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) (<i>p</i> = 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 (<i>p</i> = 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.

骨关节炎（Osteoarthritis, OA）是最常见的肌肉骨骼疾病，主要累及膝关节，引发疼痛与功能障碍。髌下脂肪垫（infrapatellar, IFP）与髌上脂肪垫（suprapatellar, SFP）是膝关节内的脂肪组织，不仅在关节活动中发挥关键力学作用，同时也是脂肪因子与细胞因子的来源，可促进骨关节炎的进展。鉴于此，本研究旨在为髌下与髌上脂肪垫在膝骨关节炎中的作用提供新的科学见解。研究人员对因终末期骨关节炎接受全膝关节置换术的患者采集的样本，通过压缩、压痕与剪切力学测试，分析了两种脂肪垫的力学性能。无约束压缩测试可计算得到能量损失、峰值应力以及初始与最终弹性模量；基于应力松弛加载条件的测试可计算得到以平衡相对刚度表征的时变力学响应；剪切测试则可获取应变能量耗散密度、峰值剪切应力与剪切模量。实验结果展现了脂肪组织典型的力学特征：随应变增加呈现非线性硬化特性，且具有时变响应。实验表明，膝骨关节炎患者的髌下脂肪垫刚度高于髌上脂肪垫：二者的最终压缩弹性模量分别为84.43 kPa与35.54 kPa（p=0.042）。在粘弹性特性方面二者较为相近：髌下脂肪垫的平衡相对刚度为0.13，髌上脂肪垫为0.11（p=0.026）。本研究结果为骨关节炎对膝关节力学特性的影响提供了新的认知，并为开发优化膝关节假体设计的计算工具奠定了基础。