Accuracy of Tibial Tuberosity-Trochlear Groove Distance, Tibial Tuberosity-Posterior Cruciate Ligament Distance In Terms of The Severeity of Trochlear Dysplasia
Authors: Dong Conglei , Zhao Chao , Li Ming , Fan Chongyi , Feng Xunkai , Piao Kang , Hao Kuo , Wang Fei 
Published: April 2021

Abstract
Purpose Increased Tibial Tubercle-Trochlear Groove Distance (TT-TG) was proposed as one of the main risk factors for patellofemoral instability (PFI). The increased TT-TG distance indicated externalization of the tibial tubercle with the reference of the trochlear groove. However, in the case of severe trochlear dysplasia, the reference point on the trochlear groove was indistinct, and the accuracy of TT-TG was controversial. The purpose of this study was to evaluate the accuracy of TT-TG and TT-PCL in consideration of the mild and sereve trochlear dysplasia . Methods From 2015 to 2020, MRI findings of consecutive knee joints with PFI symptoms diagnosed in our hospital was retrospectively analyzed. All knees with trochlear dysplasia were diagnosed by longitudinal MRI scan and lateral radiograph. The knees were classified according to the four-type classification system described by Dejour et al. 20 cases of type A (mild trochlear dysplasia), 20 cases of type B, C, and D (severe trochlear dysplasia), and 20 cases of normal type were selected and divided into normal group (normal trohlea), mild group (type A) and severe group (type B, type C, type D). Tibial Tubercle-Trochlear Groove Distance (TT-TG), Tibial Tubercle-Posterior Cruciate Ligament Distance (TT-PCL), and the Dejour classification of trochlear dysplasia was assessed by 2 experience orthopaedics. The the reliability of TT-TG diatance and TT-PCL distance was tested by intraclass correlation coefficients (ICC).Results Comparing the differences between TT-TG and TT-PCL in the normal, mild and severe groups, the TT-TG and TT-PCL in the mild and severe group show different meanings. (normal, 8.83±3.62 mm vs. 8.44±4.57mm, P>0.05；mild, 17.30±4.81 mm vs. 20.09±5.05mm, P<0.05；severe, 10.79±4.24 mm vs. 12.31±5.43mm, P>0.05). The Pearson correlation coefficient of TT-TG and TT-PCL measurements of trochlear dysplasia were r=0.480 (mild group, P=0.032), r=0.585 (severe group, P<0.001). The Intra-observer ICC of TT-TG were r=0.814 (mild group), r=0.739 (severe group). The Inter-observer ICC of TT-TG were r=0.810 (mild group), r=0.713 (severe group). In the normal knee, the Pearson correlation coefficient of TT-TG and TT-PCL was r=0.787(P<0.001), the Intra-observer ICC of TT-TG was r=0.989, and the Inter-observer ICC of TT-TG was r=0.978. Conclusion Compared with the mild trchlear dysplasia, the inter-observer and intra-observer correlations of TT-TG measurements decreased in group of sereve dysplastic trochlea (Inter-observer ICC, 0.810 vs. 0.713; Intra-observer ICC , 0.814 vs. 0.739). In the present study, the determination of TT-TG and TT-PCL distance are of great significance for patients with low grade trochlear dysplasia. And TT-PCL, without referring to the abnormal trochlear groove, is an effective indicator to measure the lateraltization of tibial tuberosity in patients with sereve dysplastic trochlea.