Implant — Keyhole Sign (Teardrop Sign)
The keyhole sign (also called the teardrop sign) indicates intracapsular rupture of a silicone breast implant. It represents silicone that has leaked through a tear in the elastomer shell and entered the potential space created by the apposed walls of a radial fold. On MRI, silicone signal intensity is seen both inside and outside the hypointense loop representing the collapsed implant envelope at the site of the fold.
Definition
A teardrop- or keyhole-shaped collection of silicone extending through a focal shell defect into the space between the two layers of an existing radial fold. In an intact implant, the two infolded shell layers are directly apposed with no interposed silicone. When the shell ruptures at or near a radial fold, silicone dissects into this potential space, producing the characteristic keyhole or teardrop configuration.
Mechanism
- A radial fold (normal shell infolding) creates a V- or U-shaped invagination where two layers of the elastomer shell lie directly apposed.
- The shell develops a tear at or adjacent to the fold — radial folds are sites of mechanical stress and therefore common rupture locations.
- Silicone gel herniates into the potential space between the apposed shell layers, separating them.
- The result is a focal silicone collection that appears keyhole-shaped on cross-section: a round or teardrop body connected to the main implant lumen through the fold.
Board Pearl
The keyhole sign specifically localizes the site of shell disruption to a radial fold. It represents an early stage of intracapsular rupture — the shell has not yet globally collapsed. Progression leads to the Implant — Subcapsular Line Sign and eventually the Implant — Linguine Sign.
Imaging Appearance
Silicone-Sensitive Sequences (Primary Detection)
- Fat- and water-suppressed silicone-specific sequences are essential for detection
- The silicone collection appears high signal (bright), extending through the low-signal loop of the collapsed shell fold
- The characteristic shape: a round or teardrop silicone focus connected to the implant lumen through the fold, with a surrounding hypointense rim of shell
- Best seen on axial images; confirm on sagittal plane
T2-Weighted (Water-Sensitive)
- Silicone is intermediate to high signal on non-fat-suppressed T2W
- On fat- and silicone-suppressed T2W, the keyhole collection appears as a signal void (silicone is suppressed), which can help confirm silicone content
- Less conspicuous than on silicone-specific sequences
T1-Weighted
- Limited utility for detecting the keyhole sign
- Silicone is intermediate signal intensity; the focal collection may not be distinguishable from adjacent implant contents
Post-Contrast
- Not the primary sequence for implant integrity evaluation
- Breast MRI performed solely to assess implant integrity does not require contrast (BI-RADS v2025)
Board Pearl
Always evaluate keyhole sign on both silicone-bright and silicone-suppressed sequences. On silicone-bright images the collection is bright; on silicone-suppressed T2W it drops out — confirming it is silicone and not a fluid collection or seroma.
Differential Diagnosis
| Finding | Appearance | Key Distinguishing Feature |
|---|---|---|
| Keyhole sign | Teardrop/keyhole silicone at a radial fold | Silicone between the apposed shell layers of a fold |
| Normal radial fold | Dark line extending inward from capsule | Apposed shell layers are directly apposed — no interposed silicone |
| Implant — Subcapsular Line Sign | Dark line parallel to capsule wall | Silicone between collapsed shell and capsule, not within a fold |
| Implant — Water Droplets | Small bright foci within implant | Water signal on water-specific sequences; seen in intact and ruptured implants |
| Gel bleed | Subtle halo of signal outside shell | Microscopic permeation through intact shell; no focal collection |
Board Pearl
The critical distinction between the keyhole sign and a normal radial fold is the presence of silicone signal between the fold layers. A normal fold has two directly apposed dark lines with no intervening bright signal. If you see bright silicone splitting the fold — it is rupture.
Co-Occurring Findings
The keyhole sign is frequently accompanied by the Implant — Subcapsular Line Sign. In the BI-RADS v2025 figure examples, most keyhole sign cases also demonstrate a concurrent subcapsular line, reflecting that once the shell is disrupted at the fold, silicone also migrates along the capsule-shell interface. The presence of both signs together strengthens diagnostic confidence for intracapsular rupture.
Water droplets may also coexist, though these can be seen in both intact and ruptured implants and are therefore not specific for rupture.
Spectrum of Intracapsular Rupture Signs
The keyhole sign represents the earliest point on the continuum of intracapsular rupture, progressing with increasing shell collapse:
- Keyhole sign — focal silicone herniation into a radial fold (earliest, focal)
- Implant — Subcapsular Line Sign — shell separating from capsule (early, more diffuse)
- Implant — Linguine Sign — shell extensively collapsed and folded (advanced)
- Complete shell collapse — shell crumpled at dependent portion (late)
Clinical Significance
- Confirms intracapsular rupture, the most common form of silicone implant failure
- Intracapsular rupture is frequently asymptomatic and an incidental MRI finding
- May progress to extracapsular rupture if the fibrous capsule is also breached, leading to free silicone in breast tissue, siliconomas, granulomatous inflammation, and silicone migration to axillary lymph nodes
- FDA screening recommendation: MRI for silent rupture beginning 5–6 years after placement and every 2–3 years thereafter
- Management varies: asymptomatic intracapsular rupture may be monitored; symptomatic rupture or extracapsular extension typically warrants surgical explantation
Pitfalls
- Mistaking a normal radial fold for the keyhole sign — The most common error. On suboptimal sequences or with partial volume averaging, a radial fold may appear to have silicone between its layers. Always confirm on thin-section silicone-specific sequences (≤3 mm).
- Double-lumen implants — The inner-outer lumen interface at a fold in a double-lumen implant can mimic a keyhole sign. Correlate with implant surgical history and look for a circumferential inner lumen boundary.
- Partial volume with adjacent silicone — On thick-section images, silicone adjacent to a fold may artificially appear to be within the fold. Use thin slices and multiplanar review.
- Overlooking a subtle keyhole — Because it represents early rupture, the sign may be small and focal. Systematically trace every radial fold on silicone-specific sequences.
- Water droplets at a fold — A water droplet near a radial fold should not be confused with the keyhole sign. Water droplets are water signal on water-specific sequences, not silicone signal.
Board Pearl
The keyhole sign may be the only sign of early intracapsular rupture when the shell has not yet globally collapsed. If the subcapsular line and linguine signs are absent, methodically trace every radial fold for interposed silicone — a single positive fold confirms rupture.