There are several ways of measuring bone mineral content. The most widely used of the current bone densitometry modalities is single-photon absorptiometry (SPA), dual-photon absorptiometry (DPA), dual x-ray absorptiometry, and quantitative CT. Quantitative CT can be called the gold standard for measuring bone mineral content, but uses an expensive piece of equipment, involves high radiation doses, and is subject to measurement errors owning to varying fat content of the marrow. SPA is used to measure bone mineral content of the distal radium and os calcis. SPA is quick and inexpensive, but investigators have found that measurements made at these distal anatomic sites do not correlate well with measurements made in the spine and hip, which are the most important sites of fracture in the osteoporotic patient. Dual-photon absorptiometry measures an integral of cortical and trabecular bone in the spine or hip, but has problems with less-than-ideal precision and image spatial resolution, along with a long examination time. Dual energy x-ray absorptiometry (DEXA) correlates well with quantitative CT and imaging capabilities are much better than DPA and more reliable.
The normal mean value of vertebral BMC for men and women in the age range of 20 to 40 years is 175 mg per cc. By the age of 70 years, it is about 110 mg per cc in men and 90 mg per cc in women. It is estimated that patients are at risk for vertebral compression fractures when the BMC becomes less than 105 mg per cc. As you can see particularly in postmenopausal women with osteoporosis bone mineral content is an important factor that should be monitored. Presently, quantitative CT is currently the most widely available means for obtaining bone density information. CT has been shown to be a better predictor of vertebral compression fracture risk than any other available densitometric method. The disadvantages with using CT include a higher radiation dose than the DEXA and SPA methods, expense of the study ($200-300), and potentially limited scanner availability. The cost of a DEXA and SPA study is approximately $65, inexpensive enough that if the study was not authorized the patient might be able to pay for it anyway.
I would recommend that any patient who is osteoporotic for whatever reason should obtain a baseline BMC before beginning treatment and should also have a follow-up BMC to determine if the patient is responding. We often recommend calcium and vitamin D for our patients with osteoporosis. How often do we actually determine if our patients are responding to treatment or are actually following our advice? Bone densitometry is an inexpensive tool that we can use both for evaluating treatment and gaining patient compliance.
Deborah Pate, DC, DACBR
San Diego, California
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