Selecting BC, D, sph or astig Rx for SCL fitting
Astig Rx
Range of Astig SCL
Standard
Sphere
+4.00 to -6.00
Astigmatism
-0.75 to -2.25
Axis
0 to 180 in 10 degrees
Custom:
Westcon, Alden
Sphere
+/- 20.00
Subtopic
Astigmatism
-10.00
Axis
1 degree steps
Assessing Fit
Stability of Lens
Stabilizing Systems
Prism Ballast
Back surface torridity
Dual Thin Zone
Eccentric Lenticulation
Proprietary Thickness
Combination
Lens Coverage
Limbal to Limbal Centration
Equilibration time: 15-20 min.
Rotation of Lens Fit
Primary gaze: Unstable if >10 degrees or >5 sec return to initial position
S/p globe rotation: <15 sec return is a good fit
>15 sec return is a tight fit
Lid Factor
Taught lids: more rotation
Flacid lids: less rotation
Thickness profile
x90 (atr)
Thickest meridian (180) parallels upper lid margin:
~lid forces lens down without rotation
~MOST stable
~soft astigmatic lens usually works best with this.
x180 (wtr)
Thickest meridian (90) perpendicular to upper lid margin:
~when lid meets thick meridian, rotation either CW or CCW
~LEAST stable
x45 (oblique)
Thickest meridian 10:00 to 4:00:
~Lid meets higher 10:00 edge --> CCW rotation
~Unpredictable rotation
~spherical GP lens usually works best with this
x135 (oblique)
Thickest meridian 2:00 to 8:00:
~Lid meets higher 2:00 edge --> CW rotation
~Unpredictable rotation
~spherical GP lens usually works best with this.
Lens surface
~Proteins
~lipids
~mucus
~debris
*will all allow for a sticky lens surface, and thus allow the lids to partake in a more significant rotational effect.
Lens power
High power = more thickness =
more lid interaction = more roation
Adjustment to Initial Fitting
LARS
CW: to examiner's left
CCW: to examiner's right
LARS Rule
Spherical Over-refraction
Spherical Trial Lens
Use Aspheric to correct for chromatic aberrations
– Ciba
• Focus Night & Day • Focus Air Optix
• Focus Dailies
Lens Power
Will be based on the patient's refraction
May be different than spectacle depending on refractive error
Take into account the vertex distance if >4D
Vertexing Back for a CL
Lens Material
Silicone Hydrogel
Wettability
Surface Treatments
Internal Wetting Agents
hybrid material
– <50% water & nonionic surface
– >50% water & ionic surface
- Increased Oxygen Transmissibility
Uses:
Dry Eye Patients
Traditional Hydrogels
Hydrophilic
Increased hydration
Water Content and Ionic Charge
Group 1: low water content, nonionic polymer
Group 2: high water content, nonionic polymer
Group 3: low water content, ionic polymer
Group 4: high water content, ionic polyer
Lens strength, deposit resistance and refractive index decrease as water content increases
Pore Size and oxygen permeability increase as water content increases
Replacement Schedule
Daily
Two Week
Monthly
Spherical or Astigmatic Cornea ?
Spherical
• Spherical & Aspheric 78%
• Astigmatic 17%
• Bifocal / Multifocal 5%
• Monovision %
Astigmatic
Type of Astigmatism
– Regular (symmetrical mires)
- Irregular (distorted mires)
– With the Rule (steep vertical cornea)
– Against the Rule (steep horizontal cornea)
ATR corneal cyl:SCL
Irregular Corneal cyl: Sph GP
-Corneal
-Lenticular
-Combined
*If <2.25 D Regular Corneal Astig
Sph GP
Any amount of Lenticular Astigmatism
Best Soft astigmatic lenses
*GP design is complicated & poor success
Steps to finding Rx :
1. Create a power cross of the glasses Rx
2. If >4D, account for vertex distance by referring to vertex chart at the back of Tylers
3. Find new contact lens Rx
If astigmatism <25% of spherical power
Spherical
If astigmatism is >25% of spherical power
Astigmatic
Initial Fitting
Base curve
Fitting Methods
Plug In Approach
Flattest Corneal Curvature
Flat: <41.75D (~8.9mm)
Median: 42.00-44.75D (~8.6mm)
Steep: >45.00D (~8.3 mm)
Logical Approach
Steps:
1. HVID+2 = Diameter
2. Match diameter to flattening factor
3. Convert flat K into diopters using Diopter Conversion table
3. Add flattening factor to the diopter value
Diameter
HVID+2
If the fit is too steep (ie: ~pooling seen under the CL
~not much movement observed
~Pt. complains of decreasing VA a
few seconds after blink)
If the fit is too flat (ie: ~wrinkle observed across lens btwn
blinks
~>1mm movement in primary gaze;
displacement onto bulbar conj
~Poor comfort; more awareness
~Pt. complains of increasing VA a few
seconds after blink)
try to keep within
+/- 2 mm range
when choosing
from Tyler's.
Measuring HVID
Palpebral Aperature
Vertical Measurement of opening between upper and lower lids
Normal range9-10.6mm
Abnormally large
>12mm need SCL dia >14.0mm
Abnormally small
<9mm need SCL diam<13.5mm
Corneal Asphericity
Asphere
• sphere @ apex
• progressive flattens to limbus
“E”ccentricity = rate of flattening to limbus
• Apex to Nasal= rapid flattening
• Apex to temporal = slow flattening
Adjustments to Initial Fitting
Sodium-fluorescein slit lamp photographs of SynergEyes Duette HD hybrid contact lens fittings with an ideal base curve and skirt curvature (A), a relatively flat fitting skirt curvature (B) and a relatively steep skirt curvature (C).
Fit Too Steep?
Observe bubble in the center or little movement or decrease VA with more wearing
Decrease diameter or increase base curve
Fit Too Flat?
Observe >0.5mm movement or patient reports VA blurry upon blinking
Increase diameter or decrease base curve