationship.
• Can be preserved with a lingual holding arch in selected mixed-dentition cases.

Exam pearl: Leeway space is greater in the mandible than the maxilla.

💡Early and late mesial shift

These describe how the permanent first molars move mesially to establish a Class I molar relationship.

Early mesial shift

The permanent first molar moves forward into the primate space, particularly the space distal to the primary canine in the mandible.

Late mesial shift

After the primary second molars exfoliate, the permanent first molars can move mesially into the leeway space because the permanent premolars are narrower than the primary molars they replace.

Memory trick:

• Early = primate spaces.
• Late = leeway space.

💡Terminal plane relationship of primary second molars

The terminal plane is the relationship between the distal surfaces of the maxillary and mandibular primary second molars.

slideshare.net

Flush terminal plane

The distal surfaces of the upper and lower second primary molars lie in the same vertical plane.

• Common in the primary dentition.
• May develop into Class I or Class II permanent molar relationships.
• A mesial shift can help establish Class I.

Mesial step

The distal surface of the mandibular primary second molar is mesial to that of the maxillary second molar.

• Small mesial step commonly develops into a Class I permanent molar relationship.
• A pronounced mesial step may be associated with a Class III tendency.

Distal step

The distal surface of the mandibular primary second molar is distal to that of the maxillary second molar.

• More likely to develop into a Class II permanent molar relationship.
• Less favourable for spontaneous development of a Class I relationship.

Quick comparison

Do not treat these as absolute predictions: facial growth, eruption pattern, arch length and other occlusal factors affect the final relationship.

Space maintainers: which appliance is used where?

A space maintainer preserves space after premature loss of a primary tooth, reducing the risk of adjacent teeth drifting into the space and causing crowding or impaction.

The choice depends on which tooth is lost, whether the permanent first molar has erupted, the time until successor eruption, occlusion, oral hygiene and patient cooperation.

A. Types and indications

High yield

• Most commonly used for premature loss of a single primary molar.
• Often used after loss of a primary first or second molar when a suitable posterior abutment is present.
• Commonly used when the permanent first molar has erupted and can support the appliance.
• A band is placed on an abutment tooth, with a loop extending across the space.

• Used after premature loss of the primary second molar before eruption of the permanent first molar.
• The intra-alveolar extension guides the eruption path of the permanent first molar.
• Requires careful placement and monitoring.
• Not suitable in every patient; medical conditions, oral hygiene and the ability to monitor the appliance matter.

• Used in the mandibular arch, usually after permanent first molars have erupted.
• Useful for maintaining lower arch length and preserving leeway space.
• Commonly used when primary molars are being lost and preservation of space is important.
• Usually consists of molar bands connected by a lingual wire.

• Used in the maxilla to help maintain molar position and preserve arch space.
• Has an acrylic button resting against the anterior palate.
• Commonly used when maxillary molars require anchorage against mesial movement.

• Connects the maxillary first molars across the palate.
• Helps control molar rotation, transverse position and anchorage.
• It is not interchangeable with a Nance appliance: a TPA generally lacks the acrylic palatal button.

• May be used for multiple missing teeth or when replacement of missing teeth is also needed.
• Requires adequate cooperation and retention.
• Can be removed for cleaning.

• Considered when the abutment primary molar requires full coronal coverage, for example because of extensive caries.
• A stainless steel crown supports the loop.

💡B. Space maintainer decision table

When might a space maintainer not be indicated?

• The successor tooth is about to erupt.
• There is no meaningful risk of space loss.
• The space is already being managed by an orthodontic treatment plan.
• The successor is absent or the treatment plan requires a different approach.
• The appliance cannot be maintained safely because of poor oral hygiene or other clinical constraints.

Always evaluate eruption, space, occlusion and the patient's ability to attend follow-up appointments.

💡Other basic orthodontic concepts

Angle's classification of malocclusion

Class II division 1: Maxillary incisors are usually proclined, often with increased overjet.

Class II division 2: Maxillary central incisors are retroclined, often with deep overbite.

Class III: May be dental, skeletal or functional, with a tendency toward reverse overjet.

💡Overjet and overbite

💡Cephalometric landmarks and measurements

Common textbook reference values are approximately SNA 82°, SNB 80°, ANB 2° and IMPA 90°, but they vary by analysis, population, age and skeletal pattern. They should not be used in isolation.

💡Eruption chronology and mixed dentition

Approximate eruption ages are commonly tested. There is individual variation.

Six-year molars: First permanent molars erupt distal to the primary second molars and do not replace a primary tooth.