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