SECEC Didier Patte Prize 2023: the ABC classification of posterior shoulder instability

Abstract

BackgroundThe ABC classification has recently been proposed as a comprehensive classification system for posterior shoulder instability (PSI). The purpose of this study was to analyze the comprehensiveness as well as inter- and intra-rater reliability of the ABC classification. MethodsIn a prospective study all consecutive patients presenting with unidirectional PSI from June 2019 to June 2021 were included. No patients were excluded, leaving a consecutive series of 100 cases in 91 patients. All recorded clinical and imaging data was used to create anonymized clinical case vignettes, which were evaluated twice according to the ABC classification at the end of the recruitment period in random sequential order by four independent raters, two experienced shoulder surgeons and two orthopedic residents in order to analyze the comprehensiveness as well as inter- and intra-rater reliability of the ABC classification for posterior shoulder instability and to describe differences in characteristics among subtypes. Group A was defined as first-time singular posterior shoulder instability event less than 3 months in the past regardless of etiology and is further subdivided in Type 1 and Type 2 depending on the occurrence of a subluxation (A1) or dislocation (A2). Group B comprises recurrent dynamic PSI regardless of time since onset and is further subdivided by the cause of instability in functional (B1) and structural (B2) dynamic PSI. Group C includes chronic static PSI with posterior humeral decentering that can either be constitutional (C1) or acquired (C2). ResultsNone of the cases was deemed unsuitable to be classified based on the proposed system by the observers. After consensus agreement between the two experts 16 cases were attributed to group A (A1: 8, A2: 8), 64 to group B (B1: 33, B2: 31), and 20 to group C (C1: 11, C2: 9). The expert raters agreed in 99% and 96% of the cases on the classification subtypes during the first and second rating (ICC: 0.998 and ICC: 0.99). The intraobserver reliability was excellent for both raters. The beginners reached the same conclusion as the consensus agreement in 94% (ICC: 0.99) and 89% of the cases (ICC: 0.97) during the first round and 94% each (ICC: 0.97) during the second round. The intraobserver reliability was excellent for both beginners. Overall, discrepancies between raters were found between groups B1 and B2 (n=14), B2 and C2 (n=4), B1 and C1 (n=1), as well as A1 and B2 (n=1). In general, each subtype shows distinctive clinical and imaging characteristics that facilitate the diagnosis. ConclusionThe presented ABC classification for PSI is a comprehensive classification with a high reliability and reproducibility. However, a gradual transition and potential progression between the subtypes of PSI must be considered. The reliable distinction between different subtypes of PSI based on etiology and pathomechanism provides a standardized basis for future investigations on treatment recommendation.