RGP Contact Lens

Fitting
OPT 424
Prof.
Prof. M.
M. O.
O. Oriowo
Oriowo
BSc(Optom),
BSc(Optom), MSc,
MSc, PhD,
PhD, FNOA,
FNOA, FAAO
FAAO
Professor
Professor of
of Optometry
Optometry & & Vision
Vision Science
Science
University
University of of Ilorin
Ilorin
Ilorin,
Ilorin, Nigeria
Nigeria
Design Choices

• Spherical back surface

practitioner designed
multiple peripheral curves
• Tricurve (2 peripheral curves)
• Tetracurve (3 peripheral curves)
• Pentacurve (4 peripheral curves)
Design Choices

• Non-spherical (aspheric) back

surface
company designed
shape derived from conic sections
more than one conic section can be
combined to form a compound
continuous curve
Relevant Conic Sections

e=1.5
e=1

e=0 e=0.5
Definitions

1 Apical Tear Layer Thickness

The depth of the tear layer trapped
between the cornea and the back of
the lens at the corneal vertex

ATLT

viewed with fluorescein

Definitions

ATLT
Definitions
2 Axial Edge Lift
Sum of the sags of all the curves on BS of the
lens which represents the vertical distance from
the edge of the lens to the extension of the base
curve

3 Radial Edge Lift

Sum of the sags of all the curves on BS of the
lens which represents the radial distance from the
edge of the lens to the extension of the base
curve
See next slide and IACLE module..
Definitions

IEA is the axial edge lift

not visible with fluorescein; function of the lens
Definitions

4 Axial Edge Clearance

The distance between the cornea and
the back of the lens at the lens edge
measured along the axis of the lens.
Relates to the lens on a known eye.

AEC

viewed with fluorescein

Definitions

AEC
Lens Design
Ocular Parameters Required
• Keratometry
limited value depending on peripheral
flattening
• Visible Iris Diameter
influences Total Diameter
• Vertical Palpebral Aperture
influences Total Diameter/fitting philosophy
• Pupil Diameter
influences BOZD
mm vs Dioptres ??

• In North America, measure cornea in

D, but fit using mm ?
• Can be confusing !
• I will rely on mm to explain points
easy translation...
mm vs Dioptres ??

• 7.80 x 8.10 • 43.25 x 41.75

• rule: • 1.50D corneal cyl
 take steep from
flat, divide by 2,
x10 = corneal cyl
 [(8.10 - 7.80)/2] x10
• 1.50D corneal cyl
Topical Anaesthesia?

• Increases initial comfort

 but can be a shock on collection of final lens !
 can soften epithelium
 comfort can be a good indicator of acceptable
fit
• Not advisable
 can be useful for situations where rapid fit
assessments are required (eg keratoconus)
Fitting Process

1 Decide on TD
2 Decide on BOZR & BOZD
3 Decide peripheral lens design
 aspheric or AEL parameters
4 Decide on BVP
5 Decide on Ct / Et
6 Decide on FS form (lencticulation)
7 Decide on Material
1. Total Diameter
Step 1: LD, TD or OAD

• PA measurement • Predicted Diameter

• <8 mm  9.0 to 9.3 mm
• 8-11 mm  9.4 to 9.6 mm
• >11 mm  9.7 to 9.9 mm
• HVID measurement • Predicted Diameter
• 10-11 mm  9.2 to 9.4 mm
• 11.5 to 12.5 mm  9.5 to 9.7 mm
• >12.5 mm  9.8 to 10.0 mm
Selection of TD

Rules of thumb
• Select Total Diameter (TD) at least
1.0-1.5mm smaller than HVID
• Select smaller TDs with increasing
astigmatism
see later
Small LD:Loose, Mobile Lens
Large LD: Tight, Immobile Lens
2. BOZR & BOZD
Central Fitting Requirements

• Aligned over central area

relative alignment with spherical lens on
aspheric cornea
• Pupil coverage for good acuity
• Adequate to promote good centration
Possible Fitting Relationships
Creating Alignment

