Aetiology of malocclusion Flashcards
Factors affecting tooth position
Skeletal pattern
Soft tissue
Dental factors
Local factors
Definition of skeletal classification
Relative positions of the jaws to one another due to the positions of the dental follicles when forming on the dental lamina. This in turn affects the relative positions of the teeth to one another.
Skeletal pattern planes of space considerations
Anterioposterior
Vertical
Transverse (looking from the front)
Anterioposterior (profile) assessment looks at
The relationship of top and bottom jaw
Can be class 1, 2, 3
Lower jaw should be slightly set back
Regions of the skull
1 and 2. skull
- Maxilla
- Mandible
- Dental bases
Class 2 aetiology
86% mandibular retrusion
14% other maxillary protrusion
Class 3 aetiology (more variation)
26% both
34% maxillary retrusion
40% mandibular protrusion
Vertical relationships splits head into 3rds
and mandibular plane ratio
Bottom of nose and bottom chin should be equal to nose to eyebrows (2&3 sections)
Frankfort mandibular plane definition
Angle formed by the intersection of the Frankfort horizontal plane and mandibular angle
Frankfort angle normal range
25 +/- 5 degrees
FMP clinical assessment
Palpate lower border of mandible and imagine ff plane
Where these 2 lines meet
Increased FMP =
Anterior open bite
Very steep mandibular plane angle
The lines meet anterior to the back of head
30 degrees or more
Decreased FMP angle =
Deep bite
Lines meet off back of the head
Reduced lower anterior face height
20 degrees or less
Crossbite comes from narrow upper or wide lower arch. It =
Upper molar buccal cusps meet in the groove of the lower molars
Asymmetry
Most patients have slight asymmetry
Asymmetry caused by
Enlargement of one side of face
Reduced size of one side of face
Teeth are in the neutral zone
Position of stability between tongue and lips
The forces balance each other
Tongue force increases during
Eating, speaking but is transient
Lip length problems
Short lips = incompetency
-can see upper gums
Gummy smile: vertical maxillary excess (when upper jaw is displaced lower than it should be)
Potentially competent lips
Lips would meet if overjet was reduced
Are digit and dummy sucking problems?
Dummy sucking not really a problem as decreases with age. 50% of babies start sucking dummy but by age 5 less than 5% still do (incisors erupt at 6&7)
Digit sucking: 1/3 of babies suck thumb. Around 20% sucking digits and 6/7 which will affect position of teeth (as this is when incisors will erupt)
Asymmetry can be caused by thumb sucking
Asymmetrical open bite
Don’t treat them until they’ve stopped sucking
Dental factors affecting tooth position
Tooth-tissue ratio
Number of teeth (missing teeth, extra teeth)
Relative position and path of eruption of teeth
Local factors influencing tooth position
Habits
Effect of labial fraenum
Pathological conditions
Skeletal class I
Basal bone of mandible is normal in relation to maxilla
Skeletal class II
Basal bone of mandible is post-normal to maxilla
Skeletal class III
Basal bone of mandible is pre-normal to maxilla
Possible causes of antero-posterior class II relationship
Discrepancy of sizes of jaws (rare) Protrusive maxilla (not common) Retrusive mandible (most common) -mandibular joint set further back on cranial base
Possible causes of antero-posterior class III relationship
Often a true size discrepancy
Skeletal pattern - vertical relation
Increased lower face height results in < OB or ANB
Decreased lower face height results in deep OB
-both difficult to treat without favourable growth
Skeletal pattern - lateral relation
Imbalance –> X-bite
-buccal
-lingual
X-bite may be result of antero-posterior discrepancy in class III skeletal pattern
Soft tissue factors - lips. Consider:
- Size and form
- competent or incompetent - Function
- lip line
Incompetent lips
Apart at rest
-rest = minimal evidence of muscular or electro-muscular activity
Up to 80% of children may have incompetent lips at some time
-may gradually become competent in time due to differential growth rates of skeletal and soft tissue elements
Aetiology of incompetent lips
Size: i.e. short upper lip
Skeletal discrepancy: as muscles, lips are related to bones of facial skeleton
-skeletal pattern may determine whether lips may be brought together completely
-may be affected by antero-posterior or vertical discrepancy
Lip seal may be obtained by 3 factors
Muscular effort
Mandibular posture
Lip/ tongue contact (depends on lip line)
What is the lip line?
