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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 occursprimarily 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
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.
Cranial Vault
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 areaof 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.