I. Theories of Tooth Eruption.
The beautiful drawings reproduced below come from a 1941 article by Massler and Schour that discusses the various theories of eruption as they were discussed in 1941 (AJO&OS 27: 552-576). You would think that so basic a process in dentistry would be fully resolved by now. It is not. A definitive article by Marks et al (Anat Rec: 245:374-393) published in 1996 still discusses theories. Briefly look over the drawings frame by frame; they nicely illustrate the eruption of the deciduous tooth, shedding, permanent tooth eruption, and--if lost--the edentulous condition.
Tooth eruption is the developmental process whereby the tooth moves in an axial direction from its location within the alveolar crypt of the jaw into a functional position within the oral cavity.
Numerous theories of tooth eruption have been proposed. These theories have involved almost all of the tissues in or near an erupting tooth. None of theories can alone account for all of the movements made by a tooth during its lifetime.
First we will briefly review theories that are NOT ACCEPTABLE.
(1) Vascular pressure and blood vessel thrust. It is well known that the teeth 'jiggle' up and down in synchrony with the arterial pulse. A logical test, therefore is to surgically remove the growing root. When this is done, the tooth continues to erupt. This explanation is not satisfactory.
(2) Pulpal pressure and pulpal growth. Multiplying cells in the root region of the tooth would seem to be an inexorable force in eruption. A test is to excise out the developing tooth, root and all, and replace it with a silicone replica. The result of such a test? The replica erupts as long as the dental follicle is intact. (The dental follicle, also called the dental sac, is the embryonic tissue which goes on to from the cementum, PDL, and the alveolar bone.)
(3) Traction by periodontal fibroblasts. Remember tractor beams in the movie Star Wars? It is that sort of idea. This theory says that fibroblasts tug and pull the developing tooth along its eruptive path. A test of this theory? The administration of drugs that block collage crosslinks have not effect. Teeth continue to erupt without the benefit of fibroblasts.
Next we briefly review some ACCEPTABLE theories.
(1) Root elongation. Root formation would appear to be an obvious cause. Yet, even rootless teeth do erupt. Some teeth will erupt along a greater distance than the length of their completed root.
(2) Alveolar bone remodeling. It is well established that alveolar bone growth, tooth development, and the eruption of the teeth are independent. Bone remodeling in response to orthodontic applicances is an every day occurrence. Also, the alveolar process forms as deciduous teeth erupt and it is lost when teeth are lost as seen most dramatically in older edentulous patients.
(3) Periodontal ligament. In humans, the periodontal ligament DOES NOT seem to be important in eruption. Animal experiments using rats show that the periodontal ligament activity seems important in rodent incisor eruption. This model does NOT apply to humans because rodent incisors grow continuously. Our teeth do not. The only remnant of such growth activity in ourselves are secondary eruption and mesial drift.
In conclusion, it appears that there is no single cause of tooth eruption. Experimental evidence clearly suggest that the dental follicle is an important element in tooth eruption. Study--and debate continue.
II. Butler's Field Theory (Concept of the Morphogenetic Field)
Why are there classes of teeth. In a single person, how do they get that way? Heterodonty, the specialization of teeth into classes has raised a number of theoretical questions. In experimental embryology, the early embryo has been viewed as a mosaic of 'organizational field', each of which pursues its own unique path.
Butler has applied this idea to teeth. It works like this: the tooth row is conceived of as three regions: the incisive region, the canine region, and the molariform. region. Butler includes premolars with molars in the molariform region.
In each region, there is a 'best copy', the standard bearer of the group. It is best illustrated with the canine since there is only one in its class. Canines are very stable teeth--and are seldom missing. Lateral incisors are variable in shape (peg-shaped) or are sometimes congenitally missing. In the molariform group, the first permanent molar is the 'best copy' of the group. Second and third upper molars are progressively smaller, the distolingual cusp tends to disappear, and the tooth retreats from being rhomboidal in shape to become a heart shape.
Going anteriorly in the tooth row, the pattern is less clear. Paleontologists tell us that the teeth we know as first and second premolars are actually the third and fourth premolars in the tooth row. Primitive mammals had four premolars. The primitive first and second have been lost. So, in a sense, the premolars most distant from the first permanent molar in the molariform group have been lost altogether.
