Gas Permeable Lens Fitting Notes

Ulster University - Gas Permeable Lens Fitting

Arnold Cochrane, Lecturer in Optometry, ulster.ac.uk

Learning Outcomes

Successful students will be able to:

  • Determine suitable gas permeable trial lens parameters
  • Describe techniques for gas permeable lens fit assessment
  • Differentiate fitting characteristics of aligned, steep, flat, and toric fitting gas permeable lenses
  • Make recommendations to improve the fit of a gas permeable lens

The Fitting Process

  • Pre-Fitting Assessment
  • Trial Lens Selection and Insertion
  • Adaptation
  • Evaluation of Lens Fit
  • Alternate Lens Selection
  • Over-Refraction
  • Lens Ordering
  • Lens Dispensing
  • Aftercare

Trial Sets

  • Trial sets with a range of BOZRs
  • Standard fixed powers
  • Reusable
  • Examples: Rose K, Keratoconus lens
  • Practitioner Fitting Guide.

GP Lens Fitting & vCJD

  • College of Optometrists Guidance regarding vCJD (SEAC1999).
  • Remote theoretical risk of transfer of vCJD.
  • Advised against reuse of trial lenses, except for complex lenses or when single-use lenses are impractical.

GP Lens Fitting - Single Use Lenses

  • Practitioner orders a diagnostic/trial lens/set empirically, based on initial measurements.
  • Lens fit and power may be adjusted following on-eye evaluation.
  • Final lens specification is then ordered from the lab.
  • For complex designs, refer to College of Optometrists Guidance (Infection Control).

Trial Lens Selection - BOZR/TD/BVP

  • Back Optic Zone Radius (BOZR): Keratometry readings – Manufacturers Fitting Guide
  • Total Diameter (TD) / Optic Zone Diameter: HVID / VPA – Pupil Diameter
  • Back Vertex Power (BVP): Patient’s Prescription

Trial Lens Selection - Back Optic Zone Radius (BOZR)

  • Preferred fitting for most GP lenses is on alignment or slightly flatter.
  • Initial lens selection based on keratometry readings – usually has BOZR nearest to “flattest K”.
  • Aspheric lenses may need flatter fitting than spherical lenses to provide alignment.

Trial Lens Selection - Back Optic Zone Radius (BOZR) - Corneal Astigmatism

  • Zero-0.75D: Fit on flattest K
  • 0.75-1.00D: Fit on flattest K to 0.05mm steeper than flattest K
  • 1.00-2.50D: Fit near flattest K to 0.10mm steeper than flattest K
  • Over 2.50D: A toric back optic zone is recommended

Trial Lens Selection - Total Diameter (TD)

  • TD based on HVID, VPA, lid position, and pupil size
  • TD typically 2mm smaller than HVID
  • Most lenses are mid-sized: 9.2-9.7mm
  • Small: < 9.0mm, Large: > 9.7mm
  • Highly toric corneas may need a smaller lens diameter to avoid excessive edge stand-off and rocking.
  • Important to consider pupil size, especially in low illumination.
  • BOZD may be specified by practitioner or predetermined by the laboratory.

Trial Lens Selection - Back Vertex Power (BVP)

  • As close as possible to the patient's prescription to provide natural vision.
  • Make adjustments for BVD where applicable.
  • Will minimize potential changes in fit from variation in power.
  • Myopes should be assessed with negative lenses, and hypermetropes with positive lenses since CoG affects fit.

Adaptation

  • Fit may be assessed once reflex tearing has stopped, normally after approximately 5 minutes.
  • Check lens stability, centration, and fluorescein pattern.
  • Consider a longer tolerance trial to allow the patient to judge their subjective response and permit some degree of adaptation.

Assessment of Lens Fit Components

  • White Light Assessment: Describes the stability of the contact lens on the eye (i.e., position and movement).
  • Blue Light & Fluorescein Assessment: Describes the relationship between the back surface of the contact lens and the anterior corneal surface.

