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Boy with right Erb palsy, photographed by French neurologist Guillaume-Benjamin Duchenne de Boulogne (before 1862)

As described by Duchenne: Posterior and external dislocation of the shoulder (subacromial dislocation), dating from birth, in a 6-year-old boy, characterized: (1) by a relief located below the posterior angle of the acromion, formed by the head of the humerus which straddles the posterior edge of the glenoid cavity; (2) by an anterior subacromial depression; (3) by the distance outside and in front of the elbow, which cannot be brought closer to the trunk; (4) by the internal rotation of the arm, which cannot be externally rotated; (5) finally by the semi-flexion of the forearm, which cannot be extended. (Source: Duchenne [de Boulogne] GB. Paralysies obstétricales infantiles des membres supérieurs avec ou sans fractures, luxations ou autres complications. In: De l'électrisation localisée et de son application à la physiologie, à la pathologie et à la thérapeutique par courants induits et par courants galvaniques interrompus et continus. Paris: J B Baillière, 1872:353-67. Translation and figure restoration and editing by Dr. Douglas J Lanska.)