Patellar translation (TT-TG distance)

Patellar translation (TT-TG distance) is an analysis performed by overlapping or superimposing axial images of the femoral condyles and tibial tuberosity. The measurements are used to quantify patellar instability.


žSuperimpose axial images of:

  • femoral condyles
  • tibial tuberosity

žDraw a line along the posterior femoral condyles, and then draw the following lines perpendicular to this line:

  • bisecting the tibial tuberosity (TT)
  • bisecting the trochlear groove sulcus (TG)
  • žmeasure the distance between TT and TG = TT-TG distance

A distance of <15 mm is considered normal, 15-20 mm is considered borderline and a distance >20 mm is considered abnormal. The distance has been shown to be slightly less when measured on MRI than CT 1.

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Article information

rID: 44662
Section: Radiography
Synonyms or Alternate Spellings:
  • Patella analysis
  • Patella tracking
  • TT-TG
  • TTT Distance

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Cases and figures

  • Figure 1: TT-TG distance measurement
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