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 mm² ± 5.3) than those at a deeper level (mean area 1.34 mm² ± 0.7, P < 0.001) and the thickness of the septa of the deepest adipose lobules (mean value 0.35 mm ± 0.32) was greater than that of the superficial one (mean value 0.29 mm ± 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 cm² ± 0.17 in extension), with a significant reduction in flexion (mean area 0.12 cm² ± 0.07, P < 0.01). The deep lobules were smaller (mean area 0.11 cm² ± 0.08 in extension) and did not change their values (mean area 0.19 cm² ± 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.

Keywords: infrapatellar fat pad; knee; microscopic anatomy; radiological anatomy; ultrasound.

Figure 1

US analysis of deformation variations of macro‐ and micro‐chambers of infrapatellar fat pad. Thickness (th) of macro‐ (th_mac) and micro‐chamber (th_mic) layers in flexion (A). Thickness (th) of macro‐ (th_mac) and micro‐chamber (th_mic) layers in extension (B).

Figure 2

Macro‐section of infrapatellar fat pad (A), consisting of white adipose tissue organised in lobules, larger near patellar tendon (B) and smaller at deep level (C). Thickness of interlobular septa of IFP was homogeneous at both levels. Haematoxylin‐eosin stain. Scale bar: 1 mm.

Figure 3

US examination of infrapatellar fat pad. Analysis of area and perimeter of adipose chamber closest to patellar tendon (PT) in extension (A) and flexion (B). Analysis of area and perimeter of deepest identifiable adipose chamber in extension (C) and flexion (D).

Figure 4

Numerical models of macro‐ and micro‐chambers, composed of interlobular septa and adipose lobules: initial conformation (A) and deformed configuration under unconfined compressive loading (B). Results from numerical analysis of compression loading. Distribution of tensile stress (C) and compressive strain (D) fields for macro‐ and micro‐chambers at same load level.