Level I Growth and Development - Unit B__formatted_raw__v1

{{PAGE_1}} LEVEL I · GROWTH AND DEVELOPMENT

Unit B

Growth and Development During the Preschool Years · Primary Tooth Eruption and Exfoliation · Development of Occlusion in the Primary and Transitional Dentition

Proffit Instruction — generated for offline reference

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{{PAGE_2}} Growth and Development During the Preschool Years Primary Tooth Eruption and Exfoliation Development of Occlusion in the Primary and Transitional Dentition

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1. Growth and Development During the Preschool Years

Introduction

This is the first of several programs that focus on specific features of human growth and development. Before viewing this program, you should have looked at the introductory programs in this series that deal with general principles in growth and the elements that control it.

This module is divided into three sections:

• Section 1 describes and sets limits on normal growth variation.
• Section 2 points out important factors that affect growth and cause variation.
• Section 3 provides details of craniofacial growth during the preschool (primary dentition) years.

Be sure you do the reading (pages 66-72 in the 5th ed or pages 72-86 in the 4th ed of Contemporary Orthodonticsand the appropriate section in your embryology text) as well as viewing this program.

Learning Objectives

The learning objectives for this module are to:

• identify the embryologic source of the facial tissues
• relate the most frequent types of facial congenital deformity to the time in embryologic development when they arose
• describe the process of fusion of the upper lip and palate, and identify the facial processes whose failure to fuse results in a cleft of the lip
• describe the changes in facial proportions from birth to early adolescence
• use percentiles to select children who need evaluation to rule out pathologic effects on growth
• describe gender differences in the rate and timing of growth
• describe the effect on growth of low birth weight, acute and chronic illness, emotional deprivation, and malnutrition
• discuss secular trends in growth and indicate their effect on the accuracy of growth charts.

Normal Growth Variations

What is Normal?

{{PAGE_4}} The brothers shown here are about the same size, but one is 6 years old and the other is 9. Can you tell which is the 9 year old? It’s the one on the right, who is slightly shorter than his brother, weighs considerably less and is slow in erupting his permanent teeth.

Do you think this represents normal variation? It will be very important in professional practice for you to know whether you are dealing with a child who is just small for his age, or one who is suffering from some condition that would influence the kind of treatment he should receive.

What is Normal? (cont.)

The children pictured here are siblings, but only a year apart in age. The girl is shorter and less mature looking than her brother. The same question as with the last picture: given the age difference, is this normal variation, or could one or both be abnormally advanced or delayed in their growth?

How could you determine that? The best way would be to see where each of them would fit on a growth chart that shows the range of normal variation.

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{{PAGE_5}} Normal Variation: Populations The normal bell-shaped curve of variability in populations, which you are familiar with by now, expresses very well the differences in physical growth of a group of children of the same age and gender. The important question in examining any child patient is whether differences from the mean of the normal curve represent just normal variation, or whether there is an indication of abnormality in growth and development.

{{PAGE_6}} Do you recognize that this graph of variation in birth length and weight, and subsequent growth, is another version of a normal distribution? If the data were plotted as numbers of children with a given birth length or weight at any specific time, a bell-shaped curve would have been generated. You would get another normal distribution if you took any other point in time. This plot shows you the percentage of children with birth length or weight above or below a certain amount and the percentage distributions at other points in time in early life.

Normal birth weights can range from 2000 grams or about 4 ½ pounds to 4000 grams or nearly 9 pounds. Birth weights outside this range would be considered at least suggestive of abnormality. In fact, it is common today to speak in terms of birth weight instead of prematurity when discussing newborns who are at risk because of their immaturity at birth. But how was that normal range established?

The Limits of Normal Variation

The normal range for physical characteristics typically extends from the 3rd to the 97th percentiles, which are outside the colored area of a chart like this.

As a general guideline, children above the 97th or below the 3rd percentile are considered possibly abnormal and in need of special investigation. It is possible that these children are simply very small or very large normals, but there is also a significant chance of an abnormality affecting growth. This is true at all stages of growth, from infancy through adolescence.

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{{PAGE_7}} Note that the percentile lines form channels. A normal child tends to stay in the same channel. In other words, a child who starts out at the 60th percentile tends to stay there during growth. So does a child who starts out at the 20th percentile. A major change in the percentile rank suggests a deviation from the normal growth pattern and a possible abnormality.

Factors Affecting Growth

Important Factors That Influence Growth Let’s look now in more detail at important factors that influence growth. Don’t forget that normal variation should go at the top of the list.

FACTORS INFLUENCING GROWTH

• Normal variation
• Heredity / genetic traits
• Changes in body proportions
• Gender
• Low birth weight
• Chronic disease
• Psychologic / emotional factors

{{PAGE_8}} Nutrition Ethnic / cultural background Urban / rural environment Socio-economic status / family size Secular trends

Heredity Heredity is an obviously important factor. The boy on the left, the youngest but tallest of this group, has the tallest father, and that isn’t coincidence. Generally speaking, large children have large parents. It is not uncommon, however, to see a small or large child in a family that has a tendency toward the opposite trait.

For all human populations, there is a modest but not perfect correlation between the size of the parents and the size of the children. The correlation coefficient (r) is 0.5 or less. R², the coefficient of determination, expresses the amount of the total variation accounted for by the factor being examined. If the correlation between parent and child size is 0.5, (0.5)² = 0.25, so heredity explains not more than 25% of the variation in size.

The bottom line: heredity is an important factor in determining how tall you are, but it’s far from the only thing.

Cephalocaudal Growth Gradient

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{{PAGE_9}} Another factor that must be considered in growth variations is changes in body proportions and changes in tissue systems during growth. Facial proportions, as you know, reflect the cephalocaudal gradient of growth (image 1). That’s another way of saying that generally structures close to the brain grow faster sooner and slower later than those further away.

Have you ever considered that you’re used to looking at differences in facial proportions as you evaluate how old a child is? You might well make the judgment “big for his age” based on that.

