US 11,596,444 B2 · Automatic recognition method for spatial position and pose of parallel external fixator for fracture reduction

Abstract

The present invention discloses an automatic recognition method for spatial position and pose of parallel external fixator, including the following steps of: installing three markers on each of the two fixation rings of the parallel external fixator; obtaining 3D images of six marker balls after scanning and reconstruction by a common 3D clinical imaging system; recognizing the sphere center coordinates of the six marker balls by sphere fitting algorithm; according to the mounting configuration of the markers on the two fixation rings, establishing coordinate systems of two fixation rings and determining the spatial position and pose of the external fixator; in addition, by obtaining the 3D images of the fracture bone segments with the 3D clinical imaging system and simulating the movement of the fracture deformity correction, the adjustment schedule of the external fixator struts can be achieved.

An automatic recognition method for spatial position and pose of a parallel external fixator, including the following steps of:

(1) installing markers, including:
defining two fixation rings of the parallel external fixator as a proximal fixation ring (210) and a distal fixation ring (211);
installing three markers on the proximal fixation ring and three on the distal fixation ring, wherein the six markers have the same structure and each comprise a marker ball and a marker pin shaft;
the distance from a sphere center of each marker ball on the proximal fixation ring to an upper surface of the proximal fixation ring is a fixed value hM;
the distance from a sphere center of each marker ball on the distal fixation ring to an upper surface of the distal fixation ring is the same fixed value hM;
the marker balls and the marker pin shafts are made of different materials, so that the marker balls can be recognized by a common three-dimensional (hereinafter referred to as 3D) clinical imaging system, and the marker pin shafts will not be recognized;
(2) acquiring 3D images, including:
3D scanning the fracture site and the parallel external fixator with installed markers;
performing threshold division and 3D reconstruction to obtain 3D images of a proximal bone segment (310), a distal bone segment (311) and six marker balls;
storing the data of the 3D images in a computer system by using a 3D point cloud format, wherein the 3D point cloud format is composed of a plurality of data points to describe the surface shape of a 3D object;
(3) sphere fitting the marker balls, including:
(a) based on the 3D point cloud data of the six marker balls and a known diameter of the marker balls, fitting the 3D point cloud of the six marker balls into six spheres by a sphere fitting algorithm;
(b) by referring to 3D images of the proximal bone segment and the distal bone segment, specifying the correspondence relationship between the six marker balls in the 3D images and the six markers on the parallel external fixator, and then storing the correspondence relationship into the computer system;
(4) specifying installation information of the markers, including:
(a) establishing a proximal coordinate system p-uvw attached to the proximal fixation ring and a distal coordinate system P-UVW attached to the distal fixation ring;
(b) according to a mounting configuration among a first marker (201), a second marker (202), a third marker (203) and the proximal fixation ring, by using the structural dimensions of the proximal fixation ring, calculating the coordinates of a first sphere center of the first marker ball M1(u1v1w1), a second sphere center of the second marker ball M2(u2v2w2), and a third sphere center of the third marker ball M3(u3v3w3) in the proximal coordinate system p-uvw, respectively;
and according to a mounting configuration among a fourth marker (204), a fifth marker (205), a sixth marker (206) and the distal fixation ring (211), by using the structural dimensions of the distal fixation ring (211), calculating the coordinates of the fourth sphere center of the fourth marker ball M4(U4V4W4), the fifth sphere center of the fifth marker ball M5(U5V5W5), and the sixth sphere center of the sixth marker ball M6(U6V6W6) in the distal coordinate system P-UVW, respectively;
the sphere center coordinates of the six marker balls constitute the markers installation information of the external fixator;
(c) inputting the markers installation information into the computer system;
(5) recognizing spatial position and pose of the coordinate systems, including:
(a) according to the plane formed by the first sphere center, second sphere center and third sphere center that is parallel to the upper surface of the proximal fixation ring, calculating a normal vector {right arrow over (pw)} of the upper surface of the proximal fixation ring by the following formula, which corresponds to the axis w of the proximal coordinate system p-uvw:
p
⁢
w
→
=
M
1
⁢
M
2
→
×
M
2
⁢
M
3
→

M
1
⁢
M
2
→
×
M
2
⁢
M
3
→

in which, {right arrow over (M1M2)} is a vector pointing from the first sphere center to the second sphere center in a point cloud reference coordinate system O-xyz, and {right arrow over (M1M2)}=(XM2−XM1YM2−YM1ZM2−ZZM1);
{right arrow over (M2M3)} is a vector pointing from the second sphere center to the third sphere center in the point cloud reference coordinate system O-xyz, and {right arrow over (M2M3)}=(xM3−xm2YM3−ym2zm3−zm2);
in which xm1, ym1, zm1 represent coordinates of the first sphere in the point cloud reference coordinate system O-xyz, xm2, ym2, zm2 represent coordinates of he second sphere in the point cloud reference coordinate system O-xyz, and xm3, ym3, zm3 represent coordinates of the third sphere in the point cloud reference coordinate system O-xyz,
(b) assuming the coordinates of an origin p of the proximal coordinate system p-uvw are pO−xyz=(XpOYpOZpO) in the point cloud reference coordinate system O-xyz, according to the coordinates M1(u1v1w1), M2(u2v2w2) and M3(u3v3w3) obtained by the step (4b) in the proximal coordinate system p-uvw, calculating the coordinates of the origin p in the point cloud reference coordinate system O-xyz by solving the following distance equations:
{

