BDS Orthodontics Syllabus

How is the ANB angle interpreted for skeletal classification?

$\text{ANB} \approx 2^{\circ}$ indicates a skeletal Class I; $\text{ANB} > 4^{\circ}$ indicates skeletal Class II (maxilla ahead of mandible); $\text{ANB} < 0^{\circ}$ (negative) indicates skeletal Class III (mandible ahead of maxilla).

What does the Frankfort mandibular plane angle (FMA / FMPA) indicate and what is its mean value?

FMA is the angle between the Frankfort horizontal plane and the mandibular plane; it indicates the vertical growth pattern. Mean is about $25^{\circ}$. A high angle suggests a vertical (hyperdivergent) growth pattern, a low angle a horizontal (hypodivergent) pattern.

State the mean values for the maxillary and mandibular incisor inclinations in the Steiner analysis.

Upper incisor to NA: $22^{\circ}$ angle and $4\text{ mm}$ linear distance; lower incisor to NB: $25^{\circ}$ angle and $4\text{ mm}$ linear distance; interincisal angle mean about $131^{\circ}$.

What is the Frankfort horizontal plane defined by?

The Frankfort horizontal plane is a line joining the porion (superior point of the external auditory meatus / bony ear canal) to the orbitale (lowest point of the bony orbital margin).

List the key steps/considerations in orthodontic treatment planning.

Establishing the problem list and diagnosis, setting treatment objectives, deciding extraction vs. non-extraction, space analysis, anchorage planning, appliance selection, sequencing of mechanics, retention planning, and interdisciplinary coordination (with reference to patient's chief complaint and growth potential).

What is a space (arch length/tooth-size) analysis and what does a discrepancy indicate?

Space analysis compares the space available in the arch with the space required (sum of mesiodistal tooth widths). A negative value (required > available) indicates crowding, while a positive value indicates spacing; it guides extraction decisions.

Differentiate removable and fixed orthodontic appliances.

Removable appliances can be taken out by the patient, apply mainly tipping (single-point) forces, are limited in the movements they achieve, and depend on patient compliance. Fixed appliances are bonded/banded to teeth, deliver controlled bodily movement, rotation and torque simultaneously, and do not rely on compliance for wear.

Name the essential components of a removable orthodontic appliance.

Active components (springs, screws, bows, elastics that move teeth), retentive components (clasps such as Adams and Southend), anchorage components, and the baseplate (connector, usually acrylic) that supports all parts.

What is an Adams clasp and on which teeth is it typically used?

The Adams clasp (modified arrowhead clasp) is the primary retentive component of removable appliances. Its arrowheads engage the mesiobuccal and distobuccal undercuts of a tooth; it is most commonly used on permanent first molars and premolars, fabricated from $0.7\text{ mm}$ stainless steel wire.

What are the main components of a fixed appliance (edgewise system)?

Brackets (bonded to teeth), molar bands/tubes, archwires (round and rectangular), and auxiliaries such as elastomeric/ligature ties, elastic chains, springs, and elastics; the bracket slot dimension (0.018 or 0.022 inch) defines the system.

How do functional appliances work to correct malocclusion?

Functional appliances harness, modify, or redirect the forces of the orofacial musculature (produced by posturing the mandible) and transmit them to the teeth and skeletal structures, thereby influencing growth, dentoalveolar position, and the sagittal jaw relationship—most effective during active growth in Class II cases.

Give three examples of functional appliances.

Activator (Andresen), Bionator, Frankel functional regulator (FR), Twin-block appliance, and Herbst appliance (a fixed functional appliance).

What is the biological basis of orthodontic tooth movement?

Application of a sustained force creates areas of pressure and tension in the periodontal ligament. On the pressure side bone resorption occurs (osteoclasts), and on the tension side bone apposition occurs (osteoblasts), allowing the tooth to move through the alveolar bone in the direction of the force.

Differentiate between the pressure side and tension side in tooth movement.

On the pressure (compression) side, the PDL is compressed, blood flow decreases, and osteoclastic bone resorption predominates. On the tension side, the PDL fibers are stretched, and osteoblastic bone deposition (apposition) predominates.

What is the difference between direct (frontal) and indirect (undermining) resorption?

Direct (frontal) resorption occurs with light, optimal forces where osteoclasts resorb the lamina dura directly, allowing smooth tooth movement. Indirect (undermining) resorption occurs with heavy forces that produce hyalinization (cell-free necrotic PDL); resorption then proceeds from adjacent marrow spaces, delaying movement.

What is hyalinization in orthodontic tooth movement?

Hyalinization is a sterile, cell-free, glass-like degeneration (necrosis) of the compressed periodontal ligament that occurs when excessive force cuts off blood supply. Tooth movement is arrested until the hyalinized tissue is removed by undermining resorption from adjacent marrow spaces.

