Question

[Smilerv]

Why do some say that low bone density doesn’t necessarily mean low bone strength? Doesn’t low bone density negate the bone strength?

[Met00]

It's well known that many people have fragility fractures with osteopenia rather than osteoporosis, and some even fracture with normal bone density, while others with very poor bone density may never fracture. That's why bone density alone doesn't tell you much about bone strength. It's normal to lose bone density as we age, yet many people don't have fragility fractures despite this. If you're able to exercise, especially high impact together with progressive weight training, you should be able to help keep your bones strong despite a gradual loss of density. Personally I think DEXA scans are leading to too many people being put on medication to prevent fractures that may never happen! The FRAX fracture risk tool, that takes into account a number of risk factors, t-scores being just one of these, is supposed to give a clearer picture of bone health, but that also has its limitations. While it seems to be fairly accurate in predicting how many in a group of people would fracture without medication, it can't determine which specific people this would apply to.

[Smilerv in reply to Met00]

Thank you for your explanation Met00.

[FrogLeg in reply to Met00]

I feel ambivalent about this:

“That's why bone density alone doesn't tell you much about bone strength.”

When we attempt to assess fracture risk, we work with probabilistic distributions across populations. There is so much complexity involved in these phenomena. Different people may get exposed to more forces than others just by their lifestyle choices. Random events may induce fractures in some and not others. Things that may be less directly associated with bone strength may have an impact as well, such as balance and coordination.

With all these confounding variables, I am not so sure it is right to say that bone density alone doesn’t tell you much about bone strength. It doesn’t tell you everything, certainly. There are architectural considerations. There are geometric physiological differences which impact bone strength. But while we can’t pin down incident fracture prediction to individuals as well as we would like, that doesn’t mean we don’t have some valuable knowledge about bone strength from BMD.

I think we can say that as BMDs decrease, the statistical likelihood of a fracture across a population increases. The individual may not know for certain with all the complexity involved whether they will actually fracture, but they know that the percentage of people in the same rough BMD group will indeed increase. And that does tell us something valuable about bone strength.

I think it is important to at least slightly separate our concept of bone strength from our concept of fracture risk or our ability to predict who exactly will fracture. We can know something about bone strength without perfectly predicting who will fracture.

[Met00 in reply to FrogLeg]

I understand what you're saying, but don't personally believe that BMD alone is useful for defining osteoporosis. For example, peak bone density varies according to ethnicity, yet, in the UK at least, your ethnicity isn't entered into the FRAX calculator. I have olive skin. If I put French as my nationality rather than British, FRAX gives me a slightly lower fracture risk, if I put Spanish it gives a significantly lower fracture risk, all using the same neck of femur BMD. Ultimately it's a matter of how each of us as individuals perceive risk. For me, a 10 % risk of fracture in the next 10 years means a 90% chance of not fracturing, so on the balance of probabilities I'd be unlikely to fracture. For anything less than 50% fracture risk this is still true, although I think I would probably want to lower my risk if it were that high! I also bear in mind that bisphosphonates, the most common osteoporosis medication, only reduce your personal fracture risk by 50%, meaning that, for example, if the risk is 10% without meds, it would be 5% with meds; or to put it another way, you would have to treat 100 people with a 10% fracture risk to prevent 5 from fracturing in ten years, yet 90 of those 100 wouldn't have fractured anyway!

I realise most of the above is focussing on fracture risk, but personally I don't believe bone density alone is all that helpful. The only thing it does show is how an individual's bone quality may be changing over time. Even then, if bone density is lost at the same time as bone-strengthening exercise is increased, bone strength may stay stable, or occasionally even improve despite the loss of density.

[Frida22 in reply to Met00]

Hi Met00 Thanks for this. I agree. I realized these 'risk factor' calculations when one of my doctors explained it. I agree - many people are probably receiving meds needlessly. I am in Canada - I think the medical rationale here is to reduce the health care burden of fracture - over medicating is not an issue from that perspective.

[UkuleleStrummer in reply to Met00]

Great post! Thanks!

[Smilerv in reply to FrogLeg]

Thank you for your explanation FrogLeg

[uncountable3]

Bone strength is a composite of two factors: bone density and bone architecture. Bone density is simply how many atoms there are. Bone architecture is how they are put together. You can have excellent bone density but if the bone is not well distributed and there are many microfractures, it may still be weak. Inversely, density could be poor but if the architecture is solid and there are few microfractures the bones could still be strong. It is a precarious situation though.

Bone density is easy to measure. At this time we don't have any direct way to measure bone architecture. We have methods like TBS that can infer some aspects of bone architecture but the correlation has limits and, unfortunately, they do best when architecture and density are similar and much less well in the more interesting cases where they are opposed.

[Smilerv in reply to uncountable3]

Thank you for your explanation uncountable3

[josephinius1]

Good question! Interesting answers. Thank you.