Tibial tubercle to trochlear groove and the roman arch method for tibial tubercle lateralization are reliable and distinguish between subjects with and without major patellar instability.

PURPOSE: Patellofemoral (PF) instability recurrence depends on several factors including the relative lateralization of tibial tubercle (TT) regarding the trochlear groove (TG). TT relative lateralization quantification has long been a topic of debate. Multiple measuring techniques have been described including TT-trochlear groove (TT-TG), TT-posterior cruciate ligament (TT-PCL) and TT-roman arch (TT-RA), with no clear consensus regarding the most reliable index or pathologic threshold. We set out to determine the normal value range of each index and their association with age, sex, and patellofemoral instability status. Also, this study aims to determine a reliable pathologic distance threshold to effectively predict patellar dislocation.

METHODS: Skeletally mature patients up to 45 years of age who presented a CT-Scan and an MRI of the same knee between 2014 and 2018 were included and divided into subgroups based on history of PF instability. Three indexes (TT-TG, TT-PCL, and TT-RA) were assessed by two independent observers blinded to instability history. ROC curves were performed for each index to obtain the cut point that better predicts instability. Univariate and multivariate models adjusted by age, sex, instability history and type of imaging technique were performed to test influence of these variables.

RESULTS: 208 patients were included. Mean age was 27.93 ± 8.48 years, 67.3% were female, and 71 patients (34.1%) presented major instability history. Good or excellent inter and intraobserver reliability was found for all three indexes. All indexes presented significantly different distributions between subjects with and without major instability (p<0.001), except for TT-PCL. Different cut point values differing between imaging modalities were found: 11.4mm for MRI TT-TG, 17mm for CT TT-TG, 15.6mm for MRI TT-RA and 18.2mm for CT TT-RA.

CONCLUSIONS: All indexes studied had good or excellent inter and intraobserver reliability. Measurements between imaging techniques (CT and MR) are not interchangeable. Both TT-TG and TT-RA correctly distinguish between subjects with and without major instability, while TT-PCL does not, recommending caution when evaluated on its own. Specific threshold values depending on imaging technique should be considered for surgical decision making.

LEVEL OF EVIDENCE: Level IV, Diagnostic Test.