Patent ID: 8197492

Claim:
An instrument for measuring an angle about a cavity axis defined by a cavity extending into a head of a bone, the bone having been resected on a plane that is inclined relative to the cavity axis, the instrument comprising: a stem arranged to be inserted into the cavity; a plane finder coupled to the stem 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 being 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; and a scale 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, wherein the plane finder comprises an elongate rocker bar pivotably coupled to the stem such that the rocker bar pivots about a pivot axis extending transverse to the axis of the stem, and further comprises a yoke coupled to the stem such that the yoke is slidable in the pivot plane of the rocker bar, the yoke comprising a pair of arms, each arm extending towards the rocker bar on opposite sides of the pivot point, wherein pivoting of the rocker bar causes the rocker bar to contact an arm of the yoke causing the yoke to slide away from the rocker bar.