4: Functional and Anatomic Aspects of the Head

Functional and Anatomic Aspects of the Head

Certain connections between functional and morphological aspects of the head and the masticatory system are described in this chapter. The morphogenesis of the craniofacial skeleton and its influence on the later morphology of the jaws are elucidated. Details of the growth of the mandible and maxilla will be discussed. Special consideration is given to the role of occlusal interdigitation in the coordination of the growth of the maxilla with that of the mandible. Backgrounds of variations in tooth position are described. The influence of hereditary and functional factors in facial growth is discussed briefly.

4.2 Morphology and function

A number of functions are localized in the head, which principally determines its size and form. The skull is the apex of the spine, and the way it is balanced by the skeletal musculature contributes to man’s posture with its manifest advantages. The brain, which is the largest and most important part of the central nervous system, lies within the skull. The auditory apparatus lies laterally behind the actual face. Vision is provided for by the eyes, placed directly under the foremost part of the brain. Below the eyes is the entrance for air and food, initially one opening. The air passages via nose and nasopharynx have appurtenances for conditioning and filtering incoming air and mechanisms for rapid closure and expulsion of undesirable material. The olfactory organ in the nose and the taste mechanisms in the mouth provide for sampling incoming air and food. The oral and facial musculature not only operates the jaws and teeth during chewing and swallowing but also allows for speech, facial expression, and some degree of manipulation where hands are inadequate. The teeth and jaws are primarily for mastication, but also provide the framework over which the facial mask is draped. In speech, the larynx and lungs of course play important roles. The beginning of the digestive system is found in the salivary and mucous glands of the mouth. In swallowing, auxiliary functions are supplied by the pharynx and the control mechanisms of the tongue, larynx, and esophagus. The external musculature of the face has a valuable function in man in supplying the highly developed characteristic of facial expression. The mimical muscles, by virtue of their typical form and attachments, are able to provide an infinite variety of expressions. Within this comes also laughing and smiling.

For the total complex of functions found in the head, a certain space, and in some ways a particular conformation, are needed. According to M. L. Moss,192 the form and development of the neurocranium and splanchnocranium are determined principally by demands established by the different functions—the “functional matrix”—and not by hereditary characteristics of the bones (“no genes for bones”). There is little doubt of the relationship between the growth of the brain and the development of the bones of the cranial vault.

In embryonic development, the processes involving the future central nervous system have decided priority. The brain has been laid down already and has been given the form destined for it, before the formation of cranial vault bones begins. Those bones are formed intramembranously around the brain and owe their form and size to the brain. The postnatal development of the cranial vault bones and the associated buildup and remodeling during their growth are regulated primarily by brain growth (Fig. 4-1).

Fig. 4-1 The brain is laid down earlier than the cranial vault bones, which form intramembranously around the brain. Thus, the form of the brain determines the form of the cranial vault bones. In the newborn child, there is normally some distance between the component bones of the vault. There is a possibility for relative movement of the bones so that the skull can be molded during birth and recover its shape subsequently. At the age of 7 years, the brain has reached 90% of its ultimate size. The cranial vault bones, which are joined by sutures, have grown correspondingly larger; the internal shape of the cranium conforms to that of the brain.

A In the period from birth until the age of 7 years, the cranial vault bones have been remarkably far displaced and have grown much larger.

B Bone growth has not occurred just at the ends of the bones. Through apposition (light stippling) and resorption (heavy stippling) the arcs of the bones now lie on curves with greater radii. The thickness of the vault bones has increased due to more apposition than resorption taking place.

4.3 Morphogenesis of the craniofacial skeleton

The cranial base and the central and caudal parts of the bones which articulate with it, upon which the brain rests, are laid down in a cartilaginous precursor. The greater part of that is already ossified by birth. Only the synchondrosis between sphenoid and occipital bones remains, as was noted earlier, to the age of 14 to 16 years.

The face also begins with a cartilaginous precursor. This, however, is not—as is the case with the cranial base and long bones—replaced by bone in the sense that the cartilage serves as a mold which changes into bone. A part of the chondrocranium forms the nasal capsule. The mandible is laid down intramembranously, lateral to Meckel’s cartilage, and beginning in the neighborhood of the branching of the inferior alveolar and mental nerves. It is suggested that the basic pattern of Meckel’s cartilage serves to determine the shape the basal bone of the mandible will acquire.

