1. Growth disposition is similar for all healthy individuals. Healthy individuals go through growth stages that are the same for everyone, according to Valadian and Porter.³ The prenatal period, from conception to birth, averages 40 weeks in length. Infancy includes the first 2 years of life after birth, and childhood ranges from 2 to 10 years for girls and 2 to 12 years for boys. The length of adolescence is the same for both sexes but comprises different years, 10 to 18 years for females and 12 to 20 years for males (Fig. 25-3).

Each growth stage is unique. Rate of size increase is most remarkable during the prenatal period and declines substantially during infancy. Generally, growth velocity plateaus during childhood and increases again during adolescence. All healthy individuals experience these growth cycles, although the various basic tissues and body parts are affected differently.

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

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.

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.

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).