15.2 Leg, Knee & Patella

Anatomy of the Knee Region

The knee is formed by the distal femur, the proximal tibia, the proximal fibula, and the patella. On the distal femur, identify the medial and lateral condyles, the medial and lateral epicondyles, and the intercondylar fossa (notch) posteriorly between the condyles. The proximal tibia carries the medial and lateral condyles, the tibial plateau, the intercondylar eminence (tibial spines), and the anterior tibial tuberosity. The fibular head, neck, and apex (styloid) sit laterally and slightly posterior. The patella is the body's largest sesamoid bone; its base is superior and its apex is inferior. Understanding that the medial femoral condyle projects lower and larger than the lateral condyle explains why the lateral knee requires a cephalic angle.

Leg (Tibia and Fibula)

The AP and lateral leg follow the long-bone rule: both the knee and the ankle joints must be included on the image. The AP places the leg with the foot dorsiflexed and no rotation, CR perpendicular to the midpoint; the lateral is a mediolateral projection with both joints demonstrated. If the leg is too long for a single field, angle the tube slightly or use two exposures, but each joint must appear on the film for the exam to accept the image.

Knee

The AP knee is the exam's most detailed central-ray question. The CR enters 0.5 inch (about 1.25 cm) below the patellar apex, and the angle is set by the ASIS-to-tabletop distance, which reflects patient thickness:

This rule keeps the joint space open regardless of body build. The AP oblique with 45 degrees medial (internal) rotation demonstrates the proximal tibiofibular articulation and the lateral condyle in profile; the 45 degrees lateral (external) rotation oblique shows the medial condyle. The lateral knee flexes the joint 20 to 30 degrees and angles the CR 5 to 7 degrees cephalad; this prevents the larger, lower medial condyle from obscuring the joint and superimposes the femoral condyles.

Intercondylar Fossa (Tunnel)

The intercondylar fossa is imaged with PA axial tunnel methods that detect loose bodies, subchondral defects, and narrowing. The Camp Coventry method is prone with the knee flexed 40 to 50 degrees and the CR angled perpendicular to the lower leg. The Holmblad method has the patient kneeling (or leaning) with the knee flexed about 70 degrees and the CR perpendicular. Both open the intercondylar fossa (tunnel). The Beclere method is the AP-axial alternative when the patient cannot assume the prone/kneeling position.

Patella

Routine patella imaging includes a PA and a lateral. The PA patella is prone with the patella parallel to the receptor and the CR perpendicular; the PA gives better recorded detail than the AP because the patella lies closer to the image receptor (less object-to-image distance, or OID). The lateral patella flexes the knee only 5 to 10 degrees, because excessive flexion draws the patella tightly into the intercondylar groove and can separate a transverse fracture. The tangential (axial) patella demonstrates the patellofemoral joint and the patella in profile. The Settegast (sunrise/skyline) method requires acute knee flexion and is contraindicated when a transverse patellar fracture is suspected; the Merchant method uses a supported axial device with about 45 degrees of flexion and is safer, while the Hughston is a prone tangential variant.

Technical Factors and Evaluation Criteria

Knee and leg work uses a grid on larger patients (above roughly 10 cm of tissue) with kVp around 70 to 80; the thin distal leg may be done tabletop. A properly positioned lateral knee superimposes the medial and lateral femoral condyles, opens the femoropatellar joint space, and shows the patella in profile; if the condyles are not superimposed, adjust rotation, and if the joint is closed, verify the 5 to 7 degree cephalad angle and 20 to 30 degrees of flexion. A well-positioned AP knee shows the femorotibial joint space open and symmetric with the fibular head slightly overlapped by the tibia. The two AP obliques are easy to reverse: 45 degrees medial (internal) rotation best demonstrates the proximal tibiofibular articulation and clears the fibular head, whereas 45 degrees lateral (external) rotation superimposes the tibia and fibula and profiles the medial condyle.

Worked Scenario

Consider a hypersthenic patient who measures 26 cm from ASIS to tabletop: the technologist angles the CR 3 to 5 degrees caudad and centers 0.5 inch below the patellar apex. An asthenic patient at 17 cm instead receives 3 to 5 degrees cephalad, and an average patient at 21 cm gets a perpendicular beam. For the PA patella, expect superior recorded detail versus the AP because the patella lies against the receptor, reducing magnification. When a transverse patellar fracture is suspected, choose the supported Merchant tangential over the acutely flexed Settegast to avoid separating fragments.

Common Traps

The classic error is mixing up the tunnel and tangential projections: Camp Coventry and Holmblad show the intercondylar fossa, while Settegast and Merchant show the patellofemoral joint. Another is reversing the AP-knee angle logic; remember that a thin patient (small ASIS-to-tabletop distance) needs cephalad angulation, while a large patient needs caudad. Finally, do not over-flex the knee for a lateral patella when a fracture is suspected, and always include both joints on a leg study.