Proportions & Systems To a major extent, the changes in body proportions with age reflect different growth rates for different tissue systems. Classically, four major tissue systems are distinguished, and a display like this of their growth pattern is called “Scammon’s curves” after the anthropologist who first published them.

The curves show the early growth of the nervous tissues (and the neurocranium), the tremendous proliferation of lymphoid tissue early in life and its “negative growth” later, and the initially slow and then very rapid growth of sexual tissues. The adolescent growth spurt, and all the things that contribute to that, will be emphasized when we study adolescence in more detail.

You need to be able to draw these curves from memory, so look at them carefully and be sure you understand what is happening during growth of each of the tissue systems.

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Proportions & Systems (cont.)

The change in facial proportions with growth reflects an interaction between the neural and general body growth curves.

As you have already learned in a different context, the growth of the maxilla is influenced to some degree by growth of the cranial base, which in turn has to grow in about the same way as the brain. Although mandibular growth is closer to the general body curve, it too is somewhat influenced by growth of the neural structures above. We can plot curves for the maxilla and mandible like the ones Scammon plotted, making this relationship clearer and clarifying why you should expect the mandible to be underdeveloped relative to the maxilla in early childhood and why it would catch up later.

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{{PAGE_11}} Lymphoid, Neural, Maxillary, Mandibular, General, Genital (growth curves by age)

Gender Another important factor in growth variation is gender. That’s more important in adolescence and adulthood than childhood, however. Boys are a little larger than girls at early ages, but at ages between 10 and 12 girls are likely to be larger, and the adult size differential isn’t there until the males have their later and more intense growth spurt. Note the similar growth velocities for boys and girls up to puberty.

{{PAGE_12}} Low Birth Weight

Low birth weight, which primarily indicates premature birth, is an important factor in growth variation early in life. Immaturity of the child’s organs, especially the respiratory tract, can greatly complicate the early weeks of life after premature birth. Perhaps it will surprise you to learn, however, that the vast majority of low birth weight children catch up and eventually grow quite normally.

This graph illustrates catch-up growth in two low birth weight groups, twins who were small for gestational age (SGA), which twins usually are because of intra-uterine competition for resources, and twins with birth weight less than 1750 grams (born quite prematurely). Note that by age 6 both groups were nearly to the norm. Low birth weight has a long-term residual effect on some children, but for most it isn’t a factor after childhood.

{{PAGE_13}} Chronic Disease Chronic illness such as congenital heart disease or an endocrine dysfunction can lead to sustained deficiencies in growth. This usually affects both height and weight, with about the same impact on all tissue systems.

This graph shows the growth of a boy with growth hormone deficiency. Note that he was below the normal range in infancy, and that his lack of growth moved him further from the normal.

But if chronic problems are corrected, catch-up growth usually occurs. Note that when injections of human growth hormone (HGH) began at age 6, this boy grew faster than normal, and almost made it to the 3rd percentile at age 14.

Successful repair of a congenital heart defect might have the same effect. Such a child probably would never get back to the mean, but might pull into the normal range.

{{PAGE_14}} Short-term Illness Short-term illness (severe influenza leading to pneumonia, for instance) causes fluctuations in growth rates, but has little long-term impact. Even more chronic but still transient illness (mononucleosis, for example) usually is followed by catch-up growth that takes a child back to the channel he was in before getting sick. One way to look at it is that absence of catch-up growth indicates a medical problem that hasn’t been corrected.

Psychologic/Emotional Psychologic and emotional factors also can affect growth. This graph shows the result of changing the social environment of two children who had fallen outside the normal range. Neither child had an identifiable organic cause for the growth problem. When each was removed from a very poor home environment and placed in a special boarding school where presumably emotional stress was reduced, the catch-up growth response was dramatic.

It takes severe levels of emotional distress to depress growth, but there’s no doubt this can occur. The mechanism is thought to be induction of a reversible growth hormone deficiency.

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Nutrition

The role of nutrition in growth is probably understood best from examining a chart of this type, which shows a partitioning of energy requirements. When nutrition is inadequate, the partitioning reflects physiologic priorities. Growth is, so to speak, taken off the top. If nutrition is barely adequate, generalized growth will be depressed. But if it’s adequate, better nutrition doesn’t lead to much if any increase in growth.

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{{PAGE_16}} Nutrition (cont.) Another way to consider nutrition and growth is to look at the amount of energy needed for growth as a percent of intake. An infant needs to devote a considerable proportion of the total toward growth, and this need is particularly acute for a premature infant. After that, the proportion devoted to growth decreases rapidly. So from a growth perspective, malnutrition at an early age would be particularly significant.

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{{PAGE_17}} Ethnic/Culture The ethnic and cultural background of an individual isn’t really a factor in growth, but it can greatly affect how growth is perceived, because the standards one is judged against can make a big difference.

For example, note the 50th percentile points for Dutch boys (blue lines) and 50th percentile points for boys from an American city (red lines), placed on this typical growth chart. At age 10, the average Dutch boy is 140 cm tall, while the American boys average 135 cm. Five cm is nearly two inches, not a trivial difference. When growth charts are used, you have to be careful about the source of the data that was used to construct them and how well the individual who is being charted fits the reference group.

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{{PAGE_18}} City/Rural

Still another potentially important variable is whether you live in the city or a rural area. This graph (image 1) shows the height of 7 year old boys in several countries, developed and underdeveloped, as of 1970. Note that the city children were consistently bigger at the same age. More recent data (image 2) shows the same trend.

The US population shows the same effect. The difference isn’t nutrition (although the vast difference for Poland may reflect a nutrition component). More services and goods—health care, sanitation, welfare—are available in the city, and that seems to overshadow any detrimental effects of city noise and pollution.

{{PAGE_19}} Image 1, height of urban vs. rural children, 1970: Children who live in cities have an advantage in terms of access to services that promote health and growth.

