Medial Accessory Anomalous Origin of the Lateral Head of the Gastrocnemius Muscle

The lateral head of the gastrocnemius muscle may have an accessory medial origin from the posterior and mid to medial aspect of the lateral femoral condyle, lateral to the popliteal vessels. This accessory bundle merges with the body of the lateral head of the gastrocnemius muscle distally and is seen in about 2% of the population.

As with any muscular variation in the popliteal fossa, we're concerned about mass effect on the popliteal vessels and popliteal artery entrapment syndrome, and the position of the popliteal vessels between the accessory bundle and the medial head of the gastrocnemius muscle makes patients susceptible to this condition.

The image above shows the medial accessory anomalous origin of the lateral head of the gastrocnemius muscle (white arrows) in a young man. The axial image shows an accessory muscle bundle originating from the posterior and mid to medial aspect of the femoral condyle. The bundle is lateral to the popliteal vessels (pink arrow). The coronal image shows the bundle merging with the body of the lateral head of the gastrocnemius muscle (LG). The popliteal vessels are located between the bundle and the medial head of gastrocnemius muscle (MG) without displacement or narrowing of the popliteal vessels. The sagittal image shows the origin of the accessory bundle in the mid aspect of the posterior femur (at the same level as the anterior cruciate ligament).

References

Kim HK, Laor T, Racadio JM. MR imaging assessment of the lateral head of the gastrocnemius muscle: prevalence of segmental anomalous origins in children and young adults. Pediatr Radiol. 2008 Dec;38(12):1300-5. Epub 2008 Oct 7.