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

References:
http://www.mivision.com.au/hybrid-contact-lenses-a-case-based-review/

http://www.optometry.co.uk/uploads/articles/cet-2013/august-9-2013-c-32856.pdf

Clinical Manual of Contact Lenses, 3E. ES Bennett, VA Henry

All Photos taken from Clinical Manual of Contact Lenses, 3E.

Done if ptnt. is unhappy with VA:
1. Refine with subjective.
2. Revert to retinoscopy if subjective unsatisfactory.
3. Determine spherical equivalent to add on top of CL Rx or trial the cross-cylinder calculation.
4. Trial the new diagnostic lens; is the ptnt. happy? Be weary of unrealistic expectations on behalf of the patient. If requests better optical quality, consider RGP?