Comprehensive Radiographic Anatomy and Positioning: Knee, Ankle, and Foot

Knee and Ankle Anatomy Review

• Bony Structures of the Distal Femur:     * Medial Epicondyle: Located on the distal end of the femur on the medial side.     * Medial Condyle: Located inferior to the medial epicondyle.     * Lateral Epicondyle: Located on the distal end of the femur on the lateral side.     * Lateral Condyle: Located inferior to the lateral epicondyle.     * Patellar Surface: The smooth anterior surface of the distal femur where the patella articulates.     * Popliteal Surface: The posterior surface of the distal femur.     * Intercondylar Fossa (or Notch): The deep depression located posteriorly between the medial and lateral condyles.

• Bony Structures of the Patella:     * Base: The superior, flattened portion of the bone.     * Apex: The inferior, pointed portion of the bone.     * Articular Surfaces: The medial and lateral surfaces on the posterior aspect of the patella.

• Bony Structures of the Proximal Tibia:     * Medial Condyle and Lateral Condyle: Large processes on the proximal end that articulate with the femur.     * Tibial Plateau (Articular Facets): The superior surfaces of the condyles.     * Intercondylar Eminences: Two small processes in the center of the plateau that form an $M$-shape.     * Tibial Tuberosity: An anterior process serving as the attachment site for the patellar ligament.

• Bony Structures of the Fibula:     * Head: The proximal end of the fibula.     * Neck: The tapered area distal to the head.     * Proximal Tibiofibular Joint: The articulation between the head of the fibula and the lateral condyle of the tibia.

Knee Joint Morphology and Soft Tissue Components

• Tibiofemoral Joint:     * Classification: Bicondylar joint.     * Function: Functions primarily as a hinge joint but also allows for circumduction motions.

• Collateral Ligaments:     * Medial Collateral Ligament ($MCL$): Located on the medial (tibial) side of the knee joint.     * Lateral Collateral Ligament ($LCL$): Located on the lateral (fibular) side of the knee joint.

• Cruciate Ligaments:     * Anterior Cruciate Ligament ($ACL$): Extends from the posterior aspect of the femur to the anterior aspect of the tibia.     * Posterior Cruciate Ligament ($PCL$): Extends from the anterior aspect of the femur to the posterior aspect of the tibia.

• Menisci:     * Structure: Medial and lateral menisci are $C$-shaped fibrocartilage structures.     * Function: Serve as cushions between the femoral condyles and the tibial plateau.

• Patellofemoral Joint:     * Classification: Saddle (sellar) joint.     * Characteristics: The patella is the largest sesamoid bone in the human body. It is designed to stay within the patellar groove (intercondylar sulcus) during knee motion.

Hip-to-Knee Thickness Ratio for Central Ray (CR) Angulation

• Purpose: To ensure the joint space is projected open regardless of the patient's body habitus, specific central ray angulations are required for AP knee projections based on the ratio of hip thickness to calf thickness.

• Thicker Hip than Calf:     * Angulation: $5^{\circ}$ cephalad.

• Equal Thickness (Hip and Calf):     * Angulation: $0^{\circ}$ (perpendicular central ray).

• Thicker Calf than Hip:     * Angulation: $5^{\circ}$ caudad.

Radiographic Projections of the Knee

• AP (Anteroposterior) Projection:     * Positioning: Patient is supine in anatomic position; the patella is centered to the midline and toes are directed forward.     * Central Ray: Angled $5^{\circ}$ cephalad, entering $\frac{1}{2}\,\text{inch}$ below the patellar apex.     * Evaluation Criteria: Open tibiofemoral joint space; proximal tibiofibular joint and fibular head visible; visible intercondylar eminences. The medial condyle will appear approximately $5^{\circ}$ to $7^{\circ}$ more inferior than the lateral condyle.

• Internal (Medial) Oblique Projection:     * Positioning: Entire leg rotated internally $45^{\circ}$.     * Evaluation Criteria: Patella is visible on the medial side; lateral condyles are in profile; proximal tibiofibular joint is open.

• External (Lateral) Oblique Projection:     * Positioning: Entire leg rotated externally $45^{\circ}$.     * Evaluation Criteria: Patella is visible on the lateral side; medial condyles are in profile; the fibular head is superimposed by the tibia.

