La Trobe UniversityBachelor of Health Sciences in Dentistry

Second Year Semester 1 2009

Dr. Robert MayneBDSc

(Melb) DCD(Ortho) MRACDS (Ortho) MOrthRCSEd

Lecture 11

Development of occlusion

Page 2

Housekeeping•

Practical

sessions:

–

Attendance and completion is a requirement of this unit–

If you havent completed a prac (and no formal notification/permission has been given) then you will fail this subject.

–

Make sure you check the LMS regularly–

Check announcements –

Participate in discussions

•

Assignment 2 –

due this Friday by 4pm–

Plagiarism –

TURNITIN. 20% upper limit.

•

Journal article access –

Text book level only expected.–

KISS principle

–

You now have access to:•

AJODO•

ADJ•

European Orthodontic Journal•

Angle Orthodontist

–

ONLINE tests –

no online test today!! –

practical only in 2 parts.•

Don’t expect to leave early today

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Development of occlusion and the dental arches

•

The ‘human dentition is in a dynamic state, constantly changing throughout life’

•

A knowledge of these changes assists the clinician in determining whether or not a specific occlusion will be:–

sustained, worsen or self correct overtime.

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Terminology

•

Intercanine width

is

measured from the cusp tips of the canines

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Terminology

•

Depends of the study,

•

Intermolar

width

is measured from the mesiobuccal cusp tips of first permanent molars

•

or the mesiolingual cusp tips of the first permanent molars

•

or the central fossa of fist permanent molars.

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Terminology

•

Arch depth

is measured at the midline from a point midway between the central incisors to a tangent touching the distal surfaces of the second primary molars or second premolars

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•

Arch length refers to

distance between central incisors to the distal of the E (or mesial of the 6)

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Terminology

•

Arch circumference is usually measured

from the distal surface of the second primary molar or mesial surface of the first permanent molar around the arch over the contact points and incisal edges in a smooth curve to the distal surface of the opposite premolar (or mesial surface of first molar)

Page 9

•

The development of occlusion occurs over three distinct stages. –

1. The deciduous dentition begins soon after birth and is complete with the eruption of all deciduous teeth.

–

2. The eruption of the first permanent molar ushers in the mixed dentition and

–

3. Loss of the last deciduous tooth denotes the transition to the permanent dentition.

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At birth

•

The maxillary and mandibular alveolar processes are not well developed at birth.

•

The upper gum pad is horseshoe-shaped with a flat shallow palate whereas the lower gum pad is more U-shaped.

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At birth•

The deciduous teeth commence formation ’in utero’

and at birth the

child is usually edentulous •

A ‘neonatal tooth’

is occasionally

present at birth and is often a supernumerary but may alternatively be a normal deciduous central incisor.

•

The tooth possibly warrants extraction if a problem to feeding, or causes ulceration of the tongue and lips.

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Deciduous Eruption•

The eruption times of primary teeth are highly variable, with acceleration or delay of up to 6 months not uncommon.

•

Factors can affect eruption time :–

Racial and socio-economic

–

nutritional deficiencies, –

endocrine disturbances,

–

mechanical or local disturbances

•

All teeth have usually erupted by 30 months of age. •

The eruption sequence though variable is usually preserved.

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Maxilla

and Mandible A B D C E

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Characteristics

•

Deciduous arches are generally ovoid in shape with spacing a normal occurrence in the anterior part of the primary dentition.

•

‘Primary spaces’

refer to distinct spaces in two locations.

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•

The maxillary primate space lies between the lateral incisors and canines

•

The mandibular primate space between the canines and first molars.

•

Developmental spaces between the incisors, may become larger as the alveolar processes expand.

•

Generalised spacing of the primary teeth is required for proper alignment of the permanent incisors

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•

The dimensions of the deciduous arches remain relatively constant from:–

the completion of the deciduous dentition •

(≈

2 years of age)

–

up until the eruption of the first permanent molars

•

(≈

6 years of age)

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Occlusal relationships•

Deciduous incisors are placed more vertical than their permanent successors demonstrating minimal overjet

and

overbite. •

Occlusal attrition commonly seen in the deciduous dentition may help to reduce a deep overbite in the years preceding exfoliation of the incisors.

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•

Overbite is not merely a product of incisor angulation and position; but also development of the vertical skeletal relationships and growth of the mandibular and maxillary dentoalveolar processes.

