Definition
The term bone density describes how much mineralized bone mass is present in a defined volume, i.e. the ratio of bone mass to bone volume. The measurement of bone density is particularly important for the diagnosis and monitoring of osteoporosis, but it can also increase or decrease in other diseases. The higher the bone density, the greater the strength and stability of the bone.
The lower the bone density, the lower the stability of the skeleton and consequently the greater the probability of fracture. The bone substance is subject to permanent remodeling. In a healthy person, about 10 percent of the skeleton is remodeled every year.
This constant rebuilding and decomposition serves to repair micro-damage and to adapt to changes in the mechanical load on the bones as well as to provide calcium quickly. This results in a complicated, hormonally controlled interaction of two different cell types, the osteoblasts and the osteoclasts (growth hormones, steroid hormones and sex hormones in both sexes). Osteoblasts build up bone substance and attach it to existing bone mass, osteoclasts break it down.
Accordingly, changes in bone density always have something to do with the altered activity of one of the two cell types. In children and adolescents, bone formation predominates, reaching a peak bone mass between 25 and 30 years. After that, bone loss gradually predominates again, which can greatly accelerate in women during the menopause, since bone formation is also regulated by estrogens, sex hormones.
If there is a sudden lack of estrogen, bone resorption predominates and osteoporosis can occur, as too many osteoclasts are formed and retained. Since men can also develop an estrogen deficiency in old age, the risk of osteoporosis also increases with age, but usually not as rapidly. The most important factor for a balanced balance between build-up and breakdown is the mechanical stress on the bone.
Here, the bone cells embedded in the mineralized bone mass probably act as a kind of mechanosensor, which then pass on information about it via messenger substances. This explains why bone mass decreases when there is no load, for example, when the patient is confined to bed for a longer period of time or stays in space. From these findings about the exact regulation of bone remodeling and the function of osteoclasts, new drugs could be developed that specifically target these points and thus form an alternative to hormone therapy for postmenopausal osteoporosis.
An increase in bone density due to increased osteoblast activity or production of bone-like material by tumor cells can be caused, for example, by benign or malignant bone tumors, osteoplastic metastases, chronic or healed inflammation of the bone marrow, an underfunction of the parathyroid glands (hypoparathyroidism), replacement tissue after a fracture or even Perthes disease. A decrease in bone density, be it due to reduced osteoblast activity or increased bone resorption, can be caused by osteoporosis, inactivity, endocrine diseases (diabetes mellitus, hyperthyroidism, Cushing’s syndrome), vitamin D deficiency, rheumatoid arthritis, hyperactivity of the parathyroid glands or by certain drugs such as steroids (cortisone) or heparin. If only individual bones are affected locally, inflammation or tumors or metastases may also be the cause.
Bone density can be measured using various methods. There is DXA (Dual energy X-ray absorption), in which the absorption of X-rays by the bone is measured and compared with reference values. The second established method is QCT (quantitative computed tomography), which is associated with a higher radiation exposure, but can detect osteoporosis at an earlier stage and also provide information about the bone structure.
–> Directly to the topic bone densitometry. In order to be able to compare the different methods with their different units, bone density is given as a so-called T-value or Z-value. The T-value describes the difference from the normal value in the form of multiples of the standard deviation.
The standard value here refers to the statistical mean value of healthy premenopausal women.Here, a standard deviation of -1 is considered normal, from – 2. 5 onward one speaks of a manifest osteoporosis. In order to adapt these values to the age-typical condition of the skeletal system – older people physiologically have a lower bone density – the Z-value has been introduced. In contrast to the T value, this refers to the mean value of bone density in the corresponding age group.