CL for astigmatism - design & fitting

what are the types of astigmatism (2)

• regular

• residual

describe regular ocular astigmatism, its different meridians and how it is recorded (6)

• represented across 2 meridians - 90 degrees apart

• meridian descriptions: refractive power (diopters) and radius of curvature (mm) (keratometry)

• astigmatism lies along power meridian - steepest meridian/smallest mm - meridian of greatest curvature/optical power

• axis meridian - meridian of least curvature/optical power

• Corneal astigmatism is an optical description

• Corneal toricity is an anatomical description

describe what residual astigmatism is (2)

• the astigmatic refractive error that is left uncorrected when a CL is already placed upon the cornea to correct the existing ametropia

• Interpret as, 'with spherical CL' unless stated otherwise

explain how we can induce astigmatism and when this is likely (6)

• Tilted &/or decentred CL

• Toricity &/or bitoricity of CL - CL itself has a toric surface

• CL mislocation (rotation &/or decentration) - laterally/clockwise/anticlockwise

• Warpage &/or flexure of CL

• This is likely in a SCL if the lens is rotated off axis

• likely to be presentable in low VA / OR still uncorrected cyl

explain how astigmatism can be due to the physiological make up of the eye and when this is likely (8)

• Un-neutralized corneal astigmatism - stock powers for CLs not coming in 0.25 steps and everything in between - if cornea has small amount of astigmatism it is not possible to correct as CL cyl start at -0.75 and go in 0.50 steps

• Posterior corneal astigmatism

• Lenticular astigmatism

• Tilted crystalline lens

• Refractive index anomalies

• Oblique aberrations

• Misalignment of ocular components

• This is unlikely in a SCL unless small astigmatism is corrected with a spherical lens - SCL not creating any tear film behind the lens to correct astigmatism in any other way

explain why toric CLs need to be stabilized (5)

• due to pressure of eye lids - lids exert pressure onto lens

• without stabilization - lens would rotate

• Maximise predictability of CL axis location - rx power needs to be at a specific axis - ensure lens is on axis

• Make CL axis location independent of Rx

• Maximise CL physiological performance

state the methods used for toric stabilization of SCL (5)

•Prism-ballast - most common

•Truncation - least utilized, more common in GP than SCL (only for customized SCL)

•Peri-ballast - popular similar to prism

•Double slab-off

•Reverse prism

• SN: these methods also apply to gas permeable lenses

describe the prism-ballast method of lens stabilization (5)

• inferior parr of lens is thickened by base down prism - 1 to 1.5D base-down

• stabilized by prism-induced CL thickness differences - uses gravity and lower lid to stabilize

• As it is thicker in inferior - that portion of CL has lower oxygen transmissibility

• discomfort with CL-lid interaction - px can feel this thickness - especially if one eye has toric lens and one has SCL

• prism ballast should not be close to pupil size - can induce prismatic effect for px

describe the truncation method of lens stabilization (4)

• measure px lid margin - remove portion of lens that is more than lower lid margin

• Truncation aligned with lower lid margin can create some CL stabilisation

• Truncation can cause some discomfort and is not always successful

• More patient visits can be required - Seldom used now

describe the peri-ballast method of lens stabilization (6)

• Minus carrier converted to a Base-down prism effect - orientation principle similar to prism-ballast

• Uses thickness ∆s as the stabilising component

• Thinner superiorly, thicker inferiorly - prevents lid rotating in lower lid margin

• Prism-free optic zone - no prismatic effect

• Can cause discomfort with CL-lid interaction at the thicker inferior half

• ↓ CL O₂ transmissibility in thicker regions

describe the double-slab off method of lens stabilization (6)

• utilizes thin and thick zones - thinner zones have a higher oxygen permeability as opposed to thicker zones - Thin zone superiorly & inferiorly

• creates thicker zones in the palprebral apperture - when lens rotates thick part knocks on lid to not allow it to rotate

• thicker zones - prevent it from going under lid - lid forces (upper & lower) maintain orientation

• Better comfort due to ↓ CL thickness

• CL is symmetrical

• Can exhibit less rotational stability is low spheres, WTR astigmatism and less successful on px with lid laxity - loose lids - elderly px

describe the reverse prism method of lens stabilization (2)

• utilizes both base up (inferiorly on lower lid margin) and base down prisms (palprebal apperture)

• Result in thinner and more comfortable lenses than just the thicker base down prism ballast design

explain how we assess the orientation of the CL (3)

• Done by observing reference points or marks on the CL to assess CL orientation in situ

• not all companies have the same markers - if a px has tight lids, markers may escape under top lid so know where the markers are before inserting them into eye

• some companies have 1 marker at 6oclock, 2 at 12 and 6, 3 at 6 etc.. - not expected to know them all

when is it applicable to fit a px with a toric CL (4)

• px with refractive astigmatism - if cyl is more than 0.75 we advise toric lenses

