How to assess a knee X-ray after Total Knee Replacement (TKR)?

How to Assess Knee X-ray After Total Knee Replacement

Standard weight-bearing anteroposterior (AP), lateral, and axial (skyline) radiographs of the knee form the foundation for post-TKR assessment, with full-length hip-to-ankle views providing optimal evaluation of mechanical axis alignment. 1

Essential Radiographic Views

Standard Three-View Series

• Weight-bearing AP view: Critical for assessing coronal plane alignment, component positioning, and joint space (polyethylene thickness) 1, 2
• Weight-bearing lateral view: Evaluates sagittal plane alignment, tibial slope, and posterior tibial subluxation 1, 2
• Axial (skyline) view: Demonstrates patellar tracking, tilt, subluxation, and patellofemoral kinematics 1
  • Weight-bearing axial views are preferred for better assessment of patellofemoral kinematics 1

Extended Views for Comprehensive Assessment

• Full-length standing hip-to-ankle radiographs: Provide optimal assessment of the mechanical axis and overall limb alignment 1, 3
  • After baseline hip-to-ankle radiograph, subsequent follow-up can use targeted knee radiographs 1
• AP views in 10° internal rotation: Improve interpretation of varus/valgus alignment compared to neutral positioning 1

Systematic Assessment Protocol

1. Component Alignment - Coronal Plane

• Mechanical axis: Should pass through center of knee joint 1
• Femoral component: Assess for varus/valgus positioning relative to mechanical axis 1
• Tibial component: Evaluate perpendicularity to mechanical axis 1
• Threshold for malalignment: >3° from neutral is considered significant 4

2. Component Alignment - Sagittal Plane

• Tibial slope: Highly reliable measurement on plain radiographs 5
• Femoral component flexion/extension: Assess relative to anatomic axis 6
• Tibial component: Evaluate for anterior or posterior tilt 6

3. Polyethylene Wear Assessment

• Joint space narrowing: Measured as minimum distance from metallic femoral condyle to tibial baseplate 1
• Annual weight-bearing radiographs recommended: Detect subclinical wear before clinical manifestation 1
• Accuracy limitation: Approximately 1mm initially, decreases with progressive wear 1
• Effusion presence: May accompany wear 1

4. Patellar Assessment

• Patellar height: Evaluate for patella alta or baja 1
• Patellar tilt and subluxation: Best assessed on axial views 1
• Patellar tracking: Dynamic assessment with weight-bearing views 1

5. Signs of Complications on Radiographs

Infection (Most Serious Complication)

• Incidence: 0.8-1.9% of TKAs 1
• Radiographic signs: Often subtle or absent in early/low-grade infection 1
• Clinical correlation essential: Pain (especially at rest or night pain), swelling, warmth, erythema 1
• Next step if suspected: Joint aspiration is appropriate initial procedure alongside radiographs 1

Extensor Mechanism Tears

• Patella alta or baja: Abnormal patellar position 1
• Localized soft-tissue swelling 1
• Posterior tibial subluxation 1
• Bony avulsions or dystrophic calcifications within tendon 1

Component Loosening

• Progressive radiolucent lines: >2mm or progressive widening 2
• Component migration: Serial comparison essential 2
• Subsidence: Particularly of tibial component 2

When Plain Radiographs Are Insufficient

Component Rotation Assessment

CT without IV contrast is the gold standard for measuring component rotation when malrotation is suspected. 1

• Indications: Persistent pain, patellar complications, suspected malrotation 1
• Femoral component rotation: Assessed relative to transepicondylar axis, Whiteside line, or posterior femoral condyles 1
  • Should be parallel to transepicondylar axis 1
• Tibial component rotation: Should be positioned in approximately 18° internal rotation relative to tibial tubercle 1
• Combined internal rotation: Directly proportional to severity of patellofemoral complications 1, 7
  • 1-4° combined internal rotation: lateral tracking and patellar tilting 7
  • 3-8° combined internal rotation: patellar subluxation 7
  • 7-17° combined internal rotation: patellar dislocation or prosthesis failure 7

Advanced Imaging Modalities

• 3D-CT reconstruction: Superior to 2D-CT for rotational measurements (ICC 0.89-0.99 vs 0.29) and component positioning 5
• MRI with metal artifact reduction: Can assess component rotation and soft-tissue abnormalities when CT unavailable 1
• CT for occult fractures: When radiographically occult periprosthetic fracture suspected 1
• Ultrasound: Limited role; useful for quadriceps/patellar tendinopathy, arthrofibrosis, and soft-tissue masses 1

Common Pitfalls to Avoid

• Non-weight-bearing films: Fail to expose true alignment, ligamentous laxity, and polyethylene wear 1, 2
• Relying solely on symptoms: Serial radiographs can detect failures before clinical manifestation 2
• Inadequate views: Single-plane assessment misses critical information 6, 2
• Ignoring subtle changes: Small progressive radiolucencies or component migration warrant close follow-up 2
• Using 2D-CT for rotation: Poor reliability (ICC 0.29); use 3D-CT reconstruction instead 5
• Ordering advanced imaging prematurely: Standard radiographs should be obtained first in all cases 1