Occlusion and Dental Developments

8/3/2019 Occlusion and Dental Developments

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Teeth erupt full size and are ideal for study throughout

life. Furthermore, in living people, dental casts and X-

ray films are obtained with relative ease. Mostimportant, age and sex can be recorded. With looseteeth in museum collections, age and sex cannot bedetermined with reliability.

When teeth erupt into the oral cavity, a new set offactors influence tooth position. As the teeth come intofunction, genetic and environment determine toothposition. For example, tooth arrangement is affected bymuscle pressure, as we will discuss shortly.

One final comment: in this course, we treat teeth as aunisex topic. In real life, however, girls shed deciduousteeth and receive their permanent teeth slightly earlierthan boys, possibly reflecting the earlier physicalmaturation achieved by girls. Teeth are slightly larger inboys that in girls; however, we cannot make thatdistinction by looking at a single extracted tooth.


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A. Occlusion defined:In dentistry, occlusion usually means the contact

relationship in function. Concepts of occlusion vary with

almost every specialty of dentistry. Here, now, is anintroductory definition for one type of occlusion, centricocclusion.

Centric occlusion is the maximum contact and/orintercuspation of the teeth.


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B. Occlusion is the sum total of many factors.

1. Genetic factors.

Teeth can vary in size. Examples are microdontia (very smallteeth) and macrodontia (very large teeth). Incidentally,Australian aborigines have the largest molar tooth size--

some35% larger than the smallest molar tooth group (Lappsand !Kung san Bushmen).

The shape of individual teeth can vary (such as third molarsand the upper lateral incisors.)

They can vary when and where they erupt, or they may not

erupt at all (impaction). Teeth can be congenitally missing (partial or complete

anodontia), or there can be extra (supernumerary) teeth. The skeletal support (maxilla/mandible) and how they are

related to each other can vary considerably from the norm.


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2.Environmental factors.

Habits can have an affect: wear, thumbsucking,pipestem or cigarette holder usage, orthodonticappliances, orthodontic retainers have an influence onthe occlusion.

3.Muscular pressure.

Once the teeth erupt into the oral cavity, the position ofteeth is affected by other teeth, both in the same dentalarch and by teeth in the opposing dental arch.

Teeth are affected by muscular pressure on the facialside (by cheeks/lips) and on the lingual side (by thetongue).


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C. Occlusion constantly changes with development,

maturity, and aging

1 . There is change with the eruption and shedding of teethas the successional changes from deciduous to

permanent dentitions take place.

2. Tooth wear is significant over a lifetime. Abrasion, thewearing away of the occlusal surface reduces crown

height and alters occlusal anatomy.


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Attrition of the proximal surfaces reduces the mesial-distal dimensions of the teeth and significantly reducesarch length over a lifetime.

(A note on terminology. Abraision and attrition havesubtle differences in meaning. Abraision is the wear ofteeth by agencies other than the friction of one tooth

against another. In contrast, attrition is the wear of teethby one tooth rubbing against another.)

3. Tooth loss leaves one or more teeth without an

antagonist. Also, teeth drift, tip, and rotate when otherteeth in the arch are extracted.


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III. Periods of Dental Development. (Stages of dentofacial

development) This presentation is about the development of

occlusion. An obvious omission is what happensbefore birth: that will be covered by others in oral

biology. You should know a few things now, however,about events before birth. The very first histologicalevidence of tooth development appear during thesecond month of intrauterine life. Calcification ofdeciduous incisors begins at 3-4 months in utero. By

the time of birth, calcification of all the deciduous teethis well under way.


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A. The pre-dentition period.

This is from birth to six months. At this stage, there are no teeth. Clinically, the infant isedentulous (without teeth, remember?).

Both jaws undergo rapid growth; the growth is in threeplanes of space: downward, forward, and laterally (to

the side). Forward growth for the mandible is greater. The maxillary and mandibular alveolar processes are

not well developed at birth. occasionally, there is a neonatal tooth present at birth. It

is a supernumerary and is often lost soon after birth.

At birth, bulges in the developing alveoli precedeeruption of the deciduous teeth. At birth, the molar padscan touch.


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B. Deciduous dentition period.

The deciduous teeth start to erupt at the age of sixmonths and the deciduous dentition is complete by theage of approximately two and one half years of age.

The jaws continue to increase in size at all points untilabout age one year.