Socioeconomic Status

Socioeconomic status (SES) and family size also can affect growth. This graph plots the height of 7 year old boys by social class and by the number of children in the family. SES is determined by the head of the household’s employment and educational status. Note that as SES declines, so does the height of the children. As the number of children in the family increases, height declines within the same SES. These differences are attributed to the amount of resources available to the individual family members.

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{{PAGE_20}} Secular Trends There has been a strong trend over the past few hundred years for humans to become larger, reach an equivalent size at an earlier age, and reach developmental milestones earlier. This has occurred throughout the world, even where nutritional status didn’t change. These changes are termed the secular trend in growth.

Menarche is a dramatic developmental milestone that tends to get recorded, and so there are particularly good data for this. It’s interesting that there has been a steady decline for at least 150 years in the age of first menstruation in girls. To some extent this probably reflects earlier acquisition of a critical body mass. All other things being equal, sexual maturation in girls and menarche occur when a critical mass of about 46-48 kg (100-105 pounds) is reached. Why the faster growth to that body mass occurs now, however, isn’t totally clear. Nutrition is a part of it but apparently not the whole story.

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{{PAGE_21}} Secular Trends (cont.) Whatever the reason for it, the secular trend needs to be taken into account when growth charts and percentile levels are used. Growth charts often are plots of data from 30 to 50 or more years ago. Plotting typical American children of the early 21st century on growth charts from 1940s puts you pretty close to the mythical city of Lake Wobegon, where all the children are above average. People like to hear that their child is above average, which may account for the popularity of old growth charts.

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Development Indicators

Earlier growth implies earlier physical maturation, and it is accompanied by quicker social and mental development too. Various biologic ages can be calculated, using developmental milestones, and it’s interesting to plot various indicators of a child’s physical and behavioral development on the same chart.

In this graph, various development indicators are plotted together for one particular girl. Like most people who are advanced (or a bit behind) in one characteristic, she was advanced in all of them.

On this chart, dental (the yellow line) represents dental age, calculated from the stage of dental development. Interestingly, dental age shows the poorest correlation with the other developmental scales of any of the biologic ages. A child who is advanced in everything else probably is advanced dentally too, but the dentist needs to remember that dental age doesn’t track very tightly with the other developmental ages.

{{PAGE_23}} Preschool Craniofacial Growth Cephalometric Overview of Craniofacial Changes

Now let’s focus more closely on craniofacial changes in the preschool years. These cephalometric tracings of the same child at age 2 (red) and age 6 (blue) are superimposed on the back end of the cranial base, at sella turcica (the cavity in the sphenoid bone that houses the pituitary gland).

Note that during this time the brain mass increased and the brain case expanded. The cranial base lengthened, and that pushed the maxilla and midface forward. The mandible moved downward and forward, and the face grew more than the cranium—all of which you would have expected from what you already know about facial growth.

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{{PAGE_24}} 2 years old 6 years old

Growth of Lymphoid Tissues

Scammon’s growth curves show a great deal of growth of lymphoid tissue in childhood. Most of this lymphoid growth occurs in areas you can’t see, but dentists get a good look at tonsils.

You’d expect the tonsils to be large in childhood and indeed they are (though usually not so large as these). Scammon’s curve predicts that they’ll normally shrink later, so large tonsils at age 6 or so often is a self-correcting condition.

{{PAGE_25}} Scammon’s curves: Growth of lymphoid tissue peaks and then retracts. Image 2, view of the tonsils: Tonsils are often enlarged near age six but then shrink.

Growth of Sex Organs and Sexual Maturation

Growth of the sex organs and sexual maturation in preschool children isn’t expected. If it occurs, which occasionally happens, something is wrong. The most likely cause is some sort of endocrine-secreting tumor, so a very sexually precocious child needs medical attention. As a dentist who treats children, you might be the first to note that your patient looked a lot more mature than one would expect at that age.

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Decline in Velocity of Craniofacial Growth

A plot of growth velocity shows that the rate of growth declines sharply from infancy to childhood. If Johnny isn’t growing as fast any more when he’s about to start to school, he’s probably perfectly normal. But notice that he’s still growing at a rate that requires new clothes and shoes every year. The rate of growth typically drops to its lowest level just before the onset of puberty and the adolescent growth.

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{{PAGE_28}} Cephalocaudal Gradient of Craniofacial Growth - Age 4

In this 4-year-old, the cranium still is large compared to the face, but the face has developed vertically as well as anteroposteriorly. The floor of the nose has descended from its earlier position just below the orbits. The maxilla and mandible have expanded laterally.

Note that the 4-year-old’s mandible has a more acute gonial angle than previously. The downward and forward growth of both jaws is evident.

{{PAGE_29}} Cephalocaudal Gradient of Craniofacial Growth - Age 6

This skull from a 6 year old child, just prior to eruption of permanent incisors, shows how the face has continued to grow faster than the cranium. The floor of the nose is now well below the orbits, and the alveolar processes of both maxilla and mandible have developed considerably.

The face still has a long way to go to catch up with the cranium, which at age 6 has reached essentially its adult size. By the time you’re six or seven, you need as large a cap as you’ll ever require, because by then brain growth is complete and the cranial vault is through expanding. In contrast, the adult mandible is twice as large as the six year old’s.

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Growth Beyond Age 6

Of course it is as important to understand growth beyond age 6 as it is to understand what happens in the pre-school years. We’ll continue into that time, with emphasis on the adolescent growth spurt and its implications for orthodontic treatment, in the next program.

Before you move on to the next module, be sure you have done the reading (pages 66-72 in the 5th ed or pages 72-86 in the 4th ed of Contemporary Orthodontics) and have used the self-test to confirm your understanding of this important information.

