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Lower Extremities

12 Knee/Patella

Anatomy

  • Tibial condyles
  • Femoral condyles
  • Femoral epicondyles
  • Patella (base, apex)
  • Tibial plateaus
  • Intercondylar eminence
  • Tibial tuberosity
  • Intercondylar fossa
  • Medial & lateral meniscus

Standard Projections (ARRT Required)


Knee – AP Projection

CR Location & Positioning

  • SID: 40″
  • Patient position: Supine, no pelvic rotation
  • Adjustments: Femoral epicondyles parallel to IR, patella slightly medial
  • CR: Directed ½ inch (1.3 cm) inferior to the patellar apex
    • <19 cm ASIS–tabletop → 3–5° caudad
    • 19–24 cm → perpendicular
    • 24 cm → 3–5° cephalad
  • Pt. Instructions: Hold still
  • Exposure: 65–70 kVp, mAs per technique (grid if tabletop >10 cm)

Evaluation Criteria

  • Coverage: Entire knee including distal femur and proximal tibia/fibula
  • Rotation checks: Femoral condyles symmetric; tibial eminence centered; slight fibular head superimposition
  • Motion checks: Sharp trabeculae, cortical margins
  • Technique checks: Joint space open, proper contrast/density, patella superimposed on femur
  • Clinical aim: Baseline view for fractures, alignment, degenerative changes

Knee – Lateral Projection (Mediolateral)

CR Location & Positioning

  • SID: 40″
  • Patient position: Lateral recumbent, affected side down, pelvis not rotated
  • Adjustments: Knee flexed 20–30° (relaxes joint & fat pads); epicondyles perpendicular to IR; patella perpendicular
  • CR: 5–7° cephalad, 1 inch (2.5 cm) distal to medial epicondyle
  • Pt. Instructions: Hold still
  • Exposure: 65–70 kVp, appropriate mAs

Evaluation Criteria

  • Coverage: Distal femur, patella, knee joint, proximal tibia/fibula
  • Rotation checks: Femoral condyles superimposed (A/P separation acceptable from beam divergence); patellofemoral joint open
  • Motion checks: Sharp bone edges, no blur
  • Technique checks: Proper penetration; soft tissues and fat pads visible
  • Clinical aim: Detect fractures, patellar dislocation, effusion

Knee – AP Oblique Projection (Medial Rotation)

CR Location & Positioning

  • SID: 40″
  • Patient position: Supine
  • Adjustments: Rotate leg internally 45°; support hip if needed
  • CR: ½ inch (1.3 cm) inferior to patellar apex; angled based on ASIS–tabletop (same rules as AP)
  • Pt. Instructions: Hold still
  • Exposure: 65–70 kVp, appropriate mAs

Evaluation Criteria

  • Coverage: Distal femur, proximal tibia/fibula, knee joint
  • Rotation checks: Proximal tibiofibular joint open; lateral condyles in profile; both tibial plateaus visible
  • Motion checks: Sharp cortical bone and trabecular pattern
  • Technique checks: Joint space visible, proper contrast/density
  • Clinical aim: Best for proximal tibiofibular articulation and lateral condyle

Knee – AP Oblique Projection (Lateral Rotation)

CR Location & Positioning

  • SID: 40″
  • Patient position: Supine
  • Adjustments: Rotate leg externally 45°; support hip/knee
  • CR: ½ inch (1.3 cm) inferior to patellar apex; angled per ASIS–tabletop (same rules as AP)
  • Pt. Instructions: Hold still
  • Exposure: 65–70 kVp, appropriate mAs

Evaluation Criteria

  • Coverage: Distal femur, proximal tibia/fibula, knee joint
  • Rotation checks: Medial condyles in profile; fibula superimposed over tibia; patella projected slightly beyond lateral condyle
  • Motion checks: Clear, sharp trabecular markings
  • Technique checks: Open joint space, correct density/contrast

Clinical aim: Demonstrates medial condyles and tibial plateau


Patella – PA Projection

CR Location & Positioning

  • SID: 40″
  • Patient position: Prone
  • Adjustments: Patella parallel to IR (heel rotated 5–10° laterally)
  • CR: Perpendicular to midpopliteal area, exiting patella
  • Pt. Instructions: Hold still
  • Exposure: 65–70 kVp

Evaluation Criteria

  • Coverage: Patella centered
  • Rotation checks: Patella not rotated; superimposed on femur
  • Motion checks: Sharp patellar margins
  • Technique checks: Adequate penetration to visualize patella through femur
  • Clinical aim: Baseline patellar imaging, sharper detail than AP

Patella – Lateral Projection (Mediolateral)

CR Location & Positioning

  • SID: 40″
  • Patient position: Lateral recumbent, affected side down
  • Adjustments: Flex knee 5–10°; epicondyles superimposed; patella perpendicular to IR
  • CR: Perpendicular, mid-patellofemoral joint
  • Pt. Instructions: Hold still
  • Exposure: 65–70 kVp

