What is osteoporosis? (Introduction Dr. Ashok Bhalla)

Dr Ashok Bhalla, consultant rheumatologist at the Royal National Hospital for Rheumatic Diseases provides an introduction to osteoporosis. He talks about why osteoporosis occurs; who is most at risk of developing the condition; types of osteoporotic fractures; the role of calcium and vitamin D in protecting our bones and the different treatments available to date.

What is osteoporosis?

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Osteoporosis literally means porous bone. The medical community defines osteoporosis as a skeletal disorder in which bone strength is reduced as a result of loss of bone mass and through the deterioration of the bone architecture. The consequence of these changes is an increased fracture risk.
Normal bone is made of an inner honeycomb-like  structure (trabecular bone)enclosed by a thick outer shell (cortical bone). In chocolate terms it is similar to a Crunchie bar or Malteser! Trabecular bone is comprised of thin struts of bony tissue which link with each other to form a strong mesh. In osteoporosis the trabeculae become thin and break leading to loss of connections between each of them. Changes also occur in the cortical bone which becomes thinner. The combined effect is that bones become less dense and more fragile. These weakened bones are more likely to fracture after a minor fall. Fractures that occur as a result of low-trauma are known as fragility fractures.

Osteoporosis is a common condition and becomes more common with increasing age. One in two women over the age of 50 is likely to experience an osteoporotic fracture in their lifetime. For men this is less common and it is estimated that one in five males over the age of 50will experience an osteoporotic fracture (National Society for Osteoporosis. January 2013).
What is bone made of?

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As mentioned above,bone consists of a tough outer  shell known as the cortex and an inner honeycomb structure known as trabecular bone. Bone is not a dead tissue. It is constantly undergoing change throughout life. In this process some bits of bone are removed by bone removing cells known as osteoclasts. At the same site, or at an adjacent site, new bone is formed by bone forming cells known as osteoblasts.
The process of bone removal and bone formation is known as bone turnover.   
The function of bone turnover is to remove bone that’s worn out and is no longer capable of providing structural support. This process helps to maintain the structural integrity and strength of bone.
When we are very young our bone density increases with age. This is because our bodies are forming more bone than is being removed. The greatest increases in bone density occur during the teenage years. In most people bone density continues to increase with ageuntil we are in our mid-to-late 20’s. There then follows a period of time when we are forming and removing bone at an equal rate. During this time our  bone density remains relatively stable. During our 30’s and beyond the amount of bone removed begins to exceed the amount of new bone formed, leading to a small net bone loss. This slight mismatch accounts for the normal age-related bone loss.
In women, at the time of the menopause, oestrogen levels fall. This oestrogen deficiency leads to an increase in bone turnover. In this state of increased bone turnover the amount of bone removed is greater than that which is formed. This leads to a rate of bone loss that is beyond normal age-related loss and together with a change in the composition of bone, results in reduced bone strength  and higher susceptibility to fractures.

The second reason why women are more at risk of osteoporosis relates to the concept of peak bone mass.  We have already said that we reach our highest bone density (peak bone mass) in young adulthood and that eventually our bone density will begin to decline.  It follows from this that the higher our peak bone mass in young adulthood, the longer it takes before our bones decline to the extent that we become osteoporotic. We know that young adult females reach a lower peak bone mass than young adult males.  This helps to explain why females are more at risk of becoming osteoporotic. 
Types of osteoporotic fracture

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The main clinical problem with osteoporosis is the occurrence of fractures. Low bone density by itself causes no symptoms. It can be interpreted like having high blood pressure. Hypertension or high blood pressure often causes no symptoms, but it is the outcome that we dread, such as a stroke or a heart attack.
Osteoporotic fractures occur most commonly at the wrist, the upper arm bone (known as the humerus), the hips, and the vertebral bodies that make up the spine. Fractures of the upper arm, wrist,  and hips, usually occur after a fall. Spinal fractures however, can occur spontaneously without trauma.
A hip fracture is probably the most devastating of all osteoporotic fractures. It requires hospitalisation and surgery to correct the fracture. It is associated with considerable risk of losing one’s independence and not being able to return to one’s own home environment.
Fractures of the spine can cause considerable pain or sometimes cause no pain, but alter the shape of the spine. The acute pain of a spinal fracture often settles over a two-month period. Multiple spinal fractures lead to loss of height and a forward stoop known as a kyphosis or Dowager’s hump. The altered shape of the spine may lead to a person developing chronic spinal pain.
How is osteoporosis diagnosed?

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At the present time bone density measurement is the best estimate we have of bone strength. Bone density can easily be measured at the hip and spine using a DXA machine. This is a painless procedure and takes no more than twenty minutes to perform.
However, we have learned that bone density does not fully explain an individual’s risk of sustaining a fracture. Many  other factors  are important which include life style factors, such as smoking and excess alcohol intake, taking certain medications such as corticosteroids, older age, a previous fracture, female sex, a parent with a hip fracture, inflammatory joint disease such as rheumatoid arthritis, premature menopause, low body weight, type 1 diabetes mellitus, overactive thyroid gland, liver disease, and bowel disorders that impair absorption of nutrients.
Taking these risk factors into account, with or without a bone density reading, allows us to predict an individual’s ten year probability of sustaining a fracture. Using such probabilities we can then decide on whether or not an individual needs treatment to reduce their risk of fractures in the future. This analysis is now done using a programme developed by WHO (World Health Organisation) called FRAX. This model requires an individual’sage, weight, height, and answers to 8 additional questions to allow the doctor to calculate the 10 year probability of fracture.
How is osteoporosis treated?

