An emerging player in knee osteoarthritis: the infrapatellar fat pad

Abstract

The role of inflammation in the development, progression, and clinical features of osteoarthritis has become an area of intense research in recent years. This led to the recognition of synovitis as an important source of inflammation in the joint and indicated that synovitis is intimately associated with pain and osteoarthritis progression. In this review, we discuss another emerging source of inflammation that could play a role in disease development/progression: the infrapatellar fat pad (IFP). The aim of this review is to offer a comprehensive view of the pathology of IFP as obtained from magnetic resonance studies, along with its characterization at both the cellular and the molecular level. Furthermore, we discuss the possible function of this organ in the pathological processes in the knee by summarizing the knowledge regarding the interactions between IFP and other joint tissues and discussing the pro- versus anti-inflammatory functions this tissue could have. We hope that this review will offer an overview of all published data regarding the IFP and will indicate novel directions for future research.

Figure 1

Current view of the infrapatellar fat pad (IFP) and its interaction with other joint tissues. The IFP (Hoffa) is a source of several soluble factors. Moreover, it is composed of adipocytes and stromal vascular cells, such as macrophages, T cells, and mesenchymal stem cells (MSCs). Cellular interactions have been described within the IFP, such as between adipocytes and macrophages or T cells but also between IFP and other joint tissues. The main interactions that have been described thus far and the cells/soluble mediators involved in these interactions as summarized in the present review are depicted with arrows. Red arrows indicate a pro-inflammatory/catabolic effect, green arrows indicate an anti-inflammatory/anabolic effect, and orange arrows indicate a mixture of these two, depending on the implicated cell in the target tissue. The interaction between IFP and bone remains to be investigated. FA, fatty acid; FCM, fat conditioned media; IL, interluekin; MSC, mesenchymal stem cell; PG, prostaglandin; TNF, tumor necrosis factor.