Lateral Knee is a radiography positioning projection of the Lower Extremity. Lateral recumbent, affected side down, knee flexed 20-30 degrees; horizontal beam if pain or trauma prevents flexing. SID 40". The centering point is located 1 inch distal to the medial epicondyle, knee flexed 20-30 degrees in true lateral. The central ray is 5-7 degrees cephalad, entering 1 inch distal to the medial epicondyle. Image-quality criteria include medial and lateral femoral condyles superimposed in true lateral, knee joint spaces open. A sample technique is 85 kVp with DR 2.5 or CR 5 mAs, at an SID of 40 inches (102 cm).
Anatomy demonstrated
- Distal femur with condyles superimposed.
- Tibial plateau in lateral relationship.
- Patella in lateral profile.
- Femorotibial joint space in lateral aspect.
- Proximal fibula visible posterior.
Patient preparation
- WARNING: if the patient cannot flex the knee because of pain or trauma, do not flex it: use a horizontal-beam lateromedial with the IR beside the knee and support under the knee (Bontrager). Added flexion may separate patellar fracture fragments.
- Verify patient identity using two identifiers.
- Remove shoes and any radiopaque items from the leg.
- Position patient lateral recumbent, affected side down.
- Flex knee 20 to 30 degrees and adjust on IR in true lateral.
- Support the opposite knee behind the affected knee to prevent over-rotation.
Position & centering point
1 inch distal to the medial epicondyle, knee flexed 20-30 degrees in true lateral.
Central ray
5-7 degrees cephalad, entering 1 inch distal to the medial epicondyle.
Exposure / technique
- kVp
- 85
- mAs
- DR 2.5 / CR 5
- SID
- 40" (102 cm)
- Notes
- Knee flexed 20-30 degrees relaxes the muscles and shows maximum joint volume (Merrill's). Bontrager prints digital 75 +/- 5 kV; the sample is Merrill's grid row. Sample technique: DR 2.5 / CR 5 mAs at 85 kVp, 40 in SID, for a 12 cm part. Adjust for your equipment.
kVp and mAs are sample values. Use caution when applying them in your department.
Image-quality criteria
- Medial and lateral femoral condyles superimposed in true lateral.
- Knee joint spaces open.
- Patella in true lateral position, not rotated.
- Fibular head partially superimposed by the tibia in a true lateral.
- Collimation: sides to skin margins (Bontrager); 1 inch anterior and posterior (Merrill's).
Common errors
- 1 Knee not flexed the 20-30 degrees the lateral recumbent view calls for.
- 2 Flexion past 30 degrees obscures joint and fat-pad detail and may separate patellar fragments.
- 3 Medial or lateral rotation distorts condylar superimposition.
- 4 CR not directed 1 inch distal to the medial epicondyle.
- 5 Without the 5-7 degree cephalad angle the distal borders of the condyles do not superimpose.
Clinical indications
- Knee trauma evaluation, posterior condyle or plateau fracture.
- Assessment of knee effusion and fat pad sign.