A trial kit for a hinged knee implant system is disclosed. The trial kit includes a tibial base plate trial, an adaptor and a tibial bearing insert trial. The adaptor and insert trial can be assembled and the assembly mounted on the tray trial by sliding the assembly in an anterior-posterior direction on the tray trial. The adaptor has a post with a cross-piece. The insert trial has openings and channels to receive the post and cross-piece. The openings are sized, shaped and oriented so that the cross-piece of the elongate post must be oriented in different directions to pass through the different openings.