Osteolysis is the process of bone loss. This usually takes place as part of normal metabolism. However, when the balance between bone resorption and bone formation is disturbed, pathological loss of bone substance may occur.
What is osteolysis?
Osteolysis is the process of bone resorption. Osteolysis is the scientific name for bone resorption, which is a normal and necessary process in bone metabolism. During this process, bones are constantly remodeled by breaking down existing bone substance and rebuilding it elsewhere. Bone metabolism is hormonally regulated in this process. Two cell types exist in bone. These are the osteoblasts on the one hand and the osteoclasts on the other. The osteoblasts are responsible for bone formation. After mineralization, they transform into osteocytes (bone cells) that are no longer able to divide. The osteoclasts, in turn, control bone resorption. These are multinucleated cells that have been transformed into macrophage-like cells by the fusion of stem cells from the bone marrow. Their task is, as so-called scavenger cells, to dissolve the bone substance. There are two forms of osteolysis. One is osteolysis in the context of bone remodeling and the other is circumscribed osteolysis. Bone remodeling is a general process in which there is a balance between bone resorption and bone formation. In circumscribed osteolysis, local bone resorption occurs, but it is pathological.
Function and task
Osteolysis is a necessary process in the organism, which simultaneously fulfills various functions. In the process of bone remodeling, bone substance must be constantly broken down and rebuilt elsewhere. This process is called bone tissue remodeling. The bone and skeletal system is the largest supporting organ of the body. The constant stress on this support system causes structural damage within the bones, which must be compensated for at all times in order to keep the musculoskeletal system functioning properly. This can only be achieved by continuously breaking down damaged bone substance and building up new bone substance. Furthermore, this process also keeps the calcium and phosphate levels constant at the same time. The skeletal system is the largest reservoir of calcium and phosphate. If, for example, too little calcium is supplied through the diet, this leads to increased bone loss. Thus, fluctuating calcium and phosphate levels can be quickly compensated. Overall, bone and calcium metabolism is subject to hormonal regulation. For example, parathyroid hormone ensures a constant calcium level in the blood by inducing increased bone resorption through stimulation of osteoclasts. At the same time, if the blood calcium level is too high, it also acts on the osteoblasts, which are responsible for bone formation. In a healthy adult human being who is still capable of procreation, bone resorption and bone formation are in balance. This constant bone remodeling process ensures a healthy musculoskeletal system.
Diseases and ailments
When the balance between osteolysis and osteogenesis (bone formation, also called ossification) is disturbed, damage occurs in the bone structure. Usually, in these disorders, the ratio of bone resorption to bone formation is shifted in favor of bone resorption. In very rare cases, however, the reverse relationship also occurs, with bone formation predominating over bone resorption. This is the case, for example, in genetic osteopetrosis (marble bone disease). The more common cases of general bone loss are due to a variety of causes. Among others, the balance may shift due to decreased activity of osteoblasts, in which case the bone-building process is disturbed. However, due to the normal process of osteolysis, general bone resorption still occurs. Furthermore, osteolysis can be increased in rheumatic processes, hormonal disorders (hyperparathyroidism), osteoporosis or metastases to the bone. Moreover, a general osteoporosis always takes place in old age. Here, bone formation slows down, while at the same time, often due to mineral deficiency as a result of resorption disorders, bone resorption is accelerated. Hormonal causes of osteoporosis are often found in women after menopause.In addition to osteoporosis, osteomalacia can also occur. Whereas in osteoporosis the entire bone substance, which is composed of organic and mineral components, is broken down, in osteomalacia only the mineral component is lost. The cause here is often insufficient calcium incorporation into the bones, due to a lack of vitamin D or disturbed phosphate metabolism. In addition to the disturbance of the balance between bone formation and bone resorption in the context of general bone remodeling, there is also circumscribed osteolysis. Circumscribed osteolysis is always pathological and describes local bone resorption due to local osteoclast overactivity without increased osteoblast activity. This means that it involves the local breakdown of bone substance, which is, however, no longer replenished. Circumscribed osteolysis occurs, among other things, in inflammatory processes, local tumors or metastases on the bone in the case of foreign tumors or constant foreign body contact of the bone. For example, implants, endoprostheses or osteosyntheses (surgical connection of several bones) can lead to osteolysis, so that the prosthesis may eventually become loose. The same applies to dental implants, which can lead to destruction of the jawbone if they are in constant foreign contact. As a result of chronic polyarthritis, bone cysts may appear near the affected joints. These bone cysts are already cavities created by osteolysis. Localized bone marrow suppuration due to bacterial infection can also lead to osteolysis. A very rare disease is the so-called Gorham-Stout syndrome. In this case, complete dissolution of a bone occurs, which then transforms into lymphatic tissue. The cause of this disease is currently unknown.
