A computer implemented medical method of calibrating a spinal cage, comprising the steps: acquiring, by using a pointer tip of a pointing device, a cage tip point at a cage tip of the spinal cage, wherein the cage tip is disposed at a first end of the cage in a length direction of the cage; acquiring, by using the pointer tip of the pointing device, a cage end point at a cage end of the spinal cage, wherein the cage end is disposed at a second end of the spinal cage, opposing to the first end, in the length direction; characterized by acquiring, by using a pointer shaft of the pointing device, at least one axis (X1, X2, X3, X4) along a side (Sl1, Sl2, St, Sb) of the spinal cage; determining a calibrated virtual model (V1, V2) of the spinal cage using the cage tip point, the cage end point and the at least one cage axis (X1, X2, X3, X4); wherein acquiring the cage end point, the cage tip point and the at least one cage axis (X1, X2, X3, X4) comprises: determining a cage-to-holder-coordinate-transformation (CageToHolder), which describes a transformation between a holder coordinate system (Holder) of a holding device and a cage coordinate system (cage) of the spinal cage; providing a pointer-to-holder-coordinate-transformation (PointerToHolder), which describes a transformation between the pointer coordinate system (Pointer) and the holder coordinate system (Holder); determining the cage-to-holder-coordinate-transformation (CageToHolder), by using the pointer-to-holder-coordinate-transformation (PointerToHolder). acquiring, by using a pointer tip of a pointing device, a cage tip point at a cage tip of the spinal cage, wherein the cage tip is disposed at a first end of the cage in a length direction of the cage; acquiring, by using the pointer tip of the pointing device, a cage end point at a cage end of the spinal cage, wherein the cage end is disposed at a second end of the spinal cage, opposing to the first end, in the length direction; characterized by acquiring, by using a pointer shaft of the pointing device, at least one axis (X1, X2, X3, X4) along a side (Sl1, Sl2, St, Sb) of the spinal cage; determining a calibrated virtual model (V1, V2) of the spinal cage using the cage tip point, the cage end point and the at least one cage axis (X1, X2, X3, X4); wherein acquiring the cage end point, the cage tip point and the at least one cage axis (X1, X2, X3, X4) comprises: determining a cage-to-holder-coordinate-transformation (CageToHolder), which describes a transformation between a holder coordinate system (Holder) of a holding device and a cage coordinate system (cage) of the spinal cage; providing a pointer-to-holder-coordinate-transformation (PointerToHolder), which describes a transformation between the pointer coordinate system (Pointer) and the holder coordinate system (Holder); determining the cage-to-holder-coordinate-transformation (CageToHolder), by using the pointer-to-holder-coordinate-transformation (PointerToHolder). determining a cage-to-holder-coordinate-transformation (CageToHolder), which describes a transformation between a holder coordinate system (Holder) of a holding device and a cage coordinate system (cage) of the spinal cage; providing a pointer-to-holder-coordinate-transformation (PointerToHolder), which describes a transformation between the pointer coordinate system (Pointer) and the holder coordinate system (Holder); determining the cage-to-holder-coordinate-transformation (CageToHolder), by using the pointer-to-holder-coordinate-transformation (PointerToHolder).