Method of calculating the required power of a toric implant
View Patent ↗A method for calculating the required power of a toric implant by using both the measured pre-operative corneal astigmatism and the predicted surgically-induced post-operative astigmatism. The surgically-induced post-operative astigmatism is predicted using power vector analysis of the surgical technique employed by the surgeon. Such a method provides a more accurate method of calculating the required post-operative refractive power of the implant. The method can be implemented manually, but preferably is automated by implementation on a computer through appropriate software.
1. A method of calculating the required power of a toric ophthalmic lens, comprising the steps of:
determining an optimum cylindrical refraction needed to correct sphero-cylindrical refractive errors in a patient's eye;
calculating a predicted surgically induced refractive change to the patient's eye;
analyzing the optimum cylindrical refraction needed to correct sphero-cylindrical refractive errors in the patient's eye in combination with the predicted surgically induced refractive change to the patient's eye to determine the required power of the toric ophthalmic lens implant at a corneal plane, comprising
describing the corneal-astigmatism and the surgically-induced astigmatism in vector terms;
calculating a predicted post-op corneal astigmatism with a vector analysis; and
converting the vector analysis results back to conventional sphero-cylindrical format
wherein the step of calculating the predicted post-op corneal astigmatism with the vector analysis comprises using the following equation:
Rx Cornea =( M,J 0 ,J 45 ); Rx SIRC ′=( M′,J 0 ′,J 45 ′)
Rx Xcyl =Rx Cornea +Rx SIRC ′=( M+M′,J 0 +J 0 ′,J 45 +J 45 ′)
wherein M=S+0.5 C, J 0 =−0.5C*cos (2α), J 45 =−0.5C*sin(2α), S=the spherical power of the implant at the corneal plane, C=the cylindrical power of the implant at the corneal plane, and α=an angle of the cylindrical correction from vertical; and
and wherein the step of converting the vector analysis results back to conventional sphero-cylindrical format comprises using the following equation:
C
=
-
2
J
0
2
+
J
45
2
;
S
=
M
-
C
2
;
α
=
{
1
2
tan
-
1
(
J
45
J
0
)
J
0
≥
0
&
tan
-
1
(
J
45
J
0
)
≥
0
1
2
tan
-
1
(
J
45
J
0
)
+
180
°
J
0
≥
0
&
tan
-
1
(
J
45
J
0
)
<
0
1
2
tan
-
1
(
J
45
J
0
)
+
90
°
J
0
<
0
&
tan
-
1
(
J
45
J
0
)
≥
0
1
2
tan
-
1
(
J
45
J
0
)
+
90
°
J
0
<
0
&
tan
-
1
(
J
45
J
0
)
<
0
wherein S=the spherical power of the implant at the corneal plane, C=the cylindrical power of the implant at the corneal plane, and α=an angle of the cylindrical correction from vertical; and
reporting a selection of a toric lens implant based on the conversion to sphero-cylindrical format, wherein the steps of the method are implemented by a computer through software.
2. A method of determining a desired toric power of an ophthalmic lens implant, comprising the steps of:
calculating the tonic power of an ophthalmic lens implant at a corneal plane (P Implant ) using the following equation
Cyl
Implant
=
∂
P
Implant
∂
P
Implant
′
Cyl
Implant
′
+
∂
P
Implant
∂
L
cornea
Cyl
cornea
wherein Cyl Implant is a cylindrical power of the lens implant at an implant plane and Cyl Cornea is a cylindrical power of the lens at the corneal plane
calculating coefficients that describe the dependence between variables in the equation of step a) using the following equation
C
1
=
∂
P
Implant
∂
P
Implant
′
=
1
(
1
-
d
n
(
P
Implant
′
+
L
cornea
)
)
2
C
2
=
∂
P
Implant
∂
L
cornea
=
1
(
1
-
d
n
(
P
Implant
′
+
L
cornea
)
)
2
-
1
(
1
-
d
n
L
cornea
)
,
wherein L cornea is a vergence at the corneal plane, d=a distance between a principal plane of the cornea near the anterior surface and a principal plane of the lens implant and n=the refractive index of aqueous humor
determining a conversion factor (CF) for translating the toric power of the lens at the corneal plane to a toric power of the lens at a desired implant plane using the following equation
Cyl Implant =( C 1 −C 2 )* C Xcyl =CF*C Xcyl , and
reporting a selection of a toric lens implant based on the conversion factor, wherein the steps of the method are implemented by a computer through software.
3. The method of claim 2 wherein the conversion factor (CF) comprising the following values:
for 0 mm≦d<0.25 mm, CF=1.00;
for 0.25 mm≦d<0.75 mm, CF=1.02-1.04;
for 0.75 mm≦d<1.25 mm, CF=1.05-1.09;
for 1.25 mm≦d<1.75 mm, CF=1.08-1.14;
for 1.75 mm≦d<2.25 mm, CF=1.11-1.19;
for 2.25 mm≦d<2.75 mm, CF=1.15-1.24;
for 2.75 mm≦d<3.25 mm, CF=1.18-1.30;
for 3.25 mm≦d<3.75 mm, CF=1.21-1.37;
for 3.75 mm≦d<4.25 mm, CF=1.25-1.43;
for 4.25 mm≦d<4.75 mm, CF=1.29-1.51;
for 4.75 mm≦d≦5.25 mm, CF=1.32-1.59;
for 5.25 mm<d≦5.75 mm, CF=1.37-1.67;
for 5.75 mm<d≦6.25 mm, CF=1.41-1.76;
for 6.25 mm<d≦6.75 mm, CF=1.45-1.86;
for 6.75 mm<d≦7.25 mm, CF=1.50-1.97.