Standard X-rays, or plain film radiography, generally cannot visualize cartilage directly. Cartilage is a flexible connective tissue found in joints, functioning as a shock absorber and providing a smooth surface for movement. Because cartilage is a soft tissue, it does not show up clearly on an X-ray image, which is designed to capture dense structures. Doctors must therefore use indirect methods or alternative imaging technologies to assess its condition.
How X-Rays Interact with Body Tissues
The ability of an X-ray to create an image depends on how different body tissues absorb the radiation beam. Tissues with high density and a high effective atomic number, such as bone, stop most X-ray photons. Bone contains calcium, which has a high atomic number, causing it to appear white on the final image; this is referred to as radiopaque.
Conversely, tissues with low density, like air and soft tissues, allow the majority of X-ray photons to pass through to the detector. Cartilage, composed mainly of water, collagen, and proteoglycans, falls into this low-density category. Since the X-rays pass through easily, the area where cartilage is located appears dark on the image, a feature known as radiolucency.
The lack of significant density difference between cartilage and the surrounding joint fluid or other soft tissues means there is little contrast for the X-ray to capture. Without this contrast, the fine structure of the cartilage remains invisible. This difference in X-ray absorption explains why a standard radiograph excels at depicting skeletal fractures but fails to detail soft tissue structures.
Interpreting Cartilage Health Through Joint Space
Although the cartilage itself is unseen, physicians use standard X-rays to assess its health by focusing on the joint space. In a healthy joint, the ends of the bones are separated by a measurable distance, which is occupied by the articular cartilage and joint fluid. When the cartilage begins to break down, the cushion between the bones is lost, causing the bones to move closer together.
This reduction in the distance between the bone ends is called joint space narrowing, and it is the primary X-ray sign of conditions like osteoarthritis. By measuring the degree of narrowing, a doctor can infer the severity of cartilage loss. Serial X-rays taken over time can track the progression of the disease and the effectiveness of treatment.
In rare circumstances, cartilage can become visible on an X-ray if it undergoes abnormal calcification or ossification. This occurs in chondrocalcinosis, where calcium crystals deposit within the cartilage, increasing its density enough to attenuate the X-ray beam. The calcified cartilage then appears as a thin white line, which is an exception to its usual invisibility.
Imaging Modalities Designed for Soft Tissue
Since standard X-rays cannot directly image cartilage, specialized techniques provide a clear view of soft tissue structures. Magnetic Resonance Imaging (MRI) is the most common method for assessing cartilage structure and integrity. MRI uses powerful magnetic fields and radio waves to generate detailed images based on the water content of tissues.
Because cartilage has a high water content, MRI produces high-resolution, multi-planar images that distinguish healthy cartilage from damaged or degenerated cartilage. This capability allows doctors to detect subtle tears, thinning, or lesions missed on a plain X-ray. MRI is considered the standard for non-invasively evaluating soft tissue injuries in joints.
Another technique is Arthrography, performed in conjunction with either a Computed Tomography (CT) scan or an MRI. Arthrography involves injecting a contrast dye directly into the joint space before the scan. This dye coats the cartilage surfaces, outlining its contours and highlighting any irregularities, erosions, or tears.
CT scans are less effective than MRI for soft tissue alone but are useful when combined with contrast dye to evaluate the joint space. CT uses X-rays and processes the data digitally to create cross-sectional images, making it excellent for viewing complex bone structures and calcified tissues. The combination of CT and contrast dye in CT Arthrography offers a high-detail view of the cartilage surface and its relationship to the underlying bone.