The bone of the maxilla in the middle face is formed intramembranously in close relationship with the cartilaginous nasal capsule. It is assumed that the later anteroposterior location of the piriform aperture of the nose in relation to the other maxillary structures is largely, but in transverse dimensions to a lesser degree, determined by the morphogenesis of the chondrocranium (Fig. 4-2).

The emphasis placed on the morphology of the chondrocranium in connection with the later form of the basal mandible and the location and width of the piriform aperture emerges largely from two findings that are of unusual clinical significance. The first is derived from the information that, after the first year of life, the form and size of the part of the mandibular basal bone at that time ossified appears to change very little, if at all. The same applies for the anteroposterior position of the piriform aperture in relation to the surrounding maxillary structures and, to a lesser degree, to the transverse dimensions of the aperture. The second finding is derived from the observation that there is little possibility to change the components just mentioned by means of facial-orthopedic or orthodontic appliances (Figs. 4-3 and 4-4).

Fig. 4-2

A With reference to the face, the chondrocranium consists principally of the nasal capsule and Meckel’s cartilage. It is suggested that the cartilaginous model is to a degree responsible for the form eventually taken by the face. (From Mugnier.197)

B Drawing of the skull of a child about 4 years of age. In the development yet to come, the form of the lower border of the mandible and the location of the piriform aperture in relation to the surrounding maxillary structures change only to a limited extent.

Fig. 4-3 Due to differences in the predetermined course of left and right mandibular inferior alveolar, and mental nerves, and in the model provided via Meckel’s cartilage, the form of the later mandibular basal bone can vary. The distance between both condyles is controlled by the positions of the articular fossae on the temporal bones. The location of these features depends once again on the form and size of the brain. The mandibles are illustrated, in caudal view, to show some examples of the variety normally available.

A Semicircular shape of the mandibular basal bone.

B Narrow anterior part with relatively small width of arch across the canine area.

C Relatively wide mandibular base in the premolar region.

D More or less rectangular form of mandibular basal bone, with a relatively large transverse dimension in the canine area.

Fig. 4-4 Diagrammatic illustration of differences in the width of the piriform aperture and the transverse dimension of the mandibular basal bone in the canine area in frontal sections.

A “Normal” situation.

B Broad piriform aperture with narrow mandibular basal bone.

C Narrow piriform aperture with narrow mandibular basal bone.

D Narrow piriform aperture with wide mandibular basal bone.

4.4 Growth of mandible and maxilla

The mandible at birth consists of the delicate basal bone with tooth germs upon it, which are encapsulated in thin bony sockets open occlusally. The ramus is short and projects only slightly above the rest of the mandible. Cartilage is still present in the midline, which offers the possibility for rapid transverse growth until it becomes ossified in the second half of the first year. The foremost part of the mandible becomes more massive, the cross-sectional size increases, and a strong cortical layer is formed; but any buildup transversely or anteroposteriorly is very limited. Mandibles become longer through additions to dorsal, not ventral (Fig. 4-5).

The enlargement of the maxilla anteroposteriorly mostly takes place dorsally, also. By formation of bone in the area of the tuberosities, the maxilla increases gradually in size. Due to the presence of the midpalatal suture, the maxilla, unlike the mandible, can increase in width until the end of the growth period. This does not apply to anteroposterior growth. Prenatally, the premaxilla has already established a bony union with the palatine process of the maxilla, though remnants of the incisive suture can frequently still be found in skulls of young individuals. However, even if that suture should remain intact, it would still not be able to contribute to anteroposterior lengthening, because uninterrupted bone runs across the buccal aspect of the dental arch.

Enlargement of the maxilla coincides with an increase in the size of the maxillary sinuses, which were already present at birth. They can vary greatly in size and shape, principally because of the external morphology of the surrounding bony structures and the proximity of the teeth. In adults the maxillary sinus begins dorsally to the canine or first premolar, and runs above the last molar up to the posterior wall of the tuberosity.

The maxilla, unlike the mandible, has the possibility to increase in width through the presence of the median suture. Because the teeth in the maxilla do not stand on a rigid unchangeable foundation, as is the case in the mandible, there is an extra possibility for lateral enlargement. Through apposition and resorption from around and above the roots of the premolars and molars, where the bony structures are generally thin-walled, the breadth of the maxilla can be modified (Fig. 4-6).