List the types of orthodontic tooth movement.

Tipping (uncontrolled and controlled), bodily movement (translation), rotation, torque (root movement/inclination change), extrusion, and intrusion.

What is optimal orthodontic force and why is it important?

Optimal force is the lightest force capable of producing maximum tooth movement (via frontal resorption) with minimal tissue damage and patient discomfort—approximately equal to or slightly greater than capillary blood pressure (~$20\text{-}26\text{ g/cm}^{2}$ of root surface). It avoids hyalinization and root resorption.

Define anchorage in orthodontics.

Anchorage is the resistance to unwanted tooth movement. It is the source against which orthodontic force reacts (Newton's third law), determining which teeth (or structures) remain stationary while others are moved.

Classify anchorage according to the number of anchorage units and site.

By site: intraoral (intramaxillary, intermaxillary) and extraoral. By manner of reaction: simple, stationary, and reciprocal. By number of units: single (primary) and compound/reinforced anchorage. Also cortical anchorage and skeletal (implant/TAD) anchorage.

What is a Temporary Anchorage Device (TAD) and what is its advantage?

A TAD is a mini-screw or mini-implant temporarily fixed to bone to provide absolute (skeletal) anchorage. Its advantage is that it provides a stationary anchor independent of patient compliance and produces no reciprocal movement of anchor teeth.

State Angle's classification of malocclusion.

Class I: mesiobuccal cusp of the upper first permanent molar occludes in the buccal groove of the lower first molar (normal molar relation, but malposed teeth). Class II: lower molar distally positioned (Division 1 = proclined upper incisors with overjet; Division 2 = retroclined upper centrals). Class III: lower molar mesially positioned relative to upper.

On what landmark is Angle's classification based, and what is a limitation?

It is based on the anteroposterior (sagittal) relationship of the maxillary and mandibular first permanent molars. A limitation is that it only assesses the sagittal plane, assumes the upper first molar is in a constant/normal position, and does not account for vertical or transverse discrepancies or skeletal patterns.

What is the difference between Class II Division 1 and Division 2 malocclusion?

In Class II Division 1 the maxillary central incisors are proclined (labially inclined) producing an increased overjet. In Class II Division 2 the maxillary central incisors are retroclined, the lateral incisors may be proclined, and overbite is typically deep with minimal overjet.

Differentiate overjet and overbite.

Overjet is the horizontal overlap of the maxillary over the mandibular incisors, measured parallel to the occlusal plane (normal ~$2\text{-}4\text{ mm}$). Overbite is the vertical overlap of the maxillary over the mandibular incisors (normal ~$2\text{-}3\text{ mm}$, roughly one-third of the lower incisor crown).

Classify the etiological factors of malocclusion into general and local factors (Graber).

General (extrinsic) factors include heredity, congenital defects, environment (prenatal/postnatal), predisposing metabolic/disease states, dietary/nutritional problems, abnormal habits, posture, and trauma. Local (intrinsic) factors include anomalies of tooth number, size, shape, abnormal frenum, premature loss or prolonged retention of teeth, delayed eruption, abnormal eruptive path, ankylosis, and caries.

How can oral habits like thumb sucking contribute to malocclusion?

Prolonged thumb/digit sucking can cause proclination of maxillary incisors, increased overjet, anterior open bite, posterior crossbite (narrow maxillary arch from altered cheek/tongue pressures), and retroclination of lower incisors—depending on duration, frequency, and intensity of the habit.

What is serial extraction and why is it performed?

Serial extraction is an interceptive procedure involving planned, sequential removal of deciduous and later selected permanent teeth (classically deciduous canines, then first premolars) in cases of severe arch-length deficiency, to guide erupting permanent teeth into better alignment and relieve crowding.

How does orthodontic treatment in a growing child differ from that in a non-growing adolescent/adult?

In growing patients, growth modification (functional/orthopedic appliances) can correct skeletal discrepancies by redirecting jaw growth, tissue turnover is faster giving quicker tooth movement, and treatment can be simpler. In non-growing (mature) patients, only dentoalveolar camouflage or orthognathic surgery can correct skeletal problems; movement is slower, periodontal support is often reduced, and there is greater relapse tendency requiring careful mechanics and prolonged retention.

Why is growth modification effective only during specific periods, and when is the ideal timing for functional appliance therapy in Class II cases?

Growth modification requires active skeletal growth to redirect; it is ineffective once growth is complete. The ideal timing is at or just before the pubertal growth spurt (peak velocity), so that the appliance can capitalize on maximal mandibular growth to correct the Class II skeletal discrepancy.