Arthritis Treatment: What Causes the Pain of Osteoarthritis?

Osteoarthritis (OA) is the most common form of arthritis. It is a disease affecting weight-bearing joints such as the neck, low back, hips, and knees.

While it may occur in younger people as a result of trauma to the joint, it is still considered a condition that accompanies aging.

OA affects more than half of adults past the age of 65. With the graying of the Baby Boomers and with the increasing incidence of obesity, that figure will continue to rise.

OA is a disease of articular cartilage, the gristle that caps the ends of long bones. Articular cartilage, or as it is referred to as hyaline cartilage is a tough connective tissue consisting of proteoglycans (complexes of proteins and sugars) as well as collagen. These coalesce to form a soupy matrix. Interspersed in this matrix are cells called chondrocytes. When cartilage is healthy, chondrocytes manufacture matrix. As osteoarthritis develops and proceeds, there is cartilage degradation with changes affecting the surrounding lining of the joint (synovium) as well as the underlying bone.

What is interesting is that cartilage has no blood vessels nor does it have any nerve fibers. So the big question had been, "What causes the pain of osteoarthritis?"

While people do have disability and pain related to OA, more than 40% of patients with OA on x-ray don't have pain!

So why do people with OA hurt?

It is likely that the cartilage wear and tear triggers other mechanisms to kick into gear. Multiple studies have identified abnormalities occurring in the synovium, bone, and other soft tissue structures that could cause pain. OA has traditionally been thought to be a non-inflammatory form of arthritis. The truth is that a substantial amount of inflammation occurs and it has effects on soft tissue structures both within as well as surrounding the joint.

Local chemical messengers known as cytokines could cause more inflammation to develop locally. The production of destructive enzymes by chondrocytes and other sources also could contribute to pain.

Magnetic resonance imaging studies have demonstrated that bone marrow edema may play a role in the production of pain.

The puzzle is this... there appears to be a disparity between the degree of joint damage and the amount of pain perceived by the osteoarthritis sufferer. This has led to more investigations into possible central causes of pain.

Functional MRI studies of the brain have shown that there are areas in the brain that appear to be responsible for pain processing.

An excellent discussion regarding this phenomenon was published recently.

(Sofat N, Ejindu V, Kiely P. What Makes Osteoarthritis Painful? The Evidence for Local and Central Pain Processing. Rheumatology. 2011;50(12):2157-2165)