01 - The Study of Physical Growth

Growth vs. Development

Introduction

Before viewing this program, you should have looked at the short video introduction to this course, Level I in the Growth and Development sequence. As with this and all the computer teaching programs, it’s important to read the assigned material in Contemporary Orthodontics (5th edition, pages 33-40; 4th ed., 40-47;) as well as view the program.

Then you will take the self-test, the last section of the teaching module, and use it to consolidate the information and to be sure that you really understand it. Remember, the test is a teaching tool. Its purpose isn’t to evaluate you and provide some sort of grade—that doesn’t happen. Its purpose is to help you master the material.

Learning Objectives

In this first computer teaching module, we will distinguish the somewhat overlapping terms of “growth” and “development” from each other, and provide an overview of methods used to study growth.

The general learning objectives of this module are to:

• Distinguish the somewhat overlapping terms of “growth” and “development” from each other
• Discuss the concepts of pattern, variability, and timing as they are applied to the study of growth and development
• Provide an overview of the methods for studying growth from which our current knowledge was derived

Growth

If you are not careful, the very word “growth” can cause some difficulties.

Growth certainly refers to an increase in size, but it’s more that that. More than anything else, growth indicates change—which is why it’s possible sometimes to speak of negative growth.

Development indicates new capabilities, often bigger and better, perhaps at some cost. In this course on growth and development, it’s important to distinguish these terms.

For our use:

Growth, most of the time, will refer to a change, almost always an increase, in size or number, generally with an anatomic reference. Occasionally, however, it will be used to indicate more of an increase in complexity than size.

Development will be used primarily to refer to an increase in complexity—especially when this carries with it an overtone of specialization and loss of potential.

Image 1	Image 2

Growth / Development: Why?

As a dental student, why should you study growth and development?

There are two important reasons:

• You have to understand normal dental, facial and psychosocial growth and development to work with children. You can’t distinguish the abnormal from the normal if you don’t understand the normal pattern of development.
• The dentist can manipulate growth to some extent, and it is important to do so in treating developmental problems in children. For example, headgear devices, like the ones shown here often are used to treat problems like protruding upper teeth and the deficient lower jaw that often is a major part of the problem.

Concepts of Pattern, Variability and Timing

Growth Pattern

Let’s start with the important concept of growth pattern. Think about the pattern for an article of clothing. As the pattern increases in size, there are changes in some areas that are larger or smaller than changes in other areas—it isn’t just blown up like a balloon. In this way, the pattern reflects a complex set of proportional relationships.

Pattern in growth is a higher level pattern, a step beyond the pattern that gets clothes to fit or describes body proportions at any one time. It refers to the predictable changes in body proportions over time as the individual grows.

Growth Pattern: Proportion Changes

There are striking changes in body proportions over time.

Note that at the end of the second month in utero, the head and face comprise almost 50% of the total body length. In contrast, the limbs are still rudimentary and the trunk underdeveloped. It may seem strange that at one time your nose was as big as your hand, but that was the case in early life.

The proportion of total body size contributed by the head and face steadily decreases after the 3rd month of fetal life. By the time of birth, greater growth of the trunk and limbs has reduced the head to 25% of the entire body. This pattern continues, so that the head and face contribute proportionally less and less to the total body length. This relative reduction in head and face size is because the craniofacial structures developed earlier. In later growth, structures away from the head grow more to catch up. This is called the cephalocaudal gradient of growth, and it’s an important part of the growth pattern that you need to always keep in mind.

Growth Pattern: Cephalocaudal Gradient

Even within the head and face, the cephalocaudal gradient of growth leads to changes in proportionality during growth. When the skull of a newborn is compared to that of an adult, it is easy to see that postnatally there’s relatively more growth of the face than the cranium. This reflects the early growth of the nervous system. The cranium, the part of the head that serves as a housing for the brain, must grow rapidly early on, and grows less later.

Within the face, structures closer to the brain grow more earlier and less later. Thus the mandible is less developed at an early point than the maxilla and grows more postnatally.

Variability

Now let’s turn to the concept of variability. Obviously, everyone is not alike in the way they grow. In dealing with a child, you’ll need to know if the size (growth) or development stage (complexity) is normal. How would you determine that?

Variability: Sample Size Effect

Normal variation leads to the well-known bell-shaped curve in the distribution of individuals within a group, so that most individuals cluster near a midpoint and few are found at the extremes. The bigger the group, the more obvious the clustering near the mean. Notice what happens when heights are plotted for these two groups.

An important concept: if you don’t see the bell-shaped curve with clustering around a mid-point, you’re not looking at a normal distribution. And then statistics based on the normal distribution no longer are appropriate.

Image 1, Distribution #1: This distribution and distribution 2 both are relatively normal, despite very different numbers of observations in each sample.	Image 2, Distribution #2: This distribution and distribution 1 both are relatively normal, despite very different numbers of observations in each sample.

