25: Growth of the Face and Dental Arches

CHAPTER 25 Growth of the Face and Dental Arches

Historically, patient care in medicine and dentistry has been oriented toward the elimination of disease and the resolution of debilitating conditions. Competent care in dentistry today includes issues related not only to disease and functional disability but also to the patient’s well-being. The appearance of the face and dentition is recognized with increasing frequency as a major factor in human psychosocial health.1

This chapter is about dental and facial malocclusion—the recognition and anticipation of malocclusion during the growing years. The dentofacial pattern can be easily and accurately assessed at chairside. In clinical terms, pertinent growth issues are discussed in relation to how growth changes the pattern of the face, occlusion, and dental arches. Knowledge of pattern appraisal and growth can be integrated into efficacious clinical decisions about a young patient. This chapter enhances the reader’s diagnostic and treatment planning skills with reference to malocclusion in the pediatric patient.

The clinician treating malocclusion is primarily interested in the growth and development of craniofacial tissues as they result in facial and dentoalveolar pattern. Our understanding of how genes express their influence on dentofacial pattern and how environment influences gene expression has advanced at a remarkable pace. How molecular mechanisms are implicated at a clinically relevant level, however, has yet to be elucidated. Mao pointed out that what we understand about induced treatment effects at the macroscopic phenotype level has been described in moderate detail at the cellular level but is only beginning to be described at the level of protein and peptide production.2 For this reason, this chapter discusses dentofacial growth and development at a macroscopic level, from the perspective of the practicing clinician.

THE NATURE OF GROWTH

Growth refers to an increase in anatomic size. Three parameters commonly used in growth literature to assess craniofacial size increase are magnitude, velocity, and direction. Magnitude refers to the linear dimension overall or the dimension of a part. Direction means the vector of size increase as might be described on a three-dimensional coordinate system. Velocity is defined as the amount of change per unit of time.

Size increase is typically illustrated in one of two ways. When growth is measured periodically and measurements are plotted as percentages of total growth, the result is a cumulative or distance curve (Fig. 25-1). A human postnatal cumulative curve is characterized by two plateaus and one period of accelerated growth. A second method of graphically demonstrating growth change is by use of an incremental or velocity growth curve (Fig. 25-2). A velocity curve plots growth increments (e.g., centimeters per year) as a function of time. Characteristic of an incremental human growth curve is rapid accelerating prenatal growth, rapid decelerating postnatal growth for the first 2 or 3 years, and a period of relatively slow incremental growth during childhood followed by growth acceleration for 2 or 3 years during pubertal adolescence.

Three observations are central to a clinically relevant understanding of growth. First, growth implies change, a transition from one condition to another. This broader meaning of growth helps define growth as a concept. Conceptual growth refers to a passage from one anatomic form (i.e., size and shape) to another. Transitions in functional stage or activity refer to development. Development, in biologic literature, usually means increased specialization or a higher order of organization and also connotes an interaction of functioning parts. Development means increased organization or specialization of functioning (physiologic) parts.

Growth is more readily understood when a physical pattern is used to describe the effects of growth change. Growth, by nature, is a relational concept. Without reference to a structural model, growth has little clinical utility or meaning. This chapter discusses growth with reference to “ideal” facial, occlusion, and dental arch paradigms.

Growth is a complex phenomenon. There is a large volume of information available on craniofacial growth. Moreover, there is little consensus in the literature as to which data or combination of data is most useful to the practitioner committed to making competent decisions about direct patient care. In light of these prevailing circumstances, the following concepts and principles about craniofacial growth are presented in a way that should be clinically useful and difficult to refute. These postulates are derived and adapted in part from widely held tenets about general human growth and development presented by Valadian and Porter.3 The discussion of general craniofacial principles is followed by application of the principles to three areas of craniofacial growth: the face, occlusion, and dental arches. The goal of this chapter is to integrate growth principles in patient appraisal to enhance diagnostic and treatment planning efficacy.

BASIC CONCEPTS OF HUMAN GROWTH

CRANIOFACIAL GROWTH PRINCIPLES

1. The basic tissue types and functioning spaces that comprise the head and face are subject to growth timing differences. The human head is composed of a variety of basic tissue types; the relative percentage of these types, at any given age, depends on timing of their growth. Neural tissue completes its growth at an early age. By contrast, general somatic tissues, such as muscle, bone, and connective tissue, mature at a slower rate. Neural tissue has attained about 60% to 70% of adult size by birth and its growth is about 95% completed by middle childhood. This is in sharp contrast to growth of other craniofacial soft tissues (Fig. 25-4). Muscle tissue is only 40% to 45% of its adult size by birth, and its growth is approximately 70% completed by 7 years of age. The size of craniofacial lymphoid tissue (tonsils and adenoids) is about 125% of adult size at 5 years of age and decreases gradually to adulthood. Linder-Aronson and Leighton have shown that functional pharyngeal space increases in relation to decreased tonsillar-adenoid mass.6

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Figure 25-4 Cumulative growth curve for craniofacial neural, muscle, and lymphoid tissues.

