Osteoarthritis: New Insight on Its Pathophysiology

Abstract

Understanding of the basis of osteoarthritis (OA) has seen some interesting advancements in recent years. It has been observed that cartilage degeneration is preceded by subchondral bone lesions, suggesting a key role of this mechanism within the pathogenesis and progression of OA, as well as the formation of ectopic bone and osteophytes. Moreover, low-grade, chronic inflammation of the synovial lining has gained a central role in the definition of OA physiopathology, and central immunological mechanisms, innate but also adaptive, are now considered crucial in driving inflammation and tissue destruction. In addition, the role of neuroinflammation and central sensitization mechanisms as underlying causes of pain chronicity has been characterized. This has led to a renewed definition of OA, which is now intended as a complex multifactorial joint pathology caused by inflammatory and metabolic factors underlying joint damage. Since this evidence can directly affect the definition of the correct therapeutic approach to OA, an improved understanding of these pathophysiological mechanisms is fundamental. To this aim, this review provides an overview of the most updated evidence on OA pathogenesis; it presents the most recent insights on the pathophysiology of OA, describing the interplay between immunological and biochemical mechanisms proposed to drive inflammation and tissue destruction, as well as central sensitization mechanisms. Moreover, although the therapeutic implications consequent to the renewed definition of OA are beyond this review scope, some suggestions for intervention have been addressed.

Keywords: inflammation; osteoarthritis; physiopathology; subchondral bone; synovium.

Figure 1

The normal joint—e.g., the knee—is composed of two articulating bones (femur and tibia), the articular cartilage, and the synovial lining of the joint cavity. A thin layer of calcified cartilage is present underneath the articular cartilage. The subchondral bone beneath the calcified cartilage is formed from cortical bone that merges into a network of trabecular bone, which is relatively porous and metabolically active. Source: original.

Figure 2

Sequential changes in the osteochondral unit during the evolution of osteoarthritis. (Left) Early OA is characterized by increased remodeling of the subchondral bone plate. With disease progression, loss of cartilage matrix proteoglycans and erosion of the collagen network led to the development of deep fissures and delamination of the cartilage, with exposure of the underlying zones of calcified cartilage and subchondral bone. In the subchondral bone, cortical plate thickness gradually increases. (Right) Chondrocytes exist mostly in clusters in late-stage OA, but chondrocyte apoptosis is also evident. In the deeper zones, chondrocytes undergo phenotypic alterations, developing features of a hypertrophic phenotype. The calcified cartilage expands and advances into the overlying hyaline articular cartilage, with duplication of the tidemark. This process is initiated by penetrating vascular elements, and accompanying sensory and sympathetic nerves, into the osteochondral junction. OA: osteoarthritis. Source: original.