Quantitative ultrasound examination

The third and last option for bone density measurement is quantitative ultrasound (QUS), in which ultrasound waves are sent through the body instead of X-rays. As a result, the radiation exposure in this procedure is zero. Ultrasound waves are also attenuated to varying degrees by tissue of varying density and can therefore provide information about the density of bone. The best regions to perform this examination are the calcaneus and the small phalanges. However, even for these regions it has not yet been proven that QUS can be used meaningfully for the purpose of disease-relevant bone density measurement.

Evaluation of the bone density measurement:

The presented procedures are different in terms of the statements that can be made by them. DEXA is used to assess the body composition of bone, muscle and fat tissue. However, it does not measure the physical density of the bone, just as no statement can be made about the three-dimensional shape of the bone.

However, it does provide a surface representation of the bone, which can also be described as surface density (kg/m2). Quantitative computer tomography, on the other hand, is much more precise than DEXA. However, QCT cannot capture the entire body composition.

This is only possible locally. However, it can be used to determine the exact physical density of the bone. QCT can therefore be used to assess bone properties such as bending strength and bone strength very accurately.

In addition, the mineral salt content of the different bone layers can be assessed individually. With DEXA, the value is represented as an average value of the entire bone. Thus, QCT is more sensitive to pathological changes in bone and can indicate osteoporosis earlier than DEXA.

Results

However, with all the above mentioned methods, the measured values obtained are not comparable with the results of other devices (nor with each other). For this reason, it has become established practice not to give absolute density values as results, but to use either a T-value or a Z-value instead. The T-value is used most frequently.

This is a dimensionless quantity that indicates to what extent the measurements deviate from the normal in multiples of a standard deviation. The T-value of the bone density measurement indicates whether and, if so, to what extent the measured bone density deviates from the average value determined for healthy men or women in their 30th year of life. The lower this value is, the higher the risk of suffering a bone fracture.

By definition (according to WHO) osteoporosis is present when the T-value is less than or equal to -2.5, i.e. 2.5 or more standard deviations below the average. Values between -1 and -2.5 are called osteopenia and all values greater than -1 are considered normal. A disadvantage in the practical handling of the T-value is that it only applies to healthy 30-year-olds.

However, since bone density naturally decreases at an advanced age, a very high proportion of these age groups would at some point be considered “ill”. Among 70-year-old women, for example, this would be just under half! For this reason, another value has been developed, the Z-value, which refers to healthy women or men of the same age.

This makes it possible to estimate whether bone density corresponds to age (and sex).A Z-value greater than -1 means that the bone density is typical for the age, values below that are pathological. In people who have a low T-value but a Z-value within the normal range, the reduced bone density is considered a normal sign of old age, and so in these cases, drug therapy is usually not required. In which cases, however, does it make sense to have a bone density measurement carried out?

The main area of application of these procedures is the diagnosis of osteoporosis. Osteoporosis is a disease also known as bone loss. It is characterized by a decrease in bone density and the loss of bone substance, which leads to an increased risk of bone fractures.

A distinction is made between primary osteoporosis (i.e. osteoporosis as an isolated disease; this form accounts for about 95% of osteoporosis patients) and secondary osteoporosis, which occurs in the context of other underlying diseases. Since bone density naturally decreases with age, osteoporosis is a disease of old age in particular, with post-menopausal women being particularly affected due to hormonal influences. Bone densitometry is used both for already diagnosed, known osteoporosis in order to assess the existing risk of fracture and for people suspected of having osteoporosis.

If a previously healthy person shows symptoms that clearly indicate osteoporosis, such as frequent bone fractures (especially if they cannot be explained by a previous accident), bone pain or a hunchback, bone densitometry can be useful. There is an increased risk of osteoporosis in people who abuse nicotine or alcohol. Even if there is a vitamin deficiency (i.e. either in conditions of malnutrition as in anorexia nervosa or diseases of the gastrointestinal tract associated with a reduced intake of food components such as chronic inflammatory bowel disease, the calcium salt content of the bones is often reduced.

Since the build-up and breakdown of bone substance is also controlled by hormones, some hormonal disorders also affect bone density. Hyperthyroidism, for example, can promote osteoporosis, and in general, women in the menopausal years (menopause) are particularly affected by this clinical picture, as the female body’s oestrogen production decreases considerably. Even if there are several known cases of osteoporosis in the family or if there is an underlying disease such as diabetes mellitus, this predisposes to the development of osteoporosis. One of the most common reasons for the development of secondary osteoporosis is long-term treatment with glucocorticoids (steroids) such as cortisol. Bone densitometry also plays an important role in the treatment of osteoporosis in the sense of monitoring its progress, in order to be able to assess whether the treatment is effective and whether or how the disease is progressing.