 Patellar instability is a condition that primarily affects young patients. A chronically unstable kneecap can predispose patients to disability and early arthritis. One measure used to gauge the risk of patellar instability is the TT-TG distance. That's the distance from the tibial tubercle to the trochlear groove. A TT-TG distance of 20 mm or more is a common indicator that the tibial tubercle is too far lateral on the tibia and thus grounds for corrective surgery. But a new study suggests there may be more than meets the eye and that the TT-TG distance may not solely or even partially represent a lateral tibial tubercle. Researchers examined knee MRIs of 89 patients with patellar instability and 92 matched control patients. Previous studies have shown that trochlear dysplasia, a condition in which the trochlea is not properly shaped to contain the patellar, is present in up to 90% of patients with recurrent patellar instability. That motivated the team to determine the effects of trochlear dysplasia on the TT-TG measurement. The researchers looked at several variables in addition to dysplasia, including external tibial rotation, patellar tilt, and the location of the trochlear groove and tibial tubercle on the femur and tibia respectively. These factors have recently been associated with recurring patellar instability as well, but few studies have closely examined their role in this context. Analysis revealed that external tibial rotation and trochlear dysplasia were strongly correlated in patients with patellar instability. In fact, among all variables analyzed, this external rotation was associated with the largest changes in TT-TG distance, with higher external rotation being linked to an elevated TT-TG distance. Meanwhile, trochlear dysplasia, as well as patellar tilt, was found to mediate this relationship. These findings paint a more nuanced picture of the processes leading to patellar instability. The researchers propose that increasing degrees of trochlear dysplasia lead to increased patellar tilt, which, through the patellar tendon, causes the tibia to rotate externally. The team also found no difference in the position of the tibial tubercle between patients with and without patellar instability. And the trochlear groove was slightly more medial at the top of the trochlea, but was basically the same at the bottom of the trochlea, close to the intercondylar notch. While more work is needed to verify this sequence of events, the overall message is clear. TT-TG distance is not a standalone measurement, but rather a multifactorial one that is influenced directly or indirectly by trochlear dysplasia. Therefore, procedures designed to correct a lateral tibial tubercle appear to be more of a compensatory procedure than a way of addressing a real pathology.