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

Friday, March 13, 2009

Calf Pain- is it my Achilles Tendon?

Achilles was a mythical Greek warrior, most famous for his role in the Trojan war. A mighty fighter, Achilles killed the Trojan Hector, as reenacted in this scene from the movie "Troy".

Achilles was the son of the nymph Thetis and Peleus, the king of the Myrmidons. When Achilles was born, his mother tried to make him immortal by dipping him in the river Styx. However, he was left vulnerable at the part of the body she held him by, his heel

The Achilles tendon is aptly named- a complete tear of this structure is a devastating injury, often career ending for many athletes.

The Achilles tendon (also known as the triceps surae), is formed by the confluence of the medial gastrocnemius, lateral gastrocnemius, and soleus muscles. Achilles tendinopathy and strains are a common problem, and can be
clinically difficult to distinguish from tears of the distal gastrocnemius muscle, particularly the medial head of the gastrocnemius muscle.

Another tendon that is in close proximity to the Achilles is a thin, rope-like tendon known as the plantaris. This tendon typically tears in the upper leg, and the examiner can be hard-pressed to distinguish between a tear of the medial head of the gastrocnemius muscle, a high Achilles strain/tear, and a plantaris tear.

In this case 36 year old male felt calf pain. His physician sent him for an MRI, with a prescription stating, "MRI right distal leg, possible Achilles tear".

The MRI technologist read the prescription, and (appropriately) imaged the distal leg:

Note that the Achilles tendon is absolutely pristine on this T1-weighted image.

It is at this point that the MRI technologist proved himself a master of his craft. He had interviewed the patient, and noted that the pain was actually in the mid calf, more than the distal calf. The technologist then slid the patient out of the MRI magnet and switched coils (a coil is a device used to pick up the MRI signal), and reimaged the patient. All this takes extra thought, extra time and extra care, but this is what separates the good MRI technologist from the excellent technologist.

This coronal STIR was then obtained, showing fluid in the upper calf, in a location and morphology highly characteristic of a tear of the plantaris tendon (Helms et. al, radiology 195:201-203, 1995):

The patient's fear of an Achilles tendon tear was assuaged, and the much more sanguine diagnosis of a plantaris tear was given.

The plantaris muscle originates from the lateral aspect of the distal femur, courses down the calf, and then inserts on to the calcaneus bone of the foot. Plantaris tendon tears are treated conservatively, and have an excellent prognosis, unlike Achilles tears, which have a much more prolonged recovery, and may need to be addressed surgically.

Vic David MD