Bone density
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 diagnosing and monitoring the progression of osteoporosis, but it can also increase or decrease in other diseases.
The higher the bone density, the higher the strength and stability of the bone.
The lower the bone density, the lower the stability of the skeleton and consequently the greater the likelihood of fractures.
Illustration structure of a bone
a - epiphysis
(Bone end)
b - metaphysis
(active growth zone)
c - diaphysis
(Bone shaft)
- Sponge-like built
Bone with red
Bone marrow -
Spongiosa substantia
+ Medulla ossium rubra - Epiphyseal line -
Epiphysial line - Dense (compact) bone -
Substantia compacta - Medullary cavity with yellow
Bone marrow -
Cavitas medullaris
+ Medulla ossium flava - Bony artery -
Nutrician artery - Periosteum -
Periosteum - Osteon (basic functional unit) -
Osteonum - Spaces filled with bone marrow
between the trabeculae -
Medulla ossium - Growth plate -
Lamina epiphysialis
You can find an overview of all Dr-Gumpert images at: medical illustrations
physiology
The bone substance is subject to permanent remodeling.
Every year around 10 percent of the skeleton of a healthy person is remodeled. This constant construction and dismantling is used Repair of micro damage and the Adaptation to a changed mechanical load the bone as well as the rapid supply of calcium.
This leads to a complicated, hormonally (growth hormones, steroid hormones and sex hormones in both sexes) controlled interplay of two different cell types, the Osteoblasts and the Osteoclasts.
Osteoblasts build up bone substance and add to existing bone mass, osteoclasts break them down.
Changes in bone density always have something to do with the changed activity of one of the two cell types to do.
So Bone structure predominates in childhood and adolescence, which has a peak value between 25 and 30 years "peak bone mass" reached. After that Gradually the bone loss predominates againwhich is common to women in the menopause can accelerate very much, because the bone structure also through Estrogens, Sex hormones, is regulated. If there is a sudden lack of estrogen, bone breakdown predominates and osteoporosis can occur because too many osteoclasts are created and retained.
Since estrogen deficiency can also develop in men in old age, the risk of osteoporosis also increases with increasing age, but usually not as rapidly.
The most important factor for a balanced balance between construction and dismantling is the mechanical stress on the bone The bone cells embedded in the mineralized bone mass presumably function here as a species Mechanosensorwho then pass on information about them via messenger substances. This explains why the bone mass decreases when there is no stress, for example if you are in bed for a long time or if you are in space.
From this knowledge about the exact regulation of the bone remodeling and the function of the osteoclasts could new drugs that specifically target these points and thus form an alternative to hormone therapy for postmenopausal osteoporosis.
Causes of change
A Increase in bone density by increased osteoblast activity or production of bone-like material by tumor cells, for example by benign or malignant Bone tumors, osteoplastic metastases, chronic or healed Inflammation of the bone marrow, one Underactive parathyroid glands (Hypoparathyroidism), by Replacement tissue after a fracture or one Perthes disease be caused.
A decrease in bone density, be it due to lower osteoblast activity or increased bone resorption, can result from, among other things osteoporosis, inactivity, endocrine diseases (Diabetes mellitus, Hyperthyroidism, Cushing's Syndrome), Vitamin D deficiency, one Rheumatoid arthritis, one Overactivity of the parathyroid glands or by certain Medication, how Steroids (Cortisone) or Heparin.
If only individual bones are locally affected, the cause can also be Inflammation or but Tumors or metastases underlie.
Bone density measurement
There are several methods to measure bone density.
So there are DXA (Dual energy X-ray absorption) in which the absorption of X-rays measured through the bone and compared with reference values.
The second established procedure is the QCT (quantitative Computed Tomography), which is associated with a higher radiation exposure, but can determine osteoporosis at an earlier point in time and can also make statements about the bone structure.
-> Straight to the topic Bone density measurement.
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Normal values
In order to be able to better compare the different methods with their different units, the bone density is given as so-called T value or Z-Value on.
The T-value describes the difference to the normal value in the form of multiples of the standard deviation. The normal value here refers to the statistical mean value of healthy premenopausal women. A standard deviation of -1 is to be regarded as normal, from - 2.5 one speaks of overt 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 was introduced. In contrast to the T value, this refers to the mean value of the bone density in the corresponding age group.
