Deep brain stimulation (DBS) is an established treatment for advanced complicated Parkinson's disease (PD). Several controlled randomized studies have given proof of an advantage for operated patients as compared to medically treated patients in terms of motor outcome, activities of daily living and health status. However these studies have addressed mostly stimulation of the subthalamic nucleus (STN). GPi stimulation has not been compared to best medical treatment (BMT) in a prospective randomized controlled trial in patients with complicated PD who are not good candidates for STN stimulation. The investigators aim assessing GPi-DBS in patients with PD who have contraindications for STN-DBS.