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2. Primary Tooth Eruption and Exfoliation

Timing and Variability of Eruption

Purpose of This Module

The purpose of this module is to review normal eruption and exfoliation of primary teeth and to identify common abnormalities associated with each process. In addition to viewing the module, be sure to read pages 86-91 (5th ed) or 97-103 (4th ed) in Contemporary Orthodontics. After viewing the teaching program and doing the reading, be sure that you are able to:

• describe the sequence and timing of the eruption of the primary teeth
• describe the pathways from the primary to permanent dentition, especially early to late mesial shift routes
• use the chronology tables to assess the dental developmental status of a child and determine the age at which dental development was affected if a problem is present
• describe the etiology, prevalence and treatment implications of the most commonly encountered disturbances of primary tooth eruption

Variability of Primary Tooth Eruption

The eruption of the primary teeth is relatively variable from child to child. Variation of 6 months of acceleration or delay is well within the range of normal.

However, the sequence of eruption of primary teeth is constant. Eruption of antimeres (i.e., left and right teeth) in the same arch usually occurs very close to the same time, so eruption can be described as symmetric in that sense. A lengthy delay in eruption between antimeres in the same arch may signal underlying pathology, such as a congenitally missing tooth or a supernumerary tooth that is blocking the path of eruption.

Eruption Sequence of Primary Teeth

Usually the first primary teeth to erupt are the mandibular lower incisors at 6-9 months of age. The maxillary central incisors then erupt and they are followed by the mandibular and maxillary lateral

{{PAGE_32}} incisors. (Image 1) After 3-4 months, the first molars erupt.

Some anxious parents may be concerned that there is a space between the newly erupted first molars and the lateral incisors. The dentist must explain that the primary canines will erupt into the space in a further 3-4 months and the space and sequence is part of the normal eruption sequence. (Image 2)

The primary dentition is completed at 24 – 30 months with the eruption of the mandibular, then the maxillary second molars. (Image 3)

{{PAGE_33}} Image 1, Newly erupted primary incisors: Erupted mandibular central incisors, maxillary central and maxillary lateral incisors in an infant of approximately 11 months of age

Image 2, Eruption of first primary molars: The first primary molars erupt before the primary canines, at around 1 year of age.

Image 3, Complete primary dental arch: Eruption of all 20 primary teeth, as seen in this 2-year-old child, is completed at approximately 24 months of age.

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Eruption Trends

Both gender and race affect the timing of eruption of the primary teeth. Eruption of primary teeth usually occurs earlier in girls than boys, and earlier in African-American children than in Caucasian children.

The eruption and development of the primary teeth serves as a template for the development of the permanent dentition. Tooth buds of the permanent teeth develop through a budding process off of the primary teeth. The succedaneous permanent teeth are intimately related to the overlying primary teeth.

Eruption Problems

Common Clinical Problems: “Teething”

A common problem related to the primary teeth is the complex of symptoms often summarized as “teething”. Eruption of primary teeth in infants is associated with drooling, increased salivation, restlessness and irritability.

In the past, systemic conditions such as croup, diarrhea and fever were also attributed to the eruption of primary teeth and dismissed as teething problems. Research has shown no association between systemic conditions (increase in fever and white blood count) and primary tooth eruption, which is a normal physiological process. Any systemic conditions that may occur during the eruption of the primary teeth should be considered coincidental, and should be investigated by a physician rather than being dismissed as a symptom of teething.

Dental intervention is usually not required during primary tooth eruption, but eruption may be hastened or the symptoms relieved by allowing the infant to chew on an approved commercially available teething aid like the one shown here.

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{{PAGE_35}} Common Clinical Problems: Eruption Hematomas

Eruption hematoma or cyst: During eruption of a tooth, as the tooth nears the epithelial surface, blood or fluid can fill the tooth follicle to form an eruption hematoma (which is also called an eruption cyst). This appears as a raised bluish lesion usually in the region of a soon-to-erupt second primary molar. The lesion is self-limiting. Once the tooth breaks the mucosa, the cyst and contents are lost and it disappears.

This picture shows bilateral eruption hematomas in the areas of the maxillary first primary molars.

Intervention is usually not required or indicated. The bluish lesion may cause some concern to untrained professionals or parents who suspect a more serious malignant pigmented lesion.

{{PAGE_36}} Common Clinical Problems: Natal Teeth

Natal and neonatal teeth: Natal teeth are teeth that are present at birth, while neonatal teeth erupt during the first 30 days after birth. The prevalence of natal teeth is approximately 1/3000 births and neonatal teeth is about 1/2000 births. In the majority of cases these teeth occur in the mandibular incisor region and are the true primary teeth and not supernumerary teeth.

These teeth can be very mobile due to incomplete root development. If there is a risk of aspiration due to displacement, extraction is indicated.

Sharp incisal edges can also cause ulceration and irritation of sublingual tissues in affected infants. In these cases extraction may also be indicated if the lesions are significant. The teeth may also cause discomfort for the mother during breast feeding.

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{{PAGE_37}} Delayed Eruption

If there are no primary teeth by 18 months, one should suspect an underlying cause and order a thorough dental and medical evaluation. A number of systemic conditions have found to be associated with delayed eruption of primary and permanent teeth. Some of these conditions include:

1. Trisomy 21 (Down Syndrome) An extra chromosome 21 is found in individuals with Down syndrome. Delayed eruption of the primary and permanent teeth are common, as is an abnormal eruption sequence. Exfoliation of primary teeth is delayed and over-retained primary teeth are common in adolescents and adults with the condition (Image 1).

2. Cleidocranial dysplasia In this condition the clavicles are either absent or rudimentary (Image 2). The primary teeth erupt without problems on the normal schedule, and since the permanent molars are formed from an extension of the primary dental lamina, they also erupt without major problems. The succedaneous teeth, however, develop normally initially but are blocked from eruption by three things: a) lack of resorption of the primary teeth, b) multiple supernumerary teeth, and c) heavy fibrotic gingiva that is difficult for the succedaneous teeth to penetrate if they do get that far. Over-retained primary teeth are common (Image 3.) In these children, extraction of the supernumerary teeth is a necessary part of treatment but by itself does not usually lead to eruption. Removal of overlying bone and incision of the gingiva also is needed. Even then, orthodontic treatment to bring the permanent teeth into the mouth usually is needed.

