Gas Permeable Corneal Lens Design and Fitting

Exam 3 material

Name some benefits of GP lenses

• better quality of vision

• safety

• comfort

• value

• durability

• can be used for irregular cornea diseases

• myopia progression

• ortho K

Who would benefit from a corneal and scleral GP?

pt with an irregular cornea (keratoconus, scars, and post transplant)

IMPORTANT: What is the percentage of GP’s prescribed out of all CL?

11% of all CLs are GPs (46% spherical)

IMPORTANT: Who can use GPs?

• all age groups

• infants/ toddlers- larger, more flexible, SiHy for EW (mostly EW GP corneal lenses)

• children: high astigmatism and refractive amblyopia

• teens: watch out for sports due to debris trapping of GPs and dislodging

• YA, middle adults, elderly, aphakic individuals

What are some disadvantages of GPs?

• disease transmission

• time-consuming fitting process

• can be blurry at first

What are some advantages of GPs?

• fewer reorders

• fewer pt revisits

• pts get final lenses more quickly

What happens at the diagnostic fitting?

• place the diagnostic lens on the pts eyes

• evaluate the fluorescein pattern and fit of each lens

• order the lens based on info gathered about best-fit

Do you need collected data before ordering a GP?

• yes, the GPs are ordered based on collected data only

What data is needed to order a GP?

• keratometry and refraction is required

• other data can be used (topography, pupil size, lid tension, etc.)

review: What is the empirical fitting?

use refraction and keratometry (also can use HVID)

no lens is placed on the eye initially but the contact is dispensed at a later appointment

Who should you try to AVOID doing empirical fittings of GPs on?

• keratoconus 

• irregular corneas 

Why should you avoid using empirical fitting of GPs on keratoconus and irregular cornea patients?

performance of these lenses can be unpredictable so you want to borrow a fitting set from a GP lab.

What are the disadvantages of empirical fitting?

• required more in-office visits

• The empirically ordered lens will be close, but a reorder is often done

What are advantages of empirical fittings?

• saves time sometimes

• the lab selects the lens

• pts prescription is the first lens you put on them

• some companies actually encourage it for their GPs

• if a pt only relays at the end of the exam they want contacts, its also beneficial

IMPORTANT: How should GPs be stored (long term)?

• store them DRY because if we store them wet for a long time the solution will dry out and lead to a BASE CURVE CHANGE (warpage)! 

How do you present corneal GPs to patients?

1. explain their condition

2. introduce GP lenses (‘awareness vs pain) (gas-perm v hard)

3. explain the benefits of GPs

4. briefly confront comfort

IMPORTANT: How can we make the initial fitting experience more comfortable for the patient? What are the advantages of doing that?

use a topical anesthetic and allow them to gradually experience lens awareness (15-20 mins) and the advantage is that it gives you time to evaluate the fluorescein pattern 

IMPORTANT: Disadvantages of topical anesthetic

• more staining from topical anesthetic use 

• may mislead the patient 

When a GP lens is placed on the cornea, what is created?

a tear lens

IMPORTANT: How is the tear lens determined?

the dioptric value and sign (±) is determined by how steep or flat the GP curvature is to the corneal curvature

IMPORTANT: What is SAM/FAP for?

The tear lens is created from the GP lens placement and what power must be added or subtracted to compensate for it

IMPORTANT: What is SAM/FAP?

• STEEPER ADD MINUS 

• FLATTER ADD PLUS

IMPORTANT: Why do we add minus when the BCR is steeper than the cornea?

when the BCR is steeper a + tear lens is created and a - power must be added to compensate for it

IMPORTANT: Why do we add plus when the BCR is flatter than the cornea?

The BCR is FLATTER and a minus tear lens is created, so we add PLUS power to compensate for it

For a spectacle Rx: +3.00D and Keratometry: 43.25 D, what tear lens is created if you add 42.75D BCR GP lens on the eye? What contact lens power is needed to compensate?

• -0.50D

• +3.50

For a spectacle Rx: +3.00D and Keratometry: 43.25 D, what tear lens is created if you add 43.50 D BCR GP lens on the eye? What contact lens power is needed to compensate?

• +0.25

• +2.75

  IMPORTANT: How do you calculate residual astigmatism?

CRA = TRA - ΔK

• CRA = calculated residual astigmatism 

• TRA = total refractive astigmatism (@ corneal plane) 

• ΔK= central anterior corneal toricity 

If the corneal Plane Rx: -3.00-1.25 x 180 and Keratometry:  42.75 @ 180; 44.00 @ 090. What tear lens is created if you place a 42.75D BCR GP lens on this eye? How would this affect your initial power selection?

• plano and -1.25 

• the power selection is -3.00 D

If the corneal Plane Rx: -3.00-1.25 x 180 and Keratometry:  42.75 @ 180; 44.00 @ 090. What tear lens is created if you place a 42.25D BCR GP lens on this eye? How would this affect your initial power selection?

• -0.50 and -1.75

• -2.50 

IMPORTANT: How do you do CRA for the pts whose refractive astigmatism and corneal astigmatism have similar axes?

