Etiology of Malocclusion

  • Polygenetic
  • Multifactorial
  • Investigation: monozygotic vs dizygotic (independent is genetic) more diff = more genetic contribution

1| Equilibrium theory: Force

  • Magnitude (6hr)
  • Duration

4| Specific causes

Soft tissue

Dentition

Vert: Opposing & PDL

Jaw

Hypo2: Red bite force = red LAFH

  • Refuted, only in CP cases

Trans: Cheek vs tongue

Hypo1: Inc usage = inc size

  • Refuted by stagnant inter-canine width
  • But genetic drift toward smaller by diet

Eg1: Soft tissue growth postures mandible causing growth at condyles

Eg2: Atrophy of temporalis after injury

AP: Lips vs tongue

Habits

Sucking (Y)

Tongue (Y/N)

Mouth breathe (N)

Effect dental:

  • Upper incisor procline
  • Lower incisor retrocline

Effect skeletal:

  • Backward mand rotation
  • Max constriction (lower tongue/cheek)

Observed in:

  • Transition infantile -> adult swallow (6y/o)
  • Displaced incisors -> achieve oral seal

Types of tongue thrust:

  • Sustained forward posture (>6hr)
  • Infantile swallow (<6hr)

Extent: Contribute but not etiology
Cause: Enlarge adenoid, nasal block, allergy

2| Malocclusion characteristics

Class I

  • Usually dental

Skeletal: Cl I or DAC Cl II/III

Class II Div I

  • Usually skeletal

Class II Div II

Class III

Soft tissue: Secondary cause

  • Median diastema is physiological to papilla
  • Migrates labially
  • Closes as #2 & #3 errupt
  • May persist: (Check Aetiology slides), frenal/thickness

Dental

  • Genetic/Environ
  • Intra-arch

Size: Tooth size, arch length

Position: Displace/Impaction

  • Common: Last of every type (#2/3, #5, #7)
  • Abnormal position of germ

Anomalies: (Macro/Micro/Hyper/Hypo)

Late lower incisor crowding

  • Mandibular growth rotation
  • Backward (Cl II): Pushed lingual by deep overbite
  • Forward (Cl III): Pushed lingual by lip

Skeletal

  • Maxilla: Protrusive
  • Mandible: Retrusive (More common)
  • Exacerbated by backward mandibular growth

Genetic

Skeletal: growth disturbances

  • Childhood jaw fracture (if scarring 5% of px)
  • RA affecting growth

Soft tissue: muscular dysfunction

  • In utero cause
  • Birth injury
  • Motor nerve damage
  • Excessive muscle contraction (eg. torticollis)
  • Decrease in tonicity (eg. CP, weakness mandible drops, supraerruption post, AOB)

Dental: disturbance

Acromegaly & Hemimandibular hypertrophy

  • Acromegaly cause: Ant pituitary excess GH, excess Mnd growth Cl III

Embryological Dev (<1%)

  • Teratogens Aspirin, smoke, alcohol)
  • Retinoic acid thalidomide
  • Intrauterine moulding (Pierre Robin seq)

Congenitally missing

  • eg. retained primaries, poor LT prog. space/drift of perm
  • Hypodontia genetic

Malformed & supernumerary

  • Size mismatch: micro/maco, spacing/crowding
  • Supernumerary: Impede eruption, median diastema, crowding

Eruption interference

  • Ankylosis of primary molars

Ecotopic erruption 6s

  • Path too M, impacted against Es
  • Damage E roots, early loss
  • Impaction of #5s

Early loss of primary #4/5

  • Mesial drift of #6, space loss
  • Impaction/displacement of successor
  • Localized molar Cl II/III

Traumatic displacement

Specific

  • Bilateral idiopathic condylar resorption

Soft tissue characteristics

  • Lower lip trap
  • Active lower lip
  • Forward resting tongue posture (DAC lower incisor proclination)
  • CIrcumoral to achieve seal / forward mandibular posture

Habits

  • Non-nutritive sucking
  • Causing proclined upper, retrocline lower

Types of Cl II malocclusion

  • True transverse: Size discrepancy
  • Relative transverse: Positioning
  • Forward mandibular growth rotation

Dental

  • Lack of occlusal stop (Lower incisor overeruption)
  • Crowding due to retroclination

Skeletal

  • Reduce VD

Soft tissue

  • Active muscular lips -> Bimax retroclination

Skeletal"

  • Mandibular prognathia (More common)
  • Maxillary retrusion
  • Exacerbated by forward mandibular growth

AOB

  • All possible

Ankylosis due to trauma
Bimaxillary proclination
Impaction

Soft tissue / Habits
Mouth breathing
Forward resting tongue posture
Thumbsucking

Skeletal

  • Hyperdivergent SB
  • Exacerbated by downward backward
  • Bilateral condylar resorption
  • Insufficient DAC

Deep OB (Q similiar to Class II Div II

  • Over-eruption of anterior teeth
  • Posterior bite collapse
  • Bruxism (1mm drop in posterior = 2mm anterior)

POB

Soft tissue

  • Macroglossia
  • Lateral tongue spread

Dental

  • Ankylosis
  • Primary failure of eruption(Idiopathic, no obstruction)
  • Mesial tipping of molars (Early exfoliation of Es)
  • Shift due to occlusal interferences

Skeletal

  • Long face pattern
  • Unilateral condylar hyperplasia (Ipsilateral)
  • Unilateral idiopathic condylar resorption (Contralateral)
  • Hemimandibular hyperplasia (Ipsilateral)

Posterior Crossbite

Dental

  • Localized displacement secondary to crowding or retained primary
  • Soft tissue
  • Usual culprits

Skeletal

  • True discrepancy (Width of arches mismatch)
  • Relative discrepancy (Size of arches mismatch)
    eg. Mandibular buccal crossbite Cl III, Scissorbite Cl II
    etc. all discrepancies... Hemimandibular elongation/hypoplasia
  • Cleft lip & Palate scarring

Genetic disorders

  • Cleidocranial dysplasia
  • Craniosynotosis
  • Treacher Collins syndrome
  • Hemifacial microsomia
  • Cleft lip and palate (30% syndromic, 70% polygenic/multifactorial)

Dental

  • Incisor position
  • Pseudo Cl III

Causes of delayed tooth eruption

Generalized (minority)

  • Familial
  • Downs/Cleidocranial dystosis
  • Rickets/Endocrine disorders
  • LT chemo
  • Hereditary gingival fibromatosis

Localized (majority)

  • Obstruction
  • Dilaceration
  • Crowding
  • Abnormal position of crypt
  • Ankylosis
  • Primary failure of eruption (genetic rare 0.06% even though straight pathway, all teeth distal to tooth will be affected, cannot be moved by ortho)
  • Cleft lip / palate
  • LT RT

Ectopic erruption of #3 (Environmental)

  • Buccal 15% assoc with overcrowding 85%
  • Palatal 85% assoc with sufficient space 83%
  • Factor #1 Long path of eruption (22mm from orbit floor)
  • Factor #2 Pegshape/small #2 fails to guide
  • Transposition (exchange places genetic between #4)

Early loss of primary #3 (More severe than loss of Es)

  • Distal Palatal drift of incisor, space loss
  • Impaction/discplacement of successor
  • Midline deviation

What causes Ankylosis

  • Avulsion/replanting

Genetics

  • OJ/OB/Molar relationship/soft tissue/skeletal
  • NOT: occlusal rltns / dental problems
  • Minor role: Arch width and length

Skeletal problems

  • Geneitc
  • Embryological defects
  • Trauma
  • Fx