EST: RGP FITTING

Question 1

what are the 2 types of designs of RGP?

Answer 1

spherical for CA <1.50

aspherical for CA >1.50

Question 2

how do we calculate BC for RGP?

Answer 2

1/3 steeper than flattest K
e.g k= 7.7 and 7.9
initial BC= 7.9 - 7.9-7.7/3

if sph RGP, fit on K
if cyl <1.50 DC, on K or 0.05-0.1mm steeper than flattest k
if cyl >1.50 to <2.50DC 0.1-0.15mm steeper than flattest K
if cyl >2.50 DC, use 1/3 rule

Question 3

how to calculate diameter for RGP?

Answer 3

HVID-2mm

anything less than this value is okay

Question 4

how to decide trial lens power?

Answer 4

+/- 3.00D
OR
+/-6.00D

Question 5

state the decentration, movement, dynamic of movement, lag, centre, mid-periphery, edge clearance and remedy needed for flat fit RGP. what kind of pattern will be seen?

Answer 5

decentration: >0.5mm
movement: >2mm
dynamic of movement: rocky, apical
lag: cross
centre: touch, absence of green
mid-periphery: pooling (clearance)
edge clearance: >0.5mm
remedy: increase BC, try aspheric design RGP

pattern: central touch

Question 6

state the decentration, movement, dynamic of movement, lag, centre, mid-periphery, edge clearance and remedy needed for optimum fit RGP. what kind of pattern will be seen?

Answer 6

decentration: <0.5mm
movement: 1-2mm
dynamic of movement: fast and smooth
lag: uncross
centre: even,
mid-periphery: even or slight touch
edge clearance: 0.3-0.5mm

pattern: diffuse

Question 7

state the decentration, movement, dynamic of movement, lag, centre, mid-periphery, edge clearance and remedy needed for steep fit RGP. what kind of pattern will be seen?

Answer 7

decentration: <0.5mm
movement: 1mm
dynamic of movement: slow
lag: uncross
centre: pooling (apical clearance)
mid-periphery: touch (absence of green)
edge clearance: <0.3mm
remedy: decrease BC, try aspheric design RGP

pattern: central pooling

Question 8

state what kind of tear lens will be formed with an on K, steep and flat RGP

Answer 8

on k: plano tear lens

steep: +ve tear lens
flat: -ve tear lens

Question 9

how to calculate tear lens?

Answer 9

0.25D ≈ for every 0.05 mm (difference between BC and K)

Question 10

how do we calculate expected over rx for rgp

Answer 10

ocular rx - (trial lens + tear lens)

ocular rx is from subjective rx

Question 11

what is the final lens to order? what rule do we use

Answer 11

trial lens power + over rx

use SAM FAP to verify Rx
steeper add minus
flatter add plus

Question 12

how can we use BC and diameter to make slight changes while maintaining an already desirable fit?

Answer 12

BC adjusted in 0.05mm steps and diameter in 0.4mm steps

e.g 7.8/9.6/-3.00 can change to 7.75/9.2/-3.00

Question 13

explain the effects of changing BC if all other parameters remained unchanged

Answer 13

increasing BC causes the fit to loosen

decreasing BC causes fit to tighten

Question 14

explain the effects of changing diameter if all other parameters remained unchanged

Answer 14

increasing dia causes the fit to tighten

decreasing dia causes fit to loosen

Question 15

what are some reasons px will complain of flare?

Answer 15

if BOZD is too small (small optic zone)
significant lens decentration
large pupil size (dim condition)
tear file deficiencies
poor lens wettability