Method of calibrating a cage

Abstract

A computer implemented medical method of calibrating a cage is presented. In particular, this calibration method calculates a virtual model of the cage based on a cage tip point and a cage end point, acquired by using a pointer tip of a pointing device, and at least one axis, acquired by using a pointer shaft of the pointing device along a side of the cage. This method allows for providing a more detailed virtual model of the cage, while being in compliance with sterility restrictions.

1 . A computer implemented method of calibrating a spinal cage, comprising:
 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;   acquiring, by using a pointer shaft of the pointing device, at least one axis along a side of the cage;   determining a calibrated virtual model of the spinal cage using the cage tip point, the cage end point and the at least one cage axis;   wherein calibrating comprises determining a spatial relation between the spinal cage and the pointing device.   
     
     
         2 . The method of  claim 1 ,
 wherein acquiring the cage end point, the cage tip point and/or the at least one cage axis comprises:   determining a cage-to-holder-coordinate-transformation, which describes a transformation between a holder coordinate system of a holding device and a cage coordinate system of the spinal cage.   
     
     
         3 . The method of  claim 2 ,
 wherein acquiring the cage end point, the cage tip point and/or the at least one axis comprises providing a pointer-to-holder-coordinate-transformation, which describes a transformation between the pointer coordinate system and the holder coordinate system;   determining the holder-to-cage-coordinate-transformation, by using the pointer-to-holder-coordinate-transformation.   
     
     
         4 . The method of  claim 1 ,
 wherein acquiring the cage end point, the cage tip point and/or the at least one axis comprises holding the pointing device onto the spinal cage until the respective cage end point, cage tip point and/or at least one axis is acquired.   
     
     
         5 . The method of  claim 1   wherein the virtual model indicates a shape, position and orientation of the cage.   
     
     
         6 . The method of  claim 1 ,
 wherein the virtual model comprises a multi-dimensional representation of the spinal cage, in particular a 3-dimensional representation of the spinal cage.   
     
     
         7 . The method of  claim 1 ,
 wherein acquiring the cage end point comprises:   acquiring a first holder point and a second holder point at opposing lateral sides of a cage holder, which holds the cage at the second end of the spinal cage, by using the pointer tip of the pointing device;   determining the cage end point as a centre of a connection line between the first holder point and the second holder point.   
     
     
         8 . The method of  claim 1 , further comprising the step:
 acquiring, by using the pointer shaft of the pointing device, at least one axis along at least one lateral side of the spinal cage in a height direction of the cage.   
     
     
         9 . The method of  claim 8 , further comprising the step:
 acquiring, by using the pointer shaft of the pointing device, at least one axis along a top side and/or bottom side of the spinal cage in a width direction of the spinal cage.   
     
     
         10 . The method of  claim 8 , further comprising the step:
 acquiring, by using the pointer shaft of the pointing device, at least one axis along a top side and/or bottom side of the spinal cage in a length direction of the spinal cage.   
     
     
         11 . The method of  claim 1 , further comprising the steps:
 acquiring, by using the pointer shaft of the pointing device,   a first axis along a first lateral side of the spinal cage in a height direction of the spinal cage;   a second axis along a second lateral side of the spinal cage opposite of the first lateral side, in the height direction of the spinal cage;   a third axis along a top side of the spinal cage in a width direction of the spinal cage; and   a fourth axis along a bottom side of the spinal cage in the width direction of the spinal cage.   
     
     
         12 . The method of  claim 1 , further comprising the steps:
 acquiring, by using the pointer shaft of the pointing device,   a first axis along a first lateral side of the spinal cage in a height direction of the spinal cage;   a second axis along a second lateral side of the spinal cage opposite of the first lateral side, in the height direction of the spinal cage;   a fifth axis along a top side of the spinal cage in a length direction of the spinal cage; and   a sixth axis along a bottom side of the spinal cage in the length direction of the spinal cage.   
     
     
         13 . The method of  claim 1 ,
 wherein, for acquiring the at least one axis in the width direction and/or height direction, the pointer shaft is held in a width axis section distant to the cage end point and the cage tip point.   
     
     
         14 . The method of  claim 1 ,
 wherein, for acquiring the at least one axis in the length direction, the pointer shaft is held in a length axis section distant to the lateral sides of the spinal cage.   
     
     
         15 . The method of  claim 1 ,
 wherein determining a calibrated virtual model comprises:   determining a cage axis of the spinal cage;   determining at least one nearest point of the at least one axis, being the nearest point of the at least one axis to the cage axis; and   determining the virtual model using the at least one nearest point.   
     
