Procedures · Extremities2 min read5-question check
Knee and patella
AP knee CR angle follows ASIS-to-tabletop thickness; lateral knee is 20 to 30° flexion, 5 to 7° cephalad.
Patient and part
For the AP knee, the patient is supine with the leg extended and rotated about 3 to 5 degrees internally so the epicondyles are parallel to the IR. For the lateral knee, the patient lies on the affected side with the knee flexed 20 to 30 degrees and the epicondyles perpendicular to the IR. More flexion pulls the patella into the intercondylar sulcus and closes the patellofemoral space. For the AP obliques, rotate the leg 45 degrees medially or laterally. For the PA axial intercondylar fossa, Camp-Coventry is prone with the knee flexed 40 to 50 degrees, and Holmblad is kneeling, leaning forward 20 to 30 degrees. For the tangential patella, Merchant is supine with both knees flexed about 40 degrees, and Settegast is prone with the knee flexed at least 90 degrees.
Central ray
Use 40 inches, about 100 cm. For the AP knee, center to half an inch, about 1.25 cm, distal to the apex of the patella, and set the angle by measuring from the ASIS to the tabletop. Under 19 cm, angle 3 to 5 degrees caudad. From 19 to 24 cm, keep the CR perpendicular. Over 24 cm, angle 3 to 5 degrees cephalad. The obliques use the same centering and angle rule. For the lateral, angle 5 to 7 degrees cephalad to 1 inch, 2.5 cm, distal to the medial epicondyle. The Merchant CR is 30 degrees caudad from horizontal.
What a good image shows
On the AP knee, the femorotibial joint space is open and the condyles are symmetric, with about half of the fibular head overlapped by the tibia. On the lateral, the femoral condyles superimpose and the patellofemoral joint is open. The medial oblique opens the proximal tibiofibular joint and frees the fibular head. The lateral oblique superimposes the fibula on the tibia. The PA axial shows the intercondylar fossa in profile, and tangential views show the patella in profile with an open patellofemoral joint.
Fixing it
On the lateral knee, offset anterior and posterior condyle borders mean rotation, while offset distal borders mean the CR angle needs adjusting. A closed AP knee joint usually means the angle did not match the patient's thickness, so remeasure ASIS to tabletop.
Clinical Example
Your patient measures 26 cm from ASIS to tabletop. You angle 5 degrees cephalad for the AP knee, and the femorotibial joint space comes out open.
Common Mistakes
- Using a perpendicular CR on every AP knee regardless of patient size.
- Doing a Settegast before a patellar fracture is ruled out; the deep flexion can displace fragments.
- Confusing which oblique frees the fibular head.
Why It Matters
The registry tests the thickness rule, the lateral knee flexion and CR angle, and which oblique shows the fibular head.
Key Relationships
- ASIS to tabletop under 19 cm: 3 to 5 degrees caudad
- 19 to 24 cm: perpendicular; over 24 cm: 3 to 5 degrees cephalad
- Lateral knee: 20 to 30 degrees flexion, 5 to 7 degrees cephalad