Variability: Distribution Measures

There are several ways to express the distribution of values within a group:

• The range is from the smallest to the largest value.
• The mean is the arithmetic average.
• The standard deviationis calculated from a mathematical formula is a way to describe the variability.

One standard deviation (S.D.) encompasses 67% of the people in a normal distribution, two S.D. includes 95% and three S.D. includes 99%. A larger standard deviation implies greater variability—but the number of individuals in the group can affect the values. With only a few individuals, each one heavily influences the values, and the standard deviation for a small group is likely to be large. Larger samples give more reliable data for mean and S.D.

Image 1, Distribution #1: The mean and standard deviation are most commonly reported to describe the central tendency and variation of a normal sample.	Image 2, Distribution #2: The mean and standard deviation are most commonly reported to describe the central tendency and variation of a normal sample.

Mean / Std. Deviation vs. Median / Percentiles

Another way to describe the central tendency of a group is to use the median, or middle person in each distribution. The median value corresponds to the 2nd quartile (Q2), and the inter-quartile range (IQR) is the measure of variation reported with the median value - just as standard deviation is reported with the mean. Usually the median is less sensitive to one or two individuals at the extremes. Often it is a better descriptor of small samples.

Percentiles also can be used to note where an individual stands relative to his or her peers.

The middle of a distribution is the 50th percentile. At the 25th percentile, 75% of the group are taller than that individual. At the 90th percentile, only 10% would be taller.

Image 1, Distribution #1: The median and IQR (inter-quartile range) are most commonly reported to describe the central tendency and variation of a non-normal sample.	Image 2, Distribution #2: The median and IQR (inter-quartile range) are most commonly reported to describe the central tendency and variation of a non-normal sample.

Growth Charts

Growth charts, expressed in percentiles, often are used to show how a patient compares to his or her peers in height and weight, and how that changes over time. The lines indicate the percentiles, the x axis is time and the y axis indicates height or weight. The charts for boys and girls are different (although the differences are surprisingly small prior to adolescence).

For this normal girl, height and weight values plot consistently near the 50th percentile, but normal variation is relatively large. As a general guideline, a child who is between the 3rd and 97th percentiles is considered within the normal range. Outside that, abnormality must be suspected.

To view this chart more clearly (it’s hard to make it large enough on the computer screen), look at Figure 2-4 in ​Contemporary Orthodontics​ (4th or 5th ed.).

Growth Charts (cont.)

Normal growth is shown on a growth chart not so much by being at the 50th percentile, as by maintaining about the same percentile over time. A child who plots at the 20th percentile consistently is just a small but normal child.

Crossing the percentiles, particularly crossing several of them, usually indicates abnormal growth and some problem. In this respect, height is a more sensitive indicator than weight.

Here we see the plot for a boy who developed a medical problem that affected growth at age 10, with partial recovery at age 13, but a long-term growth deficit. To see it better, look at Figure 2-5 in ​Contemporary Orthodontics​.

Other Charts

Many body dimensions, not just height and weight, can be plotted against time to see if growth is normal.

This graph shows the change in the height of the face for boys and girls, based on measurements from radiographs.

You could check your patient’s face height, and its change over time, against a standard graph of this type. Note that there’s a different value for boys and girls, and the male-female difference gets larger as the children get older.

This is largely due to variations in timing,which is the next concept to be discussed.

Timing: Description

Boys and girls get bigger over time, of course, but differences between individuals of the same age and gender, and differences between the sexes, are most notable near adolescence. That is because sexual maturation leads to an adolescent growth spurt. This happens at a different time in girls and boys and happens at different times in individuals of the same gender. On the average, girls have their adolescent growth spurt 2 years ahead of boys. That doesn’t mean that all girls mature faster than all boys—there’s too much individual variation for that to be true.

Timing: Description (cont.)

This diagram plots the increase in height each year for three girls, who experienced a growth spurt at different times because of a difference in timing of sexual maturation. M-1, M-2 and M-3 show for each girl the age (age range between measurements) at which menses began (called menarche). Note that the rate of growth for each girl was declining by then.

Sometimes differences in the timing of puberty, whether an individual matures unusually early or late, causes him or her to cross percentile lines during adolescence. This should not be considered an indication of a growth problem, especially if the plot for the early or late maturing preson returns toward the range where it was initially.

Chronologic vs. Biologic Age

Because of timing variability, chronologic age often is not a good indicator of an individual’s growth status. It is possible to measure age biologically, in terms of progress towards achievement of certain development markers.

For dentists and orthodontists, a good way to do this is to use the stages in maturation of cervical vertebrae, which are seen in the cephalometric radiographs that are obtained for most orthodontic patients. In these two radiographs of the same individual taken 2 years apart, you can see that the cervical vertebrae (within the box) look somewhat different. For now, you just need to understand the concept of biologic age—but soon you will be learning how to estimate a patient’s skeletal development age (one of many biologic ages that can be useful clinically in pediatric dentistry and orthodontics) from vertebral development seen in a radiograph.