     
         16 . The method of  claim 15 ,
 wherein determining the at least one nearest point comprises:   determining a length cage axis connecting the cage tip point and the cage end point;   determining a width cage axis, extending perpendicular to the length cage in the width direction through a centre of the length cage axis in the length direction;   determining at least one nearest point (N 1 , N 2 , N 3 , N 4 ) of the at least one axis (X 1 , X 2 , X 3 , X 4 ) in the width direction (W) and/or the height direction (H), being the nearest point of the at least one axis (X 1 , X 2 , X 3 , X 4 ) to the length cage axis (Al); and   determining at least one nearest point of the at least one axis (X 5  in the length direction (L) being the nearest point of the at least one axis (X 5 ) to the width cage axis (Aw).   
     
     
         17 . The method of  claim 15 , further comprising the steps:
 determining the cage coordinate system in the cage tip point, wherein the cage coordinate system comprises an x-axis, a y-axis and a z-axis, perpendicular to each other, wherein the z-axis is equal to the cage axis;   determining at least one projection point, projecting the at least one nearest point into an x-y-plane, defined by the x-axis and the y-axis;   determining a distance between the at least one projection point and an at least one planned projection point of a planned axis, wherein the at least one planned axis is a previously acquired axis for the spinal cage;   replacing the planned axis with the acquired at least one axis; if the determined distance is smaller than a predetermined threshold; and   adding the acquired at least one axis to the previously acquired axis for the spinal cage.   
     
     
         18 . The method of  claim 17 ,
 wherein replacing the planned axis with the acquired at least one axis comprises:   determining a replacement likelihood score for each planned axis; and   replacing the planned axis with the highest replacement likelihood score with the acquired at least one axis;   wherein the replacement likelihood score is determined using a weighted replacement likelihood function, using a distance of the planned axis to the cage axis and a distance of the planned axis to the acquired axis.   
     
     
         19 . The method of  claim 1 ,
 wherein determining a calibrated virtual model comprises:   determining a cage center point, representing a center of a plane determined by the at least two axis;   determining at least one correction point, being an intersection of a line through the cage center point and one of the at least two axis along a cage coordinate axis of a cage coordinate system;   determining the virtual model using the at least one correction point.   
     
     
         20 . The method of  claim 19 , wherein
 determining the cage center point comprises:   determining at least two intersection points, being an intersection between two of the at least two axis;   determining at least one middle point, being the middle point between two intersection points along one of the at least two axis;   determining the cage center point using the at least one middle point.   
     
     
         21 . The method of  claim 1 ,
 wherein determining the at least one axis comprises:   acquiring at least one shaft point relating to a shaft end distant to the pointer tip and at least one pointer tip point; and   determining the at least one axis by connecting the at least one shaft point and the at least one pointer tip point.   
     
     
         22 . The method of  claim 1 ,
 wherein determining the virtual model of the cage comprises:   receiving a basic shape of the spinal cage; and   determining the virtual model of the cage using the basic shape of the cage.   
     
     
         23 . The method of  claim 19 ,
 wherein the basic shape of the cage comprises an ellipsoid shape, a lordotic shape, a bullet shape, a round shape or a curve shape.   
     
     
         24 . The method of  claim 1 , comprising the steps:
 receiving a diameter of the pointer shaft;   determining at least one corrected nearest point by shifting the at least one nearest point of the at least one axis towards the cage axis for half of the received diameter of the pointer shaft; and   determining at least one projection point, projecting the at least one corrected nearest point into an x-y-plane, defined by the x-axis and the y-axis.   
     
     
         25 . A cage calibration system comprising
 a holding device with a holding device tracker,   a pointing device with a pointing device tracker and a tracking device, configured for tracking the holding device tracker arranged on the holding device and the pointing device, the instrument calibration system configured for executing by at least one processor, the steps of   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;   acquiring, by using a pointer shaft-of the pointing device, at least one axis along a side of the cage;   determining a calibrated virtual model of the spinal cage using the cage tip point, the cage end point and the at least one cage axis;   calibrating the spinal cage by determining a spatial relation between the spinal cage and the pointing device.   
     
     
         26 . (canceled) 
     
     
         27 . A non-transitory computer readable medium storing computer instructions which, when running on a computer or when loaded onto a computer, causes the computer to:
 acquire, 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;   acquire, 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;   acquire, by using a pointer shaft-of the pointing device, at least one axis along a side of the cage;   determine a calibrated virtual model of the spinal cage using the cage tip point, the cage end point and the at least one cage axis;   calibrate the spinal cage by determining a spatial relation between the spinal cage and the pointing device.