(From Linder-Aronson S, Leighton BC. A longitudinal study of the development of the posterior nasopharyngeal wall between 3 and 6 years of age, Eur J Orthod 5:47-58, 1983.)

Growth timing of skeletal tissues also demonstrates variation. Craniofacial bone growth is about 45% completed by birth and 70% completed by 7 years of age. In contrast, primary cartilage of the head and face has achieved approximately 75% of adult size by birth and 95% by 7 years of age (Fig. 25-5). The small amount of primary cartilage remaining in the head and face after middle childhood, however, continues to grow through puberty.

2. Growth of primary cartilage and functioning spaces has a directing influence on craniofacial pattern change. Primary cartilage is a tissue of particular interest to craniofacial growth theorists. According to Enlow and Hans, it is singular in form; has the capacity to grow from within (interstitial growth); is pressure tolerant, noncalcified, flexible, and nonvascular; and does not require a covering nutrient membrane for survival.7 Primary cartilage found in the head and face is identical to the growth plate cartilage of long bones. Scott contends that primary cartilage is genetically predisposed, acts during growth as an autonomous tissue, and is able to directly influence the craniofacial pattern.8

Sperber documents that primary cartilage first appears in the head during the fifth week prenatally.9 By the eighth prenatal week, a cartilaginous mass called the chondrocranium is present and is the precursor to the adult cranial base and nasal and otic structures. By middle childhood, most primary cartilage is replaced by bone in a process called endochondral bone formation.

The overall growth-directing influence of primary cartilage on craniofacial pattern change is most profound in early life. By birth, cartilage comprises a substantial portion of the nasal septum and cranial base. Interstitial expansion of primary cartilage probably has a direct influence on the position of the maxilla by way of the septopremaxillary suspensory ligament, as suggested by Latham10 and later contended by Gange and Johnston.11 The maxilla is most likely thrust downward and forward during infancy and early childhood. The contributions to midface growth of primary cartilage is greatly diminished after middle childhood.

The development of functioning spaces has also received considerable attention as a key concept among craniofacial growth theories.12 The head carries out numerous functions. Some functions are more essential than others, but all require the development and maintenance of spaces. Neural integration is a critical function, and space is required for the brain and central and peripheral nervous system expansion. Respiration and deglutition are also essential to life and require development of nasal, pharyngeal, and oral spaces. Sight, olfaction, hearing, and speech are important but less critical craniofacial functions that also require development of functioning spaces for operation.

According to Moss and Salentijn, a likely craniofacial growth scenario of functioning space development in head and facial patterns includes the following sequence of events.13 Rapid size increase of the brain during prenatal and early postnatal life thrusts the calvarial bony plates outward and the midface forward. Birth invokes a set of functional processes previously not essential for life (i.e., breathing and swallowing). Repositioning of the mandible and tongue takes place to ensure patency of nasal-oral-pharyngeal spaces. The mandible is depressed and thrust forward for these functions to be supported and maintained.

3. Mandibular condylar cartilage, craniofacial sutures, and appositional-resorptive bone change facilitate pattern growth of the head and face. Koski identifies the mandibular condyles, once considered growth centers with directive capacity, as an adaptive growth mechanism.14 Cartilage found at the head of the condyle is a secondary, fibrous cartilage and differs significantly from the primary, growth plate cartilage considered to be under high genetic control.15 During craniofacial growth, the mandible is repositioned continuously to its best functional advantage. Reposturing alters the anatomic position of condyle to glenoid fossa. Compensatory growth of secondary condylar cartilage is one mechanism that facilitates maintenance of mandibular position.

Koski also points out that craniofacial sutures are important growth sites that serve to facilitate calvarial and midface growth.14 Calvarial sutures close by 5 years of age, but some facial sutures remain patent through puberty. Craniofacial bones are thrust apart by primary cartilage and functioning space increases. Sutures enable osseous deposition at bone edges, which allows bones of the face and skull to adapt.

Enlow and Hans have shown that bone, unlike primary cartilage, is subject to environmental controls.16 Bone may assume many forms during growth; it is pressure sensitive, calcified, vascular, and relatively inflexible, and requires a covering membrane for survival. The craniofacial skeleton increases in size by way of surface addition only and increases in shape through differential appositional-resorptive bone growth. This differential growth process accounts for a considerable amount of size increase after middle childhood.

Growth theorists Moss and Salentijn12 believe that the general somatic tissues (i.e., bone, muscle, and connective tissue) demonstrate growth change as a consequence of supporting the functioning operations of the head. Indeed, the research evidence of Linder-Aronson17 and of Harvold and associates18 are convincing in that bone and muscle, as basic tissues, are adaptive and compensatory in nature. Understanding bone and muscle growth may come through understanding the temporal development of functioning spaces and the effects of interstitial cartilage expansion on surrounding tissues.

