An endplate for an intervertebral prosthesis includes an inner domed region and a peripheral rim wherein the inner domed region has a non-circular plan and has an area that is at least 50% of the area of the endplate. A surgical instrument for milling a recess in a vertebral endplate includes a cutting tool having a drive shaft with a positioning collar, a guard frame, and a pair of guide plates attached to the guard frame and spaced to confine the positioning collar and having guide slots for the drive shaft.