What are the signs of osteoarthritis (OA) in a knee X-ray?

Radiographic Signs of Knee Osteoarthritis

The classic radiographic signs of knee osteoarthritis include joint space narrowing, osteophyte formation, subchondral sclerosis, and subchondral cysts, which should be assessed using weight-bearing views for optimal detection. 1

Key Radiographic Features

Primary Features

• Joint space narrowing (JSN):

  • Most sensitive indicator of cartilage loss
  • Typically asymmetric, affecting medial compartment more commonly
  • Best visualized on weight-bearing views

• Osteophytes:

  • Bony projections at joint margins
  • Common locations: femoral condyles, tibial plateaus, patellofemoral joint
  • Early indicator of OA, often preceding JSN

• Subchondral sclerosis:

  • Increased bone density beneath the articular cartilage
  • Appears as whitened/increased opacity on X-ray
  • Represents bone's response to increased mechanical stress

• Subchondral cysts:

  • Fluid-filled lesions in the subchondral bone
  • Result from synovial fluid intrusion through damaged cartilage 2, 3
  • Appear as well-defined lucencies in subchondral bone

Additional Features

• Bone attrition: Loss of bone contour/flattening of articular surfaces
• Subluxation: Partial displacement of joint surfaces
• Deformity: Varus (bow-legged) or valgus (knock-kneed) alignment

Optimal Radiographic Views

For proper assessment of knee OA, specific views are recommended 1:

• Weight-bearing anteroposterior (AP) view: Essential for accurate assessment of joint space narrowing
• Lateral view: Evaluates patellofemoral joint and posterior compartment
• Patellofemoral (skyline/sunrise) view: Assesses patellofemoral compartment
• Posteroanterior (PA) flexed view (Rosenberg view): Increases sensitivity for detecting early joint space narrowing

Grading Systems

The Kellgren and Lawrence (K-L) system is commonly used to grade OA severity 4:

• Grade 0: No radiographic features of OA
• Grade 1: Doubtful narrowing of joint space, possible osteophytes
• Grade 2: Definite osteophytes, possible joint space narrowing
• Grade 3: Moderate multiple osteophytes, definite joint space narrowing, some sclerosis, possible bone deformity
• Grade 4: Large osteophytes, marked joint space narrowing, severe sclerosis, definite bone deformity

Clinical Correlation

Radiographic findings may not always correlate with clinical symptoms 1:

• Some patients with significant radiographic changes may have minimal symptoms
• Others with minimal radiographic changes may have severe pain
• Functional impairment in knee OA should be carefully assessed and monitored using validated outcome measures

Common Pitfalls and Caveats

• Non-weight-bearing views: May underestimate joint space narrowing, leading to underdiagnosis 1
• Inadequate view selection: Missing patellofemoral OA without skyline views
• Confusing OA with other conditions: Calcium pyrophosphate deposition disease (CPPD) can mimic or coexist with OA 5
• Over-reliance on imaging: Diagnosis should incorporate clinical findings; imaging is not required for typical presentations 1
• Failure to assess alignment: Mechanical axis deviation affects disease progression and treatment planning

When to Consider Advanced Imaging

Plain radiographs should be the initial imaging modality for knee OA 1. Consider MRI when:

• Symptoms are disproportionate to radiographic findings
• Suspicion of additional internal derangement (meniscal tears, ligament injuries)
• Need to assess extent of cartilage damage or bone marrow lesions
• Planning for surgical intervention

Remember that MRI may reveal incidental findings that are not clinically relevant, especially in older patients 1.