I. Genetics and Environment: Introduction

The size of the teeth and the timing of the developing dentition and its eruption are genetically determined. Teeth are highly independent in their development. Also, teeth tend to develop along a genetically predetermined course. One further comment on this issue: tooth development and general physical development are rather independent of one another. Serious illness, nutritional deprivation, and trauma can significantly impact development of the teeth. This genetic independence (and their durability) gives teeth special importance in the study of evolution.

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. Most important, age and sex can be recorded. With loose teeth in museum collections, age and sex cannot be determined with reliability.

When teeth erupt into the oral cavity, a new set of factors influence tooth position. As the teeth come into function, genetic and environment determine tooth position. For example, tooth arrangement is affected by muscle pressure, as we will discuss shortly.

One final comment: in this course, we treat teeth as a unisex topic. In real life, however, girls shed deciduous teeth and receive their permanent teeth slightly earlier than boys, possibly reflecting the earlier physical maturation achieved by girls. Teeth are slightly larger in boys that in girls; however, we cannot make that distinction by looking at a single extracted tooth.


II. Development of occlusion.

He who sees things grow from the beginning will have the best view of them.

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 an introductory definition for one type of occlusion, centric occlusion. 

Centric occlusion is the maximum contact and/or intercuspation of the teeth. Comment: this is a static definition--it describes just one position in the chewing cycle.

B. Occlusion is the sum total of many factors.

1. Genetic factors.

-Teeth can vary in size. Examples are microdontia (very small teeth) and macrodontia (very large teeth). Incidentally,

Australian aborigines have the largest molar tooth size--some35% larger than the smallest molar tooth group (Lapps and !Kung san Bushmen).

-The shape of individual teeth can vary (such as third molars and 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.

2. Environmental factors.

-Habits can have an affect: wear, thumbsucking, pipestem or cigarette holder usage, orthodontic appliances, orthodontic retainers have an influence on the occlusion.

3.Muscular pressure.

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

-Teeth are affected by muscular pressure on the facial side (by cheeks/lips) and on the lingual side (by the tongue).

Illustration of Angle classification, open bite, a mesiodens, wear, and spacing

C. Occlusion constantly changes with development, maturity, and aging.

1 . There is change with the eruption and shedding of teeth as the successional changes from deciduous to permanent dentitions take place.

2. Tooth wear is significant over a lifetime. Abrasion, the wearing away of the occlusal surface reduces crown height and alters occlusal anatomy.

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

(A note on terminology. Abraision and attrition have subtle differences in meaning. Abraision is the wear of teeth by agencies other than the friction of one tooth against another. In contrast, attrition is the wear of teeth by one tooth rubbing against another.)

3. Tooth loss leaves one or more teeth without an antagonist. Also, teeth drift, tip, and rotate when other teeth in the arch are extracted.

D. Variation is the rule. 'We are a mongrel lot.' Just look around this classroom! Seldom do we see the ideal presented in textbooks.


III. Periods of Dental Development. (Stages of dentofacial development)

This presentation is about the development of occlusion. An obvious omission is what happens before 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 histological evidence of tooth development appear during the second month of intrauterine life. Calcification of deciduous incisors begins at 3-4 months in utero. By the time of birth, calcification of all the deciduous teeth is well under way.


A. The pre-dentition period.

-This is from birth to six months.

-At this stage, there are no teeth. Clinically, the infant is edentulous (without teeth, remember?).

-Both jaws undergo rapid growth; the growth is in three planes 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 precede eruption of the deciduous teeth. At birth, the molar pads can touch.

-Comment: When crowns are fully calcified, their growth in size is completed. Therefore, it is a challenge for teeth of a fixed size to accomodate into rapidly growing jaws. One way this challenge is met is by teeth maturing progressively from anterior to posterior in their respective arches.

B. Deciduous dentition period.

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

-The jaws continue to increase in size at all points until about age one year.

-After this, growth of the arches is lengthening of the arches at their posterior (distal) ends. Also, there is slightly more forward growth of the mandible than the maxilla.

1. Many early developmental events take place.

-The tooth buds anticipate the ultimate occlusal pattern.

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

(1) mandibular central

(2) maxillary central incisors

(3) then all four lateral incisors.

-By one year, the deciduous molars begin to erupt.

-The eruption pattern for the deciduous dentition as a whole is:

(1) central incisor

(2) lateral incisor

(3) deciduous first molar

(4) then the canine

(5) then finally the second molar.

-Remember, we are talking about primary teeth!

-If deciduous teeth are retained too long, consider ankylosed teeth or missing or impacted teeth.

