10.2 Leg, Knee & Patella
Key Takeaways
- Tibia-fibula AP and lateral must include both ankle and knee joints when possible; true AP shows slight proximal tibiofibular overlap and a free distal fibula laterally.
- Knee AP is centered 1–2 cm distal to the patellar apex (through the femorotibial joint); lateral uses 5–7° cephalad CR angle with 20–30° flexion and superimposed femoral condyles.
- Medial and lateral knee obliques demonstrate the proximal tibiofibular joint or the medial/lateral femoral condyles and tibial plateaus depending on rotation direction.
- Intercondylar fossa (tunnel) projections—Camp-Coventry, Holmblad, or Béclère concepts—open the notch and free the tibial spines for loose bodies and notch morphology.
- Patella PA (or AP if needed), lateral, and tangential (Merchant, Settegast, or Hughston-type) evaluate the patellofemoral joint; cross-table lateral is preferred for trauma when flexion for skyline views is unsafe.
10.2 Leg, Knee & Patella
Quick Answer: The leg needs AP and true lateral that include both knee and ankle whenever the patient and field size allow. The knee routine is AP, lateral, and often both obliques; tunnel (intercondylar fossa) views free the notch and spines; the patella needs PA (preferred), lateral, and tangential (Merchant/Settegast-type) for the patellofemoral joint. Trauma knees get a cross-table lateral before any forced flexion. CAMRT stems link positioning errors to missed plateau fractures, joint effusions, and patellar tracking problems.
General radiography — skeletal (leg and knee) is high-volume general radiography content under RTR.4 and is tightly coupled to RTR.6 image analysis. You must know centering landmarks, rotation criteria, and when special projections add diagnostic value without unnecessary dose.
Anatomy Landmarks for Positioning
- Femoral epicondyles / condyles: AP equality of condyles = no rotation; lateral requires superimposed condyles.
- Tibial plateaus and spines: joint-space profile depends on CR angle matching the plateau tilt.
- Patella: sits in the trochlear groove; lateral shows height and joint effusion (suprapatellar bursa); tangential shows medial/lateral facets and sulcus.
- Proximal tibiofibular joint: opens on medial oblique of the knee (leg rotated medially ~45°).
- Fibular head and neck: common occult fracture site after twisting trauma—include on proximal tib-fib and knee series as indicated.
Tibia and Fibula (Lower Leg)
| Projection | Setup | CR | Evaluation |
|---|---|---|---|
| AP | Leg extended; femoral condyles equidistant from IR; foot dorsiflexed vertical | Mid-shaft of tibia (include both joints or two centered images) | Ankle and knee included when possible; slight proximal fibular overlap on tibia normal; distal fibula free laterally; no rotation of malleoli/condyles |
| Lateral | True lateral; patella perpendicular to IR; condyles superimposed | Mid-shaft | Superimposed femoral condyles; tibia and fibula largely superimposed with fibula posterior; both joints included |
Field-size reality: Adult tib-fib often requires two images (proximal and distal) with overlap so no shaft segment is missed. Never crop out a joint when the clinical question is “rule out fracture”—missed joint-including segments are a classic reject/repeat driver.
For trauma with angulation, obtain two projections as close to 90° apart as the limb allows; do not forcibly straighten a deformed leg. Support with sponges and use horizontal-beam laterals.
Knee — Routine Projections
| Projection | Setup | CR | Evaluation |
|---|---|---|---|
| AP (or PA) | Supine/seated; leg extended; epicondyles parallel to IR | Perpendicular (or 3–5° cephalad if needed) 1–2 cm distal to patellar apex (through joint) | Open femorotibial joint; symmetric femoral condyles; half of fibular head superimposed by tibia; patella centered on distal femur |
| Lateral (mediolateral) | Lateral recumbent; 20–30° knee flexion; epicondyles perpendicular | 5–7° cephalad to the joint (1 cm distal to medial epicondyle) | Superimposed femoral condyles; open patellofemoral joint; slight fibular head superimposition on tibia; soft-tissue suprapatellar effusion assessable |
| AP medial oblique (~45°) | Entire leg rotated medially 45° | Joint line | Proximal tibiofibular joint open; lateral femoral condyle and plateau demonstrated |
| AP lateral oblique (~45°) | Entire leg rotated laterally 45° | Joint line | Medial femoral condyle and medial plateau; fibula superimposed on tibia |
Why 20–30° flexion and a cephalad angle on the lateral?
Slight flexion relaxes the muscles and opens the patellofemoral relationship without drawing the patella tightly into the notch. A 5–7° cephalad CR angle compensates for the medial condyle’s slightly inferior position so the condyles superimpose. Under-flexion or wrong CR angle projects one condyle superior to the other—a rotation/angulation error that can hide plateau step-off.
Weight-bearing AP (and PA axial Rosenberg-type) bilateral knees assess joint-space narrowing under load for osteoarthritis pathways. Non–weight-bearing AP is standard for acute trauma and non-ambulatory patients.
