title: The infrapatellar fat pad should be considered as an active osteoarthritic joint tissue: a narrative review.
abstract: BACKGROUND
  Osteoarthritis (OA) of the knee joint is caused by genetic and hormonal factors and by inflammation, in combination with biomechanical alterations. It is characterized by loss of articular cartilage, synovial inflammation and subchondral bone sclerosis. Considerable evidence indicates that the menisci, ligaments, periarticular muscles and the joint capsule are also involved in the OA process. This paper will outline the theoretical framework for investigating the infrapatellar fat pad (IPFP) as an additional joint tissue involved in the development and progression of knee-OA.
  
  
  METHODS
  A literature search was performed in Pubmed from 1948 until October 2009 with keywords InFrapatellar fat pad, Hoffa fat pad, intraarticular adipose tissue, knee, cartilage, bone, cytokine, adipokine, inflammation, growth factor, arthritis, and OA.
  
  
  RESULTS
  The IPFP is situated intracapsularly and extrasynovially in the knee joint. Besides adipocytes, the IPFP from patients with knee-OA contains macrophages, lymphocytes and granulocytes, which are able to contribute to the disease process of knee-OA. Furthermore, the IPFP contains nociceptive nerve fibers that could in part be responsible for anterior pain in knee-OA. These nerve fibers secrete substance P, which is able to induce inflammatory responses and cause vasodilation, which may lead to IPFP edema and extravasation of the immune cells. The IPFP secretes cytokines, interleukins, growth factors and adipokines that influence cartilage by upregulating the production of matrix metalloproteinases (MMPs), stimulating the expression of pro-inflammatory cytokines and inhibiting the production of cartilage matrix proteins. They may also stimulate the production of pro-inflammatory mediators, growth factors and MMPs in synovium.
  
  
  CONCLUSIONS
  These data are consistent with the hypothesis that the IPFP is an osteoarthritic joint tissue capable of modulating inflammatory and destructive responses in knee-OA.
authors.affiliation: University of Antwerp, Department of Medicine, Belgium. stefan.clockaerts@ua.ac.be
authors.forename: S
authors.initials: S
authors.lastname: Clockaerts
authors.forename: Y M
authors.initials: YM
authors.lastname: Bastiaansen-Jenniskens
authors.forename: J
authors.initials: J
authors.lastname: Runhaar
authors.forename: G J V M
authors.initials: GJ
authors.lastname: Van Osch
authors.forename: J F
authors.initials: JF
authors.lastname: Van Offel
authors.forename: J A N
authors.initials: JA
authors.lastname: Verhaar
authors.forename: L S
authors.initials: LS
authors.lastname: De Clerck
authors.forename: J
authors.initials: J
authors.lastname: Somville
mesh_terms: D000273:Adipose Tissue / Q000378:metabolism / Q000503:physiopathology*; D002358:Cartilage, Articular / Q000378:metabolism; D006801:Humans; D007719:Knee Joint / Q000378:metabolism / Q000503:physiopathology*; D020370:Osteoarthritis, Knee / Q000378:metabolism / Q000503:physiopathology