Image 1, an individual at CVM 4: The yellow boxes surround the second, third, and fourth cervical vertebrae. The shape of these bones indicate this person is at stage 4 on the scale of Cervical Vertebral Maturation (CVM 4).	Image 2, an individual at CVM 5: The yellow boxes surround the second, third, and fourth cervical vertebrae. The shape of these bones indicate this person is at stage 5 on the scale of Cervical Vertebral Maturation (CVM 5).

Chronologic vs. Biologic Age (cont.)

The stages in maturation of the cervical vertebrae correlate well with the adolescent growth spurt, which is the best time for most orthodontic treatment. Stage 2 indicates that peak growth at adolescence is still a year or so ahead. Stage 3 indicates that it is less than one year to peak growth, a good place to begin treatment. Stages 4, 5 and 6 are increasing distances beyond peak growth.

For now, you just need to understand the concept of evaluating growth using biologic markers instead of years. Later you will learn more about using both cervical vertebrae and hand-wrist bones to calculate skeletal age and how to use that knowledge to improve clinical decisions.

Timing: Biologic Age

These biologic ages also could be called developmental or maturation ages. Many different types of maturation, not just the skeletal age calculated from vertebrae or other skeletal indicators, can be determined and quantified in biologic terms. Dentists use dental age all the time, judging the state of development of the dentition against the usual chronologic markers. You will be learning about that in detail later in this course.

For example, from the standard charts it would be possible to calculate a height age. When you’re the height of an average nine year old (just over 130 cms), your height age is 9, regardless of your chronologic age. Intellectual development can be calculated in the same way. Intellectual development relative to chronologic age usually is called IQ, for intelligence quotient, but it’s just another developmental age.

It is interesting that the various developmental ages tend to be highly correlated. If you’re bigger than other children the same age, you’re probably also more developed mentally and socially. Correlated, of course, doesn’t mean that this is always the case, just that it’s more likely to be that way.

Timing: Biologic Age (cont.)

The use of biologic age is a way to control timing as a variable in growth studies. Look again at the growth curves for our three girls with early, average and late menarche (image 1). Now use menarche as a biologic marker and call that time zero and plot growth increments before and after (image 2).

Note how similar the growth curves are for the group of girls after this simple transformation. They all had a very similar acceleration of growth leading up to menarche and decrease afterward. Using chronologic age instead of biologic age often can be misleading when developmental events are studied, as we will see when we consider cross-sectional vs longitudingal studies.

Image 1	Image 2

Methods for Studying Growth

Introduction: Types of Growth Data

Growth studies may either be longitudinal or cross-sectional in design. When you look at growth data, it is important to know whether the study was longitudinal or cross-sectional.

In cross-sectional studies, rather than seeing the same individual at age 9,10,11, etc., data are taken for a group of 9 year olds, a different group of 10 year olds, another different group of 11 year olds, and so on. This is quicker and easier, but the amount of individual variation tends to be understated. The mean for the group (the dark line in this figure) when chronologic age is used as the time base gives a very misleading picture of what happens to any individual.

Longitudinal studies, in which the same individual is followed over time, provide more information about individual variation, but these studies take a long time and become quite expensive. Note that longitudinal data give a much better picture of what an individual would experience.

Now remember the effect of plotting the individual data with menarche as a base. Using biologic age in that way with cross-sectional data can make it easier to avoid distortions due to different timing in different individuals.

Longitudinal Growth, Distance

This graph shows the first published longitudinal study of growth, done by the French aristocrat De Montebillard in the 1700’s. He plotted the height of his son from birth to age 18, measuring the boy repeatedly over those 18 years. Many fathers or grandfathers still do this, often with marks on a door frame as a child grows. Did somebody do this for you?

You can see that the growth in height didn’t occur in a steady fashion. It was very rapid at first, trailed off after about age 2, then accelerated from ages 14 to 16 and was still continuing at age 18.

Longitudinal Growth, Distance vs. Velocity

It’s easier to see changes in the rate of growth if the same data are plotted in a different way, showing the amount of change in each interval rather than the total height. In the plot of growth increments, note how the very rapid growth at first, slower rate in childhood prior to adolescence, and adolescent growth spurt stand out when the amount of change is plotted. The first curve is called a distance curve, the second a velocity curve.

Other Data Transformations

Various other mathematical transformations of data can make it easier to understand what is occurring during growth. In image 1, the graph showing the increase in weight of early embryos, you can see that there is an exponential acceleration in weight with increasing time.

