Why and how to measure bone density
The bone is not a solid lump of calcium but a dynamic organ constantly repairing and re-molding itself in response to growth, stress, injury, our biochemistry and hormones. As a result, bone is strongly influenced by environmental and lifestyle factors. The cells which form bone are called osteoblasts and cells which reabsorb bone are called osteoblasts and they work together to maintain bone health. As you can see from the graph, bones continue to increase in thickness (or density) until our late twenties. Bone density then stays about the same into our mid-thirties. Our bone density then slowly decreases as we get older the amount of bone removed by osteoclasts is more than the amount made by osteoblasts.
Over time, the hard, outer-shell of bones thins and the holes in cancellous bone get larger. If this level of loss is over that expected for the age, this is called ‘bone loss’ or ‘bone thinning’ or in medical terms osteopenia (mild loss) or osteoporosis (significant loss) which is associated with an increased risk of the bones becoming more fragile and breaking (fracturing) in response to much less trauma. The adjacent picture show the difference between normal bone on the left and thinned bone on the right.
What are the risks of bone loss?
The three most common osteoporotic fractures are the wrist, hip and spine as shown in the xarys. A crush fracture of the spine can occur suddenly, be very pain full and even cause nerve damage and pain. Also the spine can crumble over time causing deformity and loss of height:
How to measure bone density?
There are no blood tests available, sometimes osteoporosis can be seen on a plain x-ray but the most reliable test is a bone density scan (Dexa). This does not require an injections, is painless and safe as the amount used is much less than a normal x-ray. You just have to lie on a bed for a few minutes and the machine provides a scale of bone loss called a T-score:
- 0 to -1 normal
- -1 to -2 osteopenia
- < -2 osteoporosis
Who should have Dexa scans?
It is advisable that all elderly people have a dexa scan as lifestyle factors and specific treatments are available to reduce the risk of progression and fracture. People with the specific risk factors listed below should be tested more frequently. To arrange a dexa scan, in the UK, this usually involves a discussion with your General Practitioner.
Which factors affect bone health?
A number of factors affect bone health such as our age, gender (women more at risk after menopause), having a family history of weak bones, but common medical conditions, medications and lifestyle factors are listed below. Whatever the contributory factors, medical and lifestyle strategies can still have a strong influence on the rate of bone loss.
Medical illnesses linked to lower bone density include:
- Any chronic illnesses that cause disability and make a person less mobile.
- Spontaneous early menopause in women
- Surgery or radiotherapy to the ovaries (usually part of a medical treatment)
- Surgery or radiotherapy to the testes (usually part of a medical treatment)
- Diabetes, Asthma and Rheumatoid arthritis
- Inflammatory bowel diseases, such as Crohn’s disease and ulcerative colitis
- Chronic liver or kidney disease
- Over active thyroid disease
- Eating disorders, such as anorexia nervosa
Drugs, taken over a long time, which increase the risk of bone loss include:
- Aromatase inhibitors for breast cancer (anastrozole, letrozole and exemestane)
- LHRH agonists such as Goserelin (Zoladex®)
- Chemotherapy which damages ovarian function (in premenopausal women)
- Some chemotherapy agents specifically damage bones (e.g. methotrexate)
- Newer Immunotherapy cancer treatments ( e.g. imatinib, nilotinib and dasatinib)
- Some anti-epileptic drugs, such as phenytoin.
- Proton pump inhibitors for indigestion (omeprazole, lanzoprazole)
Lifestyle factors with increase the risk of bone loss
- Lack of physical activity
- Low body weight (BMI<20kg/m2)
- Drinking more than 3.5 units of alcohol / day
- Lack of sun exposure
- Poor diet