Aetiology of Malocclusion Flashcards
what are the possible reasons that malocclusion has increased in incidence and severity over the past 200 years
this could be because of a number of reasons
○ Mixed gene pool?
○ High survival rate of young population?
§ May cause transmission of minor genetic abnormalities which in turn then causes greater facial variation
○ Decreased jaw function due to dietary refinement?
§ We have a more refined diet, we eat softer foods so therefore the size of our jaws and the strength of our masticatory muscles has reduced gradually over the generations
§ This can lead to a mismatch between the size of the jaws and the size of the teeth and therefore result in malocclusion
All of these general factors that occurred over a long period of time may have resulted in the increase in incidence of malocclusion in modern society
what are the general aetiological factors with regards to malocclusion
There are different tissue types we need to consider with regards to malocclusion
• Skeletal
○ Size, shape and relative positions of the upper and lower jaw
• Muscular
○ Form and function of the muscles that surround the teeth ie lips, cheeks and tongue
○ The muscles which act on the jaw bones
• Dentoalveolar
○ Size of the teeth in relation to the size of the jaws
what are the different components of the facial skeleton
- Maxillary base
- Mandibular base
- Maxillary and mandibular alveolar processes
- The maxillary complex is attached to the anterior cranial base while the mandible articulates with the posterior cranial base
what causes variation in the facial skeleton
- The size and shape of the maxilla, mandibular and alveolar processes and their relationship to each other in all three planes of space
- The size and angle of the cranial base ~ can vary and cause a difference in relation to the maxilla and the mandible
what does malocclusion result from
Malocclusion results from disharmony between the components of the facial skeleton
what happens if the width of the maxilla and mandible vary in relation to each other
If they vary in relation to each other could result in crossbites and asymmetry if the mandible is off to one side or the other
Or if the maxilla and the mandible have deviated to one side together during growth
what is the aetiology of skeletal variatiob
Genetic and environmental factors
- Familial studies show strong hereditary component to shade of face and jaws
- Certain features of malocclusion are hereditary eg class III
- If a patient presents complaining of a malocclusion very often one or both of their parents, or one of their siblings or some relative will also have malocclusion
- environmental factors
what environmental factors can cause skeletal variation
○ Masticatory muscles
§ These are strong, powerful muscles that act on the jaw bones during growth so it stands to reason that these can be influencial in the way that the jaws grow
○ Mouth breathing
§ Particularly can affect the way that our faces grow in the vertical dimension
§ Obstructed nasal breathing has been linked to some extent with vertical facial growth
○ Head posture
§ The more forwards and upwards that we carry our heads, the more likely we are to have an increased vertical dimension to the face
These environmental factors may work in a subtle way together and cause variation in facial growth, particularly in the vertical dimension
how should the causes of skeletal variations be considered
Consider aetiology of skeletal variation in the 3 planes of Spee
- Antero-posterior
- Vertical
- Transverse
how do we assess jaw relationship in the antero-posterior plane
We do this by visual inspection
But also by palpating the underlying hard tissues with the fingers like so
what is a class 1 antero-posterior relationship
- cranial base angle normal (around 130-135 degrees)
- maxilla and mandible of normal size and position
- class 1 occlusion
- normal profile (which is slightly convexed)
[the ‘A’ point would be slightly ahead of the ‘B’ point]
although class 1 AP relationship usually means that the teeth have erupted into a class 1 occlusion, what is the other alternative jaw relationship that may be the case in these patients
Jaws usually correctly sized but may have bi-maxillary protrusion or retrusion
Both jaws set back together or set forward together
what is lateral cephalometry
Standardised lateral radiographs of the face and base of skull
Taken from the side in profile
what does it mean that lateral cephalometry radiographs should be reporoducible
Reproducible - patient positioned in a cephalostat a set distance from the cone and the film (ie a set distance from the x-ray source)
what is cephalometry
Cephalometry is the analysis and interpretation of these radiographs
what methods should be included in the radiographic technique
• NHP
• ALARA ○ Aluminium soft tissue fitler ○ Thyroid collar ○ Triangular collimation ○ Rare earth screen ○ LANEX screen ○ Fastest film possible (60-70kV)
• Usual methods are applied to try and minimise the radiation dose whilst still getting an image of good diagnostic quality
how are radiographs analysed
• Hand traced onto paper
○ Old fashioned using a conventional radiographic film
○ Drew lines / angles and made measurements on a light box with a bit of tracing paper
• Digitised using a computer
○ More used now a days
○ Eliminates a certain amount of error that would have been associated with doing the measurements by hand
what is SNA
Relates maxilla to SN line (the anterior cranial base)
Average value 81˚ (+/- 3˚)
Ie this is the maxilla in its normal position
what is SNB
