Contact Lens Fitting and Assessment

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Questions and Answers

What is the ideal condition for fluorescein pooling under a lens?

Fluorescein uniformly distributed across the lens

Fluorescein pooling under the center of the lens (correct)

Fluorescein accumulation at the edge of the lens

Complete absence of fluorescein under the lens

What is indicated by a lens that is described as having 'low riding' lid interaction?

The lens is positioned too low in the interpalpebral space (correct)

The lens is centered perfectly on the cornea

The lens is positioned high above the upper lid

The lens is adhered to the upper eyelid

During a dynamic fit assessment, what does 'quantifying movement in mm' refer to?

The amount of lens shift during a blink (correct)

The degree of lens rotation

The thickness of the lens material

The diameter of the lens

Which type of lens movement is most likely to occur with very flat lenses on a toric cornea?

Arcuate movement Signup and view all the answers

What factor significantly affects the centration of a lens in relation to the cornea?

Lid geometry Signup and view all the answers

What does a narrow, even band of edge clearance indicate in lens fitting?

Good alignment and fit Signup and view all the answers

What pattern is observed when using fluorescein to assess static fit in the mid periphery?

Touch, pooling, or alignment Signup and view all the answers

Why does a steep fit lens retain a similar fluorescein pattern for a longer duration?

It maintains a stable fit with minimal movement Signup and view all the answers

What is the best course of action when unsure about the alignment of a lens fit?

Use a steeper lens to confirm alignment Signup and view all the answers

Which condition requires the consideration of a toric lens?

Corneal astigmatism greater than 2.00D Signup and view all the answers

What problematic issue can arise from excessive edge clearance in the steeper meridian?

Unwanted lid interaction with the lens Signup and view all the answers

What occurs when the lens fits poorly on with-the-rule corneas?

Lens rocks along the steeper meridian Signup and view all the answers

How can lens flexure become an issue with toric corneas?

It decreases comfort for the wearer Signup and view all the answers

Which fitting option minimizes edge clearance in the steeper meridian?

Reducing the total diameter of the lens Signup and view all the answers

What is a potential outcome of using a spherical center with a toric periphery design?

Effective when the peripheral cornea is more astigmatic Signup and view all the answers

What effect does narrowing the edge lift have when using aspheric RGP lenses?

Minimizes edge clearance in the steeper meridian Signup and view all the answers

What effect does a steep lens with poor edge clearance have on fluorescein penetration?

Fluorescein may not be able to pass under the lens. Signup and view all the answers

What could be a reason for misinterpretation of fluorescein patterns?

Assessing a decentred lens. Signup and view all the answers

What is the typical measurement of vertical movement in a fluorescein pattern?

7.5 mm Signup and view all the answers

What phenomenon occurs in a static fit when a steep RGP lens is used?

Negative staining of the epithelium. Signup and view all the answers

What constitutes the typical characteristics of a 'steep' fluorescein pattern?

Central touch with very narrow edge clearance. Signup and view all the answers

During which type of fit assessment is lid interaction primarily evaluated?

Dynamic fit assessment. Signup and view all the answers

To amend a lens fit, which data should be combined with observations?

Dynamic and static fit observations. Signup and view all the answers

In a fluorescein pattern, what does 'dimple veiling' indicate?

Interaction between the lens and cornea. Signup and view all the answers

What is a key characteristic of a Bicurve lens design?

Features a sharp transition between central and peripheral curves Signup and view all the answers

Which factors are primarily responsible for controlling the design of RGP lenses?

Center of gravity and edge shape Signup and view all the answers

What advantage does a high refractive index material provide in lens design?

Allows for a thinner lens while maintaining power Signup and view all the answers

In the context of RGP lens fitting, what does 'edge clearance' refer to?

Distance between lens edge and eye surface Signup and view all the answers

Which type of lens design consists of a central radius and three peripheral curves?

Tricurve Signup and view all the answers

What is the primary purpose of using prism ballast in RGP lens design?

To ensure proper lens centration Signup and view all the answers

What does a Tricurve lens typically feature in comparison to a Bicurve lens?

Two peripheral curves instead of one Signup and view all the answers

How does capillary attraction influence RGP lens fit?