The relationship of lower lip to upper incisors
A high lip line
Severe antero-posterior skeletal discrepancy and class II div 1 incisor relationship --> lower lip behind upper incisors on palate or teeth High lip line + reduced lower face height --> class II div 2 incisor relationship since lower lip will be on labial side of upper incisors
A low lip line
Lower lip contacts tongue thus dispersing upper incisors
STRAP LOWER LIP particular problem (rare)
-hyperactive lower lip
-any OJ must be fully reduced in order to be stable
-deep mantalis fold is a sign
Soft tissue factors - tongue. Consider:
- Anatomy: extrisnic muscles of tongue are attached to hyoid, styloid and palatal bones
- Size
- Function: action usually secondary to lips but will have reinforcing effect. Adaptive in mastication, swallowing (important) and speech
Effects due to size of tongue
- apparently large tongue may improve as child matures due to > in lower face height with growth and tongue coming to rest lower in mouth
- if upper jaw > lower jaw, tongue may rest between them and prevent vertical development of posterior teeth
- large tongue may be factor in relapse after orthognathic surgery (class III cases)
How many times do we swallow per day?
How much time does it take to swallow
~1000 times
~1s
The infantile swallow
Mandible held in lower position
Tongue lower and further forward from pharyngeal wall
Infant takes in air through nose
Feeding by suckling milk
-milk ducts surrounded by smooth muscle and are stimulating by suckling
-milk directed into tongue, which is over lower lip
“Process by which babies swallow before weaning. Breathing occurs through the nose. Feeding is done by ‘suckling and not sucking. The tongue covers the lower lip during swallowing.”
The normal adult swallow
Lips are closed
Teeth in light occlusal contact
Tongue elevated to palate
Teeth clenched tightly as food is pressed between palate and floor of mouth
What age do humans change to the adult swallow
By age 6, 50% of children have adult swallow
Others are in period of transition
Types of adaptive swallow
Teeth apart (lip incompetence) Teeth together (< OB, ANB) Endogenous tongue thrust swallow
Teeth apart adaptive swallow
If tongue does not move up into upper arch, there may be a narrowing of upper arch, a buccal X-bite and lack of full development of buccal segments
-responds to tx
Teeth together adaptive swallow
Tongue is positioned forward
-responds to tx
Endogenous tongue thrust swallow
Primary an atypical neuromuscular pattern
Not the same as an infantile swallow
Occurs in <3% of cases
Not treatable
Amount of typical masticatory forces
15kg for 11min/day
Amount of typical swallowing forces
200kg for 1s
Amount of typical speech forces
100kg for 0.2s
Amount of typical force for relaxed muscles
10g for 24hrs/day
Is the amount of force or the intensity more important?
Duration > intensity
Most significant factor is continuous resting force from soft tissues
Mandibular positions
Non-occlusal
-rest
-adaptive postural
Occlusal
-RCP (condyles in most posterior position)
-ICP (ideally should be same as RCP)
-ligamentous (forced retruded position, amount limited by ligaments, usually 2mm posterior to ICP)
Mandibular rest position
Endogenous postural position
Muscles relaxed, condyles retruded, mandible unstrained
Freeway space i.e. inter-occlusal clearance
Controlled by feedback mechanisms
Not fixed
Mandibular rest position affected by
Head posture: when head is tilted back, FWS increases. When tilted forward, FWS decreases
Age
Loss of mandibular teeth brings mandible closer to maxilla
Adaptive postural positions are adopted in order to
Maintain oral seal
Achieve oral respiration
In which incisor relatioships are adaptive postural positions adopted
Class I div 1 when there is adequate lip length - mandible postured forwards
Class II div 2: lower jaw may be postured forwards for comfort and function
Class II incisors: forward posture if teeth would meet edge to edge in rest position
Mandibular movements: control
Under both voluntary control and reflex activities governed by sensory and proprioceptive impulses
Types of mandibular movements
Normal to occlusion: simple hinge from rest over 3mm
Deviation: closure from postured position
Displacement: starts from rest but may displace from normal due to premature contact
Types of displacement
Lateral displacement (crossbite)
-usually due to narrow upper arch
-posterior teeth meet cusp to cusp or crossbite
-must shift jaw right or left to get teeth into ICP
Antero-posterior. e.g. with an instanding anterior tooth (commonly a
palatal upper lateral incisor) or in a mild Class III case where the patient
can achieve an edge-to-edge incisal occlusion but then postures forward
into a more comfortable bite
Posterior. Due to overclosure with loss of posterior teeth
Tooth/ tissue ratio
Imbalance in ratio of tooth and jaw size will produce crowding or spacing of dentition
Possible aetiological genetic factors in crowding
Independent genetic control of teeth and jaws
Evolution - jaws are shrinking over time
Outbreeding - intermixing of racial/ ethnic groups
Diet - less gritty so we don’t wear out teeth as much
-wearing teeth below contact point –> enough space for teeth
Prevalence of crowding
60-70% of UK population
Depends on ethnic group
Crowding in deciduous dentition unusual
-more often expressed as lack of normal spacing
40% increase in severity of crowding over last 100,000 yrs
