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.

Figure 3.1 Human fetus at around 20 weeks of embryonic development, stained with Alizarin red and cleared.

With the exception of the clavicles, the axial and appendicular skeleton is characterized by endochondral ossification. The skull develops from a combination of endochondral and intramembranous ossification.

Courtesy of the Gordon Museum, King’s College London.

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

• Facial growth directly influences the skeletal relationship between the jaws and the occlusal position of the teeth;

• Orthodontic treatment is often carried out during a period when the craniofacial skeleton is growing and often attempts to alter or modify the pattern of jaw growth; and

• Previous patterns of facial growth can be useful for predicting future growth.

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):

• A rapid rate of growth at birth, which progressively decelerates until around 3 years of age;

• A slowly decelerating phase, which persists until the adolescent growth spurt in the early teenage years and is interrupted by a brief juvenile growth spurt at around 6 to 8 years; and

• An adolescent growth spurt, which is followed by a progressive deceleration in growth velocity until adulthood.

Figure 3.2 Height–distance curve for a male from birth to 18 years of age.

Figure 3.3 Height–velocity curve for a male from birth to 18 years of age.

Note rapid deceleration of growth during the first three years and then a gradual deceleration, briefly interrupted by a juvenile growth spurt at 8 years and the more significant adolescent growth spurt at around 13 years of age.

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).

Figure 3.4 Growth curves of different tissue types, regions of the body and organ systems.

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):

• The face of the infant skull is disproportionately small because the nasal cavity, maxilla and mandible are all poorly developed. This reduced size is compounded by the relative enormity of the cranial vault and orbits.

• All of the individual bones within the neonatal skull are smaller than those in the adult, with the exception of the ear ossicles.

• Six fontanelles or fibrous membranes are present in the neonatal skull. These regions give a degree of flexibility to the skull as it passes down the birth canal and are all closed by 18 months of age (see Fig. 2.12).

• Additional sutures are present in the neonatal skull, including the metopic suture within the frontal bone (closes at 7 years) and symphyseal within the mandible (closes at 2 years).

• The spheno-occipital synchondrosis is a cartilaginous growth plate present between the basilar region of the occipital bone and the body of the sphenoid. This region is a significant growth centre, which persists until the end of the second decade.

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.

Figure 3.6 Comparison between adult and newborn skulls.

The infant face is wide because of the precociously large cranium and orbits, but also short because of a lack of development within the nasal complex and jaws. The floor of the nasal cavity is sandwiched between the orbits and there is little vertical development of the maxilla and mandible in the neonate. The adult skull is characterized by a nasal cavity situated below the orbits and significant vertical elongation of the jaws.

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.

Figure 3.7 Growth in head circumference of a girl from birth until 18 years of age.

The upper plot is the head circumference–distance curve, whilst the lower plot is the head circumference–velocity curve. It can be seen that growth in head circumference is most rapid during the first three years of life.

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:

• Relocation; and

• Displacement of individual bones.

Figure 3.8 The mandible does not grow by a simple symmetrical enlargement (A); rather the condyle and ramus elongate in a posterior and superior direction, whilst the body of the mandible lengthens (B).

Figure 3.9 Periosteal resorption (−) and deposition (+) on the external and internal surfaces of a skull bone can produce differential changes in both size and shape, or relocation.

This relocation will follow the overall direction of external bony deposition. Displacement of individual bones under the influence of external force also takes place, occurring as an independent process but often simultaneous with relocation. However, it should be remembered that relocation and displacement can occur in opposite directions.

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 remodelling theory

The remodelling theory was presented by the anatomist James Couper Brash and represented the first attempt at a general theory explaining the fundamental mechanisms underlying craniofacial growth. This theory placed great emphasis upon remodelling as the primary mechanism by which all bones within the craniofacial complex grew. Thus, the cranial vault expanded via external deposition and internal resorption, whilst the facial bones grew downwards and forwards relative to the cranial vault by posterior resorption and anterior deposition.

The sutural theory

The sutural theory was largely the work of two anatomists, Joseph Weinmann and Harry Sicher, who suggested that primary growth of the craniofacial skeleton was genetically regulated, being controlled within the sutures and cartilages. Importantly, within this model, the sutures had an equivalent role to cartilage in being able to generate a tissue-separating force. For the cranial vault and maxillary complex, sutural growth was regarded as being the prime mediator of bony expansion and, in the case of the maxilla, downward and forward displacement relative to the anterior cranial base (Fig. 3.10).

Figure 3.10 Downward and forward growth of the nasomaxillary complex as a result of sutural growth.

The sutures are orientated parallel, downwards and forwards in relation to the anterior cranial base.

Redrawn from Sicher H (1952), Oral Anatomy (St Louis: Mosby).

The cartilaginous theory

In direct contrast to the sutural theory another anatomist, James Scott, suggested that sutures simply represented a continuation of the periosteum and endosteum of the craniofacial bones, in modified regions at their points of intersection. Growth in these regions should therefore be considered as periosteal in nature, being permissive rather than producing a tissue-separating force. Scott suggested that the bones abutting a suture could only be separated by the growth of an associated organ, such as the brain in the case of the cranial vault. Within this theory, great emphasis was placed upon the role of cartilage in producing the driving force of craniofacial growth: in particular, the nasal septal cartilage generating a downward and forward displacement of the maxillary complex, synchondroses elongating the cranial base and the condylar cartilage directing downward and forward growth of the mandible (Scott, 1953; 1954; “>1956).

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:

• A functional matrix; and

• A skeletal unit.

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:

• Periosteal matrices; and

• Capsular matrices.

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:

• Microskeletal units; and

• Macroskeletal units.

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).