•

Overbite relationship in the primary dentition offers little predictive value for that which will be seen in the permanent dentition

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•

The lower second deciduous molar is somewhat wider mesiodistally

than

the upper E, typically providing a flush terminal plane at the posterior limit of the primary dentition.

Page 23

•

This flush terminal plane is considered normal in North American Caucasoid and most European populations, acting as a guide for the permanent molars to erupt into an end –

end

relationship

•

Variations to this are mesial step and distal step relationships.

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•

For a class I permanent molar

relationship to establish, it is preferable that

•

the mandibular leeway space exceeds the maxillary one such that there is greater mesial shift of the mandibular permanent molars following loss of E’s.

•

there is greater forward growth of the mandible relative to the maxilla.

•

the lower E is lost before the upper E so that the lower first molar can move forward before the upper first molar

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MIXED DENTITION

•

The mixed dentition is the period in which both the primary and permanent teeth are present in the mouth together.

•

It typically begins with the eruption of the first permanent molars at approximately 6 years of age.

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•

The timing and sequence of tooth eruption is variable with several sequences being favourable for maintaining the length of the arch during the mixed dentition.

•

The most common pattern is one of:•

Maxilla: 6→1→2→4→5→3→7

•

Mandible: 6→1→2→3→4→5→7

Page 29

•

The eruption of permanent teeth begins when the roots are approximately two-thirds complete.

•

Moyers

(1988) postulated that the timing of eruption could not be predicted from the length of the root.

•

If a primary tooth is extracted once the permanent successor has started to erupt, the permanent tooth will erupt earlier.

•

On the other hand, if a primary tooth is extracted well before a permanent tooth is about to erupt, this may well retard eruption as the alveolar bone may reform over the successor.

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•

As a tooth erupts through the alveolus it displays a mesial drifting tendency, even before it appears in the oral cavity.

•

Once it emerges into the mouth it rapidly erupts towards the opposing arch until it is in occlusion.

•

At this point, rate of eruption mirrors that of the vertical growth of the maxilla and mandible during the adolescent growth spurt.

•

The upward forces of eruption and alveolar growth are opposed by apically directed occlusal forces from the muscles of mastication through the interdigitation of cusps.

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Incisor Liability

•

The combined width of the permanent incisors is greater when compared to those they replace.

•

Incisor liability is the term given to this size difference and varies greatly from person to person.

•

This disparity in the maxillary arch is accommodated by the interdental spaces between primary incisors, and the more labial positioning of the permanent incisors which culminates in an increase in arch depth.

Page 32

•

A transitory stage of mandibular anterior crowding occurs as a normal

developmental stage•

at 8 -

9 years of age as there is a lack of

approximately 1.6mm for the appropriate alignment of the lower incisors

Page 33

•

Incisor liability may be compensated by:–

Alveolar growth and the labial eruption of permanent teeth resulting in a 2mm increase of space to align teeth.

•

This gain is greater in the maxilla than the mandible and in boys than girls.

•

As a result, females are more likely to experience mandibular incisor crowding

–

Distal drift of the canines into the leeway space.–

Smaller permanent tooth widths than expected.

•

These occur independent of significant skeletal growth and can resolve up to 2mm of anterior crowding

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Ugly duckling stage

•

A stage characterised by the distal flaring of the lateral incisors coupled with a midline diastema in the maxillary arch

•

Central incisors in the maxilla often erupt with a diastema between them that may partially close with the subsequent eruption of the lateral incisors.

•

However a diastema is unlikely to spontaneously close if greater than 2mm in dimension

Page 35

•

As the maxillary canines erupt, they may encroach on the roots of lateral incisors applying distal pressure which results in a slight distal inclination of the lateral incisors.

•

This stage is transient and usually resolves after canine eruption is complete.

•

From here, the canines may erupt uneventfully through the alveolar process distal to the lateral incisors.

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Page 37

Leeway Space

•

The first and second deciduous molars are wider mesiodistally

than their permanent

premolar successors.•

This difference is referred to as ‘leeway space’

or ‘E space’

as most of the

difference is provided by the difference between the E’s and second premolars.

•

maxillary leeway space is 1.1mm •

mandibular leeway space and 2.4mm

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Page 39

•

The leeway space provides adequate space to resolve crowding that is present in the mixed dentition in the majority of individuals

•

“A wide range of variation exists in the amount of available bilateral mandibular E-space; consequently, assessing the relative sizes of the deciduous second molars and their permanent successors in individual orthodontic patients is necessary”

Woods 2002

Page 40

•

A conventional lower lingual arch is a passive appliance which may be used as a space maintainer until further treatment is commenced.

