Introduction

According to Todd, growth refers to increase in size. According to Proffit, it is an increase in size and number. The craniofacial complex consists of brain, face, oral cavity with bones, cartilages and soft tissues supporting them. The craniofacial growth is through the growth of bones, cartilages and the corresponding soft tissues leading to the complete facial structure. This happens in utero and the growth continues after the birth. Growth occurs in 3 planes, the transverse plane followed by horizontal and then vertical. 

Bone formation is through endochondral process and/or intramembranous process. The mechanism of bone growth can be summarised into Drift, Displacement, and Remodelling. This concept was given by Enlow.

Remodelling

It is the process of bone deposition and resorption that leads to the change in size or shape, location of the bone. The basic shape of bone is maintained.

There are 4 types of remodelling:

• Biochemical remodeling: continuous deposition and removal of ions to maintain mineral homeostasis

• Growth remodeling: constant replacement of bone during childhood

• Haversian remodeling: secondary process of cortical reconstruction of bone as primarily vascular bone is replaced

• Pathological remodeling: regeneration and reconstruction of bone during and following pathology or trauma

Drift/ Cortical drift

Drift is the growth movement of enlarging portion of the bone by remodelling action of its osteogenic tissues.

The bony cortical plate drifts by depositing and resorbing bone substance on the outer and inner surfaces  respectively, in the direction of growth. 

 If resorption and deposition take place at the same rate, the thickness of the bone remains constant. The movement is along the depositing surface of the bone if the rate of deposition is high.

Displacement / Translation

It is the change in the spatial position of the bone. It is just a physical movement of the whole bone as a unit.

It is a translatory movement of the whole bone caused by the surrounding physical forces, and is the second characteristic mechanism of skull growth.

It can be of 2 types:

• Primary displacement

• Secondary displacement

Primary displacement:

It is the physical movement of the whole bone, as the bone grows and remodels by resorption and apposition. As a bone enlarges, it is simultaneously carried away from the other bones in direct contact with it. This creates space within which bony enlargement takes place.

Secondary displacement:

It is the movement of bone related to enlargement of other bones. It is the movement of a whole bone caused by the separate enlargement of other bones, which may be nearby or quite distant. 

Example: the growth of middle cranial fossa causes the maxilla to move forward and downward.

Drift and displacement occur together and complement each other. It is very difficult to determine the separate contributions of drift and displacement to the remodeling of bone.