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PURPOSE: 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. METHODS: 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 (P25), second (P50) and third quartile (P75) above joint line of TD. RESULTS: 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 (P50) and 15 mm (P25) 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). CONCLUSION: 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. LEVEL OF EVIDENCE: Diagnostic study, Level I.
Tscholl et al. (Sat,) studied this question.