Injuries to the first rib synchondrosis are uncommon in sport. The potential for serious complications following posterior displacement is similar to that seen with posterior sternoclavicular joint dislocation. Clinical examination and plain radiography may not provide a definitive diagnosis. Computerised tomography is the most appropriate imaging modality if this injury is suspected. Posterior dislocation of the first rib costal cartilage with an associated fracture of the posterior sternal aspect of the synchondrosis has not been previously reported.