The absent bow tie sign represents the loss of the normal appearance of the menisci on parasagittal MRI images, and is suggestive of meniscal injury.
Normally the medal and lateral menisci appear as low signal bow-tie-shaped structures between the femoral condyles and tibial plateaux. As the normal meniscus laterally measures 9 - 12mm in width it should be seen on a number of adjacent images (2 - 3 images if imaged with 3mm thick slices with a 1mm skip).
An absent bow tie sign is therefore when less than two sagittal slices demonstrate normal body of meniscus, and is indicative of meniscal pathology, including:
- bucket handle tear with displacement (often with a double PCL sign)
- flap tear with displacement (i.e. bucket-handle, but the handle at one end has come loose)
- free fragment with displacement (i.e. bucket-handle, but the handle at both ends has come loose)
Other meniscal changes can however appear similar, including:
- prior resection of meniscal tear
- degenerative maceration of the inner edge of the meniscus
- small patients (children / small adults)
There presence of too many bow-ties implies a discoid meniscus.