The reaction to the field theory is usually a yawn. Consider it as a springboard for scientific investigation by formulating a hypothesis: Events at one place along the tooth row should have an effect elsewhere along the tooth row. Indeed thisis so. We saw in aprevious unit that when one or more teeth are missing, then there is a greater chance that other teeth will be missing, or at least smaller in size.
The 'field theory' is one of the old war horses in dentistry. If it is applied to experimental work, it can be given new life.
III. Malocclusion, Disuse Theory and the Begg Hypothesis.
Why are so many people wearing 'braces'? Is malocclusion the natural condition for people? Weston Price, a dentist ealier in this century, travelled the world to document the relationship between diet and dental health. His work is ignored today, dismissed for its lack of academic rigor now demanded of scientific literature.
The essence of his findings is this: The incidence of malocclusion amongst aboriginal peoples increased after contact with commerercial societies and adoption of a contemporary diet. He found that in precontact aboriginal societies that virtually all individuals show a nearly ideal occlusion. Price attributed malocclusion to a change in diet. Recent evidence suggests that this is only part of the story. Price had attributed malocclusion to nutritional change. He published his definitive work in 1939.
In the 1950s, P. R. Begg, an Australian orthodontist proposed another idea: modern diets prevented the normal wear necessary for a satisfactory occlusion. He studied Australian Aborigine dentitions and observed the considerable occlusal and proximal wear present. Begg concluded that proximal wear on the order of a half inch reduction per arch was normal--and necessary for the occlusion of the teeth. He proposed aggressive action: extraction of teeth to compensate for the lack of proximal wear. We know this theory as the Begg Hypothesis.
Corrucini has gone a step farther with these ideas by returning to one of the oldest theories in orthodontics: the disuse theory. Simply stated, it says that our jaws--particularly the mandible--do not grow to their full potential unless subjected to vigorous use in childhood.
If stated as a proper scientific hypothesis, it is subject to verification or falcification. This is the test of good science. His theory can be stated as follows: Malocclusion results from the lack of chewing stress with modern processed foods. This disuse results in less jaw growth and more malocclusion. In a series of animal experiments with rats and Old World monkeys, Corrucini has confirmed significant dental changes similar to human malocclusion when the test animals are fed soft diets that don't require chewing. His results are tantalizing and provocative. Should we feed our children beef jerky instead of Twinkies?
IV. Traces of Dental Evolutionary History
(1) our dentition has retained the ancient relationship from when teeth were used for piercing and tearing, not chewing and grinding as we do today. This pattern is called alternation. It may be said as follows: each lower tooth relates to the upper tooth of its own number and the one in front. (Note that the 'number' is the count from the midline, NOT the Universal Numbering System.)
Look at the diagram to see how it was is the crocodile and how it is in our own dentition today.
(2) A general rule in embryonic development is that development tends to proceed from the front end of an organism and progress posteriorly. We retain that pattern in a general sort of way. Anterior teeth tend to erupt first, the anterior deciduous teeth are shed first, and are replaced first (with minor exceptions). Examine this diagram of the development of the dentition..
V. The Secular Growth Trend (Also known as the Generational Growth Trend.)
Children and adults in the industrialized countries of the world are taller and heavier than their counterparts 150years ago. The trend is significant, and is documented in every country with appropriate record keeping and it is true for every ethnic group. There are three significant trends:
(1) an increase in adult height
(2) an increase in adult weight
(3) earlier maturation in both height and weight. The age of menarche has dropped by two years since the year 1800.
Examine the distance curves shown, taken from Baer. This phenomenon has attracted considerable debate. No single cause seems to be responsible for it. Numerous factors have been cited as the cause: improved nutrition, reduction in childhood infectious diseases, improved health care, an improved standard of living, and reduction in family size. There is a vigorous scientific debate as I write this: some blame it on hormone disrupters that acontaminate the environment and have entered the food chain.
There is a dental implication: teeth are a bit larger also nowadays. Many of the average dimensions cited in textbooks are for teeth extracted from individuals early in this century. The average dimensions have increased since many of those figures were first established.
