First time patellar dislocation is worrying for both parent and child. The anxiety revolves around whether this is going to recur and whether there are underlying structural problems that caused the first dislocation.
This paper compares the anatomy around the kneecap (or patella) in two groups of children - those that have experienced a 'first-time' dislocation of their patella and a control group of those that have not had such a problem. Because it is an 'MRI imaging' study, the parameters explored are limited to the structures that can be seen via MRI scans.
An exploration such as this should allow the identification of 'at risk' children. It should also give the parents and doctors an idea of whether a particular child suffering a first time patellar dislocation has underlying structural problems that might predispose to further incidents.
The TT-TG distance is a sensitive indicator of potential instability. It requires a calculation from two axial slices. Basically it is a calculation of how out of alignment the tibial attachment of the patellar tendon is.