Evaluation Criteria

  • Coverage: Patella in profile, patellofemoral joint
  • Rotation checks: Condyles superimposed
  • Motion checks: No blur
  • Technique checks: Contrast adequate for patella and femur
  • Clinical aim: Detect fractures, patellar displacement

Supplemental Projections

Knee – Weight-Bearing AP

CR Location & Positioning

  • SID: 40″ (may use 72″ to reduce magnification)
  • Patient position: Standing, equal weight on both feet
  • Adjustments: Knees fully extended, toes forward
  • CR: Perpendicular, ½ inch below apices of patellae
  • Pt. Instructions: Stand still, equal weight on both legs
  • Exposure: 65–70 kVp

Evaluation Criteria

  • Coverage: Both knees including joint spaces
  • Rotation checks: Equal distance between femoral condyles and tibial plateaus
  • Motion checks: Sharp trabeculae
  • Technique checks: Joint spaces visible, density balanced
  • Clinical aim: Detects joint space narrowing (arthritis, varus/valgus deformity)

Knee – Tunnel (PA Axial Holmblad Method)

CR Location & Positioning

  • SID: 40″
  • Patient position: Kneeling on table, standing against IR, or kneeling on stool
  • Adjustments: Flex knee 70° from full extension (20° from CR); tibia touching IR
  • CR: Perpendicular to lower leg, entering popliteal fossa, exiting patellar apex
  • Pt. Instructions: Hold still
  • Exposure: 65–70 kVp

Evaluation Criteria

  • Coverage: Intercondylar fossa, tibial plateaus, intercondylar eminence
  • Rotation checks: Symmetric condyles; centered intercondylar eminence
  • Motion checks: Crisp bony detail
  • Technique checks: Open fossa, posterior condyles visible
  • Clinical aim: Loose bodies, cartilage defects, osteochondritis

Knee – Tunnel (PA Axial Camp-Coventry Method)

CR Location & Positioning

  • SID: 40″
  • Patient position: Prone
  • Adjustments: Flex knee 40–50°; foot supported; femur flat
  • CR: Perpendicular to tibia, angled 40–50° caudad to knee joint
  • Pt. Instructions: Hold still
  • Exposure: 65–70 kVp

Evaluation Criteria

  • Coverage: Intercondylar fossa, tibial plateaus, intercondylar eminence
  • Rotation checks: Minimal tibiofibular overlap; symmetric condyles
  • Motion checks: Sharp trabeculae
  • Technique checks: Clear joint space, no patellar superimposition
  • Clinical aim: Same as Holmblad

Knee – Tunnel (AP Axial Béclère Method)

CR Location & Positioning

  • SID: 40″
  • Patient position: Supine
  • Adjustments: Flex knee until femur forms 60° angle with tibia; condyles equidistant from IR
  • CR: Perpendicular to tibia, entering ½ inch below patellar apex
  • Pt. Instructions: Hold still
  • Exposure: 65–70 kVp

Evaluation Criteria

  • Coverage: Intercondylar fossa, eminence, joint space
  • Rotation checks: Symmetric condyles, centered fossa
  • Motion checks: No blur
  • Technique checks: Patella not superimposing fossa, good contrast
  • Clinical aim: Similar to other tunnels; used when patient cannot kneel

Patella – Tangential (Settegast / Sunrise)

CR Location & Positioning

  • SID: 40″
  • Patient position: Supine or prone
  • Adjustments: Flex knee slowly until patella is perpendicular to IR; loop bandage may hold foot
  • CR: 15–20° cephalad (depending on knee flexion), through patellofemoral joint
  • Pt. Instructions: Hold still
  • Exposure: 65–70 kVp

Evaluation Criteria

  • Coverage: Patella in profile, patellofemoral joint open
  • Rotation checks: Symmetric condyles, patella centered
  • Motion checks: Sharp margins
  • Technique checks: Good contrast of patella and femoral sulcus
  • Clinical aim: Subluxation, fractures, joint space evaluation

Common Errors & Fixes in Knee & Patella Imaging

Error Seen on Image What It Means How to Fix It
Femoral condyles asymmetrical on AP knee Leg rotated internally or externally Align femoral epicondyles parallel to IR
Joint space closed on AP knee Wrong CR angle for patient size (ASIS-to-tabletop measurement) Adjust angle: <19 cm → 3–5° caudad; 19–24 cm → 0°; >24 cm → 3–5° cephalad
Medial condyle anterior to lateral on lateral knee Over-rotation toward IR Roll patient slightly back (reduce rotation)
Medial condyle posterior to lateral on lateral knee Under-rotation away from IR Roll patient slightly forward (increase rotation)
Medial condyle projects lower than lateral on lateral knee CR angle insufficient Increase cephalad angle to 5–7°
Lateral condyle projects lower than medial on lateral knee CR angle excessive Decrease cephalad angle
Patella not in profile on lateral patella Rotation error Ensure patella perpendicular to IR
Patellofemoral joint space closed on tangential view Incorrect CR angle or knee flexion Recheck flexion angle and CR alignment
Proximal tibiofibular joint not open on medial oblique Insufficient internal rotation Rotate leg medially to full 45°
Fibula not superimposed on tibia on lateral oblique Insufficient external rotation Rotate leg laterally to full 45°
Motion blur, trabeculae not sharp Patient moved or wrong exposure time Reinstruct patient, shorten exposure time
Joint spaces look normal on non–weight-bearing but narrow on weight-bearing Load-bearing arthritis changes present Always include weight-bearing studies when evaluating arthritis

