US-12616522-B2 - Method for determining the screw trajectory of a pedicle bone screw

US12616522B2US 12616522 B2US12616522 B2US 12616522B2US-12616522-B2

Abstract

A method for determining the screw trajectory of a pedicle bone screw comprises: obtaining a CT image of the target bone area intended to receive the pedicle bone screw, establishing an individualized three-dimensional geometric model of the target bone area based on the CT image, accessing a database comprising a three-dimensional bone area model; wherein the bone area model comprises a bone screw insertion surface and a pedicle traversing surface for each pedicle, morphing the bone area model to the geometric model of the target bone area generating a morphed vertebra model with a bone screw insertion surface and the pedicle traversing surface, calculating a maximum of bone density when the bone material is replaced by a bone screw for a bone screw in the morphed vertebra model of the target bone, and outputting the space vector of the screw trajectory for the bone screw together with the length and diameter of the bone screw in the morphed vertebra model of the target bone.

Inventors

Jonas Widmer
Sebastiano Caprara
Mazda Farshad

Assignees

25SEGMENTS AG

Dates

Publication Date: 20260505
Application Date: 20211213
Priority Date: 20201214

Claims (20)

1 . A method for determining an optimal screw trajectory of a pedicle bone screw comprising the following steps: obtaining a computed tomography (CT) image of a target bone area intended to receive the pedicle bone screw; establishing an individualized three-dimensional geometric model of the target bone area based on the CT image, the individualized three-dimensional geometric model including bone density information; accessing a database comprising a three-dimensional bone area model, wherein the three-dimensional bone area model comprises a bone screw insertion surface and a pedicle traversing surface; morphing the three-dimensional bone area model to the individualized three-dimensional geometric model of the target bone area generating a morphed vertebra model comprising the bone screw insertion surface and the pedicle traversing surface of the three-dimensional bone area model as well as the bone density information of the individualized three-dimensional geometric model, wherein the morphing comprises transferring the bone screw insertion surface and the pedicle traversing surface onto the individualized three-dimensional geometric model; calculating an optimal screw trajectory of the pedicle bone screw maximizing bone density when bone material is replaced by the pedicle bone screw in the morphed vertebra model of the target bone area; and outputting the optimal screw trajectory for the pedicle bone screw in the morphed vertebra model of the target bone area.
2 . The method according to claim 1 , further comprising outputting a length and diameter of the pedicle bone screw.
3 . The method according to claim 2 , wherein a first threshold value as length safety distance is provided within the determination of a screw length based on a body side surface of a body of the target bone area.
4 . The method according to claim 1 , wherein the morphing of the three-dimensional bone area model to the individualized three-dimensional geometric model comprises providing a three-dimensional pedicle traversing surface within the morphed vertebra model of the target bone, the three-dimensional pedicle traversing surface being modelled in the three-dimensional bone area model within the database and morphed with the individualized three-dimensional geometric model.
5 . The method according to claim 4 , wherein the morphing of the three-dimensional bone area model further comprises providing a two-dimensional pedicle traversing surface, wherein the two-dimensional pedicle traversing surface as the pedicle traversing surface is based on the three-dimensional pedicle traversing surface and corresponds to a plane of minimum transverse pedicle width in a pedicle.
6 . The method according to claim 5 , wherein a first threshold value as contour safety distance is provided within the determination of the two-dimensional pedicle traversing surface-, generated as a three-dimensional curve inside an outer edge of the pedicle, delimiting voxels used in the three-dimensional pedicle traversing surface and subsequently the two-dimensional pedicle traversing surface.
7 . The method according to claim 5 , wherein starting conditions of the step of calculating an optimal screw trajectory of the pedicle bone screw comprise values of a starting screw wherein the central axis of the starting screw passes through a centre point of the two-dimensional pedicle traversing surface, wherein the bone screw insertion surface of an enveloping cylinder of the starting screw is inside the bone screw insertion surface.
8 . The method according to claim 4 , wherein starting conditions of the step of calculating an optimal screw trajectory comprise values of a starting screw wherein the central axis of the starting screw is chosen to be the most centred portion of axis inside the three-dimensional pedicle traversing surface.
9 . The method according to claim 1 , wherein the step of calculating an optimal screw trajectory comprises: creating a cylinder approximating the pedicle bone screw and placing the cylinder in the morphed vertebra model along an initial screw axis calculated based on the bone screw insertion surface and the pedicle traversing surface; and calculating bone material density within the cylinder approximating the pedicle bone screw in the morphed vertebra model.
10 . The method according to claim 9 , further comprising: excluding cylinders approximating the pedicle bone screw that are perforating the morphed vertebra model; and using bone material properties within the cylinder approximating the pedicle bone screw, extracted from image voxels in the morphed vertebra model.
11 . The method according to claim 1 , wherein the step of calculating an optimal screw trajectory of the pedicle bone screw comprises: identifying a space of possible projection planes of the morphed vertebra model using the bone screw insertion surface and pedicle traversing surface; scanning possible projection planes in order to determine a set of intersection region density projections; scanning the set of intersection region density projections and calculating corresponding projected bone density scores; and determining the optimum screw trajectory as having a direction normal to the projection plane corresponding to a highest score of the projected bone density scores.
12 . The method according to claim 11 , further comprising determining the length of the pedicle bone screw using the individualized three-dimensional geometric model of the target bone area and the optimum screw trajectory, as the distance between the bone screw insertion surface and pedicle traversing surface in the direction of the optimum screw trajectory.
13 . The method according to claim 11 , wherein the step of identifying possible projection planes of the morphed vertebra model comprises: defining a sphere in the morphed vertebra model around a center of the target bone area; defining the possible projection planes as a set planes lying normal to the surface of the sphere; and restricting the possible projection planes to the set of projection planes with a positive intersection between bone screw insertion surface and pedicle traversing surface.
