Osteoporosis - Metabolic bone disorders

Metabolic bone disorders

Osteoporosis

Definition

A disease characterised by low bone mass and microarchitectural disruption. It is diagnosed if the race/sex matched bone density falls below 2.5 standard deviations from the average young bone density (WHO).

Incidence

Overall 30% of individuals will have a pathological fracture due to osteoporosis.

Age

Females over 50 and males over 55 years.

Sex

2F : 1M

Aetiology

Bone is continually being remodelled involving reabsorption and synthesis. It is thought that osteoporosis results from a long-term slight imbalance between the two processes. The risk of fractures increases with bone loss and hence with age. Factors that can affect the remodelling balance are as follows:

Sex: Females have a lower bone mass and a high rate of bone loss in the decade following the menopause. This is largely oestrogen-dependent, early menopause and ovariectomy without hormone replacement therapy predisposes. Gonadal failure and androgen insensitivity are risk factors for osteoporosis in men.

Age: Agerelated bone loss is seen in both sexes; although worse in females, this may be due to decreased calcium absorption.

Genetic factors implicated include the vitamin D receptors and collagen genes.

Diseases such as Cushing’s syndrome, type I diabetes mellitus, thyrotoxicosis, acromegaly, hyperparathyroidism, rheumatoid arthritis and chronic renal failure.

Drugs can worsen/cause osteoporosis including systemic corticosteroids, ciclosporin and cytotoxic drugs.

Smoking increases the risk of osteoporosis.

Pathophysiology

Although there is low bone mass it is normally mineralised. There is disruption of the normal architecture, with fewer and thinner bony spicules and non-supporting horizontal ‘struts’ that do not join up to any other structure. The structural integrity of the bone is reduced, causing skeletal fragility.

Clinical features

Osteoporosis is not itself painful; however, the fractures that result are. Typical sites include the vertebrae, distal radius (Colles’ fracture) and the neck of the femur. Other symptoms of vertebral involvement are loss of height and increasing kyphosis.

Investigations

Bone density scanning (dual X-ray absorptiometry, i.e. DEXA scan) is the gold standard for diagnosis.

X-ray investigation shows fractures, a bone scan can be used to demonstrate recent fractures. The generalised bone density is difficult to assess as the appearance is dependent on the X-ray penetration.

Serum calcium, phosphate and alkaline phosphatase are normal.

Bone biopsy can be performed to confirm the diagnosis.

Management

Non-pharmacological interventions include adequate nutrition (calories, calcium and vitamin D), exercise and quitting smoking.

Bisphosphonates inhibit bone reabsorption and are increasingly being used for the prevention and treatment of osteoporosis.

Oestrogen therapy in postmenopausal women is protective; however, because of the increased risk of breast cancer and thromboembolic disease it is not recommended for the prevention of osteoporosis. Males with gonadal failure benefit from androgens.