In image 2, the same data are plotted after a logarithmic transformation. This shows a straight line, indicating that the rate of multiplication of individual cells remains almost constant. It’s not that cells are dividing faster as the embryo gets older, it’s that increasingly there are more cells to divide.

Image 1	Image 2

Methods for Studying Growth: Growth Measurement Techniques

There are now four basic measurement techniques for physical growth:

1. Craniometry
2. Anthropometry
3. Cephalometric radiography
4. 3D radiography (computed tomography)

Craniometry

The science of physical anthropology began with craniometry, which is based on measurements of human skulls (or other bones, in which case it would be osteometry).

Although most craniometric studies have been done on the remains of populations from earlier times, study of contemporary skulls or other bones obviously would be possible. Distortions of the skull during growth can occur, and now are understood better after study of skeletal remains. A good example is the effect of premature fusion of sutures.

In the individual shown in image 1, the mid-sagittal suture fused prematurely and was entirely missing in adult life. Note the extremely narrow width of the cranium. In compensation, the brain and skull became abnormally long posteriorly. In image 2, note the effect of, premature fusion of sutures on the right side of the cranial base. This led to a marked asymmetry that affected both the cranium and cranial base. Once effects of this type have been noted in studies of skulls, the likely cause of similar asymmetries in living patients becomes apparent.

Image 1: In this individual, an increase in width of the cranium was not possible because of premature fusion of the mid-sagittal suture. The result was a remarkably narrow cranial vault, which forced a change in the shape of the brain.	Image 2: For this individual, premature fusion of sutures on the right side of the cranial base led to a major asymmetry that also affected the cranium.

Anthropometry

Anthropometry refers to measurements on living individuals. Various landmarks on the skull were established by investigators studying skeletal material, and the soft tissue points overlying those landmarks (or other soft tissue points that can be found repeatedly) can be used to study living individuals.

Varying soft tissue thickness introduces a source of error if the goal is to measure growth of the facial skeleton, but it is highly advantageous to follow the growth of an individual directly, making the same measurement repeatedly at different times.

This produces longitudinal rather than cross-sectional data and shows individual variation much more precisely. Much of what we know about cranial and facial growth was first deduced from anthropometric studies. Anthropometric measurements are still important in clinical examination of orthodontic patients. They are used now primarily to establish facial proportions—which are important in planning orthodontic treatment.

Image 1: Anthropometric measurement of bi-zygomatic width, which is important in clinical orthodontics because of its relationship to the width of the maxillary dental arch and in establishing face height-width proportions	Image 2: Anthropometric measurement of anterior face height, the other part of face height-width proportions. The relationship of face height to width is the “facial index”.

Cephalometric Radiology

The third important measurement technique is cephalometric radiology. Using x-ray pictures of the head and face provides a way to combine the advantages of crainometry and anthropometry.

It allows direct measurement of the skeleton because the soft tissue thickness can be ignored—but the soft tissues also are imaged and can be measured if desired.

Proper positioning of the subject in a head holder is necessary, so that repeated x-rays can be made with the head positioned exactly the same. This allows longitudinal study of growth (or treatment) changes in an individual.

Cephalometric Radiology (cont.)

The disadvantages of cephalometrics are that exposure to x-rays is required, and that the radiograph is a two-dimensional representation of three-dimensional structures. Some measurements are not possible and others are distorted in projection. The recent development of cone-beam computed tomography, which greatly reduces the radiation dose to obtain 3-D images of the head and face, means that this method is likely to replace traditional cephalometrics for many purposes.

But cephalometric radiographs give an excellent view of many skeletal and dental structures that aren’t accessible for anthropometric study, and these radiolographs are used routinely in dentistry to monitor growth and treatment of patients. A patient like this one, with too much growth of the lower jaw, has to be followed with serial cephalometric radiographs to determine the best time for treatment. You’ll be learning a lot more about how to use cephalograms for purposes like this.

Newer Imaging: Computed Tomography (CT)

With newer imaging methods like computed tomography (CT), it is possible to reconstruct facial images in three dimensions.

Axial CT, which is used in most hospital applications, is very precise but has two problems for studies of growth: it is expensive and delivers a relatively large radiation dose.

The introduction of cone-beam CT (CBCT) for images of the head has offered a significant reduction in both cost and radiation, and CBCT is now an important tool in dentistry for improved diagnosis and treatment planning. CBCT is not quite as accurate as axial CT, but its precision is adequate for most applications in dentistry, including the study of growth.

This CBCT view of a patient with an impacted canine makes the position of the affected tooth clear (indicated by the yellow arrow), and the image can be rotated and manipulated to provide other perspectives.

CB-CT Superimpositions

For growth studies, it is important to see changes over time. This requires superimposition of images of the same individual that were taken at different times. With cephalometric radiographs, tracings based on identification of landmarks typically are used.

