GROWTH OF THE CRANIOFACIAL SKELETON

1. GROWTH OF THE CRANIOFACIAL SKELETON

2. Picture illustrates the change in overall body proportions that occurs during normal growth and development: • There is progressive reduction of the relative size of the head (from almost 30% of the total body lenght at birth to about 12% of the adult) • and faster growth of the trunk and limbs ( lower limbs grow more than upper limbs). At birth, the legs represent about one third of the total body length, while in the adult, they represent about half.

3. All of these changes, which are a part of the normal growth pattern, reflect the "cephalocaudal gradient of growth“. This means that there is an axis of increased growth extending from the head toward the feet. One reason for gradients of growth is that different tissue systems that grow at different rates are concentrated in various parts of the body.

4. Picture showes the curves for growth of the four major tissue systems of the body: • growth of the neural tissues is nearly complete by 6 or 7 years of age • general body tissues, including muscle, bone, and viscera, show an S-shaped curve, with a slowing of growth during childhood and an acceleration at puberty • lymphoid tissues proliferate far beyond the adult amount in late childhood, and then undergo involution • at the same time that growth of the genital tissues accelerates rapidly.

5. the infant has a relatively much larger cranium and a much smaller face • after the age of 6 years, there is little further growth of the cranium because the brain has nearly reached its adult size • the facial skeleton grows much longer and thus in the adult forms a much larger proportion of the scull than in the child. • when the facial growth pattern is viewed against the perspective of the cephalocaudal gradient, the mandible, being farther away from the brain, tends to grow more and longer than the maxilla, which is closer.

6. The nature of skeletal growth At the cellular level, there are only three possibilities for growth: • hypertrophy is an increase in the size of individual cells • hyperplasia is an increase in the number of the cells • the third is that cells secrete extracellular material, which contribute to an increase in size independent of the number or size of the cells themselves

7. The nature of skeletal growth All three of these processes occur in skeletal growth. • Hyperplasia is a prominent feature. • Hypertrophy occurs in a number of special circumstances but is a less important mechanism. • Although tissues throughout the body secrete extracellular material, this phenomenon is particularly important in the growth of the skeletal system, where extracellular material later mineralizes

8. The nature of skeletal growth • The fact that the extracellular material of the skeleton becomes mineralized leads to an important distinction between growth of the soft tissues of the body and the hard or calcified tissues. • Growth of soft tissues is called interstitial growthand it occurs at all points within the tissue. • In contrast, when mineralization takes place, interstitial growth becomes impossible

9. The nature of skeletal growth Growth of the bone is possible: • by direct or surface apposition- formation of new cells occurs in the periosteum, and extracellular material, secreted there, is mineralized and becomes new bone • from sutures • from cartilage

10. The nature of skeletal growth The craniofacial complex can be divided into four areas that grow rather differently: • the cranial vault, the bones that cover the upper and outer surface of the brain • the cranial base, the bony floor under the brain, which also is the dividing line between the cranium and the face • the nasomaxillary complex, made up of the nose, maxilla, and associated small bones • the mandible

11. Cranial vault At birth, the flat bones of the skull are widely separated by connective tissues. These open spaces, the fontanelles, allow a considerable amount of deformation of the skull at birth.

12. Cranial vault After birth, apposition of bone eliminates these open spaces fairly quickly, but the bones remain separated by a thin, periosteum-lined suture. • Apposition of new bone at these sutures is the major mechanism for growth of the cranial vault. • There is also remodeling of the inner and outer surfaces, the bone is removed from the inner surface of the cranial vault, while at the same time, new bone is formed on the exterior surface

13. Cranial base the important growth sites are the synchondrosis: • the spheno-occipitalsynchondrosis between the sphenoid and occipital bones • the intersphenoid synchondrosis, between two parts of the sphenoid bone • the spheno-ethmoidalsynchondrosis, between the sphenoid and ethmoid bones Growth at the synchondrosis lengthens this area of the cranial base and bone remodeling on surfaces is also important.

14. The synchondrosis has an area of cellular hyperplasia in the center with bands of maturing cartilage cells extending in both directions, which will be replaced by bone.

15. Maxilla (Nasomaxillary complex) Postnatal growth occurs in two ways: • at the sutures that connect the maxilla to the cranium and cranial base • by surface remodeling. The surface changes in the maxilla are quite dramatic and as important as changes at the sutures. By these mechanisms maxilla moves downward and forward relative to the cranium and cranial base.

16. Maxilla (Nasomaxillary complex) • The surfaces are remodeled, and bone is removed from most of the anterior surface. Only a small area around the anterior nasal spine is an exception. • the bone is removed on the nasal side and added on the oral side of the palate, thus creating an additional downward and forward movement of the palate.

17. Mandible • In mandibular growth, both endochondral and periosteal activity are important. • The mandible is translated downward and forward by growing upward and backward maintaining its contact with the skull.

18. Mandible Growth occurs: • in cartilage covering the mandibular condyle • as a growth site, the chin is almost inactive • all other areas of the mandible are formed and grow by direct surface apposition and remodeling

19. Mandible • The body of the mandible grows longer as the ramus moves away from the chin, and this occurs by removal of bone from the anterior surface of the ramus and deposition of bone on the posterior surface.

20. GROWTH ROTATION Characteristic feature of facial development is a rotation of both the maxilla and the mandible. This rotation can occur in either a forward ( anterior ) or backward (posterior) direction.

