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Оглавление4 Aetiology of Malocclusion
This chapter will look at all the different types of causes for the types of malocclusions we see in orthodontics.
4.1 Skeletal Factors
Skeletal factors are categorised into all three planes of space.
4.1.1 Anteroposterior Plane (AP)
Dento‐alveolar compensation is a term used when teeth compensate for the skeletal pattern. For example, a patient may have class I incisors but be presenting with a class III skeletal pattern.
4.1.2 Vertical Plane
Growth rotations occur in the mandible. This is extra growth which can be seen once the patient has stopped growing. There are two types:
Forward growth rotation – decreased lower anterior facial height (LAFH).
Backward growth rotation – increased LAFH.
4.1.3 Transverse Plane
Absolute transverse maxillary deficiency:
This term relates to crossbites and is most commonly seen in class III cases.
Commonly seen in maxillary retrusion, this is where the maxilla is reduced in all three dimensions, which can result in crossbites anteriorly and posteriorly.
Dento‐alveolar compensation:
This term is used when teeth compensate for the skeletal pattern.
In this case transversely, teeth can compensate to result in a crossbite being present.
For example, tipping of the buccal surface of the upper molar and lingual surface of the lower molar compensates for transverse maxillary deficiency.
4.2 Soft Tissue Factors
4.2.1 Fullness and Tone of the Lips
The fullness and tone of lips can have an effect on the dentition, as they can result in positioning of the incisors in the labiolingual direction. The effects can include the following:
Lips that lack muscular tone and are flaccid – incisors tend to be proclined.
Strap‐like lips:Lips are tense and the incisors tend to be retroclined.The term highly active lip can also be used to describe this.
Lower lip line:The lower lip should cover one‐third to one‐half of the upper incisor crowns.More than one‐half coverage = retroclined upper incisors.Less than one‐third coverage = proclined upper incisors.
4.2.2 Lip Competency
Lip competency looks at how the lips are when the patient is at rest. There are two types of lip competency:
Competent lips, which is where minimal effort is required to achieve an oral seal.
Incompetent lips, which is where excessive muscular activity is required to achieve an oral seal.
With incompetent lips the following features can be found:Can often be associated with a low lower lip line.Swallowing patterns may occur to achieve an anterior oral seal. If excessive muscular activity or forward mandibular posturing is not possible to achieve, then the following swallowing patterns can occur:Tongue to lower lip results in incisor proclination.Lower lip to palate results in upper incisor proclination and lower incisor retroclination.Tongue to upper lip results in upper incisor proclination.
4.2.3 Macroglossia (Large Tongue)
The size of the tongue can affect the development of the dentition. The tongue can also impede incisor eruption and anterior open bite (AOB). The tongue can present with two types of tongue thrusts:
Adaptive tongue thrust: this can be corrected and stopped once the positions of the teeth are corrected. It is seen when the tongue is forced between the teeth.
Endogenous tongue thrust: this is where the tongue is thrusted forward between the teeth, but is a habit that may not be stopped once tooth positions have been corrected. This can result in relapse.
4.2.4 Enlarged Adenoids
Adenoids are large glands which are located by the soft palate in the roof of the mouth. Enlarged adenoids can result in the development of an AOB due to constant mouth breathing. This has the effect of the molars over‐erupting and resulting in an increased LAFH.
4.2.5 Generalised Pathology of Muscles
Muscle weakness can also have an effect on the dentition. Weak muscles enable the teeth to move due to the lack of support of the soft tissues surrounding the dentition.
4.3 Local Factors
Local factors are divided into three groups:
Variation in tooth number
Variation in tooth size
Variation in tooth position.
4.3.1 Variation in Tooth Number
4.3.1.1 Hypodontia
Hypodontia is known as missing teeth.
Hypodontia is found when there is an absence of one or more primary or secondary teeth, excluding third molars.
The most common cause can be genetic, with a family history of hypodontia.
The most common missing teeth are upper third molars (U8s), lower third molars (L8s), lower premolars (L5s), upper lateral incisors (U2s), upper premolars (U5s), and lower central incisors (L1s).
4.3.1.2 Supernumerary
Supernumerary teeth are extra teeth.
Supernumerary teeth are found when there are excessive teeth in the normal series.
The most commonly found supernumerary is in the anterior maxillary region.
The types that can be found are conical, also known as mesiodens, tuberculate, supplemental, and odontomes.
