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Updated: Jul 30 2023

TKA Axial Alignment

Images alignment corrected.jpg
  • Introduction
    • Important consideration
      • pre-op planning
      • component insertion
      • ligament balancing
      • prosthetic design selection
    • Normal anatomy
      • distal femur is ~9 degrees of valgus (anatomic axis compared to joint line)
        • 5-7 deg valgus of femur refers to difference of anatomic axis to mechanical axis
      • proximal tibia is 2-3 degrees of varus (anatomic axis to joint line)
    • Technical goals
      • restore mechanical alignment (mechanical alignment of 0°)
      • restore joint line ( allows proper function of preserved ligaments. e.g., pcl)
      • balanced ligaments (correct flexion and extension gaps)
      • maintain normal Q angle (ensures proper patellar femoral tacking)
    • Mechanical axis of Limb
      • axis from center of femoral head to center of ankle
  • Preoperative Evaluation
    • Radiographs
      • standing AP and lateral of knee
        • to evaluate for
          • joint space narrowing
          • collateral ligament insufficiency
            • lateral gapping in varus & medial gapping in valgus deformities
          • subluxation of femur on tibia
          • bone defects
      • standing full-length radiographs (AP and Lateral)
        • are indicated to determine an accurate valgus cut angle when the patient has
          • femoral or tibial deformity
          • very tall or short stature
      • extension and flexion laterals
      • sunrise view
  • Femoral Alignment
    • Anatomic axis femur (AAF)
      • a line that bisects the medullary canal of the femur
      • determines entry point of femoral medullary guide rod
      • intramedullary femoral guide goes down anatomic axis of the femur
    • Mechanical axis femur
      • defined by line connecting center of femoral head to point where anatomic axis meets intercondylar notch
      • obtaining a neutral mechanical axis allows even load sharing between the medial and lateral condyles of a knee prosthesis
    • Valgus cut angle (~5-7° from AAF )
      • difference between AAF and MAF
      • perpendicular to mechanical axis
      • jig measures 6 degrees from femoral guide (anatomic axis)
      • will vary if people are very tall (VCA < 5°) or very short (VCA > 7°)
      • can measure on a standing full length AP x-ray
  • Tibial alignment
    • Anatomic axis of tibia (AAT)
      • a line that bisects medullary canal
      • tibia medullary guide (internal or external) runs parallel to it
      • determines entry point for tibial medullary guide rod
    • Mechanical axis of tibia
      • line from center of proximal tibia to center of talus
      • proximal tibia is cut perpendicular to mechanical axis of tibia
      • usually mechanical axis and anatomic axis of tibia are coincident and therefore you can usually can cut the proximal tibia perpendicular to anatomic axis (an axis determined by an intramedullary jig)
      • if there is a tibia deformity and the mechanical and anatomic axis are not the same, then the proximal tibia must be cut perpendicular to the mechanical axis (therefore an extramedullary tibial guide must be used)
  • Patellofemoral Alignment
    • Q angle
      • Abnormal patellar tracking, although not the most serious, is the most common complication of TKA.
      • The most important variable in proper patellar tracking is preservation of a normal Q angle (11 +/- 7°)
        • the Q angle is defined as angle between axis of extensor mechanism (ASIS to center of patella) and axis of patellar tendon(center of patella to tibial tuberosity)
      • Any increase in the Q angle will lead to increased lateral subluxation forces on the patella relative to the trochlear groove, which can lead to pain and mechanical symptoms, accelerated wear, and even dislocation.
        • It is critical to avoid techniques that lead to an increased Q angle. Common errors include:
          • internal rotation of the femoral prosthesis
          • medialization of the femoral component
          • internal rotation of the tibial prosthesis
          • placing the patellar prosthesis lateral on the patella
      • Q angle management in TKA
  • Joint Line Preservation
    • Goal is to restore the joint line by inserting a prosthesis that is the same thickness as the bone and cartilage that was removed
      • this preserves appropriate ligament tension
      • if there are bone defects they must be addressed so the joint line is not jeopardized
      • elevating the joint line (> 8mm leads to motion problems) and can lead to
        • mid-flexion instability
        • patellofemoral tracking problems
        • an "equivalent" to patella baja
        • never elevate joint line in a valgus knee until after balancing to obtain full extension
      • lowering joint line can lead to
        • lack of full extension
        • flexion instability
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