A meniscal tear is signal REACHING AN ARTICULAR SURFACE on two consecutive images — and the root and ramp are where tears are missed.
Orient first
- The meniscus is normally uniformly LOW signal. A tear is intrameniscal signal that CONTACTS an articular surface. Intrasubstance signal that does not reach a surface is degeneration, not a tear, and calling it one leads to an unnecessary arthroscopy.
- The TWO-SLICE-TOUCH rule reduces false positives: the abnormal signal should be present on two consecutive images or in two planes.
- The MENISCAL ROOT is the attachment that keeps the meniscus in place. A root tear behaves biomechanically like a total meniscectomy and is missed when the coronal images are not examined at the tibial attachment.
- MENISCAL EXTRUSION beyond the tibial margin is the sign that accompanies a root tear and is visible on coronal images.
- For the ACL, the SECONDARY signs are often more reliable than the ligament itself: anterior tibial translation, a deepened lateral femoral sulcus, and the pivot-shift bone-marrow oedema pattern in the lateral femoral condyle and posterolateral tibia.
Acquire the study
- Fluid-sensitive fat-suppressed sequences in all three planes plus a non-fat-suppressed T1 or proton density for anatomy and marrow.
- Thin slices through the menisci; sagittal images are the workhorse and coronal images are what show the roots and extrusion.
- Assess the ACL on an oblique sagittal aligned to the ligament, or trace it across all three planes — a straight sagittal cuts it obliquely.
- Confirm which sequence you are on before judging any signal — see the MRI sequence primer.
The manoeuvre
- Scroll each meniscus from anterior to posterior horn and check for signal reaching the superior or inferior articular surface.
- Apply the two-slice-touch rule before calling a tear.
- Examine the meniscal ROOTS on coronal images at the tibial attachment.
- Measure meniscal EXTRUSION on a coronal image at the level of the medial collateral ligament.
- Assess the ACL for continuity, fibre orientation and signal, and look specifically at the femoral attachment.
- Look for the SECONDARY signs of ACL rupture, including the pivot-shift marrow oedema pattern.
- Assess the PCL, the collateral ligaments and the posterolateral corner.
- Assess the cartilage and the subchondral bone, and look for a displaced fragment in the notch or gutters.
What confirms it
- A meniscal tear is surface-reaching signal on two consecutive images or in two planes.
- ACL rupture is discontinuity or an abnormal orientation of the fibres, supported by the secondary signs.
What licenses you to exclude it
- An intact, uniformly low-signal meniscus with no surface-reaching signal on a good-quality study excludes a tear.
- ⚠️ A previously operated meniscus cannot be assessed by the same rule — surgical granulation tissue produces surface-reaching signal, and MR arthrography or CT arthrography is the study for a re-tear.
- Absence of secondary signs does not exclude a chronic ACL tear, where marrow oedema has long resolved.
The classic misread
- Calling intrasubstance degeneration a tear.
- Missing a root tear by not looking at the coronal tibial attachment.
- Assessing a post-operative meniscus by the standard criteria.
- Judging the ACL on a single straight sagittal image.