The 'TT-TG distance' is a measurement of the dynamic alignment of the patella.
It gives the patellar surgeon an idea of how well the groove under the patella lines up with the lower attachment of its tendon onto the shin (tibia) - or how much the anatomy is tending to pull the patella over to the outer side.
How TT-TG distance is measured
Although originally the measurements were taken from an X-ray taken with the knees bent to 30 degrees, nowadays it is always (and more accurately) derived from CT or MRI scans of the patient's knee. CT and MRI are 3-dimensional scans, which allow thin digital 'slices' of imaging information to be viewed and manipulated. Because the original scan is taken in three dimensions, the computer can 'reconstruct' views from any angle, or can overlap slices from different parts of the knee.
If the CT or MRI 'cuts' a thin axial slice here, and you laid it on a flat surface....
...it would look like this, with the patella and femur in cross-section.
A line (see the red lines) could be drawn on the slice at the back of the rounded bits of the femur (the posterior condylar line), and a second line could drawn perpendicularly from the deepest part of the groove under the patella (trochlear groove) to meet the other.
The vertical line would be the TG line, since TG means 'trochlear groove'. So the TG line marks the deepest point of the trochlear groove on the femur underneath the patella.
If a second slice was taken through the highest point of the tibial tubercle - where the patellar tendon attaches....
...and a perpendicular line dropped from there, then this would be the TT line, since TT means 'tibial tubercle'. The fibula is the skinny bone on the outer side of the shin.
Now let's superimpose the two slices and then you will be able to calculate the TT-TG distance, the distance between our two perpendicular lines.
What we are really attempting to estimate is the quality of the alignment between the groove in which the patella should run and the constraint created by the attachment of its tendon to the tibia bone.
The TT-TG distance is normally about 9mm, and if it´s more than 15-20mm then it means that the alignment is poor, and there is increased stress upon the patella simply as a result of the person's anatomy
Trochlear Dysplasia and the TT-TG Distance
The illustration to the right shows the sort of picture one might get in cases of trochlear dysplasia where this calculation really matters. You can see the much wider TT-TG Distance.
Trochlear Dysplasia is a disorder of the shape of the trochlear groove in which there is flattening and usually asymmetry of the groove - with a sort of raised 'bump' on one side, and the deepest part of the groove shifted over to the other side.
At what exact point is the femur line drawn?
Unfortunately there is controversy about exactly the point at which the upper 'slice' should be taken in order to measure the TG. The problem is that in patients with trochlear dysplasia the disturbance of anatomy is towards the upper part where a normal groove would be easily identifiable, but with this condition the trochlea at the normal reference point is flat.
A popular way to do it these days is to choose the slice where you can see both of the posterior parts of the femur condyles as well defined or the most proximal (highest) slice where you see a groove.
A 2016 publication (ref 1) suggests a more objective method, that is choosing the slice where the height of the 'Roman arch' between the condyles is 1/3 of the total height of the slice (see illustration).
2. Paiva M, Blønd L, Hölmich P, Steensen RN, Diederichs G, Feller JA, Barfod KW. Quality assessment of radiological measurements of trochlear dysplasia; a literature review. Knee Surg Sports Traumatol Arthrosc. 2018 Mar;26(3):746-755.