Assessment of Lens Fit - Equipment

  • Slit Lamp
    • Head/lids not in a relaxed position.
    • Fluorescein pattern normal even if the lens has UV inhibitor.
  • Burton Lamp
    • Permits normal head posture.
    • Ineffective when used with lenses that have UV inhibitor.

Assessment of Lens Fit - White Light Assessment

  • Lens Centration
  • Pupillary Coverage
  • Lens Movement
    • Blink
    • Version
    • Return movement after blink or with lids retracted

Assessment of Lens Fit - Fluorescein Assessment

  • Relationship between the back surface of the lens and the anterior surface of the cornea.
  • Evaluate fit/alignment over center, mid-periphery, and edge.
  • Corneal clearance indicated by fluorescein pooling (green).
  • Corneal touch indicated by total absence of fluorescein (blue/black).
  • Aligned, steep, or flat.

Aligned Fit - Fitting Characteristics

  • Good centration in primary gaze and on lateral eye movements.
  • The lens should not extend beyond the limbus even with wide excursions of the eye.
  • Pupillary coverage: The lens should remain centered over the visual axis in both primary and four principal directions of gaze.
  • Minus lenses may be lid-attached and move with the lid but should still cover the pupil.
  • Movement of 1-1.5mm on blink and version.
  • The lens should drop smoothly in a straight line following blinking or with lids retracted.
  • Alignment or hint of apical clearance over central 6-7mm.
  • Mid-peripheral alignment over an approximate 1mm band.
  • Edge clearance over peripheral 0.5mm band.

Steep Fit - Fitting Characteristics

  • Well-centered in both primary and lateral gaze.
  • Minimal movement on blink and version.
  • Apical clearance, central pooling of fluorescein – Denser pooling = greater degree of steepness – Air bubble = excessive central clearance.
  • Bearing at the transition seen as an area of blue touch beyond the central pooling.
  • Minimal edge clearance at the lens periphery.
  • Dimple Veiling.

Flat Fit - Fitting Characteristics

  • Lens unstable and decenters, often inferiorly and to one side.
  • Pupillary coverage is intermittent/variable.
  • Excessive, rapid movement on blink and version.
  • The lens may override limbus on lateral gaze, causing significant discomfort.
  • Arcuate movement when the lens drops between blinks.
  • The central area of touch is surrounded by fluorescein to the edge of the lens.
  • Excessive edge clearance is indicated by a wide area of fluorescein pooling at the periphery of the lens.

Fitting Modifications

To correct a steep fit:

*   Use a flatter BOZR
*   Decrease TD to improve lens mobility
*   Use flatter peripheral curves

To correct a flat fit:

*   Steepen the BOZR to improve centration
*   Increase TD to improve lens stability
*   Use tighter peripheral curves
*   Use a thinner lens to reduce mobility

Clinical Equivalents

  • The principle applies where two lenses of the same design and material, which have differing but related parameters, give the same fitting/characteristics on the eye (fluorescein patterns).

  • Rule of Thumb: An increase in TD / BOZD of 0.5mm requires flattening of BOZR by approximately 0.05mm to maintain the same fluorescein pattern. E.g.,

    • 7.95/9.0=8.00/9.57.95/9.0 = 8.00/9.5
    • 7.95/9.0=7.90/8.57.95/9.0 = 7.90/8.5

Assessing Toric Fits - Things to Consider

  • Is the cornea spherical or astigmatic?
  • Is the astigmatism WTR, ATR, or oblique?
  • What trial lens would best match the corneal shape?
  • How, in theory, will my trial lens match corneal contour?
  • What do I expect with regard to alignment, touch, and clearance?
  • Does the fluorescein pattern conform to expectations?
  • What impact do the lids have on this fit?
  • Is there excessive lacrimation as a result of edge-stand off?
  • Is the fluorescein pattern relatively stable?
  • Is the lens stable, or does it rock with blinking?

