Introduction To investigate if increased Tibial tubercle – Trochlear Groove distance (TT-TG) measured on axial MRI is due to lateralization of the tibial tubercle or medialization of the trochlear groove.

Methods 65 knees (28 normal (NK), 25 with trochlear dysplasia (TD) and 12 with patellar dislocation without TD (PD)) were examined. The medial border of the posterior cruciate ligament (PCL) was chosen as the central anatomical landmark. The distance from the tibial tubercle (TT) to PCL (TT-PCL) was measured to examine the lateralization of the tibial tubercle. The distance from the trochlear groove to PCL (TG-PCL) was measured to examine the medialization of the trochlear groove. Between group differences was investigated by use of one-way ANOVA.

Results The mean (SD) values for TT-TG were 8.5 mm (3.6) for NK, 11.4 mm (6.2) for PD and 17.1 mm (4.8) in the TD group (p<0.01). The mean (SD) values for TT-PCL were 19.5 mm (4.2) for NK, 17.0 mm (5.0) for PD and 20.2 mm (5.0) in the TD group (p=0.10). The mean (SD) values for TG-PCL were 10.5 mm (3.7) for NK, 5.8 mm (4.9) for PD and 3.9 mm (3.9) in the dysplastic group (p<0.01).

Conclusion TD knees had increased TT-TG compared to NK and PD. The TT-PCL distance did not differ significantly between groups, whereas the TG-PCL distance declined with increased TT-TG. The present results indicate that increased TT-TG is due to medialization of the trochlear groove and not lateralization of the tibial tubercle.