Lecture no. (2 + 3 + 4)
Done by: Meyassarah Samman
N.B.: Notes are in green italic
The etiology of the orthodontic problems
Orthodontics: straight teeth + face
What are the possible causes of malocclusion
Thinking carefully about what may be causing a certain malocclusion, this will:
1-allow us to point out one (or multiple) causal explanation
2-enable us to predict them (we may not be able to prevent them)
3-we may try to alter causal conditions to block undesirable results
Embryologic disturbances *functional influence
Skeletal growth disturbances *equilibrium theory
Disturbances of dental development
normal occlusion 35%
known cause 5%
the etiology of
1-Specific causes of malocclusion:
1-disturbances in embryologic development:
Ex. Teratogens are chemicals and drugs producing embryologic defects if given any critical
times (table 5-1 in the book)
the dr. mentioned: thalidomide>>microsomia, trearchercollin
syndrome /Aspirin>> cleft lip and palate / Cigarette, alcohol, dilantin
2-skeletal growth disturbances:
macrognathia and micrognathia
Pressure against the face (blamed on forceps use)
Pierre robin syndrome is rare
(Extreme mandibular deficiency
sometimes with cleft palate)
childhood fractures of the jaws:
breakage the nick of the condyle:
( because it is very thin)
almost 25% of the early fracture will
develop a problem in mandibular growth
>>ankylosis>>growth in one side
(the chin points toward the
affected side)=unilateral fracture cause asymmetric growth
, and 75% go unnoticed due to complete
regeneration of the condyle
Growth can be affected by:
1-muscle attachment and its activity can affect the formation of bone
lead to atrophy of bone because of muscle dysfunction)
2-muscles are part of the total soft tissue matrix, carries the jaws downward and forward
3-damage may result from nerve injury to muscles (at birth)
Ex. Muscle contraction i.e. torticollis (sternocleidomastoid) >> facial asymmetry.
Muscle dystrophy (reduced tonicity) drop of the mandible away from the facial skeleton
leading to increased lower face ht., excessive eruption of posterior teeth, narrow maxilla…
Anterior pituitary tumor >> excessive secretion of growth hormone >> excessive growth of
mandible (class III malocclusion)
5-Hemimandibular hypertrophy (condylar hyperplasia):
Unknown lateral excessive growth of the mandible,
ttt: total removal or reconstruction
6-Disturbance of dental development:
1. Congenitally missing teeth:
causes spaces, over eruption and drifting
Anodontia: total absence of teeth
Oligodontia: absence of many teeth
Hypodontia: absence of only few teeth (common)
the most common: #2 + #5
Multiple missing teeth =cleidocrainal dysplasia
2. Malformed and supernumerary teeth:
Peg-shaped laterals, selective small or large teeth >> "tooth size discrepancy"
Fusion (separate pulp)
Germination (common pulp of teeth)
3. Supernumerary teeth:
Mesiodens (most common)
in the midline
Cleidocranial dysplasia (multiple supernumeraries)
4. Interference with eruption:
can be caused by any lesion like cyst or tumor
Ex. Supernumerary teeth
Heavy fibrous gingival as in cleidocranial dysplasia (mechanical obstruction)
Drifting of permanent teeth
Ankylosed primary teeth
5. Ectopic eruption:
Eruption of a permanent tooth in the wrong place (common in first permanent
Transposition of teeth, ex. Canine and premolars
6. Early loss of primary teeth:
Ex. Mesial eruption of permanent molars after early loss of 2
primary molar >>
space loss >> crowding
it also can block the permanent teeth from eruption, so to
prevent this we must use a space maintainer
7. Traumatic displacement of teeth:
Direct injury to permanent teeth
if there is a trauma and the tooth goes up in the
gingiva and bone we must pull it before its ankylosed
Damage to permanent tooth bud (injury to primary teeth)
Drift of permanent tetth after premature loss of primary teeth
Trauma to primary teeth during formation of permanent teeth
During crown formation after crown formation
Crown malformation displacement of crown relative to the root(dilaceration)
It is apparent that certain types of malocclusion run in families.
Ex: the Hapsburg German royal family. (prognathic mandible)
(Class3 which is the most
The pertinent question for the etiologic process of malocclusion is not whether there are
inherited influences on the jaw and teeth because obviously there are, but whether
malocclusion is often caused by inherited characteristics.
(The whole case is not transmitted to the child, ex. The mother has a small jaw and the
father has large teeth, this will produce a new type of malocclusion other than that of the
Malocclusion could be produced by inherited characteristics in two major ways:
Inherited disproportion between jaw size and teeth size.
Inherited disproportion between upper and lower jaws.
Question: are the jaws and teeth size inherited independently or are dentofacial
characteristics linked together?
Example: inheritance of large upper jaw and small lower jaw
old human remains show absence of malocclusion, maybe due the homogeneity of
1. Evolutionary reduction in the jaw/teeth size
ةرب نم اوجوزتي
3. Twin studies
1. Evolutionary reduction in the jaw/teeth size
as well as the number of the teeth
Ex: missing third molars, phasing out lateral incisors and2nd premolars.
2. Out breading:
Slow genetic drift cannot explain the increase of malocclusion
(it takes years to see the
change in generation)
Question: could malocclusion be a disease of civilization?
If out breading is a cause of malocclusion, then we could predict that modern urban
populations would have high prevalence of malocclusion.
Results were short extremities and under developed midface due to achondrroplasia
rather than "inheritance"
In 1930,animal studies by Prof.stockard :breading experiments in dogs:
Sever malocclusion resulted from carrying the achondroplasia gene.
