Updated: 9/6/2018

TKA Instability

Review Topic
  • Introduction
    • incidence
      • common cause of early failure following total knee arthroplasty
      • accounts for 10-20% of revisions
    • types
      • extension (varus-valgus) instability
      • flexion (anteroposterior) instability
      • mid-flexion instability
      • genu recurvatum
      • global, multiply-operated instability

  • History
    • previous operations
    • indication for initial replacement
    • original implant information
    • comorbidities including
      • connective tissue disease
      • inflammatory diseases
      • diabetes, Charcot arthropathy
    • history of trauma
  • Symptoms
    • pain, instability or both
    • timeline as to start of symptoms, what worsens/improves
  • Physical Examination
    • overall gait, observe for valgus/varus thrust
    • ligamentous examination throughout range of motion, attempt to reproduce symptoms
    • flexion instability test
      • positive posterior sag with the knee flexed to 90 degrees
    • overall strength
    • extensor mechanism competency
    • patellar tracking
  • Radiographs
    • recommended views
      • weightbearing AP
        • used to assess joint line symmetry
      • full-length AP
        • used to assess overall mechanical alignment
      • lateral
        • used to assess tibial slope, tibial subluxation, recurvatum
    • findings
      • extension instability
        • excessive distal femoral resection
        • oversized femoral component
      • flexion instability
        • overresection of posterior femoral condyles
        • undersized femoral component
        • increased tibial slope
      • mid-flexion instability
        • anterior or proximal placement of femoral component
      • genu recurvatum
  • Computed tomography
    • can offer information regarding component rotation
  • Serum labs
    • CBC, ESR, CRP, must rule out infection as potential cause
  • Knee aspiration
    • to rule out infection via cell count and culture
Extension (varus-valgus) Instability
  • Definition
    • varus/valgus instability
    • types
      • symmetrical
        • caused by excessive distal femoral resection, causing flexion/extension gap mismatch
      • asymmetrical
        • more common
        • ligamentous asymmetry caused by failure to correct deformity in the coronal plane
  • Treatment
    • symmetrical instability
      • distal femoral augments to tighten extension gap
      • upsizing poly will fail as it affects both flexion and extension gaps
    • asymmetrical instability
      • balance ligaments accordingly
        • controlled release of soft tissue on contracted side
        • if ligamentously insufficient, varus/valgus constrained device needed  
      • if caused by, intraoperative MCL transection/deficiency 
        • suture repair or suture anchor reattachment, use of either CR or PS implant, hinged knee brace for 6 weeks postoperatively
        • use of unlinked constrained prosthesis
Flexion (anteroposterior) instability 
  • Definition
    • occurs when the flexion gap exceeds the extension gap
  • Treatment
    • over resection of posterior femoral condyles  
      • treat with posterior augments
    • undersizing femoral component
      • upsize femoral component
    • excessive tibial slope
      • decrease slope and consider posterior-stabilized prosthesis
    • excessive posterior femoral condyle cuts
      • augment posterior condyles of distal femur
    • posterior cruciate ligament insufficiency following a cruciate-retaining arthroplasty 
      • convert to posterior-stabilized prosthesis

Mid-flexion instability
  • Causes
    • controversial topic, poorly understood
    • associated with modification of the joint line
    • involves malrotation when the knee is flexed between 45 and 90 degrees
    • potential contributing factors
      • femoral component design in sagittal plane
      • attenuation of anterior MCL
      • overall geometry of the tibiofemoral joint
  • Treatment
    • typically, full revision is required
    • goals
      • restoration of joint line
      • equalize flexion and extension gaps
Genu recurvatum
  • Definition
    • fixed valgus deformity and iliotibial band contracture
  • Causes
    • associated with poliomyelitis, rheumatoid arthritis, or Charcot arthropathy
    • poliomyelitis
      • patient walks with knee locked in hyperextension, ankle in equinus due to quadriceps weakness
  • Treatment
    • typically long-stemmed posterior stabilized, or varus/valgus constrained implant
    • rotating-hinge reserved for salvage as residual hyperextension may occur, leading to early failure
Global, multiply-operated instability
  • Definition
    • laxity of both flexion and extension gaps, as well as varus/valgus instability
    • can be associated with severe bone loss
  • Presentation
    • multidirectional ligamentous instability with recurvatum gait
  • Treatment
    • varus/valgus constrained prosthesis at minimum
    • typically, hinged prosthesis with or without augments, sleeves, cones
    • severe bone loss situations may require endoprosthetic replacements

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(OBQ09.230) A 66-year-old female presents with knee instability going down stairs 18 months after a posterior cruciate retaining total knee arthroplasty. She reports having recurrent effusions. Radiographs are shown in Figure A. What is the most likely cause for her instability? Review Topic

QID: 3043

Intraoperative rupture of the patellar tendon




Alteration of the joint line




Posterior cruciate insuffiency




Anterior cruciate insufficiency




Catastrophic component loosening



ML 1

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