Bone mineral density is exactly what it sounds like. It’s a measure of how much mineral, primarily calcium, is packed into a given area of bone. It is expressed in grams per square centimetre (g/cm²), and it is the foundational number from which everything else on your DEXA report is calculated.

How DEXA measures it

DEXA stands for dual-energy X-ray absorptiometry. It’s a technique that uses two low-dose X-ray beams of different energy levels to scan your bones. By measuring how much of each beam is absorbed by the bone, the machine calculates how dense the bone tissue is. The scan is quick, painless, and uses a very small amount of radiation. Far less than a standard chest X-ray.

The result is a BMD value for each site scanned. That value is then compared to reference populations to produce the T-score and Z-score you see on your report.

What a typical BMD value looks like

BMD values vary by site and by individual, but as a general reference, a healthy young adult woman might have a lumbar spine BMD of around 1.0–1.2 g/cm². A woman with osteoporosis might have a value of 0.7–0.8 g/cm² at the same site. The lower the number, the less mineral density and the more fragile the bone.

Why BMD alone doesn't tell the whole story

Bone strength is not determined by density alone. The architecture of bone (how the internal lattice of strands and spaces is structured) also matters. Two bones can have similar BMD values but very different structural integrity. This is one reason that fracture risk cannot be predicted from BMD alone, and why tools like FRAX incorporate other factors alongside the scan result.

Research is ongoing into imaging techniques that can assess bone quality beyond density. However, DEXA BMD remains the most widely used, validated, and accessible measure available in clinical practice.

Why tracking BMD over time matters

One of the most useful things your BMD value does is give you a baseline. When you have a second DEXA scan, typically one to two years after starting treatment, your doctor will compare the new BMD values to your original ones. A stable or improving BMD is a sign that treatment is working. A declining BMD may prompt a change in approach.

This is why keeping your DEXA reports (and photographing them in Lenari) gives you something genuinely valuable over time: a record of how your bones are responding.