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Updated: Jun 11 2021

TKA Instability

  • Summary
    • TKA Instability is a common cause of early failure following total knee arthroplasty.
    • Diagnosis can be made clinically with presence of a varus/valgus thrust during ambulation, positive posterior sag with the knee flexed to 90 degrees and overall laxity of the knee on exam.
    • Treatment depends on severity of symptoms, direction of instability and the type of TKA prosthesis present in the knee. 
  • Epidemiology
    • incidence
      • common cause of early failure following total knee arthroplasty
      • accounts for 10-20% of revisions
  • Etiology
    • Types
      • extension (varus-valgus) instability
      • flexion (anteroposterior) instability
      • mid-flexion instability
      • genu recurvatum
      • global, multiply-operated instability
  • Presentation
    • 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
  • Imaging
    • 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
  • Studies
    • 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
      • knee dislocation
        • posterior stabilized knees can "jump the post" resulting in dislocation
    • 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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