Chapter 3

Bone and Cartilage

3.1 Introduction

The craniofacial skeleton is a complicated assembly of parts, and its growth is by no means simple. Different types of bone growth, contributing to the result, will be outlined. Some special features, which should be recognized in the growth and morphology of bone and which are of significance for a good understanding of facial growth, will be developed further. The morphogenesis of bone and the properties of bone and cartilage will be discussed. Apposition and resorption of bone will be examined. The relationship which, as suggested, exists between bone morphology and rate of growth will be discussed and some related propositions will be introduced. In addition, traction and spicule formation will be examined.

In this chapter the author—despite the concomitant objections—has used a number of assumptions that are supported inadequately by research data. He believes it permissible to do so in the expectation that these assumptions can lead to a better understanding of facial growth and the possibilities of influencing it.

3.2 Morphogenesis of bone

The greater part of the skeleton is originally laid down in cartilage. This cartilage is then fairly quickly replaced by bone, except in those places where joint surfaces develop or where cartilage is to function as a region of growth. In this manner, long bones are first formed in cartilage and subsequently cartilaginous discs remain as growth regions near the ends of the bones until the growth in length has been completed. Only after that are those cartilaginous epiphyseal discs replaced with bone. The cartilage on the articular surfaces of the joints, on the contrary, remains as such over the entire life span, under normal circumstances.

In the head, various parts are first established in cartilage, notably the cranial base. At birth the greater part of the base is already ossified. Cartilage is retained in places where rapid growth still must occur. This happens at the spheno-occipital synchondrosis, which remains active more or less until the conclusion of growth in the cranial base. It ossifies a few years after its growth has ceased at the age of 14 to 16 years.174 The cartilage in the condyle and the nasal septum and the elastic cartilage of the pinna of the ear normally remain present throughout life.

The greater part of the calvaria and facial skeleton, just as a part of the mandible and clavicle, is not formed through enchondral ossification of a cartilaginous model of the bone; rather, it forms directly from a condensation of mesenchyme with a fibrous structure. One or more primary centers of ossification develop in that tissue by differentiation of mesenchymal cells into osteoblasts. As soon as growth of the centers of ossification occurs, the surrounding membrane develops into a layer of periosteum from which osteoblasts lay down periosteal bone. This is called intramembranous ossification.

3.3 Characteristics of bone and cartilage

Some important differences exist between bone and cartilage. Unlike cartilage, bone is calcified and rigid. Therefore, it cannot expand, in the true sense, from within itself. Bone, as with enamel, dentin, and cementum, has no potential for interstitial growth. All other tissues, including uncalcified cartilage, have that ability. Cartilage cells can, as a rule, multiply and lay down extracellular material (matrix), thus quickly forming new tissue. In addition, cartilage can grow appositionally by activity of the perichondrium.

Bone can increase or decrease in size only through cellular activity on the surfaces, including the interfaces between bone and cartilage. Bone apposition and resorption occur externally where the bone is covered with periosteum, internally in the medullary cavity and spongy cancellous bone where the lining is endosteum, on the surfaces of sutures, and at the attachment of the periodontal ligament between teeth and alveolar bone. The potential for the greatest growth exists through the cartilaginous precursor, such as exists with epiphyseal discs and synchondroses. The intense proliferation of cartilage offers the possibility there for relatively rapid growth.

As distinct from bone, cartilage can form new tissue under relatively high levels of pressure (i.e., forces that exceed capillary pressure). Once formed, the calcified part (not membranous) of a bone can indeed withstand force very well. This applies, however, even more to cartilage, which, as has been pointed out, forms the load-bearing surfaces of joints and remains intact there. Growing cartilage can proliferate while under pressure. The cells of the cartilaginous tissue which must divide and produce matrix material are not actually on the surface of the tissue and thus are not directly under load. Above all, cartilage is avascular and receives nutriment by diffusion of tissue fluid. The external surfaces of bony structures, surfaces which are covered by periosteum, are indeed directly exposed to pressure and tension and are dependent on vascularization for nutrition. Pressure readily affects the ability of a vascular bed to carry nutriment, to the detriment of the tissue it was supplying.

The typical properties of cartilage make it possible not only to provide considerable growth in a short time, but also to grow despite the pressures operating on the cartilage, such as static and dynamic forces due to gravity, body mass, and muscle force.