• Lateral Projection:     * Positioning: Patient rolled onto the affected side; knee flexed; femoral condyles should be superimposed.     * Central Ray: Angled $5^{\circ}$ cephalad, entering $1\,\text{inch}$ below the medial condyle.     * Evaluation Criteria: Patella seen in profile anteriorly; patellofemoral joint space open; tibial tuberosity in profile; fibular head superimposed posteriorly.

• Intercondylar Notch (Tunnel) Projection:     * Positioning: Knee flexed to a comfortable degree.     * Central Ray: Directed perpendicular to the long axis of the lower leg, passing through the intercondylar fossa at the level of the patellar apex.     * Evaluation Criteria: Demonstration of an open tunnel between the femoral condyles; patella is centered.

• Tangential Patella (Sunrise/Settegast) Projection:     * Positioning: Knee flexed $40^{\circ}$ to $45^{\circ}$.     * Central Ray: Angled approximately $5^{\circ}$, skimming the patellofemoral joint space.     * Evaluation Criteria: Clear visualization of the patella, the open patellofemoral joint space, and the intercondylar sulcus.

Ankle Anatomy and Spatial Orientation

• Bony Landmarks of Distal Tibia:     * Medial Malleolus: The distal medial process.     * Anterior Tubercle: A small process on the anterior distal tibia.     * Anterior Crest: The "shin" border of the tibia.

• Bony Landmarks of Distal Fibula:     * Lateral Malleolus: The distal lateral process.     * Apex, Head, and Neck: Proximal components (referenced for orientation).

• Ankle Joints:     * Distal Tibiofibular Joint: Between the distal tibia and fibula.     * Tibiotalar Joint: Between the distal tibia and the talar dome.     * Fibulotalar Joint: Between the lateral malleolus of the fibula and the talus.     * Mortise Joint: The entire ankle joint complex ($3$ to $4\,\text{mm}$ space).

• Spatial Planes:     * Intermalleolar Plane: This plane lies $15^{\circ}$ posterior to the coronal plane.     * Rotation for Imaging: A $0^{\circ}$ foot position aligns the coronal plane to the Image Receptor ($IR$). A $15^{\circ}$ internal rotation aligns the intermalleolar plane parallel to the $IR$ (Mortise view). A $45^{\circ}$ internal rotation opens the distal tibiofibular joint.

Radiographic Projections of the Ankle

• AP Ankle Projection:     * Positioning: Patient supine, knee extended, and foot dorsiflexed without force.     * Central Ray: Perpendicular to the $IR$, entering midway between the malleoli.     * Evaluation Criteria: Distal $\frac{1}{3}$ of tibia and fibula, both malleoli, talus, and proximal half of metatarsals. Medial mortise joint is open; lateral mortise joint is closed due to fibular superimposition.

• Mortise Oblique ($15^{\circ}$ internal) Projection:     * Positioning: Leg and foot internally rotated $15^{\circ}$.     * Central Ray: Perpendicular, entering midway between malleoli.     * Evaluation Criteria: Uniform joint space of $3$ to $4\,\text{mm}$ around the entire talus; minimal superimposition at the distal tibiofibular joint.

• $45^{\circ}$ Internal Oblique Projection:     * Positioning: Leg and foot internally rotated $45^{\circ}$.     * Evaluation Criteria: Distal tibiofibular joint is open; lateral malleolus is free of superimposition; medial malleolus is partially superimposed on the talus.

• External Oblique Projection:     * Positioning: Foot externally rotated $45^{\circ}$.     * Evaluation Criteria: Medial malleolus is free of superimposition; fibula overlaps the lateral tibia.

• Lateral Ankle Projection:     * Positioning: Patient on the affected side; foot dorsiflexed to $90^{\circ}$.     * Central Ray: Perpendicular to the medial malleolus.     * Evaluation Criteria: Distal fibula superimposed over the posterior half of the tibia; tibiotalar joint open; calcaneus in profile. A dark area posterior to the Achilles tendon may indicate joint effusion.

Tibia and Fibula (Lower Leg) Projections

• AP Tib-Fib Projection:     * Setup: Requires inclusion of both knee and ankle joints. May require diagonal $IR$ placement or increased Source-to-Image Distance ($SID$) to capture the full length.     * Central Ray: Directed to mid-tibia/fibula. Use of the anode heel effect (placing the thicker part of the leg under the cathode) provides uniform exposure.     * Evaluation Criteria: Entire tibia and fibula, including both proximal and distal tibiofibular joints and the adjacent knee/ankle joints.

