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Geodes, also known as subchondral cysts or pseudocysts, are well-defined lytic lesions at the periarticular surfaces.
Geode, meaning a crystal-lined hollow rock, may be the preferred term over subchondral cyst, meaning epithelial-lined fluid-filled lesion as these two latter features are absent in these lesions, however, it should be noted that both are widely used in the literature 4,7.
Geodes themselves are generally not considered symptomatic but given they are associated with joint pathology they are often associated with non-specific symptoms such as pain, swelling or reduced range of motion 9.
Geodes are seen in a small group of disorders including:
There are two main theories of geode pathogenesis 4,7,9:
synovial fluid is forced through the hyaline cartilage into the subchondral bone due to elevated intra-articular pressure, resulting in a cystic collection of joint fluid
cystic necrosis development post subchondral fracture
Location specific lesions
Usually seen in the setting of degenerative joint disease, geodes are periarticular lesions that are round-to-oval with thin sclerotic margins 8,9.
Well-defined periarticular lesions with internal signal intensity as follows 8,9:
T1: low signal; can be high signal if proteinaceous
PD: low-intermediate signal
T2: high signal
T1 C+ (Gd)
none or thin peripheral enhancement
internal enhancement can be present due to fibrous content, enhancing synovial fluid, or contrast diffusion
ill-defined perilesional enhancing bone marrow edema may be present
Treatment and prognosis
They rarely cause problems by themselves but are often misdiagnosed as something more sinister and an unnecessary biopsy of a geode might be performed on the basis of the differential of an epiphyseal lesion.
History and etymology
Geode is a term borrowed from "geology", where it refers to rounded formations in igneous and sedimentary rocks.
Dominant subchondral geodes can be confused with lytic epiphyseal lesions such as 7:
intraosseous ganglion: adjacent degenerative joint disease absent otherwise indistinguishable on imaging 10