Aetiology of Malocclusion I Flashcards
What is malocclusion, and why is it prevalent in Western developed countries?
Malocclusion refers to the misalignment or incorrect relation of the teeth when jaws are closed.
Prevalence reasons:
- Mixed gene pool.
- High survival rates of young populations.
- Decreased jaw function due to dietary refinement.
What are the general aetiological factors of malocclusion?
- Skeletal: Size, shape, and relative positions of the upper and lower jaws.
- Muscular: Form and function of muscles surrounding teeth (lips, cheeks, tongue).
- Dentoalveolar: Size of teeth in relation to jaw size.
Name the components of the facial skeleton.
Maxillary base.
Mandibular base.
Maxillary and mandibular alveolar processes.
Maxillary complex attachment to the anterior cranial base.
Mandibular articulation with the posterior cranial base.
How does variation in the facial skeleton lead to malocclusion?
Malocclusion results from disharmony between components of the facial skeleton, such as size, shape, and relationships of the maxilla, mandible, and alveolar processes in all three planes of space.
Influencing factors include the cranial base size and angle.
What are the genetic and environmental factors contributing to skeletal variation?
Genetic:
- Strong hereditary component observed in familial studies.
- Class III malocclusion often has hereditary features.
Environmental:
- Masticatory muscle function.
- Mouth breathing.
- Head posture.
What are the three planes in which skeletal variation is classified?
Antero-posterior.
Vertical.
Transverse.
Describe the antero-posterior relationship for a Class I jaw.
Mandible is normally related to the maxilla, allowing teeth to erupt into Class I occlusion.
Both jaws are typically correctly sized but may exhibit bi-maxillary protrusion or retrusion.
What is lateral cephalometry, and why is it important?
Lateral cephalometry involves standardised lateral radiographs of the face and skull base.
Importance:
- Reproducible results. (patient positioned in a cephalostat at a set distance from the cone and film)
- Aids in analysis and interpretation of skeletal and dental relationships.
What is the key radiographic technique used in lateral cephalometry?
Use of Natural Head Position (NHP).
Adherence to the ALARA principle (minimise radiation exposure).
Aluminium soft tissue filter and thyroid collar.
Triangular collimation.
Rare earth or LANEX screens with the fastest possible film (60-70kV).
What methods are used for cephalometric analysis?
- Hand tracing onto paper using a light box.
- Computer digitisation for precise measurement.
- Software analysis, e.g., OPAL, providing detailed metrics.
Explain the cephalometric angles SNA, SNB, and ANB for a Class I relationship.
SNA: Relates the maxilla to the anterior cranial base.
- Average value: 81° ± 3°.
SNB: Relates the mandible to the anterior cranial base.
- Average value: 78° ± 3°.
ANB: Relates the mandible to the maxilla.
- Average value: 3° ± 2°.
Describe the antero-posterior relationship for a Class II skeletal pattern.
what are the causal factors and what happens to teeth
Mandible: Positioned posteriorly relative to the maxilla.
Causal factors:
- Mandible too small (most common).
- Maxilla too large.
- Combination of both.
- Cranial base angle may be obtuse.
Teeth: Erupt into post-normal (Class II) occlusion.
What are the cephalometric changes observed in a Class II skeletal pattern?
SNA: Average or increased if the maxilla is prognathic.
SNB: Usually decreased.
ANB: Greater than 5°.
Describe the antero-posterior relationship for a Class III skeletal pattern.
what are the causal factors and what happens to teeth
Mandible: Positioned anteriorly relative to the maxilla.
Causal factors:
* Maxilla too small (most common).
* Mandible too large.
* Combination of both.
* Cranial base angle may be acute.
Teeth: Erupt into pre-normal (Class III) occlusion.
What are the cephalometric changes observed in a Class III skeletal pattern?
SNA: Decreased if the maxilla is deficient.
SNB: Often average but may increase if the mandible is prognathic.
ANB: Less than 1° or negative.
What is dento-alveolar compensation?
A mechanism where dento-alveolar structures compensate for skeletal discrepancies.
Forces from lips, cheeks, and tongue incline teeth into a position of soft tissue balance.
How does dento-alveolar compensation manifest in severe Class III malocclusion?
Upper incisors: Proclined.
Lower incisors: Retroclined.
Helps disguise underlying skeletal discrepancies.
Define the Frankfurt and mandibular planes and their meeting point.
Frankfurt plane: Lower orbital rim to the superior border of the external auditory meatus.
Mandibular plane: Lower border of the mandible.
Intersection point: External occipital protuberance.
What are the clinical parameters used to assess vertical jaw relationships?
Upper anterior face height (UAFH): Brow ridge (glabella) to the base of the nose.
Lower anterior face height (LAFH): Base of the nose (subnasale) to the inferior aspect of the chin (soft tissue menton).
Proportional ratio: LAFH to TAFH (total anterior face height) = 50%
What are the average cephalometric values for vertical jaw relationships?
Frankfurt plane: Orbitale to Porion.
Mandibular plane: Menton to Gonion.
Average FMPA: 27° ± 4°.
LAFH: Nasion to Anterior Nasal Spine.
TAFH proportion: LAFH/TAFH = 55%.
Describe the characteristics of a short facial type.
Proportions: LAFH < 55%; FMPA < 23°.
Jaw alignment: Tendency towards parallelism.
Growth pattern: Forward mandibular growth rotation.
Bite tendency: Deep overbite.
Describe the characteristics of a long facial type.
Proportions: LAFH > 55%; FMPA > 31°.
Mandibular plane: Steeply inclined.
Growth pattern: Backward mandibular growth rotation.
Bite tendency: Anterior open bite.
What causes transverse arch width discrepancies?
what does this result in?
Disproportion between maxillary and mandibular dental arches.
Results in unilateral or bilateral buccal segment cross-bites, often exaggerated by antero-posterior discrepancies.
Explain mandibular displacement due to transverse discrepancies.
what may this lead to
Inter-arch width discrepancies cause cusp-to-cusp occlusion of posterior teeth.
Mandible deviates to one side to achieve inter-cuspation.
May lead to temporomandibular disorders (TMD).
What are common causes of facial asymmetry?
Dental causes: Displacement of the mandible due to unilateral cross-bite.
True mandibular asymmetry:
- Hemi-mandibular hyperplasia or elongation.
- Condylar hyperplasia.
Severe cases: Mild expressions of hemi-facial microsomia.
What factors contribute to dento-alveolar disproportion?
Crowding:
- Small jaws, normally sized teeth.
- Large teeth (macrodontia).
Spacing:
- Large jaws, normally sized teeth.
- Small teeth (microdontia).
