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Snapping pes anserinus syndrome is an often encountered cause of medial knee snapping. It results from impingement and translation of the gracilis tendon or semitendinosus tendon over the osseous structures of the knee during active flexion and extension. Ultrasonography is often the diagnostic imaging test of choice in cases of mechanical snapping. We report 2 cases of painful snapping pes anserinus and highlight the value of dynamic ultrasound in making an accurate diagnosis so as to direct care.

Update This article was updated on January 14, 2020, because of a previous error. On page 1, in the Abstract and the section entitled “Case Reports,” the word that had read “Genentech“ now reads “Genetech.” An erratum has been published: JBJS Case Connect. 2020 Feb 5;10(1):e0363ER. Case: The use of biologics is rapidly expanding. Over the past decade, there has been a significant increase in the number of cash-based “stem cell”/regenerative medicine clinics in the United States. These clinics provide cash-based services touting stem cell injections to cure a myriad of conditions. Largely, these clinics are unregulated and using injections in a non–Food and Drug Administration–approved manner. We report on 2 patients who presented with symptoms suggestive of septic arthritis following stem cell injections by cash-based local stem cell clinics. Case 1 involved a patient who developed septic arthritis following an injection of umbilical cord blood–derived cellular products (Genetech) and required an antibiotic spacer followed by a total hip arthroplasty. Case 2 involved a patient who developed a likely immune-mediated reaction following an injection of morselized human placental allograft tissue by a local chiropractic office at a cost of approximately $8,000. Conclusions: We present these cases to bring increased awareness to the issue and call for increased regulation of this practice.

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