Pathophysiology
Summary
Joints serve as the articulation points between bones, with the articular cartilage providing a smooth gliding surface for joint movements. Articular cartilage lies over the subchondral bone plate and is composed of a matrix of type II collagen secreted by chondrocytes. Within the joint capsule is the synovial membrane, which produces synovial fluid—a lubricant and shock absorber enriched with hyaluronic acid.
Osteoarthritis (OA) is a degenerative joint disease that evolves from mechanical wear and tear. The disease begins with the release of cytokines from chondrocytes, resulting in inflammation and damage to articular cartilage. As the damage ensues, chondrocytes proliferate and increase their activity, promoting the production of more cartilage in order to repair the affected regions. These activated chondrocytes continue to release cytokines that cause further injury, as well as matrix metalloproteinases (MMPs), which are enzymes that degrade the extracellular matrix, including type II collagen. This cycle of inflammation and cartilage degradation underscores the pathogenesis of osteoarthritis.
As OA advances, MMPs persistently deteriorate the cartilage matrix, prompting chondrocyte death despite their efforts to produce new cartilage. Clinical presentations of OA are various, including subchondral sclerosis, osteophytes (bony spurs), subchondral cysts, enlarged joints, as well as the formation of a layer of granulation tissue, known as a pannus, that exacerbates cytokine release and perpetuates cartilage degradation.
Risk factors for OA include age (> 50 yrs), obesity, and prior traumatic joint injuries. Patients with OA present with pain during weight-bearing activities that primarily affects the hip joints, lower cervical & lumbar spine, finger joints, and base of the thumb. The pain & stiffness in OA typically worsens in the evenings after extended activity and is alleviated with rest. Other manifestations include crepitus, swollen nodules on the distal (Heberden’s nodes) & proximal (Bouchard’s nodes) interphalangeal joints, and limited joint mobility in advanced stages.
Lesson Outline
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FAQs
Articular cartilage is composed of a type II collagen matrix secreted by chondrocytes and overlies the subchondral bone plate, ensuring smooth gliding movements in joints. In osteoarthritis, a cycle of chronic local inflammation results in chondrocyte activation. These activated chondrocytes release cytokines and matrix metalloproteinases (MMPs) that degrade the cartilage's extracellular matrix, including type II collagen. This degradation initiates the pathological progression of osteoarthritis, compromising joint function.
The synovial membrane is located within the joint capsule and is responsible for producing synovial fluid—a lubricant and shock absorber for joints enriched with hyaluronic acid. In osteoarthritis, chronic local inflammation results in the formation of a layer of granulation tissue, known as a pannus. This pannus exacerbates inflammation by releasing more cytokines, further damaging the articular cartilage and undermining joint health.
Osteoarthritis often presents asymmetrically—presenting as joint pain and stiffness that intensifies with weight-bearing activities. In contrast to RA, the pain and stiffness in osteoarthritis worsens by the end of the day and can be alleviated with rest. Other manifestations include crepitus, swollen nodules on the distal (Heberden’s nodes) and proximal (Bouchard’s nodes) interphalangeal joints, and restricted range of motion in advanced stages.
Osteophytes, or bony spurs, are bone and cartilage protrusions formed as chondrocytes attempt to mend damaged cartilage. In osteoarthritis, osteophytes can emerge in areas such as the hip joints and spine, potentially leading to spinal foramina impingement and subsequent nerve root compression.
Age, specifically being over 50, is a primary risk factor for osteoarthritis. Obesity and prior traumatic joint injuries also increase risk. Osteoarthritis often manifests as pain during weight-bearing activities and predominantly affects areas like the hip joints, lower cervical and lumbar spine, finger joints, and the base of the thumb.
