5.1.3 Chooses, fits and orders rigid lenses.

Question 1

RGP lenses should be first choice:

Answer 1

• Patients with irregular astigmatism due to corneal damage or keratoconus will only attain satisfactory vision with RGP lenses.
• Existing rigid lens wearers may not be satisfied with the visual performance of soft lenses.
• Patients with particularly exacting visual requirements will probably see better with RGPs.
• High minus prescriptions will result in lenses that are thickest at the edge of the optic zone. An RGP lens will leave the metabolically active limbal area uncovered and not restrict oxygen supply.

Question 2

HVID

Answer 2

The horizontal visible iris diameter (HVID) is usually measured with a ruler (figure 1) but greater accuracy can be achieved by using a measuring graticule on a slit-lamp, thus eliminating parallax errors. However, even the cruder version is probably more measurement than we really need. The fact is that all ‘system’ lenses and most custom ones have total diameters between 8.5 and 10.00mm, which is going to be 1.50 to 2.0mm less than the normal range of HVIDs anyway, and selection of the total diameter has more to do with interaction of the lens with the eyelids than with corneal diameter. The only RGP patients for whom HVID is worth recording are those with abnormally large or small ones (megalo- and micro-corneal subjects).

Question 3

VPA

Answer 3

The visible palpebral aperture (VPA) is also measured, and it does at least give some indication as to how small a lens needs to be to be interpalpebral. Experienced practitioners rarely measure the VPA unless it is exceptional, and rather more useful information can be recorded by drawing the position of the lids in relation to the cornea (figure 2).

The position of the upper lid can influence the degree of lid attachment obtainable with a given diameter. The position of the upper lid prior to fitting may also be worth noting because long-term rigid lens wear can induce ptosis. The shape of the lids can influence horizontal centration, particularly if the lids are tight. The lower lid, if particularly low, can be associated with inferior corneal drying and with dropping lenses, particularly with prism-ballasted torics and translating multifocals.

Question 4

Pupil diameter

Answer 4

The pupil diameter is also measured, both in normal ambient conditions and in poor light, using a Burton lamp when available, or a pupil guide if not (figure 3). The idea is to try to prevent flare from unwanted reflections from the peripheral curves of the lens by ensuring the back optic zone diameter (BOZD) of the lens is larger than the pupil.

Question 5

Selecting the diameters

Answer 5

Here are the main points from the provided text:

• Lens diameter affects lid/lens interaction:
  • Smaller, interpalpebral lenses depend on corneal interaction for centration.
  • May be uncomfortable due to edge contact with lid margin during blinking.
• Tucking lens edge under upper lid:
  • Increases comfort by avoiding lid margin contact.
  • Allows lid to help hold lens in place and promote tear circulation during blinks.
• Larger optic diameters on low-astigmatism corneas:
  • Help distribute lens weight and blink force over a wider area.
  • Increase capillary attraction, improving lens retention.
  • Reduce visual flare due to a larger optic zone.
• BOZD (Back Optic Zone Diameter):
  • Can be custom-ordered to any size.
  • Must allow space for peripheral curves to prevent abrupt transitions.
  • Abrupt transitions may impair tear exchange.
  • Typically, BOZD is \~1.5 mm smaller than TD (Total Diameter).
• Lens diameter selection guideline:
  • Default to a larger diameter (around 9.60 mm).
  • Unless there are specific reasons not to (not yet listed).

Question 6

Good reasons not to selec a large diameter:

Answer 6

Here are the main points from the provided text:

• Astigmatism:
  • Larger diameters make it harder to achieve good alignment in both meridians.
  • Smaller BOZD can accommodate larger corneal cylinders effectively.
• Flexure:
  • Depends on material and thickness.
  • Lenses may flex up to 30% of corneal toricity.
  • Causes uncorrected astigmatism in over-refraction.
  • If the lens flexes during blinking and returns to shape, it may cause suction, affecting the epithelium.
• Lid interference:
  • Tight lids can prevent proper centration.
  • May require a smaller TD than vertical palpebral aperture (VPA) to reduce lid interaction.
  • However, this may lead to discomfort.
• Neovascularization:
  • In former soft lens wearers, a smaller lens may avoid areas of new vessel growth.

