Surgical cutting guide, surgical alignment guide and surgical instrument system

A surgical cutting guide (8) is described which comprises a body section delimiting a recess (14) extending into the body section from an external surface. The recess has a central axis and is for receiving an attachment protrusion (18) from a surgical alignment guide (6). The recess comprises: a first portion (36) having a first constant cross section along the central axis; and a second portion (38) having a second constant cross section along the central axis which is larger than the first constant cross section. The first and second portions are coaxial with the central axis and the second portion is closer to the external surface than the first portion. This configuration creates a stepped surface in the recess which is engaged by corresponding features on a protrusion of a surgical alignment guide. The attachment protrusion (18) of the alignment guide (6) has a central axis and comprises: a first portion (20) having a first constant cross section along the central axis; and a second portion (22) having a second constant cross section along the central axis that is larger than the first constant cross section. The first portion is further away than the second portion, and the first and second portions are coaxial with the central axis. The surgical cutting guide and surgical alignment guide form a system in which the alignment guide can be removed more easily from the cutting guide because there is more freedom of movement once the stepped surfaces are disengaged.