UNIT 4: Astigmatism & Presbyopia Flashcards
Types of Astigmatism: Corneal
- Regular: Meridians 90 Degrees Apart
- WTR: Flattest K @ 180 Degrees ( +/- 30 Degrees)
Example: 41.00@180/43.00@90 - ATR: Flattest K @ 90 Degrees ( +/- 30 Degrees)
Example: 45.00@180/42.00@90 - Oblique: Flattest K between 30&60 or 120&150
degrees
Example: 42.50@35/44.75@125
- WTR: Flattest K @ 180 Degrees ( +/- 30 Degrees)
- Irregular: Flattest & steepest meridians are
notably more or less than 90 degrees apart.
Example: 41.00@180/42.00@60
Astigmatism By Meridian
Total/ Refractive Astigmatism
- The amount of astigmatism in the spectacle
prescription. It represents the combined effect of
the cornea and internal astigmatism. - Minus cylinder axis 180 (+/- 30 degrees) corrects
WTR astigmatism.
Example: -3.00 -1.00 x 180 - Minus cylinder axis 90 (+/- 30 degrees) corrects
ATR astigmatism.
Example: -3.00 -1.00 x 90
Residual Astigmatism
- Astigmatism that remains after a contact lens
is fit to the cornea. - Spherical rigid lenses will neutralize corneal
astigmatism with the tear layer behind the lens.
Any internal astigmatism will remain and may
create optical problems. - Soft lenses conform to the cornea and do not
neutralize corneal astigmatism.
Calculating Residual Astigmatism
- The amount of residual astigmatism may be
calculated by comparing the “K” readings to the
spectacle prescription in minus cylinder form. - With the spectacle prescription in minus cylinder
form determines the amount and type of
refractive astigmatism. - If they are both the same type ( WTR, ATR,
Oblique) subtracts refractive astigmatism for
the corneal astigmatism.
Example: “K”:41.50@180/43.50@90 = 2.00D WTR
RX: -3.50 -1.50x180 = 1.50D WRT
____________
0.50 CRA - If they are opposite types ( WTR, ATR,
Oblique) add refractive astigmatism for
the corneal astigmatism.
Example: “K”:41.50@180/43.50@90 = 2.00D WTR
RX: -3.50 -1.50x090 = 1.50D ATR
____________
3.50 CRA
Soft Lens Correction
- Spherical:
- Rx Astigamtism =/< 1.00D
- Rx Sphere to Cyl. Ratio >/= 3 to 1
- Toric Designs:
- Front surface
- Back surface
- Stock
- Custom
Soft Toric Stabilization
- Prism Ballast
- Truncation
- Double slab off/ Thick&thin zones
- Posterior Toric
- Aspheric Posterior
Soft Toric Lens Fitting
- Patient selection
- Lens selection
- Insertion and Equilibration
- Lens Evaluation
- Axial Alignment
- Compensation ( LARS )
- Order
Soft Toric Lens Evaluation
- Movement
- Axis Rotation
- V/A
- Comfort
Soft Toric Lens Problems
- Discomfort
- Axis Mislocation
- Edema
- V/A
Rigid Lens Correction
- Spherical:
- Residual Astigmatism </= 1.00D
- Corneal Astigmatism < 3.00D
- Toric Designs:
- Front Surface
- Back Surface
- Bitoric
Fitting Front Surface Torics
- Select spherical diagnostic lens 0.75 - 1.5^
- Evaluate fluorescein pattern & axis
- Sphero-cyl overrafracion
- Add overrafraction to diagnostic lens
power and compensate for rotation
Fitting Back Surface Torics
- Rigid contact lens index of refraction = 1.49
versus Cornea = 1.375 - Approx: 2/3 of the corneal toricity may be placed
on the posterior lens surface - K’s: 42.00/45.00@90
- Rx: -2.00 - 3.00 x 180
- CPC = 42.00 x 44.00
- Power -2.00 -3.00 x 180
Fitting Bitoric Lenses
- Calculate CPC and Power for each meridian
separately. - Fit the flattest meridian CPC 0.25 D flatter than
flat “K”. Fit the steepest meridian 0.50 to 0.75D
flatter than steepest “K”. - Determine the total power in each meridian.
Compensate of the power is > 4.00D - Use FAP rule to adjust power
- Order each meridian separately
Presbyopic Contact Lens Fitting: Distance Vision with Readers
- Advantages:
- No lens design or material limitations
- Good vision possible at all distances
- Disadvantages:
- Does not fulfill cosmetic needs
- Inconvenient
Presbyopic Contact Lens Fitting: Monovision
- Advantages:
- No lens design or material limitations
- Inexpensive
- Can be worn part-time
- Disadvantages:
- Loss of depth perception
- Headaches
- Limited success with higher add powers
- Prolonged near work difficult
Presbyopic Contact Lens Fitting: Bifocal
- Advantages:
- Binocular distance and near correction
- Fulfills patient’s cosmetic needs
- Disadvantages:
- Cost
- Success Rate
Presbyopic Patients Wearing Contact Lenses (CL Spectrum)
- Practitioner preference for presbyopic contact
lens patients:- 59% multifocals
- 27% Monovision
- 14% Over-Spectacles
- In practice, Presbyopic Patients:
- 66% Glasses
- 16% Monovision CL
- 15% Soft Multifocal CL
- 3% GP Multifocal CL
Patient Selection
- Motivation
- Visual Demand
- Astigmatism
- Intended Use
- Distance Rx and Add Power
- Lid Position
- Tear Quality & Quantity
- Pupil Size/ Change in Size
Lid Position & Pupil Size
Lid Position & Pupil Size
Pre- Corneal Film Assessment
- Schirmer Test
- Break Up Test (B.U.T.)
- Rose Bengal
- Lacrimal Lake
Tear Break Up Time
Corneal Dryness
Cornea: Keratitis Sicca
- Keratoconjunctivitis Sicca
- Several Possible causes:
- Reduced function of lacrimal glands
- Excessive evaporation of tears
- Mucin deficiency
- Tests:
- Rose Bengal, Schirmer
- B.U.T.
- Lacrimal Lake
Keratitis Sicca with Rose Bengal
Types of Bifocals: Visual Mechanism Simultaneous
- Light passes through distance & near portions
simultaneously. - Two bundles of light contribute to the retinal
image at all times. - Patient learns to ignore one and concentrate on
the other.
Types of Bifocals: Visual Mechanism - Alternating/ Translating
- Wearer uses one portion of the lens for distance
vision and another portion for near vision. - As the eye moves down to read the lower lid
moves the CL up, so that the reading segment
lies in the pupillary area. - Truncation and/or prism is usually required to
stabilize lens rotation.
Cast Molded Advantages & Disadvantages
- Advantages:
- Lower unit cost
- Surface quality
- Reproducibility
- Disadvantages:
- Initial set-up costs
- Limited parameters
- Edge finish
Types of Bifocals: Lens Designs- Segmental/ Alternating
- Advantages:
- Clearer image higher add powers
- Disadvantages:
- Must translate, comfort problems, Edema
- Examples:
- Tangent Streak (GP) - Firestone Optics
- Solitaire (GP) - Truform Optics
- Solution (GP) - XCell
- Trition (Soft) - Gelflex
Triton Gelflex
Segmental/ Alternating: Truncation