• If spherical SCLs failed to mask corneal astigmatism - unsatisfactory VA with best sphere

• px may switch to toric if - GP CLs leave residual astigmatism due to physiological aspects - GP CLs cause discomfort

• if px has a large rx (-12/-8 etc) - they won’t have perfected vision in gls – not necessary to incorporate all of cyl power as it won’t have impact on vision

explain how we select a trial for toric CLs (6)

• Measure Rx & vertex distance

• Select  CL power (BVP) to match corneal-plane Rx

• Vertex correct each meridian >4D

• Select BOZR &/or TD — {(K1 + K2)/2} +0.9mm — HVID +2mm

• Always lowest minus possible - for CYL component as only come in 0.50 steps (if px has a -1.00 cyl we would select a -0.75 cyl not -1.25 - select lowest minus)

• 5 degrees in refraction show patient choice of 10 degrees either way - E.g. refraction 45, decide on preference between 40 or 50 in trial frame

what do the stock powers and axis go up in for toric lenses (4)

• start at -0.75 and go up in 0.50 steps to -2.25

• axis goes up in 10 degree steps

• some companies do go above -2-.25 to the likes of to -5.75 - in this case the axis do then start going up in 5 degree steps - however this is not the norm

• Online calculators can generate the order for u from px prescription

what makes a good, tight and loose fit for toric CL

Good fit: 

◦Full corneal coverage, good centration & movement, quick reorientation after blinking/lateral gaze

◦Full corneal coverage, 0.2 - 0.5 mm movement

Tight fit: 

◦Good centration, initially comfortable, little or no movement. Slow reorientation if mislocated

Loose fit: 

◦Excessive movement, poor centration,
 uncomfortable.

◦CL orientation unstable & inconsistent

what do the markers on the CL show and how do we assess them (3)

• CL marks are for reference only, they do not indicate the axes of cylinder optically, they of no particular significance

• Measure rotation using: narrow slit-lamp beam with protractor scale

• align the beam with the marker and read the angle off the scale of slit lamp

how do we interpret the orientation of markers what does it mean - what do we do with this information (5)

• if marker is exactly at 6 o clock - orientation is perfect no need to re order trial lens

• if the orientation is 4 degrees or less - cannot order new trial as only go up in 10 degree steps - if 5 or over then need to alter lens - rotation is too much for px visual outcome using:

• LARS - left = add / right = subtract

• if the marker is 15 degrees to the RIGHT from where it should be (6 o clock position) - then subtract 15 from the axis of trial lens - eg if trial lens was 70 - 15 = 65, however axis come in 10 degree steps so it would be 60.

• if the marker was 15 degrees to the LEFT - add 15 to the trial lens axis - if trial lens axis was 180 add 15 - would be 15 but would order 10 as only come in 10 degree steps

what patients do not do well with toric CL and why (4)

• Low spherical component, esp. WTR astigmatism: e.g. +0.25 / –2.25 x 180

• Oblique cylinders: e.g. –2.00 / –1.75 x 50

• above 2 is due to - the rotation and stability of CL – more difficult to generate a stable lens due to curvature

• High cylinders: any movement off-axis will generate poor vision - e.g. +2.00 / –5.75 x 80

what should we do if the lens rotates again on the new trial order (5)

• LEAVE IT

• Once you have compensated for the rotation in the first trial, the second lens should fit the same orientation - it will still be rotated - not at 6 o clock - it is meant to be rotated - only now the rotation has already been accounted for

• The lens acts as intended (based on spectacle refraction) even if the reference marker is rotated

• This only holds true if the rotation is the SAME as the first trial

• This is often due to a persons anterior eye surface and not the lens design

power crosses do at home on paper notes - but key points (6)

• only need to vertex correct if either meridian is greater than 4

• use the formula to vertex correct

• draw power crosses to help

• remember if the stock power for cyl is over -2.25 we can now offer axis in 5 degree steps - would show px either one and ask which one they prefer

• if the calculation says -1.00 cyl or 172 axis - remember things like cyl only starts in -0.75 so that’s what would acc be ordered and then 170 for axis as axis only go in 10 degree steps

• plus powers become more plus - minus powers become more minus - due to focal lengths

when do we need to over refract for toric lenses

Only over-refract a lens that has less than 5 degrees rotation - to determine if optimal power

if u try to OR a lens that has not been compensated for - no point - only OR on a compensated lens

explain OR of a toric lens (6)

• Use loose lenses in a trial frame

• Check sphere power - BVS

• Check axis and cylindrical power - JCC

• Ensure if there is an over-refraction that the change can be accommodated in a new lens - 0.50 jumps in soft lenses - use the 0.50 JCC as no point using the 0.25 ones

• 10 degree jumps in soft lenses

• Consider extended range if the astigmatism is not generating the BCVA in spectacles - communicate this may take a while to the px

routine for toric SCL assessment