The Effect of Bone Strontium on BMD for Different Manufacturers' DXA Systems

Osteoporotic patients treated with strontium ranelate show relatively large increases in bone mineral density (BMD) measured by dual-energy X-ray absorptiometry (DXA) due to the replacement of some of the calcium atoms in bone by strontium. A study published by Pors Nielsen and colleagues reported that replacement of 1% of calcium atoms by strontium causes a 10% increase in BMD. 

We refer to the ratio of the percentage increase in BMD to the molar percentage of strontium in bone as the strontium ratio. Theoretically it is expected that the strontium ratio should vary between different manufacturers' DXA equipment depending on the effective photon energy of the device, an effect that arises because of the proximity of the X-ray energies produced by lower energy devices to the strontium K-edge at 16 keV. 

In this study we report theoretical estimates of the strontium ratio for two axial DXA systems and two peripheral DXA devices based on their broad spectrum X-ray emission. The theoretical figures were verified in an experimental study in which the strontium ratio for each device was measured using phantoms containing mixtures of hydroxyapatite and strontium hydrogen-phosphate. 

The theoretical values/experimental results of the strontium ratio were: 

11.0/11.2 for the Hologic Discovery,

9.9/9.9 for the GE-Lunar Prodigy, 

9.1/8.6 for the Demetech Calscan, and 

8.5/6.3 for the Osteometer Dexacare G4. 

The results confirm both theoretically and experimentally that the effect of bone strontium on BMD measurements is different for different DXA systems. In the future it might be possible to exploit this effect to make a non-invasive estimate of average bone strontium content in groups of patients receiving strontium medication for osteoporosis.

https://pubmed.ncbi.nlm.nih.gov/20699129/

What Determines How Much Strontium Is in Your Bones?

It is generally said that, for every 1% of strontium incorporated into bone crystal, BMD is overestimated by 10%. However, we do not know how much strontium has been incorporated into each of your bones. 

The amount of strontium in your bones will depend on several factors: 

(1) the dosage (680 mg, as in the COMB study; 340 mg, which is a half dose; or 450 mg, the dosage used in the MOTS study; 

(2) the length of time you have been taking strontium (The COMB and MOTS studies lasted only one year. Therefore, the strontium effect on those DXA scans was less than had the subjects been studied for three years or longer. 

(3) how well you absorb the strontium (Absorption of strontium salts is about 25% for most people with normal absorption, but, as we age, our ability to absorb nutrients, supplements, and medications is often reduced.); 

(4) the individual bones (Some bones absorb strontium better than others.); 

(5) the DXA equipment used to measure BMD.