The tibial tubercle trochlear groove distance (TT-TG) is one of the major risk factors for patellar instability. This distance can be increased in case of medialized trochlear groove (TG), lateralized tibial tubercle (TT) or intra-articular torsion. The aim of this study is to define the origin of an increased TT-TG.

Seventy MRI performed for lateral patellar dislocation (PFD) and trochlear dysplasia were retrospectively analysesd and compared with an age and gender matched control group without trochlear dysplasia (Ctrl, n=70). TT-TG distance was measured on the most proximal image where the TG was first seen, to the center of the patellar tendon tibial on its tibial insertion. The medialization of the TG was measured in percentages along the posterior condylar line, with the maximal condylar width of the distal femur as reference. The position of the TT was measured using the TT-PCL distance. Intra-articular femoro-tibia torsion was measured using the angle between the posterior condylar lines of the femur and tibia (pcl-a).

The mean TT-TG was 15.9±4.9mm in PFD and 8.2±3.8mm in Ctrl (P<0.564) than in Ctrl. Forty-one knees of the PFD had a TT-TG>15mm (PFD+), and 29 below (PFD−). TG in PFD+ was 3.1mm more medialized (P<0.123) than in PFD−. In the PFD+ with a TT-PCL>21 (n=27), TG was 3.4mm more lateralized (P<0.035) than in PFD+ with a TT-PCL<21 (n=14). The pcl-a showed an increased tibial internal rotation in Ctrl (9.4°±5.5°) compared to PFD (2.6°±5.7°, P<0.001), whereas no difference in the pcl-a between PFD+ and PFD− was found.

Increased TT-TG in PFD can only partially be explained by osseous measurement on the femur and the tibia. Intra-articular rotation might play an important role. In PFD with increased TT-TG, medialized trochlear groove might be the most important factor. Indication for TT osteotomy should be evaluated carefully when using TT-TG.