Even though bone is a strong and rigid structure, it exhibits changes both internally and externally in response to normal function. The internal changes consist, among other things, of reconstruction processes which, for example, lead to compact cortical bone existing adjacent to spongy bone. Both internal and external changes are made possible by the bone cells: osteoblasts and osteoclasts.

3.4 Apposition and resorption of bone

Osteoblasts are responsible for the synthesis of extracellular bone matrix and for the initiation of calcification. In general, osteoblasts do not divide. New osteoblasts are derived from differentiation of mesenchymal cells. Osteoblasts are more or less round and tablet-shaped, and are typical surface cells. They are most common in the deeper layers of the periosteum, in the endosteum and around trabeculae, on the walls of actively forming Haversian canals, and on the remnants of the walls of the lacunae of the previously hypertrophied and disintegrated cartilage cells in the transition zone between cartilage and bone. During the deposition of matrix, which initially occurs unilaterally, the osteoblasts withdraw and thus remain at the surface. However, after some time the unidirectional deposition becomes general and the osteoblasts surround themselves with matrix. An osteoblast is called an osteocyte after it has been built into bone. The life of an osteoblast is estimated to be 20 days.

An osteoblast has a limited capacity for matrix deposition.23 24 In this chapter, 10 µm per day is accepted value, leaving the possibility open that it may be greater However, that is not required for the validity of the further argument. This limited capacity for matrix deposition plays an essential role in the speed with which bone can be laid down on surfaces. This applies also for the mechanisms nature has at its disposal to obtain still greater bone growth. This will be further detailed later in this chapter.

Osteoclasts are responsible for the resorption of bone. They can remove bone from any surface. Through their activities, the typical resorption pits, called Howship’s lacunae, are formed. Osteoclasts can be exceptionally large. Usually, they have multiple nuclei. It is assumed that a generation of osteoclasts comes about through the fusion of numbers of small cells, probably mononuclear phagocytes (monocytes, macrophages). The life of osteoclasts is estimated to be 15–20 days22; they show no mitosis.

3.5 Maintenance of form and proportions in bone growth

As a rule, the general configuration of a whole bone alters little in overall form during growth, although some change does occur. Individual bones, long bones, and groups of several bones, such as is the case in the skull, retain their own “pattern” during their development to maturity. This constancy in shape during growth is made possible by apposition and resorption of bone on those surfaces which adapt in order to compensate for disproportionate buildup or undesirable displacement of the bone in question. This phenomenon is denoted by Enlow as “remodeling.”76 He defines remodeling as the process by which the changes which must occur as direct consequence to the increase in size of a bone take place. Remodeling must accompany bone enlargement. It is a mechanism of sequential progressive adjustment that functions to maintain the shape and proportions of the bone throughout its growth period. Enlargement of a bone in a particular region requires corresponding changes in the configuration of other parts. The process of remodeling embraces a continuous change in shape and size of the bony compartments of the skull to compensate for the unceasing changes in location and proportions of those components in the overall growth of the head. Enlow illustrates the principle of remodeling by using a long bone, as well as others. He indicates in this connection the resorption that takes place in the circumference of the shaft of a long bone in the region adjacent to the epiphyseal disc. The epiphyseal disc is situated in a part of the bone broader than the shaft. The length increase occurring in the disc, together with the relatively small increase in the diameters of the epiphyseal disc, leads consequently to formation of bone of a larger diameter than is needed for the narrower shaft. The excess size of this part of the bone is simultaneously reduced by the resorption mentioned above, through which the original shape of the long bone is maintained (Fig. 3-1).

Two basic processes play a role in the movement of a bone. They are termed “drift” (transformation) and “displacement” (translation), as is explained in Figure 3-2. Apposition and resorption are not only found in the compensation for deviating forms which otherwise would come about for the growth at epiphyseal discs, but they are also necessary in order to maintain the relationship of a particular bone which is part of a complex with the other parts of that structure. A good example is the relationship of the zygomatic arch to the rest of the craniofacial skeleton. In the growing head, the zygomatic arch is moved laterally and downwards by resorption and apposition (Fig. 3-3). The extent and type of remodeling depends on the variations in relative movement of particularly bony structures. A maxilla which is moved downwards principally by apposition of bone on the palatal side and resorption on the nasal side will show compensatory resorption and apposition on the ventral side. To a lesser extent, this applies when the increase in height of the middle face happens not mainly through changes in the palate and nasal floor, but by changes at the sutures (Fig. 3-4).