Summarized by Dr.Hani Alhebshi ..
The aim of this summary is to outline the basic mechanisms and regulation theories to be as a quick reference of craniofacial growth and development. More details can be found in the bottom of this summary when references are listed.
Craniofacial Mechanisms are devided into 4 components :
- Growth Mechanisms ( How bone is formed ).
- Growth Pattern ( change of size and shape of bone).
- Growth Rate ( Speed at which bone is formed).
- Regulation Mechanisms that regulate those three factors.
Growth Mehanisms :
Ossification of bone occurs in two ways :
- Endochondral Ossification : Cartilage transformed into bone.
- Intramembranous Ossification : Mesenchymal connective tissue is transformed into bone by deposition of bone on existing bone surfaces.
Three basic phenomena are involved in growth mechanisms :
I – In Cartilage Conversion :
They are remnant cartilages in the cranial base. Spheno-occipital Synchondrosis is the most important one. It is completely covered with bone by age 12-13 years in girls and some years later in boys. They are considered as special joints enabling growth to take place at younger ages. Spheno-occipital Synchondrosis acts like a growth center not site. (Note : The only cartilage that is established to be a growth center is the epiphyseal plate in long bones ).
*Nasal Septal cartilage
Nasal cartilage growth is concluded in literature as secondary to and compensatory for a prior passive displacement of midfacial bones but plays very important role in maintaining normal midfacial form.It is concluded to be a growth site not center.
Previously it has been claimed that it was expansive like epiphyseal plate ( Growth Center) thereby pushing mandible forward.However , It is considered to participate in regional adaptive growth ( i.e Condyle is a growth Site).
II – Sutural Deposition
Sutures have fibrous components that unites the bones together and permitting tissues to grow.Thy ossifies with age and eventually cranial growth ceases.
III – Periosteum
It has the greatest significance for the change in size and shape of the bones. It depends on pressure and tension sites where osteoclasts appear in pressure site and osteoblasts in the tension site.Regenerative capacity is extremely high in the young child.
Growth Pattern :
Change in bone’s shape and size takes place on the basis of several basic principles:
Modeling : Surface specific apposition and resorption that produce a change in size and shape.
Remodelling : Reconstruction of existing osseous tissue ( Haversian system).
Cortical Drift “Relocation ” : Relative migration of bone due to remodeling ( migration in relation to a fixed structure).
Displacement : Two types :
- Primary Displacement ( Translation ) :
Process of physical movement related to a bone’s own enlargement. As bone enlarges it is simultaneously carried away from other bones in direct contact with it. This creates the “space” within which bony enlargement takes place. It always takes place in the direction opposite to the vector of bone growth.
- Secondary Displacement :
one bone displaces another bone as the first bone is moved
V – Principle :
Many facial and cranial bones or parts of bones have a V-Shaped configuration. Bone deposition occurs on the inner side of the “V” and resorption occurs on the outer side of the “V”.
The direction of movement is toward the wide end of the “V”.
Growth of Nasomaxillary Comolex “NMC” :
Growth in NMC occurs by displacement , cortical drift ( remodeling ) and sutural movement.Nasal cartilage is considered to play a role in the development of the midface through vomer groove.As the cartilage of nasal bone enlarges downward and forward it slides along the vome rine groove and pulls the maxilla.
Primary displacement by bone remodeling superior and posterior to tuberosity so displace maxilla forward and downward.The displacement caused by expansive growth of the soft tissues in the midfacial region.That will trigger new bone growth at sutural contacts with cranial floor.
Secondery displacement by enlargement of nasal , orbital and cranial bones.
Sutural Growth in sutures that attaches the maxillary complex to cranium and facial bones.
The alveolar base is elongated creating space for posterior and late erupting teeth.
Movement of nasal floor is by drifting owing to remodeling growth and by displacement owing to movement of maxilla as a whole.
Lowering of Orbital floor is limited by bone deposition toward the orbits.
While Nasal floor continues to be lowered by nasal resorption with simultaneous bone deposition of bone orally on the palate.
The nasal floor drops 61% while orbit lowered only 25%.
Growth of Mandible :
Displacement both primary and secondary displacements are involved in addition to cortical drift owing to remodeling in all directions.
Ramus remodeling (Ramus Relocation) : Bone is deposited on posterior of the ramus and resorption anterior to it.So Ramus will be relocated more posteriorly as it remodels. it also becomes wider and longer.
Buccal drift of the rami because bone deposition occurs buccally and resorption lingually.
Primary Displacement and Seconderay Displacement are involved in the mandibular growth.
Condylar and coronoid processes are formed in cartilage. wedge of this cartilage converts to bone except at articular surface.This remaining surface has the local potential to grow and its growth direction will determine the autorotation of mandible ( i.e long or short face).
It was previously believed that growth of condylar cartilage was the cause of mandibular displacement. However, they are not a type of control center of whole mandible ( It’s rather a site that functions locally ).So Condylar growth is a secondary phenomenon to adapt for the space in the temporomandibular joint region that was created by enlargement of soft tissues.
Whole mandible is displaced away from its articulation in each glenoid fossa by the growth enlargement of the composite of soft tissues in growing face. As this occurs , the condyle and ramus grow upward and backward into the space created by the displacement process.
Bjork showed condyles grow in different directions.
As condyles grow posteriorly mandible is thrust forward.so gonial angle remain obtuse. Mandible is long.
When condyle grow upward and forward the result is an acute gonial angle and short effective length of mandible (between condyle and chin).
Growth Rate :
The Growth rate is not constant but it varies at different times during the development of the child.
Regulation Mechanisms ( Theories of Growth Control ) :
1-Sicher’s hypothesis ( Sutural Dominance 1940 ):
He claimed that there is innate genetic mechanisms are primary in skeletal formation.
2- Scott’s hypothesis ( Nasal Septum ):
displacement of cranium is secondary to brain mass growth..
Midface growth is secondary to chondrochranium above all nasal septum. Nasal cartilage is center in his hypothesis.
Mandibular growth is result of condylar cartilage.
3- Moss’ hypothesis ( Functional Matrix Theory) :
He hypothesized that bone has no genes ( osteogenic tissue has no genetic control ).
Craniofacial complex classified as compnenets ( matrices) with carry out the function.
Functional Matrix : comprising tissues and cavities and skeletal unit consisting of bone, cartilage, and tendons which protects and support the matrix.
Periosteal Matrix : is a part of functional matrix where bone can be remodeled in the muscle insertions and the teeth in alveolar process through the periosteum .
Capsular Matrix : is a broader effect is achieved by tissues and functional cavities like brian mass and respiratory function which produce the movement of the whole bone.
4- Van Limborgh’s theory :
Postnatal faical growth is controlled by by a multifactorial system that is influenced by :
- Intrinsic genetic factors : within cells ( cell differentiation ).
- Epigenetic factors : factors that are determined genetically but effective outside the cells and tissues.
- Local epigenetic : eg. Embryonic induction infuences.
- General epigenetic : eg, sex and growth hormones.
- Environmental factors :
- Local : eg ,muscular force.
- General : eg , food , oxygen supply.
5- Petrovic’s Hypothesis ( Servosystem 1990 ) :
Physiologic effect of factors controlling the facial growth is not limited to simple commands but includes relays, implying interactions and feedback loops. Occlusal adjustment play the role in controlling masticatory muscles enabling lower dental arch to adjust to the optimal occlusal position. The increased muscle activity induces posterior growth rotation of the mandible and growth rate of the condyle.