The tibial-tubercle trochlear groove distance in patients with trochlear dysplasia: the influence of the proximally flat trochlea.

[trochlear dysplasia]

Increased tibial tubercle trochlear groove distance (TT-TG ) is frequently associated with trochlear dysplasia (TD ). Since the trochlear groove appears more distally in patients with TD compared to controls , it is unknown whether TT-TG might be comparable and meaningful .Fifty patients with TD were retrospectively analysed and compared to 52 age- and gender-matched patients (CG ). TT-TG was measured on transverse MR images , as the distance between the trochlear groove of minimal 2 mm depth proximally and the centre of the patellar tendon at its distal insertion . The height of the femoral reference point above joint line was recorded for both groups . TT-TG measurement was repeated in CG using the first (P 25 ), second (P 50 ) and third quartile (P 75 ) above joint line of TD .Patients with TD had a significantly smaller vertical distance between the most proximal trochlear deepening and the femorotibial joint line (20 .6 mm , range 10 .3 -30 .9 ) compared to CG (33 .8 mm , range 25 .4 -41 .1 ; p < 0 .001 ). TT-TG values measured at 20 mm (P 50 ) and 15 mm (P 25 ) proximal to the femorotibial joint line were significantly smaller compared when measured with the most proximal reference point [1 .8 mm (95 % CI 1 .3 -2 .3 , p < 0 .001 ) and 2 .4 mm (95 % CI 1 .9 -3 .0 , p < 0 .001 )] in CG . The inter-rater reliability was excellent (ICC 0 .99 ).TT-TG distance depends significantly on the femoral reference point . Since the trochlear groove is seen more distally in patients with TD compared to controls , TT-TG of the patients with highest risk of recurrent patellar instability might be underestimated .Diagnostic study , Level I .