What causes the empty notch sign on MRI of the knee?
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Answer:
The empty notch sign is the absence of the normal low-signal anterior cruciate ligament fibres in the lateral part of the intercondylar notch on coronal or axial MRI, so that the notch appears empty and filled only with fluid or oedematous soft tissue. It is one of the secondary appearances of a complete anterior cruciate ligament tear and is most useful in the acute knee, where haemarthrosis and oedema obscure the ligament on sagittal images. The primary signs remain the more reliable: discontinuity of the fibres, an abnormally horizontal ligament with a Blumensaat angle less than the slope of the intercondylar roof, and increased signal within the ligament. Secondary signs, which should be assembled together, are anterior tibial translation of more than 5 to 7 mm, the deep lateral femoral notch sign, the pivot-shift bone contusion pattern of the posterolateral tibial plateau and mid lateral femoral condyle, a Segond fracture, buckling of the posterior cruciate ligament and an uncovered posterior horn of the lateral meniscus. The differentials for an apparently empty notch are a chronic tear in which the ligament has resorbed, previous reconstruction, a mucoid degenerated ligament and partial volume artefact from an oblique acquisition, which is why a dedicated oblique coronal or sagittal plane along the ligament is helpful when the standard planes are equivocal.
Why is it called so?
The intercondylar notch, which normally contains the ligament, is empty on the image.
Pathophysiology
The anterior cruciate ligament runs from the posteromedial aspect of the lateral femoral condyle to the anterior intercondylar area of the tibia, occupying the lateral part of the notch. In a pivot-shift injury the tibia translates anteriorly and rotates internally on a flexed knee, and the ligament fails, most often in its proximal third or at the femoral attachment. The torn fibres retract, and the resulting gap fills with haemarthrosis, so the region that normally contains a taut low-signal band contains only fluid. The same displacement drives the lateral femoral condyle against the posterolateral tibia, producing the kissing contusions, and stretches the anterolateral structures, producing the Segond fracture, which is why these findings cluster.
Alternative names: Empty lateral notch; absent ACL sign
Other associated named signs: Segond fracture, the deep lateral femoral notch sign and kissing contusions
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