14 . The method according to claim 11 , wherein the step of scanning of possible projection planes comprises: selecting a number of discretely distributed projection planes within the possible projection planes; determining an intersection region of the bone screw insertion surface and pedicle traversing surface for each of the number of discretely distributed projection planes; and summing of all voxels of the morphed vertebra model representing bone density within the intersection region and normal to the projection plane to obtain respective intersection region density projections.
15 . The method according to claim 11 , wherein scanning the set of intersection region density projections and calculating corresponding projected bone density scores comprises summing of voxels of the intersection region density projection within an area delimited by one or more possible bone screw diameter for each possible position of a bone screw within the corresponding intersection projection, wherein the optimum screw trajectory has an axis crossing the center of the screw diameter corresponding to the highest score of the projected bone density scores.
16 . The method according to claim 1 , wherein the calculating step is provided with starting parameters of screw length, screw diameter with boundary conditions of a predetermined maximum length, and a predetermined maximum diameter, in the individualized three-dimensional geometric model of the target bone area.
17 . The method according to claim 1 , wherein the morphing of the three-dimensional bone area model comprises providing a sagittal plane within the geometric morphed vertebra model to determine the pedicle traversing surface.
18 . The method according to claim 17 , wherein the sagittal plane of a body of the target bone area provides a first threshold value which the contour and the tip of the pedicle bone screw has not to pass for any one pedicle bone screw to be introduced into a same body or for only one of two pedicle bone screws to be introduced into the same body in a way that the two pedicle bone screws do not occupy the same place.
19 . The method according to claim 1 , wherein the morphing of the three-dimensional bone area model comprises determining a vertebral foramen within the geometric morphed vertebra model to determine the pedicle traversing surface as smallest bone material diameter on sides of the vertebral foramen.
20 . The method according to claim 1 , wherein the step of calculating an optimal screw trajectory of the pedicle bone screw is repeated iteratively for maximizing bone density when bone material is replaced by the pedicle bone screw in the morphed vertebra model of the target bone area.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS The present application is a U.S. National Phase filing of International Application No. PCT/EP2021/085554, filed on Dec. 13, 2021, and claiming priority to European Patent Application No. 20213746.9 filed Dec. 14, 2020. The present application claims priority to and the benefit of all the above-identified applications, which are all incorporated by reference herein in their entireties. TECHNICAL FIELD The present disclosure relates to a method for determining the screw trajectory of a pedicle bone screw, a data processing system comprising means for determining the screw trajectory of a pedicle bone screw as well as a computer program product comprising instructions which, when the program is executed by a computer, cause the computer to plan the screw trajectory of a pedicle bone screw. BACKGROUND OF THE DISCLOSURE CN 110946652 A discloses a screw trajectory determining method of a bone screw, comprising the steps of establishing an individual three-dimensional geometric model of a target bone, and a three-dimensional bone density model containing spatial distribution information of bone density, setting bone screw parameters, and calculating the space vector of a bone screw trajectory, calculating a space helical line of bone threads as a bone screw thread curve; and placing the bone screw thread curve into the individual three-dimensional geometric model of the target bone, extracting the bone density at points of contact with bone screw threads in the individual three-dimensional geometric model of the target bone, and calculating an average value of the bone density at the contact positions; and determining a bone screw trajectory according to the bone screw parameters, the bone density and the distribution of the bone density. According to the method, by obtaining coordinates of the bone and bone screw contact positions on the three-dimensional scale, the bone density around the screw threads is determined accurately and allows determining a screw trajectory of the bone screw according to the fixation performance of the bone screw. US 2017/112575 A1 discloses a non-transitory computer readable media embodying a program of instructions executable by machine to perform operations for pedicle screw positioning, the operations comprising: receiving image data of at least a portion of a spine; segmenting at least one vertebra of interest in the image data; determining two pedicle regions within the segmented vertebra of interest; determining one or more safe regions within the segmented vertebra of interest; generating two optimal insertion paths within the one or more safe regions, wherein the two optimal insertion paths pass through respective centres of the pedicle regions; and displaying the two optimal insertion paths for pedicle screw positioning. The method uses voxels and distances of the voxels within a segmented vertebra of interest from a nearest vertebral edge to determine the safe region for a screw to be implanted, by assigning the voxel to the safe region if the distance to such an edge being greater than a threshold distance. US 2004/240715 A1 discloses a method for determining the placement of a pedicle screw comprising determining a trajectory for the placement of such a pedicle screw in a patient from a set of 2D images, wherein the step of determining the trajectory comprises: computing an optimum implant trajectory of the pedicle screw by determining the minimum transverse pedicle width for all of the 2D image slices that contain the vertebra under study, and determining for the vertebra under study the overall minimum transverse pedicle width of the minimum transverse pedicle for all of the 2D images slices. CN 109199604 A discloses a pedicle screw optimal entry point positioning method based on feature vectors of a three-dimensional mesh model as input into a decision tree classification model. SUMMARY OF THE DISCLOSURE Based on this prior art, the present disclosure provides an alternative method to optimize screw parameters, comprising size and orientation, based on patient-specific bone properties, especially bone density of the replaced screw volume. According to the present disclosure, this is addressed by the features of the independent claim 1. In addition, further advantageous embodiments follow from the dependent claims and the description. In particular, this is addressed by a method for determining the screw trajectory of a pedicle bone screw comprising the following steps: obtaining a CT image of the target bone area intended to receive the pedicle bone screw,establishing an individualized three-dimensional geometric model of the target bone area based on the CT image including bone density information,accessing a database comprising a three-dimensional bone area model; wherein the bone area model comprises a bone screw insertion surface and a pedicle traversing surface,morphing the bone area model to the geometric model of the tar