Classification of Human Bones

Bones can be classified by many methods. Here we discuss them one by one

According to Shape

1. Long bones

Each long bone has an elongated shaft or diaphysis and two expanded ends (epiphyses) which are smooth and articular. The shaft typically has 3 surfaces separated by 3 borders, a central medullary cavity, and a nutrient foramen directed away from the growing end. Examples of typical long bones are humerus, radius, ulna, femur, tibia and fibula, metacarpals, metatarsals and phalanges

Short bones

Their shape is usually cuboid, cuneiform, trapezoid, or scaphoid. Examples: carpal and tarsal bones.

Flat Bones resembles shallow plates and form boundaries of certain body cavities. Examples: bone in the vault of the skull, ribs, sternum and scapula.

Irregular Bones vertebra, hip bone, and bones in the base of the skull.

Pneumatic Bones Certain irregular bones contain large air spaces lined by epithelium. Examples: maxilla, sphenoid, ethmoid, etc. they make the skull light in weight, help in resonance of voice, and act as air conditioning chambers for the inspired air.

Sesamoid Bones.

These are bony nodules found embedded in the tendons or joint capsules. They have no periosteum and ossify after birth. They are related to an articular or nonarticular bony surface, and the surfaces of contact are covered with hyaline cartilage and lubricated by a bursa or synovial membrane.

Examples: patella, pisiform, fabella, etc.

Function of the sesamoid bones are not definitely known. Their possible function are:
* To resist pressure;
* To minimize friction;
* To alter the direction of pull of the muscle; and
* To maintain the local circulation.

Accessory (Supernumerary) Bones

These are not always present. They may occur as ununited epiphyses developed from extra centres of ossification. Examples: sutural bones, os trigonum, os vesalianum, etc. In medicolegal practice, accessory bones may be mistaken for fractures. However, these are often bilateral, and have smooth surfaces without any callus.

Developmental Classification

1 Membrane (Dermal) Bones ossify in membrane (intramembranous or mesenchymal ossification), and are thus derived from mesenchymal condensations. Examples: bones of the vault of skull and facial bones.

A defect in membranous ossification causes a rare syndrome called cleidocranial dysostosis. It is characterized by three cardinal features:

* Varying degrees of aplasia of the clavicles;
* Increase in the transverse diameter of cranium, and
* Retardation in fontanelle ossification. It may be hereditary or environmental in origin.

Cartilaginous Bones ossify in cartilage (intracartilaginous or endochondral ossification), and are thus derived from performed cartilaginous models. Examples: bone of limbs, vertebral column and thoracic cage.

A defect in endochondral ossification causes a common type of dwarfism called achondroplasia, in which the limbs are short, but the trunk is normal. It is transmitted as a Mendelian dominant character.

Membrano-Cartilaginous Bones ossify partly in membrane and partly in cartilage. Examples: clavicle, mandible, occipital, temporal, sphenoid.

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