Rigid Gas Permeable (RGP) contact lenses (1)
Lens Materials
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Rigid Gas Permeable (RGP) Lenses
RGPs are made from silicone acrylates and fluorosilicone acrylates and are identified by the suffix –focon, while soft lenses have the suffix –filcon.
Examples: Boston EO (Dk 44), Boston XO (Dk 75), Boston XO modified (Dk 100).
Advantages: Good oxygen permeability, wettability, and resistance to warpage.
Astigmatism Considerations
Not all astigmatism is corneal; lenticular astigmatism may occur.
Calculations are essential for distinguishing types of astigmatism and determining appropriate lens fitting (spherical vs. non-spherical RGP).
Criteria for toric lens fitting: needed if corneal astigmatism > 2.50D.
Fitting Process
Gather Rx, K readings, and pupil size to document specifications for lens fitting.
Steps include fitting lens, assessing clearances, refining fit based on a sphero-cylindrical over-refraction.
Ensure proper assessment using white light and fluorescein to check centration, movement, and fit characteristics.
Selection of RGP Lenses
Ideal for new wearers needing significant correction, dry environments, or those with irregular corneas.
Benefits include stable vision, ease of handling, lower complications, and cost-effectiveness over time.
Astigmatism Considerations
Is All Astigmatism Corneal?
The assumption is that all astigmatism is corneal is overly simplistic.
Lenticular astigmatism is common:
A spherical lens might yield unexpected refractive results when fitted due to lenticular components.
Example Calculation:
K readings: 7.80 @ 180 and 7.50 @ 90; Rx: -3.00/-1.25x180.
Astigmatism calculation: 7.80 - 7.50 = 0.30; 0.30D = 0.05mm gives 0.30/0.05 = 6; 6 x 0.25 = 1.50D.
Conclusion: Was the Rx astigmatism corneal?
Corneal Astigmatism Cases
Further example for clarity:
K readings: 7.70 @ 180 and 7.00 @ 90; Rx: -3.00/-1.00x180.
Astigmatism Calculation:
7.70-7.00 = 0.70; 0.50D = 0.1mm gives 0.7/0.1 = 7; 7 x 0.50 = 3.50D.
Conclusion: Not all astigmatism is corneal; non-spherical RGP lens may be needed for fitting.
Fitting Toric Lenses
Criteria for toric lens fitting:
Required if corneal astigmatism is > 2.50D or if cylindrical correction is primarily not corneal.
Inadequate tear lens might lead to poor visual results due to lenticular astigmatism.
Example of complex Rx:
K readings: 7.00 @ 90 and 7.20 @ 180 with an Rx of -1.00/-1.00x35.
Outcomes suggest a soft toric lens might be preferable.
Versatility of RGPs
RGPs offer multiple lens types:
Front surface toric
Back surface toric
Bitoric lenses
Considerations:
Front surface torics may not perform well for corneal astigmatism.
Back surface torics could complicate lenticular astigmatism corrections.
Induced astigmatism may arise from refractive index differences between tears and lens materials, necessitating potential bitoric fitting.
Midpoint Summary
Key points to remember:
2 RGP rules.
Basic contact lens design principles.
Information needed for RGP specification.
Understanding of tear lens functioning.
Calculation of astigmatism and potential corrections.
RGP Lens Fitting Process
Preliminary Measures
Gather Rx, K’s, pupil size, etc.
Despite idealism, individual corneal characteristics necessitate adjustments.
Document lens specifications clearly:
Manufacturer, design, material, BOZR, TD, BVP.
Example Specification:
B&L Maxim 7.80/9.30/-3.00 Boston EO with blue visibility tint.
Initial Lens Fit Selection: EUROPA SCLERAL
Procedure:
Place spherical fitting set lens on eye starting from 500 g.
Analyze clearances and landing zones, adjust BC as needed.
Introduce toric haptic lens if issues persist.
Final adjustment via sphero-cylindrical over-refraction to finalize lens power.
Insertion and Removal of RGPs
Instructions available in video format (https://www.youtube.com/watch?v=WJVcmfa8JTU).
Note: RGP insertion/removal differs from soft lenses.
Fit Assessment Steps
Ensure lens is sterile and wet before insertion.
Monitor adaptation time for settling.
Perform white light then fluorescein assessments – NaFL should be introduced with the lens.
Evaluate fit based on at least 8 features (e.g., steepness, alignment) avoiding terms like 'tight' and 'loose'.
White Light Assessment Criteria
Use medium magnification and broad beam.
Evaluate:
Centration: check if lens is central, excessive droop, or stabilized.
Movement upon blink: average movement for RGP, pattern observations.
Coverage: Ensure complete pupil coverage.
Lag/Limbus interaction not required for RGPs.
Push-up tests not applicable for RGP lenses.
Further White Light Assessment Indicators
Lid Interaction: observe movement with lids and its independence during blinking.
Wettability and Deposits: watch for dry patches and deposit formation even within 5-10 minutes.
Fluorescein Assessment Steps
After white light assessment, utilize fluorescein to evaluate:
Edge lift: Look for appropriate tear reservoir edge.
Fluorescein pattern: crucial for detection of lens steepness/flatness and alignment.
Over Refraction Practices
Perform over-refraction as needed, noting that keratometry reflects corneal curvature, not the overall Rx.
Change of 0.05mm fits correlates with 0.25D over-refraction changes.
Next Steps After Fit Adjustment
Order appropriate lens for trial, considering minimum specification requirements:
Manufacturer, design, material, radii, diameters, BVP, tint/engraving if relevant.
When to Select RGP Lenses
Situations ideal for RGPs include:
New wearers requiring significant corneal correction or those in dry environments.
Patients with irregular corneas or poor prior handling of soft lenses.
Benefits of RGP Lenses
Advantages include:
Crisp, stable vision.
Ease of handling, lower complications, and long-term comfort.
Cost-effective over time.
Final Summary Considerations
Recall necessary measures for correct RGP fittings.
Understand insertion/removal differences from soft lenses.
Be prepared to comment on at least 8 fitting characteristics.
Familiarize with fluorescein patterns.
Identify common patient profiles benefiting from RGP lenses.
Adhere to fundamental rules for RGP fitting adjustments.