A bone spur on the knee, medically known as an osteophyte, is an abnormal growth of bone tissue that forms along the edges of the knee joint. The question of what a knee bone spur “looks like” is complex because the appearance is mostly internal and not easily visible without diagnostic tools. These bony projections develop as a biological response to underlying joint instability and damage, often occurring gradually over time. While some bone spurs grow large enough to be felt as a bump under the skin, their true size and shape are confirmed through medical imaging. Understanding this condition requires focusing on its structure, the symptoms it creates, and how it appears in a clinical setting.
Defining a Knee Bone Spur
A knee bone spur is a hard, smooth deposit of extra bone that forms where bones meet within the joint capsule. These outgrowths are composed of calcified bone tissue and are not sharp pieces of broken bone, despite the common term “spur.” The formation typically occurs at the margins of the three bones that make up the knee: the femur (thigh bone), the tibia (shin bone), and the patella (kneecap).
Osteophytes are a biological attempt by the body to repair and stabilize a joint that has sustained chronic damage. When the protective cartilage in the knee begins to wear down, the body tries to compensate for the loss of cushioning by depositing new bone material at the stressed joint edges. This abnormal bone remodeling process attempts to increase the joint’s surface area, theoretically distributing pressure more widely. However, the resulting growth can interfere with normal joint function.
Symptoms Experienced by the Patient
While the physical appearance of a bone spur is internal, the effects it has on the surrounding joint tissues are often noticeable to the patient. A common complaint is localized pain, which may worsen during or immediately after physical activities like walking, squatting, or going up stairs. This discomfort occurs when the bony growth rubs against adjacent soft tissues, such as ligaments, tendons, or the joint lining.
Many people also experience significant joint stiffness, particularly after periods of rest, such as first thing in the morning. The presence of a bone spur can physically limit the knee’s range of motion, making it painful or difficult to fully extend or bend the leg. In some cases, a clicking, popping, or grinding sensation, known as crepitus, can be felt as the spur interferes with the smooth movement of the joint surfaces. If a spur grows large enough to compress nearby nerves, patients may also report symptoms like tingling, numbness, or muscle weakness in the lower leg.
How a Bone Spur Appears on Imaging
The most direct answer to what a knee bone spur looks like is found on a standard X-ray image, which is the primary tool for visual confirmation. Since bone tissue is dense, it appears bright white on the radiograph, clearly silhouetting the osteophyte against the darker background. When viewed on an X-ray, the bone spur appears as an irregular, often claw-like or hooked projection extending outward from the otherwise smooth contour of the bone end.
These projections are typically observed at the joint line, where the femur and tibia meet, and sometimes beneath the kneecap. The size and shape can vary widely, from small, pointy growths to large, prominent outgrowths, and their presence is often graded using systems like the Kellgren and Lawrence classification for osteoarthritis severity. While an X-ray provides an excellent two-dimensional view of the bone structure, a computed tomography (CT) scan can offer a more detailed, cross-sectional view, and a magnetic resonance imaging (MRI) scan can show how the bony growth is affecting surrounding soft tissues.
Primary Causes of Knee Bone Spur Formation
The underlying reason for knee bone spur formation is almost universally linked to joint degeneration, most commonly Osteoarthritis (OA). Osteoarthritis involves the progressive breakdown of articular cartilage, the slick, protective tissue that covers the ends of bones. As this cartilage thins and erodes, the joint loses its cushioning, leading to increased friction and stress on the underlying bone.
The body interprets this sustained mechanical stress and instability as a form of injury that needs to be stabilized. In response, specialized bone-forming cells, known as osteoblasts, are activated to deposit new bone material at the stressed joint margins. This process is essentially a misguided attempt at repair, creating the osteophyte in an effort to stabilize the hypermobile joint. Other contributing factors can include chronic joint instability from previous ligament injuries, such as an ACL tear, or repetitive, high-impact stress on the knee over many years.