Osteoarthritis (deforming arthrosis) is a degenerative dystrophic disease, in which the articular cartilage and subchondral bone are primarily affected with the gradual involvement of surrounding tissues in the pathological process. Osteochondrosis proliferations - osteophytes - are formed along the edges of the articular cartilage; subchondral sclerosis of the bone and its cystoid degeneration are increasing.
Osteoarthritis is the most common joint disease. The development of the disease is typical after 40-50 years, but osteoarthritis may occur even in adolescence and childhood. The older the age group, the more likely is the development of osteoarthritis. Among people of 50 years of age, its frequency reaches 27%, and over 70 years of age - already 97%. This results from the fact that degenerative dystrophic changes in osteoarthritis are an involutive process caused by "chronic joint overload".
Most often and most severely, deforming arthrosis affects the joints of the lower extremities, primarily the hip and knee. The first metatarsophalangeal joint, interphalangeal joints of the hands are affected relatively often. An increase in the osteoarthritis occurrence of other localization may be associated with professional or sports activities. Initially, one joint is affected. Subsequently, other joints may be involved in the process, more often the symmetrical joints. Cases of polyosteoarthrosis are not uncommon.
Etiology
The disease is polyethological. The development of the process is associated with a number of factors. The value of the mechanical factor is the most proven. It may include macro- or microtraumatization with subsequent development of degenerative changes or joint statics impairment, which leads to an increase in the load on certain areas of the articular surface and destruction of the articular cartilage. An important role in the development of osteoarthritis is assigned to the condition of the subchondral bone, changes in which lead to trophic disorders in the articular cartilage. The importance of angiotrophic factor is indisputable. Microcirculation disorders, in particular venous stasis, may cause or contribute to the development of degenerative cartilage changes. Change in the cartilage tissue metabolism in case of deforming arthrosis is considered to be proven. Already with the initial degenerative changes in the cartilage, the chondroitin sulfate level decreases, the sulfo-polysaccharide level in the synovial fluid decreases sharply.
Change in the synovial fluid structure and deterioration of its "lubricating" and shock-absorbing functions is of great importance. Involutive tissue changes, neurogenic disorders, heredity, chronic infection, endocrine shifts, obesity, etc., should be pointed out among other reasons contributing to the osteoarthritis development.
In cases where the loads exceed the norm due to professional, sports, or household activities, these loads may become prohibitive for a previously unchanged ("healthy") joint. This indicates primary osteoarthritis. However, the overload may also occur with a physiological (normal) load, which the articular cartilage "cannot handle", as it is modified by a previous disease or damage, dysplasia or a statics impairment. Osteoarthritis, which develops with an underlying previous disease or injury, is called secondary osteoarthritis.
Thus, оsteoarthritis develops when chronic mechanical stress becomes over-extreme for a particular joint.
Pathogenesis and pathomorphology
Changes in articular cartilage. At the initial stage of osteoarthritis, articular cartilage degeneration is a priority change. Its changes are associated with dysmetabolism of its main substance with proteoglycan deficiency. As a result of proteoglycan depolymerization, protein-polysaccharide complexes with a lower molecular mass are formed, easily leaving the cartilage. As a result of the proteoglycan loss, the cartilage becomes less hydrophilic.
Subchondral bone. The role of the subchondral bone in the osteoarthritis development has long been underestimated. Today, many authors believe that its changes in particular may serve as the primary factor, initiating the cartilage tissue degradation due to penetration of proinflammatory cytokines and growth factors produced in the subchondral bone.
Synovial fluid. Due to its elasticity and viscosity, synovial fluid performs the following main functions:
- shock absorption, damping the mechanical load on the cartilage and bone;
- lubrication, providing optimal friction characteristics of the joint;
- protection against inflammatory mediators.
In case of osteoarthritis, the concentration and molecular mass of hyaluronic acid in the synovial fluid decrease, which causes a decrease in all its specified protective functions, leading to further reduction of the cartilage ability to endure mechanical stress. Articular cartilage is thinning, cracking. Its surface fades, becomes uneven, rough.
"Joint mouse". The desquamated cartilage fragments become loose bodies in the joint cavity, forming so-called "joint mice". They relocate into the joint cavity, interfere with movements, injure cartilage, on which ulcerative defects with bone exposure develop over time.
Subchondral osteosclerosis develops with an increase in the load on the subchondral bone, as well as due to the loss of shock absorbing properties by cartilage and synovial fluid. Bone degeneration entails an increase in intraosseous pressure due to venous stasis. This results not only in cartilage trophism impairment on bone side, but also in bone tissue ischemia. As a result of local ischemia, oval defects (cysts) of cancellous bone tissue develop, filled with sluggish, poorly vascularized granulation tissue. Superficially located cysts may rupture into the joint cavity - erosive osteoarthritis. Distortion of the trabecular structure leads to a decrease in bone strength. In the most loaded sections, micro-cracks occur with depression, indentation of the articular surface - impression. Incongruence of the articular surfaces develops, which is enhanced by debilitation of the ligamentous apparatus and atrophy of the surrounding muscles. This may result in distortions of the limb or segment axis.