X-rays of the knee joint are requested frequently, particularly at the Emergency Assistance department. They are used primarily to confirm/exclude a fracture. Or to assess the level of osteoarthritis in the knee joints (= gonarthrosis).
To establish the presence of a fracture, as in each conventional X-ray, the knee should be imaged in at least two directions. A standard examination includes an anterior-posterior image and a lateral image. Additional directions may be added when indicated. The most commonly used examinations are explained below.
The front-to-back or anterior-posterior knee image can be made in both supine and standing positions (fig. 1 & 2). In supine position, the X-rays pass through the knee from anterior to posterior (= AP image). An alternative to the supine position is the standing AP image. The knee is fully extended and imaged in the craniocaudal direction under a 10° angle. Additionally, a standing posterior-anterior image (= PA image) may be opted for, also known as the Rosenberg method. In the Rosenberg method, knees should be flexed at 45° (fig. 2).
Standing images have the advantage over supine images that by the additional load on the knee joint they more reliably detect reduced joint space caused by meniscus and cartilage disorders (see also the Pathology section).
Figure 1. Technique for supine anterior-posterior (AP) image.
Figure 2. Technique for standing posterior-anterior (PA) image and standing anterior-posterior (AP) image.
Lateral images are made in the supine position with the knee flexed to 30°. The X-rays pass through the knee joint from medial to lateral (fig. 3).
Figure 3. Technique for lateral image of the right knee (mediolateral projection).
In a good lateral image, the medial and lateral femoral condyles project over each other and the patellofemoral joint is projected free. Where necessary, oblique images can be obtained by exorotation of the knee from the neutral supine position (= lateral oblique image) or endorotation (= medial oblique image). In a trauma setting, an image using a horizontal x-ray beam may be preferred over the standard lateral image in order to establish lipohemarthrosis (see Pathology section). The knee is fully extended and the X-rays pass through the knee from lateral to medial (fig. 4).
Figure 4. Technique for lateral knee image using a horizontal X-ray beam.
The axial image is also termed the sunrise image and provides information on the patellofemoral joint. Additionally, patellar pathology (fracture & subluxation/luxation in particular) can be identified. There are several techniques to make an axial image of the patella. One commonly used technique is the inferosuperior projection. The patient is in the supine position and flexes the knee to 40-45° using knee support (fig. 5).
Figure 5. Technique for axial image (inferosuperior projection). The patient holds the X-ray plate.
In a tunnel view, the intercondylar fossa is projected free. It is used primarily to identify a free body or osteochondral defect (see Pathology section). There are various techniques to make a tunnel image. One commonly used technique is the axial projection. The patient is in the supine position and flexes the knee to 40-45° using knee support (fig. 6). X-rays pass through the knee from anterior to posterior at a 90° angle to the lower leg.
Figure 6. Technique for tunnel view (axial projection).