The infrapatellar fat pad and the synovial membrane: an anatomo-functional unit

Abstract

The infrapatellar pad, a fibro-adipose tissue with peculiar microscopic and mechanical features, is gaining wide attention in the field of rheumatological research. The purpose of this descriptive review is to summarize the most recent published evidence on the anatomic, physiologic and biomechanical inter-relationship between the infrapatellar fat pad and the knee synovial membrane. As an extrasynovial tissue, the infrapatellar fat pad does not directly interact with the articular cartilage; based on its location in close contact with the synovial membrane, and due to the metabolic properties of adipose tissue, it may influence the behavior of the synovial membrane. In fact, considering evidence of macroscopic and microscopic anatomy, the infrapatellar fat pad is the site of insertion of the infrapatellar and medial synovial plicae. Also biochemically, there is much evidence highlighting the interaction among these two structures; in the case of inflammation, the mutual interplay is ascribable to the release of pro-inflammatory mediators stimulating the proliferation of inflammatory cells and promoting tissue modifications in both. All these assumptions could support the emerging idea that the infrapatellar fat pad and the synovial membrane may be considered a morpho-functional unit.

Keywords: infrapatellar fat pad; knee plicae; osteoarthritis; synovial membrane.

Figure 1

Microscopic characterization of infrapatellar fat pad (IFP) and anatomic evidence of the interplay with synovial membrane. (Aa) Sagittal section of the knee in magnetic resonance imaging showing the localization of the IFP and highlighting the relation with the synovial membrane (red dotted line). (Ab,c) The ultrastructural appearance of the IFP (b), in which the presence of clusters of spherically shaped adipocytes is clearly recognizable. A network of collagen fibres among the adipocytes is visible [c, white arrow; scanning electron micrographs, magnifications: 250× (b); 700× (c)]. (B) Dissection of the knee in partial flexion (with the patella reflected superiorly) showing close relation between the synovial membrane and the deep surface of the IFP (a). The microscopic images show the course of the synovial membrane (S) over the IFP with the presence of a cleft (b); the connection between an interlobular septum and the synovial membrane is evident (*). The subsynovial layer is characterized by collagen (Coll) (c) or adipose tissue (AT) (d). Patellar tendon (PT). Haematoxylin‐eosin, magnifications: 1.25× (b); 10× (c, d). Human tissues were sampled from cadavers managed by the Body Donation Programme of the Section of Human Anatomy, University of Padua (De Caro et al. 2009).