Relates mandible (B point) to SN line (anterior cranial base)
Average value 78˚(+/- 3˚)
what is ANB
Difference between the SNA and SNB angles gives you the ANB angle
This gives a measure of the relative position of the maxilla to the mandible in the antero-posterior plane
= Related mandible to maxilla
Average value 3˚ (+/- 2˚)
What is MxP/MnP
Then in the vertical dimension you are looking at MxP/MnP
The maxillary to mandibular plane angle
Gives a measure of how much the jaws diverge from each other vertically
what does SN stand for
sella to nasion
what is the normal inclination of the upper incisors
inclination of the upper incisors
(A line down the long axis relative to the maxillary plane compared to the angle of the upper incisors)
The normal is about 109˚
what is the inclination angle of the lower incisors
Inclination of lower incisors
Relative to the mandibular plane
Normally around 90-93˚
what is point A
Point A is the deepest concavity on the maxilla above the upper incisors
what is point B
Point B is the deepest concavity below the lower incisors on the mandible
what is class 1 occlusion
- Mesio-bucca cusp of upper 6 is occluding in the middle of the lower 6
- Upper canine occluding between the lower canine and lower 4
- Incisor relationship is class I
what is a class II antero-posterior relationship
- cranial base angle increased (obtuse angle)
- maxilla prognathic / sits too far forward
- class II occlusion (post-normal)
- mandible placed posteriorly relative to maxilla
- convex profile
what are the reasons that the mandible might be placed posteriorly relative to maxilla
○ Mandible too small (most commonly)
○ Or maxilla too large
○ or combination of both
○ Alternatively the mandible is normally sized but placed too far back due to obtuse cranial base angle ie glenoid fossa is set further back and in which case the mandible is articulating further back than the maxilla
what happens when the lower lip is tucked behind the lower incisors
causes a lip trap
what is the SNA, SNB and ANB usually like in class II AP relationship cephalometrics
• SNA usually average but may be increased if maxilla prognathic
Ie if the maxilla is set too far forward
• SNB usually decreased
Because the mandible is set too far back
• ANB > 5˚
what is a class III antero-posterior relationship
- cranial base angle decreased (acute angle)
- maxilla small and retrognathic
- class III occlusion (pre-normal)
- mandible placed anteriorly relative to maxilla
- concave profile
what are the reasons that the mandible may be placed anteriorly relative to maxilla
○ Maxilla too small (most commonly)
~ Maxilla is either deficient or set back
○ Or mandible too large
~ Prognathic and set forwards
○ Or combination of both
○ Alternatively: normally sized jaws but mandible positioned too far forwards due to acute cranial base angle
Angle more closed so carried mandible forwards into a more prominent position
what is the SNA, SNB and ANB usually like in class III AP relationship cephalometrics
- Expect SNA to be decreased if maxilla deficient
- SNB often average but may be increased if mandible prognathic
- ANB < 1˚or negative
what can disguise underlying skeletal discrepancy
Dento-alveolar structures may disguise underlying skeletal discrepancy
what do forces from lips, cheeks and tongue tend to do
Forces from lips, cheeks and tongue tend to incline teeth towards a position of soft tissue balance
what is dento-alveolar compensation
In class III cases in particular, as the class III skeletal pattern develops during growth there is a tendency for the soft tissues and the muscles surrounding the teeth (the lips and the tongue) to tip the teeth towards each other in order to minimise the reverse overjet This is what we call compensation
what do we use to measure vertical facial dimension
To measure vertical facial dimension we use both angular and proportional measurements
what is the frankfurt plane
○ Lower orbital rim to superior border of external auditory meatus
○ This is the angular measurement most commonly used
○ Extends from the inferior orbital rim (orbitale) to the superior border of the external auditory meatus (which is represented by the tragus of the ear clinically)
what is the mandibular plane
○ Lower border of mandible
○ Extends from gonion (a point at the angle of the mandible) through to the mentum (which is at the inferior border of the symphysis of the mandible)
where does the frankfurt plane and mandibular plane normally meet
Planes normally meet at the external occipital protuberance
Clinically these 2 planes normally meet around the back of the head at the external occipital protuberance
where is the upper anterior face height measured from
Brow ridge (glabella) to base of nose
where is the lower anterior face height measured from
Base of nose (sub nasale) to inferior aspect of chin (soft tissue menton)
what are the average clinical values of the vertical jaw relationships
Average ratio of LAFH to TAFH = 50%
TAFH = Total Anterior Face Height
This is what a ‘normal face’ ratio would be
what is the average value of FMPA
Average value of FMPA = 27˚ (+/- 4˚)
Same value as the maxillary-mandibular plane angle
The 2 are used inter-changeably (although there can be variation between them particularly because of tipping of the maxillary plane)
what are the proportions of the upper anterior face height
○ Nasion to Anterior Nasal Spine
○ Instead of glabella the nasion is used ~ this is the frontal nasal suture
○ Anterior nasal spine is at the front of the maxillary plane
what are the proportions of the lower anterior face height
Anterior Nasal Spine to Menton
