The infrapatellar fat pad in inflammaging, knee joint health, and osteoarthritis

Abstract

Osteoarthritis (OA) is the most common form of arthritis and accounts for nearly $140 billion in annual healthcare expenditures only in the United States. Obesity, aging, and joint injury are major risk factors for OA development and progression, but the mechanisms contributing to pathology remain unclear. Emerging evidence suggests that cellular dysregulation and inflammation in joint tissues, including intra-articular adipose tissue depots, may contribute to disease severity. In particular, the infrapatellar fat pad (IFP), located in the knee joint, which provides a protective cushion for joint loading, also secretes multiple endocrine factors and inflammatory cytokines (inflammaging) that can regulate joint physiology and disease. Correlates of cartilage degeneration and OA-associated disease severity include inflammation and fibrosis of IFP in model organisms and human studies. In this article, we discuss recent progress in understanding the roles and regulation of intra-articular fat tissue in regulating joint biology and OA.

Fig. 1. Sagittal view of IFP in the knee joint.

The IFP (Hoffa’s fat pad) is positioned at the front of the knee, behind and below the patella, and lined by the synovial membrane on its posterior side facing the joint space. The IFP is arranged into discrete lobules that are surrounded by thin connective tissue, and each lobule consists of unilocular adipocytes that occupy the majority of the volume, along with blood vessels, nerve fibers, and several types of immune cells. The IFP serves to reduce knee load, enlarge synovial space, support patella tendon blood supply, and distribute lubricant in the joint space.

Fig. 2. Inflammatory milieu of IFP.

a Schematic of IFP as both a site and a source of inflammation in the joint, and a contributor to the inflammatory synovial milieu in OA. In addition, IFP can be a rich source of MSCs that can modulate inflammation and potentially play a role in joint tissue repair. b Sagittal proton density fat-suppressed Magnetic Resonance image demonstrating altered signal within Hoffa’s (yellow arrow) and the suprapatellar fat pads (white arrow) in a patient with knee OA. (MR image courtesy of Christine Chung, MD, Professor of Radiology, University of California San Diego (UCSD) Department of Radiology.