Subchondral Bone in Osteoarthritis: What's the Connection?

Osteoarthritis (OA) is commonly referred to as wear-and-tear arthritis. However, researchers now understand that it is not entirely about worn cartilage. There are other elements of joint anatomy, such as subchondral bone tissue, that play a significant role in OA.

Additionally, there are other subchondral bone conditions, including subchondral sclerosis and the formation of subchondral cysts, that are associated with OA.

This article explains what the subchondral bone is and how subchondral bone damage contributes to OA. It will help you to better understand what happens to the subchondral bone in osteoarthritis.

What Is Subchondral Bone?

Subchondral bone is the layer of bone just below the cartilage in a joint. "Chondral" refers to cartilage, while the prefix sub means below.

Subchondral bone serves as a shock absorber in weight-bearing joints like the knees. The bone has many blood vessels to supply it with nutrients and oxygen, and to take away wastes.

Cartilage doesn't have its own blood supply, so the close association with the highly vascular subchondral bone is what keeps the cartilage nourished. These vessels provide over half of the hydration, oxygenation, and glucose for the cartilage.

Therefore, the health and function of the subchondral bone make a difference in the health of the cartilage that lies just above it.

How Osteoarthritis Affects Subchondral Bone

In osteoarthritis, the subchondral bone becomes thicker than usual. Research evidence suggests this thickening happens before the articular cartilage at the joint starts to wear away—an indication that subchondral bone plays a role in the development of osteoarthritis.

Studies have found that before osteoarthritis develops, there can be an increased rate of cell turnover in the subchondral bone, as measured by a bone scan technique called scintigraphy. This indicates a process at work that causes the bone to excessively break down and repair itself.

Subchondral bone attrition is the flattening or depression of the bone surface that forms part of a joint. This occurs in early knee osteoarthritis and indicates the potential for cartilage loss and misalignment of a knee compartment.

Subchondral Conditions Associated With OA

Several other subchondral conditions are associated with osteoarthritis. These conditions are typically seen as part of a pattern with OA that leads to a diagnosis.

Subchondral Bone Marrow Edema

Subchondral bone marrow edema, a buildup of fluid also known as bone marrow lesions, is often seen together with subchondral bone attrition.

One theory is that wear and tear on the knee causes microfractures that require a constant state of repair. This cellular activity produces enzymes and proteins that may have further effects on the adjacent cartilage.

Researchers have also discovered that subchondral bone tissue produces several similar cytokines and growth factors (proteins) that can cause inflammation and changes in cartilage tissue. More study is needed to identify which chemicals might be responsible and how they interact.

Subchondral Cyst

A subchondral cyst is often seen in osteoarthritis. This is a fluid-filled sac that extends from the joint, sometimes called a bone cyst.

When viewed by X-ray or magnetic resonance imaging (MRI), these cysts appear like small, hollowed-out spots in the subchondral bone. Subchondral cysts may cause pain and joint stiffness, but not everyone has symptoms—and some people don't develop the cysts at all.

However, cartilage and subchondral bone damage are key factors leading to the development of subchondral cysts.

Subchondral Sclerosis

Subchondral sclerosis occurs because of increased bone density or thickening in the subchondral layer. When seen on X-ray or other imaging, the changes to cartilage in a knee or other affected joint are considered a hallmark finding for osteoarthritis.

One study suggests that the tiny microfractures of early OA, to which the body responds with frequent and repetitive repair activity to preserve cartilage, may lead to subchondral sclerosis. This, in turn, can lead to deformity and subchondral bone irregularity.