Implant — Radial Folds
A radial fold is an infolding of the intact elastomer shell of a breast implant. No silicone is present interposed between the directly apposed infolded layers of the shell. This is a normal finding and does NOT indicate intracapsular rupture. Radial folds may be present in both silicone and saline implants.
Radial folds are classified under intracapsular silicone implant findings in the BI-RADS v2025 MRI lexicon (Section J.4.a) and are explicitly labeled as a normal variant.
Definition
- Mechanism: The implant shell buckles inward, creating a V- or U-shaped invagination
- Key feature: The two layers of infolded shell are directly apposed — no silicone separates them
- Occurrence: Common in both prepectoral and retropectoral implants
- Implant types: Seen in silicone and saline implants alike
- Clinical significance: None — entirely benign, no follow-up required
Imaging Appearance
Silicone-Specific Sequences (Fat- and Water-Suppressed)
- Hypointense linear infolding projecting into the bright silicone signal
- The two shell walls are seen as a single dark line or closely apposed parallel dark lines
- No silicone signal between the infolded layers
- Best visualized on axial and sagittal silicone-specific images
T2-Weighted (Non-Fat-Suppressed, Silicone-Suppressed)
- Dark line(s) within the implant on silicone-suppressed T2W sequences
- Surrounding silicone is suppressed, making the fold less conspicuous
- Useful as a complementary sequence to confirm no silicone leak into the fold
Key Imaging Features
- Folds originate from the implant periphery and extend inward
- May be single or multiple
- Can change in appearance with patient positioning and compression
- Typically thin and smooth, without nodularity along the fold
Radial Fold vs Signs of Rupture — Critical Distinction
| Feature | Radial Fold (Normal) | Keyhole Sign (Rupture) | Subcapsular Line (Rupture) | Linguine Sign (Rupture) |
|---|---|---|---|---|
| Shell integrity | Intact | Disrupted | Disrupted | Extensively collapsed |
| Silicone between layers | No | Yes — fills the fold | Yes — between shell and capsule | Shell fragments float in silicone |
| Appearance | Thin dark line, V/U-shaped | Teardrop/keyhole of bright signal in fold | Dark line paralleling capsule | Multiple wavy dark lines |
| Diagnosis | Normal variant | Intracapsular rupture | Intracapsular rupture | Intracapsular rupture |
| Management | None | Surgical consultation | Surgical consultation | Surgical consultation |
Board Pearl
Radial folds are normal — do not confuse them with the keyhole sign of rupture. The critical distinction: in a radial fold, the two layers of shell are directly apposed with no silicone between them. In a keyhole sign, silicone extends into the potential space created by the fold. If you see bright signal within a fold, it is no longer a simple radial fold — it is a keyhole sign indicating intracapsular rupture.
Relationship to the Keyhole Sign
The keyhole sign (also called the teardrop sign) represents silicone that has leaked through a break in the shell into the potential space created by the apposed walls of a radial fold. In other words, a keyhole sign is a radial fold that has been infiltrated by silicone due to shell rupture. Understanding radial folds is essential to recognizing when one has been converted into a keyhole sign.
Board Pearl
Think of the keyhole sign as a “radial fold gone wrong” — the fold itself is a normal anatomic feature, but when silicone fills the space between the apposed shell walls, it transforms from a normal variant into a sign of intracapsular rupture.
Differential Diagnosis
| Finding | Key Distinguishing Feature |
|---|---|
| Radial fold (normal) | Thin dark line, no silicone between apposed layers |
| Implant — Keyhole Sign (rupture) | Silicone signal (bright) fills the fold — teardrop/keyhole shape |
| Implant — Subcapsular Line Sign (rupture) | Dark line of shell paralleling the fibrous capsule peripherally |
| Double-lumen implant inner shell | Concentric inner shell — correlate with implant history/type |
| Artifact | Chemical shift or susceptibility artifact at shell — does not follow fold morphology |
Pitfalls
- Overcalling rupture: The most common error — misinterpreting a normal radial fold as a sign of rupture. Always look for silicone signal between the layers before diagnosing rupture
- Partial volume averaging: On thick-slice images, a radial fold may appear to contain silicone signal due to volume averaging. Use thin-section silicone-specific sequences to resolve
- Double-lumen implants: The inner shell of a double-lumen implant can mimic intracapsular findings. Always review the patient’s surgical history and implant type before interpretation
- Positional change: Radial folds can shift with patient repositioning — if a “fold” is seen in one plane but not orthogonal views, consider artifact
- Multiple folds mimicking linguine sign: Several radial folds in close proximity may superficially resemble the linguine sign, but radial folds originate from the periphery and maintain shell contour, whereas the linguine sign shows free-floating collapsed shell fragments
Board Pearl
When evaluating implants on MRI, always use both silicone-specific and silicone-suppressed T2W sequences. Radial folds are best seen on silicone-specific sequences as dark lines in bright silicone. The silicone-suppressed sequence helps confirm that no silicone has leaked outside the shell.
Clinical Significance and Management
- No clinical significance — radial folds are a normal variant requiring no intervention
- No follow-up needed for isolated radial folds
- Report as: “Radial fold(s) — normal finding” to avoid clinical confusion
- Important to document explicitly so that future comparisons do not misinterpret a stable fold as a new finding
Reporting
When describing radial folds in a structured BI-RADS MRI report:
- List under Intracapsular silicone findings: “Radial folds (normal variant)”
- Confirm implant is intact if no other signs of rupture are present
- Note the number and location if prominent, for future comparison
Related
- Implants and Augmentation
- Implant — Keyhole Sign
- Implant — Subcapsular Line Sign
- Implant — Linguine Sign
- Implant — Water Droplets
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Enrichment summary: expanded from 33 lines to ~105 lines. Added:
- **Imaging Appearance** across silicone-specific and T2W sequences
- **Comparison table** (radial fold vs keyhole vs subcapsular line vs linguine)
- **DDx table** with 5 entities and distinguishing features
- **5 Pitfalls** (overcalling rupture, partial volume, double-lumen, positional change, multiple folds)
- **3 Board Pearls** (fold vs keyhole distinction, keyhole as "fold gone wrong", dual-sequence technique)
- **Relationship to Keyhole Sign** section explaining the anatomic continuum
- **Reporting** guidance for structured BI-RADS reports
- Cross-links to all 4 related implant descriptor pages