Bone function

The human body comprises 208 bones. The axial skeleton includes the skull, vertebral column and the thoracic cage, while the appendicular skeleton consists of the bones in the upper and lower limbs. Cortical bone is found in the shafts of long bones of the appendicular skeleton. Cancellous or trabecular bone is found in the marrow cavity. Seventy per cent to 80% of bone by mass is cortical or compact bone. The remaining 20%-30% is trabecular or spongy or cancellous bone. Bone marrow consists of stroma, myeloid tissue, fat cells, blood vessels, sinusoids and lymphatic tissue.

The structural unit of cortical bone is the osteon. This consists of irregular, branching and anastomosing cylinders, which are composed of a central neurovascular Haversian canal. The canal is surrounded by concentric lamellae of cell permeated layers of the bone matrix. The cylinders are between 200 and 300 mm in diameter. Six to seven concentric osteocyte rings comprise up to 20 lamellar plates.

The osteons are oriented in the long axis of the bone. They are connected to one another by perpendicularly oriented Volkmann's canals. Cortical bone is a complex structure of adjacent osteons and their interstitial and circumferential lamellae.

Bone is a dynamic tissue with the capacity for continuous remodelling. It has the following functions:

Provision of mechanical support and the structural framework for the body;

Vitamin D disorders

These may be produced by:

Alterations in availability of vitamin D;

Altered conversion of vitamin D3 to 25-hydroxyvitamin D3;

Altered conversion of 25-hydroxyvitamin D3 to 1,25-dihydroxyvitamin D3 and/or

24,25-dihydroxyvitamin D3; Altered end organ responsiveness.

n Provision of a site for muscle attachment;

d g Protection of the viscera;

i ne Contains the bone marrow and is a site for haematopoiesis; y Provides a metabolic reserve of calcium and phosphate ions, thereby contri-

io buting to mineral metabolism.

y Remodelling provides a constant and rapid source of calcium for the main tenance of homeostasis of serum levels, and allows enhancement of skeletal strength and elasticity. It is an integrated process of coupled bone resorption and bone formation that results in the maintenance of net skeletal mass with the renewal of mineralised matrix. This allows regeneration in response to injury, particularly micro-damage from exposure to physiological loads. The remodelling sequence can be considered to be one of activation, resorption and formation.

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