Mechanical insights into fat pads: a comparative study of infrapatellar and suprapatellar fat pads in osteoarthritis

Osteoarthritis (OA) is the most common musculoskeletal disorder, primarily affecting knee joints and causing pain and disability. The infrapatellar (IFP) and the suprapatellar (SFP) fat pad are knee adipose tissues that play essential mechanical roles during articular activity but are also sources of adipokines and cytokines, contributing to OA progression. For this reason, this work aims to provide new insights into IFP and SFP implications in knee OA. IFP and SFP tissue mechanical properties were studied through compression, indentation and shear mechanical tests performed on samples collected from patients who underwent total knee arthroplasty surgery due to end-stage OA. The energy loss, peak stress, and initial and final elastic moduli were calculated from the unconfined compression tests. The time-dependent response, evaluated in terms of equilibrium relative stiffness, was computed from stress-relaxation loading conditions. Considering shear tests, they provided strain-energy dissipation density, peak shear stress, and the shear moduli. Experimental results showed the typical adipose tissue mechanics features: non-linear stiffening with strain and time-dependent response. Experimental results showed that OA IFP is stiffer than OA SFP, indeed IFP final compression elastic modulus was greater than the SFP (84.43 kPa vs 35.54 kPa respectively) (p = 0.042). Regarding the viscoelastic properties they were comparable: the equilibrium relative stiffness was reported as 0.13 for IFP and 0.11 for SFP (p = 0.026). These outcomes provide new insights into the OA influence on knee mechanics and lay the basis for developing computational tools to improve knee prosthesis design.