Contribution of Infrapatellar Fat Pad and Synovial Membrane to Knee Osteoarthritis Pain

Abstract

Osteoarthritis (OA) is the most common form of joint disease and a major cause of pain and disability in the adult population. Interestingly, there are patients with symptomatic OA displaying pain, while patients with asymptomatic OA that do not experience pain but show radiographic signs of joint damage. Pain is a complex experience integrating sensory, affective, and cognitive processes related to several peripheral and central nociceptive factors besides inflammation. During the last years, the role of infrapatellar fat pad (IFP), other than the synovial membrane, has been investigated as a potential source of pain in OA. Interestingly, new findings suggest that IFP and synovial membrane might act as a functional unit in OA pathogenesis and pain. The present review discuss the role of IFP and synovial membrane in the development of OA, with a particular focus on pain onset and the possible involved mediators that may play a role in OA pathology and pain mechanisms. Inflammation of IFP and synovial membrane may drive peripheral and central sensitization in KOA. Since sensitization is associated with pain severity in knee OA and may potentially contribute to the transition from acute to chronic, persistent pain in knee OA, preventing sensitization would be a potentially effective and novel means of preventing worsening of pain in knee OA.

Figure 1

Infrapatellar fat pad and synovial membrane in healthy and osteoarthritic condition. In osteoarthritis there is an increase of fibrosis, lymphocytic infiltration, and vascularization compared to normal tissues. These changes contribute to OA pain.

Figure 2

Anatomy of the infrapatellar fat pad (IFP). (a) Sagittal section of magnetic resonance, showing the IFP and its location with respect to patella (p), patellar tendon (pt), and tibia (t). (b) Microscopic sagittal image of the IFP, showing the organization in small lobuli and thin interlobular septa (is, black arrow) by Hematoxylin and Eosin staining. Scale bar = 1,200 μm.

Figure 3

S-100 immunostaining of infrapatellar fat pad (on the right) and synovial membrane (on the left) in normal subjects and in osteoarthritic patients. Synovial membrane (a) and infrapatellar fat pad (b) of a healthy subject. Synovial membrane (c) and infrapatellar fat pad (d) of an osteoarthritic patient. In both tissue samples from OA patients immunostaining highlighted an increase of nervous fiber compared to controls. Original magnification 20X.