A knee X-ray, or radiograph, is a common, non-invasive imaging method that uses electromagnetic waves to create a picture of the internal structures of the knee. This technique is highly effective for visualizing dense tissues, such as bone, which appears bright white because its high calcium content absorbs the radiation. Soft tissues, like muscles and fat, absorb less radiation and appear in various shades of gray. The primary purpose of this procedure is to assess the condition of the bony anatomy and the overall alignment of the joint.
Identifying the Key Bony Landmarks
The radiograph clearly displays the four main bones that comprise or surround the knee joint, each with distinct features. The largest bone visible is the distal end of the femur, or thigh bone, which culminates in two rounded projections known as the medial and lateral femoral condyles. These condyles articulate with the top surface of the shin bone, the tibia.
The tibia’s wide, flat top section is called the tibial plateau, and it supports the body’s weight. Between the two concave surfaces of the plateau is a slight bony rise called the intercondylar eminence. The fourth major bone is the patella, or kneecap, which is classified as a sesamoid bone because it is embedded within a tendon. The patella is situated directly in front of the joint, protecting it and increasing the mechanical advantage of the thigh muscles.
The fibula, the smaller bone of the lower leg, is also partially visible on the outer side of the knee. The head of the fibula appears next to the tibia’s lateral condyle. The fibula does not bear significant weight or directly articulate with the femur to form the main knee joint. The smooth, continuous outline of these bones and the uniform spacing between them are the primary indicators of a healthy knee anatomy.
Why Different Views Are Necessary
X-rays produce two-dimensional images of three-dimensional structures, requiring different angles to fully visualize all areas of the knee without overlap. Standard protocol includes an Anterior-Posterior (AP) view, a Lateral view, and often a specialized Patellofemoral view. The AP view, taken from front to back, provides a clear, frontal image of the overall alignment between the femur and tibia. This projection is ideal for assessing the width of the joint space and the contours of the tibial plateau.
A Lateral view is necessary to clearly visualize structures that are superimposed in the AP projection. This side view is particularly useful for evaluating the alignment and height of the patella in relation to the femur. The lateral projection is also reliable for detecting joint effusions, which are abnormal accumulations of fluid within the joint capsule.
The Patellar view, often called a Sunrise or Merchant view, requires the knee to be flexed to a specific angle, usually between 20 and 45 degrees. This specialized projection allows the radiologist to look tangentially through the patellofemoral joint space. Its purpose is to evaluate the articulation between the kneecap and the groove in the femur, which is often missed or poorly seen on the other standard views.
What the X-ray Reveals About Joint Health
Interpreting a knee X-ray involves looking for deviations from the normal appearance of the bony landmarks and joint alignment. A primary finding is the presence of fractures, which appear as a distinct break or discontinuity in the bright white line of the bone’s cortex. Alignment issues, such as a joint subluxation or malalignment, are also readily apparent when the normal parallel relationship between the femur and tibia is distorted.
For chronic conditions like osteoarthritis, the X-ray reveals indirect signs of cartilage loss, as the cartilage itself is not directly visible. This loss is indicated by joint space narrowing, where the gap between the femur and tibia appears reduced compared to a healthy joint, particularly in weight-bearing views. This degenerative process also includes the formation of small, bony growths called osteophytes, or bone spurs, typically seen along the margins of the joint.
Patients should understand the limitations of the radiograph, as it is primarily a tool for assessing bone. Soft tissue structures, including the ligaments, tendons, and menisci, do not show up clearly on a standard X-ray. While effects like joint swelling may be visible, the specific cause, such as a ligament tear, requires further imaging modalities like Magnetic Resonance Imaging (MRI) for a definitive diagnosis.