Concepts of growth& development II ORTD 431, lecture 4 Pages 35-48, 94-98, 108-112 3rd addition Proffit

The Nature of Skeletal Growth At the cellular level , there are only three possibilities for growth 1. Hypertrophy increase in cell size 2. Hyperplasia increase in cell No. 3. Secretion of extracellular matrix (ECM) In fact all three processes occur in the skeletal growth where hyperplasia is the most prominent followed by hypertrophy then EMC Secretion of ECM is most important in skeletal system growth

Thus there is a distinction between growth of soft or non mineralized tissue and hard , calcified tissues. Hard tissue includes ( bone, teeth and sometimes cartilages) Soft tissues are every thing else In most instance cartilage behaves like soft tissue and should be included in soft tissue group

Interstitial Growth - Growth of soft tissues occurred by combination of hyperplasia and hypertrophy this phenomenon go every where within the tissues the result is interstitial growth - It occurs in at all points within the tissue - Is characteristic of nearly all the soft tissues and of uncalcified cartilage within the skeletal system.

Direct or surface apposition of bone Appositional growth  growth by addition at the periphery of a particular part. Interstitial growth  that occurring in the interior of structures already formed. appositional growth  growth by addition at the periphery of a particular part. interstitial growth  that occurring in the interior of structures already formed.

The periosteum play an important role in adding thickening of bone and reshaping the outer contour of bone. As long as the rate of proliferations of cartilage cells equal or faster than maturation and secretion of ECM are high , there is growth as they slow growth will slow too

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 no cartilage model (direct apposition of bone in the ECM). Sites : mandible, maxilla, and cranial vaults.

Remodeling Growth process by which apposition (bone addition) takes place in one area and resorption ( bone removal) takes place in another area. Function of remodelling : 1- Create the changing size of the whole bone. 2- Relocate each parts of the whole bone to allow bone enlargement. 3- Shape the bone to accommodate its functions. 4- Provide fine tune fitting to all bone parts to each other and to the surrounding soft tissues. 5- Carry out continuous structural adjustments to adapt to intrinsic and extrinsic changes in condition.

Development of mandible In the mandible , bone formation begins just lateral to meckels cartilage and spreads posteriorly along it without any direct replacement of the cartilage by the newly forming bone of the mandible . Meckels cartilage disintegrate except some remnants which stay as 1. Sphenomandibular ligaments 2. Tow of conductive ossicles.

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 capsules . The growth cartilage contribute to lengthening of the head and anterior displacement of the maxilla. An accessory cartilage ( zygomatic or molar cartilage), which forms in the developing malar process , disappears and is totally replaced by bone before birth

Site and types of growth in the craniofacial complex To understand growth in any area of the body , it is necessary to understand: 1. Site or location of growth 2. Type of growth occurring on that location 3.The determinant or controlling factors in that growth

The site growth in craniofacial complex The cranial vault , the bone that cover the upper and outer surface of the brain The cranial base , the bony floor under the brain, which is also the dividing line between the brain and the face. The nasomaxillary complex made up from the nose, maxilla, and associated small bones The mandible

Cranial Vault Bone formation occurs via intramembranous pathway ( by periostum) At birth, the flat bones of the skull are widely separated by relatively loose connective tissues the Fontanelles allows a considerable deformation of the skull at birth this allows the relatively large head to pass through the birth canal.

Cranial Vault After birth, apposition of bone along the edges of Fontanelles. Remodeling at the sutures is the major mechanism for growth of the cranial vault. In addition there is tendency for remodeling on the outer & inner surfaces of the flat bone , which allows changes in the contour during growth.

The Cranial Base Bony floor under the brain Bones are formed initially in cartilage and later transformed by endochondral ossification to bone. In general, cranial base is a midline structure grow throw the endochondral pathway (cranial base )and as you move laterally ,at sutures and surface remodeling become more important.

The Cranial Base At synchonrosis , a band of immature proliferating cartilage cells is located between the centers of ossification.

Growth at intersphenoid synchonrosis A band of immature proliferating cartilage cells is located at the center of the synchonrosis, while a band of maturing cartilage cells extend in both directions away from the center , and endochondral ossification occurs at both margins.

The cranial base Growth at the synchondrosis lengthens this area of cranial base . Even within cranial base , bone remodeling on surface is also important it is the mechanism by which the sphenoid sinus enlarges, for instance.

Maxillary (nasomaxillary complex) growth The maxilla develops postnatally entirely by intermembanous ossification. No cartilage replacement Growth occur in two ways: 1) Apposition of new bone at the sutures. 2) Surface remolding .

Maxillary (nasomaxillary complex) growth As growth of the surrounding soft tissues translates The maxilla downward and forward , opening up Space at its superior and posterior sutural attachments, new bones is added on both sides of sutures.

Mandible Hyperplasia, hypertrophy, and endochodral replacement occur in the condylar cartilage all other areas of the mandible are formed by direct surface apposition and remodeling. The chin moves downward and forward. The correct concept of mandibular growth is that the mandible translated downward and forward and grows upward and backward. the actual growth occurs at condyle and along the posterior surface of the ramus (removal of bone from anterior surface of ramus and deposition of bone on the posterior surface make the mandible grow longer

Theories of growth Three major theories in recent years have attempted to explain the determinants of craniofacial growth: 1) Bone , like other tissues, is the primary determinant of its own growth 2)Cartilage is the primary determinant of skeletal growth ,while bones responds secondarily and passively. 3) The soft tissue matrix in which the skeletal elements are embedded is the primary determinant of growth , and both bone and cartilage are secondary followers. Sites Vs. center of growth Functional matrix theory.