Reconsideration of radiographic axial imaging techniques for the evaluation of patellofemoral alignment: skyline view vs. merchant view.

Background: This study aimed to compare two common radiographic axial imaging techniques, the skyline view (SV) and the merchant view (MV), and to reveal the variability of acquired images in patients with recurrent patellar dislocation (RPD). Methods: Thirty consecutive patients with 34 affected knees of RPD were included in this study (4 males, 26 females; mean age, 26.4 years). Both SV and MV were measured at 30°, and 45°of knee flexion in each patient. The congruence angle (CA) and the lateral patellofemoral angle (LPA) were evaluated for lateral shifts and patellar tilts in each view. Result: The axial alignment differed significantly between the SV and MV at both 30° and 45° of knee flexion. The MV had higher CA and lower LPA values than the SV. Intraclass correlation coefficient analysis demonstrated excellent-to-good intra- and inter-observer reliabilities. A discrepancy in acquired images between SV and MV was demonstrated, with SV underestimated patellar malalignment in both lateral tilt and shift in patients with RPD. Conclusion: Since axial patellar alignment varies significantly depending on the radiographic technique, MV imaging is preferable to prevent the underestimation of patellofemoral instability. Surgeons should be aware of the characteristics of each imaging method and choose the appropriate technique for accurate evaluation.