-Eruption times can be variable. If you see a child who is unusually early or late in getting their teeth, inquire about older siblings or parents. (Remember our remarks about tight genetic control?) It applies to patterns of eruption, too.

2. Occlusal changes in the deciduous dentition.

-Comment: we here introduce the terms overjet and overbite. The key to understanding them is another basic element of human occlusion. The size of the maillary arch tends to be greater than that of the mandibular arch. Therefore, when they are 'superimposed' one atop the other, the maxillary teeth 'overhang' the mandibular teeth. Examine the diagram to clearly understand this concept.

Illustration of maxillary/mandibular superimposition, overjet/overbite, and the progressive wear of deciduous teeth the flat plane occlusion

-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: clinicians often regard such wear as excessive. In Stone Age dentitions, however, it is a frequent finding.)

-Often the deciduous teeth are badly chipped and worn. The 'flat plane' occlusion', unusual in our fast food culture was the norm in food forager and early agricultural societies when food contained grit from mill stones.

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

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

-Primate spaces occur in about 50% of children. They appear in the deciduous dentition. The spaces appear between the upper lateral incisor and the upper canine. They also appear between the lower canine and the deciduous first molar. In great apes and some monkeys, the primate spaces are a special type of diastema that function to accomodate the large canines.

Illustration of the primate spaces seen in 50% of human dentitions

C. Mixed Dentition Period.

-Begins with the eruption of the first permanent molars distal to the second deciduous molars. These are the first 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 to spacing. In the older literature, is called by the 'ugly duckling stage.' With the eruption of the permanent canines, the spaces often will close.

Illustration of molars and incisors

-Between ages 6 and 7 years of age there are:

20 deciduous teeth

4 first permanent molars

28 permanent tooth buds in various states of development

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. Partial or total anodontia. When yousee missing teeth in children, always ask about the family history. Heredity is significant.

Illustration of supernumerary incisor

c. Supernumerary teeth.

d. Microdontia

e. Macrodontia

F. Microdontia

-Comment: here are two terms that appear in older dental literature and in National Boards from many years ago. They are presented here--because they aren't in most dictionaries. Hypodontia is a term used by British dentists. Hypodontia describes what we call partial anodontia (one or more missing teeth, remember?). The term oligodontia is used by the British when more than half the teeth are missing.

2. Errors in skeletal alignment. Malpositioned jaws disrupt normal 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.

-Comment: why does malocclusion occur? The incidence of malocclusion tends to be greater in 'civilized' societies with Western diets and living styles. It is a complex problem with no single answer. For a challenging and provocative discussion, see Weston Price: Nutrition and Physical Degeneration.

Illustration of maxillary lateral incisor variability and an unerupted maxillary central incisor

-In the mixed dentition, the deciduous second molars have a special importance for the integrity of the permanent dentition. Consider this: The first permanent molars at age six years erupt distal to the second deciduous molars.

-Permanent posterior teeth exhibit physiological mesial drift, the tendency to drift mesially when space is available. If the deciduous second molars are lost prematurely, the first permanent molars drift anteriorly and block out the second premolars. Check out the diagram shown above to the right. Solution? If a second deciduous molar is extracted prematurely, a space maintainer can be fabricated by the dentist.

Illustration of the consequences of premature loss of the lower second deciduou molar

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

-Important: The deciduous anteriors--incisors and canines are narrower than their permanent successors mesiodistally.

-Important: The deciduous molars are wider that their permanent successors mesiodistally.

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

-Examine the diagram. The leeway space in the lower arch** is approximately 3.4 mm.

-The leeway space in the upper arch is approximately 1.8 mm.* In normal development, the leeway space is taken up by the mesial migration of the first permanent molars.

D. Permanent dentition period.

-Maxillary / mandibular occlusal relationships are established when the last of the deciduous teeth are lost. The adult relationship of the first permanent molars is established at this time.

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

-Occlusal and proximal wear reduces crown height to the permanent dentition and the mesiodistal dimensions of the teeth.

-The proximal wear results in a decrease in arch length. In some persons, the proximal wear per arch can be dramatic. Losses of 1 cm or more have been reported.

-Comment: occlusal and proximal wear also changes the anatomy of teeth. As cusps are worn off, the occlusion can become virtually flat plane. Some speculate that this is the 'normal' condition for an adult occlusion--something we do not see because of soft contemporary diets. Teeth worn to flat plane have exposed dentin, something we consider pathological, but is the normal circumstance amongst many precontact aboriginal peoples. There is a practical advantage to proximal wear: less incisor crowding and fewer impacted third molars. In our culture, we need orthodontic retainers and the skills of oral surgeons.

