Knee Imaging: Systematic Interpretation

The MRI criterion for a definite meniscal tear is abnormal signal within the meniscus that contacts the articular surface on at least two consecutive images in one plane. The signal should be intermediate to high on proton density sequences - the normal meniscus is uniformly dark on all sequences. I grade meniscal signal as: Grade 1 is intrameniscal signal not reaching the surface (degeneration, not a tear); Grade 2 is linear signal reaching one surface but not through; Grade 3 is signal definitely contacting the articular surface, which represents a tear. Common tear patterns include: vertical/longitudinal tears that run parallel to the long axis and may displace as bucket handle tears; horizontal tears that run parallel to the tibial surface, creating superior and inferior flaps, typically degenerative; radial tears that extend perpendicular to the free edge, disrupting the hoop stress mechanism; complex tears with multiple components; and root tears at the meniscal attachment which are functionally equivalent to meniscectomy.

Primary signs of ACL tear on MRI include: complete discontinuity of the ligament fibers, abnormal orientation where the ACL is not parallel to the Blumensaat line (roof of the intercondylar notch), diffuse T2 hyperintensity throughout the ligament in acute tears, non-visualization of the ligament, and an irregular wavy contour rather than the normal taut appearance. The ACL is best assessed on sagittal images oriented parallel to the intercondylar notch. Secondary signs are equally important and include: the characteristic bone bruise pattern with contusions of the lateral femoral condyle and posterolateral tibial plateau from the pivot shift mechanism; anterior tibial translation greater than 7mm; uncovering of the posterior horn of the lateral meniscus; buckling of the PCL; the Segond fracture which is a lateral tibial avulsion fracture pathognomonic of ACL injury; and the deep lateral femoral notch sign (sulcus greater than 2mm). These secondary signs can suggest ACL injury even when the ligament itself is difficult to visualize.

For knee osteoarthritis assessment, I request weight-bearing views as they are essential - non-weight-bearing films significantly underestimate joint space narrowing. My standard series includes: weight-bearing AP of both knees on a single cassette for comparison, lateral view in 30 degrees flexion, and importantly the Rosenberg view which is a PA view in 45 degrees flexion, weight-bearing. The Rosenberg view is the most sensitive for early medial compartment OA as it loads the posterior aspect of the joint where cartilage loss often begins. I also include a sunrise or Merchant view to assess the patellofemoral joint. On these images, I assess the four cardinal features of OA: joint space narrowing (the most important feature, measure in mm), osteophytes (marginal spurs), subchondral sclerosis (increased bone density), and subchondral cysts. I also assess alignment - varus deformity typically accompanies medial compartment OA, valgus with lateral OA. I grade severity using the Kellgren-Lawrence classification from 0 to 4, where grade 4 represents bone-on-bone contact with large osteophytes.