Patellar translation (TT-TG distance)

Last revised by Craig Hacking on 21 Dec 2022

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 femur and tibial tuberosity on axial images. The measurements are used to quantify patellar instability.


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


  • 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 1.

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Cases and figures

  • Case 1: TT-TG distance measurement
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  • Case 2
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  • Case 3: TG
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  • Case 3: TT-TG distance
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