{{PAGE_38}} Children with Down syndrome have over-retained primary teeth and an abnormal eruption pattern.

Delayed Eruption (cont.)

1. Hypothyroidism This condition results from a deficiency in thyroid hormone production. A congenital form is seen if affected infants are not treated with thyroid hormone replacement

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{{PAGE_39}} medications. In these individuals delayed eruption of primary teeth and delayed exfoliation of primary teeth can result. In the juvenile form of the condition that develops in childhood, delayed exfoliation of primary teeth and eruption of permanent teeth also often occurs.

2. Hypopituitarism This condition develops due to a deficiency in growth hormone or an abnormality in the pituitary gland. Patients have deficient physical growth along with delayed eruption of primary teeth and delayed exfoliation of primary teeth.
3. Acondroplastic Dwarfism
4. Osteopetrosis (dense alveolar bone)
5. Ectodermal dysplasia (congenital absence and retained primary teeth)

Early Eruption of Primary Teeth Systemic conditions have also been associated with the early eruption of primary teeth. These are usually hormonal in nature and can include:

1. Hyperpituitarism Hyperpituitarism is due to an abnormality in the pituitary gland and excessive production of growth hormone. In these patients, increased growth is accompanied by early eruption of primary teeth.
2. Hyperthyroidism Excessive production of thyroid hormone also can be associated with early eruption of primary teeth.

Ankylosis of Primary Teeth Ankylosis occurs when a tooth becomes fused to the adjacent bone, instead of being separated from the bone by an intact periodontal ligament. Once ankylosis occurs, the affected tooth can no longer erupt.

In both the primary and mixed dentition, primary teeth may become ankylosed. The affected teeth are sometimes errantly referred to as submerged teeth because they appear to be submerging into the alveolar bone (Image 1.) This term is not acceptable. In reality, the ankylosed tooth remains static while the adjacent teeth continue to erupt and bring bone with them as jaw growth creates space between the upper and lower jaws (Image 2).

Primary teeth are more commonly affected than permanent teeth. The prevalence in Caucasian children is approximately 4%. It is lower in African-American children at approximately 1%. Mandibular teeth are more commonly affected than maxillary teeth, and the most commonly ankylosed tooth is the mandibular first primary molar. Ankylosis commonly occurs bilaterally and can affect multiple teeth in a single quadrant.

{{PAGE_40}} Image 1, Ankylosed primary second molars: The maxillary and mandibular second primary molars are ankylosed and significantly below the plane of occlusion, especially the first permanent molars. Image 2, Multiple ankylosed primary teeth: Multiple ankylosed primary teeth that are below the plane of occlusion. Coincidentally four permanent premolars appear to be congenitally missing.

Ankylosis of Primary Teeth (cont.)

Pathophysiology of Ankylosis: Resorption of primary teeth is a normal physiologic process that includes osteoclastic and osteoblastic activity near the root surfaces of teeth. In areas of recent osteoclastic resorption, osseous bridging may develop that results in fusion between the alveolar bone and root surface in the periodontal ligament space.

Diagnosis of Ankylosis: The best diagnostic criterion is evaluation of tooth position relative to the occlusal plane. If the primary tooth is below the occlusal plane and sufficient mesio-distal space exists for it to be positioned more superiorly, the tooth is diagnosed as ankylosed. An additional indication of ankylosis is the sound produced by tapping on the affected tooth. Ankylosis usually produces a sharp higher pitched sound that resonates through the alveolar bone. Normal teeth produce a more cushioned sound. Radiographs may reveal a reduced periodontal ligament space if the area of fusion occurs in a plane perpendicular to the central X-ray beam, but in the majority of cases, such radiographic evidence is not found.

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Sequelae and Treatment of Ankylosis

Sequelae of Ankylosis: Ankylosis of primary teeth leads to a posterior open bite in that area, and can significantly disrupt normal vertical development of the alveolar process. As the vertical discrepancy increases, the normal interproximal contacts are lost between adjacent teeth, and the adjacent teeth can tip mesially or distally over top of the ankylosed primary tooth, which reduces arch length (image 1).

The majority of ankylosed primary teeth eventually exfoliate without intervention, but exfoliation may be delayed. In some cases succedaneous permanent teeth may be absent underneath ankylosed primary teeth. Research has shown that this is coincidental and not a cause and effect relationship.

Treatment of Ankylosis: Usually no treatment is indicated. However in cases with loss of arch length and tipping of adjacent teeth, which typically occurs with significant submergence of the primary tooth, extraction of the tooth along with space maintenance or space regaining may be required (image 2). Even without space loss, if the ankylosed tooth is not exfoliated before it would be completely submerged, it should be extracted.

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{{PAGE_42}} Image 1: Radiograph showing ankylosed second primary molars allowing first permanent molars to tip mesially. Image 2: Radiograph of a space maintainer placed after extraction of ankylosed primary molars.

Resorption and Exfoliation

Root Resorption and Exfoliation of Primary Teeth

The process by which primary teeth are lost and replaced by the succedaneous permanent teeth is called exfoliation. Exfoliation, of course, occurs when the remaining root structure of the primary tooth is no longer sufficient to retain it in the dental arch. This panoramic radiograph of a nine year old boy shows erupting mandibular canines and first premolars and resorption of the overlying primary teeth.

Two processes occur during exfoliation: resorption of the overlying alveolar bone and primary tooth roots, and eruption of the permanent teeth. The degree of resorption of the primary teeth and the amount of root development of the permanent tooth can be valuable in estimating when the eruption of a permanent tooth is likely to occur, and this is the best way to determine dental age in children.

The resorption of the primary roots and alveolar bone occurs due to the action of osteoclastic cells.

{{PAGE_43}} Early Exfoliation and Systemic Conditions

Early exfoliation or loss of primary teeth can be associated with a number of systemic conditions, some of which can be very serious in nature. Early loss of multiple primary teeth in an atypical sequence is especially a concern as it may be a sign of a serious medical condition. For patients with abnormalities in loss of primary teeth or eruption of permanent teeth, a review of family history and genetic testing may be appropriate.

