The epiphysis is barely ossified at birth, and the diaphysis barely more so. The junction between these two portions is the growth zone, a few millimeters thick and more or less circular in shape.

This zone has two sides, one towards the epiphysis, the other towards the diaphysis, and it is this latter side that produces bone.

The epiphyseal cartilage at this level undergoes a gradual transformation: cells grow, then form columns. They are then invaded by capillaries, calcified and ossified.

This area is relatively fragile, and many childhood traumas manifest themselves as “epiphyseal detachment”, but as this area is extremely active, repair is also very rapid.

Bone thus grows in length at the junction of these two zones (one at each end), but it also grows in thickness through a different mechanism of peripheral apposition and internal resorption, a mechanism that allows a certain spontaneous correction of deformities after fractures, for example.

Growth does not take place identically at each extremity or on each bone. Growth is most active near the knee and far from the elbow. The lower end of the femur accounts for 70% of bone length, while the upper end of the humerus accounts for 80%.

The ossification defect in Ollier enchondromatosis lies somewhere in the epiphyseal plate, in the zone where cartilage cells differentiate to calcify and then ossify. Damage to the elbow (a region of moderately active growth) is likely to be less severe than damage to the upper end of the humerus or the lower end of the femur.

The three diagrams of a growing femur show this unequal activity of the two ends, which can be experimentally demonstrated in animals by fixing two markers in the bone; these do not move apart during growth, but the quantity of bone produced is not the same at each end.