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Background Radiological assessments are considered an important part of the management of patellar instability (PI). However, PI measurements are influenced by the knee position, which cannot be guaranteed to be the same for each examination. Therefore, we aimed to determine the reliability of common PI measurements on magnetic resonance imaging (MRI). Methods Two MRI examinations within a 6-month period were obtained from 51 knees. The common PI measurements were quantitatively determined and re-evaluated. The intraclass correlation coefficients (ICC), Bland–Altman plot, standard error of measurement (SEM), and minimal detectable change (MDC) were used to determine the intra-observer, inter-observer, and inter-scan reliability. Results Adequate intra- and inter-observer reliability was obtained for all PI measurements (all ICCs > 0.8). For patellar positional parameters, the inter-scan reliability was adequate for the angle of Fulkerson, angle of Laurin, patellar tilt angle (PTA), lateral patellar displacement (LPD), and bisect offset ratio (BSO; ICCs = 0.723–0.897), although it was inadequate for the angle of Grelsamer and the congruence angle (CA; ICCs = 0.325–0.380). All parameters of trochlear dysplasia showed adequate inter-scan reliability (ICCs = 0.793–0.915). Nearly all patellar height parameters showed adequate inter-scan reliability (ICCs = 0.700–0.903), except the patellar trochlear index (PTI; ICC = 0.655). Conclusion All PI measurements showed adequate intra- and inter-observer reliability on MRI. Most measurements showed adequate inter-scan reliability, with the exception of the angle of Grelsamer, CA, and PTI. Electronic supplementary material The online version of this article (10.1186/s12891-019-2697-7) contains supplementary material, which is available to authorized users.

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