A reamer guide is described for reaming a cavity within a tibia. The reamer guide comprises: a plate having a circular aperture there through and defining a plane; and a disc rotatably mounted in the aperture, the disc having a bushing therein defining a reamer axis offset from the center of the disc for receiving a reamer in use and in which the disc is rotatable about an axis transverse to the plane of the plate. Thus a cavity larger than the reamer can be accurately and easily formed.