It is hard tissue menton instead of soft tissue ie the bottom of the bony mandibular symphysis on the lateral ceph
what are is the average value of LAFH to TAFH with regards to average cephalometric values
Average value of LAFH to TAFH = 55%
This gives a different ratio
why is there a difference between the proportions clinically compared with cephalometric
Reason for difference is our eyes can easily divide something into 2 (50;50) but the computer allows for more sophisticated measurements
what is long face syndrome
- significantly increased LAFH
- Has a very convex profile
- There is an appearance of the chin being quite set back and down in comparison to the normal position
- In addition he is trying to posture his lips together because they would tend to be incompetent at rest ie the lips would not sit together normally unless he uses the mentalis muscle to posture the lower lip up into contact with the upper lip
what happens to the occlusion if the mandibular has grown down and back in quite an extreme way
the alveolar processes are very deep
The upper molars have erupted a great deal more than you would normally expect
Anteriorly the incisors have erupted as much as possible but not enough to actually come into occlusion so the patient has a skeletal anterior open bite
what happens with long facial type in a vertical jaw relationship
Increased vertically
• LAFH to TAFH proportion > 55% ~ increased
• FMPA > 31˚
○ Outside the standard deviation of 27˚ +/- 4
- Steeply inclined mandibular plane
- Backward mandibular growth rotation
- Anterior open bite tendency
what are the features when the UAFH is greater than the LAFH
○ The UAFH is greater than the LAFH = reduced LAFH
○ The angle of the lower border of the mandible is much flatter
○ Instead of the chin being set back, there is more of well defined chin point
○ These are hallmarks of a more anterior direction of mandibular growth
○ The lower lip appears relaxed and fits comfortably with the upper lip at rest rather than the patient having to strain the mentalis muscle
what happens to the occlusion in patients with short facial types
The dento-alveolar processes are much shallower as the molars haven’t had to erupt as far to come into occlusion
Instead of an anterior open bite, the incisors have erupted past each other into an increased overbite
what is short facial type clinically
- LAFH to TAFH proportion < 55%
- FMPA < 23˚
- Tendency to parallelism of jaws
- Forward mandibular growth rotation
- Deep overbite tendency
what is short facial type like in cephalometric values
• Cephalometric Values
○ FMPA = 20˚
○ Can see how far towards the back of the head it would take these lines to travel before they would meet
○ Jaws much more parallel
○ And the LAFH should be 55% because it is a lateral ceph so this is a reduced proportion
what are arch width discrepancies
Disproportion of maxillary and mandibular dental arches widths
This directly reflects the width of the underlying skeletal base
what does arch width discrepancies cause
Causes unilateral or bilateral buccal segment cross-bites
Affects occlusion
what can exaggerate arch width discrepancies
Often exaggerated by antero-posterior discrepancies
○ If you have 2 dental arches that fit nicely together that you then move into a class III position that you then create a posterior cross-bite
○ It is possible to have correctly proportioned dental arches but with buccal segment cross-bites because of the underlying antero-posterior discrepancy
○ Then for example if there was a narrow maxilla as well as a class III jaw discrepancy the tendency is exaggerated
what happens with a narrow maxilla and a normal width mandible
narrow maxilla and possibly a normal width of mandible
Result is bilateral buccal segment cross-bites
what happens with a broad maxlla and a contracted / smaller mandibular arch
broad maxillary arch as well as a contracted mandibular arch
Result = sitted bite (where upper teeth are biting entirely outside the lower teeth)
Not very common
when does mandibular displacement occur
Occurs when inter-arch width discrepancy causes upper and lower posterior teeth to meet cusp to cusp
Mandible forced to deviate to one side to achieve position of inter-cuspation
Deviating mandible following initial contact of teeth
what can mandibular displacement have a possible association with
Possible association with TMD
what is the dental cause of facial asymmetries
Displacement of normal mandible due to unilateral cross-bite
Ie this can be caused by the situation which we have just looked at where there is not actually an asymmetry of the jaw but the patient appears to have a jaw asymmetry because they are placing their mandible off to one side to gain a comfortable occlusion
Important to assess whether the asymmetry is a dental cause or whether it is a true mandibular asymmetry
what are true mandibular asymmetries
○ Hemi-mandibular hyperplasia / elongation
§ More non-specific cause
§ No clearly defined cause but are more like asymmetrical growth of the mandible
○ Condylar hyperplasia
§ Probably the most common cause of true mandibular asymmetry
○ These different conditions tend to occur together in various combinations
They are not always completely alone in their cause of an asymmetry
what is dento-alveolar disproportion
Discrepancy between size of teeth and jaws
what is crowding caused by
○ Small jaws, normally sized teeth
○ Large teeth (macrodontia) ~ Not particularly common
what is spacing caused by
○ Large jaws, normally sized teeth
○ Small teeth (microdontia)
§ More common
§ Particularly seen accompanying hypodontia or missing teeth
