BMD by DEXA = real bone strength?

[Rev. Cheeseman]

TL;DR Summary

Areal bone mineral density by DEXA, is it really an indicator of bone strength?

BMC is a measurement of bone mineral found in a specific area and is measured in grams (g). BMC can be measured in a specific location (i.e., arm, leg, etc.) or for the total body. BMD, on the other hand, is the amount of bone mineral in bone tissue. It is derived by dividing the BMC (g) by the area (cm2).

So, for example, a total body bone mineral content is 5200 grams divided by the area of the total body which is 2500 cm^2 and the areal BMD which is measured by using DEXA will be 2.08 g/cm^2.

But a total body bone mineral content of 5500 grams which is heavier than the previous example divided by the total body area of 2800 cm^2 which is basically bigger bone structure is equal to 1.96 g/cm^2 BMD which is less than the former.

According to logic, the latter which is bigger and heavier will be the stronger bone than the former. So, is areal BMD giving a false impression that it is actually stronger than it actually is?

 

[berkeman]

Can you give us some links to your reading please? Thanks.

 

[Rev. Cheeseman]

Can you give us some links to your reading please? Thanks.

First, the strongman Brian Shaw's bone mineral content and areal bone mineral density

Second, a bodybuilder Nsima Inyang's bone mineral content and areal bone mineral density

Nsima Inyang, despite being smaller than Brian Shaw have higher areal bone mineral density at 1.922 g/cm2 which we can get from dividing the bone mineral content with his total body skeleton size. His bone mineral content is 11.5 lbs which if we converted to grams will be 5216 grams. So, 5126 grams divided by 2713 cm which is the size of the total body skeleton is 1.922 g/cm2.

Brian Shaw, who have heavier bone mineral mass and larger bone, have less areal BMD (which is around 1.800 g/cm2 despite having heavier bone) than Nsima. Does that means Brian's bones are less stronger than Nsima despite Brian's bones being much bigger and heavier? Why is that?

Even my previous examples that I made up myself earlier, which is...

"So, for example, a total body bone mineral content is 5200 grams divided by the area of the total body which is 2500 cm^2 and the areal BMD which is measured by using DEXA will be 2.08 g/cm^2.

But a total body bone mineral content of 5500 grams which is heavier than the previous example divided by the total body area of 2800 cm^2 which is basically bigger bone structure is equal to 1.96 g/cm^2 BMD which is less than the former."

also shows some strange conclusion using the DEXA areal bone mineral density formula. Common sense tells us the bigger and heavier bone will be the stronger one.

 

[Rev. Cheeseman]

Brian Shaw's areal BMD is less because Brian's bones are big in comparison to his bone mineral mass therefore the DEXA scan machine registered a lesser areal BMD, and Nsima's bones are smaller in comparison to his bone mineral mass resulting in higher areal BMD.

But in absolute terms, comparing these two Brian's bones will be much stronger as his bones are bigger and heavier.

 

[Rev. Cheeseman]

Hi, I tried to add all the BMD numbers to get the total number like what is shown in the picture above and I get a wrong answer. What is the formula on how to find the total BMD with using the numbers from each regions?

 

[berkeman]

View attachment 334760

Hi, I tried to add all the BMD numbers to get the total number like what is shown in the picture above and I get a wrong answer. What is the formula on how to find the total BMD with using the numbers from each regions?

That "total" looks to be the average of the 7 numbers, although I get 1.967 for the average, which is a little off. There may be some weighting also being considered, where larger structures contribute more to the average. Is it not described in the text?

 

[Rev. Cheeseman]

That "total" looks to be the average of the 7 numbers, although I get 1.967 for the average, which is a little off. There may be some weighting also being considered, where larger structures contribute more to the average. Is it not described in the text?

No, it is not. Many other DXA reports also seem to contained similar errors especially with the total numbers. Also, the number for trunk part which already included ribs, spine and pelvis is already there. Do we need to treat ribs, spine and pelvis as separate parts from the trunk? Not sure. Very confusing.

 

[Rev. Cheeseman]

That "total" looks to be the average of the 7 numbers, although I get 1.967 for the average, which is a little off. There may be some weighting also being considered, where larger structures contribute more to the average. Is it not described in the text?

Anyway, how do you managed to find the total? What is the formula?

 

[berkeman]

Anyway, how do you managed to find the total? What is the formula?

If you're asking me, I said I took the average of the 7 numbers, so the sum of the 7 numbers divided by 7.

Does the text (or video, whatever) really not discuss how that "total" is calculated?

It looks like they are doing something similar with the Body Fat % numbers in their other table. It's most likely a weighted average based on volume, so you will need to find out the weights they are using for each section.

 

[Rev. Cheeseman]

If you're asking me, I said I took the average of the 7 numbers, so the sum of the 7 numbers divided by 7.

Does the text (or video, whatever) really not discuss how that "total" is calculated?

It looks like they are doing something similar with the Body Fat % numbers in their other table. It's most likely a weighted average based on volume, so you will need to find out the weights they are using for each section.

The source of the DEXA result picture above is from Instagram and I can no longer find the original account, but I managed to find this report online https://www.body-comp.com/wp-content/uploads/2016/12/body-comp-sample-report.pdf I tried to find how they calculate the total but still can't find it.

 

[256bits]

TL;DR Summary: Areal bone mineral density by DEXA, is it really an indicator of bone strength?

According to logic, the latter which is bigger and heavier will be the stronger bone than the former. So, is areal BMD giving a false impression that it is actually stronger than it actually is?

Stronger, or weaker, depending.
From reading up on the subject.
Both bone mineral content and bone size would be determining factors for bone strength. But they are independant from one another, and, in addition, the aBMD bone strength correlates inversely to a bone size strength ( as you are mentioning ).

From Wiki - dual-energy X-ray absorptiometry - Scoring-
Also, there are other variables in addition to age that are suggested to confound the interpretation of BMD as measured by DXA. One important confounding variable is bone size. DXA has been shown to overestimate the bone mineral density of taller subjects and underestimate the bone mineral density of smaller subjects. This error is due to the way by which DXA calculates BMD. In DXA, bone mineral content (measured as the attenuation of the X-ray by the bones being scanned) is divided by the area (also measured by the machine) of the site being scanned.

It is important for patients to get repeat BMD measurements done on the same machine each time, or at least a machine from the same manufacturer. Error between machines, or trying to convert measurements from one manufacturer's standard to another can introduce errors large enough to wipe out the sensitivity of the measurements.[citation needed]
NOTE: Citation for this comment flagged as needed by Wiki, so take it as a possible another difficulty with aBMD if being used as an absolute bone strength indicator.

A study discusses this same problem.
https://bmcmusculoskeletdisord.biomedcentral.com/articles/10.1186/s12891-019-2785-8

Background​

Areal bone mineral density (aBMD) estimated by dual-energy X-ray absorptiometry (DXA) is used to estimate peak bone mass, define osteoporosis and predict fracture. However, as aBMD is calculated as bone mineral content (BMC) divided by the scanned area, aBMD displays an inverse relationship with bone size. In a skeleton that is increasing in size, this is a problem, as bone size is an independent factor that determines bone strength. It could therefore be questioned whether peak aBMD is the period with greatest bone strength, a period that in the hip then would occur in ages 16–19. The aim of this study was to evaluate whether there are changes in bone size in men after age 18 that may influence peak bone strength. Another aim was to provide updated normative DXA data.