Bone Mineral Apparent Density

BMAD means "Bone Mineral Apparent Density" which is important when measuring bone density in children or in patients with short stature. Another term for this concept is "volumetric bone density".

The reference range for this calculation is by Bachrach. The explanation for the method is given below.

The DEXA technique analyzes the attenuation of xrays as they pass through an area of the body. The method cannot detect the depth of the bone which is being measured, and thus is actually an "areal" density in g/cm2 rather than a "volumetric" or Archemdean density in g/cm3. As bones grow, the volume increases at a faster rate than the area, so the areal bone density will increase even if the volumetric density remains stable.

Figure showing principles of BMAD

The graph is from our study of young primate vertebra, in which volumetric density did not show much change with age, although the areal bone density suggested an increase with age.

Vertebrae are not simple cubes, but complex shapes. In our study, we measured both the vertebral dimensions and the volume of the entire vertebra, and found that the volume was predicted quite well by simply multiplying the height x width x depth of the vertebral body. However, when measuring bone with DEXA, the depth is not known. The depth of a single vertebra can be estimated as the square root of the area, which is the basis for calculating volumetric BMD. The formula is:

BMC/area^1.5       (where area^1.5 is the area times the square root of the area).

In our studies we calculate the volumetric BMD for each lumbar vertebra and take the average. At Stanford, they took the results for 3 vertebrae (L2-4) as a block, and called this the BMAD. The BMAD is therefore lower than the volumetric BMD by a factor of 1.73 (the square root of 3). The average BMAD value at age 16 in a non-Black female is 0.16 0.02 g/cm3.

Another problem with estimating the volume from the area is that the shape of the vertebra changes with growth - in other words, the growth rate of the height of the vertebrae is different from the width of the vertebrae. The best volumetric bone density is measured using QCT, but that technique has other problems with radiation and reproducibility.

BMAD probably does not predict fractures in postmenopausal women any better than areal BMD, so it is not necessary to use BMAD clinically for ordinary-sized adults. But in adults shorter than 5 feet, the areal BMD can be very misleading. For example, we studied patients who were very short due to mutations in the growth hormone receptor, and found that the BMAD was similar to unaffected family members, whereas the areal BMD was much lower. The bone volume/tissue volume from bone biopsies in these patients was normal, providing validity to the BMAD results.

The term BMAD was defined by Carter DR, Bouxsein ML, Marcus R in: New Approaches for Interpreting Projected Bone Densitometry Data. J Bone Min Res, 7:137-145, 1992. We had previously estimated the "volumetric bone density" (JCE&M, 66:501, 1988 ) based on dimensions on xrays and calculated volume as: height x width x depth x pi/4. Several other investigators have used similar volume calculations.

Updated 10/8/05