Possible effects of crowding on the dentition
Overlapping and displacement
Impaction
Mesial movement of teeth
Causes of generalised spacing
Hypodontia Small teeth (narrower)/ well developed arches Unusual in UK population (localised spacing = diastema)
Missing teeth: terms used for congenital absence
Anodontia: all teeth missing
Oligodontia: several teeth missing
Hypodontia: a few teeth missing
Aetiology of congenital absence of teeth
May be linked to hereditary syndrome e.g. ectodermal dysplasia
-sex-linked recessive condition
-lack of sweat glands, sebaceous glands and hair follicles
-some degrees of anodontia or malformed teeth
Cleft lip and palate: absence of teeth may be due to surgery carried out in region of developing tooth germs
Often family history of some missing teeth
Most common missing teeth
Lower 8s: 13.6% Upper 8s: 11% Lower 5s: 3% Upper 2s: 1.8% Upper 5s: 1.1% Lower incisor: 0.3%
Effects of developmental hypodontia
- Form of teeth: often other teeth may be smaller or malformed (e.g. peg shaped laterals)
- Position of teeth: may be affected but will be minimal if there are only one or two teeth missing
- Growth: severe hypodontia may affect alveolar bone and > FWS (but will not > rest position of mandible)
Missing teeth can be due to
Congenital absence
Acquired loss of teeth
Acquired loss of teeth refers to
Premature loss of deciduous teeth
Loss of permanent teeth
Retention of deciduous teeth (can lead to upper 2s palatal deflection)
Delayed eruption of permanent teeth (normal variation, missing teeth, impeded eruption, trauma)
Extra teeth can be either
2% of pts Supplemental Midline (premaxillary) supernumeraries -conical (mesiodens) -tuberculate
Supplemental teeth
- what are they
- where do they occur
- who gets them
- what do they lead to
- treatment
Normal form
Usually occur at end of series
Europeans may have extra upper or lower 2
Africans have > prevalence of 9s and extra premolars
May > crowding potential of dentition
Usually treated by removal of suitable tooth
Conical midline supernumeraries (mesiodens) characteristics and treatment
Develops early Root formation coincides with upper central May erupt Usually displace other teeth Does not delay eruption May be inverted Tx: extract supernumerary unless it is high, inverted or removal would damage other teeth Usually between 1/1
What do tuberculate teeth represent and why?
Third dentition
- develops later than upper 1
- does not usually erupt
- delays eruption of 1
- other teeth may move into space
- barrel shaped
Development of tuberculate teeth
On palatal aspect of upper central
May be unilateral, bilateral or not associated with other supernumerary teeth
Treatment of tuberculate teeth
Extract ASAP but other teeth often remain high when erupted or require ortho
NB essential to carry out full ortho assessment before dealing with supernumeraries
-space must be created for permanent teeth to erupt into
-any associated malocclusion must be dealt with
Position and form of teeth affected by
- Shape and size
- germination
- fusion
- macrodontia (teeth fused together)
- microdontia (e.g. peg shaped laterals) - Transposition (teeth swap places) and displacement (especially upper 3s)
- very difficult to correct - Traumatic displacement e.g. primary tooth pushed up into gum –> damage to adult teeth
- Impaction
Examples of habits
Thumb/ finger/ dummy sucking
Force and habits
According to equilibrium theory, force duration more important than magnitude
To affect teeth significantly, force applied for min 6hrs/day
Prevalence of habits
> 50% infants suck digits between 6m-2yrs
Usually stopped by 4 years
Effects of habits
No long term effects in deciduous dentition
In permanent dentition
-flared, spaced upper incisors
-lingually inclined lower incisors (but dependent on method of sucking - may be proclined)
-< OB or AOB
-narrowed upper arch
Does not affect molar relationship
Treatment for habits
If habit stops early, OJ may < within 1 yr
Tongue may have adapted to < OB and prevent closing
Development of labial frenum
Remnant of tecto-labial bands
Precedes tooth development (and therefore transeptal fibres)
Attached to papilla at birth and migrates with eruption of deciduous and permanent molars
Usually attaches to muscosa over labial plate
-sometimes attaches to crest of alveolar bone before tooth eruption (determined by 7 weeks IUL when there is separation of lip and alveolus by invaginating lip furrow band, familial tendency)
Histology of labial frenum
Not muscle
Consists of vascularised collagen fibres
Interrupts transeptal fibres
Effects of labial frenum
Midline spacing often associated with large fleshy fraenum
Space closure will relapse in some cases unless fraenetomy performed
Diagnosis of low labial frenum attachment
Blanching on palatal mucosa when fraenum put under tension
V-shaped notch on radiograph
Treatment of low labial frenum attachment
Fraenectomy
-not until upper 2s and 3s present (since 90% diastemas close spontaneously)
Either close space before or after surgery: most people recommend space closing first
Bonded retainer???
Local factors: pathology
Rare and usually self-evident
- fracture
- cyst
- neoplasm
- inflammatory condition
Infraoccluded teeth
Due to missing permanent tooth OR ankylosed root to bone (primary tooth)
Can submerge too much and drop below gum line –> surgical extraction
In which stage of dentition should the problem of a sucking habit be dealt with?
Late mixed dentition
