Lower Extremity Radiography: Knee and Patella Projections

Principles of Oblique Knee Radiography

• Anatomical Indicators of Leg Rotation:

  • The fibula is situated on the lateral aspect of the lower leg.
  • A properly positioned lateral oblique projection separates the fibula from the tibia, opening the proximal tibiofibular joint space between the head of the fibula and the lateral tibial condyle.
  • The position of the patella serves as an exact indicator of rotational direction:
    • Lateral Oblique Projection: The patella projects laterally toward the fibular side.
    • Medial Oblique Projection: The patella projects medially toward the medial tibial condyle side.

• Joint Space and Condylar Changes in Medial Rotation:

  • When the leg is turned medially, the lateral joint space opens up, while the medial joint space closes.
  • The medial condyle and medial epicondyle of the femur become superimposed and progressively obscured.
  • The medial tibial plateau and medial femoral condyle boundaries become more difficult to distinguish as medial rotation increases.

• Evaluation of Rotational Errors:

  • Normal Medial Oblique: The patella is displaced medially past the margin of the femoral condyle, and the proximal tibiofibular joint remains superimposed.
  • Over-Rotated Lateral Oblique: When over-rotation occurs laterally, the fibula becomes superimposed directly down the center of the shaft of the tibia, and the patella becomes difficult to distinguish within the surrounding bony anatomy.

Intercondylar Fossa Projections (Tunnel Views)

• Definition and Diagnostic Purpose:

  • Tunnel views project the central ray directly through the intercondylar fossa (the deep depression or "cave" located on the posterior aspect of the distal femur).
  • These projections utilize controlled elongation and distortion to open up the joint space and stretch the intercondylar fossa for maximum visibility of loose bodies, joint pathology, and bony spines.

• Holmblad Method (PA Axial):

  • Patient Positioning Options:
    • Option 1 (Upright on stool): Standing with the affected knee flexed and resting on an image receptor placed on a stool. Note: Contraindicated for elderly, unstable, or wobbly patients due to fall risks and safety incidents.
    • Option 2 (Upright at bucky): Standing erect facing the vertical bucky, with the affected knee brought forward and flexed directly against the bucky face.
    • Option 3 (Kneeling on table): Kneeling on all fours on the radiographic table with the affected knee centered directly over the image receptor on the tabletop.
  • Flexion Requirements: The patient's knee must be flexed exactly $70^\circ$. When kneeling, the upper torso must lean forward; leaning backward creates an unwanted $90^\circ$ right angle.
  • Central Ray: Perpendicular ($0^\circ$ angle) to the image receptor, directed through the popliteal fossa (the posterior crease of the knee) and exiting the anterior knee joint space.
  • Radiographic Appearance: The intercondylar fossa appears wide, elongated, and completely open. Both the medial and lateral femoral condyles and epicondyles are demonstrated, along with the tibial plateaus and sharp intercondylar eminence (tibial spines).

• Camp Coventry Method (PA Axial):

  • Patient Positioning: The patient lies prone on the radiographic table with the knee flexed $40^\circ$ to $50^\circ$.
  • Support: A radiolucent sponge, cushion, or firm rest is placed under the ankle or foot to stabilize the limb and eliminate lateral swaying. Standard pillows are inadequate because they compress too flat.
  • Central Ray Angle Rule: The central ray angle matches the degree of knee flexion exactly ("Match the degree to the bend of the knee"):
    • If the knee is flexed $40^\circ$, the tube is angled $40^\circ$ caudad.
    • If the knee is flexed $50^\circ$, the tube is angled $50^\circ$ caudad.
  • Central Ray Path: The beam enters the posterior knee through the popliteal fossa and exits at the patellar apex.
  • Visualized Anatomy: Demonstrates an open intercondylar fossa, the tibial spines, and the condyles of the femur and tibia. If exposure factors are too high (excessive $\text{mAs}$), burnout of soft tissues and the fibular head will occur due to overpenetration rather than mispositioning.

• Béclère Method (AP Axial):

  • Patient Positioning: Patient is placed supine on the radiographic table with the knee flexed $40^\circ$ to $45^\circ$ (previously described historically up to $60^\circ$).
  • Image Receptor Elevation: The image receptor is elevated on sponges beneath the flexed popliteal region to minimize Object-to-Image Distance ($\text{OID}$), as the tabletop prevents direct flat contact.
  • Central Ray: Angled cephalad to match the angle of the lower leg ($40^\circ$ to $45^\circ$), entering perpendicular to the long axis of the tibia at a point $1/2\text{ inch}$ inferior to the patellar apex.
  • Degree Summary of Tunnel Views:
    • Camp Coventry: $40^\circ$ to $50^\circ$ knee flexion, $40^\circ$ to $50^\circ$ caudad tube angle.
    • Béclère: $40^\circ$ to $45^\circ$ knee flexion, $40^\circ$ to $45^\circ$ cephalad tube angle.
    • Holmblad: $70^\circ$ knee flexion, $0^\circ$ perpendicular central ray.

Patellar Radiography: Standard Projections

• PA Patella:

  • Positioning: Patient is positioned prone on the table (or erect facing the vertical bucky). The patella is centered directly to the image receptor.
  • Heel Rotation: The patient's heel is rotated $5^\circ$ to $10^\circ$ laterally (externally) to rotate the patella parallel with the image plane.
  • Central Ray: Directed perpendicular to the popliteal depression, exiting through the center of the patella.
  • Diagnostic Superiority: Compared to an AP projection, the PA projection significantly reduces $\text{OID}$, which minimizes geometric magnification and increases spatial resolution/recorded detail.
  • Superimposition & Pathology: The patella (a sesamoid bone) is superimposed directly over the distal femur. Fracture lines running transversely or longitudinally through the patella can be identified, though open epiphyseal growth plates in pediatric patients must not be misidentified as fractures.

