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

Authors’ Response:

We thank Dr Nizić 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 Nizić 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] = 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 = 0.8) and TT-TG distance (AUC = 0.8) had higher capacity than TT-PCL distance (AUC = 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.

Zijie Xu, MD Pei Zhao, MD Yi-Fan Song, MD Hai-Jun Wang, MD Aiguo Zhou, MD Jia-Kuo Yu, MD Beijing, PR China