Chung et al: (Hawaiian studies): Hawaiian is exceptionally heterogeneous modern
population, a mixture of Polynesian, Chinese, European, Japanese and others.
Question: if the teeth and jaws characteristics inherited independently, what would we
expect the prevalence of malocclusion to be?
If the child have a large jaw from the dad and
a small teeth from the mom is he going to have severe spacing?
: the effect of interracial crosses appear to be more additive than multiplicative
Sever malocclusion not observed.
3. Familial studies:
Classically determined through twin studies
Monozygotic twins (identical)
most important study so we can know the inherited
Monozygotic twins are genetically identical i.e. any differences should be solely as a
result of environmental influences.
Dizygotic twins are not similar more than ordinary siblings i.e.
They are only share intrauterine and family environment.
By Lauweryns et al: 40 % of dental and facial variations that lead to malocclusion
(and 60% environmental)
Corrucini et al: with correction of environmental differences within twin pairs, some
hereditability of dental characteristics is almost zero.
Studies of estimating the influence of hereditary in family members ,similarities and
differences in mother-child, father-child and siblings pairs.
For most facial skeletal measurements, the genetic influence correction was high (0.5).
high as it reaches 1)
Dental correction was variable (0.5-0.15).
Maximum 50% and minimum 0% of the genetic influence.
Longitudinal studies of cephalometric radiograph and dental casts of siblings(Bolton-
Brush Growth Study):Haries and Jonson reported:
high craniofacial hereditability.
low dental (occlusal) hereditability.
Summary of inherited influences:
Not more than 50% of occlusal traits are inherited.
Certain types of malocclusion, especially class 3 mandibular prognathism, have a
strong inherited component.
Malocclusion cannot be explained as caused by genetic drift during evolution or by
increased out breading.
The equilibrium theory and the development of dental occlusion.
Functional influences on dentofacial development.
1-The equilibrium theory:
If a force is more than the other movement occur
If the 2 forces are equal there is no movement
The equilibrium theory and the development of the dental occlusion:
Therefore, the dentition is in equilibrium since teeth are subjected to a variety of forces but
do not move.
, swallowing, speaking …
The equilibrium effect on dentition:
Because of the biologic response, "the
of the force is more important than the
(Forces depend on: direction-magnitude-duration"the most important in ortho")
Why don’t teeth move during the heavy f
orce of chewing?
Heavy masticatory force on teeth
Shock absorber effect of PDL
No tooth movement
Heavy masticatory forces
More than a few seconds
Release of biting forces
Duration threshold in humans is: 6
Maintenance of equilibrium between the light sustained pressures from lips, checks and
tongue ensures stability of teeth.
Pressures from lips, cheeks and tongue are much lighter in magnitude than forces of
However, much greater in duration.
Light sustained pressures from lips, checks and tongue are in equilibrium.
Intermittent short-duration presences during swallowing do not have impact on tooth
Tongue don’t move teeth normally because it’s a short duration but if this increased (tongue
thrusting) this will cause moving
Duration threshold in humans is: 6
The equilibrium theory:
Loss of muscle activity pressure from the tongue
Flaring of the teeth buccally and lingually
Habits effect on the equilibrium:
For any intraoral habit, its effect on the position of the teeth is determined not by the force it
applies to the teeth but by how long that force is sustained.
Pressure on teeth
Teeth movement no movement
If pressure is sustained this will cause movement
Vigorous thumb sucking but not for a long duration>> no movement
Equilibrium effects on jaw size and shape:
The functional processes of bones will be altered if function is lost or changed.
the alveolar process exists to support teeth …
if they don't implant within 4-6 months
Failure of eruption
Bone never forms
alveolar process never formed
Extraction of tooth in one jaw
super eruption of opposing tooth with new alveolar bone
- Therefore, the position of the tooth, not the functional load on it, determines the shape
of the alveolar ridge.
- The location of the muscle attachments is more important in determining bone shape than
mechanical loading or activity.
So some people have strong or square jaw that coz their
- Muscle growth can produce change in jaw shape particularly at the coronoid process and
the angle of the mandible.
-intermittent pressure of forces has little or NO EFFECT on either the position of the
teeth or size and shape of the jaw.
- The equilibrium influences will affect the vertical and horizontal position of teeth.
- The equilibrium influence on the jaw is the positional changes affecting the functional changes
affecting the functional processes including the condyler process.
Pressure generated by chewing activity will affect dentofacial development in two ways:
a-Function and dental arch size:
Animal experiments: soft diet changed jaw morphology in pigs.
Question: Does masticatory effect influences the vertical jaw relationship?
YES, clearly are affected by muscular activity (the effect on tooth eruption)
- In humans it is unclear (dental arch dimensions are established early in life)
A genetic drift towards smaller jaw size + diet changes may be an answer.
An experiment on humans, some of them had soft diet and others had regular diet
they've noticed a difference.
In comparison of the primary arch to the permanent arch
the primary have a larger arch
length than the permanent Because the lee way space will decrease after permanent
eruption. (Leeway space: the difference between the deciduous molars and the permanent
b-Biting force and eruption:
Force exerted by the masticatory muscles is not a major environmental factor in controlling
tooth eruption and is not an etiologic factor for most patients with deep or open bite.
2-Sucking and other habits:
-thumb sucking during the PRIMARY dentition has NO or little long term effect.
PERSISTENT thumb sucking through PERMENENT dentition causes MALOCCLUSION.
-VIGOROUS sucking but INTERMITTENT may NOT displace the teeth
. (Beyond 6 hours>>