Question 7

Keratometry and Corneal Topography

Answer 7

Here are the main points from the provided text:

• Keratometry:
  • Still the most common method of measuring corneal shape in optometric practice.
  • Historically the only measurement used until recently.
• Corneal shape:
  • The cornea is not spherical, generally shaped as a prolate ellipse (flattens toward the periphery).
  • The degree of flattening (asphericity) is described by:
    • Eccentricity (average: 0.39)
    • Shape factor (average: 0.85)
• Shape variation:
  • Flattening rate varies between individuals (shape factors range: +0.50 to -0.10).
  • Negative shape factor means the cornea steepens peripherally (seen in \~3% of population).
• Limitations of keratometry:
  • Poor correlation between keratometry readings and actual flattening rate.
  • Two eyes with the same K reading can have very different flattening rates, even in the same person.
  • Flattening rates may differ in the horizontal vs. vertical meridians.

Question 8

Modern topographical studies have identified five groups of astigmatism:

Answer 8

• Round (23% symmetrical, low astigmatism).
• Oval (21% asymmetric, low astigmatism).
• Symmetrical bow-tie (17.5% symmetrical astigmatic).
• Asymmetric bow-tie (32% asymmetric astigmatic).
• Irregular (7%, no pattern).

Question 9

Asymmetric corneas:

Answer 9

• Have different flattening rates in upper vs. lower cornea.
• Can restrict lens movement.
• Astigmatic bow-tie patterns show toricity differences between upper and lower cornea.
• Can cause poor centration of back surface toric lenses.

Question 10

Keratometer limitations:

Answer 10

• Measures curvature ~3mm apart on either side of the visual axis.
• Single or averaged readings provide limited info on overall corneal shape.
• Does not assess shape factor, which affects lens fitting.

Question 11

Choosing the Back Optic Zone Radius

Answer 11

Here are the main points from the provided text:

• Cornea and back optic zone relationship:
  • Dependent on fitting philosophy.
  • Held in place by eyelids and surface tension at the lens edge.
• Edge clearance:
  • Excessive clearance: No meniscus and no surface tension.
  • Reduced clearance and edge thickness: Increased surface tension.
• Fitting types:
  • Lid attachment:
    • Lens movement controlled by lids.
    • Can lead to lens riding high and lower corneal exposure stain.
  • Interpalpebral:
    • Small TD and BOZD to minimize lid interference.
  • Alignment fit (most common today):
    • Goal: Align the back optic zone with the corneal surface, with a uniform thin tear film.
    • Advantages:
      • Even distribution of lens weight and blink forces.
      • Minimizes lens flexure for better visual performance and reduced corneal stress.
      • Efficient tear replenishment and oxygen transmission, preventing barrier effects.
      • Flatter fit usually works best for most patients (fitting on flattest K).
      • For larger diameters, need to flatten the lens to maintain a clinically equivalent fit.
• Fitting adjustments:
  • Steeper than alignment fit might help centrate the lens without significant compromise.
  • Avoid steepening the BOZR based on corneal toricity to prevent central clearance.
  • Back surface toric may be an alternative for centring issues.
• Aspheric design fitting:
  • Follow manufacturer recommendations for BOZR based on eccentricity or shape factor of the lens design.

Question 12

The Periphery of rgp

Answer 12

Here are the main points from the provided text:

• Periphery of the lens:
  • Generated by progressively flatter spherical curves or an aspheric curve.
  • Purpose: Important for tear circulation under the lens.
• Edge clearance:
  • Insufficient clearance leads to:
    • Loss of lubricative effect of tear film, causing lens adherence to the epithelium and potential mechanical damage.
    • Loss of oxygen to the cornea under the lens. This is problematic for lenses with low oxygen transmission.
  • Adequate peripheral clearance ensures proper tear exchange and prevents hypoxia of the central cornea, protecting epithelial integrity.
• Other reasons for peripheral clearance:
  • Allows the eyelids to dislodge the lens during blinking.
• Modern system lenses:
  • Designed with a smooth progression in the periphery.
  • Spherical curves: Transitions between curves are polished to create a continuous surface.
  • Typically suitable for the majority of patients.
  • For unusual corneal shapes, the edge clearance can be adjusted by laboratories while keeping the optic zone unchanged.
• Edge clearance vs. edge lift:
  • Edge clearance: Gap between the front surface of the cornea and the back surface of the peripheral curves. Observable with fluorescein.
  • Edge lift: A geometrical characteristic of the lens, defined in axial or radial terms. Not the same as edge clearance.