CRA= TRA - ΔK

IMPORTANT: For patients whose refractive astigmatism and corneal astigmatism are 90 degrees apart

CRA= TRA + ΔK

What is the ideal pt for a spherical GP lens?

a pt with

• -0.75 x WTR corneal astigmatism

• -0.75 x WTR refractive astigmatism

• similar axes

Why is that the ideal patient?

• -0.75 x WTR corneal astigmatism

• -0.75 x WTR refractive astigmatism

• similar axes

1. movement would be up and down (along the steepest corneal meridian) using the flattest as a fulcrum

2. A -1.00 DS GP contact lens with a BCR of 44.25 D would correct both meridians! and Refractive astigmatism is corrected by the tear lens

How does a GP lens want to move ATR astigmatic cornea?

• in the path of least resistance 

• nasal or temporal 

• with the blink (leading to decentration complications)

Which corneal astigmatism is preferable for a spherical GP lens?

WTR is preferred over ATR for a spherical GP lens

• ATR can be fitted, just be aware of the movement (lateral(horizontal))

What are if the keratometry readings are spherical?

• there is no corneal meridian to serve as a fulcrum (prop) and the lens can move all over

• mainly displacing superior-temporally after the blink leading to GLARE complaints

• also tucking under the upper lid to help keep it centered

IMPORTANT: What if the keratometry readings show high astigmatism?

it is acceptable to use spherical/aspheric GP lenses up to about 2 D of corneal astigmatism

• around 2 D or more of corneal astigmatism the lens can rock too much and become uncomfortable

• potential for FLEXURE

• so consider a GP with a toric back surface

IMPORTANT: When would you consider using an aspheric back surface on a corneal, single vision, GP lens and why?

good for patients with 1.00-2.00 D of corneal astigmatism, because it’s a little more forgiving in the periphery at the 3 and 9 o’clock.

What are the two main corneal GP philosophies?

1. Lid attachment (larger >9.5 mm and fit flatter)

2. interpalpebral (smaller <9.5 mm and fit steeper)

How do you decide between interpalpebral and lid-attachment?

let the lid be your guide (esp. upper lid)

What is the average fissure width?

9-11mm 

IMPORTANT: If the upper lid covers the superior cornea from 10-2 o’clock and the fissure width is normal, how should you fit the lens? (top picture) 

lid attachment fit 

IMPORTANT: if there is a a narrow palpebral fissure, how should you fit the lens? (middle picture)

interpalpebral fit

A patient comes in looking like the lowest picture, their upper lid is at or above the limbus, how should you fit the lens?

interpalpebral fit because it won’t get lid attached.

What are some other reasons why one would choose lid attachment over interpalpebral fit?

• lid attachment is more comfortable and tucked under the upper lid leading to less contact of the upper lid and lens edge 

• less corneal disruption at 3 and 9 o’clock

What does lid attached lenses cause more than smaller lenses?

corneal molding and distortion

What must the relationship between the OZD and pupil size be?

the OZD MUST be LARGER than the pupil size in DIM light. if not the pts will return and complain of flare at night and in dark environments

• check pupil size and compare it to OZD

• check for lens decentration (if not centered on the cornea pt will experience flare when the OZD nears the pupillary margin)

What is the center of gravity?

property that treats an object as though all of its weight were concentrated at one point

What is the exact position of the COG dependent on?

• diameter

• power

• BCR

• lens thickness

Where is the COG?

• anterior surface or somewhere inside the lens

If the COG is anterior the lens has a higher tendency to 

drop 

If the COG is posterior the lens has a higher tendency to 

to adhere (stick)

The COG can be moved anteriorly with smaller or larger diameter lenses?

smaller and this results in a lower lens position 

The COG can be moved posteriorly with smaller or larger diameter lenses?

larger resulting in a higher lens position

Why do plus lenses tend to drop after the blink?

they are thicker and have a higher mass and lead to inferior decentration

What do minus lenses tend to do after a blink?

they are thinner and lower in mass than plus lenses so they stay with the upper lid (lid attached) or get pushed inferiorly when the upper lid hits the thick lens edge 

The current trend is to fit larger OADs (9.6 mm or larger) to optimize initial comfort, where does the improved comfort come from?

• less lens movement

• less lid/lens interaction

• good centration

What is the diameter selection OAD/OZD consideration?

The OAD should be 1.4 mm larger than the OZD to give room for application of peripheral curves (0.7 mm on each side)

If the OZD of 8.1 is desired, what should the OAD ordered be?

9.5 mm 

What do you change the OAD/OZD by for a clinically significant difference in fit?

Change the OAD/OZD by at least 0.3mm for a clinically significant difference in fit.

What is the BCR primary purpose?

to optimize the fitting relationship of contact lens to cornea

What are some factors affecting the BCR?

• corneal curvature

• desired lens to cornea relationship

• observed fluorescein pattern

How do you convert D to mm? 

337.5 (337.5/D=mm) or (337.5/mm = D)

What is the Base curve selection equation?

