gas permeable lens designs and ordering of parameters

outline the routine examination protocol for a gas permeable lens

what should gas permeable lenses provide (5)

◦Good VA - in most cases as good as gls or better - and better than soft spherical lenses in terms of VA

◦Comfort and adequate wearing time

◦No ocular insult, maintain corneal integrity - lens should not damage any surface of eye

◦Maintenance of normal tear flow behind and over the lens

◦Normal facial appearance (posture) and invisibility - no head tilt - lenses should be essentially invisible

who should we fit GP lenses for/patient considerations (6)

◦Optimal anterior eye characteristics

◦High motivation

◦usually have moderate to high Rx

◦Corneal toricity: >0.25 D & <2.00 D - can be beyond this but basic fitting capped at 2

◦Residual cylinder <0.75 D

Specialised fitting (theme 3)

what are the benefits of GP lenses (7)

• can be modified - can make adjustments once already ordered: change power up to +0.75D / peripheral curves can be flattened to increase EL / total diameter can be reduced / edges can be reshaped and polished

• reduced contamination risk - reduces risk of infection - generally non-compliant with lens maintenance - even non-compliant have less risk

• life style issues - sports

• better VA - for all px - particularly: astigmatic px / irregular astigmatism / low levels of uncorrected cyl with soft CLs

• less deposit build up

• less surface dehydration

• some conditions can only be fitted with GP - only these lenses will work for them as they have irregular corneas - keratoconus / traumatised corneas / post-grafts

what are the potential disadvantages of GPs we need to consider (8)

• Initial discomfort - adaption time of 2 weeks - for particularly sensitive corneas 3-4 weeks

• Precise fitting required, skill level higher and requires more time to fit - higher risk of loss and of px blinking/rubbing lens out / OZ - flare and glare when blinking

• Stringent regulations concerning the use and disinfection of diagnostic lenses

• Foreign bodies - will scratch the cornea

• Not without complications : 3 & 9 o’clock staining / Lens adhesion / GPC / Corneal warpage

• Breakage & scratching - more costly to replace - although a stronger material can be scratched with incorrect cleaning - need good px education

• Greasing with some patients

• Instability of some materials - parameters can be changed with incorrect cleaning (too much pressure)

what is the most basic of GP designs used

• tri curve lenses - base curve (central curve) / secondary/intermediate curve / peripheral curve

• Mono- and bicurve lenses can be manufactured but are not often fitted

• Curves can be added, called multicurve lenses - can have ones with 6/7 curves etc…

explain how we can customise the parameters (2)

• Each surface can be cutomised independently as toric or aspheric or toric aspheric, as can each curve separately - could have a toric base curve but a spheric front curve - designed to fit the shape/curve of the cornea

• Objective: measure ocular dimensions to aid in the design of a GP lens

define the parameters in this diagram

green solid lines - front curves

red dash lines - back curves

optic zone - centre green circle at front and red dash circle at back

total diameter - full width of lens

explain what BOZD, FOZD, and BPZD are

• BOZD - back optic zone diameter - diameter of the central back optic zone (the base curve area) - Determines how much of the central cornea is covered by the base curve - affects fit, centration, and tear pattern

• FOZD - front optic zone diameter - The diameter of the front optic zone (on the front surface) - Determines the optical area for vision - needs to cover the pupil in all lighting conditions - The usable “seeing” zone on the front of the lens.

• BPZD - back peripheral zone diameter - The diameter out to the end of the peripheral curve(s) on the back surface - Controls edge lift - affects tear exchange and comfort - How far the peripheral curves extend from the centre

what is an aspheric surface in terms of eccentricity (3)

• a surface that as an e value (eccentricity)

• an e value indicates the rate of change of that cornea from central cornea to peripheral cornea

• determines the flattening from the center to the periphery

what e value would indicate a cornea is significantly aspheric (2)

• 0.7 or more

• in this case we would match the CL asphericity to the cornea

what would the e value be of a perfectly spheric eye

• e value = 0

• no asphercity - working with a circle/perfectly round - r value (radius of curvature) is equal on every surface

what is responsible for tear exchange (4)

• radial and axial edge clearance

• always a tear layer between cornea and lens

• size of this gap = clearance

• clearance is what allows tears to move in and out under the lens → tear exchange (oxygen + debris removal)

describe the terms radial and axial edge clearance

• The terms axial & radial clearance are used to describe the distance between the CL & the cornea for each of the CL zones - this clearance is responsible for tear exchange