Question 13

Edge profile

Answer 13

Here are the main points from the provided text:

• Edge shape: Important for lens comfort, especially during the early stages of wear.
• Laboratory standard designs: Often used, but if lenses are uncomfortable, a different form can be requested.
• For alignment fit with partial lid attachment:
  • Comfort is determined by the interaction between the lens edge and the eyelids, rather than the cornea.
  • Rounding of the anterior edge (front edge) is more important for comfort than rounding the posterior edge (back edge).

Question 14

Centre thickness

Answer 14

Here are the main points from the provided text:

• Modern lenses: Tend to flex due to their material properties and thin design, especially in low minus-powered lenses.
• Flexure factors: Increases with Dk (oxygen permeability) and corneal toricity (degree of astigmatism of the cornea).
• System lenses: Typically account for flexure automatically.
• Astigmatism considerations:
  • For corneal astigmatism greater than 2.00D, an additional 0.02mm thickness may be needed.
  • Fitting an astigmatic cornea with a spherical lens may require ordering a slightly thicker lens, which may compromise oxygen transmission.

Question 16

Back Vertex Power

Answer 16

Here are the main points from the provided text:

• Contact lens power calculation:
  • The spectacle prescription is adjusted for vertex distance.
  • The tear lens formed between the contact lens and the cornea affects the final lens power.
  • If the lens is in perfect alignment with the central cornea, the tear lens power is zero.
• Tear lens power adjustment:
  • A steeper lens than the cornea creates a positive tear lens power.
  • A flatter lens than the cornea creates a negative tear lens power.
  • Rule of thumb:
    • For every 0.05mm the BOZR (back optic zone radius) is steeper than K, the tear lens power increases by +0.25D.
    • To counteract this, subtract -0.25D from the contact lens power.
    • If the contact lens is flatter than K by 0.05mm, add +0.25D to the contact lens power.
• Example 1:
  • Spectacle prescription: -5.00DS at a back vertex distance of 10mm.
  • K readings: 7.80mm in all meridians.
  • The lens selected has a BOZD of 7.75mm (0.05mm steeper than K).
  • Adjust spectacle prescription for the corneal plane:
    • Effective spectacle power: -4.75D.
    • Tear lens power: +0.25D (due to the steeper fit).
    • Final contact lens power: -5.00D.
      Here are the key points from the provided example:
• Initial Power Calculation:
  • The spectacle lens at 10mm BVD has an effective power of -4.75D in the corneal plane.
  • The selected BOZR of 7.75mm is 0.05mm steeper than K, creating a tear lens power of +0.25D.
  • To counter this, subtract -0.25D from the contact lens power.
  • The final contact lens power to order is -5.00D.
• Adjusting to a Flatter Lens:
  • If a flatter BOZR of 7.85mm is selected (2 x 0.05mm flatter than the previous lens), the tear lens power will be -0.50D.
  • To compensate, add +0.50D to the contact lens power.
  • The final contact lens power to order is -4.50D.

Question 17

Binocular considerations

Answer 17

• Myopes are often exophoric, especially at near fixation distances.
  • Wearing spectacles for near tasks gives them base-in prism for near vision.
  • Without this base-in prism, decompensation of near ocular motor balance may occur, leading to:
    • Variable vision
    • Asthenopic symptoms (eye strain)
    • Diplopia (double vision).
  • Over-correction of myopia can sometimes help by stimulating accommodation and inducing accommodative convergence.
• AC/A Ratio:
  • Normal AC/A ratio is around 4.
  • Even a -0.25D over-correction can significantly affect the oculomotor balance without overloading accommodation.
• Accommodative Demand:
  • Myopes experience higher accommodative demand with contact lenses compared to spectacles.
  • Myopic patients who are used to reading without spectacles may struggle with the extra accommodative demand when their myopia is corrected with contact lenses.
  • Over-correction is not appropriate for such patients.
• Hyperopic Patients:
  • Hyperopes often find near vision easier with contact lenses because their accommodative and convergence demands are reduced.
  • Practitioners should be cautious of hyperopes with significant latent hyperopia, as their latent element can become manifest after contact lens fitting.