4.3.1.3 Early Loss of Deciduous Teeth
There could be numerous reasons for early loss of deciduous teeth.
Common causes could be caries, trauma, or root resorption.
Early loss could result in crowding and space loss, which could possibly lead to impaction.
For example, early loss of a lower deciduous E can result in mesial movement of the lower first molar, making no space for the lower premolar to erupt, resulting in impaction.
4.3.1.4 Extraction of Permanent Tooth
Most commonly a first permanent molar.
Extraction of a first permanent molar can result in crowding and space loss, which could possibly lead to impaction.
Reasons for a first permanent tooth being extracted could be poor diet and oral hygiene, which can lead to caries.
4.3.2 Variation in Tooth Size
4.3.2.1 Macrodont
A large tooth is known as a macrodont.
This tooth will present with a large clinical crown.
Macrodonts can be associated with supernumaries.
A clinical feature that can result from macrodonts is dental crowding.
4.3.2.2 Microdont
A small tooth is known as a microdont.
This tooth will present with a small clinical crown.
Microdonts can often be associated with hypodontia.
A clinical feature that can result from microdonts is spacing.
4.3.2.3 Dento‐alveolar Disproportion
This is a term that relates to the teeth and jaws.
Dento‐alveolar disproportion is found when there is a mismatch in tooth and jaw size.
For example, patients could present with big jaws but small (microdontia) teeth or small jaws and big (macrodontia) teeth.
The clinical result that can be found with this is crowding or spacing.
4.3.2.4 Bolton Discrepancy
This is an analysis which determines the size discrepancy between the size of the maxillary and mandibular teeth.
It looks at the mismatch in tooth size and helps to analyse the optimum inter‐arch relationship.
Measurements are taken of the mesio and distal widths of all the teeth.
This analysis was developed to achieve the ideal occlusion and inter‐digitation.
4.3.3 Variation in Tooth Position
4.3.3.1 Infraocclusion (Ankylosis)
Infraocclusion is also known as ankylosis.
Ankylosis is when failure of eruption of a tooth occurs due to the anatomical fusion of cementum and alveolar bone.
When a tooth is ankylosed, the tooth will submerge relative to its neighbours. It is important to know that when this happens the tooth is not sinking, its neighbouring teeth are erupting alongside the alveolar complex.
The most commonly affected deciduous teeth are the first and second deciduous molars (Ds and Es).
The most common cause of this is genetic and the condition has a high occurrence in patients with hypodontia.
4.3.3.2 Ectopic Tooth
This is a term used to describe a certain type of eruption of a tooth.
An ectopic tooth is a tooth that manages to erupt, but may erupt at an angle or in an aberrant position, rather than emerging in its correct position.
4.3.3.3 Impacted Teeth
Impaction occurs when there is failure of eruption due to an obstruction such as tissue, bone, or another tooth.
It occurs mostly when crowding is present.
The most common teeth that are impacted are the upper canines (U3s), upper central incisors (U1s), and lower first permanent molars (L6s).
4.3.3.4 Transposition
Transposition is a term used when the anatomical positions of teeth are interchanged.
The most commonly affected teeth are the maxillary canines and first premolars and mandibular lateral incisors and mandibular canines.
There are two types of transposition:True transposition is found when the roots and crowns of the teeth have completely interchanged.False transposition is found when the roots are in the correct position but the crowns have interchanged.
4.3.3.5 Primary Failure of Eruption
This is seen when a tooth fails to fully erupt.
It usually has a strong genetic basis.
The most commonly affected teeth are the first and second permanent molars.
4.4 Habit
A thumb or finger sucking habit may exist until at least the age of 6–7 years. It can have a significant effect on occlusion, depending on the duration and intensity. Forces (intensity of the habit) acting more than six hours a day result in tooth movement such as:
AOB
Increased overjet
Buccal crossbites.
4.5 Fraenal Attachments
4.5.1 Upper Labial Fraenum
Patients who have a low fraenal attachment may present with a maxillary midline diastema.
If the patient has an alveolar cleft between the incisors, a fraenectomy may be needed at the end of treatment.
If the patient does not have a fraenectomy, this can result in the diastema reopening post‐treatment.
4.5.2 Lower Labial Fraenum
A lower fraenal attachment can result in the following:
Diastema
Poor oral health
Recession at the gingival margin.