• axial edge clearance - measured centrally under the BOZD - straight down the OA — too much AC - excessive pooling (steep fit) — too little AC - bearing/touch (flat fit) — bright green pool centrally = increased AC / dark/black touch = decreased AC

• radial edge clearance - occurs in mid-peripheral and peripheral zones

explain the relationship between edge lift and tear reservoir

• wide edge - increase the volume of tears stored in the reservoir - excessive reservoir - stability of CL fit can be affected adversely - loose fit

• with a narrow edge - decrease volume of tears stored in reservoir - inadequate reservoir - tight fit

• A balance between these forces is required.

what is the ideal edge lift width

0.2-0.4mm

explain edge shape vs lens comfortability

• round round is the most comfortable edge shape - however note it is 8/10 - not 10/10 - so can still be felt by the px

what measurements do we need to take when fitting a GP lens (4)

diameters:

• BOZD - known as OZ

• TD - total diameter

radii:

• BC - base curve

• e value

how do we obtain the diameter measurements (3:3)

BOZD/OZ:

◦the actual prescription/power is here that the px sees through

◦For this reason, the pupil needs to be inside the OZ

◦Scotopic pupil +1mm (minimum) - average OZ = 7-8mm

TD:

◦HVID-2mm (rule of thumb)

◦The lens is smaller than the cornea

◦Most common is 9.3-9.4mm, up to 10.5mm - more complex the eye larger the TD is

how do we obtain the radii measurements (4:4)

BC:

◦Taken from K readings or SimK

◦Responsible for centration

◦0.1 steeper than flattest K for spherical or low astigmatic eyes - for example k readings of 7.4/7.6 - flattest k is 7.6. 0.1 steeper is 7.6-0.1 = 7.5

◦High astigmatism >=2.5 DC use toric base curve - On flattest K  - (bc1) // 1/3(K1-K2)+ Steepest K - (bc2)

E value:

◦Based on topographical overview

◦Aspheric design are meant for alignment fits - very little tear film behind the lens

◦Can mask astigmatism to a greater degree than spherical BC - (up to 4D)

-Often chosen for abnormal or irregular corneas

ordering template for GP lenses

RE	Parameter	LE	Calculation
	BC (radius)		0.1 steeper than flattest K
	OZ (diam)		Scotopic pupil +1mm minimum TD-1.4mm as a maximum
	2R		Often 1mm flatter than BC
	2CD	OZ + 0.8mm	(TD-OZ)/2 in your design
	PCR		Can alter as per fit, 0.05mm increments Often 1.5mm flatter than secondary curve
	TD		HVID-2mm as a guideline Manufacturer’s guide selection
gRey for Right eye	Colour	bLue for Left eye	Visitint recommended to differentiate lenses
Dk/t mid to high	Material	e.g.  A brand Boston	Mid Dk/t is sufficient in trials High Dk/t for final lens
Tear film calc	Prescription		Not related to spectacle refraction
e.g. FST	Design	e.g. bitoric	Dependent on visual and fit requirements

give an example of when we would have to calculate a toric base curve

• when high astigmatism - 2.50D+ and if there is a large difference between 2 k readings of 0.4 or more - this is because 0,4mm = 2.50D difference

• lens will not fit well - will start to rock - so need to calc curvatures to fit lens better to cornea which is a back surface toric - will have 2 diff curvatures on back surface of OZ

• K - RE 7.2/7.7@180 - can see there is a 0.5mm difference

• bc1 - will be the flattest k = 7.7

• bc2 - 1/3(7.7-7.2) +7.2 = 7.37

example of an order form trial lens

Patient information

◦HVID = 12mm

◦Pupils = 3/5mm

◦K - RE 7.2/7.7@180 / LE 7.4/7.6@180

◦Rx - RE -8.00/-2.50 x 180 / LE -9.00/-0.75 x 180

RE	Calc	Parameter	LE	Calc
7.7/7.37	•On flattest •1/3(7.7-7.2) +7.2	BC	7.5	0.1 Steeper than 7.6
6mm	5+1mm	OZ	6mm	5+1mm
10mm	12-2mm	TD	10mm	12-2mm
Grey		Colour	Blue
-7.25	Assuming an alignment fit	Power	-8.00	Vertex corrected (each eye)
80+ Dk/t	With tear exchange the requirement for Dk/t is lower	Material	80+ Dk/t	Your lab will have a choice
BST	DC= 2.50 lens would rock The front could be adjusted if the lens fitted and the O/R indicated a need for visual improvement	Design	Tricurve	Unlikely to have to adjust either surface for toricity.  -0.75 DC will be absorbed by tear film

what are the different lens designs we can fit (4)

what to order when - explain what the back and front surface are for

back surface is for fit:

• BC is designed to follow the cornea

• Tear film will fill up any irregularities

• Higher corneal cylinder will require a toric back surface – so lens fits cornea better (2.50+)

front surface is for vision:

• Once the initial fit for the back surface is determined, the over-refraction will determine if optimal vision is obtained with spheres or spherocylindrical powers

• >-0.75 Cylinder over-refraction requires a front surface toric

• however - if low vision/irregular cornea – if no visual improvement then do  not make front surface toric – as slightly uncomfortable –so if not needed don’t do – need a prism ballast to keep lens at visual axis - added thickness

what is a bitoric design

If the back surface is toric and the front surface is toric, the design is BITORIC

explain the significance of the tear film with hard lenses in comparison to soft contact lenses (4)

• In hard lenses in situ - can have a significant tear lens depending on its fitting relationship to the cornea

• power the tear film contributes - is enough to add to the optic system – can acc allow us to see very well

• If a rigid CL decentres, any tear lens (spherical or sphero-cylindrical) can acquire a prismatic component as well – induced prismatic effect – visual disturbance

• however in SCL - soft lens drapes cornea – tear film very thin – in a SCL has no contribution to optics

explain how the BC determines the tear film that is created (3)

• if we choose a lens with BC steeper than K = create a positive power tear layer

• alignment fit BC - tear lens same thickness - no significant tear film power

• flatter than K = negative power

explain why we usually choose a BC that is steeper in relation to adherence incidence (3)

• Adherence – lens binds to eye and doesn’t move - we do not want this to be the case - need movement in a lens still

• as shown in graph - aligned and flat fit have a higher incidence than steeper

• this is why we accept steeper fits to avoid adherence

what is the rule of thumb of the tear lens

• When the BOZR of a rigid CL is FLATTENED by 0.05 mm: the tear lens power is altered (decreased) by -0.25D

• When the BOZR of a rigid CL is STEEPENED by 0.05 mm: the Tear Lens power is altered (increased) by +0.25D

• Additional optical power of opposite sign is required in the CL to off-set the effects of the induced tear lens.

• Note: The Tear Lens is treated as being THIN in these calculations 

explain how corneal astigmatism is neutralized in relation to the tear lens (4)

• base curve of contact lens - back surface can be spherical (the OZ)

• this back surface generates the front surface of the tear film power

• in this way the tear lens is sphericalised - by the back surface of a spherical rigid CL

• These factors combined - a major reduction in corneal astigmatism when a spherical rigid CL is worn - a large amount of corneal astigmatism is absorbed

what are the limits however in neutralizing corneal astigmatism (3)

• gross differences in CL & corneal curvatures are dysfunctional/physically unacceptable - cannot have a spherical surface on a largely toric eye - lens will not fit

• practical limits apply to the amount of corneal astigmatism that can be corrected by a simple rigid spherical CL

• practical limits depend on the topography of the cornea but a limit of about 2.5 - 3.0 D is widely accepted

how much astigmatism is neutralized by a spherical rigid CL (5)

• RULE-OF-THUMB: Approximately 90% of anterior corneal astigmatism is neutralised by the presence of a spherical rigid CL.

• However, all other sources of ocular astigmatism remain unaltered - astigmatism in other than the cornea

• if there is a small amount of astigmatism elsewhere will still come through in the residual astigmatism.

• If the anterior cornea is astigmatic but the manifest ocular Rx is spherical - a spherical rigid CL in situ will disclose the ‘internal astigmatism’ as a residual astigmatism - removal corneal component and internal component remains

• with an axis at 90 ° to the corneal astigmatism & of about 90% its magnitude (90% of the corneal cylinder magnitude as residual astigmatism)

explain the over refraction of rigid CLs (3)

• impossible to separate tear lens power from OR

• final rx to order - final fit – only order one lens power which is made up of : Ocular Rx = BVP_Trial + Tear Lens Power + Over-Rx

• final lens power is comprised of multiple things cannot separate from each other

application of tear film optics - what do we do if we make a BC FLATTER (3)

• FAP

◦If you change the BC flatter you will need to compensate for the negative tear lens created by adding plus power to the lens you order

◦FLAT ADD PLUS

◦For every +0.1mm you add to the BC to flatten it, add +0.50 to the script

application of tear film optics - what do we do if we make a BC STEEPER (3)

• SAM

◦If you change the BC steeper you will need to compensate for the positive tear lens created by adding minus power to the lens you order

◦STEEP ADD MINUS

◦For every -0.1mm you take away from the BC to steepen it, add -0.50 to the script