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Recently, dual energy x ray absorptiometry (DXA) was presented by Murphy et al as a new method for assessing periarticular bone mineral density (BMD) at the knee joint.1 Precision errors for BMD measured at the patella, femur, and tibia were reported for 14 subjects. The paper highlights the emerging importance of measurement of radiological data and attention to regional density characteristics in bone and joint diseases. Subchondral bone mineral density of the proximal tibia has previously been assessed by dual photon absorptiometry (DPA)—the precursor of DXA. Relations between subchondral BMD of the proximal tibia and age, height, weight, and BMD of the lumbar spine and femoral neck were examined by Bohr and Schaadt.2 Petersen et alpresented subchondral BMD values for a large number of healthy subjects and for patients with various orthopaedic conditions.3Bohr and Lund and Petersen et al examined changes in BMD at the proximal tibia after knee arthroplasty.4 5 Petersen et al examined subchondral BMD after meniscectomy,6and Madsen et al reported data for subchondral BMD measured in several subregions of the proximal tibia in healthy subjects and in subjects with osteoarthritis of the knee.7 Moreover, Petersen et alstudied relations between bone strength and BMD assessed by DPA and DXA in the proximal tibia.8 Other related studies could be mentioned. Unfortunately, none of these studies was referred to by Murphy et al.1
We appreciate the interest shown in our work1-1 by Dr Marsden. We acknowledge that subchondral bone mineral density has previously been assessed by dual photon absorptiometry. However, we do not feel that this is particularly relevant to our paper. The purpose of our study was to develop and validate a method for measurement of periarticular bone mineral density at the knee joint using the technique now recognised as the gold standard for the assessment of bone mineral density—that is, dual energy x ray absorptiometry (DXA). With the exception of one study,1-2 the studies referred to by Dr Marsden use only dual photon absorptiometry.
As mentioned by Dr Marsden, Petersen et almeasured bone mineral density of small regions of interest within the proximal tibia by DXA and used these measurements to investigate the relation between trabecular bone strength and bone mineral density in the proximal tibia.1-2 However, it is not clear from this paper how many measurements were taken for calculation of precision values as the paper concentrates on the use of, rather than the validation of, this technique. Furthermore, the DXA measurements were performed only on postmortem sections of tibial bone obtained at necropsy. Finally, unlike our study, which showed how to measure bone density of periarticular bone, the regions of interest selected by Marsden et al did not include the periarticular surface of tibia, but rather were confined to small areas within the subchondral bone. Thus the areas measured consisted primarily of trabecular bone.
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