Types of Skeletal Growth

Types of Skeletal Growth

Interstitial vs. Appositional
Endochondral vs. Intramembranous
Remodeling
Interstitial vs. Appositional
Interstitial – “inside the tissues”
Growth that occurs by a combination of hyperplasia, hypertrophy, and secretion of extracellular matrix
Occurs at ALL points within the tissue
Examples:
All soft tissues
Cartilage
Cranial base is formed by interstitial growth

Interstitial growth

Resting cells in cartilage
HYPERPLASIA
proliferating cells
(cell division)
HYPERTROPHY
Increase in cell size
secretion
Extracellular matrix
If the extracellular matrix is NOT mineralized,
interstitial growth may continue in soft tissues and cartilages.
CARTILAGE


Types of GrowthInterstitial vs. Appositional
Appositional – “addition to surfaces”
Growth that occurs by a combination of hyperplasia, hypertrophy, and secretion of extracellular matrix
Occurs ONLY at the SURFACE OF BONES
Addition of new bone to the surface of existing bone
Occurs through the activity of cells in the periosteum
Examples:
Bony surfaces of cranial vault
Bony surfaces of maxilla and mandible

Appositional growth

Resting cells in periosteum
HYPERPLASIA
proliferating cells
(cell division)
HYPERTROPHY
Increase in cell size

mineralization

Extracellular matrix
When the extracellular matrix IS mineralized, bone is formed.
Interstitial growth CAN NOT occur within bone.


PERIOSTEUM
BONE
secretion
Cartilage can grow intestitially, but bone can’t.
Once the cartilage is transformed into bone, it continues to grow only by apposition.

Types of GrowthEndochondral vs. Intramembranous

Endochondral
Bone growth within cartilage
Ossification centers occur within cartilage
Cartilage is transformed into bone
Examples
Chondrocranium: ethmoid, sphenoid and basioccipital bones

Ethmoid, sphenoid, petrous part of temporal bone and basioccipital bones

Types of GrowthEndochondral vs. Intramembranous
Intramembranous
Secretion of bone matrix within and between connective tissue membranes
No intermediate formation of cartilage
It does NOT replace cartilage
Examples
cranial vault (brain case), maxilla and mandible



Cranial vault (brain case), maxilla and mandible

Intramembranous bone formationThe Mandible

Mandible forms by intramembranous bone formation just lateral to Meckel’s cartilage

Condylar cartilage of mandible

It does NOT arise from Meckel’s cartilage
It develops from an independent secondary cartilage
Fuses with the mandibular ramus

Types of Growth

Surface Remodeling
Change in shape of bones
The result of bone removal (resorption) in one area and bone addition (apposition) in another
Occurs at the surfaces of growing endochondral AND intramembranous bones
Examples
Bony surfaces of cranial vault, maxilla and mandible
Internal Remodeling
Vascular channels within bones bring osteocytes to the area
Allows the bone to adapt to mechanical stress
Allows for exchange of Calcium and phosphate
Internal remodeling does not make bone grow or change its shape


Sites of Growth in Craniofacial Complex
Cranial vault
Cranial base
Nasomaxillary complex (maxilla)
Mandible
The Cranial Vault (brain case)
Flat bones formed by intramembranous formation from ossification centers
No cartilaginous precursors
Periosteal activity results in bone remodeling both at sutures and at inner and outer surfaces



Brain case grows primarily by apposition of new bone at sutures, but also by remodeling of inner and outer surfaces of bones.
Cranial sutures and fontanelles: lined by periosteum

The Cranial Base

Bones of the base of the skull formed by endochondral ossification
Formed initially in cartilage then transformed to bone
Ossification centers result in ethmoid, sphenoid and basi-occipital bones.



Synchondroses of the cranial base
Cartilaginous joints between the bones of the cranial base
Immovable joints
Most important synchondrosis: spheno-ethmoidal, inter-sphenoidal, and spheno-occipital

Synchondroses of the cranial base

Endochondral growth occurs within the synchondroses
Growth at the synchondroses lengthens the area of the cranial base


*Bone surface remodeling also occurs in the cranial base: note the enlargement of the sphenoid sinus (S)
Syncondrosis vs. Suture
• Synchondrosis
• Suture
• Junction between adjacent bones
• Junction between adjacent bones
• Cartilage
• Connective tissue
• Endochondral ossification
• Direct ossification
• Active growth (primary)
• Re-active growth (secondary)


The Nasomaxillary complex
Formed entirely by intramembranous ossification
No cartilaginous precursor
Growth occurs by apposition and surface remodeling

Circum-maxillary suture system

Translation of maxilla
Maxilla is translated downward and forward- away from cranial base
Apposition of bone occurs at circum-maxillary sutures, which elongates the maxilla

Surface remodeling of maxilla

Bone in the anterior surface of the maxilla is resorbed
Apposition of bone occurs at the maxillary tuberosity

Bone is resorbed in the floor of the nose

Apposition of bone occurs at the palate and alveolar process
As a result, palate moves mostly downward and widens transversely

The Mandible

Endochondral growth occurs at the cartilage that covers the mandibular condyle
All other areas grow by surface apposition and remodeling

Translation of mandible

Mandible is translated downward and forward- away from cranial base
Principal sites of growth: posterior surface of ramus and the condylar and coronoid processes

Surface remodeling of mandible

Body of mandible grows longer
Ramus of mandible grows higher
More room in the body of mandible for molars to erupt

Theories of Growth control

Site of growth is a location at which growth occurs.
Center of growth is a location at which independent growth occurs.
“All centers of growth are also growth sites, but the reverse is not true.”