Patellar translation (TT-TG distance)

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Patellar translation can be measured using the tibial-tuberosity to trochlear groove distance (TT-TG distance) by calculating the transverse length between the trochlear groove on the femoral and tibial tuberosity on axial images. The measurements are used to quantify patellar instability.

Measurement

Superimpose axial images of:

  • femoral condyles
  • tibial tuberosity

Draw 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)
  • measure the distance between TT and TG = TT-TG distance

Interpretation

  • normal: <15 mm
  • borderline: 15-20 mm
  • abnormal: >20 mm

The distance has been shown to be slightly less when measured on MRI than CT .