Friday, March 27, 2009

Dislocated Sesamoid Bone

What is the largest sesamoid bone in the body?

Medical and biologic mavens will regard this question as a "gimme"— the largest sesamoid bone is the patella. A sesamoid bone is a bone embedded within a tendon. These bones often resemble a sesame seed, hence the name.

Sesamoid bones are said to increase the moment arm of the tendon in which they are embedded.
The patella can sometimes dislocate from its normal position in the trochlear groove of the femur. This dislocation event is typically lateral in direction, and can be confused clinically with a tear of the anterior cruciate ligament. The MRI findings of lateral patellar dislocation events have been described in several excellent articles. These findings can include osteochondral injuries to the medial patella and lateral femoral condyle, tears of the medial patellofemoral ligament, and bone bruises of the inferomedial patella and anterolateral femoral condyle.

In this case, a forty-year old male sustained a knee injury while doing martial arts 1 week ago. Axial T2-weighted image with fat saturation identifies bone bruises (red arrows) in the medial patella and anterolateral femoral condyle.

An axial intermediate-weighted image depicts a large osteochondral defect in the patella (red arrows). Note the undamaged patellar cartilage (green arrows) in the lateral patella:

Interestingly, there is a corresponding large chondral defect (red arrows) in the lateral femoral condyle:

This chondral defect is at the posterior margin of the bone bruise, a characteristic location described by Sanders et al. (AJR 187:1332, 2006). Chondral debris (white arrow) is found in the suprapatellar joint recess. There is also a tear of the anterior horn of the lateral meniscus (yellow arrow) in this patient:

An axial T2 weighted image with fat saturation better defines the jagged nature of this chondral lesion:

Thus, shearing cartilage injuries of the lateral femoral condyle can occur in the setting of lateral patellar dislocation events. This lesion is more common in young patients, but can occasionally be seen in older individuals as well, as this case illustrates.

Vic David MD

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