Patellar tendinitis (jumper’s knee) is demonstrated by MRI as an area of edema within the patellar ligament (i.e., tendon) at its patellar (Fig. 6-28) or tibial tuberosity attachment. There is also associated edema in the adjacent subcutaneous fat or the infrapatellar fat pad.
FIGURE 6-28. Sagittal T2-weighted fat suppressed image of patellar tendinitis. The arrow points to the increased signal intensity within the proximal tendon fibers and the adjacent infrapatellar fat pad. .
Ischemic necrosis about the knee most commonly involves the weight-bearing surface of the medial femoral condyle, and its MRI findings are as described for the hip.
Osteochondritis dissecans occurs mainly in adolescents and involves a partial or total separation of a segment of articular cartilage and subchondral bone from the underlying bone (39). It commonly involves the intercondylar portion of the medial femoral condyle articular surface. It is visualized on T1-weighted MR images as a low–signal-intensity region in the subchondral bone with or without disruption of the overlying articular cartilage (Fig. 6-29A,B). If the involved osteochondral segment becomes completely separated from the underlying bone, it becomes an intra-articular loose body. The role of MRI in osteochondritis dissecans is mainly to determine the stability of the fragment, because the treatment hinges on that.
FIGURE 6-29. Osteochondral lesion (previously osteochondritis dissecans). T2-weighted fat suppressed (A) and PD (B) sequences. A: There is a hyperintense T2 signal at the interface of the OC lesion and the adjacent cortex compatible with an unstable fragment. B: The lesion is well demarcated by a hypointense rim (short arrows).
Chondromalacia patella can be diagnosed and graded noninvasively by MRI (40). In stage I, the posterior patellar articular cartilage demonstrates local areas of cartilage swelling with decreased signal intensity on both T1- and T2-weighted images. Stage II is characterized by irregularity of the patellar articular cartilage with areas of thinning. Stage III demonstrates complete absence of the articular cartilage with synovial fluid extending through this cartilaginous ulcer to the subchondral bone (Fig. 6-30).
FIGURE 6-30. Grade IV chondromalacia. Sagittal PD sequence with grade IV chondromalacia. There is a full thickness defect (arrow) with subchondral bone sclerosis and early subchondral cyst formation within the proximal patella.
Popliteal (i.e., Baker’s) cysts and other synovial cysts about the knee appear hyperintense on T2-weighted images (Fig. 6-31A–D). They can be visualized on axial, sagittal, or coronal images. Popliteal cysts are usually an enlargement of the semimembranosus-gastrocnemius bursa, which is located between the tendon of insertion of the semimembranosus and the tendon of origin of the medial head of the gastrocnemius. Popliteal cysts may communicate with the knee joint and therefore may be caused by chronic knee joint pathology that produces effusion. A previously undescribed bursa is now known to be consistently present be tween the tibial collateral ligament and a major slip of the semimembranosus tendon that extends beneath it, and may serve to clarify many cases of previously unexplained medial knee pain (46). Inflammation of this bursa is well demonstrated by MRI (Fig. 6-32A,B).
FIGURE 6-31. Baker’s cyst. A: A T1-weighted axial MRI demonstrates a hypointense Baker’s cyst (arrowheads) in the interval between the semimembranosus (SM) and the medial head of the gastrocnemius (MG). T1-weighted (B) and T2-weighted (C) sagittal MR images through Baker’s cyst (arrowheads). Note that the hypointense fluid in the cyst in the T1-weighted image becomes hyperintense on the T2-weighted image. D: A coronal T1-weighted image locates the cyst between the SM and the MG.