Postnatal growth of the craniofacial region

3 Postnatal growth of the craniofacial region

The adult skull is composed of twenty-eight individual bones and represents one of the most complex regions of the body. The skull bones either develop from a cartilaginous template, ossify directly from membrane, or are composite, being formed following contributions from both mechanisms (Fig. 3.1). Growth of this region therefore represents a combination of endochondral and periosteal modes of osteogenesis.

An understanding of the mechanisms underlying craniofacial growth is important for the orthodontist:

General growth of the body

A simple plot of height versus age (or height-distance curve) for either males or females reveals a relatively smooth and constant increase that occurs from birth to the late teenage years and results in an approximate threefold increase in height (Fig. 3.2). However, the height versus age curve does not demonstrate the dynamic changes in growth rate or velocity that occur from birth to adulthood. To do this, an incremental plot of height change, or a height–velocity curve is required, which shows three general phases in the growth curve (Fig. 3.3):

Whilst the general trends associated with height change are similar in males and females, some fundamental differences do occur between the sexes. In particular, the adolescent growth spurt is greater and occurs later in males, giving them a longer overall period of growth, greater acceleration during adolescence and generally an increased overall height.

In contrast, other body tissues demonstrate quite different patterns of growth in comparison to height. For example, the central nervous system is well developed at birth and grows rapidly during the early years of life, being essentially complete by approximately 10 years of age; whilst the reproductive organs do not begin to increase in size until puberty (Fig. 3.4).

The skull at birth

One of the most striking features of a newborn child is the large size of the head in relation to the rest of the body (Fig. 3.5). This is because at birth, the cranial vault is approximately two-thirds of its final dimension, due to extensive prenatal growth and development of the brain. However, despite this large size the skull of the neonate differs significantly from that of an adult (Fig. 3.6):


Figure 3.5 Changes in body proportions from the fifth month postconception to maturity.

Note the large size of the head in relation to the rest of the body at birth.

Redrawn from Medawar PB (1945), The shape of the human being as a function of time. Proc R Soc Lond 132:133–141.

Rapid growth of the cranial vault continues for the first year after birth, but this progressively decreases during the next two years and remains at a low level until adulthood (Fig. 3.7). However, by 5 years of age around 90% of the adult cranial dimension has been attained. Significantly, the dimensions of the cranium are not affected by the pubertal growth spurt.

In contrast to the cranial vault, the face is subject to a more significant change in postnatal dimension, which takes place over a longer period of time and does come under influence of the pubertal growth spurt. Facial growth results in anterior and vertical development of the nasal cavity and jaws relative to the cranial base and a significant change in overall proportions of the skull. By the mid-teenage years the cranial vault has attained adult dimensions, whilst the face is around 95% of its final size.

Mechanisms of craniofacial bone growth

It is clear from the direct comparison of different skull bones that postnatal growth of the craniofacial region does not result from a simple proportional enlargement of each individual bony element (Fig. 3.8). Endochondral bone growth occurs through cartilaginous replacement, whilst intramembranous bones grow as a result of periosteal remodelling. The complexity and diversity of the skull arises because the constituent bones enlarge differentially, in both a temporal and spatial manner (Fig. 3.9). The basic mechanisms underlying growth of the craniofacial region reflect this and produce:

The relocation of a bone takes place via differential changes in both size and shape, which are mediated by surface deposition and resorption. This remodelling occurs on both the outer (periosteal) and inner (endosteal) surfaces of each bone and the relocation, or cortical drift, will follow the direction of external bony deposition.

The displacement of individual bones as single units also takes place, occurring as an independent process and often simultaneously with relocation. Displacement is mediated by the soft tissues, which apply external forces upon the bones, resulting in their displacement away from each other. Compensatory growth at the sutures maintains articulation of the bones as they move. The soft tissues include craniofacial muscles and connective tissues, primary and secondary cartilages and organs such as the brain and eyes. The relative importance and influence of these different forces upon craniofacial growth is controversial and they form the basis of several fundamental theories of growth control in this region.

Theories of craniofacial growth

Several theories that attempt to explain the mechanisms controlling postnatal growth of the craniofacial skeleton have been proposed. These theories have placed varying degrees of emphasis upon the role of genetic and environmental factors, or the significance of different tissues within this region; some being based upon experimental and biological observation and others having a more theoretical basis (Carlson, 2005).

The functional matrix theory

The functional matrix theory of Melvin Moss describes bone growth within the craniofacial skeleton as being influenced primarily by function (Moss and Salentijn, 1969). In contrast to both the sutural and cartilaginous theories, this theory does not assume that growth of the skull is genetically determined. Indeed, this theory suggests that genes play no major role in determining postnatal growth of the craniofacial region.

Moss suggests that the head simply represents a region where a number of specific functions occur, each being carried out by a ‘functional cranial component’. Functional cranial components consist of two elements:

The functional matrix represents all the tissues, organs and spaces that perform a given function, whilst skeletal units are the bones, cartilages and tendons that support this function. Two types of functional matrix exist:

The periosteal matrix consists of the soft tissues intimately related to a skeletal unit, such as muscles and tendons; whilst capsular matrices are the organs and tissue spaces associated with specific regions within the skull, such as the neurocranium, orbits and oropharynx.

Skeletal units are also further subdivided into:

Each skeletal unit does not necessarily represent an individual bone within the skull, some bones being composed of several microskeletal units or several bones uniting together to function as a single cranial component or macroskeletal unit. In general, the periosteal matrices primarily determine growth of microskeletal units, influencing the size and shape of bones; whilst macroskeletal growth is influenced more by the capsular matrices, producing displacement of cranial regions, such as the nasomaxillary complex or cranial vault (Fig. 3.11).

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Jan 1, 2015 | Posted by in Orthodontics | Comments Off on Postnatal growth of the craniofacial region

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