That doesn’t work with 3-D images, but now it is possible to superimpose on surface contours, and changes between sequential 3-D images can be discerned by color maps. This allows you to look at the amount of change at thousands of points that can be viewed from any orientation, instead of being limited to tens of points seen from one orientation in a cephalometric radiograph. The superimposed images here show changes in the position of the jaws created in an adult by orthognathic surgery to reposition his maxilla and mandible, but the same approach can be used in growth studies, and such studies now are being undertaken.

Experimental Methods in the Study of Growth

The second way to study growth is with marker techniques. These are classic experimental methods for studying growth. Using these methods, it is possible to follow either processes or physical changes by visualizing the marker. Most of these techniques are used in animal studies because the analysis may be destructive.

Those techniques include:

1. Vital staining
2. Autoradiography
3. Cephalometric superimposition on Implants
4. Molecular biology

Vital Staining

In vital staining, dyes that are incorporated into bone (or sometimes other tissues) are injected into animals. Bone that was growing at the time the dye was injected is stained, and the amount of growth between two injections or since the last injection can be seen.

The technique goes back to the famous English anatomist John Hunter, who observed that the dyes in textile wastes fed to pigs stained their bones in interesting ways. He experimented to find which dyes were best for staining bone. Alizarin proved to be best.

This young rat had four injections of alizarin dye at 2-week intervals, first red, then blue, then red again, then blue again. The dye is excreted rapidly, so the staining occurs during only a few hours after each injection.

As you look carefully at the mandible (image A, the condylar process is in the center), you can see the sequential bands of color. The white bone at the tip of the condylar process grew there since the last injection. Both the sites of growth and the rate of growth in various areas can be seen. The sites of growth are the areas where new bone is being formed, or removed by the remodeling that accompanies bone growth. The rate of growth is shown by the distance between bands of color that were deposited at known times when the dye was injected.

Image B shows the zygomatic arch. Note that new bone is being formed on the outside of the arch, while bone is being resorbed on the inside. The arch is growing outward by being constantly remodeled. In a view like this one, you can not only see where changes are occurring, you can tell how quickly if you know the timing of injections of the dye.

Autoradiography

Autoradiography is based on causing tissues to take their own pictures. Radioactively labeled substances are injected, tissue specimens are prepared, and when photographic film is exposed by placing it over the tissue specimen in the dark, the location of radioactive materials in the tissue is revealed as radiation activates silver grains in the film layer.

This is an autoradiograph of a small bone that was growing in organ culture. Tritium-labelled thymidine was incorporated into the culture medium, and was taken up when DNA of new cells was formed at the time of cell division. The black dots are nuclei that contain the labeled thymidine, which means they must have been formed by cell division that occurred since the tritiated thymidine was added to the culture medium.

In this experiment, carbon 14-labelled proline also was added to the medium. Proline, an amino acid, is a major constituent of collagen, which in turn is the major component of connective tissues and bone matrix. The dark label away from the cell nuclei shows where proline is being incorporated into newly-formed collagen, and therefore shows where collagen formation is taking place. Note that it is most active in the area where maturing collagen is being replaced by bone spicules. With radioactive labeling, essentially any substance can be turned into the equivalent of a vital stain.

Implant Radiology

Implant radiology is based on visualizing metal pins placed in the skeleton. Perhaps you know that when bones are fractured, metal pins often are placed to hold them while they heal, and often the pins are left in place. A long time after the fracture has healed, the pin will still be visible on x-rays as a marker of the fracture site. It is possible experimentally to place small metallic implants in bones anywhere in the skeleton, including the face and jaws, as permanent markers.

This girl has small pieces of tantalum wire in her jaws, placed in areas where little growth occurs, as markers, so that the changes in jaw contours as she grows can be followed more precisely. The implants were placed from inside the mouth by anesthetizing the muscosa over the area where the implant is to be placed, then using small spring-loaded device to drive the implant through the soft tissue into the bone. It is a simple and painless procedure. When another cephalogram is taken later, one can superimpose on the markers, and then changes due to remodeling of the surfaces of the bone can be seen clearly.

Implant Radiology (cont.)

Implants have been placed in a number of children whose growth pattern was followed longitudinally, using cephalometric x-rays to detect the location of the pins. Then the pins were used to superimpose the outline of the jaws, so that growth changes could be seen relative to the pins.

This method was originated by Professor Bjork in Copenhagen, and has been responsible for much of our current understanding of jaw growth patterns. In this superimposition on pins in the mandible, you can see that bone was added to the posterior surface of the ramus and to the condylar and coronoid processes, but was removed from the gonial angle area. Note also that a great deal of remodeling of the condyles occurred. What was once the condyle at age 4 became a part of the ramus at age 10, and then was partially resorbed away as the condylar process continued to grow upward. Note also how the teeth erupted upward and forward relative to the pins in stable areas of the bone. Until cephalometric superimpositions on markers was done, the large amount of remodeling of the bone as the jaws grew had not been appreciated.

