What causes Absent bow-tie sign in the knee on sagittal MRI?
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Answer:
The absent bow-tie sign is caused by loss or displacement of the normal meniscal body such that fewer than the expected two consecutive sagittal MR images show the continuous low‑signal “body” of the meniscus; this most commonly reflects a longitudinal (bucket-handle) meniscal tear with central fragment displacement into the intercondylar notch, but can also result from flap or free fragments, prior partial meniscectomy, or severe degenerative maceration of the inner meniscal substance.
Why is it called so?
On sagittal MR images the intact meniscal body normally appears as two adjacent low‑signal rounded segments resembling a bow tie across consecutive slices; when the expected paired “bow‑tie” appearance is missing (absent), the descriptor “absent bow‑tie sign” succinctly names that loss of the bow‑tie appearance.
Pathophysiology
A longitudinal (vertical) tear through the meniscal body allows the inner (central) fragment to displace away from its normal position between the femoral condyle and tibial plateau; when that displaced fragment translates centrally into the intercondylar notch (or becomes a free/flap fragment), the contiguous bulk of the meniscal body is absent on the usual two or more adjacent sagittal slices, producing the imaging finding of fewer than two consecutive bow‑tie shaped segments. Reduced meniscal width (small patient, discoid variants, or prior resection) or macerative degeneration can mimic this appearance by physically removing or thinning the central meniscal substance that normally creates the bow‑tie silhouette.
Pathophysiology: A longitudinal tear permits central displacement of the meniscal fragment (bucket‑handle), removing the contiguous meniscal body from its expected sagittal positions and so eliminating the pair of bow‑tie images across adjacent slices; alternatively, resection or degeneration reduces meniscal bulk and yields the same absent appearance.
Alternative names: Absent bow tie sign (no widely used alternate eponym).
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