Good alignment results in enhanced stability Signup and view all the answers

In RGP lens fitting, what signifies an 'alignment fit'?

Smooth vertical movement with blink and no limbal overlap Signup and view all the answers

What aspect of lens design can impact the comfort and removal of RGP lenses?

The smoothness of the edge shape Signup and view all the answers

What does an increase in aqueous component do to the tear film during RGP lens wear?

Decreases centration ability Signup and view all the answers

Which lens design results in minimized thickness for high power prescriptions?

Multicurve Signup and view all the answers

What is the effect of using a steeper lens design on a contact lens fit?

Creates negative pressure for alignment Signup and view all the answers

What is indicated by a fixed power in lens fitting?

Limited adaptability to different corneal shapes Signup and view all the answers

Study Notes

RGP Lens Design and Fitting

RGP stands for rigid gas permeable lenses.
The presentation discusses the impact of lens design on fitting.
Factors influencing RGP fitting include design principles, fitting sets vs empirical fitting, and forces controlling lens design.

Principles of RGP Design

Single curve: First lens design (1947), PMMA material, fit 0.3mm flatter than the flattest K reading. Limited success due to physiological issues, lenses didn't fit properly. The cornea isn't spherical, increasing flattening towards the limbus. Improvements by adding peripheral curves to the lens.
Bicurve (C2): Consists of a central radius and one flatter peripheral curve. Example: 7.80/8.60/-3.00D. A sharp transition between curves.
Tricurve (C3): Central radius and flatter peripheral curves. Example: 7.70 / 8.50 / 9.65 /-3.00D. Modern lenses feature a final peripheral curve flatter than the first peripheral radius.
Tetracurve/Multicurve (C4): Combination of multiple curves
Aspheric: Advanced designs
Constant Axial Edge Lift (CAEL): A specific design characteristic

Fitting Sets vs Empirical Fitting

Fitting Sets: Fitted on the spot in one session, limited BOZR (0.10), usually fixed power and base curve.
Empirical Fitting: Orders are made to suit specific patient needs, more than one session, various BOZRs (0.05), and various TDS, adjustable power.

Forces Controlling Lens Design

Center of gravity: Affects lens centration.
Frictional forces: Viscosity of tear film, thinning or increase in aqueous component, reduces centration ability.
Capillary attraction: Good alignment leads to capillary attraction/stability. Flat lenses have reduced capillary attraction. Steep lenses achieve a negative pressure (suction effect) by creating more capillary attraction. Tear meniscus helps lens centration, larger meniscus aids better centration.
Specific gravity: Lenses' different densities can affect fitting and placement, gravity can pull lenses down
Thickness and lenticulation: Adjusting thickness controls the mass and profile, lenticulation controls zone diameter.
Refractive index of materials: Higher index materials result in thinner lenses. Example materials include PMMA (1.49), CAB (1.47), and Silicone (1.43).
Edge shape: Smooth and well-finished edge helps in comfort and removal.

RGP Lens Design

Edge clearance (EC): Gap between the cornea and the back surface of the peripheral curve (observed during fluorescein fit).
Radial edge lift (REL): Measured along radius.
Axial edge lift (AEL): Measured along axis of lens.

Static and Dynamic Fit Assessment

Static fit (Fluorescein patterns): Assess with fluorescein and cobalt blue or Burton lamp; central, mid-periphery, periphery are assessed, checking for pooling or other issues.
Dynamic fit: assesses centration, lid interaction, movement on blink, and edge position.

Toric Corneas

Toric corneas have varying curvature in different meridians.
Considerations include comfort, edge clearance, centration, type of contact lens. Options include altering BOZR, adjusting TD, using aspheric and toric lens design, changing the type of lens.

Fit Conclusion

Important to carefully evaluate the fluorescein fit, deciding if the lens is flat, aligned, or steep
Recommendations on how to amend the fitting or select new lens specifications.

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Description

This quiz covers essential concepts related to contact lens fitting, including fluorescein pooling, lens movement, and the impact of corneal shape on lens centration. Ideal for those studying optometry and lens assessment, it addresses common challenges and considerations when fitting lenses to different corneal types.