E2642. Too Much of a Good Thing: Accessory Muscles Around the Knee and Potential Clinical Implications
  1. Sydney Payne; Stanford University Medical Center
  2. Shelby Payne; Stanford University Medical Center
  3. Jennifer Padwal; Stanford University Medical Center
  4. Kathryn Stevens; Stanford University Medical Center
Accessory muscles in the knee are commonly overlooked on MRI studies as they have similar signal characteristics to adjacent muscle groups or surrounding structures on fat suppressed images. The most common accessory muscles around the knee include the accessory gastrocnemius muscle, which is present in 2 - 5.5% of the population, the accessory plantaris muscle which is seen in 6.3% of the population, and the tensor fascia suralis muscle, which is present in 1.3% of the population. Most of these accessory muscles around the knee are asymptomatic but can become clinically significant if they cause mass effect on adjacent neurovascular structures or result in altered biomechanics of the knee. Occasionally accessory muscles can also be mistaken for neoplasm on clinical examination. Identifying accessory muscle tissue may also be clinically relevant in planning a donor site for autograft harvest. Correct identification of the accessory muscles of the knee and their relationship to other knee structures will facilitate more accurate interpretation and help target treatment of associated knee injuries.

Educational Goals / Teaching Points
The goals of this educational exhibit are to review the anatomy of accessory muscles around the knee and to discuss potential associated pathology.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
The presence of an accessory muscle in the knee may be related to clinically significant knee pathology such as popliteal artery entrapment, patellar maltracking disorders, and iliotibial band friction syndrome. Variant knee muscles may also be misinterpreted as a mass on clinical examination.

The clinical importance of accessory muscles in the knee likely depends on their anatomic relationship to adjacent knee structures. This educational exhibit will provide an anatomic review of the accessory muscles around the knee, which will aid diagnostic accuracy on MRI, and help identify potential related pathology.