A medical data processing method for assisting the positioning of a first medical structure relative to a second medical structure in at least one of two directions, the method being constituted to be executed by a computer and comprising the following steps: - receiving, at a digital processor of a computer, two-dimensional projection image data comprising two-dimensional projection image information describing a two-dimensional projection image of at least the first medical structure, and taken in a spatial direction that is perpendicular to said two directions; - receiving, at the processor, registration data comprising registration information describing a registration of the two-dimensional projection image with respect to the first medical structure, wherein the registration is based on the spatial direction the two-dimensional image is taken in; - determining, by the processor and on the basis of said two-dimensional projection image data, two-dimensional position data comprising position information describing, in the two-dimensional projection image, the position of a base point assigned to the first medical structure in an invariant positional relationship, and a reference point assigned to the second medical structure in an invariant positional relationship; - determining, by the processor and on the basis of the two-dimensional position data, positional relationship data comprising positional relationship information describing the directional distance between the base point and the reference point in said at least one of two directions of the two-dimensional projection image; - receiving, at the processor, three-dimensional position data comprising position information describing the position of the base point and the reference point in the three-dimensional anatomical space; - determining, by the processor and on the basis of the positional relationship data, the registration data and the three-dimensional position data, three-dimensional reference point correspondence data comprising correspondence information describing whether or not the directional distance between the base point and the reference point in said at least one of two directions within the three-dimensional anatomical space corresponds to the directional distance between the base point and the reference point in the two-dimensional projection image; - receiving, at a user-interface of a medical navigation system, the three-dimensional reference point correspondence data, for providing information to a user as to whether the position of the second medical structure relative to the first medical structure fulfils the condition that the directional distance between the base point and the reference point in said at least one of two directions within the three-dimensional anatomical space corresponds to the directional distance between the base point and the reference point in the two-dimensional projection image. - receiving, at a digital processor of a computer, two-dimensional projection image data comprising two-dimensional projection image information describing a two-dimensional projection image of at least the first medical structure, and taken in a spatial direction that is perpendicular to said two directions; - receiving, at the processor, registration data comprising registration information describing a registration of the two-dimensional projection image with respect to the first medical structure, wherein the registration is based on the spatial direction the two-dimensional image is taken in; - determining, by the processor and on the basis of said two-dimensional projection image data, two-dimensional position data comprising position information describing, in the two-dimensional projection image, the position of a base point assigned to the first medical structure in an invariant positional relationship, and a reference point assigned to the second medical structure in an invariant positional relationship; - determining, by the processor and on the basis of the two-dimensional position data, positional relationship data comprising positional relationship information describing the directional distance between the base point and the reference point in said at least one of two directions of the two-dimensional projection image; - receiving, at the processor, three-dimensional position data comprising position information describing the position of the base point and the reference point in the three-dimensional anatomical space; - determining, by the processor and on the basis of the positional relationship data, the registration data and the three-dimensional position data, three-dimensional reference point correspondence data comprising correspondence information describing whether or not the directional distance between the base point and the reference point in said at least one of two directions within the three-dimensional anatomical space corresponds to the directional distance between the base point and the reference point in the two-dimensional projection image; - receiving, at a user-interface of a medical navigation system, the three-dimensional reference point correspondence data, for providing information to a user as to whether the position of the second medical structure relative to the first medical structure fulfils the condition that the directional distance between the base point and the reference point in said at least one of two directions within the three-dimensional anatomical space corresponds to the directional distance between the base point and the reference point in the two-dimensional projection image.