Chronic Pain, Osteoporosis, and Bone Density Testing
Osteoporosis is a disease manifested by low bone density and poor quality of bone, resulting in skeletal fragility and increased risk of fracture.1 While osteoporosis is generally a silent and asymptomatic disease until a fracture occurs, pain and osteoporosis are often associated. Fractures usually cause sudden and severe pain, with non-union fractures and some vertebral fractures resulting in chronic pain. Recent evidence suggests that pressure-induced tibial pain may be an indicator of low bone density in patients without fracture.2 Some metabolic disorders that cause low bone density, such as vitamin D deficiency and osteomalacia, can cause bone and muscle pain,3 proximal muscle weakness, and postural instability4 in the absence of fracture. Chronic pain is associated with many risk factors for osteoporosis and fragility fractures. These risk factors may be categorized according to whether they are due to the underlying disease, the pain itself, or the treatment for pain (see Table 1).
Diseases associated with chronic pain and osteoporosis include prevalent vertebral fracture, rheumatoid arthritis, inflammatory bowel disease, multiple myeloma, and insulin-dependent diabetes5 with diabetic neuropathy. Regional bone loss may occur with painful disorders such as reflex sympathetic dystrophy6 (Sudeck’s atrophy, algodystrophy) or immobilization of a limb due to trauma — with or without fracture.7
Chronic pain and its associated diseases may result in poor nutrition, impaired cognition, elevated serum cortisol8 or high levels of inflammatory cytokines,9 with potential adverse effects on bone density.
Some treatments for chronic pain disorders, such as glucocorticoids10 and anticonvulsants,11 may be harmful to bone. Other medical treatments, such as narcotics and antidepressants, may impair balance and mobility, resulting in increased risk of falls and fractures.12 Hypogonadism, another risk factor for osteoporosis, has been reported in men13 and women14 treated with opioids.
The consequences of a fracture may include additional acute and chronic pain, limited ambulation, disability, loss of independence, increased risk of future fractures and death.15 Chronic pain patients at risk for osteoporosis should be considered for bone density testing so that appropriate therapeutic intervention may be started to prevent fractures and their clinical consequences.
“...dual-energy X-ray absorptiometry (DXA) of the spine and hip is the recommended method for diagnosing osteoporosis and monitoring the effects of therapy."
Bone Density Testing
Bone density testing is a non-invasive technique used to diagnose osteoporosis or low bone density, predict the risk of fracture, and monitor the effectiveness of therapy for osteoporosis. While measurement of bone density at peripheral skeletal sites with a variety of technologies is useful to increase osteoporosis awareness and predict fracture risk, dual-energy X-ray absorptiometry (DXA) of the spine and hip is the recommended method for diagnosing osteoporosis and monitoring the effects of therapy. The key to effective clinical management is the identification of high risk patients before the first fracture occurs, so that therapy can be initiated to reduce the risk of fracture.
Dual-energy X-ray Absorptiometry
DXA is used to measure bone mineral density (BMD) at the spine and proximal femur. With appropriate software, many DXA instruments can also measure BMD at the forearm and total body. DXA measures areal BMD (aBMD in g/cm2) by using ionizing radiation with photon beams of two different energy levels. DXA is the “gold-standard" method for the diagnosis of osteoporosis and monitoring the effects of therapy for the following reasons:
- biomechanical studies have shown a correlation between mechanical strength and BMD measured by DXA,16
- epidemiological studies have established a strong relationship between fracture risk and BMD measured by DXA,17
- the World Health Organization (WHO) classification of BMD for the diagnosis of osteoporosis and osteopenia is based on reference data obtained by DXA,18
- randomized clinical trials showing a benefit with pharmacologic intervention have selected subjects based on low BMD measured by DXA,19
- there is a relationship between reduction in fracture risk with pharmacologic therapy and BMD increase as measured by DXA,20
- the accuracy and precision of DXA is excellent.21
DXA is widely available in the United States, with an estimated 10,000 instruments in operation. Radiation exposure from DXA is extremely small,22 typically about the same as the normal daily level of background radiation. Conventional radiography, on the other hand, is an insensitive and subjective technique for evaluating bone density at any skeletal site, requiring 30-40% bone loss before a problem is detected. The best use of standard X-ray in the management of osteoporosis is to diagnose fractures, to monitor the healing of fractures, and to evaluate for some secondary causes of osteoporosis. If an X-ray is suggestive of low bone density, a quantitative measurement of BMD by DXA should be done.
When to Order a Bone Density Test
As with any clinical test, bone density measurement should only be done when the potential benefits outweigh the risks, and when the results are likely to play a role in making patient management decisions. The risks of bone density testing are extremely low. Pregnancy should be considered an absolute contraindication to doing any X-ray-based bone density test. Many organizations have developed guidelines to aid in the selection of those at risk for low BMD who most likely to benefit from knowledge of the results. The most comprehensive guidelines are those of the International Society for Clinical Densitometry23 upon the recommendation of an international panel of experts (see Table 2).