An astute dentist may be the first health care provider who suspects a systemic condition. Some of these conditions include:

1. Cherubism (images 1 and 2)
2. Acrodynia
3. Hypophosphatasia
4. Familial hypophosphatemic vitamin D-resistant rickets
5. Cyclic neutropenia (image3)
6. Progeria
7. Leukemia
8. Langerhan’s cell histiocytosis
9. Congential agranulocytosis

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{{PAGE_44}} Image 1, Cherubism: teeth: Panoramic radiograph of a patient with cherubism. The significant feature is bilateral multilocular cystic lesions of the mandible that result in expansion of the mandibular borders.

Image 3, Neutropenia: These radiographs show extensive alveolar bone loss in the primary dentition in a patient with neutropenia. The primary teeth seem to floating in air and will be lost prematurely.

Lingual Eruption of Permanent Incisors

The mandibular permanent incisors develop lingual to the primary incisors, and in some cases they erupt behind the primary teeth without loss of the primary incisors. First-time parents may be alarmed by this and seek treatment for the “two rows of teeth”.

In most cases intervention is not required. As the permanent teeth erupt further, the tongue will exert a forward force on the newly erupted permanent incisors that moves them forward; this forward movement stimulates resorption of the primary incisors and leads to their eventual loss. But, if there is not good spacing between the primary incisors, crowding of the permanent incisors is likely.

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Ectopic Eruption

The eruption of permanent teeth along a deviated path is called ectopic eruption. This is relatively common with the eruption of mandibular lateral incisors, occurs sometimes with maxillary first molars, and can occur in the eruption of any permanent tooth.

Mandibular permanent lateral incisors may erupt distally, causing resorption and early loss of the primary canine (image 1.) This usually is a sign of a crowding problem in the mandibular arch. Early unilateral loss of a primary mandibular canine results in a shifting of the dental midline and retroclining of the mandibular incisors (image 2), which usually requires intervention.

{{PAGE_46}} Image 1, radiograph showing ectopic eruption: The eruption of the permanent right lateral incisor resulted in the unilateral loss of the right primary canine. As a result the permanent incisors have shifted to the right. Image 2, dental cast showing ectopic eruption: This mandibular cast of the same patient shows the shifting of the lower midline to the right after early loss of the right primary canine.

Ectopic Eruption: Maxillary Molars

The second most frequently affected teeth for ectopic eruption are the maxillary first permanent molars.

In approximately 3% of children, mesial eruption of the maxillary first permanent molars causes significant resorption of the second primary molars. It is not uncommon to discover the ectopic eruption on routine bitewing radiographs (image 1.) Even though the resorption can be extensive, in the majority of cases the teeth are asymptomatic. Clinically, ectopically erupting maxillary first molars can appear partially erupted with only the distal portion of the occlusal surface visible (image 2). The primary molars can become mobile and can be lost prematurely due to the resorption (image 3).

{{PAGE_47}} Image 1, Radiograph of ectopic molars: The ectopically erupting maxillary first permanent molars are resorbing the distal surfaces of the second primary molars bilaterally. Image 2, Clinical presentation of ectopic molars: On clinical exam, an ectopically erupting maxillary may look partially or incompletely erupted. A bitewing radiograph would confirm the diagnosis and reveal any resorption of the adjacent primary molar. Image 3, Severe resorption: An ectopically erupting first permanent molar has caused severe resorption of the upper right primary second molar. On the left side, the maxillary primary second molar has been lost prematurely.

Ectopic Eruption: Treatment

In approximately 2/3rds of the children with an ectopic maxillary first molar, the affected permanent molar is able to jump the area of resorption, which allows eruption to occur (image 1) so the problem self-corrects.

After the initial diagnosis in a child age 7-9, a period of observation (3-4 months) is usually recommended to allow self-correction to occur. If it doesn’t occur, intervention will be required to actively disengage the two teeth to facilitate eruption of the permanent molar. Techniques such as a brass separating wire, orthodontic separator, or an active appliance may be indicated. (image 2).

{{PAGE_48}} Image 1, Self-correction: Serial bitewing radiographs taken 3-4 months apart show the self-correction of an ectopically erupting maxillary molar.

Image 2, Treatment: Appliance with a band cemented on the second primary molar and an adjustable spring used to correct an ectopically erupting maxillary molar.

Image 3, Correction of an ectopically erupting maxillary molars: Bitewings showing the correction of bilateral ectopically erupting maxillary molars

Normal vs Pathologic Root Resorption

It is important for a dentist to be able to differentiate normal physiologic root resorption from pathologic inflammatory resorption. As with permanent teeth, pathologic resorption can occur either externally or internally on a primary tooth.

Internal (image 1) or external resorption (image 2) occurs due to inflammatory changes in the pulpal tissue or periapical tissues. The inflammation usually is a sequel to carious involvement of the pulp or trauma to the tooth.

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Summary

This module reviewed normal eruption and exfoliation of primary teeth and identified common abnormalities associated with each process. A number of these conditions have serious medical prognoses and may initially be diagnosed through dental findings.

You should now be prepared to describe and discuss: a). Timing and variability of primary tooth eruption b). Eruption problems in the primary dentition c). Resorption and exfoliation of primary teeth d). Ankylosis of primary teeth e). Ectopic eruption

Be sure you have read pages 86-91 (5th ed) or 97-103 (4th ed) in Contemporary Orthodontics. Then take the self-test to be sure you have mastered this material.

