Soft Contact Lens Dynamics and Fitting

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Questions and Answers

What is the primary impact of high water content lenses on eye opening?

They lose more water than low water content lenses. (correct)

They enhance corneal deswelling.

They reduce the availability of oxygen to the cornea.

They prevent lens shrinkage when worn.

Which design consideration is most likely to affect the centration and comfort of a lens?

The permeability of the lens.

The thickness of the lens.

The water content of the lens.

The movement of the lens. (correct)

How does the lens thickness influence tear film interaction?

Thicker lenses increase the likelihood of edema. (correct)

Thicker lenses promote better oxygen transmission.

Thicker lenses enhance tear film stability.

Thicker lenses do not affect tear film interaction.

What is the consequence of lens shrinkage in high water content lenses?

Detrimental effects on the tear exchange rate. Signup and view all the answers

How does the Dk/t value relate to lens design in terms of preventing edema?

Higher Dk/t values increase the amount of oxygen available. Signup and view all the answers

What is the maximum recommended frequency for intermittent extended wear of contact lenses?

1-2 nights per week Signup and view all the answers

What is the main consequence of wearing contact lenses longer than recommended?

Chronic hypoxia Signup and view all the answers

How does the thickness of a contact lens affect oxygen transmission?

Thicker lenses reduce oxygen availability Signup and view all the answers

Why is tear mixing considered minimal under soft contact lenses?

They create a seal over the cornea Signup and view all the answers

What is a characteristic of overnight edema caused by contact lens wear?

It can lead to persistent daytime edema Signup and view all the answers

What should be the focus concerning corneal swelling when wearing contact lenses?

Dk/t and local lens thickness are important Signup and view all the answers

Which statement about daily wear versus extended wear contact lenses is correct?

Extended wear requires greater oxygen transmissibility than daily wear Signup and view all the answers

What happens to the edema cycle with continuous contact lens wear?

Edema persists without oxygen supply Signup and view all the answers

What primarily prevents an instantaneous response from the soft lens during fitting?

Visco-elastic properties of the lens Signup and view all the answers

How does lens relaxation affect the post-lens tear film after blinking?

It can change the volume of the tear film Signup and view all the answers

What role does tear film thickness play in lens movement dynamics?

Lens movement is proportional to tear film thickness Signup and view all the answers

What characteristic of hydrogel lenses is influenced during blinking?

Their alignment with the cornea Signup and view all the answers

Which aspect of a soft lens fitting can be described by the acronym BOZR?

Base Curve and Optical Zone Radius Signup and view all the answers

Which property of soft lenses causes a slower response during the fitting process?

Material elasticity Signup and view all the answers

What happens to the post-lens tear film when the lens is too thick?

It may become unbalanced Signup and view all the answers

What effect does the lack of lens movement have on tear mixing under soft contact lenses?

Can lead to poor mixing of tears Signup and view all the answers

Study Notes

Soft Contact Lens Tear Mixing

Hydrogel lens deformation during blinking and lens alignment with anterior eye topography induce visco-elastic forces
These forces cause a lag in lens relaxation behind lid retreat
The force to move the lens is inversely proportional to the tear film thickness
Visco-elastic properties of the lens prevent an instantaneous response to lid retreat
The lens relaxation may involve movement or volume change of the post-lens tear film
A thin post-lens tear film is mainly composed of viscous mucin and lipids

Soft Contact Lens Fitting Philosophy

Intended lens usage is relevant to fitting considerations as extended wear (EW) and daily wear (DW) lenses require different levels of oxygen permeability
For EW: cornea’s minimum oxygen requirement must be met due to the longer wear time, which exacerbates oxygen deprivation
For DW: less open eye requirements apply
EW always requires higher oxygen transmission than DW

Oedema Cycle

Oedema occurs whenever a contact lens is worn, as the lack of oxygen causes corneal swelling
Overnight wear results in corneal oedema
Incomplete deswelling during the day leads to persistent chronic residual daytime oedema

Overnight Lens Wear

Less oxygen is available with overnight lens wear
Oedema depends on the Dk/t of the lens
Dk/t depends on the lens thickness and thicker lenses allow for less oxygen transmission

High Water Content Lenses

These lenses lose more water than low water content lenses on the eye
They lose water even when worn in a high humidity environment
Lens shrinkage on eye affects the tear distribution and base curve radius (BOZR)
This impacts fit and needs to be taken into account

On Eye Opening

Corneal deswelling occurs, improving the condition of the eye as more oxygen becomes available
Hydrogel lenses allow for 8% oxygen permeation (Dk/t)
Rigid gas permeable (RGP) lenses with Dk/t of 10-11% provide better oxygen transmission than hydrogels
RGP lens performance stems from greater permeability, tear pump action

Preventing Oedema

Daily wear (DW) lenses require a Dk/t of 24 to allow for 9.9% oxygen transmission
Extended wear (EW) lenses need a Dk/t of 87 to allow for 17.9% oxygen transmission
All currently available hydrogel lenses cause greater than 8% overnight oedema

SCL Design Considerations

Centralization of the lens is important for vision, comfort, and mechanical function
Lens movement is beneficial for debris clearance and comfort
The 4C’s relate to the quality of the lens fit: Coverage, Centration, Comfort, Clarity

Tear Mixing

Studies show little tear mixing occurs under soft contact lenses
Corneal swelling is related to local Dk/t, which is determined by the thickness of the lens at a specific point
Lens thickness is a crucial factor in contact lens fitting

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Description

This quiz explores the mechanics of soft contact lenses, including how tear mixing and lens deformation affect lens performance. It delves into the fitting philosophies for extended and daily wear lenses, highlighting the importance of oxygen permeability. Understand the underlying principles that professionals consider when fitting contact lenses for optimal vision and comfort.