• In principle
 match the sag of the lens to the sag of the
cornea over the BOZD
• Radius selected depends on:
 corneal curvature
 BOZD
• In general, select first lens on/around “flat
K”
 assess fit and refine as necessary
Characteristics of Aligned Fit
Fluorescein Pattern Lens Movement
• Minimal under the • Reasonable centration
optic zone • Good movement
 15-20 microns
• When pushed up with
 barely visible with
lower lid:
blue light
 falls straight down
• no pooling towards
the centre
Alignment Fits
Characteristics of Steep Fit
Fluorescein Pattern Lens Movement
• Visible under the optic • Generally centration is
zone reasonable
 depth depends on relative • Limited movement
steepness
• When pushed up with
• More fluorescein
towards centre of optic lower lid:
zone, less towards  falls straight down but
junction of BOZD slowly
Steep Fits
Characteristics of Flat Fit
Fluorescein Pattern Lens Movement
• None visible under • Poor centration
optic zone  often up and out
 may look “aligned” • Excessive movement
• In extreme cases • When pushed up with
 more fluorescein towards
edge of optic zone, less
lower lid:
towards corneal apex  falls in an arc as
periphery of optic zone
bears on cornea and
acts as pivot point
Flat Fits
Selection of Lens BOZR/BOZD

Rules of thumb
• In broad terms clinical equivalent fits
are linked by the following rule:
0.05mm BOZR:0.50mm BOZD
flatten BOZR with increasing BOZD
Selection of Lens BOZR/BOZD

Rules of thumb (0.05:0.50)

7.55:8.00 7.45:7.00

7.50:7.50
Selection of Lens BOZR/BOZD

Rules of thumb
• Select BOZR to match flattest K-reading
• Select BOZD to be larger than largest
pupil diameter
 select smaller BOZD with increasing
astigmatism
• To achieve alignment (with BOZD’s
typically being 7.80-8.00mm) require a
BOZR flatter than k
BOZR for TD = 9.2/9.4

• Corneal Cylinder • BOZR

 0.00 to -1.00 D  on flat K
-1.12 to -2.00 D  ¼ K + flat K
-2.12 to -2.75 D  1/3 K + flat K

>-2.75 D  back toric

In some instances may need to revert to BST sooner

Fitting the Toric Cornea
Options

• Fit “aligned” to flattest meridian

• Reduce total diameter
• Use toric periphery lenses
• Use fully toric lenses
Toric Fits

• NB WTR vs ATR fits

• always look for depth/width of clearance around lens when assessing fit
 3. Peripheral Design
Adequate Peripheral Clearance
Requires either:
flattening aspheric curve
• e-value governs rate of flattening
• may be enhanced by additional bevel curve
series of flatter spherical curves
• number of curves and their relative diameter
Ideally want 80 - 100 m
Aspherics

• Rate of flattening of aspheric curve

is usually dictated by laboratory
Therefore
• Axial edge lift and axial edge
clearance pre-determined
Characteristics of
Adequate Peripheral Clearance
Fluorescein Pattern Lens Movement
• Bright even band of • Smooth movement of
fluorescein around lens with eye versions
the lens periphery • No significant
• Approximately reduction in peripheral
0.5mm in width fluorescein in
secondary positions of
gaze
• No touch with
peripheral cornea
Adequate Peripheral Clearance
Characteristics of
Inadequate Peripheral Clearance
Fluorescein Pattern Lens Movement
• Narrow band of • Maybe smooth but
fluorescein around may be limited by
edge of lens peripheral corneal
• Fluorescein is limited touch with lateral eye
to the extreme edged movement
of the lens • Fluorescein rapidly
diminishes in
secondary positions of
gaze
Inadequate Peripheral Clearance
Characteristics of
Excessive Peripheral Clearance
Fluorescein Pattern Lens Movement
• Deep fluorescein • Mobile lens hitching to
pooling at lens edge lid
• Extends well into mid • Often high riding
periphery of lens • Lens may be prone to
towards optic zone fall out
• Bubbling under lens
edge with blinks
• Tear break up over
lens edge after blink
Excessive Peripheral Clearance
Peripheral Fit

