Approach to KNEE radiograph
- Dr Ayush Srivastava
- Oct 2
- 2 min read
Updated: Oct 2
1. Standard Views:
AP View: Best for assessing the overall alignment (varus/valgus), joint space narrowing.
Lateral View: For evaluating the patellofemoral joint, knee effusion (Lipohemarthrosis), and the alignment of the femoral and tibial condyles.
Patellar View (Sunrise/Merchant View): For assessing the patellofemoral joint congruency and evaluating for patellar fractures or osteochondral defects.
2. Alignment and Geometry Check
A. Mechanical Axis (AP View)
The mechanical axis should pass from the center of the femoral head to the center of the ankle joint, passing through the center of the knee joint.
Normal: The mechanical axis passes centrally through the knee.
Genu Varum (Bow-legged): Weight-bearing axis shifts to the medial (inner) compartment.
Genu Valgum (Knock-kneed): Weight-bearing axis shifts to the lateral (outer) compartment.
B. Patellofemoral Alignment (Lateral View)
Patellar Height: The position of the patella relative to the femur is assessed using the Insall-Salvati Ratio.
Insall Salvati ratio:
Normal: 0.8 to 1.2
Patella Baja (<0.8): Low-lying patella.
Patella Alta (>1.2): High-riding patella.
Patellar Tilt/Subluxation (Patellar View): Assess for lateral subluxation or tilt, which is often a contributing factor to patellofemoral pain syndrome.
3. Joint Space and Cartilage
Evaluate the width and symmetry of the femorotibial and patellofemoral joint spaces.
Femorotibial Joint:
Look for asymmetric narrowing (e.g., medial compartment narrowing in early osteoarthritis).
Symmetry suggests a uniform process like inflammatory arthritis (rheumatoid arthritis) or normal cartilage.
Patellofemoral Joint:
Assess for superior or inferior joint space narrowing and osteophytes.
Degenerative Changes: Look for the characteristic triad of Osteoarthritis (OA):
Joint space narrowing (often asymmetric).
Osteophytes (bone spurs) at the joint margins.
Subchondral sclerosis (increased bone density) and/or subchondral cysts.
4. Bony Survey: Search for Fractures
Systematically inspect the distal femur, proximal tibia/fibula, and patella for fracture lines, subtle contour breaks, or impaction injuries.
A. Tibial Plateau Fractures:
Often difficult to see on the AP view. Look for a lip of the plateau driven down or a step-off.
Check the Lateral View for subtle lip of the plateau driven down or a Lipohemarthrosis (fat-fluid level).
The presence of a fat-fluid level (Lipohemarthrosis) in the suprapatellar bursa on a horizontal beam lateral radiograph is pathognomonic for an intra-articular fracture.
B. Patellar Fractures:
Inspect on the Lateral and Patellar views. They are typically transverse, stellate, or vertical. Differentiate from a bipartite patella (a normal, often bilateral developmental variant).
C. Osteochondral Injuries:
Look for fragments of bone/cartilage within the joint space (loose bodies).
Assess for a characteristic osteochondral defect on the lateral aspect of the medial femoral condyle associated with Osteochondritis Dissecans (OCD).
5. Soft Tissues
The soft tissues are a critical source of indirect signs of injury.
Knee Effusion: Look for fullness or displacement of the suprapatellar recess on the Lateral View.
Lipohemarthrosis: As noted above, the fat-fluid level on the horizontal beam lateral view indicates an intra-articular fracture.
Ligamentous Injury (Indirect Signs):
Segond Fracture: A small avulsion fracture of the lateral tibial plateau near the joint line. It is highly associated with an Anterior Cruciate Ligament (ACL) tear and a meniscal tear.
Arcuate Complex Fracture (Avulsion of the fibular head): Associated with injury to the Posterolateral Corner (PLC) structures.
Calcifications: Note any periarticular calcifications, which may suggest conditions like Calcium Pyrophosphate Deposition (CPPD) disease (Chondrocalcinosis, often seen in the menisci) or tendinitis/bursitis.
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