21. GROWTH ROTATION Forward rotation is the more common case : • the ascending ramus is long • gonial angle is small • symphysis menti is tilted backwards • chin is jutting out • there is apposition under the symphysis and resorption under the gonion so the lower border of mandible is S-shaped.

22. GROWTH ROTATION Backward rotation is characterized by increased anterior facial height resulting from a marked anterior lowering of the mandible in particular The following features are typical: • the ascending ramus is short • gonial angle is larger than normal • symphysis menti is tilted forwards

23. GROWTH ROTATION Most individuals show an anterior rotation pattern. The type of rotation has decisive influence on dental development, as extreme rotations influence the space conditions and vertical relations within the dentition. Maxilla rotates in the same direction but not to the same degree as the mandible, on which the typical features are more marked

24. Theories of growth control Growth is strongly influenced by genetic factors, but it also can be significantly affected by the environment. Exactly what determines the growth of the jaws, remains unclear. Three major theories try to explain the determinants of craniofacial growth: • bone, like other tissues, is the primary determinant of its own growth • cartilage is the primary determinant of skeletal growth • the soft tissue matrix in which the skeletal elements are embedded is the primary determinant of growth, and both bone and cartilage aresecondary followers. .

25. Theories of growth control The major difference in the theories is the location at which genetic control is expressed. In contemporary thought, the truth is to be found in some synthesis of the second and third theories, while the first, which was dominant until the 1960s, has largely been discarded

26. Sites versus centers of growth A site of growth is merely a location at which growth occurs, whereas a center is a location at which independent (genetically controlled) growth occurs All centers of growth also are sites, but the reverse is not true.

27. Sites versus centers of growth The sutures between the membranous bones of the cranium and jaws were considered growth centers. If this theory were correct, growth at the sutures should occur largely independently of the environment. But it can be seen that growth at sutures will respond to outside influences. What is more, if an area of the suture is transplanted to another location, the tissue does not continue to grow. So the sutures are sites of growth but are not growth centers.

28. Cartilage as a determinant of craniofacial growth • If cartilaginous growth were the primary influence, the cartilage at the condyle of the mandible could be considered as a pacemaker for growth of that bone, and the remodeling of the ramus could be viewed as secondary to the primary cartilaginous growth. • Growth of the maxilla is more difficult but not impossible to explain on a cartilage theory basis. Although there is no cartilage in the maxilla itself, there is cartilage in the nasal septum, and the nasomaxillary complex grows as a unit. The sutures of the maxilla serve as reactive areas and they respond to the maxillary translation by forming new bone when the sutures were pulled apart by forces from the growing cartilage

29. Cartilage as a determinant of craniofacial growth Transplantation experiments demonstrate that not all skeletal cartilage acts the same when transplanted. • Nasal septal cartilage grows nearly as well in culture as epiphyseal plate cartilage. • Cartilage from the mandibular condyle shows significantly less growth in culture than the other cartilages.

30. Cartilage as a determinant of craniofacial growth Experiments to test the effect of removing cartilages are also informative. • Removing a segment of the cartilaginous nasal septum causes a considerable deficit in growth of the midface. • In children, after a fracture of the mandibular condyle, all of the original bone and cartilage resorb, and a new condyle regenerates directly from periosteum. There is an excellent chance that the condylar process would regenerate to approximately its original size and a small chance that it would overgrow after the injury. In 15% to 20% of children who suffered a condylar fracture, there was a reduction in growth after the injury. Whether it occurs is dependent on the severity of the soft tissue injury.

31. Cartilage as a determinant of craniofacial growth In summary, it appears that epiphyseal cartilages and (probably) the cranial base synchondroses can and do act as independently growing centers, as can the nasal septum (perhaps to a lesser extent).

32. Functional matrix theory of growth • If neither bone nor cartilage was the determinant for growth of the craniofacial skeleton, it would appear that the control would have to lie in the adjacent soft tissues. • This point of view was put formally in the 1960s by Moss, in his "functional matrix theory" of growth. • Growth of the face occurs as a response to functional needs and is mediated by the soft tissue in which the jaws are embedded. The soft tissues grow, and both bone and cartilage react.

33. Functional matrix theory of growth The growth of the cranium illustrates this view of skeletal growth very well. Pressure created by the growing brain separates the cranial bones at the sutures, and new bone passively fills in at these sites so that the brain case fits the brain. This phenomenon can be seen in humans in two experiments of nature. • First, when the brain is very small, the cranium is also very small, and the condition of microcephaly results. • A second case is the condition called hydrocephaly. Uncontrolled hydrocephaly may lead to a cranium two or three times its normal size.

34. Functional matrix theory of growth • Another, excellent example is the relationship between the size of the eye and the size of the orbit. An enlarged eye or a small eye will cause a corresponding change in the size of the orbital cavity. • According Moss the major determinant of growth of the maxilla and mandible is the enlargement of the nasal and oral cavities, which grow in response to functional needs. The theory does not make it clear how functional needs are transmitted to the tissues around the mouth and nose.

35. In summary: • it appears that growth of the cranium occurs almost entirely in response to growth of the brain. • Growth of the cranial base is primarily the result of endochondral growth and bony replacement at the synchondroses, which have independent growth potential but perhaps are influenced by the growth of the brain.

36. The extent to which growth of cartilage of the nasal septum leads to translation of the maxilla remains unknown, but both the surrounding soft tissues and this cartilage probably contribute to the forward repositioning of the maxilla. • The mandible is translated in space by the growth of muscles and other adjacent soft tissues,