 

 

Hows & Whys of Knee Radiography

ŸAnatomy

  • What are the menisci, and what is their function?
    Crescent-shaped fibrocartilage that deepen the tibial plateaus and absorb shock.
  • What structures form the intercondylar fossa?
    The posterior aspects of the femoral condyles.
  • Where is the tibial tuberosity located, and what attaches there?
    On the anterior proximal tibia; the patellar tendon inserts here.
  • Which part of the patella is superior, and which is inferior?
    The base is superior; the apex is inferior.
  • What is the relationship of the medial vs. lateral femoral condyles?
    The medial condyle extends more distally and is larger.

Positioning

  • Why is the CR angled 3–5° cephalad for patients with a thick pelvis (>24 cm)?
    To project the tibia and femur joint surfaces open, compensating for natural slope of the tibia.
  • Why is the knee flexed 20–30° for the lateral projection?
    To relax muscles/fat pads and open the joint space without displacing fractures.
  • Why is only 5–10° of flexion used for the lateral patella?
    To avoid tightening the quadriceps tendon, which could displace a fracture fragment.
  • Why is the leg rotated internally 45° for the medial oblique knee?
    To open the proximal tibiofibular joint and show lateral condyles in profile.
  • Why is the leg rotated externally 45° for the lateral oblique knee?
    To demonstrate the medial condyles in profile and superimpose fibula over tibia.
  • Why must the intermalleolar line be parallel to the IR for the mortise ankle but not the knee?
    The knee joint space requires CR angle correction instead of patient rotation due to tibial slope.
  • Why is the prone PA patella preferred over AP?
    To reduce OID, giving sharper detail.
  • Why must the CR be angled 15–20° cephalad in the Settegast (Sunrise) projection?
    To align the beam with the patellofemoral joint space.
  • Why are weight-bearing AP or Rosenberg knees performed?
    To evaluate joint space narrowing under load, useful in arthritis.

Technique & Image Evaluation

  • How do you know an AP knee was not rotated?
    Femoral condyles symmetric; tibial eminence centered; slight fibular head overlap.
  • How do you know the CR angle was correct on an AP knee?
    The femorotibial joint space is open and symmetric.
  • How can you tell the lateral knee is correctly positioned?
    Condyles superimposed; open patellofemoral joint space; tibia and fibula slightly overlapped.
  • What error is indicated if the medial condyle appears anterior to the lateral on a lateral knee?
    The knee was over-rotated toward the IR.
  • What error is indicated if the medial condyle appears posterior to the lateral on a lateral knee?
    The knee was under-rotated away from the IR.
  • How can you tell the CR angle was insufficient on the lateral knee?
    Medial condyle projects inferior to lateral condyle.
  • How can you tell the CR angle was excessive on the lateral knee?
    Lateral condyle projects inferior to medial condyle.
  • How can you confirm correct positioning on a medial oblique knee?
    Proximal tibiofibular joint open, lateral condyles in profile.
  • How can you confirm correct positioning on a lateral oblique knee?
    Medial condyles visualized, fibula superimposed on tibia.
  • How do you know the intercondylar fossa (tunnel view) is open?
    Fossa is unobstructed, intercondylar eminence centered, patella not superimposing fossa.
  • How can you confirm patient cooperation on knee images?
    Sharp trabecular markings, no motion blur.

Clinical Applications

  • Which projections best demonstrate the patella in profile?
    Lateral and Sunrise (tangential).
  • Which projections demonstrate the patellofemoral joint space?
    Lateral and tangential (Settegast, Hughston, Merchant).
  • Which projection best demonstrates the intercondylar fossa?
    Tunnel views (Holmblad, Camp-Coventry, Béclère).
  • Why is the weight-bearing AP knee essential in arthritis studies?
    Reveals joint space narrowing not visible on non–weight-bearing views.
  • Why are tunnel views helpful in evaluating knee trauma?
    They can show “joint mice” (loose bodies), cartilage defects, or osteochondritis dissecans.
  • Why should flexion be limited in suspected patellar fracture?
    Excess flexion could displace fracture fragments.

 

License

Radiographic Procedures Review Guide Copyright © 2025 by Carla M. Allen and Taylor M. Otto. All Rights Reserved.