An assembly for use in cutting a bone during a surgical procedure comprises a cutting tool and a guide block. The cutting tool includes a blade and a drive unit for imparting a cutting motion to the blade. The guide block can be positioned against the bone and has a reference surface for guiding the blade during the cutting step, the guide block having a screen surface which provides a point of reference to indicate proper alignment of the blade. The cutting tool includes means for directing a collimated beam of light in a direction parallel to the blade, on to the screen surface on the guide block when the blade is in contact with the guide surface, the distance between the blade axis and the light beam being equal to the distance between the reference surface and the point of reference on the screen surface.
