Snap, Crackle, Pop: Atypical Baker’s Cyst Causing Snapping Hamstring

Case Diagnosis: Baker’s cyst with associated snapping semimembranosus

Case Description or Program Description: A 69-year-old male with a past medical history of total hip arthroplasty presented with right knee discomfort for 2 months duration. The patient described medial knee pain associated with a snapping sensation when straightening his knee. He denied knee catching or locking. The snapping was most prominent during hamstring curls and when straightening his leg (while walking and transitioning from sitting to standing). The pain was improved with rest. Physical exam of the right knee was significant for reproducible knee discomfort and snapping of the medial/distal hamstring tendons with end range of knee extension while weight bearing.

Setting: Outpatient sports medicine clinic

Assessment/Results: Point of care ultrasound (POCUS) revealed a baker's cyst measuring 1.7cm x 1.7cm x 0.8cm. Rather than its typical location between the semimembranosus and medial gastrocnemius, it was medial/superficial to the semimembranosus. With dynamic imaging during resisted knee flexion, the semimembranosus passed inferior and lateral to the baker’s cyst and was associated with the patients feeling of snapping. X-ray of the right knee was insignificant. Baker’s cyst aspiration with steroid injection was performed on the right knee with near complete resolution of pain and snapping. The patient was able to resume his workouts at the gym.

Discussion (relevance): Atypical Baker’s cysts can cause mechanical symptoms beyond simple posterior knee pain. In this case, the cyst’s medial displacement contributed to semimembranosus tendon snapping, leading to discomfort and activity limitations. POCUS was instrumental in identifying the cyst’s unusual location and guiding targeted intervention.

Conclusions: This case exemplifies an atypical presentation of a baker’s cyst associated with a rare complication of a snapping hamstring. This emphasizes POCUS as an essential tool in facilitating a complete workup and proper treatment of abnormal knee pain presentations.