Distinguishing Subchondral Cysts from Erosions on Imaging
The key distinction is that erosions show loss of the dark subchondral cortical line with hypointense signal on T1-weighted MRI compared to adjacent bone marrow, while subchondral cysts maintain an intact or partially intact cortical margin and typically appear well-defined with variable signal characteristics.
Critical Imaging Features on MRI
Erosions 1
An erosion is definitively characterized by:
- Complete full-thickness loss of the normal dark subchondral cortical line at its expected anatomic location
- Hypointense (dark) signal on T1-weighted non-fat-suppressed sequences compared to the surrounding bright normal bone marrow
- Represents actual cortical and trabecular bone loss
- Defect in the subchondral bone itself
Subchondral Cysts 2
Subchondral cysts demonstrate:
- Preserved or only partially disrupted cortical margin (the dark cortical line may be thinned but not completely absent)
- Well-defined, rounded or oval lucent lesions
- Variable signal intensity depending on cyst contents (fluid vs mucoid material)
- Often associated with surrounding sclerosis (hypointense rim on all MRI sequences)
Optimal Imaging Protocol
T1-weighted non-fat-suppressed sequences are essential for distinguishing these lesions 1. This sequence best visualizes:
- The integrity of the subchondral cortical line (appears dark/hypointense)
- Normal bone marrow (appears bright/hyperintense)
- Structural damage patterns
CT without contrast is highly effective for visualizing subchondral cysts and assessing cortical integrity 2, with specificity of 99% for detecting bone defects.
Key Distinguishing Algorithm
Assess the subchondral cortical line on T1-weighted images:
- Completely absent = erosion
- Intact or partially thinned = likely cyst
Evaluate signal characteristics on T1-weighted sequences:
- Hypointense relative to marrow = erosion
- Variable signal (often similar to fluid) with defined margins = cyst
Look for associated features:
- Erosions: May show bone marrow edema on STIR/fluid-sensitive sequences, irregular margins
- Cysts: Often have sclerotic rim (hypointense on all sequences), smooth margins, may communicate with joint space via small fissure 3
Common Pitfalls to Avoid
Fat metaplasia in an erosion cavity ("backfill") can mimic other lesions 1. This shows:
- Hyperintense signal on T1-weighted images (like fat)
- Complete loss of the subchondral cortical line
- Irregular hypointense band of sclerosis at the margin (representing the original erosion boundary)
- This represents a healed erosion, not an active one
Large parameniscal cysts can cause erosions 4. When cysts are significantly enlarged (mean 38mm vs 12mm for non-erosive cysts), they may create secondary erosive changes in adjacent bone.
Cyst formation can follow erosion enlargement 5. In inflammatory arthritis, cysts frequently develop as erosions enlarge and may eventually replace the erosion entirely, creating diagnostic confusion.
Clinical Context Matters
In inflammatory arthropathies (rheumatoid arthritis, spondyloarthritis): Erosions are the primary pathologic feature indicating active bone destruction 1, 6.
In degenerative joint disease: Subchondral cysts are characteristic findings along with osteophytes and sclerosis, while erosions are typically absent 7.