Question 18

Markings

Answer 18

• Previous Markings on Lenses:
  • Lenses used to be marked with ‘R’ (right) and ‘L’ (left), and some even had BOZR and TD printed on them.
  • This was helpful during aftercare, especially for non-fitting practitioners.
• Disadvantages of Engravings:
  • Engravings can contribute to deposits forming on the lens.
  • They may also weaken the lens mechanically.
• Changes in Lab Practices:
  • After issues with lenses splitting along the vertical line of the engraving, many labs shifted to a simpler marking system.
  • Most lenses now have only a small dot on one lens, typically the right lens.
• Impact on Patients:
  • Many patients, particularly those with early presbyopia, struggle to identify which lens is for which eye.
  • This can lead to patients attending aftercare appointments with either lens in either eye, or two identical lenses.

Question 19

Tints

Answer 19

• Handling Tint:
  • Some standard lenses come with a handling tint to help detect dropped lenses.
  • This works well when white bathroom suites are common, but less so with colors like avocado or beige.
  • The handling tint is generally not an option but a standard feature of certain lenses.
• UV Inhibitor:
  • Many lenses have a UV inhibitor incorporated into the lens polymer.
  • Occasionally, the UV inhibitor is offered as an optional extra.

Question 20

For a cornea with low toricity:

Answer 20

• Total Diameter (TD):
  • Should typically be large (9.60mm) unless specific reasons dictate otherwise.
• Back Optic Zone Diameter (BOZD):
  • Should be set to flattest K or slightly flatter.
  • Be aware that keratometer readings vary by instrument (due to mire separation), so know your device.
• Edge Lift:
  • Use standard edge lift unless patient-specific needs require adjustment.
• Lens Edge Design:
  • Must be well-rounded—especially the front edge—for comfort and function.
• Centre Thickness:
  • Use standard thickness unless fitting a spherical lens on a significantly astigmatic cornea, in which case add 0.02mm.
  • Manufacturers often factor in material flexibility when setting standard thickness.
• Power:
  • Base on the spectacle correction (spherical equivalent), adjusted for vertex distance and tear lens power.
  • Only modify this further if managing oculomotor balance.

Question 21

Insertion (Application)

Answer 21

• Lower Eyelid Control:
  • Use a finger next to the one holding the lens to control the lower lid.
  • This steadies your hand and protects the cornea if the patient jerks forward.
• Speed of Approach:
  • Avoid approaching too slowly—this gives the patient time to panic and allows the solution/tear film to dry, causing lens adherence to the finger.
• Target Area:
  • Aim for the upper cornea using an oblique approach (not along the visual axis).
  • This uses gravity and avoids provoking lid closure and Bell’s phenomenon.
• Visual Axis Approach Risks:
  • May trigger defensive blinking and eye rolling upward, causing lens misplacement onto the lower sclera.
  • Can cause discomfort or corneal insult from lens edge.
• Fixation Strategy:
  • Aiming at the upper cornea means Bell’s phenomenon helps the lens land on the central cornea.
  • Proper fixation ensures gentle lens positioning and expels air bubbles.
• Lens Positioning:
  • Aim for \~25% of the lens overlapping the limbus for ideal placement.
• Patient Comfort:
  • Talk continuously to reassure and distract the patient, reducing discomfort, tearing, and blinking.
  • Use positive suggestion (e.g., “look down… that probably feels better already”).
• If Patient Doesn’t Respond:
• There is an air bubble trapped under the lens
• The lens edge is damaged
• There is a foreign body under the lens
• The patient is reacting to the conditioning solution

Question 22

Should the lens settle on the sclera,

Answer 22

no attempt should be made to move it onto the cornea. The reduced edge clearance of modern RGP designs make this a very dubious procedure now, and the likely result is damage to the limbal area from the edge of the lens. Any damage to the cornea increases the risk of serious infection and the limbal area is where the corneal stem cells reside. If the lens displaces onto the sclera, take it out and reinsert onto the cornea. You might have to push it round to the temporal sclera first in order to be able to get at it. The ‘pinch’ technique is appropriate here to remove the lens.

Question 23

Removal

Answer 23

• Lens Removal Overview:
  • Multiple techniques exist; each suits different patients depending on lid anatomy, tear film properties, and reflexes.

