Surgical intervention: The surgical intervention consists of replacement of the degenerative intervertebral lumbar disc with a artificial lumbar disc device (ProDisc II, Spine Solutions Inc. , New York, NY). The ProDisc consists of three pieces, two metal endplates and a polyethylene core that is fixed to the inferior endplate when the device is implanted, and is implanted through a retroperitoneal (or transperitoneal) access. Depending on the surgeon, a Pfannenstiel incision or a para median incision is used. A fluoroscope is used to ensure that the prosthesis is placed in the midline and sufficiently towards the posterior edge of the corpus vertebrae. All hospitals participating in the study will use the same artificial lumbar disc device, and standard instruments from the manufacturer are used for inserting the disc. Postoperatively, orthoses will not be used, but patients are given restrictions on heavy lifting and excessive movements the first 6 weeks after surgery. Otherwise, no instructions will be given. Six weeks after surgery, patients will be referred to physiotherapy.