Additional considerations in site selection for diagnosis

As noted in Chapter 2, the PA lumbar spine is commonly affected by dystrophic calcification in older adults, increasing the measured BMD and T-score. In particular, after the age of 60, the percentage of women having osteophytes in the lumbar spine is estimated at 61% (32). Calcification of the aorta, facet sclerosis, and intervertebral disk calcification can all have a significant effect on bone density measured at the PA lumbar spine (33-38). As a consequence, for diagnosis, the PA lumbar spine is less useful in individuals age 60 and over. This age cutoff is somewhat arbitrary but generally appro priate. In individuals under age 60, the PA lumbar spine is an entirely appropriate skeletal site for the diagnosis of osteoporosis with bone densitometry.

In the proximal femur, the total hip, femoral neck, and trochanter are all useful sites. The proximal femur is less affected by dystrophic changes than the PA spine, but it is not entirely free of such affects. Although the hip joint itself is not measured in a DXA proximal femur study, severe osteoarthritis of the hip joint can affect the bone density of the proximal femur (39,40). Therefore, if severe unilateral osteoarthritis is known or suspected, the proximal femur on the unaffected side should be measured. If this is not possible, the trochanteric region becomes the preferable region of interest in the proximal femur as the effects of osteoarthritis of the hip joint are most notable in the femoral neck and Ward's area, both of which are included in the total hip region of interest.

If either hip has been fractured or undergone surgical instrumentation, it is not suitable for diagnostic assessment by densitometry. An additional consideration is the presence of scoliosis. In a study from Hans et al. (41), femoral neck bone density averaged 4.2% lower on the side of the spine convexity in a small study of 15 women with scoliosis. Consequently, the measurement of the proximal femur on the side of the convexity in scoliotic patients may provide the "worst case" bone density between the two femurs.

New developments in densitometry applications have made possible the rapid study of both proximal femurs. Although this may be desirable in specific circumstances, the question has arisen as to whether study of both proximal femurs should always be done when a proximal femur bone density measurement is performed to diagnose osteoporosis. Bone density in the various regions of the right and left femurs in individuals has consistently been found to be highly correlated when measured by DXA (42-45). Similarly, the average absolute differences in bone density in g/cm2 between the right and left proximal femoral regions have been small and generally not statistically significantly different. Based on comparisons of both femurs in 198 women with an average age of 32.6 years, Bonnick et al. (42) found that the average difference was 0.7%, 0.2%, and 1.9% for the femoral neck, Ward's area, and trochanter, respectively. Only the difference observed at the trochanter was statistically significant, but all the differences were well within the precision error for the region of interest. Rao et al. (43) evaluated 131 women with an average age of 61 years. No significant difference was found between the femoral neck BMD in the right and left femur in this study as well, with a mean difference of about 1%. The differences in BMD between the right and left femur at Ward's area and the trochanter were statistically significant but small, averaging 2 to 2.5%. In a very large study of 2372 women with an average age of 56.6 years, Petley et al. (44) did find a statistically significant difference between the right and left proximal femurs in the femoral neck, but again, the difference was quite small and not clinically relevant. The mean BMD of the right femoral neck in this study was 0.840 g/cm2, whereas the mean BMD in the left femoral neck was 0.837 g/cm2. The mean difference was 0.003 g/cm2. The maximum difference observed in this study was 0.249 g/cm2. Based on these findings, all of these authors concluded that there was little justification for the routine measurement of both proximal femurs in clinical practice. In a study of 61 women from Mazess et al. (45), however, it was noted that 32% of the women had differences between the right and left femoral necks of 0.5 SD and that 5% of the women had differences that exceeded 1 SD. In those 5% of women, a difference of 1 SD, or an entire T-score unit, could clearly affect not only the assignment of diagnostic category but treatment decisions as well. Nevertheless, it is difficult to justify routine measurement of both femurs in all women on this basis. It is possible that a difference of even 0.5 SD could alter the assignment of diagnostic category when the WHO Criteria are employed. If such an alteration will reasonably affect decisions to intervene, then the measurement of both femurs may be justified.

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Arthritis Relief Now

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