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