Diagnostic Tools for Osteoporosis in Older Adults
Low bone density is major risk for osteoporotic fracture. In older adults special precautions apply in interpreting bone mineral density measurements (either by central dual energy X-ray absorptiometry [DXA] or peripherally with calcaneal ultrasonography). Clinical assessment for vertebral fractures is an important part of the management. Therapeutic regimes for osteoporosis treatment are complicated and require repeated reinforcement to ensure long term compliance. Adequate compliance (80%) is required for optimal therapeutic benefit.
Key words: calcaneal ultrasonography, central dual energy x-ray absorptiometry (DXA), bone mineral density (BMD), older adult, special precautions.
Introduction
Osteoporosis is a disorder of bone microarchitecture resulting in an increased susceptibility to fracture. Bone strength is dependent on both bone mineral density (BMD) and bone quality. At present, we only have tools to measure BMD, and we use this information as a major factor to help us define a patient’s fracture risk. The only clinically available indicator of bone quality is the presence of a prior “fragility fracture” in adulthood (usually after age 40). A fragility fracture is one occurring with low trauma, such as might occur after a fall from a standing height or less.
Measuring Bone Mineral Density
Bone mineral density can be measured both peripherally (appendicular skeleton) and centrally (axial skeleton). Central measurement of spine and hip BMD, with dual energy x-ray absorptiometry (DXA) is the most widely used technology and has the most data validating it as a predictor of fracture risk. Bone mineral density as measured by DXA is actually an estimate of density extrapolated from a two dimensional x-ray projection and expressed as grams of calcium per square centimetre. An individual’s result is then expressed as the number of standard deviations from the mean BMD of a healthy young-adult reference population (known as the T-score). Based on this measurement patients are categorized by the World Health Organization (WHO)1 into normal, low bone mass (formerly “osteopenia”), or osteoporosis categories (Table 1); this information is used to assess relative fracture risk.
A variety of methods are available to measure bone in the appendicular skeleton, the most popular being ultrasound. These methods are often used for screening programs in community settings, such as drugstores. However, in some communities where DXA is not readily available, these tools may also be used for diagnosis. Diagnostic cut-offs are not comparable to the WHO diagnostic criteria as the latter are based on central DXA measurement. Diagnostic cut-offs are usually defined per individual instrument. A recent position statement was published by the International Society for Clinical Densitometry (ISCD)2 on the use of quantitative ultrasonography (QUS) tools in the management of osteoporosis. The ISCD statement concluded that the heel is the best site for fracture risk assessment, and that validated QUS devices predict fragility fracture in postmenopausal women and men over the age of 65, independently of central DXA BMD. They also stated that osteoporosis cannot be diagnosed by QUS according to the WHO classification for DXA, but that each instrument can identify specific thresholds. Quantitative ultrasonography cannot be used to monitor the skeletal effects of osteoporosis treatment. There are also many technical challenges with QUS that can lead to errors, including positioning and the coupling medium used. The authors concluded that QUS use is only justified in situations where central DXA is unavailable.