BONE HEALTH
Osteoporosis is defined by the National Osteoporosis Foundation (NOF) as a bone mineral density (BMD) by DEXA T-score of less than -2.5. A T-score compares the density of your bones against the average healthy 30 year old person. An osteopenic patient with a major osteoporotic fracture at the distal radius, hip, or spine is also considered osteoporotic. Both of these categories of osteoporotic patients warrant pharmacologic therapy as the benefit of treatment far outweighs the overall risk. Over recent years, the idea that BMD is the best and only reliable predictor of future fracture risk has been challenged by many. Much of this criticism stems from the observed incidence of low-energy fractures among patients who are osteopenic (t-score -1.0 to -2.4) but not frankly osteoporotic. In fact, a majority of fractures deemed to be considered the result of low bone mineral density occur in osteopenic patients, not osteoporotic patients. This is in part due to the vastly greater prevalence of osteopenia. This places a challenge on care providers attempting to prevent a fracture through risk stratification using BMD alone. In 2008 a tool was launched by the World Health Organization Center for Metabolic Bone Diseases called FRAX. The FRAX algorithm is based on 10 years of meta-analysis data that quantifies the added risk of fracture in an osteopenic patient with other specific co-morbid conditions, medication history, and family history. These include previous use of glucocorticoids, rheumatoid arthritis, secondary osteoporosis, parental history of hip fracture, and current alcohol/smoking habits. FRAX gives clinicians the ability to assess the risk of an osteopenic patient of having a future fracture. If the patient meets a certain threshold, pharmacologic treatment is warranted. Most recently, the focus of metabolic bone disease researchers has shifted toward identification of poor bone quality over low bone mineral density. This is especially important in patients with medical conditions such as type II diabetes, kidney disease, and chronic conditions that require the use of glucocorticoids as these have been shown to have a more significant effect on bone quality and not as significant an effect on bone mineral density. In one study of patients with chronic kidney disease, BMD variations among patients who fractured vs. patients who did not fracture were not significant. This reinforces the idea that BMD measurements are not able to measure cancellous bone microarchitecture (bone quality) which plays a large role in overall bone strength and subsequent fracture risk. One way to measure the bone's microarchitecture is through an iliac crest bone biopsy and histology analysis, but this is not a pleasant or feasible task in an outpatient clinic setting. The newest and most promising technology in assessment of bone quality uses 2D DEXA images of the lumbar spine and runs the images through software that quantifies grey-level variations to produce a trabecular bone score (TBS) that is an estimate if 3D bone architecture. This measurement is an indirect means of quantifying bone quality that can be easily computed in seconds from a DEXA image. Early studies have found that TBS can predict fracture risk independent of BMD. TBS may give the clinician another tool is assessing overall fracture risk in patients with osteopenic or normal BMD scores with the goal of reducing fractures that occur due to poor bone quality. For care and treatment of patients with certain conditions or patients that take medications known to increase the risk of low-energy fractures, assessment of bone quality over bone density may become the new standard.
Reilly Loomis, PA-C