Initial BCR = Mean K - 0.50D

Why do we use -0.50 D?

adjustments for corneal asphericity by going flatter

When do we consider a bitoric?

if >2.00 D of corneal astigmatism

Find the initial base curve selection.

42.50 @ 162; 43.50 @ 072 (1.00D)

Mean K = (42.50 + 43.50) ÷ 2 = 86.00 ÷ 2 = 43.00D

Initial BCR = 43.00 - 0.50 = 42.50D (on “K”)

When fitting a spherical GP, we refer to fitting of the lens in relation to the

flatter k reading or “K”

• larger lid attached = steeper lens

• smaller interpalpebral lens = slightly flat lens

For every 0.5 mm change in the OZD; the BCR should adjust by?

0.25 mm 

Increase the diameter by 0.50 mm, then

flatten the BCR by 0.25 mm

decrease the diameter by 0.50 mm, then

steepen the BCR by 0.25 mm

What do you change the BCR by to have a significant effect on the lens?

0.50 D

What is tricurve vs tetracurve?

tricurve is two peripheral curves while tetracurve is three peripheral curves 

tricurve: Base Curve Radius, Secondary Curve Radius, and Peripheral Curve Radius 

tetracurve: Base Curve Radius, Secondary Curve Radius, Intermediate Curve Radius and Peripheral Curve Radius 

Which one do you select a smaller OAD for? Tricurve or Tetracurve

tricurve

__________ is always larger than edge clearance, why?

Edge lift is always larger than edge clearance because the cornea is aspheric.

lift = lens

clearance = cornea

What is the appropriate amount of AEL?

0.10-0.12 mm which results in 0.50 mm wide band of fluorescein around the lens periphery. 

How do you increase the edge lift of a lens?

• flatten the peripheral curve radius

• increase the peripheral curve width

What has more of an effect on the edge of lift?

Changing the PCW or PCR has more of an effect on edge lift than changing the secondary or intermediate curves.

IMPORTANT: In order to make a clinically significant change what do we change the PCR and PCW by?

PCR by at least 1.0 mm 

PCW by at least 0.2 mm 

What are consequences of too little edge lift?

• difficult to remove the lens

• trapping of debris

• increase risk of cornea adherence

• VLK 3 and 9 o’clock staining

If you have too much edge lift you may see?

• 3 and 9 o’clock staining ‘wicking of tears at lens edge, decreased blink rate

• lens decentration

• dislodging

• decrease in comfort

IMPORTANT: What is a blend?

allows more tear flow and gets rid of a sharp edge (causes metabolic debris, prevent tear flow, be uncomfortable) 

IMPORTANT: What is the minimum blend that can be ordered to enhance debris removal?

Medium (B)

IMPORTANT: what is the blend ordered most of the time?

heavy (C) blend is mostly ordered

A thin lens is more likely to

• position superiorly

• warp

• flex

a thick lens is more likely to

• decenter inferiorly 

• lead to complications (injection, corneal desiccation, variable vision) 

How do you decrease the risk of flexure?

At least a 0.03mm change in center thickness should to be made for a clinically significant change.

What has more of an effect on mass than O2?

in GPs, center thickness has more of an effect on mass than O2, so a thicker lens will decrease O2 by about 1% but more mass 24%

If you have a patient complaining of discomfort with GP lenses and you can’t figure out why, what may be the solution?

edge polishing (edge design and shape) may be the primary variable affecting comfort and lens positioning 

IMPORTANT:

Edge thickness and center thickness are approximately equal at -2.00 D (standard lens design).

a minus lenticular can be added to do what to edge thickness?

increase edge thickness

A minus lenticular design is recommended with minus powers between -1.50 and plano and on all plus powers.

When should a plus lenticular be considered?

for a minus lens power of -5.00 D or greater 

• decreases edge thickness and minimizes inferior decentration

IMPORTANT: What is plasma treatment?

A Super cleaning of the lens surface! using radio waves and oxygen increasing the attraction of liquids. Making lens more wettable, improving comfort, and reducing fogging. Do not use GP polish, anything abrasive or alcohol based

• only on new lenses

• ships wet this time ;)

• last 6 months

What is tangible Hydra-PEG?

polymer coating for CL

• improves wettability, lubricity, water retention, minimizes protein and lipid deposits

• no alcohol or abrasive cleaners or tap water

What is needed by the lab to order a lens?

• lens type 

• BCR 

• power 

What is the ultimate goal in ordering a lens?

• moves vertically with a blink

• centers after a blink

• not harmful to the cornea

• comfortable

• good vision

What is the lacrimal lens equation?

CLP = CPR - OR - LLP

CLP = Contact Lens Power

CPR = Corneal Plane of Refraction

OR = Over-Refraction

LLP = Lacrimal Lens Power

Is PMMA good for a GP?

• no, its not O2 permeable and can cause edema 

• refit with a low-Dk FSA material allowing for more gradual corneal rehab process and pts will be less likely to damage the lens when cleaning 

What is the most popular spherical GP design at NSUOCO?

megathin Essilor