How to Read a Knee X-Ray: A Systematic Approach
Systematic knee radiograph interpretation — technical adequacy, bone assessment, joint space, soft tissue, alignment, and clinical correlation.
Why a Systematic Approach Matters
The most common error in radiograph interpretation is not misidentifying a finding — it is failing to look for it. A systematic approach ensures that every clinically relevant structure is evaluated on every film, regardless of the presenting complaint. The surgeon who reads every knee radiograph the same way will miss fewer findings than the surgeon who only looks at the area of suspected pathology.
This guide presents a structured five-step approach applicable to standard knee radiographic series: anteroposterior (AP), lateral, and where available, skyline (Merchant) views.
Step 1: Assess Technical Adequacy
Before interpreting any findings, confirm the radiograph is adequate for diagnosis.
Patient identification — verify the correct patient name and date. In a busy clinic, image-patient mismatch is a real risk.
Projection and positioning — on the AP view, the tibial spines should be centred between the femoral condyles, indicating proper rotation. The fibular head should overlap the lateral tibial plateau by approximately one-third. On the lateral view, the femoral condyles should be superimposed (or nearly so).
Exposure — trabecular bone detail should be visible in both the femur and tibia. If the film is too dark (overexposed), subtle fractures and early osteophytic changes may be obscured. If too light (underexposed), soft tissue detail is lost.
Coverage — the AP view should include the distal femoral shaft, the entire knee joint, and the proximal tibial shaft including the fibular head. The lateral view should include the suprapatellar pouch.
Step 2: Evaluate Bone
Trace the cortical outline of each bone systematically and continuously. The femoral condyles should show smooth, convex articular surfaces without cortical irregularity, step-off, or lucent lines that might indicate fracture. Continue along the tibial plateau — both medial and lateral surfaces should be clearly defined. Trace the fibular head, the proximal tibial shaft, and the patella (on lateral view).
A cortical break — a discontinuity in the white cortical line — is the hallmark of fracture. Tibial plateau fractures, distal femoral fractures, and patellar fractures can all be subtle on plain radiographs. If clinical suspicion is high despite a negative radiograph, cross-sectional imaging (CT or MRI) is warranted.
Look for focal bone lesions (lytic or scite areas), periosteal reaction (new bone formation along the cortex, suggesting infection, stress fracture, or tumour), and bone density changes (generalised osteopenia or focal sclerosis).
Step 3: Evaluate the Joint Space
The joint space width is assessed on the AP weight-bearing view. Compare the medial and lateral compartments — in a normal knee, both compartments should be approximately equal in width, with the lateral space being slightly wider.
Asymmetric narrowing is the hallmark of osteoarthritis. Medial compartment narrowing is most common (reflecting the higher load borne by the medial compartment). Lateral compartment narrowing, while less common, occurs in valgus-aligned knees. Patellofemoral narrowing is assessed on the lateral and skyline views.
Step 4: Look for OA Features
If joint space narrowing is present, systematically assess the four cardinal radiographic features of osteoarthritis, which form the basis of the Kellgren-Lawrence grading system:
Osteophytes — bony spurs at the joint margins. Look at the medial and lateral femoral condyle margins, the tibial plateau margins, and the superior and inferior patellar poles. Osteophytes are the most sensitive early radiographic sign of OA.
Subchondral sclerosis — increased whiteness (density) of the bone immediately beneath the joint surface, typically in the weight-bearing zone of the affected compartment. This reflects bone remodelling in response to abnormal mechanical loading.
Subchondral cysts appear as well-defined lucent (dark) areas within the subchondral bone. They indicate advanced disease and are most commonly found in the medial tibial plateau and medial femoral condyle.
Deformity — in advanced OA, bone loss and remodelling can alter the joint contour. Flattening of the medial tibial plateau, femoral condyle erosion, and subluxation all indicate severe disease (KL grade 4).
Step 5: Check Alignment
On a standard AP knee radiograph, a rough assessment of alignment can be made by evaluating the femorotibial angle (FTA). The anatomical axis of the femoral shaft and the tibial shaft should intersect at an angle of approximately 5–7 degrees of valgus. Significant varus (bow-legged) or valgus (knock-kneed) alignment should be documented, as it influences both OA progression and surgical planning.
For precise alignment measurement (LDFA, MPTA, HKA), a full-length standing leg radiograph is required — standard knee films are insufficient for accurate mechanical axis determination.
The Lateral View: Additional Information
The lateral view provides information not available on the AP. Evaluate the suprapatellar pouch for effusion (a dark stripe of fluid above the patella). Assess the patella position relative to the femoral trochlea — patella alta (high-riding patella) is associated with patellar instability; patella baja (low-riding patella) may indicate patellar tendon scarring or prior surgery.
The skyline view, when available, shows patellofemoral joint space, lateral patellar tilt, and trochlear dysplasia — all relevant to anterior knee pain assessment.
AI-Assisted Interpretation
Artificial intelligence is increasingly being used to augment radiograph interpretation. Deep learning models trained on large annotated datasets can automatically detect and classify OA severity, measure joint space width, and identify alignment abnormalities — providing a quantitative, reproducible second opinion that complements the surgeon's clinical assessment.
The Salnus Surgeon Portal integrates AI-assisted OA screening and measurement directly into the DICOM viewing workflow. Upload a knee radiograph, and the system provides automated KL grading, JSW measurement, and a GradCAM heatmap showing which regions of the image influenced the AI's assessment.
Disclaimer: This article is for educational purposes only and does not replace formal radiology training. All radiograph interpretations should be performed by qualified healthcare professionals.
References:
- Helms CA. Fundamentals of Skeletal Radiology. 5th ed. Elsevier; 2019.
- Kellgren JH, Lawrence JS. Radiological assessment of osteo-arthrosis. Ann Rheum Dis. 1957;16(4):494-502.
Reviewed by the Salnus biomedical engineering team.