The Nature of Skeletal Growth Mohammad Almohaimeed BDS, SSC(Ortho)

The Nature of Skeletal Growth At the cellular level: there are three possibilities for growth: hypertrophy (increase in size of cells), hyperplasia (increase of cell number), and secretion of extracellular matrix (ECM). The extracellular matrix in the mineralized tissues differs than in soft tissue in that it becomes mineralized.

The Nature of Skeletal Growth Growth of soft tissues including cartilage occurs by interstitial growth, i.e growth occurs at all points at the same time (by hyperplasia, hypertrophy and secretion of ECM). In mineralized tissues, growth occurs by direct or surface apposition, in which growth occurs at the surface, not within the mineralized mass, through the activities of cells at the periosteum.

The Nature of Skeletal Growth In general bone formation in the body occurs primarily through two main scenarios: 1. Endochondral bone formation (a transitional cartilage is formed). Sites: chondrocranium and long bones 2. Intramembranous bone formation (direct apposition of bone in the ECM). Sites: mandible, maxilla, and cranial vault.

Development and maturation of the chondrocranium At 8 weeks, It appears as solid bar of cartilage extends from the nasal capsule anteriorly to the occipital area posteriorly. At 12 weeks, ossification centers appear in the midline cartilage structures, and in addition, intramembranous bone formation of the jaws and brain case starts.

Development and maturation of the chondrocranium From this point on, bone replaces cartilage of the original chondrocranium rapidly, so that only the small cartilaginous synchondroses connecting the bones of the cranial base remain.

Development of Mandible In the mandible, bone formation begins just lateral to Meckel's cartilage and spreads posteriorly along it without any direct replacement of the cartilage by the newly forming bone of the mandible. Meckel’s cartilage disintegrate except some remnants which stay as sphenomandibluar ligaments and two of the conductive ossicles.

Development of Mandible The condylar cartilage (secondary cartilage) develops initially as a separate area of condensation from that of the body of the mandible, and only later is incorporated within it. fusion of the cartilage with the mandibular body occurs at 4 months. But the condylar cartilage persists after birth.

Development of Maxilla The maxilla also forms initially as mesenchymal condensation lateral to the nasal capsule. An accessory cartilage (Zygomatic or malar cartilage), which forms in the developing malar process, disappears and is totally replaced by bone before birth (unlike the mandible)

Sites and Types of Growth in the craniofacial complex

The Skull

The Skull

The Skull

The Skull

The Skull

Cranial Vault

Cranial Vault Bone formation occurs via intramembranous pathway. Fontanelles allow a considerable deformation of the skull at birth. Remodeling at the sutures is the major mechanism for growth of the cranial vault. In addition there is a tendency for remodeling on the our ant inner surfaces of the flat bones, which allows changes in the contour during growth.

The Cranial Base In general, midline structures grow through the endochondral pathway (cranial base) and as you move laterally, growth at sutures and surface remodeling become more important

The Cranial Base

The Cranial Base At synchondrosis, a band of immature proliferating cartilage cells, located at the center of the synchondrosis, while a band of maturing cartilage cells extends in both directions away from the center, and endochondral ossification occurs at both margins. Growth at the synchondrosis lengthens this area of the cranial base.

Cranial Base Even within the cranial base, bone remodeling on surfaces is also important-it is the mechanism by which the sphenoid sinus enlarges, for instance.

Maxillary Growth As growth of surrounding soft tissues translates the maxilla downward and forward, opening up space at its superior and posterior sutural attachments,new bone is added on both sides of the sutures. Sutural growth and surface remodeling are the two mechanisms for maxillary growth.

Mechanisms of Bone Growth All bone growth is a complicated mixture of two basic processes: deposition and resorption which are carried out by growth fields (the soft tissues investing the bone). Because the fields grow and function differently on different parts of the bone, the bone undergoes remodeling (shape changes)

Mechanisms of Bone Growth A. Deposition and resorption Deposition occurs on the surface facing the direction of growth while resorption is seen on the surface facing away. Enlow’s “V” principle is useful in understanding deposition and resorption in complicated remodeling during growth in length

Growth in width : V-principle

Mandible

Mandible Remodeling is done by resorption in the anterior part of the ramus and deposition in the posterior part of the ramus

Mandibular Chin

Mechanisms of Bone Growth B. Growth fields These include muscles, mucosa, blood vessels, nerves, connective tissue, brain, etc. These are the determinants of bone growth and its type (deposition or resorption).

References Contemporary Orthodontics; W. R. Proffit Mosby, 4th Edition; 2007.

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