Osteoporosis : bone densitometry is the diagnostic

GILE/SIPA

Published the 21.10.2017 at 11h27

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Keywords :

OstéoporosediagnosticDensitométrie bone

Bone densitometry measures the bone mineral density of the bones, that is to say, their mineral content. This is an x-ray examination is painless, very little radiating (tenth of an x-ray of the lung) and quick (under 15 minutes), which takes place on a special machine in the supine position.

This measure is carried out mainly on the two parts of the body : the lumbar spine and the femoral neck. The wrist is no longer measured only in some diseases like hyperparathyroidism, linked to a hyper-functioning of the glands in the back of the thyroid, and regulate calcium in the blood and, therefore, its exit from the bone.

The test assesses fracture risk

The bone is made up of 2 components : a part mineral, the bone crystal, and a part consisting of a frame made of proteins. To make it simple, the os is like “a roof” a ” frame structure in proteins “, on which come to ask ” tiles, which are the crystals of bone “. Bone strength is related of course to the amount of material, and therefore weight, that will make the bone density, but also to the manner in which it is built, the architecture of the structure, and the quality of the materials. It is the bone density test which is the exam that will accurately measure the ” total weight of the frame, and tiles “.

The part of bone mineral, which contains mostly calcium, but also phosphorus, determines bone mass and could explain 70% to 80% of bone strength. The ” quality of the os “, which is commensurate with the quality of the protein plot bone, quality of bone crystal and the internal architecture of the os would be responsible for 20 % to 30 % of the bone strength. The abnormalities in bone quality explain, however, why some people who have a bone density low, fracture not necessarily, and why others who have a bone density a little lowered fracture to minimal trauma. But, in the end, the assessment of fracture risk achieved by bone densitometry is more accurate than the assessment of cardiovascular risk from cholesterol !

The bone density test can ascertain the level of risk

The measures of densitometry are reliable if the machines are regularly calibrated and monitored. This quantitative evaluation allows us to estimate the bone strength from the accurate measurement of bone density. The bone density test compares your bone mineral density measured at a reference value in the same population (same ethnic group), same age and same sex.

This allows to calculate a first index, the “T-score” which is the comparison between the density measured in the individual, and the bone density average for the population at the age of 30 years old, the age where bone density is at its maximum (” peak ” bone mass).

A side of the T score, one is interested in the “Z score” which is the comparison of the bone density is measured relative to the average density in the general population of women (or men) of the same age.

The T-score gives an indication of bone loss compared to the peak bone mass, while the Z-score gives an idea of the normality or the abnormality of the measurement (it is normal to lose bone as one gets older, the question remains whether it is excess or not).

The World Health Organization has set benchmarks

The T-score is used to classify the individuals evaluated in a category according to the WHO classification :

• T score greater than – 1 ds : normal density,
• T-score between – 2.5 and – 1 ds : “osteopenia” or moderate decrease in bone density,
• T score less than or equal to – 2.5 ds : “osteoporosis density” or significant decrease in bone density,
• T score less than or equal to – 2.5 ds with one or more fractures: osteoporosis fracture “.
  It should be remembered that a “osteoporosis density” is not strictly equivalent to a ” osteoporosis disease “, since it is usually a combination of T-score and risk factors for osteoporosis, or a fracture of fragility, which will trigger the prescription of an eventual treatment.
  Moreover, the bone density test allows to follow over time the evolution of the bone mineral density in cases of progressive disease or in case of treatment with anti-osteoporotic. In this case, it is not necessary to re-examined every year, but rather every 3 to 5 years.

The bone density test is used for a treatment decision

Osteoporosis density is classically defined as a T score less than -2.5 ds on one of the 3 following sites : lumbar spine, femoral neck or femur total. This threshold diagnostic is not, however, a threshold of initiation of an anti-osteoporotic (” threshold for therapeutic intervention “).
The decision of initiation of treatment is determined based on the “overall risk of fracture”, which depends on both :

• Of the age,
• A personal history of fracture low trauma,
• Measurement of bone density,
• Risk factors of fall,

And, in borderline situations, the estimation of the absolute risk of fracture from the score, FRAX®.

The bone density test is, therefore, useful in people who have risk factors medical of osteoporosis.

We can remake a bone density test

A second bone density test may be proposed three to five years after completion of the first, depending on the appearance of new risk factors. It may also be useful to repeat it when treatment for osteoporosis is started and one wants to monitor its effectiveness. In this case, given the constraints of the technology and the time to onset of effectiveness of the treatment, a period of two years is necessary between the two measures. Bone densitometry is supported by health Insurance in some indications.

Video course of bone densitometry

https://www.pourquoidocteur.fr/GesteSante/Les-examens/43-Passer-une-osteodensitometrie