A cutting guide instrument for use in a cutting step in an orthopaedic surgicalprocedure comprises a reference component comprising a first element (26) whichcan be attached or otherwise aligned relative to the bone in a predetermined arrangementand a locator arm (30) . The invention further comprises a cutting block (28) comprisinga reference surface (34) which can be engaged by a cutting tool to define the locationand orientation of the tool during the cutting step. The cutting block furthercomprises at least one fixation bore (33) for receiving a fixation pin to fix thecutting block to the bone . The locator arm can be fitted against the referencesurface so that the reference component and cutting block are arranged in a predeterminedmanner .
