Types of bone formation

Endochondral bone formation

Diaphysis

Epiphysis

epiphyseal plate

The epiphyseal plate is responsible for almost all growth in length of long bones. The periosteum on the bone surface

plays an important role in adding thickness and in reshaping the external contours.

Near the outer end of each epiphyseal plate is a zone of actively dividing cartilage cells. Some of these undergo

hypertrophy, secrete an extra-cellular matrix and eventually degenerate as the matrix begins to mineralize and then is

rapidly replaced by bone.

Intramembranous bone formation

In this type of formation, it is possible for bone to form by secretion of bone matrix directly within the

connective tissue without any intermediate formation of cartilage. Ex. the cranial vault and both jaws.

The condylar cartilage develops initially as an independent secondary cartilage, which is separated by a considerable gap from the body of

the mandible. Early in fetal life, it fuses with the developing mandibular ramus.

Whatever the location for intramembranous bone formation, interstitial growth within the mineralized mass

is impossible and the bone must be formed entirely by

apposition of new bone to free surfaces.

Its shape can be changed through removal (resorption) in one area and addition (apposition)

of bone in another. This process is called

remodeling.

To understand growth in any area in the body, it is necessary to understand: 1-The site or location of growth. 2-The type of growth occurring at that location. 3-The determinants or controlling factors in that growth.

the craniofacial complex will be divided into four areas that grow differently:

1-Cranial vault 2-Cranial base

3-Naso-maxillary complex. 4-The mandible.

The cranial vault is made up of a number of flat bones that are formed directly by intramembranous bone formation

Cranial base

the bones of the cranial base are formed initially from cartilage; hence endochondral ossification

takes place.

Centers of ossification appear early in the embryonic life in the chondrocranium indicating

the location of basioccipital, sphenoid and ethmoid bones that form the cranial base.

As ossification proceeds bands of cartilage called synchondrosis remain between the centers of

ossification.

There are four synchondrosis: 1- Spheno-ethmoidal: It closes about 5-7years. 2- Inter-sphenoidal: It persists till the beginning of adult life. 3- Spheno-occipital: It persists to 18-20 years of age. 4- Intra-occipital: It closes 3-5 years.

A synchondrosis looks like two-sided epiphyseal plate

area of cellular hyperplasia in the center with bands of maturing cartilage cells extending in both directions

A significant difference from the bones of the extremities is that immovable joints develop between the bones of the cranial base in contrast to the highly movable joints

of the extremities. In addition, the periosteum-lined sutures, containing no cartilage, are quiet different from

the cartilaginous synchondrosis.

Naso-maxillary complex

Intramembranous ossification takes place. Growth occurs in two ways:

1- By apposition of bone at the sutures.

2- By surface remodeling.

the roof of the mouth is carried downward and forward. Since the anterior part of the alveolar process is resorptive,

some of the forward growth is cancelled.

Mandible In contrast to the maxilla, both endochondral

and periosteal activity are important in

mandibular growth.

The principal sites of growth are the posterior surface of the ramus and the condyle and coronoid processes.

Anterior resorption & posterior deposition

Determinants of growth

Bone is the primary determinant of its own growth.

Cartilage is the primary determinant of skeletal growth, while bone responds secondarily and

passively.

The soft tissue matrix is the primary determinant

and both cartilage and bone are secondary

followers.

Sites versus centers of growth Site is merely a location at which growth occurs; while a center is a location at which independent (genetically controlled) growth occurs. All centers

of growth are sites but the reverse is not true.

Cartilage as a determinant of craniofacial growth

The mandible is a modified long bone

Functional matrix theory

In this concept, soft tissues grow and both

cartilage and bone react to this growth.

The relationship between the size of eye and the orbit .

Mechanical restriction caused by scar tissue in the TMJ due to condylar fracture is the reason for growth

deficiency causing hemifacial microsomia

If there is a growth deficiency in the brain, the brain case

will be small causing microcephaly.