What Knee Cyst is Visible on X-ray?
Ganglion cysts are the primary knee cysts that can be visible on plain radiographs when they contain calcifications, though most knee cysts including Baker's cysts, meniscal cysts, and ganglion cysts are typically NOT visible on standard x-rays and require ultrasound or MRI for diagnosis. 1, 2, 3
Understanding X-ray Visibility of Knee Cysts
The critical distinction is that most knee cysts are radiolucent (not visible) on plain radiographs because they contain fluid without calcium deposits. 1
Ganglion Cysts with Calcification
- Ganglion cysts may occasionally be visible on x-ray if they contain calcifications, appearing as rounded, amorphous opacities adjacent to bony structures 3
- These calcifications appear as hyperechoic areas with posterior acoustic shadowing on ultrasound 3
- Ganglion cysts can occur in various locations around the knee including the infrapatellar fat pad, cruciate ligaments, and superolateral patellar region 2, 4, 5, 6
Why Most Knee Cysts Are NOT X-ray Visible
Baker's cysts (popliteal cysts) are the most common knee cysts but are not visible on plain radiographs because they are fluid-filled without calcification 1
Meniscal cysts similarly contain synovial fluid and require MRI or ultrasound for detection 5, 7
Recommended Diagnostic Approach
First-Line Imaging
High-frequency ultrasound (≥10 MHz) is the first-line imaging modality for evaluating suspected knee cysts, as it can:
- Detect even small fluid collections missed on x-ray 1, 8
- Identify calcifications within cysts that may be visible on radiographs 3, 8
- Characterize cyst location and relationship to surrounding structures 2, 8
- Diagnose Baker's cysts by visualizing the comma-shaped extension between the medial gastrocnemius and semimembranosus tendon 1, 8
When to Use Plain Radiographs
- Plain radiographs can detect larger calcifications within bursae or ganglion cysts but are less sensitive than ultrasound for smaller deposits 3
- X-rays are useful as confirmatory imaging when calcifications are suspected 3
Advanced Imaging
MRI without contrast is appropriate when ultrasound is inconclusive or when evaluating:
- Ganglion cysts in atypical locations 2
- Associated joint disorders 2
- Meniscal cysts and their relationship to meniscal tears 5, 7
- Cruciate ligament ganglion cysts 5, 7
Critical Clinical Pitfalls
- Never assume a cyst is absent based on negative x-ray alone - most knee cysts require ultrasound or MRI for detection 1
- Failing to distinguish between tendon calcification and bursal calcification on x-ray can lead to incorrect treatment approaches 3
- Overlooking small calcifications that may be missed on plain radiographs but are clearly visible on ultrasound 3, 8
- When evaluating for Baker's cyst, failure to identify the comma-shaped extension on ultrasound can lead to misdiagnosis 1, 8