Joint Mouse

Joint mouse, mus articularis, is a free body that has separated from the synovial membrane or articular cartilage and more or less freely moves within the joint cavity. Joint mice occur in numbers from one to several tens and even hundreds. They are most commonly found in the knee joint, then in the elbow, less frequently in the shoulder, hip, and wrist joints, and very rarely in the jaw and metacarpophalangeal joints. Their size varies from a lentil seed to a pea to the size of a patella.--From an etiological standpoint, two forms of free bodies should be distinguished: 1) on the basis of former arthropathy, 2) on the basis of trauma. The first group includes joint processes that result in the precipitation of fibrin, which under the influence of movements forms clots and organizes; this group should also include the detachment of hypertrophied and hyperplastic villi of the synovial membrane, the deposition of salts and their impregnation of areas of the synovial membrane, and the separation of pieces of cartilage and osteophytes. In the second group, the cause is trauma, under the influence of which pieces of cartilage and bone detach from the articular surfaces during sharp movements, strains, fractures, and dislocations. A number of authors (Schmieden, Lexer, Payr, etc.) recognize only a purely traumatic origin of free bodies, in contrast to others (Franz König, Hildebrand, Konjetzny, Axhausen), who argue against the primary origin of free bodies depending on trauma. The latter authors believe that the formation of joint mice is preceded by a destructive process in the joint, osteochondritis dissecans (König). Axhausen reduces this destructive process to aseptic epiphyseal necrosis on the basis of white mycotic thrombosis. From a pathoanatomical standpoint, joint mice are distinguished as fibrinous, fibrous, fatty, cartilaginous, and bony. Purely traumatic joint mice consist of normal cartilage with included living bone covered with connective tissue. The nutrition of such joint mice occurs either through the synovial fluid or due to a connection with the synovial membrane from which it has not yet completely separated. In those cases where joint mice form through a destructive process in the articular ends and only slowly separate from their bed, in addition to changes in the cartilage, necrosis of bone tissue is observed, and at the site of their detachment, connective tissue and fibrocartilage develop.--In form, free bodies show great diversity; they may be: 1) similar to grains of rice (rice-shaped bodies, corpuscula oryzoidea), pumpkin or melon seeds; 2) round or flask-shaped; 3) in the form of polyhedral formations resembling gallstones; 4) in the form of shapeless fragments and pieces of cartilage and bone. The clinical picture is not uniform. Often subjective phenomena are expressed extremely weakly, so that the patient may not suspect the presence of joint mice. Sometimes free bodies are found accidentally at autopsy. In other cases, the phenomena are very distinct. The clinical picture depends on whether the free bodies separate from their bed immediately or after some time. In the first case, there is a sudden disruption of joint movements, in the second, the disturbances are minor and usually taken for rheumatic pains or ligament strain. The most characteristic symptom for joint mice that have completely separated or are still attached by a pedicle is joint locking. It occurs during sharp movement, falling, or impact and depends on the entrapment of the joint mouse between the articular ends or between the capsule and the articular end. This entrapment causes the most acute pain, which can disappear as quickly as it appeared after the reduction of the mouse into the free joint cavity through appropriate movements. The described picture is completely similar to joint locking in meniscus rupture. Very often after such locking, acute synovitis with serous effusion develops, which resolves within 1-2 weeks, although not completely. Attacks of entrapment may recur, and then more persistent changes develop in the joint up to villous hyperplasia and deforming arthritis. If there are many bodies in the joint, the entire joint capsule represents a bag filled with characteristic formations, sometimes quite dense and hard, sometimes softer and with more indistinct outlines. In other cases, these bodies produce true crepitus. Zakharov in his case compared the sensation on palpation of the knee joint to palpating a pocket filled with nuts. The diagnosis is usually not difficult, as joint mice can be palpated during their accidental or intentional displacement, and patients sometimes come to the doctor with a ready-made diagnosis. Symptoms of entrapment give reason for careful examination of the joint. If nothing can be palpated, and the free body contains bone tissue or is heavily impregnated with lime salts, then a radioscopic image can resolve the difficulty. This is especially true of formations not yet separated from their bed. When examining knee images, one should not forget about the possibility of the so-called iabella bone-cartilage body behind the articular ends, analogous to a sesamoid bone. When making a diagnosis, damage to the meniscus, hyperplasia of villi should be excluded. The first is characterized by the presence of painful points when pressing with a finger along the line of the joint space medially or laterally from the patella, limitation of extension, weakening of the m. rectus femoris. The second disease is distinguished by less intensity of pain during entrapment, on palpation there is no definite configuration, there is no such easy mobility as with a mouse, and therefore this formation does not disappear from under the hand.--The prognosis is generally satisfactory, since timely removal of free bodies and their bed in osteochondritis dissecans gives a favorable result (60% of favorable results in followed cases, according to Payr).--Treatment is only operative. Anesthesia is best spinal; local does not always give sufficient freedom of action, especially when it is necessary to examine the entire joint. On the elbow, a lateral longitudinal incision above the head of the radius is recommended, on the knee-Payrov's longitudinal. One should not limit oneself to removing the mice, but should examine the place of their origin and not leave altered areas of cartilage and bone in order to avoid the development of deforming arthritis. Joint mice in the so-called dry arthrites are not subject to removal.