Osteoporosis causes bones to become weak and brittle — so brittle that a fall or even mild stresses like bending over or coughing can cause a fracture. Osteoporosis-related fractures most commonly occur in the hip, wrist or spine.
Bone is living tissue that is constantly being broken down and replaced. Osteoporosis occurs when the creation of new bone doesn’t keep up with the removal of old bone.
Osteoporosis affects men and women of all races. But white and Asian women — especially older women who are past menopause — are at highest risk. Medications, healthy diet and weight-bearing exercise can help prevent bone loss or strengthen already weak bones.
Your bones are alive and constantly growing — not static, like you see them drawn in books. Bones continually change throughout your life, with some bone cells dissolving and new bone cells growing back in a process called remodeling. With this lifelong turnover of bone cells, you replace most of your skeleton every 10 years.
But for people with osteoporosis — a thinning of the bones — bone loss outpaces the growth of new bone. Bones become porous, brittle, and prone to fracture. Look at an X-ray of a hip with normal bone density, and you see a dense matrix of bone cells. But look at a hip withosteoporosis, and you see mostly air. The bony matrix has all but dissolved, with only a few thin strands left.
It is increasingly being recognized that multiple pathogenetic mechanisms interact in the development of the osteoporotic state. Understanding the pathogenesis of osteoporosis starts with knowing how bone formation and remodeling occur.
Normal bone formation and remodeling
Bone is continually remodeled throughout our lives in response to microtrauma. Bone remodeling occurs at discrete sites within the skeleton and proceeds in an orderly fashion, and bone resorption is always followed by bone formation, a phenomenon referred to as coupling.
Dense cortical bone and spongy trabecular or cancellous bone differ in their architecture but are similar in molecular composition. Both types of bone have an extracellular matrix with mineralized and nonmineralized components. The composition and architecture of the extracellular matrix is what imparts mechanical properties to bone. Bone strength is determined by collagenous proteins (tensile strength) and mineralized osteoid (compressive strength). The greater the concentration of calcium, the greater the compressive strength. In adults, approximately 25% of trabecular bone is resorbed and replaced each year, compared with only 3% of cortical bone.
Osteoclasts, derived from mesenchymal cells, are responsible for bone resorption, whereas osteoblasts, from hematopoietic precursors, are responsible for bone formation (see the images below). The 2 types of cells are dependent on each other for production and linked in the process of bone remodeling. Osteoblasts not only secrete and mineralize osteoid but also appear to control the bone resorption carried out by osteoclasts. Osteocytes, which are terminally differentiated osteoblasts embedded in mineralized bone, direct the timing and location of bone remodeling. In osteoporosis, the coupling mechanism between osteoclasts and osteoblasts is thought to be unable to keep up with the constant microtrauma to trabecular bone. Osteoclasts require weeks to resorb bone, whereas osteoblasts need months to produce new bone. Therefore, any process that increases the rate of bone remodeling results in net bone loss over time.
There typically are no symptoms in the early stages of bone loss. But once bones have been weakened by osteoporosis, you may have signs and symptoms that include:
- Back pain, caused by a fractured or collapsed vertebra
- Loss of height over time
- A stooped posture
- A bone fracture that occurs much more easily than expected
DEXA Scan (Dual X-ray Absorptiometry)
The most common osteoporosis test is dual X-ray absorptiometry — also called DXA or DEXA. It measures people’s spine, hip, or total body bone density to help gauge fracture risk. Read more.
Beyond DEXA: Other Bone Mineral Density Tests
Various methods can check bone density, including ultrasound and quantitative computed tomography (QCT). Bone density scores and cost may vary by testing method. Learn about these tests.
Treatment recommendations are based on an estimate of your risk of breaking a bone in the next 10 years using information such as the bone density test. If the risk is not high, treatment might not include medication and might focus instead on lifestyle, safety and modifying risk factors for bone loss.
For both men and women at increased risk of fracture, the most widely prescribed osteoporosis medications are bisphosphonates. Examples include:
- Alendronate (Fosamax)
- Risedronate (Actonel, Atelvia)
- Ibandronate (Boniva)
- Zoledronic acid (Reclast)
Side effects include nausea, abdominal pain, difficulty swallowing, and the risk of an inflamed esophagus or esophageal ulcers. These are less likely to occur if the medicine is taken properly. Intravenous forms of bisphosphonates don’t cause stomach upset. And it may be easier to schedule a quarterly or yearly injection than to remember to take a weekly or monthly pill, but it can be more costly to do so.
Using bisphosphonate therapy for more than five years has been linked to a rare problem in which the middle of the thighbone cracks and might even break completely. Bisphosphonates also have the potential to affect the jawbone. Osteonecrosis of the jaw is a rare condition that can occur after a tooth extraction in which a section of jawbone dies and deteriorates. You should have a recent dental examination before starting bisphosphonates.
Estrogen, especially when started soon after menopause, can help maintain bone density. However, estrogen therapy can increase a woman’s risk of blood clots, endometrial cancer, breast cancer and possibly heart disease. Therefore, estrogen is typically used for bone health only if menopausal symptoms also require treatment.
Raloxifene (Evista) mimics estrogen’s beneficial effects on bone density in postmenopausal women, without some of the risks associated with estrogen. Taking this drug may also reduce the risk of some types of breast cancer. Hot flashes are a common side effect. Raloxifene also may increase your risk of blood clots.
In men, osteoporosis may be linked with a gradual age-related decline in testosterone levels. Testosterone replacement therapy can help increase bone density, but osteoporosis medications have been better studied in men with osteoporosis and are recommended alone or in addition to testosterone.
Other osteoporosis medications
If you can’t tolerate the more common treatments for osteoporosis — or if they don’t work well enough — your doctor might suggest trying:
- Denosumab (Prolia). Compared with bisphosphonates, denosumab produces similar or better bone density results and reduces the chance of all types of fractures. Denosumab is delivered via a shot under the skin every six months. The most common side effects are back and muscle pain.
- Teriparatide (Forteo). This powerful drug is similar to parathyroid hormone and stimulates new bone growth. It’s given by injection under the skin. After two years of treatment with teriparatide, another osteoporosis drug is taken to maintain the new bone growth. This drug is reserved for patients with severe osteoporosis.