Due to differences in bone composition and turnover rates in diverse skeletal locations, bone mineral density (BMD) in the same individual will vary depending on the site(s) of measurement. It is difficult to predict, based on dual-energy X-ray absorptiometry (DXA) or other methods, the BMD at one site based on the measurement at another.1 In this small cross-sectional study by Wilson et al low-energy proximal humeral fractures were more strongly associated with low BMD at the distal radius than with decreased BMD at the hip, lumbar spine, or a combination of the latter two. Site-specific BMD measurement at the location of a possible fracture is most predictive of such fracture compared with measurements at other skeletal sites.2 However it is not practical to measure all sites of possible fracture, but risk of upper extremity fracture, as this paper illustrates, may not be adequately captured by central (axial) DXA alone.
In a comprehensive study by Clowes, Eastell and Peel3 using age-adjusted standardised odds ratios, DXA at the lumbar spine was found to be the best predictor of proximal humeral fractures and not DXA of the forearm. Possible explanations include differences in subjects compared with the current investigation and the use of different devices for lumbar spine and forearm measurements in the Clowes et al study. These somewhat confounding results emphasise the inherent difficulty in using surrogate sites. Nevertheless, there are several key implications to be derived from the conclusions drawn by Wilson and co-workers, particularly related to fracture prediction and the distal radius as a measurement site for BMD.
In the upper extremity it is likely that, as is the case for hip and spine, prediction of future fracture is best performed by BMD measurement at or near the site of interest. However, BMD measurements, when obtained at skeletal locations with similar trabecular or cortical composition to the site of interest, do not better predict the risk of fracture.3 In contrast, skeletal locations with a greater proportion of trabecular bone (such as the distal radius) do have a stronger association with all fractures compared with measurements at predominately cortical sites.3
In the current study, consideration of BMD measurements at the hip, spine, and distal radius identified only 66% of study subjects as having osteoporosis by WHO criteria. This finding illustrates the limitations of DXA as a modality that can evaluate all aspects of bone strength and quality. Bone microarchitecture, a major contributor to bone strength, is not reflected by quantification of BMD. Bone densitometry has high specificity and low sensitivity, but in general the lower the DXA BMD the greater the likelihood of fracture.4,5 Currently, the International Society for Clinical Densitometry advocates for forearm BMD measurement only when the hip and/or spine cannot be measured or interpreted, in hyperparathyroidism, or in very obese patients that are over the weight limit for the DXA table.6 However, BMD of the distal radius tends to be less than that at the spine or hip, and is particularly responsive to age-related and metabolic changes;7 therefore the inclusion of its measurement with that at other sites would increase diagnostic value for osteoporosis as well as predictive value for estimating risk of fracture.
1. Gramp S, Genant HK, Mathur A, et al. Comparisons of noninvasive bone mineral measurements in assessing age related loss, fracture discrimination, and diagnostic classification. J Bone Miner Res 1997;12:697-711.
2. Marshall, D, Johnell, O, Wedel, H. Meta-analysis of how well measures of bone mineral density predict occurrence of osteoporotic fractures. BMJ 1996;312:1254-9.
3. Clowes JA, Eastell R, Peel NF. The discriminative ability of peripheral and axial bone measurements to identify proximal femoral, vertebral, distal forearm and proximal humeral fractures: a case control study. Osteoporos Int 2005;16:1794-1802.
4. Blake GM, Fogelman I. The role of DXA bone density scans in the diagnosis and treatment of osteoporosis. Postgrad Med J 2007;83:509-17.
5. Compston J. Guidelines for the management of osteoporosis: the present and the future. Osteoporos Int 2005;16:1173-6.
6. Lewiecki EM, Gordon CM, Baim S, et al. Special report on the 2007 adult and pediatric Position Development Conferences of the International Society for Clinical Densitometry. Osteoporos Int 2008;19:1369–78.
7. Arlot ME, Sornay-Rendu E, Garnero P, Vey-Marty B, Delmas PD. Apparent pre-and postmenopausal bone loss evaluated by DXA at different skeletal sites in women: the OFLEY cohort. J Bone Miner Res 1997;2:683-90.
Pignolo RJ, MD, PhD, Director
Ralston-Penn Clinic for Osteoporosis & Related Bone Disorders, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania, USA