The pain arising from a fractured bone can be reduced by immobilising the bone in a plaster cast such as might occur after a wrist fracture. In some instances surgery may be required to align the bones in their correct position. Fractures of a major bone, such as a hip, often require surgery. Fractures of the spine, causing  back pain can sometimes be helped by surgical procedures known as vertebral kyphoplasty or balloon kyphoplasty.  These treatments will not be required for the majority of patients.
Other treatments that can help to reduce pain arising from fractures include variouspainkillers.,Non-drug based treatments, such as hydrotherapy (which is pool based exercise),can help to reduce muscle spasm and pain, as well as improve an individual’s mobility. An alternative treatment is to use TENS (Transcutaneous electrical nerve stimulation).
It is important to mobilise individuals as quickly as possible since bed rest increases bone loss and therefore increases the risk of future fractures. Apart from reducing pain there are a number of medications available to treat the condition of osteoporosis. These medications fall into two groups. The first group work to reduce bone loss and are known as antiresorptive drugs. The second group, fewer in number, help to stimulate new bone formation and are known as anabolic drugs.
The most commonly used medications are the antiresorptives. These include the bisphosphonates, Denosumab, hormone replacement therapy, and Raloxifene.  Of the antiresorptive drugs the bisphosphonates (Alendronate, Risedronate, and Ibandronate) are the drugs of choice.  They are usually taken by mouth  once a week or once a month. Some bisphosphonates can also be given through an injection into the vein every three months (Ibandronate)or once a year (Zoledronate). When taken by mouth, it is important to adhere to the instructions strictly, since these types of medications are not absorbed very well if there is food present in the stomach. Therefore most of the time it's advisable that these medications are taken on an empty stomach and that one waits for about 30 to 45 minutes before eating. This helps to aid absorption of the medication. 

Common side-effects of the oral bisphosphonates include problems with the upper digestive tract such as indigestion, heartburn, and swallowing problems.  Aching of the muscles and joints can be seen with both the oral and injectable forms of bisphosphonates.

In all patients, the need for on-going bisphosphonate therapy should be reviewed regularly.

A new type of drug in the antiresorptive class has recently being licenced and works through a different pathway from the bisphosphonates. The drug is called Denosumab and it is given as an injection under the skin every 6 months. It seems to be as effective as the bisphosphonates in reducing fractures.  
Strontiumranelate is another medication that can be taken by mouth.  This seems to beas effective as the bisphosphonates.  It is not clear exactly how it works but it may work to reduce bone loss as well as increasing bone formation.  

Hormone replacement therapy (HRT) was a popular treatment for osteoporosis in the past and has been shown to reduce the risk of hip and spinal fractures.  It is generally not used as first-line therapy now as it has been superseded by the bisphosphonates.  However, it can be considered in younger postmenopausal females (less than 60 years of age) if they have been unable to tolerate other osteoporosis treatments or have other reasons for needing to take HRT.

Raloxifene is another medication that falls in the antiresorptive class of drugs.  It works by mimicking the action of oestrogen to protect bone.  It has been shown to reduce the risk of spinal fractures in women.  It is not suitable for individuals with a history of, or at risk of, blood clots. Like HRT, it is not a first-line therapy for osteoporosis.
In order for osteoporosis medications to work effectively patients should also be given supplements of calcium and Vitamin D.
For patients who fail to respond to the bisphosphonates or anti-resorptive drugs, and by failure we mean those in whom the bone density is continuing to fall, or those who have sustained further fractures after one or two years of treatment, then one can  consider using anabolic (bone-forming) drugs.
At the present time there are only two anabolic drugs licensed. These are drugs that go under the name of parathyroid hormone (PTH). They have to be given by daily injection under the skin for a period of 24 months (see also Medication for osteoporosis). These medications are expensive and so NICE (National Institute for Health and Care Excellence) have laid out strict criteria to be fulfilled before these medications will be funded by the NHS.

Bone acts as a vast reservoir for minerals especially calcium. The body loses a certain amount of calcium in urine and faeces every day. This needs to be replaced by what we take through our diet. If the dietary intake is inadequate to replace the amount lost, then the difference is obtained from the skeleton and this leads to bone loss.
The absorption of calcium from the gut into the body is aided by Vitamin D and therefore it is important that there is enough Vitamin D in our blood to help with this process.
Calcium is obtained from dairy products. Vitamin D is synthesised in the skin from sunlight exposure. Individuals who are housebound or those who cover their skin are likely to be Vitamin D deficient. For these individuals a supplement from a tablet may be necessary to maintain their Vitamin D levels at an adequate level.
In the absence of gravity astronauts lose a lot of calcium from their bone and also a lot of bone density. Fortunately, when they come back to Earth and experience gravity this reverses. This tells us that it is important to do weight bearing exercises, as exercise helps maintain bone health.



Last reviewed July 2013
Last updated October 2013


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