-In the absence of rapid wear, overbite and overjet tend to remain stable.

-Mesio-distal jaw relationships tend to be stable, except for changes due to disease such as acromegaly (an endocrine disease) or accident.

-Teeth are lasting longer nowadays, thanks to fluoride and advances in dental care.

-With aging, the teeth change in color from off white to yellow. smoking and diet can accelerate staining or darkening of the teeth.

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

Illustration of the upper anterior teeth with proximal root caries

.-Periodontal disease affects more people. Why? part of it is increased awareness on the part of dentists and their patients. Also, again, people are keeping their teeth longer. Check out the root caries shown in the diagram.

-Some crowding or spacing may occur after the age of 40 years.

-For most people, teeth become less sensitive cosmetically since the patient to hot and cold, thanks to secondary dentin. With gingival recession, some patients have sensitivity due to exposed dentin at the cemento-enamel junction.

-Comment: Americans in midlife want more and better care for their teeth. They will be increasingly have dental insurance and will be more affluent.


IV. Curve of Spee.

-The cusp tips and incisal edges align so that there is a smooth, linear curve when viewed from the lateral aspect. The mandibular curve of Spee is concave whereas the maxillary curve is convex.

-It was described by Von Spee as a 4" cylinder that engages the occlusal surfaces.

-It is called a compensating curve of the dental arch.

Illustration of the Curve of Spee seen from a lateral perspective and the Curve of Willson viewed from the distal end of the lower dentition

-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.


V. Overjet and overbite.

-These are genetic in origin, but can be altered by environment Excessive thumbsucking can increase overjet, for example. In deciduous teeth, they tend to diminish with age. In our culture, they tend to be stable in adults. They diminished in Stone Age man as the teeth were worn to a flat plane. The overwhelming mechanical reason for overjet is that the upper arch is larger and overlies the lower.


VI. Compensating curvatures of the individual teeth.

-This describes the gentle curvature of the long axes of certain posterior teeth to exhibit a gentle curvature.

-These are probably analogous to the trabecular patterns seen in the femur and therefore reflect lines of stress experienced during function.

-Spirals and helices are often encountered in nature. Examples are pubic hair, untrimed finger and toe nails, tusks in elephants, and growing incisors in rodents.


VII. Posteruptive tooth movement. These movements occur after eruption of the teeth into function in the oral cavity. These movements, known collectively as occlusomesial forces are discussed below. It is important for you to learn them and understand them now. In recent literature, the two forces listed below are collectively described as 'occlusomesial' forces. For clarity, we here used the older terms.

Illustration of compensating curvatures of molar teeth, continuous eruption, the direction and intensity of mesial drib, and the Angle Class I molar relationship as shown with the right first permanent molars seen in lateral perspective


A. Continuous tooth eruption is just what the term says: the ongoing eruption of teeth after coming into occlusion. This process compensates for occlusal tooth wear. We mentioned before that cementum thickens in older teeth. Cementum deposition and progressive remodelling of the alveolar bone are the growth processes that provide for continuous

B. Physiological mesial drift describes the tendency of permanent posterior teeth to migrate mesially in the dental arch both before and after they come into occlusion. Clinically, it compensates for proximal tooth wear. -Both continuous tooth eruption and physiological mesial drift are well documented, but the underlying mechanisms are poorly understood. Here are some general observations on physiological mesial drift that will help you in your clinical career.

(1) It describes the tendency of posterior teeth to move anteriorly.

(2) It applies to permanent teeth, not deciduous teeth. (See the remarks at the end of this section on why this is so.)

(3) The further you go distally, the stronger is the tendency for drift.

(4) It compensates for proximal wear.

(5) In younger persons, teeth drift bodily; in older persons, they tip and rotate.

(6) Forces that cause it include occlusal forces, PDL contraction, and soft tissue pressures. There may be other more subtle factors as well.

(My note: Why is mesial drift not a significant force in the deciduous dentition? A possible functional explanation is this: Deciduous molars are wider than their permanent successors. If they experienced considerable wear on their mesial and distal surfaces, malocclusion of the permanent dentition would be an inevitable result)


VIII. Normal or 'Ideal' Occlusion

-The ideal molar cusp and buccal relationship as described by Angle (the 'father' of modern orthodontics) is for the mesial cusp of the upper first molar to fit into the buccal groove of the lower first molar. Examine the diagram above so that you clearly picture this relationship.

.....CJ '98