Roger Smith PhD FRCP, Nuffield Orthopaedic Centre, Oxford
Osteoporosis is the most common metabolic bone disease which particularly affects the spine. This is because the trabecular bone of the vertebral bodies is metabolically very active. An imbalance between bone formation and breakdown in favour of breakdown rapidly causes vertebral bone loss. This contributes to vertebral fracture.
Osteoporosis can be defined in two ways: qualitatively as a disorder in which there is a loss of bone, without a change in its composition, with micro-architectural failure leading to fragility; and quantitatively as a reduction in the amount of bone (expressed as bone mineral content or density) by more than 2.5 standard deviations below peak bone mass. The amount of bone is measured by dual x-ray absorbitometry (DXA) usually a the spine and hip.
To understand osteoporosis it is important to realise that bone is a living tissue whose amount, shape and strength depend on the activity of specialised bone cells, osteoblasts (which form bone), osteoclasts (which resorb bone) and osteocytes (which probably detect mechanical signals within bone).
The most frequent cause of osteoporosis is lack of oestrogen in post menopausal women. This leads particularly to fractures of the spine, hip and wrist. Fractures related to osteoporosis cause considerable disability and shorten life.
The treatment of osteoporotic related fractures cost millions of pounds annually in the UK. Hip fracture is the most costly and disabling result of osteoporosis and tends to occur in older women than does spinal fracture.
Osteoporotic spinal fractures can occur soon after menopause. They may follow injury to the spine after lifting heavy loads, or apparently spontaneously. They are usually painful and cause a loss of height and deformity with kyphosis. The loss of height affects the upper segment (crown-pubis) and alters the normally equal relationship between height and span.
Radiographs show variable asymmetric vertebral collapse. Significant loss of height in a post-menopausal woman is most commonly due to osteoporotic collapse but can also occur from degenerative changes in the spine and intervertebral discs.
Back pain and loss of height should not be assumed to be due to osteoporosis, especially where it occurs inappropriately, for instance in young men or young persons. It is therefore important to consider other causes; for instance osteoporosis with spinal disease can rarely occur in children (idiopathic juvenile osteoporosis : leukaemia should be excluded), in pregnancy, in osteogenesis imperfecta and in Cushings syndrome.
Sex hormone deficiency in young women with anorexia nervosa or young men with hypogonadism both cause bone loss. Multiple myeloma imitates osteoporosis and must be excluded, as must secondary neoplastic deposits.
Since some back pain and loss of height are nearly universal with advancing years it is difficult to know how these should be investigated. A good history (including previous illnesses, operations such as gastrectomy and hysterectomy, family history and present health) and physical examination (to confirm height loss, deformity and to exclude serious disease such as breast cancer) are essential.
The clinician must then decide whether to investigate further. In hospital practice blood tests for haematology and biochemistry would be usual, and also radiographs of the spine (the lateral film is more informative than the PA to show vertebral collapse). A preferred alternative which gives more information is the MRI scan.
Provided that serious conditions are excluded, the most frequent problem is the prevention and treatment of post-menopausal osteoporosis where fractures has already occurred. Hormone replacement therapy (HRT) is the first choice and bisphosphonates (such as Etidronate and Alendronate) the second. General measures such as exercise, avoidance of smoking, adequate vitamin D and calcium are important.
This is a complex and sometimes controversial subject, and larger works should be consulted (see references). Useful information is available from the National Osteoporotic Society.
References
Marcus R, Feldman D & Kelsey J: Osteoporosis 199 Academic Press New York
Smith R, Harrison J & Cooper C: Shared care for osteoporosis 1998 Isis Medical Media Oxford
National Osteoporotic Society PO Box 10, Radstock, Bath BA3 3YB, UK