The mature bone consists of two main components: osteoid (organic matrix) and hydroxyapatite (inorganic substance). The osteoid consists primarily of the collagen protein. Hydroxyapatite - a complex substance, which includes calcium, phosphate (acidic phosphoric acid residue) and hydroxyl groups (OH). In addition, it contains some magnesium. In the process of bone formation, hydroxyapatite crystals are deposited in the osteoid matrix. The outer part of the bone consists of a dense cortical bone tissue; the internal structure is represented by a more loose spongy tissue and contains many cells filled with red bone marrow - tissue involved in the production of blood cells.
Maintaining a bone
Neither cortical nor spongy bone is inert. Even after the completion of growth, they retain metabolic activity and are constantly reconstructed. This coordinated process, in which parts of the bone dissolve and replace with a new tissue, is necessary to maintain bone health. The formation of bone tissue is regulated by specialized cells - osteoblasts. They synthesize the osteoid and provide the formation of hydroxyapatite. For the resorption of bone tissue, cells called osteoclasts are responsible.
Bone diseases
The bone is susceptible to damage by many pathological processes. It can be broken mechanically (fracture), often becomes a place of localization of secondary tumors (especially in breast, lung and prostate cancer), bone metabolism can also be disturbed. There are many metabolic bone diseases. Osteoporosis is a condition in which simultaneous loss of osteoid and mineral component of bones occurs. This process inevitably occurs with aging, but with estrogen deficiency in women in menopause it is markedly accelerated. The main reason for the development of osteoporosis is the imbalance between the rate of destruction and the formation of bone tissue. Its main effect is the weakening of bone tissue, predisposing to fractures (especially the hips, wrists and vertebral bodies), which often result from even minor injuries.
Osteomalacia
When osteomalacia, the mineralization of bones is disturbed, as a result of which they soften and can deform, causing acute pain or fractures. Osteomalacia is usually associated with a deficiency of vitamin D or disorders of its metabolism, leading to a lack of calcium to form bones. It is treated by the appointment of vitamin D and calcium preparations.
Paget's disease
This bone disease mainly affects the elderly. The reason is unclear, but it is known that in this disease, the activity of osteoclasts increases, which leads to an acceleration of bone resorption. This, in turn, stimulates the formation of more new bone tissue, which, however, is softer and less dense than normal bone. Pain in Paget's disease is due to stretching of the periosteum, a membrane covering the outer surface of bones, abundantly innervated by pain receptors. Analgesics are used to relieve pain, and the disease itself can be treated with bisphosphonates, which slow the bone resorption.
Renal osteodystrophy
It is observed in patients with chronic renal failure. The most important factor in this disease is the breakdown of vitamin D metabolism. During the processes taking place in the liver and kidneys, vitamin D is converted to calcitriol, a hormone that regulates calcium absorption. With chronic renal failure, production of calcitriol is reduced. The condition is treated by the appointment of calcitriol or similar drugs. Methods such as fluoroscopy, isotope scanning and histological examination of bone tissue samples are important components of bone disease diagnosis. Valuable diagnostic information about bone diseases, with the exception of osteoporosis, can often be obtained also in blood tests.
Blood tests
The most important tests are measurements of the concentration in the plasma of calcium and phosphate, as well as the activity of alkaline phosphatase, an enzyme that is produced by osteoblasts. Calcium concentration in plasma Normally varies between 2.3 and 2.6 mmol / l. The level of calcium is regulated by two hormones - capcitriol (a derivative of vitamin D) and parathyroid hormone. It decreases with renal osteodystrophy, and also in most cases of osteomalacia and rickets. In osteoporosis and Paget's disease, the calcium concentration is kept at a normal level (although with Paget's disease, if the patient is immobilized, it can rise). An increased concentration of calcium in plasma is observed with primary hyperparathyroidism (usually caused by a benign tumor of the parathyroid glands). Parathyroid hormone activates osteoclasts, but clinical manifestations of bone disease in this disease are not frequent. A high level of plasma calcium is also common in cancer patients. In some cases, this is due to the destruction of the bone by metastases, in others because of the synthesis by the tumor itself of substances similar to parathyroid hormone (GPT peptides). The concentration of phosphate in the plasma is normally between 0.8 and 1.4 mmol / l. Increased concentration is observed in renal failure (when the concentration in the plasma of urea and creatinine, the products of metabolism, usually excreted from the body with urine, is sharply increased), and decreased - with osteomalacia and rickets. With Paget's disease and osteoporosis, the concentration of phosphate in the plasma is usually within the normal range. Plasma alkaline phosphatase activity The increased activity of this enzyme is observed in osteomalacia, Paget's disease and renal osteodystrophy. With effective treatment, it decreases. Especially alkaline phosphatase is useful as a marker of the effectiveness of treatment in Paget's disease. The plasma alkaline phosphatase level also increases in certain diseases of the liver and the bile duct system, but usually in this case there are no difficulties with diagnosis.
Other blood tests
If necessary, the concentration in the blood of vitamin D can be measured. A low level indicates osteomalacia or rickets. None of the tests described above can detect osteoporosis, because the imbalance between the rate of formation and destruction of bone with this usually slowly progressing disease is relatively small. The diagnosis can be confirmed with the help of special X-ray methods. Normal dense bone on the radiographs is clearly outlined, with osteoporosis, bone tissue becomes less dense and looks darker in the picture. To measure bone mineral density, a two-photon X-ray densitometry method is used that can confidently diagnose osteoporosis. Doctors are in urgent need of simple methods to identify people with osteoporosis or those at increased risk of developing this disease, as well as monitoring the effectiveness of treatment.