Bone Ages Differently in Different Places

Bone Ages Differently in Different Places

A bone density report does not carry one number. The lumbar spine is listed separately from the femur, and the figures differ. Many people ask which one to believe.

I tell them both are correct. The two numbers measured different places.

The Scan Measures Several Places Separately by Design

A bone density scan does not take the body as one. It generally measures the lumbar spine and the upper femur separately, and reports each value on its own.

There is a reason. Bone differs by place. The lumbar spine holds more of the porous inner structure and turns over faster; the upper femur carries a larger share of dense outer bone. The loads they take differ, and so does the pace at which they change.

One thing should be understood, though. With age the lumbar value can read higher than it truly is. Bony outgrowths, hardening as the joints wear, and calcium deposited in the large vessel passing in front are all captured in the same image. So when the lumbar value looks acceptable and the femur comes back low, I read the femur more heavily.

Even within one person, bone is not a single piece.

Which Is Why the Values Disagree

This is not unusual. Values measured at several sites landing on the same verdict is, if anything, the less common outcome. In one analysis, the proportion in which all three sites agreed was very small.

So in practice the judgment is generally made from the lowest site. It errs on the safe side. But that approach, in holding onto the one low value, leaves the state of the other places further back.

Values That Disagree Are a Record of a Life Lived

I do not read that difference as a measurement error. I take it to hold how much load a person has put through each place.

Bone keeps what it is asked to keep. Places that walk and bear weight keep receiving the signal; places long spent lying or sitting receive less of it. Someone who has favoured one leg for years may show a lower value on that side. Values disagreeing means the body did not age evenly — and uneven ageing usually has a life behind it.

The State of the Place, More Than the Low Number

In the consulting room I ask, before "what is the score," which place is low, and how has it been used: whether the person has sat for long years, which leg has been favoured, what walking is like.

Raising a number is slow work. The direction, though, is fairly clear: putting load back through that place — walking, standing, stepping — is a signal to bone. If one place came back low, I look at whether that place has been doing less of the work.

That said, if height has noticeably dropped, if a bone has broken from a light knock, or if the back has rounded with continuing pain — then, before weighing places, having it examined comes first.

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Several numbers on a bone density report do not mean the scan is confused. It means bone has lived differently in each place.

If the spine and the hip disagree, neither is wrong. The two places have come through different stretches of time.

Related Reading

• An Average Does Not Know the Place — which place a test reflects
• The Body Does Not Age All at Once — different clocks in different places

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Sources

• T-scores from the lumbar spine, femoral neck and total hip rarely give concordant diagnoses, so the diagnosis depends strongly on which site was measured — A Statistical Approach Regarding the Diagnosis of Osteoporosis and Osteopenia From DXA, 2021
• When both hips are measured, the verdict can differ between left and right as well — Dual-energy X-ray Absorptiometry of Both Hips, 2017

Written by Dr. Heo Ji-young (PhD in Korean Medicine Pathology, Kyung Hee University · former Research Professor of Herbology, Kyung Hee University)