You’ll be seeing more tracings of cephalometric radiographs superimposed on implants in the future, and you must understand the concept.

Molecular Techniques

It also is possible to use the techniques of molecular biology to study growth, and this holds great promise for the near future. An example of such methods is diagrammed on this slide. Genetic information carried by plasmids can be injected into the nucleus of a mouse egg, which is then implanted into a pseudopregnant mouse.

When the mouse gives birth, if the new genetic information is active and has been copied, the progeny have foreign DNA and are known as transgenic mice. These genetically-altered mice can be examined for morphologic differences, extracts can be made to test for inactivated hormones or enzymes, etc.

Molecular Techniques (cont.)

One of these mice strains was injected with a plasmid carrying genes coding for growth hormone. The impressively large size of the animal on the left confirms the activity of this genetic information in these transgenic mice. Because normal growth patterns are so precise and specific, genetic control is obvious.

Experiments of this type have great promise in explaining exactly how the control is mediated.

Summary

In studies of growth and development:

• growth (increase in size) and development (increase in complexity with specialization and loss of potential) are different concepts
• growth pattern refers to predictable changes in proportions with growth
• children outside the 3rd and 97th percentiles on standard growth charts may be beyond normal variation
• the timing of adolescence is a major contributor to variability in growth
• biologic ages based on physiologic events can be used to reduce variation related to timing

Summary (cont.)

Important measurement techniques to know:

• Craniometry
• Anthropometry
• Cephalometric radiology
• Cone-beam CT

Remember: the timing of adolescence is a major contributor to variability in growth

Cross-sectional growth studies:

• quicker, less expensive, less precise in detecting typical timing fluctuations

Longitudinal growth studies:

• slow, expensive and hard to maintain, very efficient and excellent in showing details

Experimental marker techniques to know:

• vital staining
• autoradiography
• implant superimposition

Molecular biology techniques: The way of the future in growth studies.

Referral to Self-Test

The self-test section of this program is designed to help you be sure you have understood the material.

Now that you have gone through the module, do the assigned reading in Contemporary Orthodontics(pages 20-33 in the 5th ed.; pages 27-40, 4th ed.) Then take the self-test, and use it as a guide for further study and review. You should be sure you understand the correct answer to all questions that you didn’t get right on your first try.

Copyright 2013, UNC Dept. of Orthodontics

Self-Test

Question 1

(A) Anthropometric data are preferred over craniometric data in the study of growth because (B) soft tissue rather than hard tissue landmarks are used in craniometric studies.

1. A true, B true, A and B related
2. A true, B true, A and B not related
3. A true, B false ✓
4. A false, B true
5. A and B false

Correct

That’s right, anthropometric data are preferred over craniometric data in growth studies, not because soft tissue landmarks are used in anthropometry, but because they are (or can be) longitudinal rather than cross-sectional. Craniometric data, which are obtained from skulls (no soft tissue available), always are cross-sectional. The soft tissue landmarks are a disadvantage of anthropometry for studies of skeletal growth.

Incorrect

That’s wrong! It’s true that anthropometric data are preferred over craniometric data in growth studies, and false that craniometry uses soft tissue landmarks. Anthropometric data are presferred, not because of the soft tissue landmarks, but because they are (or can be) longitudinal rather than cross-sectional. Craniometric data, which are obtained from skulls (no soft tissue available), always are cross-sectional. The soft tissue landmarks are a disadvantage of anthropometry for studies of skeletal growth.

Question 2

In fetal and postnatal growth, what is meant by the “cephalocaudal gradient of growth?”

1. head grows faster than trunk and limbs
2. trunk and limbs grow faster than head ✓
3. cephalic structures are more important for growth
4. caudal structures are more important for growth

Correct

That’s right. In very early (embryonic) development, the cranium represents 25% of the whole individual, and the head about half. From then on, the trunk and limbs grow faster than the head, and the face more than the cranium.

Incorrect

No, that’s wrong. In very early (embryonic) development, the cranium represents 25% of the whole individual, and the head about half. From then on, the trunk and limbs grow faster than the head, and the face more than the cranium. The cranium and head aren’t more or less important than the other structures, they just grow at different times.

Question 3

How would you establish a child’s reading age?

1. compare against standards for social development
2. compare against standards for reading development ✓
3. evaluate with scores of standard IQ tests
4. not possible, developmental ages relate to physical characteristics only

Correct

That’s right, there are standards for reading development just as there are standards for development on other physical and social-psychological scales. A child can be advanced or slow in reading just as he or she could be in physical development. It’s interesting that the different types of development are highly correlated.

Incorrect

No, that’s incorrect. There are standards for emotional development just as there are standards for development on other physical and social-psychological scales, and a child can be evaluated in relation to those scales. A child can be advanced or slow in emotional development just as he or she could be in physical development.

