MECHANISMS OF EARLY NORMAL GROWTH OF THE CRANIOFACIAL SKELETON

define isometric growth VS allometric growth

• isometric growth: implies progressive proportional increase in all organs and systems with time

• allometric growth: describes the differences in relative rates of growth between one part of the body and another

  • clearly demonstrated in changes of body proportion between foetuses, neonates, children and adults

state the change in head proportion from foetus to adult

• between 6 and 7 weeks post fertilisation the head is nearly ½ of the total length of the embryo

• it then grows proportionally more slowly until the head is 1/8 the length of the body in adults

the skull _________ to the coronal suture is derived from ________

the skull posterior to the coronal suture is derived from mesoderm

where are the bones anterior to the coronal suture derived from 

ectomesenchyme from the neural crest

define condensation

condensation: both types of bone formation start with the coalescence of mesodermal or ectomesenchymal cells into dense groups

» mechanisms of craniofacial growth

what happens after condensation

• the condensed cells may differentiate into chondroblasts that form a cartilaginous template of the bone

• the cartilage is eventually replaced by bone

  • this process is called endochondral ossification

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• alternatively, bone may be formed directly in the condensation by intramembranous ossification

» mechanisms of craniofacial growth

outline interstitial growth and appositional growth (mechanisms of craniofacial growth)

• most organs grow by interstitial growth which results from cell division and matrix proliferation throughout their structure

• the mineralised matrix of bones prevents interstitial growth (growth in length from within)

• therefore bones grow by addition of new material onto existing surfaces which increases thickness and diameter - this is appositional growth

which bones can grow via interstitial growth

• bones developing by endochondral ossification can grow by interstitial growth

• because cartilage is growing during this process and cartilage can grow by interstitial growth

how are adult size and proportions of a bone achieved

• by a combination of surface deposition and resorption

  • this is known as remodelling

• during growth periods, virtually all internal and external bone surfaces undergo remodelling

• growth sites are surfaces/ cartilages where large amounts of growth take place

what are causes of growth of the craniofacial skeleton

• generally: hormonal, nutritional, genetic and socioeconomic influences

• locally: growth pattern (somatic, neural), capsular matrices (enclosed tissues), periosteal matrices (muscle attachments, teeth)

outline the size of the skull at birth

• proportionally large - this reflects early cerebral maturation

• the facial skeleton is relatively small (1/8 of cranium) compared with an adult (1/2 of cranium)

• ossification is incomplete in many sites to allow future growth

what is the smallness of the face at birth due to

• the smallness of the face at birth is due to the rudimentary stage of development of the mandible and maxillae - because teeth are unerupted

postnatal growth of the skull 

• most of the postnatal growth of the skull is concerned with the growth of the facial skeleton

what is the first feature to appear on the forming skull

the cartilaginous precursors of the cranial base

which ossification process are the bones of the cranial base formed by

the bones of the cranial base are formed by endochondral ossification

what are in between the bones of the cranial base

• cartilaginous joints known as synchondroses

  • these are remnants of the cartilaginous precursors

• growth can occur at synchondroses

in which area does the growth cause most of the cranial lengthening

growth in the cranial base results in most of the cranial lengthening

which synchondrosis is the most important site of growth on the skull

• the spheno-occipital synchondrosis

• growth here continues until approx. 18yo although it can stop earlier around 15yo

what is the importance of cranial base elongation

elongation of the cranial base allows room for the elongation of the maxillae to accommodate the 2nd and 3rd permanent molar teeth

what ossification process are bones of the cranial vault formed by

the bones of the cranial vault are formed by intramembranous ossification

what does the growth of the cranial vault follow

the growth of the cranial vault follows the neural growth pattern of the enclosed brain, which is rapid prenatally

outline the growth of the cranial vault

• growth continues rapidly in the first year then at a slower rate until the 7th year

