Findings: An abnormal soft tissue "mass" which follows muscle on all imaging sequences is located just deep to the vastus intermedius muscle on sagittal spin density weighted image (A), coronal spin density weighted image (B) and axial T2 weighted image (C). There is no abnormal signal in the surrounding soft tissue and this structure does not displace the adjacent muscles.

Diagnosis: Articular muscle of the knee.

Discussion: Well known to anatomists but not well recognized in the imaging anatomy literature, this muscle serves to support the suprapatellar bursa and the superior joint capsule of the knee. It originates from the anterior aspect of the distal femur and travels inferomedial (B) to insert into the suprapatellar bursa. It has a symmetric appearance from side to side with a variable number of muscle slips. The articularis muscle is distinct from the vastus musculature.

Reference:
Puig S, Dupuy DE, Sarmiento A, Boland GW, Grigoris P, Greene R.
Articular Muscle of the Knee: A Muscle Seldom Recognized on MR Imaging.
AJR 1996;166:1057-1060.

DiDio LJA, Zappala A, Carney WP. Anatomic-Functional Aspects of the
Musculus Articularis Genus in Man. Acta Anat (Basel) 1967;67:1-23.

The Lower Limb. In: Woodburne RT ed. Essentials of Human Anatomy.
7th ed. Oxford Press, New York, NY; 1983:532-556.

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