*4.1. Study Population*

Patients undergoing ACLR were enrolled at the IRCSS Rizzoli Orthopedic Institute (Bologna, Italy), while patients undergoing TKR for end-stage OA were enrolled at the Orthopaedic Clinic (University-Hospital of Padova, Padova, Italy). The Local Ethical Committees approved the study protocol and all patients signed written informed consent (approval code: 4510/AO/18, approval date: 18 July 2019). Patients or the public were not involved in the design, or conduct, or report, or disseminate plans of our research. Patients with previous knee surgery or other significant pathologies were excluded from the study. For each patient, demographic and clinical data were recorded.

Small biopsies of IFP were obtained during knee arthroscopy for ACLR or TKR surgery.