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3. Development of Occlusion in the Primary and Transitional Dentition

Primary Dentition Occlusion

Purpose of This Module

The purpose of this module is to review the development of occlusion in the primary and transitional dentition. In addition to pages 86-91 (5th Ed) or 97-103 (4th Ed) in Contemporary Orthodontics, carefully review Table 3-1 (5th ed)/Table 3-2(4th ed), Figure 3-2 (5th ed)/Fig 3-13 (4th ed), and Fig 3-35 (5th ed)/Fig 3-43 (4th ed). After this preparation and viewing the teaching program, be sure you are able to:

• describe the primary to permanent dentition pathways, especially the early and late mesial shifts
• describe the location and significance of dental spacing in the primary dentition
• discuss the pre-eruptive position of the permanent teeth and its significance
• identify the canine and molar relationships that can occur in the primary dentition and discuss the implication of each for development of the permanent dentition occlusion

Pre-Eruptive Development of the Dental Arches

All infants have a small, relatively under-developed mandible that gives them a very convex profile (image 1). As growth progresses, the mandible will grow forward more than the maxilla. This leads to a straighter facial profile and increased mandibular projection compared to the maxilla.

Intra-orally the maxillary and mandibular arch forms have different shapes in the edentulous infant. The maxillary arch is ovoid in shape (image 2), while the mandibular arch is V-shaped (image 3). The alveolar ridges contain the developing primary and permanent tooth buds.

Initially the upper and lower gums pads touch together with no vertical space. Vertical growth of the maxilla and mandible produces an increase in face height which accommodates the eruption of the primary teeth. The erupting teeth don’t push the jaws apart; the jaws grow apart which creates space for the eruption of the teeth.

{{PAGE_51}} Image 1, Infant Profile: The facial profile in an infant is very convex with the mandible being much less developed than the maxilla.

Image 3, Infant mandibular arch form: The mandible of an infant is V-shaped in comparison to the maxilla arch.

Dental Spacing

The eruption of the primary dentition is usually complete by 24-30 months of age. Spacing is common and desirable in the primary dentition and is usually noticeable in two locations called primate spaces (image 1). Why that name for these spaces? Because they are also found in non-human primates.

In the maxillary arch the primate spaces are found between the lateral incisors and the canines. In the mandibular arch they are found between the canine and first molar (image 2).

{{PAGE_52}} Dental Spacing (cont.) Spacing between the primary incisors is the normal condition in young children. As they grow and the alveolar processes develop, the spaces may become larger (image 1).

Lack of spacing in the primary incisor region is abnormal (image 2) and indicates future crowding problems in the transitional and permanent dentitions as the larger permanent incisors erupt.

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{{PAGE_53}} Incisor spacing in the maxillary and mandibular arches is common and helps accommodate the larger permanent incisors.

This child has no interdental spacing in the primary dentition, which indicates there will be future crowding problems when the larger permanent teeth erupt.

Incisor Liability

The combined mesiodistal width of the permanent incisors (centrals and laterals) is larger than the width of the primary incisors. This size differential is called the incisor liability. In the maxilla the four permanent incisors are on average 7.6 mm wider than the primary incisors; in the mandible the permanent incisors are 6.0 mm wider.

In the maxillary arch the sum of the widths of the permanent incisors is approximately 7.5 mm greater than the sum of the widths of the primary incisors.

In the mandibular arch the sum of the widths of the permanent incisors is approximately 6 mm greater than the sum of the widths of the primary incisors.

Overcoming Incisor Liability

If the permanent incisors are larger than the primary incisors, where does the space come from to allow their eruption? This lack of space appears also to be compounded by the fact that most jaw growth occurs posteriorly in the ramus of the mandible and the tuberosity region of the maxilla, and not anteriorly in the dental arches.

{{PAGE_54}} In the maxillary arch the larger permanent incisors are accommodated by:

1. Utilization of any interdental spaces between the laterals and centrals. (image 1)
2. Utilization of the maxillary primate spaces between the laterals and canines. (image 2)
3. Labial eruption of the incisors increasing arch length and circumference. (image 3)
4. Increase in width of the dental arch across the canines (intercanine width)

In the mandibular arch the larger permanent incisors are accommodated by:

1. Utilization of any developmental spaces between incisors.
2. Increase in intercanine width.

Image 1, Interdental spaces: Interdental spaces between the primary incisors help to accommodate eruption of the larger permanent incisors.

Image 2, Primate spaces: In the maxillary arch primate spaces between the primary lateral incisors and canines help accommodate the eruption of the larger permanent incisors.

Image 3, Labial eruption of the incisors: In the maxillary arch the larger permanent incisors also erupt labially into a wider arc which increases arch circumference.

Classification of Molar Occlusion in the Primary Dentition

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{{PAGE_55}} Flush terminal plane (image 1) The distal surfaces of opposing primary second molars are in the same vertical plane.

Mesial step terminal plane (image 2) The mandibular primary second molar terminal plane is located mesial to the terminal plane of the maxillary primary second molar. This corresponds to Angle Class I, the normal relationship in the permanent dentition.

Distal step terminal plane (Image 3) The mandibular primary second molar terminal plane is located distal to the terminal plane of the maxillary primary second molar terminal plane. This corresponds to Angle Class II, a major type of malocclusion characterized by protruding upper incisors, in the permanent dentition.

In the primary dentition the distribution of the categories is as follows: Flush terminal plane – 37% Mesial terminal plane – 49% Distal terminal plane – 14%

{{PAGE_56}} Molar: Flush Terminal Plane

Molar: Mesial Step

Molar: Distal Step

Image 1, Flush Terminal Plane: Flush terminal plane relationship with the distal surfaces of opposing primary second molars are in the same vertical plane.

Image 2, Mesial Step: In a mesial step relationship, the mandibular primary second molar terminal plane is located mesial to the terminal plane of the maxillary primary second molar.

Image 3, Distal Step: In a distal step relationship, the mandibular primary second molar terminal plane is located distal to the terminal plane of the maxillary primary second molar terminal plane.

Canine Relationships in the Primary Dentition

The antero-posterior relationships of the upper and lower primary canines are also used to classify occlusion in the primary dentition, just as they are in the permanent dentition. There are four categories. The first two are:

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1. End-to-end (image 1) The long axes of the upper and lower canines are in the same anteroposterior plane.
2. Class I (image 2) The cusp tip of the upper canine occludes in the embrasure between the lower canine and lower first molar.

