PURPOSE OF REVIEW: The tibial tubercle-to-trochlear groove (TT-TG) distance and tibial tubercle-to-posterior cruciate ligament (TT-PCL) distance have both been proposed for use in the evaluation of patients with patellofemoral instability. RECENT FINDINGS: While the TT-TG value may be confounded by several factors, including age, gender, body mass index, and varying degrees of knee flexion on cross-sectional imaging, recent literature supports its utility for differentiation of patients with and without patellofemoral instability, and stratification of patients with and without coronal malalignment. The TT-PCL describes pure lateralization of the tibial tubercle, and may be used in conjunction with the TT-TG to better understand the path taken by the extensor mechanism of the knee joint. The TT-TG measurement is superior to the TT-PCL measurement for differentiating patients with patellofemoral instability from their stable counterparts, though TT-PCL may be useful in conjunction with TT-TG for better understanding the path of the extensor mechanism across the knee joint.