• Lateral Tib-Fib Projection:     * Positioning: Patient rolled to the side; foot dorsiflexed.     * Central Ray: Perpendicular to mid-tibia/fibula.     * Evaluation Criteria: Tibial tuberosity in profile; fibular head superimposed by the tibia; posterior borders of the femoral condyles superimposed.

Anatomy and Joint Classification of the Foot

• Phalanges:     * Structure: Distal, middle, and proximal segments (total of $14$ phalanges per foot).     * Hallux (First Digit): Contains only proximal and distal phalanges.

• Metatarsals:     * Structure: Five bones labeled $1$ through $5$ (medial to lateral), each having a head, shaft, and base.     * Fifth Metatarsal: Features a distinct lateral tuberosity.

• Tarsals ($7$ bones):     * Medial Cuneiform, Intermediate Cuneiform, Lateral Cuneiform.     * Cuboid (lateral side).     * Navicular (medial side).     * Calcaneus (heel bone).     * Talus (articulates with leg).

• Joint Classifications:     * Interphalangeal Joints ($IP$): Ginglymus (hinge) joints.     * Metatarsophalangeal Joints ($MTP$): Condyloid (semi-ellipsoidal) joints.     * Tarsometatarsal ($TMT$) and Intertarsal Joints: Plane (gliding) joints.

• Longitudinal Arch: Extends from the head of the first metatarsal to the calcaneus.

Radiographic Projections of the Toes

• AP Toe Projection:     * Central Ray: Angled $10^{\circ}$ to $15^{\circ}$ toward the calcaneus, entering the $MTP$ joint of interest.     * Evaluation Criteria: Frontal view of phalanges and distal halves of metatarsals; open joint spaces; equal concavity on both sides of phalangeal shafts (no rotation).

• AP Oblique Toe Projection:     * Positioning: Foot rotated $30^{\circ}$ to $45^{\circ}$ medially (for digits $1$ through $3$) or laterally (for digits $4$ and $5$).     * Central Ray: Perpendicular, entering the $MTP$ joint.     * Evaluation Criteria: Increased concavity on the side away from the $IR$. Minimal metatarsal overlap.

• Lateral Toe Projection:     * Central Ray: Perpendicular, entering the $IP$ joint (first digit) or proximal $IP$ joint (digits $2$ through $5$).     * Evaluation Criteria: True lateral profile; increased anterior concavity on the distal phalanx; posterior concavity on the middle and proximal phalanges.

Radiographic Projections of the Foot

• AP or Dorsal-Plantar Foot Projection:     * Central Ray: Angled $10^{\circ}$ toward the calcaneus, entering the base of the third metatarsal.     * Evaluation Criteria: Visualization of all phalanges, metatarsals, and tarsals; nearly equal spacing between metatarsals $2$ through $5$; intertarsal joint between the first and second cuneiforms demonstrated.

• AP Oblique Foot ($30^{\circ}$ internal) Projection:     * Positioning: Foot internally rotated $30^{\circ}$.     * Central Ray: Perpendicular, entering the base of the third metatarsal.     * Evaluation Criteria: Third through fifth metatarsals free of superimposition; sinus tarsi and calcaneal tuberosity visible.

• Lateral Foot Projection:     * Positioning: Patient on side; foot dorsiflexed $90^{\circ}$.     * Central Ray: Perpendicular, entering the medial cuneiform.     * Evaluation Criteria: Open tibiotalar joint; distal fibula superimposed posteriorly on the tibia; longitudinal arch in profile.

Radiographic Projections of the Calcaneus

• Plantar-Dorsal Axial Projection:     * Positioning: Foot dorsiflexed to $90^{\circ}$.     * Central Ray: Angled $30^{\circ}$ cephalad, entering at the base of the third metatarsal.     * Evaluation Criteria: Entire calcaneus visible from tuberosity up to the talocalcaneal joint; open joint space; sustentaculum tali visible.

• Lateral Calcaneus Projection:     * Central Ray: Perpendicular, entering $1\,\text{inch}$ inferior to the medial malleolus.     * Evaluation Criteria: Calcaneus in profile; open talocalcaneal and calcaneocuboid joints; sustentaculum tali demonstrated.