Question 4

Which of the following is the most appropriate description of the concept of pattern in growth?

1. proportional size relationships
2. changes in proportions over time ✓
3. conservation of proportions as size increases
4. changes in proportions as size increases

Correct

That’s right. Pattern in growth refers to the way in which proportional relationships change (or are maintained) as the individual changes over time (whether or not he gets bigger). The changing proportions of the face describe its growth pattern.

Incorrect

No, that’s wrong. Pattern in growth refers to the way in which proportional relationships change (or are maintained) as the individual changes over time (whether or not he gets bigger). There is a pattern in the size relationships of parts at any one time, but that isn’t the growth pattern. And everything doesn’t have to get bigger as you grow. The changing proportions of the face describe its growth pattern.

Question 5

On growth charts with 6-month data plots, what is the significance of a child “crossing the percentiles” downward?

1. indicates growth disturbance, perhaps chronic illness or chronic problem ✓
2. Indicates increasingly active growth, perhaps tumor
3. no significance over this period of time, quick changes more important
4. no significance, changes in channels typically occur during normal growth

Correct

That’s right, a downward movement across the percentile channels over a 6 month period indicates a growth disturbance and raises the question of some chronic illness or other chronic problem. A normal child usually stays within his or her channel.

Incorrect

No, that’s wrong. A decrease in the percentile channels over a 6 month period indicates a growth disturbance and raises the question of some chronic illness or other chronic problem. A normal child usually stays within his or her channel. Note the effect of growth hormone (HGH) deficiency in the plot for this child, and then the catch-up effect of administering the hormone.

Question 6

(A) Girls grow earlier than boys at adolescence because (B) girls mature sexually earlier than boys.

1. A true, B true, A and B related ✓
2. A true, B true, A and B not related
3. A true, B false
4. A false, B true
5. A and B false

Correct

That’s right. As a group, girls grow earlier than boys at adolescence, and the reason is their earlier sexual maturation. In an important sense, adolescence and adolescent growth is a sexual phenomenon.

Incorrect

No, you’re wrong. As a group, girls grow earlier than boys at adolescence, and the reason is their earlier sexual maturation. In an important sense, adolescence is a sexual phenomenon. The group tendency, of course, doesn’t mean that every individual is that way. Some boys mature before some girls.

Question 7

Which of the following is not appropriately described as growth?

1. increase in size
2. increase in number
3. increase in complexity
4. increase in specialization ✓

Correct

That is correct. Growth refers to an increase in size, number or occasionally complexity, but not to increased specialization. The term “development” often is used to describe an increase in specialization of biologic functions, but sometimes it is used to describe an increase in complexity—so sometimes there is an overlap with growth, sometimes there isn’t.

Incorrect

That’s wrong. Growth refers to an increase in size, number or occasionally complexity, but not to increased specialization. The term “development” often is used to describe an increase in specialization of biologic functions, but sometimes it is used to describe an increase in complexity—so sometimes there is an overlap with growth, sometimes there isn’t.

Question 8

The value of implants, such as metallic pins placed in the jaws, in radiographic studies of growth is that:

1. the implants mark areas of bone that are growing
2. the implants mark areas of bone that are remodeling
3. the implants mark areas of bone that are not growing or remodeling ✓
4. the implants outline areas of bone of particular interest so they can be evaluated

Correct

That’s correct, the implants mark areas that are not growing or remodeling. This provides stable areas for superimposition of longitudinal x-rays, and then you can see which other areas are changing and how much, and which areas are not changing. As this superimposition on implants in the mandible shows, implant studies are an excellent way to determine the changing proportions that define the growth pattern.

Incorrect

No, that’s not the best answer, implants do that only indirectly. The implants mark areas that are not growing or remodeling. This provides stable areas for superimposition of longitudinal x-rays, and then you can see which areas are changing and how much, and which areas are not changing. As this superimposition on implants in the mandible shows, implant studies are an excellent way to determine the changing proportions that define the growth pattern.

Question 9

What percentage of a normally distributed population is within two standard deviations of the mean?

1. 50%
2. 66%
3. 78%
4. 87%
5. 95% ✓

Correct

That’s right, 95% of the population are within two standard deviations of the mean, and 99% are within three standard deviations. But when you’re treating patients with problems, you’re likely to encounter individuals who are at the extremes of the normal distribution.

Incorrect

No, that’s wrong. 95% of the population are within two standard deviations of the mean, and 99% are within three standard deviations. But when you’re treating patients with problems, you’re likely to encounter individuals who are at the extremes of the normal distribution. Sometimes doctors begin to think that almost everybody is abnormal, because they see so many of the extreme variations.

Question 10

How do vital stains contribute to the study of growth?