Assessing Toric Fits - Example

  • K Readings: 7.90 @ 5, 7.50 @ 95
    • Astigmatism?
    • Trial Lens: 7.90/9.00/-3.00D

Over-Refraction

  • Can be used to indicate whether a particular lens is steep, flat, or aligned.
  • A BOZR 0.1mm steeper than central corneal curvature will induce a +0.50DS tear lens and will thus require a -0.50DS over-refraction.
  • A BOZR 0.1mm flatter than central corneal curvature will induce a -0.50DS tear lens and will thus require a +0.50DS over-refraction.

Worked Example

  • Consider a -4.00DD myope (BVD=12mm) with spherical K-readings noted at 43.27D. A gas permeable trial lens of parameters 7.70/9.0/-3.00D is inserted into the eye. After a suitable period of adaptation, the fit of the lens is deemed to be acceptable, and an over-refraction is undertaken. Calculate the over-refraction result and record the final lens parameters to be ordered.

  • Keratometry Reading = 43.27D
    F=n’–1rF = \frac{n’ – 1}{r}
    r=n’–1F=1.3375–143.27=0.0078mr = \frac{n’ – 1}{F} = \frac{1.3375 – 1}{43.27} = 0.0078m

  • Keratometry Reading = 7.80mm

Worked Example

  • Ocular Refraction, Rxo = -3.75DS
  • BOZR 0.1mm steeper than K Induced tear lens of +0.50DS
  • Over Ref + Trial Lens + Tear Lens = Ocular Ref
  • Over Ref = Ocular Ref - Trial Lens - Tear Lens
  • Over Ref = -3.75 – (-3.00) – (+0.50) = -1.25DS
  • Final Lens Parameters 7.70/9.00/ -4.25D

Lens Ordering

  • Example: Bausch & Lomb Quantum II
  • RE: 7.85 / 9.6 / -1.25D
  • LE: 7.90 / 9.6 /-2.50D
  • Consider:
    • Center Thickness
    • Blue Handling Tint
    • Engrave
    • Lens Verification

Summary

  • GP fitting is a slightly more technical process than soft lens fitting.
  • GP fitting is becoming a bit “niche.”
  • GP lenses often provide good acuity and low complication rates, therefore potentially a good option for patients.
  • Allow tearing to subside before assessing and remember the impact of lids.
  • Seeking an aligned fit or best compromise.
  • Fit is based primarily on fluorescein pattern, therefore easy to recognize.
  • Nothing to be afraid of!

Lens Dispensing

  • Slit Lamp Assessment
  • Fitting Assessment
  • Visual Assessment
  • Lens Handling
  • Lens Care Advice
  • Lens Wearing Schedule
  • Symptoms & Precautions
  • Aftercare Schedule
  • Patient Declaration

The Fitting Process

  • Pre-Fitting Assessment
  • Trial Lens Selection Insertion
  • Adaptation
  • Evaluation of Lens Fit
  • Alternate Lens Selection
  • Over-Refraction
  • Lens Ordering
  • Lens Dispensing
  • Aftercare

Further Reading

  • Veys J, Meyler J & Davies I (2002). Essential Contact Lens Practice (Chapter 6). Butterworth Heinemann
  • Optician Evans K & Hiscox R. REVISED Essential Contact Lens Practice Part 9 Rigid Gas Permeable CL Fitting. Available at: https://www.opticianonline.net/cpd-archive/5999/
  • Gasson A & Morris J (2010). The Contact Lens Manual (Chapters 7, 8, 9, 10, 11). Butterworth Heinemann
  • Efron N (2024) Contact Lens Practice 4th Edition (Chapters 15 & 16) Butterworth Heinemann
  • Franklin A & Franklin N (2007) Eye Essentials: Rigid Gas Permeable Lens Fitting (Chapters 3 & 4). Elsevier Butterworth Heinemann