MSK for Friday, April 11th, 2025

History

13 year old female with intermittent pain at the lateral aspect of the left knee.

Images (Click any image to enlarge)

Question

Based on these images, and given a lack of additional findings at the knee, what rare finding may be the cause of this patient's symptoms.

Your answer

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Correct answer

Symptomatic cyamella

Discussion

MR images show a small, ovoid sesamoid bone at the posterolateral aspect of the lateral femoral condyle, within the proximal popliteal tendon, consistent with a cyamella. The presence of surrounding fluid signal, in the absence of other internal derangement suggests the cyamella is symptomatic.

The cyamella is a sesamoid bone positioned within the proximal popliteus tendon, seen in less than 2% of humans. It should not be confused with the fabella, which is a sesamoid bone positioned within the lateral head of the gastrocnemius muscle.  Although typically asymptomatic, case reports describe rare cases of symptomatic cyamella, with suggested mechanisms such as repetitive microtrauma, transient dislocation of the popliteus tendon / cyamella, and twisting injury to the knee overloading the popliteus tendon and damaging the cyamella.

References

• Berthaume MA, Bull AMJ. Cyamella (a popliteal sesamoid bone) prevalence: A systematic review, meta-analysis, and proposed classification system. Clin Anat. 2021 Jul;34(5):810-820. doi: 10.1002/ca.23743. Epub 2021 May 4. PMID: 33905585.
• Benthien JP, Brunner A. A symptomatic sesamoid bone in the popliteus muscle (cyamella). Musculoskelet Surg. 2010 Dec;94(3):141-4. doi: 10.1007/s12306-010-0083-6. Epub 2010 Nov 21. PMID: 21104175.
• Mishra AK, Jurist KA. Symptomatic cyamella. Arthroscopy. 1996 Jun;12(3):327-9. doi: 10.1016/s0749-8063(96)90068-7. PMID: 8783827.