After this, growth of the arches is lengthening of thearches at their posterior (distal) ends. Also, there is

slightly more forward growth of the mandible than themaxilla.


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1. Many early developmental events take place.

The tooth buds anticipate the ultimate occlusal pattern.

Mandibular teeth tend to erupt first. The pattern for thedeciduous incisors is usually in this distinctive order:

(1) mandibular central


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(2) maxillary central incisors

(3) then all four lateral incisors.

By one year, the deciduous molars begin to erupt.


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Upper Primary Teeth Development Chart

Upper Teeth When tooth emerges When tooth falls out

Central incisor 8 to 12 months 6 to 7 years

Lateral incisor 9 to 13 months 7 to 8 years

Canine (cuspid) 16 to 22 months 10 to 12 years

First molar 13 to 19 months 9 to 11 years

Second molar 25 to 33 months 10 to 12 years

Lower Primary Teeth Development Chart

Lower Teeth When tooth emerges When tooth falls out

Second molar 23 to 31 months 10 to 12 years

First molar 14 to 18 months 9 to 11 yearsCanine (cuspid) 17 to 23 months 9 to 12 years

Lateral incisor 10 to 16 months 7 to 8 years

Central incisor 6 to 10 months


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Remember, we are talking about primary teeth!

If deciduous teeth are retained too long, considerankylosed teeth or missing or impacted teeth.

Eruption times can be variable. If you see a child who isunusually early or late in getting their teeth, inquireabout older siblings or parents. (Remember our remarksabout tight genetic control?) It applies to patterns oferuption, too.


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2. Occlusal changes in the deciduous dentition.

We here introduce the terms overjet and overbite.The key to understanding them is another basicelement of human occlusion. The size of the

maxillary arch tends to be greater than that of themandibular arch. Therefore, when they are'superimposed' one atop the other, the maxillaryteeth 'overhang' the mandibular teeth. Examine the

diagram to clearly understand this concept.


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Illustration of maxillary/mandibularsuperimposition, overjet/overbite, and the

progressive wear of deciduous teeth the flatplane occlusion


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The overjet tends to diminish with age. Wear and

mandibular growth are a factor in this process.

The overbite often diminishes with the teeth being worn to

a flat plane occlusion. (My comment: cliniciansoften regard such wear as excessive. In Stone Agedentitions, however, it is a frequent finding.)

Often the deciduous teeth are badly chipped and worn.

The 'flat plane' occlusion', unusual in our fast food culturewas the norm in food forager and early agriculturalsocieties when food contained grit from mill stones.

Spacing of the incisors in anticipation of the soon-to-eruptpermanent incisors appears late.

Comment: Permanent anterior teeth (incisors and canines)are wider mesiodistally than deciduous anterior teeth. Incontrast, the deciduous molar are wider mesiodistally thatthe premolars that later replace them.


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Primate spaces occur in about 50% of children. Theyappear in the deciduous dentition. The spaces appearbetween the upper lateral incisor and the upper canine.

They also appear between the lower canine and thedeciduous first molar. In great apes and some monkeys,the primate spaces are a special type of diastema thatfunction to accomodate the large canines.


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C. Mixed Dentition Period.

Begins with the eruption of the first permanent molarsdistal to the second deciduous molars. These are thefirst teeth to emerge and they initially articulate in an'end-on' (one on top of the other) relationship.

On occasion, the permanent incisors 'spread out' due tospacing. In the older literature, is called by the 'uglyduckling stage.' With the eruption of the permanentcanines, the spaces often will close.


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1. Errors in development. These are usually genetic.a. Variability of the individual teeth. In general, the teeth

most distal in any class are the most variable.b. Hypodontia

This somewhat common condition is probably of geneticorigin. The teeth most often missing are the thirdmolars, second premolars, and maxillary lateral incisors;other teeth may be reduced in size. Several teeth maybe absent in patients with Down syndrome andectodermal dysplasia.

Many permanent teeth are missing, including upper lateral incisors and lower centralincisors (hypodontia)
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1071315/figure/fig4/


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c. Supernumerary teeth.

Supernumerary teeth are erupting palatal to upper centralincisors


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d.Microdontia

Teeth, especially the third molars, may vary in size,form, and structure because of genetic factors.Microdontia (teeth smaller than usual) is largely ofgenetic origin and usually affects the lateral incisors,which are conical or peg-shaped. Teeth that are larger

than normal (megadont) are uncommon. Double teethoccur occasionally, most often in the primary dentition.In the succeeding permanent dentition, they are likely

followed by extra tooth elements.


