|Calculation of the TT-TG distance is a normal part of the evaluation of a person with an unstable kneecap.|
|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.|
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.