1. stain everything that was present when they were administered
2. stain everything that was growing when they were administered
3. stain everything that grew after they were administered
4. stain specific things that were growing at the time they were administered ✓

Correct

That’s correct. A vital stain marks specific things that were growing when it was administered, for instance, the bone that was being laid down at that time. Different vital stains are used for different tissues. If it stained everything, you couldn’t distinguish one structure from another.

Incorrect

No, that’s wrong. A vital stain marks specific things that were growing when it was administered, for instance, the bone that was being laid down at that time. Different vital stains are used for different tissues. If it stained everything, you couldn’t distinguish one structure from another.

Question 11

Within the face, the postnatal cephalocaudal gradient of growth predicts that:

1. the face grows faster than the cranium
2. the mandible grows faster than the maxilla
3. the maxilla grows faster than the cranium
4. all of the above ✓
5. none of the above

Correct

That’s right. The cephalocaudal gradient of growth predicts (correctly) that a structure further away from the cranium grow faster than one closer to it. So in postnatal life both jaws grow faster than the cranium and the mandible grows faster than the maxilla.

Incorrect

No, that’s wrong. The cephalocaudel gradient of growth predicts (correctly) that the structures further away from the cranium grow faster than those closer to it, so in postnatal life both jaws grow faster than the cranium and the mandible grows faster than the maxilla.

Question 12

If a girl’s bone age is 12, what is her chronologic age most likely to be?

1. 10
2. 11
3. 12 ✓
4. 13

Correct

That’s right, the greatest probability is that the developmental age matches the chronologic age. That’s how the developmental age was established. But for any individual, the chronologic and developmental ages may or may not coincide.

Incorrect

No, that’s wrong. The greatest probability is that the developmental age matches the chronologic age. That’s how the developmental age was established. But for any individual, the chronologic and developmental ages may or may not coincide.

Question 13

Which of the following is the major disadvantage of cephalometric radiography in the study of growth?

1. requires damaging x-rays
2. requires complex head positioning equipment
3. makes it difficult to see soft tissue landmarks
4. offers a two-dimensional view of three-dimensional objects ✓

Correct

That’s right. All of the responses are disadvantages to some extent, but the biggest disadvantage is the two-dimensional view that the radiograph provides. Changes in distances between points that are not in the same plane will be distorted. The compensating advantage is that longitudinal data for skeletal dimensions can be obtained by superimposing tracings of serial cephalometric radiographs.

Incorrect

That’s wrong. All of the responses are disadvantages to some extent, but the biggest disadvantage is the two-dimensional view that the radiograph provides. Changes in distances between points that are not in the same plane will be distorted. The compensating advantage is that longitudinal data for skeletal dimensions can be obtained by superimposing tracings of serial cephalometric radiographs.

Question 14

The difference between a growth distance curve and a growth velocity curve is that:

1. distance curve plots total achieved, velocity curve plots increment ✓
2. distance curve plots increment, velocity curve plots total achieved
3. both curves plot increment, but velocity curve magnifies it
4. both curves plot total achieved, but distance curve magnifies it

Correct

That’s right. Distance curves plot the total growth achieved, velocity curves plot the growth increment since the last measurement. Both types of plots are valuable, depending on what you’re trying to observe—but velocity plots make it easier to appreciate the changes that are occurring, as at adolescence.

Incorrect

No, that’s incorrect. Distance curves plot the total growth achieved, velocity curves plot the growth increment since the last measurement. Both types of plots are valuable, depending on what you’re trying to observe—but velocity plots make it easier to appreciate the changes that are occurring, as at adolescence.

Question 15

(A) Longitudinal growth data are quite inefficient in terms of the amount of information obtained from each subject because (B) the longitudinal data highlight individual variations.

1. A true, B true, A and B related
2. A true, B true, A and B not related
3. A true, B false
4. A false, B true ✓
5. A and B false

Correct

That’s right. Longitudinal data are very efficient, not inefficient, for growth studies, so the first statement is false. It’s true that the longitudinal data highlight individual variations, so the second statement is true. That’s another advantage of longitudinal versus cross-sectional data.

Incorrect

No, that’s wrong. Longitudinal data are very efficient, not inefficient, for growth studies, so the first statement is false. It’s true that the longitudinal data highlight individual variations, so the second statement is true. That’s another advantage of longitudinal versus cross-sectional data.

Question 16

What does it mean to say that a child is in the 95% percentile for height?

1. she’s in the shortest 5% of the population her age
2. she’s in the tallest 5% of the population her age ✓
3. she’s more than two standard deviations below the mean
4. she’s more than two standard deviations above the mean

Correct

That’s correct, she’s in the tallest 5%, taller than 95% of her age group. But that puts her within two standard deviations of the mean, in her case, exactly two above.

Incorrect

That’s wrong. She’s in the tallest 5% of her age group, taller than 95%. But that puts her exactly two standard deviations above the mean, not more than two above.