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e.Macrodontia

Macrodontia is a condition in which any tooth or teethappear larger than normal for that particular type oftooth.
http://www.juniordentist.com/wp-content/uploads/2011/07/Macrodontia.jpg


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2. Errors in skeletal alignment. Malpositioned jaws disruptnormal tooth relationships.

3. Soft tissue problems. Ocasionally, the proper eruption of a tooth is prevented

by fibrous connective tissue over the crown of the tooth. In the mixed dentition, the deciduous second molars

have a special importance for the integrity of thepermanent dentition. Consider this: The first permanentmolars at age six years erupt distal to the seconddeciduous molars.

Permanent posterior teeth exhibit physiological mesialdrift, the tendency to drift mesially when space is

available. If the deciduous second molars are lostprematurely, the first permanent molars drift anteriorlyand block out the second premolars.


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An incisor diastema may be present. The plural fordiastema is diastemata.

Important: The deciduous anteriors--incisors andcanines are narrower than their permanent successorsmesiodistally.

Important: The deciduous molars are wider that their

permanent successors mesiodistally.

This size difference has clinical significance. Thedifference is called the leeway space.


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Examine the diagram. The leeway space in the lower

arch** is approximately 3.4 mm.The leeway space in the upper arch is approximately1.8 mm.* In normal development, the leeway space istaken up by the mesial migration of the first permanentmolars.


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D. Permanent dentition period.

-Maxillary / mandibular occlusal relationships are

established when the last of the deciduous teeth arelost. The adult relationship of the first permanent molarsis established at this time.

A lateral view of the molars, right side, during succession to thepermanent dentition


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Occlusal and proximal wear reduces crown height to thepermanent dentition and the mesiodistal dimensions of theteeth.

The proximal wear results in a decrease in arch length. Insome persons, the proximal wear per arch can bedramatic. Losses of 1 cm or more have been reported.

Occlusal and proximal wear also changes the anatomy of

teeth. As cusps are worn off, the occlusion can becomevirtually flat plane. Some speculate that this is the 'normal'condition for an adult occlusion--something we do not seebecause of soft contemporary diets. Teeth worn to flatplane have exposed dentin, something we consider

pathological, but is the normal circumstance amongstmany precontact aboriginal peoples. There is a practicaladvantage to proximal wear: less incisor crowding andfewer impacted third molars. In our culture, we needorthodontic retainers and the skills of oral surgeons.


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In the absence of rapid wear, overbite and overjet tendto remain stable.

Mesio-distal jaw relationships tend to be stable, exceptfor changes due to disease such as acromegaly (anendocrine disease) or accident.

Teeth are lasting longer nowadays, thanks to fluorideand advances in dental care.

With aging, the teeth change in color from off white toyellow. smoking and diet can accelerate staining ordarkening of the teeth.

Gingival recession results in the incidence of more rootcaries. People are keeping their teeth longer, thus thereis more opportunity for root caries


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IV. Curve of Spee.

The cusp tips and incisal edges align so that there is asmooth, linear curve when viewed from the lateral

aspect. The mandibular curve of Spee is concavewhereas the maxillary curve is convex.

It was described by Von Spee as a 4" cylinder thatengages the occlusal surfaces.

It is called a compensating curve of the dental arch. There is another: the Curve of Wilson. Clinically, it

relates to the anterior overbite: the deeper the curve,the deeper the overbite. The Curve of Wilson is referred

to only infrequently in dental literature. If the Curve of Spee is flattened out during orthodontic

treatment, it tends to come back.


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Illustration of the Curve of Spee seen from a lateralperspective and the Curve of Willson viewed from the

distal end of the lower dentition


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Some crowding or spacing may occur after the age of40 years.

For most people, teeth become less sensitivecosmetically since the patient to hot and cold, thanks tosecondary dentin. With gingival recession, somepatients have sensitivity due to exposed dentin at the

cemento-enamel junction. Americans in midlife want more and better care for their

teeth. They will be increasingly have dental insuranceand will be more affluent.

Periodontal disease affects more people. Why? part of itis increased awareness on the part of dentists and theirpatients. Also, again, people are keeping their teethlonger.


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THE

END.

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Occlusion and Dental Developments

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