•

A lip bumper, fixed appliances or an active lingual arch may be used to gain additional arch by distalising

the first permanent

molars before the eruption of the second permanent molars.

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Pseudo Class III

•

Pseudo class III malocclusion is characterised by the presence of an anterior crossbite caused by a mandibular displacement

•

It is recommended that pseudo class III malocclusion should be treated early to eliminate functional shift of the mandible whilst improving the alignment of incisors

•

If correction of the displacement is undertaken before the eruption of the permanent canines and premolars,

•

this allows the development of a proper class I anterior relationship in the correct mandibular relationship.

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Page 48

Mixed dentition problems

Ankylosis•

the teeth fuse to the bone which can prevent them from erupting.

•

Limits dentoalveolar development•

With a permanent successor in a normal position, there should be no more than a 6 month delay in exfoliation of an ankylosed deciduous molar

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•

Ectopic eruption of teeth–

Canines

–

1st

molars

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PERMANENT DENTITION•

Bishara

et. al. (1988) studied the outcomes of different deciduous molar

relationships in 121 patients on subsequent permanent first molar relationships.

•

It was reported that a:

•

Distal step relationship in the primary occlusion

always progressed to a full or partial class II molar relationship.

•

Terminal plane developed into:–

Class I in 56% of cases, Class II in 44% of cases

•

1.0mm mesial step produced:–

Class I in 76% of cases, Class II in 23% of cases, Class III in 1% of cases

•

2.0mm mesial step resulted in:–

Class I in 68% of cases, Class II in 13% of cases, Class III in 19% of cases

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Early signs of crowding•

Advanced eruption of the permanent second molars ahead of the second premolars may decrease the space available for the premolar to erupt into.

•

Concurrent eruption of the upper canine and first premolar often forces the canine labially.

•

A labially displaced canine may also be the result of insufficient space at the time of eruption

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Early signs of crowding

•

Premature loss of E’s may result in mesial eruption of migration of the first permanent molars.

•

This decreases the leeway space available to align the remaining unerupted

premolars and canines once they

erupt.

•

Space maintenance may be an option if the permanent molar has erupted and the E is lost

•

Unilateral or bilateral loss of deciduous canines with the eruption of lateral incisors is an incipient sign of crowding.

Page 72

Arch dimensional changes

Page 73

Intercanine width•

observed that the greatest increase in canine width occurred from birth to 2 years of age; –

5mm in the maxilla and 3.5mm in the mandible.

•

The intercanine

width continues to increase up to 13 years of age in the maxilla and 12 years of age in the mandible.

•

With the eruption of the permanent canines, there may also be a small increase in intercanine

width as they

attain a more buccal position in the dental arch relative to their predecessors

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Intermolar width

•

Sillman

(1964) reported that intermolar width increased from 3 -

12 years of age at

a rate of:–

0.5mm per year in the maxilla

–

0.2mm per year in the mandible, •

with no significant changes thereafter.

Page 76

Arch depth•

Moorrees

(1969) noted a significant decrease in arch

depth from the age of 4 to 6, explained by the closing of spaces between the deciduous molars and canines as the first permanent molars erupt.

•

The permanent incisors erupt at a greater angulation and more labial position compared with their predecessors.

•

As the deciduous molars are replaced by their smaller permanent successors this leads to a significant reduction in arch depth.

•

This results in a significant increase in arch length in the maxilla and a small increase in the mandible.

•

arch depth is shorter at 18 years of age than at 4 years of age.

Page 77

Arch Length

•

From 3 -

13 years of age:–

Maxillary arch length increased significantly in males (4.0mm) and females (2.4mm)

–

At age 8, the mandible had reaches its peak arch length

–

From 8 -

13 years of age, mandibular arch length decreased 2.4mm in males and 3.2mm in females.

–

This difference may possibly be due to the E spaces being on average, larger in the mandible compared with the maxilla

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Arch circumference

•

Moorees

(1969) reported that between the ages of 5 -

18 years, maxillary arch circumferences

increased slightly (1.3mm in males; 0.5mm in females),

•

while mandibular arch circumference displayed marked decreases (3.4mm in males; 4.5mm in females).

•

Both arches would be expected to show decreased arch circumference from late adolescence into adulthood as arch depth and arch length both decrease during this time