• Lateral Patella (Mediolateral):

  • Positioning: Patient is turned onto the affected side with the knee flexed only $5^\circ$ to $10^\circ$.
  • Pathology Warning: If injury or a suspected fracture is present, do not flex the knee.
  • Anatomical Dynamics of Flexion: Flexing the knee beyond $10^\circ$ pulls the patella downward into the intercondylar sulcus (trochlear groove), which completely closes the patellofemoral joint space.
  • Central Ray: Centered perpendicularly directly to the mid-patellofemoral joint space without tube angulation.
  • Radiographic Appearance: Patella is visualized in true lateral profile anterior to the femur, with a well-defined, open patellofemoral joint space.

Tangential Patellar Projections (Sunrise Views)

• Anatomical Concept: Tangential beams skim across the anterior aspect of the distal femur and patella to project the patella in profile above the femoral condyles, resembling a sun rising above mountains.

• Settegast Method:

  • Clinical Indication: Used primarily to detect vertical fractures or rule out transverse fractures of the patella, as well as evaluating patellofemoral subluxation.
  • Patient Positioning: Can be performed prone (leg bent backward with a traction strap/sheet) or supine/seated (patient seated upright holding the image receptor superior to the distal femur).
  • Radiation Safety Warning: The seated/supine technique delivers higher direct scatter and primary beam exposure to radiosensitive organs including the breasts, thyroid, and eyes.
  • Technique: The knee is slowly flexed until the patella is perpendicular to the image receptor (minimum $90^\circ$ flexion if tolerated). The central ray is angled approximately $15^\circ$ to $20^\circ$ cephalad, skimming through the patellofemoral joint space.
  • Structural Landmarks: The anterior groove is the intercondylar sulcus (trochlea), whereas the posterior groove is the intercondylar fossa. The position of the fibula confirms the lateral femoral condyle.

• Merchant Method:

  • Equipment: Requires a dedicated Merchant cassette holder attached to the foot of the radiographic table.
  • Patient Positioning: Patient is supine with lower legs hanging over the end of the table, supported by the device to establish an exact $40^\circ$ knee flexion.
  • Central Ray: Angled $30^\circ$ caudad from the horizontal plane (entering the superior aspect of the patellae and skimming toward the shins).
  • Bilateral Assessment: Routinely performed bilaterally on a single image receptor with "R" and "L" markers to compare the alignment and patellofemoral joint spaces of both knees simultaneously.

Questions & Discussion

• Question: What oblique projection of the knee is demonstrated when the patella projects laterally?

  • Response: Lateral oblique. If the patella projects toward the fibular side, it is a lateral oblique projection. Over-rotation will cause the fibula to superimpose directly over the center of the tibia.

• Question: What oblique projection of the knee is demonstrated when the patella projects medially?

  • Response: Medial oblique. The patella moves over to the medial tibial side, and the lateral joint space opens.

• Question: Which tarsal bone forms the ankle joint?

  • Response: The talus.

• Question: Which tarsal bone is the largest?

  • Response: The calcaneus.

• Question: What is the specific joint located in the great toe (first digit)?

  • Response: The interphalangeal (IP) joint. The great toe contains only two phalanges (proximal and distal), so it has only a single IP joint, unlike digits two through five which have proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints.

• Question: How many phalanges are in the great toe compared to the other digits?

  • Response: Two phalanges (proximal and distal) in the great toe; three phalanges in each of the remaining toes.

• Question: How can you determine if a foot radiograph is an AP axial projection rather than an AP projection?

  • Response: By the open joint spaces. An angulation of the central ray opens the tarsometatarsal and intertarsal joint spaces that are closed on a perpendicular AP view.

• Question: How is an AP projection of the distal femur distinguished from an AP projection of the lower leg?

  • Response: The entire lengths of the tibia and fibula are not present on a distal femur projection; only the distal shaft and condyles of the femur along with the proximal tibial plateau are visible.

• Question: How is an AP projection of the distal femur distinguished from a lateral projection of the distal femur?

  • Response: On an AP projection, both femoral condyles are seen symmetrically in profile parallel to the image receptor. On a lateral projection, the condyles are superimposed and perpendicular to the image receptor.

• Question: How can an oblique toe projection be differentiated from an AP toe projection?

  • Response: By the concavity/curvature of the phalangeal shafts and the visualization of the sesamoid bones free from excessive superimposition.

• Question: How is an AP ankle projection differentiated from a Mortise oblique projection and a $45^\circ$ medial oblique ankle projection?

  • Response: On a standard AP ankle, the distal fibula superimposes the distal tibia and the talus, meaning not all three borders of the joint space are open. On a Mortise projection ($15^\circ$ to $20^\circ$ medial rotation), the entire mortise joint space (all three borders: superior, medial, and lateral) is open and visualized without superimposition. On a $45^\circ$ medial oblique ankle, the medial joint space closes and narrows, while the lateral tibiofibular syndesmosis opens widely.

• Question: Why are bilateral AP knee projections often performed upright?

  • Response: To demonstrate joint space narrowing under physiological weight-bearing conditions.

• Question: On a lateral leg projection, which bone is positioned anteriorly and which is posteriorly?

  • Response: The tibia is positioned anteriorly, and the fibula is positioned posteriorly.

• Question: Which anatomical structures are visualized on a medial oblique projection of the foot?

  • Response: Rotating the foot medially places the lateral structures into profile. The tuberosity at the base of the fifth metatarsal, the cuboid bone, the lateral metatarsals, and the sinus tarsi are clearly displayed without superimposition.