5.1.3 Chooses, fits and orders rigid lenses.

Question 1

Material

Answer 1

• 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
  1. Medium oxygen delivery (Dk/t does not need to be as high in RGPs as with softs as do not cover entire cornea)
  1. Slightly lower Dk allows for good protein/lipid resistance
  1. B&Ls most popular material

Question 2

Lens design

Answer 2

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

Question 3

Spec Rx – allows us to choose BVP

Answer 3

• </= 2.50DC = spherical back surface (tears will fill gaps)
• > 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 4

Cornea designs based off of type of RGP lens chosen

Answer 4

• CORNEA: toric
• Rx: toric
• RGP: spherical
• CORNEA: spherical
• Rx: toric
• RGP: Front surface toric
• CORNEA: toric
• Rx: spherical
• RGP: bitoric
• CORNEA: spherical
• Rx: spherical
• RGP: spherical

Question 5

Keratometry – dictates BC

Answer 5

• 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 6

HVID & VVID – dictates TD

Answer 6

• HVID dictates the total size of the lens
• TD ~ 1.5 – 2mm smaller than HVID
  1. HVID generally 11-12mm
• Generally, 9.30 and 9.80 TDs available
• VVID gives us indication of the fit
  1. Lid attached; minus lens more likely to lid attach as it is thicker in the periphery
  1. Interpalpebral
• Higher powers require greater td for stability
  *

Question 7

Pupil diameter – dictates BOZD

Answer 7

• 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 8

Over-refraction

Answer 8

Flat Fitting Lens
- NaFl pattern → central touch, large edge lift.
- Flat fit creates a negative tear lens → results in a plus over-refraction (Rx).

Steep Fitting Lens
- NaFl pattern → central pooling (smaller pooling area = steeper lens).
- Steep fit creates a positive tear lens → results in a negative over-refraction (Rx).

Lens Adjustment Rules
- 0.25DS over-refraction = 0.05mm BOZR change.
- Increasing diameter by 0.5mm → flatten base curve (BC) by 0.05mm.
- Increased diameter = increased sagittal depth (sag).
- As the cornea is aspheric, increasing diameter steepens the lens, so BC must be flattened to maintain the same Rx.

Question 9

Lens fit

Answer 9

• White light
  1. Centration
  1. Lid adherence or interpalpebral
  1. Covers pupil in all POG
  1. Movement on blink (1-1.5mm)
• Nafl pattern
  1. Steep = central pooling /narrow edge lift / very steep = air bubble
  1. Flat = central touch / large area of edge left
  1. Alignment = hit of apical clearance over central 7mm, mid peripheral alignment 1.50mm, edge clearance 0.4mm
  1. Toric cornea c spherical lens = dumbbell patter, alignment with flattest k only

Question 10

Difference in K’s:

Answer 10

• Difference in K’s is 0.15-0.25= then steepen(reduce) by 0.05
• Difference in K’s is 0.3-0.45, then steepen(reduce) by 0.10
• Difference 0.5 or greater= back toric/ bitoric/ smaller diameter RGP

Question 11

steeper corneas

Answer 11

Tetracurve (Maxim) & Aspheric (Quantum) (better for steeper corneas) (matches shape of cornea better so better for unusual corneas)

Question 12

Rules of thumb:

Answer 12

• Every +0.50mm increase in TD, increased BOZR by 0.05mm
• Every 0.05mm lens is flatter than corneal radius = -0.25D tear lens = +0.25 OR
• Every 0.05mm lens is steeper than corneal radius = +0.25D tear lens = -0.25 OR

Question 13

Apical clearance

Answer 13

central vault over corneal apex & bearing toward the paracentral cornea causing some NaFl to build up underneath but not as excessively as a steep fit