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Updated: Jun 15 2023

TKA Patellofemoral Alignment

Images Implant Rotation Illustration small_moved.jpg Rotation Illustration smallcopy_moved.jpg lateralization illustration small_moved.jpg
  • Introduction
    • Abnormal patellar tracking is the most common complication of TKA.
      • the most important variable in proper patellar tracking is preservation of a normal Q angle.
    • Abnormal Q angle
      • an increase in the Q angle will lead to
        • an increased lateral subluxation forces on the patella relative to the trochlear groove
        • possible pain, mechanical symptoms, accelerated wear, and even dislocation.
      • it is critical to avoid techniques that lead to increase Q angle. Common errors include
        • internal rotation of the femoral prosthesis
        • medialization of the femoral component
        • internal rotation or medialization of the tibial prosthesis
        • placing the patellar prosthesis lateral on the patella
  • Anatomy
    • Q Angle
      • the Q angle is defined as angle between
        • axis of extensor mechanism (ASIS to center of patella)
        • axis of patellar tendon (center of patella to tibial tuberosity)
  • Imaging
    • CT scan
      • malrotation of components is best diagnosed with CT scan of the knee
  • Femoral Prosthesis
    • There are three reference axis that one may use:
      • anteroposterior axis
        • defined as a line running from the center of the trochlear groove to the top of the intercondylar notch
        • a line perpendicular to this defines the neutral rotational axis
      • transepicondylar axis
        • defined as a line running from the medial and lateral epicondyles the epicondylar axis is parallel to the cut tibial surface
        • a posterior femoral cut parallel to the epicondylar axis will create the appropriate rectangular flexion gap
      • posterior condylar axis
        • defined as a line running across the tips of the two posterior condyles
        • this line is in ~ 3 degrees of internal rotation from the transepicondylar axis, the femoral prosthesis should be externally rotated 3 degrees from this axis to produce a rectangular flexion gap
        • if the lateral femoral condyle is hypoplastic, use of the posterior condylar axis may lead to internal rotation of the femoral component
        • WARNING: the average posterior condylar twist angle is 3º but the range is 1-10º. Therefore vary angle of femoral rotation based on variances in femoral anatomy.
    • Internal Rotation of Femoral Prosthesis will Increase Q angle
      • by internally rotating the femoral prosthesis, you are effectively bringing the groove and the patella medially. This will increase the Q angle to the tibial tubercle
      • will also make the medial compartment tight in flexion with subsequent TKA stiffness
    • Medialization of the Femoral Prosthesis will Increase Q angle
      • a medialized femoral prosthesis will bring the trochlear groove to a more medial position, and thus bring the patella medial with it, thus increasing the Q angle
      • therefore, you want the femoral component to be slighly lateral if anything
  • Tibial Prosthesis
    • The preferred rotation of the tibial component is neutral, with no internal or external rotation.
      • the best way to obtain this is to have the tibial component centered over the medial third of the tibial tubercle
      • this may leave a portion of the posteromedial tibia uncovered and some overhang of the prosthesis over the tibia on the posterolateral tibia.
    • Internal Rotation of Tibial Prosthesis will increase Q angle
      • internal rotation of the tibial component effectively results in relative external rotation of the tibial tubercle and an increase in the Q angle
    • Medialization of tibia will increase Q angle
  • Patellar Prosthesis
    • The preferred position of the patellar prosthesis is to be either centered over the patella or medialized
      • medializing the patellar component is one strategy to decrease the Q angle.
      • results in uncoverage of lateral facet. Consider removing to lessen risk of lateral facet syndrome.
      • another alternative is use of an oval shaped patella with the apex medialized.
    • Lateralization of the patellar prosthesis will increase the Q angle and increase maltracking
    • Intraoperative lateral subluxation of the patella
      • if patella laterally subluxes intraoperatively during trialing, deflate tourniquet and recheck before performing a lateral release
    • Indications for resurfacing
      • absolute
        • inflammatory arthritis
        • patella maltracking
        • patellofemoral arthritis as the main indication for TKA
    • Options for resurfacing during TKA
      • always resurface
      • never resurface
        • option to perform patelloplasty
          • excision of marginal osteophytes, reshaping of patella
      • selective resurfacing
    • Patella resurfacing vs. Non-resurfacing
      • less anterior knee pain with resurfacing
      • less revision rates with resurfacing
      • inferior results with secondary resurfacing
      • similar patient satisfaction rates
      • trochlear design important: “patellar friendly”
        • thinner anterior flange
        • anatomic trochlear groove
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