Radiographic Procedures · ARRT Radiography
Knee, Leg & Patella
The AP knee ASIS angle rule, the 5-7 degree lateral knee, 45 degree obliques, tunnel views (Camp Coventry, Holmblad, Beclere), and tangential patella methods for the ARRT exam.
Overview
Knee, Leg and Patella belongs to the ARRT Procedures domain: 66 of the 200 scored questions under the 2022 specification, in force until 28 February 2027, and 72 from 1 March 2027. Nearly every projection carries a number, and two anatomical facts explain most of them: the tibial plateau slopes 10 to 20 degrees posteriorly, and the medial femoral condyle extends farther distally than the lateral one because the femoral shaft leans about 10 degrees from vertical.
The AP knee angle comes from a measurement, not a guess. Measure from the ASIS to the tabletop: under 19 cm takes 3 to 5 degrees caudad, 19 to 24 cm takes a perpendicular ray, and over 24 cm takes 3 to 5 degrees cephalad. The lateral knee flexes 20 to 30 degrees and angles 5 to 7 degrees cephalad to superimpose the condyles, and the adductor tubercle on the medial condyle is the landmark that tells you which way a rotated lateral went. Both knee obliques rotate the leg 45 degrees.
The named methods do the heavy lifting. Tunnel views of the intercondylar fossa: Camp Coventry prone at 40 to 50 degrees of flexion, Holmblad kneeling at 60 to 70, and Beclere supine at 40 to 45. Weight-bearing work includes the bilateral AP and the Rosenberg method, with knees flexed 45 degrees and the CR 10 degrees caudad. For the patella, the Merchant method flexes 40 degrees with a 30 degree caudad ray at a 48 to 72 inch SID, while Settegast requires at least 90 degrees of flexion and waits until a patellar fracture is excluded by the PA and lateral.
On an initial lower leg exam the rule is firm: both the knee and ankle joints go on the image, each 1 to 2 inches from the receptor ends. The patella is the largest sesamoid bone, about 2 inches across, and articulates with the femur only.
What you’ll learn in this chapter
The 32 lessons in this chapter break down as follows. The full lesson content is unlocked when you start a free account.
Knee, Leg & Patella
- Knee, Leg and Patella
Anatomy
- Tibia and Fibula
- Distal Femur and Its Angles
- Quiz
- Patella, Joints and Soft Tissue
Technique
- Technique for the Knee and Leg
Lower leg
- Tibia and Fibula: AP and Lateral
- Quiz
Knee
- AP Knee
- Knee Obliques
- Quiz
- Lateral Knee
- Reading Rotation, and the Trauma Knee
- Quiz
Weight-bearing
- Weight-Bearing Knees
- Quiz
Intercondylar fossa
- Tunnel Views
- Quiz
Patella
- PA and Lateral Patella
- Merchant Method
- Quiz
- The Other Tangential Methods
Summary
- Every Angle in One Place
Image critique
- Critiquing a Knee Film
- Quiz
Clinical indications
- What You Are Looking For
Recap
- What to Carry Out of This Chapter
- Quiz
- Quiz
- Quiz
- Quiz
- Quiz
Key terms in this chapter
These are the 7 terms most likely to appear on the ARRT registry from this chapter. Use them as a flashcard pre-quiz.
- ASIS-to-Tabletop Rule
- Sets the AP knee central ray: under 19 cm takes 3 to 5 degrees caudad, 19 to 24 cm takes a perpendicular ray, over 24 cm takes 3 to 5 degrees cephalad. Thin points down, thick points up.
- Adductor Tubercle
- A raised area on the posterolateral medial femoral condyle that, when visible, tells the condyles apart on a rotated lateral knee. A posterior tubercle means the knee was under-rotated.
- Camp Coventry Method
- PA axial tunnel view taken prone with the knee flexed 40 to 50 degrees and the CR perpendicular to the lower leg. Shows the intercondylar fossa in profile, free of the patella.
