DXA Bone 'Density'

[Guest guest]

Hi folks:

The following link (by far the best source I have found to date

regarding bone density ...... if anyone has a better source please

post) explains why DXA output units are 'grams per SQUARE

centimeter': " Bone mineral density – an `area density'. BMD is

defined by m/a where m is the mass (in grams) of calcium in the path

of an X-ray beam passing through a bone and a is the cross-sectional

area (in cm2)of the beam. "

http://www.schoolscience.co.uk/content/5/biology/mrc/osteopor/page4.ht

ml

http://snipurl.com/dwu6

So, as previously suspected, DXA output is essentially a measure of

**MASS**. Not a measure of density, let alone a measure of porosity

(which is what osteoPOROSIS is about). It seems obvious that an X-

ray beam of given cross section will not be able to distinguish

between a smaller solid bone and a larger porous bone - since it is,

presumably, simply measuring how much of the beam is not intercepted

on the way through the bone. And a smaller solid bone would absorb

the same amount of X-rays as a larger more porous one.

This may have relevance for those whose bones are smaller than

average because their BMI is lower than average.

All else equal, smaller bones are presumably less strong than larger

bones. But they likely are nowhere near as weak as porous bones.

----------------------

So it appears that DXA measures bone **MASS**, not bone porosity, nor

fracture risk. This raises the question as to how relevant DXA

output is for fracture risk in people with low BMIs.

A QCT scan, on the other hand, does purport to measure bone

**DENSITY**. Presumably, although I have not yet found any sources

that show evidence to confirm this, a QCT measure of density may be a

lot more closely related to porosity and fracture risk than DXA

output because, one would think, porous bones will be less dense.

I **have** found a serious source (on Pubmed, sorry did not make a

note of it) which concluded that output from a nuclear magnetic

resonance (NMR) scan is very closely correlated (R-squared = .98)

with porosity. This study did not tackle the issue of correlation

with fracture risk, but logically it seems likely it will be well

correlated with that also.

So, I am coming to the tentative conclusion that for people with BMI

below 25 DXA can be a very misleading measure of bone health. So

much so that it may be a waste of time, expense and radiation

exposure, in my present opinion. Whether or not it is misleading in

my case (BMI = ~22.5) remains to be seen.

This may be the last thing I post on bone density, unless I find much

more authoritative information on the relevance of QCT and NMR.

Rodney.