Synovial cysts and juxta-articular bone cysts (geodes)

Abstract

D EFINITIONS: Synovial cysts are cavities filled with products of synovial fluid and lined either (1) by true secretory synovial membrane (as in distended bursae and tendon sheaths or in herniations of the joint capsule) or (2) by reactive fibrous tissue or (3) by preexisting anatomical structures such as bone, muscle, or tendon; (2) and (3) imply that these cavities are pseudocysts, since they lack a true epithelial lining. The term Baker’s cyst is today synonymous with type 1 synovial cyst. It describes popliteal cysts’ and cysts related to other joints2 The eponym is not justified, Baker’s descriptions having been antedated by those of Adams,3 Foucher,4 and others (e.g., Billroth5), but it seems unlikely that it will be discarded. Synovial bursae are spaces containing small amounts of synovial fluid and lined by a form of synovial membrane. They are either primary, developing during fetal life, or adventitious, developing after birth in response to local pressure or repeated friction. Juxta-articular bone cysts are pathological cavities in the bone connected to the joint, filled with fibrin or other debris from the synovial fluid. The terms geode and microgeode are preferable to bone cyst because of the lack of a defined epithelial lining and because these differ from solitary bone cysts. Geode derives from the Greek words ge (earth) and eidos (form, i.e., spherical) and is used as a geological term for small gas-filled cavities in rocks. Acute synovial rupture is the bursting of a distended joint capsule or communicating cyst or bursa allowing leakage of joint fluid directly into the surrounding tissue. It may be followed by healing of the site of rupture and either absorption or encapsulation of the extravasated fluid. Chronic joint rupture is the persistent leakage of joint fluid from the joint capsule or communicating bursa directly to the surrounding tissue spaces and is thought to cause some instances of persistent periarticular edema.