RGP TORIC CORRECTION
Ed Bennett, O.D., M.S. Ed.
Objectives of this lecture:
1. To be able to understand the applications of front surface toric, back surface toric and bitoric lenses
2. To be able to determine lens powers for back surface and bitoric lenses
I. RESIDUAL ASTIGMATISM
Residual Astigmatism = Spectacle Cylinder - Corneal Cylinder
Example One: -1.25 = -1.25 - 0
(predicted) (-3-1.25x180) (45DS)
Example Two: -1.25 = 0 - 1.25 (predicted) (-3.00DS) (43.75/45.00)
Example Three: Rx = +2.00 - 3.50 x 180
K’s = 41.50 @ 180/44.50 @ 090
CRA = -3.50 x 180 - (-)3.00 x 180 => -0.50 x 180
B. Correction
1. Spherical Rigid Lens
a. ARA differs from CRA
Example Four: Rx: -1.50 -1.50 x 180
K’s: 42 @ 180/ 44.50 @ 090
CRA = -1.50 x 180 - (-) 2.50 x 180 => +1.00 x 180 or +1.00 -1.00 x 090
Dx Lens: -3.00D, 42.50 BCR LLP = 42.50 - 42 = +0.50D
OR = Rx - (CLP + CCP) + delta K
-1.50 -1.50 x 180 = -3.00 + (+) 0.50 + (-) 2.50 x 180
=> +1.00 + 1.00 x 180 or +2.00 -1.00 x 090
but ARA (via O.R.) = -0.50 x 090
b. VA not decreased significantly
c. Low Critical Vision Demand
d. Lens flexes to decrease RA - when the corneal toricity is WTR and the RA is ATR, a thin spherical lens will flex and reduce the amount of RA.
Example Five: Rx = -1.25 -1.00 x 180
K’s = 42 @ 180/ 44 @ 090
CRA = -1.00 x 180 - (-) 2.00 x 180 => +1.00 -1.00 x 090
Thin lens will flex to correct 0.50D
2. Soft Lenses
Example Six: Spherical/Rx =
-3.00 -0.25 x 090 K’s = 42/43.25
Example Seven: Toric/Rx = -3.00 -1.25 x 090 K’s = 42/42.25
3. Prism Ballast
-3/4 to 1PD for high minus lenses; 1 1/4 - 1 1/2 PD for low minus/plus lenses; vice versa if also truncated
Example Eight: CT x 100 = Prismatic Power x OAD (Borish)
If 1 PD added to 9mm lens, .09mm would need to be added to conventional CT
Example Nine: Rx = -1.50 -1.50 x 090
K’s = 43.25DS
Dx Lens = 43.50/-3.00
RA = -1.50 x 090
Toric Lens = +0.25 OR = +1.25 -1.50 x 090 (rotates 10 degrees)
= -1.75 -1.50 x 080 or -3.25 +1.50 x 170
Contact Lens Order:
|
BCR: 43.50 (7.76) |
SCR/PCR: 8.80/0.3, 10.80/0.3CT: 0.24mm |
Verification:
a. Radiuscope: rotate until meridians at 090 and
180; mark with red pencil along one meridian
b. Lensometer: orient line and read both powers
Problems:
|
1. Blurred Vision |
5. Can’t Modify Front |
HIGH ASTIGMATISM
1. Problems
a. Residual Astigmatism/Flexure
Minimized via:
1. Lower Dk material
2. Flatter BCR (0.50D minimum)
3. Increase CT (0.03mm minimum)
4. Decrease OZD (0.4mm minimum)
b. Decentration
c. Rocking
d. Desiccation
e. Discomfort
2. Aspheric Design (Boston Envision)
a. Boston RxD material
b. biaspheric design
c. 0.1mm BCR steps
d. EnVision vs. Spherical Lens on a 3D astigmat
B. Toric Peripheral Curves
Example Ten: K’s = 43 (7.85)/45.25
(7.46)
Lens Design: OAD = 9.0/OZD = 8.0/6.8
BCR = 43.50 (7.76) Rx = -1.50
SCR = 8.85/8.46 PCR: 10.85/10.46
C. Back Surface Toric
1. Determination of BCR (Several methods)
Remba
|
Corneal Astigmatism |
Flat Meridian |
Steep Meridian |
|
2- 2.75D |
Fit 0.25 Flat |
Fit 0.25D Flat |
2. Determination of Toric Powers (Sarver)
Fs = Ff + Kf - Ks where
Fs = BVP in Steep Meridian
Ff = BVP in Flat Meridian
Ks = BCR in Steep Meridian
Kf = BCR in Flat Meridian
Example Eleven: Rx = +1.00 -4.00 x 180
K’s = 41 @ 180, 45 @ 090
Kf = 41 + (-) 0.25 = 40.75
Ks = 45 - 0.75 = 44.25
Ff1 = +1.00 + (+0.25) (LLP) = +1.25
Fs1 = +1.25 +40.75 - 44.25 = -2.25
BCR Rx SCR PCR
40.75/44.25 +1.25 9.00 11.00
(8.28) (7.63)
OAD = 9.0 CT = .20
3. Contact Lens Power Conversion Factors:
From: To:
Multiply by:
|
Kcl back surface |
CLRx in Air |
1.452 |
= 1-1.49 1-1.3375 |
|
Kcl |
CLRx in Fluid |
0.456 |
= 1.336-1.49 1-1.3375 |
|
CLRx in Air |
CLRx in Fluid |
0.314 |
= 1.336-1.49 1-1.49 |
|
CLRx in Fluid |
CLRx in Air |
3.19 |
= 1-1.49 1.336-1.49 |
1:2:3 Principle
"Induced" cylinder = one-half radiuscope valve, one- third lensometer valve. It will be a minus cylinder axis along flatter meridian of toric back surface.
