What causes behind the knee pain after total knee replacement?

Common Causes of Behind the Knee Pain After Total Knee Replacement

Behind the knee pain after total knee arthroplasty (TKA) is most commonly caused by aseptic loosening, instability, arthrofibrosis, or soft tissue impingement, with each requiring specific diagnostic imaging for confirmation. 1

Primary Causes of Posterior Knee Pain Post-TKA

1. Aseptic Loosening

• Most common cause of late-stage TKA failure (>2 years post-surgery) 1
• Results from:
  • Inadequate primary fixation
  • Mechanical stresses
  • Particle debris causing osteolysis
  • Poor bone stock
• Diagnostic approach:
  • Initial radiographs (rated 9/9 for appropriateness) 1
  • CT scan without contrast (rated 7/9) for suspected loosening 1

2. Mechanical Instability

• Represents the third most common cause of prosthesis failure (7.5% of cases) 1
• Often results from:
  • Surgical error
  • Poor prosthesis selection
  • Ligamentous imbalance
• Diagnostic approach:
  • Standard radiographs first (rated 9/9) 1
  • Fluoroscopy (rated 7/9) to assess abnormal motion 1
  • CT without contrast (rated 5/9) for standard instability 1

3. Soft Tissue Abnormalities

• Includes:
  • Quadriceps or patellar tendinopathy (0.17-2.5% incidence) 1
  • Arthrofibrosis (4.5-6.9% of failures) 1
  • Soft tissue impingement
• Diagnostic approach:
  • MRI without contrast (rated 5/9) 1
  • Patients with keloids have increased risk of arthrofibrosis 1

4. Osteolysis

• Higher incidence in cementless TKA compared to cemented TKA 1
• Common locations:
  • Femoral condyles near collateral ligament attachments
  • Periphery of components
  • Access channels to cancellous bone
• Diagnostic approach:
  • CT without contrast (rated 8/9) for suspected osteolysis 1
  • MRI without contrast (rated 6/9) 1

Inflammatory Causes

• Persistent low-grade inflammation can cause chronic pain
• Patients with high chronic postoperative knee pain show increased levels of high-sensitivity C-reactive protein (4.3 mg/L vs. 1.7 mg/L in low pain groups) 2
• Diagnostic approach:
  • Laboratory tests for inflammatory markers
  • Joint aspiration if infection suspected (rated 9/9) 1

Neurogenic Pain

• Neuropathic pain peaks between 6 weeks and 3 months post-operatively 3
• High correlation between pain scores and neuropathic pain scores at 3 months, 1 year, and 3 years post-TKA 3
• May present as burning, shooting, or electric-like pain

Diagnostic Algorithm for Behind the Knee Pain

1. Initial evaluation:

   • Standard radiographs (AP, lateral, and axial views) 1, 4
   • Physical examination to assess range of motion (limited ROM is associated with higher pain) 2

2. If radiographs are negative:

   • For suspected mechanical issues: CT without contrast (rated 7-8/9) 1
   • For suspected soft tissue problems: MRI without contrast (rated 5-6/9) 1

3. For suspected infection:

   • Joint aspiration for culture and cell count (rated 9/9) 1
   • If aspiration is negative but infection still suspected: Three-phase bone scan with WBC imaging 1

4. For suspected instability:

   • Fluoroscopy to assess dynamic motion (rated 7/9) 1
   • CT for component rotation assessment (rated 9/9) 1

Common Pitfalls in Diagnosis

• Failure to consider multiple contributing factors - most painful TKAs have more than one cause 5
• Over-reliance on bone scans too early after surgery - positive bone scans are noted in 20% of asymptomatic knees 1 year after surgery 1
• Not distinguishing between mechanical and inflammatory causes of pain
• Overlooking patellofemoral issues which require specific diagnostic pathways 6

Key Considerations for Clinicians

• Component alignment is critical regardless of surgical approach 4
• Soft tissue balance significantly affects outcomes even with perfect bone cuts 4
• Pain catastrophizing is associated with higher levels of chronic pain after TKA 2
• Approximately 20% of TKR patients experience more pain post-operatively than pre-operatively 7

By following this diagnostic algorithm and understanding the common causes of posterior knee pain after TKA, clinicians can more effectively identify and address the underlying issues, improving patient outcomes and quality of life.