Age / Sex : 14 / M

Age / Sex : 14 / Male

Chief complaint : Knee pain

What is your impression?

Two weeks later, you can see the final diagonosis with a brief discussion of this case (Please submit only one answer).

Courtesy : Wan Tae Kim, Veterans Health Service Medical Center

Discussion

Answer:  Accessory third head of the gastrocnemius

Findings:

Axial T2-weighted MR image shows a thick band like muscular structure arising near the midline of the distal femur and coursing anterolateral toe the popliteal vessels.

Coronal PD-weighted MR image shows a third head of gastrocnemius lateral to the popliteal vessels and medial to the lateral head of the gastrocnemius.

Sagittal T2-weighted MR image shows the aberrant third head which will join the lateral head more distally.

Diagnosis:  

Accessory third head of the gastrocnemius

Discussion:

The third head of the gastrocnemius was initially described in 1813 by Kelch.

The third head of the gastrocnemius which joins the medial aspect of the lateral head is a not uncommon finding on MR examinations of the knee.

The frequency of third head of the gastrocnemius within the population is estimated to be between 2.9% and 5.5%.

The size of the third head may be quite variable, ranging from a small threadlike muscle to a large bulky muscle.

An anomalous slip of the lateral head of the gastrocnemius originating from the midline posterior cortex of the distal femoral metaphysis can cause popliteal vascular entrapment.

References:

1. Koplas MC, Grooff P, Piraino D, Recht M. Third head of the gastrocnemius: an MR imaging study based on 1,039 consecutive knee examinations. Skeletal Radiol.2009 Apr;38(4):349-54.


2. Bergman RA, Walker CW, el-Khour GY. The third head of gastrocnemius in CT images. Ann Anat. 1995 May;177(3):291-4.