Ross & Wilson Anatomy and Physiology in Health and Illness (194 page)

Anterior tibialis

This originates from the upper end of the tibia, lies on the anterior surface of the leg and is inserted into the middle cuneiform bone by a long tendon. It is associated with dorsiflexion of the foot.

Soleus

This is one of the main muscles of the calf of the leg, lying immediately deep to the gastrocnemius. It originates from the heads and upper parts of the fibula and the tibia. Its tendon joins that of the gastrocnemius so that they have a common insertion into the calcaneus by the calcanean (Achilles) tendon. It causes plantarflexion at the ankle and helps to stabilise the joint when standing.

Diseases of bone

Learning outcomes

After studying this section you should be able to:

explain the pathological features of osteoporosis, Paget’s disease, rickets and osteomalacia

outline the causes and effects of osteomyelitis

describe abnormalities of bone development

explain the effects of bone tumours.

Osteoporosis

In this condition, bone density (the amount of bone tissue) is reduced because its deposition does not keep pace with resorption. Although the bone is adequately mineralised, it is fragile and microscopically abnormal, with loss of internal structure. Peak bone mass occurs around 35 years and then gradually declines in both sexes. Lowered oestrogen levels after the menopause are associated with a period of accelerated bone loss in women. Thereafter bone density in women is less than in men for any given age. A range of environmental factors and diseases are also associated with decreased bone mass and are implicated in development of osteoporosis (
Box 16.1
). Some can be influenced by changes in lifestyle. Exercise and calcium intake during childhood and adolescence are thought to be important in determining eventual bone mass of an individual, and therefore the risk of osteoporosis in later life. As bone mass decreases, susceptibility to fractures increases. Immobility causes reversible osteoporosis, the extent of which corresponds to the length and degree of immobility. For instance, during prolonged periods of unconsciousness, osteoporotic changes are uniform throughout the skeleton, but immobilisation of a particular joint following fracture leads to local osteoporotic changes in involved bones only.

Box 16.1
Causes of decreased bone mass

Common features of osteoporosis are:

•
skeletal deformity – gradual loss of height with age, caused by compression of vertebrae

•
bone pain

•
fractures – especially of the hip (neck of femur), wrist (Colles’ fracture) and vertebrae.

Paget’s disease

Paget’s disease is a disorder of bone remodelling, where the normal balance between bone building and bone breakdown becomes disorganised and both osteoblasts and osteoclasts become abnormally active. The bone deposited is soft and structurally abnormal. This predisposes to deformities and fractures, commonly of the pelvis, femur, tibia and skull. Most cases occur after 40 years of age and the incidence increases with age. The cause is unknown and it often goes undetected until complications arise. The disease increases the risk of osteoarthritis (
p. 424
) and osteosarcoma.

Rickets and osteomalacia

The underlying abnormality in osteomalacia and rickets is inadequate mineralisation of bone, usually because of vitamin D deficiency, or sometimes because of defective vitamin D metabolism. In children, whose bones are still growing, this leads to characteristic bowing and deformity of the lower limbs, and is called rickets. In the adult, who still requires vitamin D for normal turnover of bone, deficiency leads to osteomalacia, which is associated with increased risk of fracture and bone pain.

Deficiency may be caused by poor diet, or by limited exposure to sunlight (needed for normal vitamin D metabolism). Some people are genetically unable to metabolise vitamin D normally, leading to poor bone mineralisation, and occasionally malabsorption may also lead to deficiency.

Osteomyelitis

The general term for bacterial infection of bone is osteomyelitis. This may follow an open fracture or surgical procedures, which allow microbial entry through broken skin. It may also be a consequence of blood-borne infection from a focus elsewhere, such as the ear, throat or skin; this is most commonly seen in children. If promptly and adequately treated, the infection can resolve without permanent damage, but if not, it may become chronic, with sinus formation draining pus to the skin, fever and pain.