X-rays are a common diagnostic tool for knee pain, providing valuable insights into bone structures. This imaging technique uses radiation to create black-and-white images of the body’s interior. For knee pain, X-rays visualize the bones of the knee joint, including the femur (thigh bone), tibia (shin bone), fibula (calf bone), and patella (kneecap). Denser materials like bone appear white, while less dense tissues appear in shades of gray or black.
What X-rays Reveal
X-rays are effective at visualizing conditions affecting the bones within the knee joint. They can clearly show acute breaks or cracks in the bones, known as fractures, which appear as distinct lines or disruptions. X-rays can also identify subtle fractures, such as stress fractures.
Changes associated with arthritis are readily apparent on knee X-rays. Joint space narrowing, which indicates a loss of cartilage, is a key sign of osteoarthritis. X-rays can also reveal bone spurs, called osteophytes, which are bony growths that form at the edges of joints. Subchondral sclerosis, an increase in bone density beneath the cartilage, and subchondral cysts, fluid-filled sacs within the bone, are additional indicators of arthritic changes.
An X-ray can assess the alignment of the knee joint, showing if bones are out of their normal position, as seen in dislocations or partial dislocations (subluxations). This imaging can also detect abnormal bone growth, such as bone tumors, or evidence of bone destruction that might suggest an infection within the bone. X-rays can also identify foreign objects within the joint space.
What X-rays Do Not Reveal
While X-rays are excellent for bone structures, they have limitations with soft tissues. The radiation used in X-rays does not effectively visualize soft tissues like ligaments, tendons, and cartilage, which appear as indistinct gray or black areas. Therefore, X-rays cannot directly show tears or sprains in the major knee ligaments, such as the anterior cruciate ligament (ACL) or medial collateral ligament (MCL).
Similarly, damage to the meniscus, the C-shaped cartilage pads that cushion the knee joint, cannot be directly seen on an X-ray. X-rays are also unable to visualize inflammation or tears in tendons, such as patellar tendinitis or quadriceps tendinitis. Conditions like bursitis, which is the inflammation of fluid-filled sacs around the knee, or muscle strains, are not directly detectable on X-ray images.
Although severe swelling might be inferred from soft tissue changes on an X-ray, the specific cause of fluid accumulation or inflammation cannot be directly identified. A normal X-ray does not mean there is no injury, as significant soft tissue damage can be present even when X-ray images appear clear. This limitation often necessitates further imaging if soft tissue injury is suspected.
When X-rays are Ordered
X-rays are frequently a first-line imaging test for knee pain due to their accessibility and ability to quickly assess for serious bone conditions. They are typically ordered after trauma or an injury, especially when a fracture is suspected following a fall, direct impact, or twisting injury. A healthcare provider may also recommend an X-ray if a patient cannot bear weight on the injured leg, as this can be a sign of a significant bone injury.
For persistent knee pain that does not improve with conservative treatments, an X-ray can help identify underlying bone issues. Visible changes like swelling or deformity in the knee also prompt X-ray imaging to check for bone involvement. X-rays are also routinely used in pre-surgical planning to evaluate the bone structure before certain orthopedic procedures.
Complementary Imaging Techniques
When X-rays do not provide a complete picture, particularly when soft tissue injuries are suspected, other imaging modalities complement their findings. Magnetic Resonance Imaging (MRI) uses magnetic fields and radio waves to create detailed images of both hard and soft tissues. MRI is highly effective for visualizing ligaments, tendons, menisci, and cartilage, making it suitable for diagnosing tears or damage to these structures. It can also detect subtle bone bruises or stress fractures that might not be visible on an X-ray.
Ultrasound uses sound waves to produce real-time images. It is particularly useful for evaluating superficial soft tissue structures around the knee, such as tendons and ligaments close to the skin surface, and for identifying fluid collections like Baker’s cysts. Ultrasound can also guide injections into the knee joint. These advanced imaging tests are often ordered after an initial X-ray if symptoms persist, or if a soft tissue injury is strongly suspected despite a normal X-ray result.