The characteristics of the lobular arrangement indicate the dynamic role played by the infrapatellar fat pad in knee kinematics

Abstract

The infrapatellar fat pad (IFP) is an intracapsular but extrasynovial structure, located between the patellar tendon, the femoral condyles and the tibial plateau. It consists of white adipose tissue, organised in lobules defined by thin connective septa. The aim of this study is the morphometric and ultrasonographic analysis of IFP in subjects without knee pathology during flexion-extension movements. The morphometric study was conducted on 20 cadavers (15M, 5F, mean age 80.2 years). Ultrasound was performed on 24 volunteers with no history of knee diseases (5M, 19F, mean age: 45 years). The characteristics of the adipose lobules near the patellar tendon and in the deep portion of the IFP were evaluated. Numerical models were provided, according to the size of the lobules. At histological examination, the adipose lobules located near the patellar tendon were larger (mean area 12.2 mm 2 AE 5.3) than those at a deeper level (mean area 1.34 mm 2 AE 0.7, P < 0.001) and the thickness of the septa of the deepest adipose lobules (mean value 0.35 mm AE 0.32) was greater than that of the superficial one (mean value 0.29 mm AE 0.25, P < 0.001). At ultrasound, the IFP was seen to be composed of very large lobules in the superficial part (mean area 0.29 cm 2 AE 0.17 in extension), with a significant reduction in flexion (mean area 0.12 cm 2 AE 0.07, P < 0.01). The deep lobules were smaller (mean area 0.11 cm 2 AE 0.08 in extension) and did not change their values (mean area 0.19 cm 2 AE 0.52 in flexion, P > 0.05). In the sagittal plane, the reduction of thickness of the superficial layer (with large adipose lobules) during flexion was 20.6%, whereas that of the deep layer (with small adipose lobules) was 1.3%. Numerical simulation of vertical loads, corresponding to flexion of the knee, showed that stress mainly developed within the interlobular septa and opposed bulging of the lobules. The characteristics of the lobular arrangement of the IFP (large lobules with superficial septa in the superficial part and small lobules with thick septa in the deep one), significant changes in the areas and perimeters of the superficial lobules, and the reduced thickness of the superficial layer during flexion all indicate the dynamic role played by the IFP in knee kinematics.

A numerical investigation of the infrapatellar fat pad

“…The procedure adopted for the histomorphometric analysis of healthy IFP has been described by Macchi and colleagues. 2,16 Moreover, some notes describing the development of the computational analysis are reported in the following text. The specimens of IFP were sampled from bodies or body parts without a history of OA provided by the Body Donation Program ''Donation to Science'' of the University of Padova.…”

“…The IFP consists of white adipose tissue organized in well-defined lobules by connective septa. 2,16 The size of such lobules varies throughout the tissue ( Table 1). The major and minor axes of the adipose lobules closest to the patellar tendon were greater than those of the deepest adipose lobules (p \ 0.01) and the thickness of the septa of the deepest adipose lobules was greater than that of the superficial one (Table 1).…”

“…The major and minor axes of the adipose lobules closest to the patellar tendon were greater than those of the deepest adipose lobules (p \ 0.01) and the thickness of the septa of the deepest adipose lobules was greater than that of the superficial one (Table 1). 16 A specific algorithm was implemented in MatLab R2019 (MathWorks, Inc., Natick, MA, USA) to identify and measure the thickness of the septal walls and the dimension of adipocyte lobules that were approximated to ellipses.…”

“…For example, through computational meso-models based on different mechanical and geometrical properties, in terms of lobules dimension and septa thickness, it has been shown that IFP provides cushioning to the knee, thus providing an important mechanical role. 5,6,15 These analyses consider a homogeneous distribution of the adipose chambers although recently our group has demonstrated that IFP presents an inhomogeneous distribution of adipose lobules in micro-and macro-chambers, 16 similar to other adipose tissues that play a mechanical role. [17][18][19][20] As an example, the calcaneal fat pad is composed of small adipose chambers located close to the dermis, while their dimension progressively increases toward the calcaneus.…”

“…21 The different conformations of micro-and macrochambers, in terms of adipose lobules and interlobular septa, provide different contributions to the overall mechanical properties of the tissue. To correlate the histo-morphological configuration of the IFP 16 with its mechanical response, 20 finite element models of the knee are developed, considering the structural conformation and the mechanical behavior of the biological system. Hyperelastic constitutive formulations for the different biological tissues are adopted.…”