Procedures · Lower Extremity

Knee Positioning Series

Explore the knee's anatomy, then use Practice mode to position the AP, oblique, and lateral projections yourself.

Knee · Extended

Right

StudyBucky

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Hover a bone, name it, then click to check · drag to rotate

The routine knee series

The projections this region is examined with. Set them up yourself in Practice positioning.

Anteroposterior projection · leg rotated internally 3–5°

Demonstrates: The distal femur, proximal tibia and fibula and the femorotibial joint space. Correct positioning shows the femoral and tibial condyles and the joint space symmetric, about the medial half of the fibular head superimposed by the tibia, and the intercondylar eminence centred in the intercondylar fossa.
Part position: Supine with the leg extended and rotated internally 3–5°, until the interepicondylar line is parallel to the image receptor. That small internal rotation is what makes it a true AP — the leg does not sit there naturally.
Central ray: Angled from the ASIS-to-tabletop measurement — 5° caudad under 19 cm, 0° between 19 and 24 cm, 5° cephalad over 24 cm — and centered ½ inch (1.25 cm) distal to the apex of the patella.
Why it is positioned this way: This is the one projection in the body whose central-ray angle is chosen by measuring the patient rather than by a fixed number. The angle exists to run the beam parallel to the tibial plateau, and how much tilt that takes depends on the thickness of the thigh and buttocks — so the ASIS-to-tabletop distance stands in for it.

AP weight-bearingThe same projection taken erect under body weight, to assess joint alignment and the joint space under normal physiologic load.

AP oblique projection · leg rotated internally 45°

Demonstrates: The lateral condyles of the femur and tibia in profile, with the proximal tibiofibular joint open and the fibular head clear of the tibia.
Part position: Supine as for the AP, then rotate the entire leg internally 45°, which puts the interepicondylar line at 45° to the image receptor. Stabilise the foot and ankle there if needed.
Central ray: Angled 0° on an average patient, centered ½ inch (1.25 cm) distal to the apex of the patella.
Why it is positioned this way: Turning the leg inward swings the fibula clear of the tibia and throws the lateral half of the joint into profile — the half the AP superimposes.

AP oblique projection · leg rotated externally 45°

Demonstrates: The medial condyles of the femur and tibia in profile, with the patella superimposing the lateral femoral condyle.
Part position: Supine as for the AP, then rotate the entire leg externally 45°, again putting the interepicondylar line at 45° to the image receptor.
Central ray: Angled 0° on an average patient, centered ½ inch (1.25 cm) distal to the apex of the patella.
Why it is positioned this way: It is the mirror of the medial oblique and exists for the opposite half of the joint: the medial condyles, which a medial rotation superimposes.

Mediolateral projection · knee flexed 20–30°

Demonstrates: The knee in true lateral profile with the femoral condyles superimposed, the femorotibial joint space open, and the patella in profile with the patellofemoral joint space clear.
Part position: Lateral recumbent, affected side down, with the knee flexed 20–30°. Adjust the rotation of the body and leg until the knee is a true lateral — the femoral epicondyles directly superimposed and the plane of the patella at 90° to the image receptor.
Central ray: Angled 5–7° cephalad, centered 1 inch (2.5 cm) distal to the medial epicondyle.
Why it is positioned this way: The cephalad angle exists to stop the medial femoral condyle projecting inferior to the lateral one, which is what superimposes the condyles and opens the joint space. The flexion is not optional either: beyond about 30° the patella is drawn tight against the femur and the patellofemoral space closes.