Arthroscopic Infrapatellar Fat Pad Release and Mesenchymal Stem Cell Mobilization

Cell-based orthobiologic strategies commonly require tissue harvesting from an extra-articular donor site, ex vivo processing, and subsequent intra-articular administration. The infrapatellar fatpad is a local intra-articular adipose tissue source containing cells with mesenchymal stem cell-related properties. However, it remains unclear whether cells with a mesenchymal stem cell immunophenotype can be directly mobilized into the joint fluid during routine knee arthroscopy and whether this mobilization is accompanied by an immediate molecular response.

This was a prospective, single-center, paired-sample clinical laboratory study involving adults undergoing routine knee arthroscopy for meniscal pathology. Participants were prospectively enrolled between January 2024 and January 2025. The study was designed as a within-participant comparison of paired intra-articular fluid samples collected immediately before and after controlled mechanical release of the infrapatellar fat pad.

Knee arthroscopy was performed through standard anterolateral and anteromedial portals. Sterile 0.9% saline was initially introduced to distend the joint and obtain adequate visualization. After visualization of the joint, fluid inflow was stopped, and no intra-articular structure was manipulated before baseline sampling. A 20-mL intra-articular fluid sample was aspirated using a sterile syringe and divided into two equal 10-mL aliquots as the pre-release sample.

Following baseline sampling, a motorized arthroscopic shaver was introduced through the anteromedial portal. With the shaver suction turned off, the infrapatellar fat pad was gently and selectively shaved for approximately 1 minute. Care was taken to avoid contact with the menisci, articular cartilage, cruciate ligaments, and synovial structures outside the intended release area.

Immediately after controlled mechanical release of the infrapatellar fat pad, and before further irrigation or continuation of the planned arthroscopic procedure, a second 20-mL intra-articular fluid sample was collected under the same conditions. This sample was divided into two equal 10-mL aliquots as the post-release sample. No additional fluid was introduced into the joint between the pre-release and post-release sampling procedures. After collection of the post-release sample, the released tissue was removed using routine arthroscopic suction and irrigation, and the planned arthroscopic procedure was completed.The timing, sample volume, portal use, release technique, and specimen handling were standardized across participants.

Paired samples were analyzed by flow cytometry using standardized processing and acquisition procedures. The final population selected for quantitative analysis was defined as LIN-negative, CD29-positive, CD73-positive, CD13-positive, CD105-positive, CD90-positive, and CD31-negative, representing cells with a mesenchymal stem cell immunophenotype.

Quantitative real-time polymerase chain reaction was performed separately on the total cellular fraction of each pre-release and post-release sample without flow cytometric sorting or enrichment of the mesenchymal stem cell-immunophenotype population. Expression levels of IL-1β, IL-6, IL-10, TGF-β, and CD24 were evaluated, with GAPDH used as the reference gene. Relative gene-expression changes were calculated using the comparative 2^-ΔΔCt method.

The primary objective was to compare the number of cells with a mesenchymal stem cell immunophenotype in standardized intra-articular samples collected before and after controlled mechanical release of the infrapatellar fat pad. The secondary objective was to compare the expression of the selected molecular markers between paired pre-release and post-release samples. The study evaluated immediate cellular and molecular changes and was not designed to assess the persistence, functional activity, or clinical efficacy of the mobilized cell population.