1. The ‘Pinch’ Technique

• How it works:
  • Use thumb and finger on upper and lower lid margins.
  • Start with lids wider than lens diameter → gently squeeze lids together.
  • Lid margins tuck under the lens periphery and dislodge it.
• Advantages:
  • Minimal force needed if done correctly.
  • Provides a controlled and direct removal.
• Disadvantages:
  • Can induce blepharospasm in sensitive patients due to lid margin contact.

2. The ‘Pull and Blink’ Method

• How it works:
  • Patient fixates a target to center lens.
  • Eyes opened wide; outer canthus is pulled laterally to tension lids.
  • Tension plus blinking breaks tear film’s surface tension, dislodging lens.
• Advantages:
  • Useful for patients intolerant of pinch technique.
  • Works well with lenses that have larger peripheral clearance.
• Disadvantages:
  • May fail if lid tension isn’t sufficient.
  • Requires precise eye positioning and coordination.

3. The ‘V’ (or ‘>’) Method

• How it works:
  • Eye abducted (looking outward).
  • Practitioner tensions lids into a V shape (apex at inner canthus).
  • Patient looks inward; cornea moves lens into lid junction where it dislodges.
• Advantages:
  • Effective for patients where other methods fail.
  • Particularly useful when lids are tight or movement coordination is difficult.
• Disadvantages:
  • Requires good instruction and practitioner control of lids.
  • Less intuitive for patient self-removal.

Would you like these summarized as a printable comparison chart or patient instruction guide?

Question 24

Assessing the fit
When assessing the fit of a rigid contact lens, textbooks tell us that we must consider two things:

Answer 24

1 The dynamic fit, or how the lens moves and centres on the eye.

2 The static fit, or how the back surface of the lens relates to the cornea

Question 25

Material

Answer 25

* Most B&L RGPS are fluorosilicone acrylate * Silicone is softer so scratches more easily, hence modern RGPs are only on 1–2-year replacement schedules. * The addition of fluoropolymers to silicone acrylate increases the wettability & less susceptible to protein deposition * We use ML92 which is a mid DK (33) fluorosilicone acrylate * Medium oxygen delivery (Dk/t does not need to be as high in RGPs as with softs as do not cover entire cornea) * Slightly lower Dk allows for good protein/lipid resistance * B&Ls most popular material *

Question 26

Lens design preference

Answer 26

**Maxim (1st choice)** * Spherical design, back surface multi-curve * Straight forward fitting – fit to flattest K * Available in toric **Quantum (2nd choice)** * Spherical centre with aspheric edge * Issue with aspheric is that you don’t get full Rx when looking away from the centre * Fit 0.05 – 0.1mm steeper than the flattest K due to aspheric geometry

Question 27

Spec Rx – allows us to choose BVP

Answer 27

* 2.50 DC = back surface toric * Spherical back surface & front surface toric = if lenticular astig, prism ballast to stabilise * Bitoric/ FSTx +BSTx = if both corneal & lenticular astigmatism = stable

Question 28

Keratometry – dictates BC

Answer 28

* Ks taken to allow RGP to match corneal curvature; allows us to choose base curve * Ks look at central 3mm (as this is what keratometers are designed to measure) * Ks become flatter in periphery as cornea is aspheric * Spherical lenses – fit to the flattest K * BOZR = flat K – (flat k-steep k / 3) *

Question 29

Pupil diameter – dictates BOZD

Answer 29

* BOZD = area with Rx (periphery of lens is for stabilisation) * BOZD must be > pupil size in scotopic (dim light) to ensure pupil is smaller than BOZD in all lighting conditions * If pupil > BOZD = issue with flare / haloes around lights * High myopes may need larger BOZDs to avoid flare *

Question 30

Flat fitting lens

Answer 30

 Nafl = central touch, large area of edge lift  Flat fit creates a negative tear lens, resulting in plus over Rx

Question 31

Steep fitting lens

Answer 31

 Nafl = central pooling (smaller area of pooling = steeper lens)  Steep fit creates a positive tear lens resulting in a negative over Rx  0.25DS over Rx = 0.05 BOZR change  Increase in diameter by 0.5 = flatten BC by 0.05 o Increased diameter = increased sag o Because cornea is aspheric, increasing diameter = steepening lens, therefore you need to flatten it to keep Rx same

Question 32

Types of designs

Answer 32

The most common rigid design used in this country is a tricurve back surface design. Bicurve designs are occasionally used with smaller designs (<8.40mm) while tetracurve designs are sometimes used with larger designs (>9.50mm). Aspheric designs are often used with branded, stock designs (eg Quantum, Conflex) as opposed to custom designs. The perceived advantages of aspheric designs are: * Thinness * Comfort due to smooth back surface * Aspheric optics (but this also gives induced astigmatism on decentration).