Intercondylar Fossa (Tunnel) Projections
Tunnel views demonstrate the intercondylar notch, tibial spines, and loose bodies that hide on routine AP/lateral.
| Method (concept) | Patient position | CR idea | Goal |
|---|---|---|---|
| Camp-Coventry (PA axial) | Prone; knee flexed 40–50°; foot supported | Angle caudad to match flexion (e.g., 40° caudad for 40° flexion), entering popliteal crease | Open notch; spines free; intercondylar eminence profiled |
| Holmblad (PA axial) | Kneeling on IR or partially standing with femur 70° to IR | Perpendicular to lower leg / IR through crease | Same notch demonstration with less CR angle dependence |
| Béclère (AP axial) | Supine; knee flexed 40–45°; IR under knee | Perpendicular to lower leg, entering 1–2 cm distal to patellar apex | Alternative when prone methods are not feasible |
Know the concept: flex the knee, direct the CR perpendicular to the lower leg (or angled to match), and open the fossa without excessive condylar rotation. Exact departmental names vary; competency is matching flexion and CR so the notch is clear and spines are not obscured.
Patella
| Projection | Setup | CR | Evaluation |
|---|---|---|---|
| PA (preferred) | Prone; patella parallel to IR (slight lateral rotation of heel often ~5–10°) | Perpendicular to mid-patella | Sharp patellar cortex; no rotation; femorotibial joint may be included per protocol |
| Lateral | True lateral; 5–10° flexion only for non-trauma patella series when possible | Mid-patellofemoral joint | Patella in profile; joint spaces open; assess alta/baja and effusion |
| Tangential (Merchant) | Supine; knees flexed ~45° over table end; IR held against shins/perpendicular setup per device | Angled ~30° from horizontal caudad to patellofemoral joints (device-dependent) | Bilateral sulcus and medial/lateral facet relationships; subluxation assessment |
| Tangential (Settegast) | Prone or seated; knee flexed ≥90°; patient holds IR | Angled 15–20° to joint (perpendicular to space) | Patella free of femur; sulcus visible—contraindicated if transverse patellar fracture suspected until fracture ruled out |
Hughston and other inferosuperior/superoinferior skyline variants follow the same idea: beam tangent to the patellofemoral joint. Choose the method the department stocks equipment for and that the patient’s flexion tolerance allows.
Trauma rule for the patella and knee
If transverse patellar fracture or acute hemarthrosis is possible, obtain AP (or PA) and cross-table lateral first. Do not force deep flexion for Settegast or other high-flexion skylines until fracture is excluded—flexion can distract fragments. Merchant devices use moderate flexion and may still be inappropriate in acute unstable trauma; follow protocol and clinical guidance.
Cross-Table Lateral Trauma Knee
For stretcher and acute injury patients:
- Leave the leg as found; support with sponges.
- Place IR vertically against the medial or lateral aspect of the knee (grid if required by part thickness/technique).
- Direct a horizontal beam through the joint, matching cephalad angle principles when possible for condyle superimposition.
- Include distal femur and proximal tibia-fibula.
Cross-table laterals also show lipohemarthrosis (fat-fluid level in the suprapatellar bursa)—a critical sign of intra-articular fracture even when the fracture line is subtle. Vertical-beam laterals with the patient rolled may obscure this layered fluid-fat sign.
Common Knee & Leg Injuries (Critique Context)
| Injury / finding | Why it matters | Positioning link |
|---|---|---|
| Tibial plateau fracture | Joint depression; surgical decisioning | True AP/lateral; obliques; cross-table lateral for lipohemarthrosis |
| Patellar fracture | Transverse vs stellate; extensor mechanism | PA/AP + lateral; avoid forced skyline if unstable |
| Fibular head/neck fracture | Associated with knee or Maisonneuve patterns | Include proximal fibula on knee/tib-fib series |
| Joint effusion | Soft-tissue diagnosis on lateral | True lateral with visible suprapatellar region |
| Loose body in notch | Locking symptoms | Tunnel projection |
| Patellar subluxation | Tracking disorders | Merchant/tangential comparison |
Maisonneuve-pattern thinking: proximal fibular fracture with ankle injury means the full-length tib-fib and ankle series may both be needed—do not mentally isolate “ankle only” when the requisition or clinical note spans both regions.
Technique Tips
Knee and proximal tib-fib thickness often warrants a grid (or AEC with appropriate chamber) per department chart; distal tib-fib may be non-grid. Use adequate kVp for penetration of the distal femur without losing soft-tissue effusion visibility on lateral. Collimate to the joint or shaft segment of interest; bilateral Merchant views should still collimate per knee when possible. Marker placement must not cover the joint line or fibular head.
CAMRT Application Focus
When a stem shows a trauma knee that “cannot flex,” choose cross-table lateral plus AP without forcing tunnel or Settegast flexion. When the clinical question is osteochondral loose body or notch morphology, add a tunnel method with CR matched to flexion. When patellofemoral tracking is the question in a stable outpatient, Merchant-type tangential bilateral comparison is high-yield. When a tib-fib requisition arrives, ensure both joints are represented—even if that means two overlapping fields. Safe adaptation, correct rotation criteria, and pathology-linked projection choice are the RTR.4/RTR.6 behaviors the CAMRT exam rewards.
For a routine mediolateral knee projection in a cooperative non-trauma patient, which combination is most appropriate?
Which projection is primarily used to open the intercondylar fossa and free the tibial spines for evaluation of loose bodies?
A patient arrives on a stretcher with an acutely swollen knee after a fall. Which lateral approach is preferred for the initial trauma series?
On a correctly positioned AP knee radiograph, which evaluation criterion is expected?