The Unexplored Role of Intra-articular Adipose Tissue in the Homeostasis and Pathology of Articular Joints

Abstract

Intra-articular adipose tissue deposits known as articular fat pads (AFPs) are described to exist within synovial joints. Their assumed role in normal joint biomechanics is increasingly objectivized by means of advanced methods of functional imaging. AFPs possess structural similarity with body subcutaneous white adipose tissue (WAT), however, seems to be regulated by independent metabolic loops. AFP dimension are conserved during extreme WAT states: obesity, metabolic syndrome, lipodystrophy, and cachexia. Hoffa fat pad (HFP) in the knee is increasingly recognized as a major player in pathological joint states such as anterior knee pain and osteoarthritis. HFP contains numerous population of mesenchymal and endothelial progenitors; however, the possible role of mature adipocytes in the maintenance of stem cell niche is unknown. We propose that AFP is an active component of the joint organ with multifunctional roles in the maintenance of joint homeostasis. Endowed with a rich network of sensitive nervous fibbers, AFPs may act as a proprioceptive organ. Adipokines and growth factors released by AFP-resident mature adipocytes could participate in the maintenance of progenitor stem cell niche as well as in local immune regulation. AFP metabolism may be locally controlled, correlated with but independent of WAT homeostasis. The identification of AFP role in normal joint turnover and its possible implication in pathological states could deliver diagnostic and therapeutic targets. Drug and/or cell therapies that restore AFP structure and function could become the next step in the design of disease modifying therapies for disabling joint conditions such as osteoarthritis and inflammatory arthritis.

Keywords: Hoffa fat pad; cellular therapies; intra-articular fat pad; osteoarthritis; white adipose tissue.

Figure 1

Anatomy of the HFP. The HFP (Hoffa fp) is limited anteriorly by the patellar tendon (PT) (Pat ten) and the joint capsule, superiorly by the inferior pole of the patella (Pat) (A), inferiorly by the proximal tibia (Tib) and the deep infrapatellar bursa (asterisk), and posteriorly by the synovium (arrows) and femur (Fem). It is attached directly to the anterior horns of the menisci (Med men, Lat men) (B). Normal vascular supply consists of two vertical arteries, posterior and parallel to the lateral edges of the PT (C). Courtesy of Draghi et al. from Insights into Imaging 2016 (15).

Figure 2

Ultrasound image of infrapatellar fat pad of a normal knee joint White arrow indicates the fat tissue with distinct ultrasound features compared to the underlying patellar tendon (PT) and synovial tissue (Syn). HFP, Hoffa fat pad; FC, femoral condyle; T, tibia. (A) Sagittal view, (B) coronal view.