TDR was performed in 457 patients from 21 sites ( 261 patients in the investigational group ( Kineflex-L Disc ; metal-on-metal design anchored with keels , 204 randomized and 57 nonrandomized training cases ) , and 196 in the control group ( CHARITE artificial disc ; metal with polyethylene core with teeth for anchoring ; 190 randomized and 6 nonrandomized training cases ) .