We describe a case of medial clavicular physeal separation with posterior displacement of the metaphysis in a 13-year-old girl, focusing on the role played by ultrasound in the diagnosis, planning of treatment and post-reduction follow-up. On clinical examination and conventional radiography, the injury is essentially indistinguishable from a sternoclavicular dislocation or a fracture of the medial aspect of the clavicle; however, the pathogenesis is different, consisting in medial physeal separation and 'degloving' of the inner cancellous bone of the metaphysis from the surrounding periosteal collar with posterior metaphyseal dislocation. In our case, attempted closed reduction failed, and the injury required open reduction with relocation of the clavicle into the periosteal sleeve followed by suturing of the periosteal tear.