- Rosenberg Method
- PA axial weight-bearing projection with both knees flexed 45 degrees against the upright bucky and the CR 10 degrees caudad. Demonstrates cartilage loss as joint space narrowing under load.
- Merchant Method
- Bilateral tangential patella projection: knees flexed 40 degrees over the table end, CR 30 degrees caudad, SID 48 to 72 inches. The quadriceps must be relaxed or the reading is false.
- Settegast Method
- Prone tangential patella projection requiring at least 90 degrees of knee flexion. Never attempted until a patellar fracture has been excluded by the PA and lateral.
- Intercondylar Fossa
- The deep notch between the posterior femoral condyles, also called the intercondylar notch. It is the target of all three tunnel views.
Sample practice question: Extremity
One free sample from the 377-question Extremity bank. See the format, the rationale style, and the difficulty before you sign up.
A patient presents with wrist pain after falling on an outstretched hand. The PA, lateral, and oblique wrist projections appear normal but clinical suspicion for scaphoid fracture remains. What additional projection should be performed?
Show answer and rationale
A, Incorrect: The carpal tunnel projection demonstrates the carpal tunnel itself, not the scaphoid in particular.
B, Correct: Correct. The Stecher method (PA wrist with ulnar deviation, central ray angled 20° proximally toward the elbow) elongates the scaphoid and reveals fractures missed on routine PA. It is the standard supplementary view for suspected scaphoid fracture.
C, Incorrect: AP oblique medial rotation is for the elbow, not the wrist. Wrist obliques are routinely lateral rotation.
D, Incorrect: Radial deviation does not elongate the scaphoid. Ulnar deviation is required for the Stecher.
Hands-on
Practice the positioning
Open the Positioning Lab to drill body position, central ray, anatomy, and common errors for each projection in this chapter.
- AP Leg (Tibia & Fibula) Supine, leg extended on IR, foot true AP. SID 40".
- Lateral Leg (Tibia & Fibula) Turned onto the affected side, knee slightly flexed, foot in true lateral. SID 40".
- AP Knee Supine, leg fully extended on IR. SID 40".
- Lateral Knee Supine rotated lateral, knee flexed 20-30 degrees. SID 40".
- AP Weight-Bearing Knees Upright standing, both feet flat, knees extended. SID 40".
- PA Axial Intercondylar Fossa Standing Holmblad Standing facing IR, knee flexed ~70 degrees, CR perpendicular to lower leg. SID 40".
- PA Axial Intercondylar Fossa Camp-Coventry Prone, knee flexed 40-50 degrees, CR 40-50 degrees caudad. SID 40".
- AP Axial Intercondylar Fossa Beclere Supine, knee flexed 60 degrees, CR 60 degrees cephalad. SID 40".
- PA Patella Prone, leg extended with the patella flat against the IR. SID 40".
- Lateral Patella Supine rotated lateral, knee flexed 5-10 degrees. SID 40".
- Tangential Patella Settegast Prone, knee acutely flexed at least 90 degrees, CR tangential to the patellar underside. SID 40".
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Frequently asked questions
What does the ARRT Radiography Radiographic Procedures category cover?
Knee, Leg and Patella belongs to the ARRT Procedures domain: 66 of the 200 scored questions under the 2022 specification, in force until 28 February 2027, and 72 from 1 March 2027. Nearly every projection carries a number, and two anatomical facts explain most of them: the tibial plateau slopes 10 to 20 degrees posteriorly, and the medial femoral condyle extends farther distally than the lateral one because the femoral shaft leans about 10 degrees from vertical.
How many lessons are in the Knee, Leg & Patella chapter?
This chapter contains 32 lessons across 12 sections, plus a knowledge-check quiz at the end. The full lesson content is unlocked with a Premium subscription. The free tier includes 3 complete chapters.
Is this chapter aligned with the current ARRT Content Specifications?
Yes. Every chapter on this site maps directly to the ARRT Radiography Content Specifications in effect for your exam window. This chapter falls under the Radiographic Procedures domain of the official ARRT exam blueprint.