p
⁢
M
1
→

2
=
u
1
2
+
v
1
2
+
w
1
2

p
⁢
M
2
→

2
=
u
2
2
+
v
2
2
+
w
2
2

p
⁢
M
3
→

2
=
u
3
2
+
v
3
2
+
w
3
2
(c) in the point cloud reference coordinate system O-xyz, assuming the value of unit vector {right arrow over (pu)} of the proximal coordinate system p-uvw is {right arrow over (pu)}=(xpuypuzpu);
respectively taking the scalar products of vectors {right arrow over (pM1)}, {right arrow over (pM2)} and {right arrow over (pM3)} of the first sphere center, the second sphere center and the third sphere center with the unit vector {right arrow over (pu)}, the outcomes are equivalent to the vector components u1, u2 and u3 of the first sphere center, the second sphere center and the third sphere center along the axis {right arrow over (pu)}, expressing as the following:
{
p
⁢
M
1
→
·
p
⁢
u
→
=
u
1
p
⁢
M
2
→
·
p
⁢
u
→
=
u
2
p
⁢
M
3
→
·
p
⁢
u
→
=
u
3
in which, the vectors {right arrow over (pM1)}, {right arrow over (pM2)} and {right arrow over (pM3)} are determined by, {right arrow over (pM1)}=(xM1−xpOym1−ypOZm1−zpO), {right arrow over (pM2)}=(xM2−xpOYM2−YpOzM2−zpO) and {right arrow over (pM3)}=(xm3−xpOYM3−YpOzm3−zpO), respectively;
solving the above equation set to determine the value of unit vector {right arrow over (pu)} of the proximal coordinate system p-uvw;
(d) calculating the unit vector {right arrow over (pv)} of the proximal coordinate system p-uvw according to the right-hand rule, using the following equation:
{right arrow over (pv)}={right arrow over (pw)}×{right arrow over (pu)}
(e) repeating step (5a) to step (5d), according to the coordinates of the fourth sphere center M4(U4V4W4), the fifth sphere center M5(U5V5 W5) and the sixth sphere center M6(U6V6W6) in the distal coordinate system P-UVW, calculating the coordinate of an origin P of the distal coordinate system P-UVW, in the point cloud reference coordinate system O-xyz, PO-xyz=(xPOYPOZPO), and determining the value of unit vectors {right arrow over (PU)}, {right arrow over (PV)} and {right arrow over (PW)} of the three coordinates axes of the distal coordinate system P-UVW in the point cloud reference coordinate system O-xyz;
(f) determining the position and pose of the proximal coordinate system p-uvw relative to the point cloud reference coordinate system O-xyz by using the origin coordinate pO-xyz and the unit vectors {right arrow over (pu)}, {right arrow over (pv)}, {right arrow over (pw)};
determining the position and pose of the distal coordinate system P-UVW relative to the point cloud reference coordinate system O-xyz by using the origin coordinate PO-xyz and the unit vectors {right arrow over (PU)}, {right arrow over (PV)}, {right arrow over (PW)};
and storing the spatial position and pose of the proximal and distal coordinate systems into the computer system;
(6) determining the spatial position and pose of the external fixator, including:
(a) calculating the relative position of the proximal fixation ring and the distal fixation ring by using the origin coordinate PO-xyz of the proximal coordinate system and the origin coordinate PO-xyz of the distal coordinate system, and expressing the relative position with a position vector {right arrow over (rPp)};
as the following:
{right arrow over (rPp)}=pO-xyz−PO-xyz=(xpO−xPOypO−yPOzpO−zPO)
(b) calculating the relative pose of the proximal fixation ring and the distal fixation ring by using the unit vectors {right arrow over (pu)}, {right arrow over (pv)}, {right arrow over (pw)} of the proximal coordinate system and {right arrow over (PU)}, {right arrow over (PV)}, {right arrow over (PW)} of the distal coordinate system, and expressing the relative pose with a pose matrix RPp as the following:
R
P
p
=
[
p
⁢
u
→
·
P
⁢
U
→
p
⁢
v
→
·
P
⁢
U
→
p
⁢
w
→
·
P
⁢
U
→
p
⁢
u
→
·
P
⁢
V
→
p
⁢
v
→
·
P
⁢
V
→
p
⁢
w
→
·
P
⁢
V
→
p
⁢
u
→
·
P
⁢
W
→
p
⁢
v
→
·
P
⁢
W
→
p
⁢
w
→
·
P
⁢
W
→
]
(c) determining the spatial position and pose of the external fixator according to the position vector {right arrow over (rPp)} and pose matrix RPp of the proximal fixation ring relative to the distal fixation ring;
(7) determining the adjustment values of the external fixator for fracture reduction, including:
displaying the 3D images of the proximal bone segment and the distal bone segment on the screen of the computer system, moving the 3D images displayed on the screen according to the desired fracture reduction movement, achieving the correct alignment of the proximal bone segment and the distal bone segment in the screen display;
automatically calculating and storing, by the computer system, the translation and rotation movement values of the proximal bone segment 3D image relative to the distal bone segment 3D image in the point cloud reference coordinate system O-xyz during the reduction, wherein the translation and rotation movement values of the proximal bone segment 3D image relative to the distal bone segment 3D image are equal to the translation and rotation adjustment values of the proximal fixation ring relative to the distal fixation ring during the reduction, thus the adjustment values of the external fixator for fracture reduction is determined;
and
(8) according to the recognized spatial position and pose of the external fixator, the adjustment values of the external fixator for fracture reduction, and by using a kinematics algorithm in the field of robotics, the adjustment schedule of the external fixator for fracture reduction is obtained;
adjusting the external fixator referring to the adjustment schedule, such that the fracture reduction is achieved.

265 more elements

Automatic recognition method for spatial position and pose of parallel external fixator for fracture reduction

Life of this patent

Automatic recognition method for spatial position and pose of parallel external fixator for fracture reduction

Life of this patent

Abstract

Claims8