Pure and Mixed Imaging Tests, Precision, and Sex Dependence in Patellofemoral Imaging: Response

Abstract

We thank Dr Nizic ´for his interest for our recently published study. 6 The tibial tubercle-Roman arch (TT-RA) distance has landmarks on the tibia and femur. Therefore, the translation of the tibia or the rotation of knee joints would alter the value of TT-RA distance. We have also performed studies to investigate the difference in the TT-RA distance between CT and MRI. 4 We found that the value of TT-RA distance is affected by the rotation or flexion of the knee joint, which is also the case for the current gold standard tibial tubercle-trochlear groove (TT-TG) distance. 1,4,6 Therefore, the TT-RA distance is a mixed imaging test.We do agree with Dr Nizic ´that a test of tibial tuberosity lateralization that is independent of femorotibial rotation may be a more reliable assessment. Some measurements such as tibial tuberosity-tibial intercondylar midpoint (TT-TIM) distance, 2 or tibial tubercle-posterior cruciate ligament (TT-PCL) distance 3 can evaluate pure tibial tubercle lateralization. However, the clinical value of these measurements remains uncertain. TT-PCL distance reflects the true lateralization of the tibial tubercle, and the TT-TG distance reflects the mixed tibial tubercle lateralization. We compared the clinical application value between TT-TG and TT-PCL distance in our previously published study and found the difference in the TT-PCL distance between patients with patellar dislocation and healthy controls to be only 1.11 mm. 5 In addition, the TT-PCL distance was found to have a poor capacity to predict patellar dislocation (area under the receiver operating curve [AUC] ¼ 0.627). Therefore, the role of true tibial tubercle lateralization in the evaluation of patellar dislocation patients warrants further assessment. As mixed imaging tests, the TT-RA distance (AUC ¼ 0.8) and TT-TG distance (AUC ¼ 0.8) had higher capacity than TT-PCL distance (AUC ¼ 0.6) to distinguish patients with patellar dislocation from healthy individuals, 5,6 which indicate that the mixed imaging tests would be more suitable to evaluate patellar dislocation.As for the question of the accuracy of the TT-RA distance. We used the intraclass correlation coefficient to evaluate the precision of the measurements, and found that the TT-RA distance showed higher inter-and intraobserver reliability than the TT-TG distance (Table 2 of our study 6 ), especially in patients with patellar dislocation (Figure 6 of our study 6 ). When regarding the precision of TT-RA distance to distinguish patellar dislocation patients from healthy individuals, the AUC was comparable to that of the TT-TG distance (Figure 8 of our study 6 ).The sex difference of TT-RA distance was not investigated in the present study. It is hard to determine any sex-based differences in the TT-RA distance with a small sample size.Thank you for your kind thoughts and interest.