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  • Patella baja is characterized by lowering of the patella relative to its normal position
    • may be congenital or acquired (this topic)
  • Pathophysiology of acquired patella baja
    • common causes include
      • proximal tibial osteotomy
        • patella baja is the most common complication seen following proximal tibial opening-wedge osteotomy    
        • may be caused by shortening of the patellar tendon during tibial osteotomy or from scarring of the patellar tendon post-operatively
      • tibial tubercle slide or transfer  
      • trauma to the proximal tibia
      • technical error during primary total knee replacement (joint line elevation)
      • ACL reconstruction
  • Associated conditions
    •  total knee arthroplasty
      • patella infera is an important consideration when performing total knee arthroplasty
        • improper technique may cause patella baja
        • special techniques must be utilized when performing TKA in patients with patella baja from congenital or acquired (tibial osteotomy, prior TKA) causes
  • Symptoms
    • anterior impingement knee pain
    • knee stiffness
  • Physical exam
    • mechanical block to full flexion 
      •  limited flexion due to patellar impingement on the tibia in extremes of flexion
  • Radiographs
    • recommended views
      • AP and lateral views of the knee
      • lateral view of the knee in 30 degrees of flexion
        • used to measure Insall-Salvati ratio
          • measures ratio patellar tendon length to patellar bone length 
          • normal Insall-Salvati is 1:1 between length of the patellar tendon length to patellar bone length
    • findings 
      • lateral view in extension
        • distal positioning of the patella in relation to the trochlear groove 
      • Insall-Salvati ratio of < 0.8 is consistent with patella baja
  • Nonoperative
    • activity modifications, physical therapy
      • indications
        • mild symptoms in younger patients
  • Operative
    • total knee replacement
      • indications
        • severe impingement in older patients with osteoarthritis
  • Total knee arthroplasty in patient with patella baja
    • methods to address patella infera during TKA
      • place patellar component superiorly
        • indications
          • mild patella baja 
        • technique
          • use a smaller patellar dome placed on superior aspect of patella
          • trim inferior bone to decrease flexion impingement
      •  lower joint line 
        • indications
          • moderate patella baja  
        • technique
          • add distal femoral augmentation  
          • cut more proximal tibia to lower joint line (lower tibial cut)
          • avoid bone cuts that raise the joint line
            • raising the joint line will effectively increase the patella baja deformity 
          • may require revision knee system
      • transfer tibial tubercle to cephalad position
        • indications
          • moderate patella baja 
        • technique
          • technique is difficult due to complexity of a tibial transfer in proximity to a cemented tibial component
        • outcomes
          • unpredictable bone healing leads to variable, and often poor, outcomes
          • patients may be left with extensor lag 
      • patellectomy 
        • indications
          • severe patella baja
        • techniques
          • alters the tension in the anterior knee mechanism
            • therefore recommended to use use a cruciate substituting system
          • consider partial patellectomy in which patella is resected to a width of 10-12mm.
            • lessens impingement & crepitus while maintaining some of the fulcrum of the patella.

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