mechanisms of early normal growth of the craniofacial skeleton

define isometric growth?

* progressive proportional increase in all organs and systems with time

define allometric growth?

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

where is allometric growth shown?

is clearly demonstrated in the changes of body proportion between fetuses, neonates, children and adults.

describe the head at 6-7 weeks post-fertilisation

the head is nearly half of the total length of the embryo.

grows until the head is 1/8th of body size in adults

What bones of the skull are derived from mesoderm and ectomesenchyme?

- The skull posterior to the coronal suture is derived from mesoderm
- The bones anterior to this suture are derived from ectomesenchyme from neural crest

What is the process of condensation?

Coalescence of mesodermal or ectomesenchymal cells into dense groups

This is the START of the 2 types of bone formation

What are the 2 types of bone formation?

**Endochondral Ossification (ECO)** - condensed cells differentiate into chondroblasts - forms cartilage template. Then cartilage replaced by bone



**Intramembranous Ossification (IMO)** - bone formed directly from condensed cells

what do most organs grow by?

**Interstitial growth** - results from cell division and matrix proliferation throughout their structure

what prevents interstitial growth in bones?

therefore what does bone do in order to grow?

* the mineralised matrix of bones,
* Therefore, bones grow by addition of new material onto existing surfaces. = APPOSITIONAL GROWTH

why do bones developing from ECO not have the limitations of appositional growth?

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

How are the adult size and proportion of bone achieved?

Process of remodelling - combination of surface deposition (osteoblasts) and resorption (osteoclasts)

What bone surfaces undergo remodelling during growth periods too?

all internal and external bone surfaces undergo remodelling

describe growth sites

are surfaces or cartilages where large amounts of growth take place

What general factors Affect Growth of the Craniofacial Skeleton?

* hormonal influences
* nutritional influences
* genetic influences
* socioeconomic influences - links to nutrition

What localised factors affect Growth of the Craniofacial Skeleton?

1) Growth Patterns

* Somatic (bones whose growth is in relation to the rest of the body)
* Neural (growth of the bones surrounding the brain follows the pattern of growth of the brain).



2) Capsular Matrices- bone around enclosed soft tissues need to be remodelled in relation to soft tissue e.g bones around the eye, the brain



3) Periosteal Matrices- Joints, Muscular Attachments and Teeth a have an influence on the bones of the craniofacial skeleton

describe the skull and facial skeleton at birth?

skull = proportionally large •

facial skeleton = relatively small compared with an adult - due to early stages of development of the mandible and maxillae

Based on looking at skull at birth what reflects early cerebral maturation?

* the large size of the calvaria (top of skull)
* ossification is incomplete in many sites

when is there the biggest growth of the facial skeleton?

during childhood and puberty.

What is cranial base?

Bones at bottom of skull

How is cranial base formed?

Bones of cranial base formed from cartilaginous precursors therefore via ECO

In between the bones of the cranial base are **cartilaginous joints** (remnants of the precursors) known as **synchondroses**

What does growth of cranial base result in?

* Cranial lengthening
* Important site of growth : spheno-occipital synchondrosis -continues until \~18 years old
* Elongation of the cranial base allows room for elongation of the maxillae to accommodate the 2nd and 3rd permanent molars.

What is cranial vault?

Part of the Skull Overlying the Brain

How are bones of cranial vault formed?

* Via IMO (no cartilage precursor)
* Follows neural growth pattern - this is rapid prenatally
* Grows from 1-7 yrs starts rapidly but slows down
* Grows via appositional growth (bone onto bone) and remodelling in 1st year
* fontanelles gradually fuse by ossification

what are the facial bones formed by?

* IMO
* Occurs over long period (well into adulthood)
* Orbital and upper nasal cavity growth is achieved by **deposition of bone at sutures**


* More deposition occurs on the **facial aspect** of sutural junctions -resulting in the maxilla being carried **downwards and forward**

what are some of the post natal sites of growth

* Frontal suture
* Internasal suture


* Symphysis menti - joins 2 halves of mandible together

\


Deposition of bone at the sutures is associated with an increase in width of the facial skeleton.

What direction does facial skeleton always form?

downwards and forwards

How does the maxilla grow forwards?

Arrow in pic shows direction of growth

* deposition = on the palate and rapid on the tuberosities


* resorption = labial area

\
Deposition posteriorly is greater than resorption anteriorly = forward growth

What causes downward growth of maxilla?

* Resorption of bone in upper aspect of nasal cavity
* Deposition of bone on palate

How does mandible form embryologically?

forms by IMO - bone forms around nerve canal (IAN,incisive,mental) and ramus formed by backward spread

\
After bone is formed secondary cartilages appear in mandible

Secondary cartilages = growth sites - produce bone by ECO

How does the mandible grow?

Condylar cartilage is one of the main sites of bone deposition in the mandible:

* active throughout the growth period to maturity
* as cartilages grows it is replaced inferiorly by bone



Narrow zone of growth cartilage persists beneath the articular surface of the condylar head.

* means that changes in the mandible and condyle can occur throughout life.

How do teeth influence growth of mandible?

* Eruption of primary dentition 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

what is the position of the TMJ altered by?

The position of the TMJ is altered by growth in the lateral cranial base:

As skull widens, underside of glenoid fossa moves outwards

So condylar process of mandible needs to widen so that they align

\
This is done by selective resorption and deposition in ramus

How is length of ramus increased and what for?

* increased by bone deposition along posterior border
* resorption along the anterior border
* accommodates for eruption of permanent teeth

Summary for why mandible growth needed

occurs to accommodate eruption of the permanent dentition, to maintain alignment of the TMJ and to keep upper and lower teeth aligned.

When is there the most change in growth of facial soft tissues?

* most change occurs around puberty
* They may mask or enhance hard tissue changes

What change happens in nose?

grow in a downward and forward direction at least until early adulthood.

yearly increase of 1–1.3 mm in overall length

describe the growth of the lower lips

lower lip grows more than the upper lip

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

Why is there increased chin projection in males?

due to mandibular growth rather than soft tissue change.