Question 33

MULTICURVE (MAXIM): Design

Answer 33

**Design 9.30mm & 9.80mm diameter**s • Spherical optic zone with well blended multicurve or aspheric (Ultra) periphery • BOZD does not change with TD so no need to alter BOZR when changing diameter • Constant average thickness and Dk/t across power range • Constant edge clearance from cornea **Fit** • Fit on flattest K, BOZR <7.60 use 9.30mm; >7.60 use 9.80mm • Difference in K readings 0.3mm use a toric back surface

Question 34

QUANTUM

Answer 34

**Design 9.00mm, 9.60mm, 10.00mm diameters** • An advanced polynomial aspheric back surface for precise corneal alignment • Thickness control to optimise lens thickness across power range **Fit** • First choice diameter 9.60mm (9.00mm and 10.20mm options) • 0.1mm steeper than flattest K will give an alignment-like fluorescein pattern • Adjust power of sphere in minus cyl refraction by-O.50D • No need to change BOZR with diameter, Fit on flattest K, BOZR <7.60 use 9.30mm; >7.60 use 9.80mm • Difference in K readings 0.3mm use a toric back surface

Question 35

RGP Materials

Answer 35

* BOSTON XO2; Hyper Dk Fluorosilicone Acrylate ; Outstanding oxygen permeability without compromising wettability, stability, or comfort. Excellent deposit resistance. * BOSTON XO Super Dk Fluorosilicone Acrylate High Oxygen delivery, stability equalling that of lower Dk materials. * QUANTUM II (ML210) Super Dk Fluorosilicone Acrylate High Oxygen Delivery. Good Protein resistance. * BOSTON EO High Dk Fluorosilicone Acrylate Excellent wetting/deposit resistance. * BOSTON EQUALENS; High Dk Fluorosilicone Acrylate Fluorinated polymer for improved oxygen delivery. Contains UV absorber. * QUANTUM I; (ML92) Mid Dk Fluorosilicone Acrylate Medium oxygen delivery. Good protein resistance. * BOSTON ES Low-Mid Dk Fluorosilicone Acrylate ; Exceptional durability and modulus. Exceptional wetting and deposit resistance.

Question 36

The most common rigid design

Answer 36

• The most common rigid design used in this country is a tricurve back surface design. Bicurve designs are occasionally used with smaller designs (<8.40mm) while tetracurve designs are sometimes used with larger designs (>9.50mm). • Aspheric designs are often used with branded, stock designs (eg Quantum, Conflex) as opposed to custom designs. • The perceived advantages of aspheric designs are: - Thinness - Comfort due to smooth back surface - Aspheric optics (but this also gives induced astigmatism on decentration).

Question 37

Higher DK RGP means

Answer 37

increased deposition, more susceptible to scratches, thicker lenses and lower modulus, therefore, poorer correction of astigmatism

Question 38

Silicone Acrylates

Answer 38

- Copolymer of silicone & acrylate (PMMA) - Pros - low-medium Dk (12-60), Good dimensional stability, Less lens flexure, Soft & flexible - Cons - Attracts protein deposits but less lipids, Not suitable for EW due to Dk, Decreased wettability & potentially Optical quality

Question 39

Flurosilicone Acrylates

Answer 39

- Fluorine monomer added to silicone acrylate - Pros - High Dk (40-100+), Good for EW, Better wettability, Attracts less protein - Cons - Less lipid resistance, Greater lens flexure, Scratches more easily so replaced more often

Question 40

Ordering RGP

Answer 40

What to put on the order form? - Manufacturer - Lens name - Material - Measurements - BOZR (write their K's for bitoric as it's being fitted directly to their corneal curvature), TD, Power