An instrument for measuring an angle about a cavity axis defined by a cavity extending into a head of a bone is provided, the bone having been resected on a plane that is inclined relative to the cavity axis. The instrument includes a stem arranged to be inserted into the cavity, a plane finder and a scale. The plane finder is coupled to the stem and arranged to contact the resection surface as the stem is inserted into the cavity and to move relative to the stem until the plane finder lies in a plane parallel to the resection surface. Movement of the plane finder relative to the stem is indicative of the angle about the cavity axis between a radial position indicated by part of the stem and a line extending perpendicularly from the resection surface. The scale is configured such that movement of the plane finder relative to the stem is indicated by the position of a portion of the plane finder relative to the scale.