Example Twelve: I.C. = .456 x K(back surface)
.456 x -3.50 x 180
-1.60D x 180
If +1.60D is added to front surface induced cylinder is corrected BST is lens of choice when corneal toricity is ATR and RA is one-half back surface toricity. (i.e., patient has ATR cylinder and refractive cyl = 1 1/2 times back surface toricity).
D. Bi-Toric
Uses:
1. Centration
2. Vision
3. Corneal Integrity
Example Thirteen: Rx = -4.25 -3.00 x 010
Vertex = -4.00 -2.75 x 010
K’s = 42.75 @ 010, 45.50 @ 100
CRA = 0
Kf = 42.75 - (+) 0.25 = 42.50 (7.94)
Ks = 45.50 - (+) 0.25 = 45.25 (7.46)
Ff1 = -4.00 - (-) 0.25 = -3.75D
Fs1 = Ff1 + Cyl = -3.75 -2.75 or -6.50D
Contact Lens Order:
BCR = 7.94/7.46
SCR/W = 8.94/.3, 8.46/.3
PCR/W = 10.94/.3, 10.46/.3
Rx = -3.75/-6.50
OAD = 8.8, CT = .13
Induced Cylinder = .456 x back surface toricity = .456 x -2.75 = approx. -1.25D
Spherical Power Effect - when induced cylinder (only) is applied on front surface, the lens can rotate freely without any effect on vision
Polycon SPE: Astigmatism Select
1.25-2.87D 2D
3.00-4.87D 3D
5.00 or more 4D
Base Curve Selection: Flat Meridian = 0.12-0.50D flatter than "K"
1. Refract with spheres over Dx lens
2. Add O.R. to powers in flat and steep meridians of diagnostic lens
Example Fourteen: K’s = 42.50/45.50
Rx = -3.75-3.50 x 180
Vertex - -3.75 -3.00 x 180
Dx Lens = 42 (pl)/45(-3.00)
O.R. = -3.25DS
Final Rx = 8.04/7.50 (-3.25)(-6.25)
If spherical O.R. results in blurred vision, use C.P.E. (cylinder power
effect).
METHOD ONE: (Polycon Fitting Guide)
1. Sph-Cyl OR over SPE Dx lens
2. Add sphere to both meridians
3. If O.R. Axis is within 15 degrees of flat meridian of Dx lens, add cyl to steep meridian, if O.R. Axis is within 15 degrees of steep meridian of Dx lens, add cyl to flat meridian.
Example Fifteen:
1. Dx Lens = 8.04/7.50 (pl) (-3.00)
Flat Mer. = Axis 010
2. O.R. = -2.00 -1.25 x 180
Add -2.00 to both meridians = -2.00/-5.00
3. Add cyl to steep meridian -1.25 + -5.00 = -6.25
Final Order: 8.04/7.50 (-2.00)(-6.25)
Example Sixteen:
1. Dx Lens = 8.04 x 7.50 (pl) (-3.00)
2. OR = -2.00 -1.25 x 090
3. Add Sphere to both meridians -2.00/-5.00
4. Add cyl to flat meridian -2.00 +(-) 1.25 = -3.25
Final Order: 8.04/7.50 (-3.25)(-5.00)
METHOD TWO: (Silbert) - If axes are at or near the principal corneal meridians, add appropriate power in refraction to air power of corresponding meridian in the diagnostic lens, and order.
Example Seventeen: Dx SPE = 42/45 (pl)(-3.00)
O.R. = -1.00 -1.25 x 180
Add -1.00 to 180 Meridian and -2.25 to 090 Meridian
Final Order: 42/45 (1.00)(-5.25)
POOR CANDIDATE: A difference of 15 degrees or more between corneal cyl axis and spectacle axis.
Examples:
1. Typical Fit (7.6 x 8.0mm BCR)
2. Too Steep
3. Not Enough Back Surface Toricity
4. Too Much Back Surface Toricity
Verification
1. Radiuscope: rotate until 2 principal meridians
are at 90 and 180 then record base curves.
2. With red grease pencil, lens edges are marked along one of the two
meridians, indicating whether this is flatter or steeper meridian.
3. The lens is turned over and a line is drawn, connecting the 2 edge marks.
4. The lens is oriented on the lensometer and the 2 powers read.
5. This is differentiated from a warped lens which will be spherical.