Canine Relationships in the Primary Dentition (cont.)

The third and fourth categories are:

3. Class II (image 1) The cusp tip of the upper canine occludes mesial to the lower canine in the embrasure between the lower canine and lateral incisor.

4. Class III (image 2) The cusp tip of the upper canine occludes distal to the embrasure between the mandibular first primary molar and canine.

Different relationships can exist on the right and left sides of the mouth. As well, different combinations of molar and canine relationships are possible. These asymmetric differences may be due to differences in skeletal growth, tooth size differences, and different spacing on the left and right sides of the dental arches.

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{{PAGE_58}} Image 3: In a Class II primary canine relationship, the cusp tip of the upper canine occludes mesial to the lower canine in the embrasure between the lower canine and lateral incisor. Image 4: In a Class III primary canine relationship, the cusp tip of the upper canine occludes distal to the embrasure between the mandibular first primary molar and canine.

Primary to Permanent Dentition

Transitions from Primary to Permanent Dentition

The relationship of the primary second molars and canines, and the position of the primate spaces, greatly influence the eruption of the first permanent molars.

One would expect that:

• Mesial terminal plane → Class I molar permanent molars (image 1A)
• Flush terminal plane → End-to-end permanent molars (halfway between Class I and II, image 1B)
• Distal terminal plane → Class II molar permanent molars (image 1C)

However this is too simplistic. Sometimes it turns out that way, but often it doesn’t.

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Leeway Space

The difference in the combined mesial-distal width of the primary canine, first molar, and second molar and the succedaneous permanent canine, first premolar, and second premolar is called the leeway space. Surprisingly, the width of these three primary teeth is larger than the three permanent teeth. In the mandibular arch the leeway space is on average approximately 2.5 mm per side (5.0 mm per arch) and 1.5 mm per side (3.0 mm per arch) in the maxillary arch. Most of the leeway space is due to the size differential between the mandibular primary second molar and the succedaneous mandibular second premolar.

The variability in leeway space between individuals is quite large due to variability in tooth size.

Early Mesial Shift

Patients with a flush terminal plane molar relationship in the primary dentition can, and often do develop into a Class I permanent molar relationship when the first permanent molars erupt.

Recall that primate spaces exist distal to the mandibular primary canines. At 6 years of age, when the mandibular first permanent molars erupt, they may shift forward, closing the mandibular primate space. This results in a mesial shifting of the lower molar relative to the upper molar and early establishment of a Class I molar relationship. This is called the early mesial shift.

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{{PAGE_60}} Late Mesial Shift

With the exfoliation of all the primary second molars (age 11-12 years of age), the permanent maxillary and mandibular molars shift mesially into the leeway space. Because the leeway space is larger in the mandibular arch, the permanent mandibular molars shift more mesially than the permanent maxillary molars. This is called the late mesial shift. This allows a flush terminal plane in the mixed dentition to transition to a Class I molar relationship in the permanent dentition.

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Flush Terminal Plane Transition

Let’s start with a child with a flush terminal plane, the most frequent relationship in the primary dentition.

For a child with a flush terminal plane, which is the norm in the primary and mixed dentition, the transition to the normal Class I relationship in the permanent dentition requires a one-half cusp (3-4 mm) relative forward movement of the lower molar. This is accomplished most readily by a combination of more mandibular than maxillary jaw growth and a mesial shift of the mandibular molars (the solid green arrow).

If there is a mesial shift (early or late) (dashed green arrow) but no more mandibular than maxillary growth, there will be about half as much movement in the direction of Class I. If neither growth nor a mesial shift occurs, an end-to-end relationship of the permanent molars would persist.

About 75% of children with a flush terminal plane in the primary dentition will develop into a Class I molar relationship in the permanent dentition. Because of unfavorable jaw growth and/or minimal mesial shift, the other 25% will not complete the transition to a complete Class I relationship.

{{PAGE_63}} Distal Step Transition

With differential growth of the mandible and mesial shift of the molars, a distal step terminal plane in the primary dentition may develop into an end-to-end relationship in the permanent dentition (which is usually described as a ½ cusp Class II), but that is about as much improvement as one could expect. If minimal differential jaw growth occurs, a full cusp Class II relationship in the permanent dentition will result. It is highly unlikely that a distal step terminal plane will develop into a Class I relationship in the permanent dentition.

This means that children with a distal step relationship in the primary dentition are quite likely to require appropriately timed orthodontic intervention in the future to have a Class I molar relationship in the permanent dentition.

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Mesial Step Transition

For most children, a mesial step relationship in the primary dentition usually was due to an early mesial shift, and it develops into a Class I molar relationship in the permanent dentition—unless there is significantly more mandibular than maxillary growth during the transition period.

However, if excessive mandibular growth occurs, a mesial step may develop into a Class III molar relationship and Class III malocclusion (lower teeth in front of the upper teeth) in the permanent dentition.

In the primary and early mixed dentitions (before the period of transition from the mixed to the permanent dentition), mesial step relationships are quite uncommon and usually are a sign of excessive mandibular growth or deficient maxillary growth. These growth patterns usually persist during the transition period into the permanent dentition years, often changing the mesial step into a Class III occlusal relationship that becomes more severe with increasing age.

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{{PAGE_65}} Summary The purpose of this module was to review the development of occlusion in the primary and transitional dentition. You should now understand: a) Dental spacing in the primary dentition b) Classification of occlusion in the primary dentition c) Implications of occlusion in the primary dentition d) Possible pathways from the primary to the permanent dentition

In addition to pages 86-91 (5th Ed) or 97-103 (4th Ed) in Contemporary Orthodontics, carefully review Table 3-1 (5th ed)/Table 3-2(4th ed), Figure 3-2 (5th ed)/Fig 3-13 (4th ed), and Fig 3-35 (5th ed)/Fig 3-43 (4th ed) before you take the self-test. It’s important to be sure you understand these important concepts.

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