TG = trochlear groove, and for the measurement it is identified at its deepest point. TT = tibial tubercle, where the tendon attaches.
Two imaging 'slices' from different levels (one at the TG level and the other at the TT level) are overlaid to measure this distance.
Calculation of the TT-TG distance is a normal part of the evaluation of a person with an unstable kneecap.
How to measure TT-TG distance
The TT-TG distance is the distance measured from side to side, as if you dropped a 'plumb line' down from the deepest part of the groove.
What is the normal TT-TG Distance?
It is normally about 9mm, and if it´s more than 15-20mm then it means that the alignment of the groove with the tendon attachment is poor, tending to pull the kneecap over to the outer side.
The TT-TG Index: a new knee size adjusted measure method to determine the TT-TG distance. Hingelbaum S, Best R, Huth J, Wagner D, Bauer G, Mauch F. Knee Surg Sports Traumatol Arthrosc. 2014 Oct;22(10):2388-95.
Tibial tubercle-trochlear groove distance: defining normal in a pediatric population. Dickens AJ, Morrell NT, Doering A, Tandberg D, Treme G. J Bone Joint Surg Am. 2014 Feb 19;96(4):318-24.
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