To be experienced in msk anatomy means to know also the anatomical variants; today I show you the pictures of a young football player with a “bad feeling” on the postero-lateral compartment of the knee, where a palpable nodularity is noted during the physical exam. With dynamic ultrasound investigation a well defined muscular structure adjacent to the lateral gastrocnemius insertion is noticed; mri scans confirmed the presence of the accessory muscle belly.

Coronal T2 weighted (left) – Sagittal T1 weighted (right) Mri scans (0.3 Tesla) of the same patient.

Comparison between Mri and ultrasound imaging modalities.

The accessory plantaris muscle is most frequently located with the origin of the normal plantaris muscle, but in this case merged with the origin of the lateral head of the gastrocnemius. These anatomical variants are frequently asymptomatic, but their relationship and effect on adjacent structures can mimic different kind of pathologies.