An X-ray uses electromagnetic radiation to create images of dense structures inside the body. Low-dose radiation passes through the knee, and the calcium in bone absorbs the energy, making it appear white on the resulting image. Since bone is the densest tissue in the joint, the X-ray is considered the standard first-line imaging tool for quickly evaluating knee pain, primarily focusing on the bony components.
Diagnosing Acute Traumatic Injuries
When sudden trauma occurs, such as a fall or a sports accident, X-rays are typically the first step to assess the damage. The primary role of this imaging is to identify a fracture, which can range from obvious displacements of bone segments to subtle hairline cracks.
The X-ray allows medical professionals to visualize the alignment of the knee joint following a high-impact injury. This view is important for checking for a dislocation, which is a complete displacement of the tibia and femur. Even if a joint has spontaneously relocated, the initial images can sometimes show signs of the severe trauma that led to the event.
Another finding in acute trauma is an avulsion fracture, where a tendon or ligament pulls a small piece of bone away from the main structure. For example, a Segond fracture (a fragment pulled from the lateral tibial plateau) strongly suggests an underlying anterior cruciate ligament (ACL) tear, even though the ligament is not visible. These small bony fragments act as an indirect sign pointing toward a significant soft tissue injury. The presence of a fat-fluid level, called lipohemarthrosis, on a horizontal X-ray beam also indicates a fracture that has bled into the joint space.
Detecting Degenerative and Chronic Conditions
For individuals with long-term or persistent knee pain, the X-ray is invaluable for detecting structural changes that develop slowly over time. The most common chronic condition identified is osteoarthritis (OA), which is characterized by the progressive degeneration of the joint. Since X-rays do not directly image cartilage, its loss is inferred by observing joint space narrowing, where the gap between the femur and tibia decreases due to the worn-down articular surface.
A weight-bearing X-ray view is often used because the natural pressure of standing accentuates this loss of joint space, making it easier to assess the severity of cartilage wear. In response to the increased stress from cartilage loss, the bone attempts to repair and restructure itself, leading to the formation of bone spurs, known as osteophytes, along the joint margins. These bony projections are a hallmark finding of degenerative joint disease.
The bone directly beneath the cartilage, called the subchondral bone, may also show increased density, which is visible as a whitened area on the X-ray, known as subchondral sclerosis. Occasionally, fluid-filled cavities called subchondral cysts, or geodes, may form in this dense bone layer. The X-ray also helps evaluate long-term structural alignment issues like genu varum (bow-legged) or genu valgum (knock-kneed) deformities, which can contribute to chronic, uneven joint wear.
X-rays can reveal other chronic bone pathologies that cause persistent pain, such as benign or malignant tumors that alter the normal bone density and structure. Signs of chronic infection, like osteomyelitis, may also be visible through changes in bone texture. X-rays are also highly effective at locating foreign bodies, such as metal shards or old, retained surgical hardware, within the joint space or surrounding tissues.
Understanding the Limitations of X-Rays
While X-rays are excellent for visualizing bone, their primary limitation is the inability to clearly image soft tissues. Structures like ligaments, tendons, muscles, the meniscus, and articular cartilage are not dense enough to block the radiation effectively, appearing as indistinct gray shadows. This means a standard X-ray cannot definitively diagnose injuries such as a torn anterior cruciate ligament (ACL), a meniscal tear, or most cases of tendinitis.
A patient may have a “normal” X-ray, showing no fracture or dislocation, yet still have a significant soft tissue injury causing severe pain and instability. For instance, a complete tear of the medial collateral ligament (MCL) or posterior cruciate ligament (PCL) will not be directly visible. These injuries are often suspected based on the mechanism of trauma or clinical examination but require a different imaging modality for confirmation.
Consequently, if a soft tissue injury like a ligament or meniscal tear is suspected, a medical professional will often order follow-up imaging, most commonly a Magnetic Resonance Imaging (MRI) scan. The MRI is superior because it uses magnetic fields and radio waves to create detailed pictures of soft tissues. A Computed Tomography (CT) scan may also be used in some cases to better characterize complex bone fractures that were seen initially on the X-ray but are not fully understood.