VI. The Paramasticatory Functions of Teeth
In the daily practice of dentistry, we recognize the principle functions of teeth as mastication, esthetics, and speech. We also use teeth for purposes beyond their conventional role in our lives. We call these paramasticatory uses of teeth. Such functions might best be described as the use of teeth as tools. You have done it when you have opened a restaurant package of crackers or nipped off a piece of thread when sewing a button. One of the most bizzare demonstrations occurred when a student a few years ago brought a videotape of a modern-day Eskimo opening a 55 gallon drum with his teeth!
Neandertals (and more modern peoples since them) used their front teeth as a built-in tool, serving as pliers to hold one end of some material such as wood or hide so that one hand would be free to cut, scrape or pierce the material with a stone implement. Wear patterns on the worn incisors of some specimens suggest that the Neandertals softened animal hides by chewing them.
Workers in more recent times have left evidence of their handling of plant fibres. Years of drawing them between their teeth has left characteristic grooves on the occlusal surfaces.
In ancient times, the dutiful Eskimo wife would chew her husband's boots to soften them in preparation for the next day's hunt. How is that for aparamasticatory use of teeth!
VII. Age and Aging of Teeth
(1) Tooth developmental charts offer many clues, including stages of development for the individual teeth. The neonatal line is a clue to death before birth.
(2) The specific gravity of teeth increases with age.
(3) Individual teeth offer clues as to the age of the individual from whence they came, including clues to age such as blunderbuss root ends, excessive wear, reduced pulp chambers, sclerotic dentin, heavy accretions of dentin.
(4) There is an increase in the hardness with aging.
(5) Tooth wear is increased with age. First permanent molars wear more than seconds; seconds more than thirds.
(6) Sclerotic dentin increases with age. You have seen sclerotic dentin in your laboratory dissections.
(7) The thickness of cernentum increases with age. A very rough rule of thumb is that it doubles in thickness between 25 and 75 years of age.
(8) Teeth tend to darken and acquire stains with age.
(9) Secondary dentin increases with age.
VIII. The Self Protective Features of the Dentition
(1) The axial position of the tooth is important. It is easy to lose sight of this fact when we work with teeth in the laboratory. In general, there is a slight mesial, buccal or lingual inclination of the tooth in its proper jaw relationship. To see this clearly, examine the drawing to the right of this text. Very likely, the long axisof each tooth is positioned to best withstand the occlusal forces imposed upon it. Another idea to understand is this: Axial tooth position changes the effective crown form. A tilted axis can increase or decrease effective tooth contours.
(2) In normal function, the teeth serve to protect two important areas of the periodontium: The gingival tissues and the attachment apparatus.
Wheeler's Tooth Deflection Theory says that 'convexities in the gingival third of the tooth serve to protect against the gingival impaction of food.
Examine the diagram below. Observe how the form and alignment of the teeth serve to shelter the vulnerable gingival crevice** from damage.
You won't find the 'Wheeler Theory' in the literature any more; however it is implicit in all of current clinical science. Here we give it a name.
Incidentally, the facial and lingual contours are convex to about 0.5 mm. on the facial surfaces, the height of contour is at or is within the gingival 1/3rd. on the lingual surface, the height of contour for posterior teeth is in the middle third. Why are anterior teeth excluded? They have cingula!
(3) Normal saliva flow and muscular activity is importantfor cleansing of teeth.
The Morris Muscle Action Theory says that 'teeth are self cleansing by action of cheek musculature and saliva flow over the smooth tooth surfaces. You won't see this one in current literature, either. It is implicit nowadays in clinical theory. You will become aware of its significance when you encounter patients with reduced saliva flow (often due to radiation or chemotherapy) or inadequate muscular activity (seen in stroke victims or cerebral palsy).
VIII. Mesial and Distal Curvature of the Cervical Line
This short unit is about the height of curvature of the cervical line (CEJ) as it appears on the mesial and distal surfaces. You will understand it best if you examine the two sets of drawings shown here.
The rules are as follows: the proximal CEJ curvature is greatest anteriorly and the least posteriorly. They are about the same on mesial and distal surfaces that face face each other. On any given tooth, the height of curvature is greater on the mesial aspect of that tooth than it is on the distal.