Bone erosions, or simply, erosions, refer to focal bony defects in the subchondral bone plate or cortical bone and the adjacent trabecular bone. Unlike bone cysts, the cortical or subchondral bone is also affected. They are a feature seen in inflammatory arthritides and erosive osteoarthritis and can also occur in crystal deposition disease, metabolic, autoimmune, or other inflammatory disorders or tumours.
Contrary to scalloping, erosions are characterised by a more active rapid bone loss 1.
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Pathology
Bone erosions are bony defects developing as a result of excessive local bone resorption and inadequate bone formation 1,2. They are commonly associated with synovitis and osteitis 1.
Aetiology
Bone erosions can arise from a variety of causes which include the following 1-3:
septic arthritis (early stage of medullary involvement and osteomyelitis)
Location
Bone erosions are typically described in articular (central or marginal) and periarticular (juxta-articular) locations and can affect the subchondral bone plate or the cortical bone. This might happen in central subchondral locations of joints usually covered by articular cartilage, in the bare areas within the joints or outside of the joints at the entheses or other locations (e.g. tumour, metabolic).
Subtypes
The following types of bone erosions are described 3:
subchondral or central bone erosions: typically seen in osteoarthritis
marginal erosions: a typical feature of rheumatoid arthritis, seen in the bare area of joints
juxta-articular erosions: a typical feature in gout, seen outside the joint capsule
erosions at the entheses: a typical feature of spondyloarthritis
Radiographic features
Plain radiograph
Bony erosions are characterised by small bony lucencies or breaks in the subchondral or cortical bony surface often associated with alterations of the adjacent trabecular bone 4.
Ultrasound
Ultrasound has a better capability than radiographs in the detection of bone erosions 3-5, which can be nicely depicted as a bony surface discontinuity or defect visible in two different planes 3,6.
CT
CT allows for a reliable evaluation of cortical bone and subchondral bone plate in multiple different planes at various locations and thus the detection of bony erosions.
MRI
Bone erosions are characterised by a loss of the dark signal or disruption of the subchondral bone plate or the cortical bone.
They can be readily visualised on MRI, which is considered a very sensitive technique for their detection 2, as well as visualising joints, which are hardly or not accessible by ultrasound. Especially high-resolution thin-slice sequences show a high sensitivity, which is considered comparable or superior to CT 7-9.
T1: variable
T2: hyperintense
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