A 42-year old female patient with degenerative disc disease received implantation of an artificial disc at L5-S1 (Prodisc, Synthes). Postoperative acute bilateral S1-pain without motor deficit could be explained by posterior dislocation of the inferior plate and a subsidence of the cranial plate of the disc prosthesis (a, b). This could clearly be visualised on plain radiographs. No dislocation of the PE inlay occurred which can be seen in the radiographic marker, being still in place. This patient was treated with a conversion into anterior fusion at L5-S1 with pain relief at the 1 year follow-up.