4. Growth of the head and face tends to demonstrate relative equivalency. Humans tend to grow with relative consistency. A percentile growth chart is a valuable instrument for assessing growth consistency over a time period (Fig. 25-6). Percentile charts are customarily divided into the following seven percentile levels: 97th, 90th, 75th, 50th, 25th, 10th, and 3rd. Healthy children tend to maintain a similar percentile level through successive stages of development. Deviations during growth of more than two percentile levels may indicate developmental problems, such as illness or disease.

Attributes (craniofacial parts) that are structurally related also maintain a consistent relationship throughout successive stages of growth after infancy. Enlow and Hans16 identify the dental arches of the maxilla and mandible as an example of a structural part-counterpart relationship. An Angle class II skeletal pattern at 3 years of age is maintained into adulthood without corrective therapy. Both dental arches in healthy individuals tend to increase in size at about the same rate. Hence, balanced or equivalent growth tends to maintain architecturally related structures of any craniofacial pattern that is present after 2 years of age.

BASIC CONCEPTS OF CRANIOFACIAL GROWTH

1. Different parts of the craniofacial complex grow at different times. The head takes on appearance characteristics unique to each particular growth stage. Different parts of the face experience differences in growth timing as well. The infant has a disproportionately large calvaria and forehead compared with the adult because growth of the neural tissue takes place earlier in life than facial growth.

Size increase of the face and calvaria in the three spatial planes is a differential growth process. Scott,19 Meredith,20 and Ranly21 have contributed to an understanding of this process. By birth, the cranial height dimension has attained about 70% of its adult status; cranial width, 65%; and cranial length or depth, 60% (Fig. 25-7). In contrast, only 40% of facial height and 45% of facial length (depth) has been achieved by birth. Face width (i.e., bizygomatic and bigonial), on the other hand, has attained about 60% of adult stature. Growth in face width actually falls between the classic neural and general somatic growth curves.

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Figure 25-7 Cumulative growth curve for calvaria and face in width, height, and depth.

(From Scott JH. The growth of the human face, Proc R Soc Med 47:5, 1954; Meredith HV. Changes in form of the head and face during childhood, Growth 24:215-264, 1960; Ranly DM. A synopsis of craniofacial growth. New York, 1980, Appleton & Lange.)

After birth, a pattern in facial growth timing emerges. The anterior cranial base completes most of its growth during infancy and early childhood, but frontal and nasal bones continue outward expansion through appositional-resorptive bone growth.22 Growth magnitude and duration are greater for the anterior maxilla than for the forehead but less than for the anterior mandible. The posterior face demonstrates the greatest incremental growth during late puberty.

2. Differences in growth size, direction, velocity, and timing are observed among individuals. Bergersen has also noted large variations in growth patterns among individuals and has shown that any measured attribute will demonstrate a range of expression about a central tendency.23 Incremental growth curves for healthy males and females will demonstrate the same general disposition but may show marked differences in maturation timing (Fig. 25-8). Generally, females mature 2 years earlier than males, but Valadian and Porter have indicated that variations are so great that an early-maturing boy may mature earlier than a late-maturing girl.3 Males tend to grow larger in size than females.
3. The heads and faces of no two humans are exactly the same. Brodie pointed out that no two humans are exactly the same.24 This fact is no more clearly evident than when one compares, at any given age, a measured attribute shared by healthy individuals. Most attributes have a range of expression that can be graphically illustrated by a normal distribution curve (Fig. 25-9).

If the same attribute was measured in a population of individuals, the most frequently occurring value (mode), middle value in the series (median), or arithmetic average of all the measured values combined (mean) would represent the central tendency of the population. Central tendency is often referred to as normalcy. Another way to describe attribute distribution is by using percentile equivalents. The 50th percentile indicates the center of the distribution, the 25th percentile the lower one fourth, and so on.

A third statistical parameter often used in growth literature to indicate distribution is the standard deviation. A standard deviation (SD) of ±1 includes about 68% of the entire population; ±2 SD and ±3 SD are equivalent to approximately 95% and 99% of the distribution, respectively. The mean values and SDs for a normative population are invaluable aids in describing a patient’s condition. By comparing a patient’s value to a population value for the same trait, the clinician can make statements about relative largeness or smallness. Generally, measurements beyond ±2 SD are considered clinically important because those values fall outside 95% of the population on which the normative value is based.

In the remainder of this chapter, references are made to craniofacial growth principles and concepts in discussing growth of the face, occlusion, and dental arches.

Jan 14, 2015 | Posted by in Pedodontics | Comments Off on 25: Growth of the Face and Dental Arches
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