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Procedures · Lower Extremity
Femur Positioning Series
Explore the femur's anatomy, then use Practice mode to position the AP and lateral projections yourself.
Femur · Extended
Right
StudyBucky
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The routine femur series
The projections this region is examined with. Set them up yourself in Practice positioning.
AP femur — mid and distal
Anteroposterior projection · leg rotated internally 5°
- Demonstrates
- The distal two-thirds of the femur with the knee joint on the image. Correct positioning shows the femoral and tibial condyles symmetric in size and shape, the outline of the patella slightly toward the medial side of the femur, and about the medial half of the fibular head superimposed by the tibia.
- Part position
- Supine, with the femur centered to the midline of the table and aligned to the long axis of the image receptor. Rotate the leg internally about 5°, until the interepicondylar line lies parallel to the image receptor. Set the receptor so the knee joint is included once the divergence of the beam is allowed for — its lower margin sits about 2 inches (5 cm) below the knee joint, which leaves the joint space at least 1 inch (2.5 cm) clear of that margin.
- Central ray
- 90° to the long axis of the femur and to the image receptor, centered to the midpoint of the image receptor.
- Why it is positioned this way
- This is the one exam in the region centered to the receptor rather than to a palpable landmark. The receptor is chosen long enough to carry the knee joint, so centring to its midpoint is what puts the central ray over the middle of the field. The same geometry produces a finding that reads as an error and is not: the knee joint space will not appear fully open, because the beam reaches a joint at the end of a long field already diverging. That is not rotation, and repositioning will not fix it — a dedicated knee projection is what opens that joint.
Also covered by this projection
Proximal femur insteadWhen the site of interest is the proximal femur, a unilateral hip routine or a pelvis is recommended rather than this projection, and the leg is rotated internally 15–20° instead of 5°, to bring the femoral neck into profile.
Both joints on an initial examDepartmental routines commonly include both the hip and the knee joint on an initial femur examination. On a large adult that takes a second, smaller receptor for whichever joint did not fit.
Lateral femur — mid and distal (mediolateral)
Mediolateral projection · knee flexed 45°
- Demonstrates
- The distal two-thirds of the femur in profile with the knee joint on the image. Correct positioning superimposes the anterior and posterior margins of the medial and lateral femoral condyles and leaves the patellofemoral joint space open, with the patella in profile.
- Part position
- Lateral recumbent on the affected side, with the femur centered to the midline of the table. Flex the knee about 45° and draw the unaffected leg behind the affected one. Adjust the rotation of the body until the femoral epicondyles sit at 90° to the tabletop. The receptor again projects about 2 inches (5 cm) beyond the knee joint.
- Central ray
- 90° to the long axis of the femur and to the image receptor, centered to the midpoint of the image receptor.
- Why it is positioned this way
- The beam enters the medial surface and exits the lateral, which is what makes this a mediolateral projection rather than a lateromedial one. The epicondyles are the positioning check, because a body that looks lateral can still leave the femur rotated: the rotation is adjusted until they sit at 90° to the tabletop, and the finished image confirms it when the anterior and posterior margins of the condyles superimpose. Drawing the unaffected leg behind the affected one is what stops the pelvis rolling past that point. The knee joint again will not appear open, and the distal margins of the condyles will not quite superimpose — beam divergence, as on the AP.
Also covered by this projection
Trauma lateromedial projectionWhen the injury rules out turning the patient onto the affected side, the same lateral is taken supine: the leg and knee are supported, the receptor stands on edge against the medial surface of the thigh, and a horizontal beam is directed from the lateral side. That reverses the projection — the beam now enters lateral and exits medial, so the image is the mirror of a routine mediolateral. Watch for grid cutoff whenever a grid cassette is stood on edge like this.