• Options
add constant amount
order constant axial edge lift
order constant axial edge clearance
• Each will result in differing
peripheral fit appearance
Constant amount addition

• Decide on BOZR & BOZD

• Decide on TD & interim diameters
• Make SCR 0.8 - 1.2mm flatter than BOZR
• Make PCR 2.2 - 3.0 flatter than BOZR
• Will result in altered AEL across lenses
 greater for steep lenses
 less for flat lenses
Lens Design - CAEL types

• Constant axial edge lift does not

result in constant axial edge
clearance in different patients
• Distribution of edge lift across
curves affects fit
Lens Design with 0.10 AEL

7.80 7.70 8.40 8.50 8.90 8.80 11.00 9.30

AEL 0.023 0.016 0.066 (0.105)
7.80 7.70 8.10 8.50 8.40 8.80 12.50 9.30
AEL 0.012 0.010 0.081 (0.103)
7.80 7.70 8.60 8.50 9.40 8.80 10.00 9.30
AEL 0.030 0.022 0.051 (0.103)
Lens Design with 0.10 AEL

Lens 1 :
Normal distribution

Lens 2 :
Minimal mid-peripheral clearance

Lens 3 :
Generous mid-peripheral clearance
CAEL designs

• Use sag tables

• Use computer program...
 4. BVP
Selection of Lens BVP
Rules of thumb
If lens is aligned to flat meridian of
cornea then:
Lens BVP = Spectacle Rx
on least minus meridian (corrected for BVD)
Selection of Lens BVP

-2.00/-1.75x180

7.80 al 180 7.45 al 90

7.80:7.40 TD9.20 -2.00

Selection of Lens BVP

Rules of thumb
• In broad terms BVP and fit are linked
by the following rule:
0.05mm BOZR : 0.25D
flatter BOZR needs more plus power
steeper BOZR needs more minus power
Selection of Lens BVP

Rules of thumb (0.05mm:0.25D)

7.55:7.50 -2.75 7.45:7.50 -3.25

7.50:7.50 -3.00
 5. Ct & Et
Step 5A: CT
Plano  Approx. 0.20 mm

Minus  subtract 0.02 per D, up to a

limit of CT = 0.10mm
 often order ~ 0.15
 add 0.02 per D, up to a limit
Plus
of CT = .30 mm, then
lenticulate
Lens Thickness

• Determined by:
rigidity
permeability
BVP
• Considerations:
on-eye lens flexure
correction of corneal astigmatism
Dk/t
5B: Edge Thickness
Lens Comfort vs Apex Location
 6. Front Surface Form

Front Surface Design

FOZD affects: • Lenticulation
vision affects:
lid interaction centre thickness
• comfort lens mass
• movement O2 transmission
• centration
comfort
Front surface forms
Regular (plano) design -ve carrier +ve carrier

edge thickness

junction thickness
Lenticular Carriers

(Or
Plus)
7. Material Selection – check Lecture on
Material

Typical RGP lens Specification

European System
C3: Wondergas RGP Blue
7.80:7.50/8.35:8.40/9.35:9.20
BVP -3.00 CT 0.17

North American System

C3: Wondergas RGP Blue
7.80 8.35/0.45 9.35/0.40 9.20 7.50
BVP -3.00 CT 0.17
Tips on Fit Assessment

• Always instil fluorescein

 can use white light light if required
• Look at pattern in primary position
 lens in habitual position
• Get patient to make version movements
 check edge clearance over peripheral cornea
• Use lids to assess central fit
 push lens up with lids - check central fit and movement
on drop
 assess lid interaction with lens
• centration change with/without lids
5 Components of a Desirable Fit

• central and mid-peripheral alignment

 apical alignment or alignment to the flat
meridian of a toric cornea
• moderate edge width and clearance
• smooth movement on blink
• well centred
 upper lid just over the top edge of the lens
• “adequate” total diameter
Descriptors

• Central fit
 flat, aligned, apically clear, steep
• Peripheral fit
 remember width & depth
 excessive, adequate, inadequate
• Movement
 excessive, adequate, inadequate
• Centration
 high, low, nasal, temporal, central
• Diameter
 too large, adequate, too small