• during the first year, growth occurs primarily by appositional growth (growth of bone in thickness and diameter), accompanied by remodelling to adapt to continually altering curvatures

outline the growth of fontanelles

• the fontanelles seen at birth are gradually closed by ossification of the bones around them

• the bones meet but do not fuse

• they remain separated by fibrous joints/ sutures which are essential growth sites in the immature skull

• the sutures will fuse in maturity

which ossification process do the facial bones formed by 

the facial bones are formed by intramembranous ossification

growth of the facial skeleton occurs…

growth of the facial skeleton occurs over a long time period (well into adulthood)

how is growth of the orbital and upper nasal cavity achieved

orbital and upper nasal cavity growth is achieved by deposition of bone at sutures

where on sutural junctions does the deposition of bone favour

more deposition occurs on the facial aspect of sutural junctions

outline the growth of the maxilla

• the maxilla is carried downwards and forwards by expansion of the orbits and nasal septum and by sutural growth

• there is deposition at the inner aspect and resorption at the outer aspect - suggests posterior growth

• rapid deposition at the tuberosities » forwards translocation

is there more deposition posteriorly or anteriorly in the maxilla

• deposition posteriorly is greater than resorption anteriorly

• therefore there is growth as well as translocation

growth at which sites on the skull cause an increase in width

growth of the maxilla at sub-adult and adult ages

outline growth in the bones of the nasal cavities and the palate

• upper aspect of bones in floor of nasal cavities are resorbed

• bone is deposited on the oral surface of the palate

• direction of growth of maxillary arch is downwards

what ossification process is the mandible formed by

embryologically, the mandible forms by intramembranous ossification

outline the growth of the mandible

• ossification occurs around the mental, incisive and inferior alveolar nerves

• the ramus is formed by rapid backward spread of ossification

• secondary cartilages appear in the mandible

  • they are not primary cartilages, which are formed in the pharyngeal arches

  • 2° cartilages are growth sites, producing bone by endochondral ossification

outline the growth of the condylar cartilage

• one of the main sites of bone deposition in the mandible

• it is active throughout the growth period up to maturity

• as the cartilages grows it is replaced inferiorly by bone

• a narrow zone of growth cartilage persists beneath the articular surface of the condylar head

what does eruption of the primary dentition stimulate

• enables infant to chew food

• stimulates development of muscles of mastication

• increases size of their attachment sites to mandible

• angle of mandible must be altered so that occlusal surfaces of upper and lower teeth are parallel

image showing mandibular age changes

what happens to the mandible in older, edentulous patients

mandibular thinning

how is the position of the TMJ altered

the position of the TMJ is altered by growth in the lateral cranial base (mandibular/ glenoid fossae)

how is the position of the condylar head adjusted

selective resorption and deposition in the ramus adjusts the position of the condylar head

how is the rami of the mandible elongated

• growth at the condylar cartilages elongates the rami of the mandible

• length of the ramus and the body is also increased by bone deposition along the posterior border of the ramus and resorption along the anterior border

what does an increase in the length of the mandible help accommodate

eruption of the permanent dentition

image showing mandibular growth

the growth of facial soft tissues may mask or enhance…

hard tissue changes

when does most change of facial soft tissue occur

around puberty

outline the growth of the nose

• the nose continues to grow in a downward and forward direction at least until early adulthood though it can continue beyond this

• does not seem to be an appreciable decrease in rate of nasal growth - typical for skeletal structures

by how much does the nose grow yearly

• average yearly increase of 1-1.3mm in overall length of external nose is almost the same for males and females

outline the growth of the lips 

• the upper and lower lips grow more than the skeletal lower face in children

• the lower lip grows more than the upper lip

• upper lip shows rapid increase in length from 1-3yrs, slows between 3-6yo and then increases up to 15yrs

the increase in lip length is proportional to what

• increase in lip thickness at the vermillion border is proportional to increase in lip length

outline the growth of the chin

• increased chin projection in males during growth is due to mandibular growth rather than soft tissue change

» notable when deciding orthodontic treatment as the mandible will start growing after the maxilla

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