rigid lens principals and products

Rigid lens principals Rigid lens principals and productsand products

Week 9

Soft CL VS gas perm CLSoft CL VS gas perm CL

Rigid gas perm lens is a hard material that allows 02 to pass through the material to the cornea.

It is smaller than a soft contact lens.

It is fit differently and more precisely.

It “reshapes” the cornea to eliminate the astigmatism.

Rigid lens principalsRigid lens principals

Gas perm lenses are made from SA (silicone acrylate) and FSA (fluorinated silicone acrylate).

These material allow O2 to permeate the lens to the cornea, reducing edema and increasing wear time for the patient.

It must be fit more precisely because it reshapes the cornea.

Hard lenses,PMMA (polymethylmethcryla

te) have poor O2 transmission.

1st generation “hard” lens.

Long lasting and very durable.


Dk ValueDiffusion coefficient is

the value of O2 permeability of the lens material.

The higher the Dk value the higher the O2 permeation to the cornea.

A higher Dk value will lower the chance of corneal edema.

Dk can be expressed in the center thickness (L)of the material by the equation DkL.

This will define the O2 transmissibility of the material through the center thickness.


Two corneal shapes

SphericalAspherical (toric)

Spherical Aspherical 44.00/44.00@90 46.00/42.00@90


Spherical Cornea

Implies that this corneal has minimal topographic variation.

The K readings will be very similar horizontally and vertically.

There will be minimal corneal astigmatism.

Aspherical Cornea

Implies that the cornea has a variance in corneal curvature and power.

K readings will be different horizontally and vertically. The more the readings vary, the more astigmatism there will be.

This is corneal astigmatism.


Aspherical corneas

Aspherical corneas will has corneal astigmatism.

With the rule astigmatism

Against the rule astigmatism

Lenticular astigmatismOblique astigmatism


Base Curve (BC)The primary curve

of the lens.It has to be

designed to contour the anterior corneal surface and tear film.


The BC’s relationship to the cornea is referred to as:

BearingAlignmentClearanceThese are

adjectives used to describe how the CL fits on the cornea.


Bearing

Suggests that the lens is fitting flatter on the cornea than the keratometric readings.

If the lens is bearing centrally, it is flat centrally, pushing the tear film to the periphery.

If the lens is bearing peripherally, there will be no tear film peripherally.


ClearanceSuggests that the

lens is fitting steeper on the cornea than the keratometric readings.

If the lens has clearance centrally, the lens fits steeper than K centrally and the tears will pool in the center.


AlignmentSuggests that the lens

perfectly contours the cornea and the tear film is evenly distributed on the corneal surface.

The lens will not be bearing or have clearance.

Usually used for patients with little or no corneal astigmatism, fit on “K”.


The base curve is selected based on the “K” readings.

Rigid lenses are fit in three ways.

On “K” Steeper than “K” Flatter than “K” This is ALWAYS based on

corneal astigmatism, not refractive astigmatism.

When fitting any of these, use the flattest “K” readings taken for that eye.

Example……..

OD = 39.00/41.75@90 You want to fitter steeper

than “K”. (even though it is a flat cornea)

Use 39.00 as the starting “K” for this eye.

Fit steeper than 39.00 depending on the amount of corneal astigmatism.


On “K” (alignment) Flatter than “K”

Implies that the curvature is equal to the flattest “K” reading.

This method is appropriate for low cylinder patients.

Implies using a base curve flatter than the flattest”K” meridian.

It will exhibit a bearing effect on the corneal cap allowing increased lid interaction. Bearing will cause a minus tear layer effect requiring a plus power compensation.

(FAP) flatter, add plus.


Steeper than “K” Tear film layer

Implies using a base curve steeper than the flattest “K”. This allow central clearance of the lens. The lens will position centrally, along with an increase of tear film.

This tear film will cause a plus lens effect, requiring a minus power compensation.

(SAM) Steeper add minus.

The tear film created by fitting steeper or flatter than “K” re shapes the cornea and the tear layer fills in the gaps to help correct for the cylindrical power not put into the CL.


Tear film layerFitting Flatter than

“K” causes a minus tear layer effect.

The tear layer is pushed to the periphery creating a minus lens effect.

This will have to compensated with flatter add plus when figuring the lens power.


Tear film layerFitting steeper than

“K” will cause a plus tear layer effect.

The tear layer will pool in the center of the lens and this will need to be compensated by steeper add minus to the power of the lens.


Determining diameterHorizontal visible iris diameter

The CL diameter should be 2.0-2.5 mm smaller than the visible iris diameter or 1.2 – 1.5 mm from the limbus to the edge of the lens.

Normal rigid lens D is 8 to 10.5 mm, much smaller than a soft CL.


Determining diameterCorneal Alignments fit, flatter than “K”

Large diameters will be fit flatter than “K”.

This is called corneal alignment fit or Korb method.

Corneal alignment fit rides high on the cornea and fit UNDER the upper lid. Sometimes referred to lid attachment fit.

FAP rule applies.


Determining diameterApical fit, steeper than “K”

Smaller diameter corneas are fit steeper than “K”

This is called an Apical lens design or interpalpebral lens design and fits in between the lids, centrally on the cornea.

SAM rule applies.


Determining diameterDon’t worry, there’s a chart for everything!!!

Corneal alignment fitFlatter than “K”

Large diameter

Apical fitSteeper than “K”

Smaller diameter


Let’s review what you learned today!!!

Rigid lens principals reviewRigid lens principals review

Rigid lens principals reviewRigid lens principals review

Bearing means?Alignment means?Clearance means?FAP means?Why do we need to

use FAP?SAM means?Why do we need to

use SAM?

What type of fit is steeper than “K”?

Is that for a large or small diameter corneal?

What does on”K” mean?

Rigid lens principals

Great job!

See you next week for rigid lens fitting.

Test in 2 weeks.

rigid lens principals and products

Documents

lens material

generation hard lens

soft contact lens

rigid lenses

minimal corneal astigmatism

corneal curvature

chance of corneal edema

anterior corneal surface