A typical long bone ossifies in three parts, the two end from secondary centres, and the intervening shaft from a primary center. Before ossification is complete the following parts of the bone can be defined.

Epiphysis

The end and tips of a bone which ossify from secondary centres are called epiphyses. These are of the following types:

• Pressure epiphyses is articular and takes part in transmission of the weight. Examples: head of femur; lower end of radius, etc.
• Traction epiphysis is nonarticular and does not take part in the transmission of the weight. It always provides attachments to one or more tendons which exert a traction on the epiphysis. The traction epiphyses ossify later than the pressure epiphyses. Examples: trochanters of femur and tubercles of humerus.
• Atavistic epiphysis is phylogenetically an independent bone which in man becomes fused to another bone. Examples: coracoid process of scapula and os trigonum.
• Aberrant epiphysis is not always present. Examples: epiphysis at the head of the first metacarpal and at the base of other metacarpal bones.

Diaphysis

It is the elongated shaft of a long bone which ossifies from a primary centre.

Metaphysis

The epiphysial ends of a diaphysis are called metaphyses. Each metaphysis is the zone of active growth. Before epiphysial fusion, the metaphysis is richly supplied with blood through end arteries forming ‘hair-pin’ bends.

This is the common site of osteomyelitis in children because, the bacteria or emboli are easily trapped in the hair-pin bends, causing infraction. After the epiphysial fusion, vascular communications are established between the metaphysial and epiphysial arteries.

Now the metaphysis contains no more end-arteries, and is no longer subject to osteomyelitis.

Epiphysial Plate of Cartilage

It separates epiphysis from metaphysic. Proliferation of cells in this cartilaginous plate is responsible for lengthwise growth of a long bone. After the epiphysial fusion, the bone can no longer grow in length.

The growth cartilage is nourished by both the epiphysial and metaphysial arteries.

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