CL for astigmatism

What is the power meridian

• meridian of greatest curvature/optical power

• steepest >D or smallest mm

Axis meridian

• meridian of least curvature/optical power

What is residual astigmatism

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

How can residual astigmatism be induced?

• Tilted &/or decentred CL

• Toricity &/or bitoricity of CL

• CL dislocation → rotation and/or decentration

• Warpage and flexure of CL

(likely in a soft CL if lens is rotated off axis)

Physiological residual astigmatism

• un-neutralised corneal astigmatism

• posterior corneal astigmatism

• lenticular astigmatism

• tilted crystalline lens

• Refractive index anomalies

• oblique aberrations

What is the purpose of toric CL stabilisation?

• maximise the predictability of CL axis location

• max CL physiological performance

what resists the lens moving on your eye ?

• Lids

• forces applied by upper lid more significant than lower lid

What is Prism-Ballast for Toric stabilisation

• 1 to 1.5 D Base-down prism (thicker at the bottom)

• stabilised by prism-induced CL thickness differences

• however decrease in CL O2 transmissibility

• discomfort with CL-lid interaction

What is truncation → Toric SCL stabilisation

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

• Truncation can cause some discomfort

• Truncation not always successful

• more px visits can be required

• not used now

What is Peri-Ballast → Toric SCL stabilisation ?

• minus carrier converted to base down prism effect

• uses thickness prisms as stabilising component → thinner superiorly , thicker inferiorly

• prism free optic zone

• can get discomfort with CL-lid interaction at the thicker inferior half

• decrease in CL O2 transmissibility in thicker regions

What is the double slab-off → toric SCL stabilisation?

• better comfort due to a decrease in CL thickness

• Thin zones Superiorly AND Inferiorly

• Lid forces (upper and lower) maintain orientation

• CL is symmetrical

• can exhibit less rotational stability if low spheres, WTR astigmatism

What is Reverse prism method for Toric SCL stabilisation?

• BOTH base up and base down prism

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

What is Bausch and Lomb reference marks on a CL

• 3 lines around 6 O’clock

• 30 degree separation (15 each)

• Purevision Toric, Purevision 2 toric

• Optima Toric

• Soft CL toric

Reference markers on Alcon CL?

• 3,6 & 9 O’clock Positions

• 6 O’clock line thicker

• Air Optic for astigmatism CL

Cooper vision reference markers on CL

• single vertical line @ Base of CL → 6 O’clock position

• Biofinity toric

• Avaira Toric

• Clarity Toric

Johnson and Johnson CL reference markers

• Vertical lines at 6 & 12 O’clock position

• 1 Day Acute moist for Astigmatism

• Acute Oasys + Advance for astigmatism

What are the indications of fitting a Toric CL

• when spherical CLs fail to mask corneal stigmatism → unsatisfactory VA with BVS

• GP CLs leave residual astigmatism due to physiological aspects

• GP CLs cause discomfort

• High sphere to Cyl ratio

• cyl a third or more of total refraction

What to do when selecting a trial for Toric CL ?

• 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 if possible

• if refraction 45 e.g decide on preference between 40 or 50 in trial frame

What stock do CL come in

start at 0.75 and go up in 0.50 steps

-0.75

-1.25

-1.75

-2.25

• IN 10 DEGREE axis jumps

What is a good fit of a CL

• full corneal coverage , good centration and movement , quick reorientation if mislocated

• 0.2-0.5mm movement

Describe a tight fit CL

• Good centration , initially uncomfortable , little or no movement

• slow reorientation if mislocated

Describe a loose fit CL

• excessive movement , poor centration and uncomfortable

• CL orientation unstable & inconsistent

LARS rule

• Right = subtract

• Left =Add

• if contact lens moved to the right and was 70

• subtract how much it moved by → 10 degrees example

• 70-10=60

• reorder at 60

What should you do if the reordered CL rotates again

LEAVE IT

• second lens should fit in the same orientation

• could be due to persons anterior eye surface and not lens design

Hyperopia CL

• CL have shorter distance over which to focus light than spectacles

• Therefore, the contact lens power must be MORE POSITIVE than the glasses prescription.

Myopia CL

• CL have a longer distance over which to focus light than spectacles

• Therefore, the contact lens power must be LESS NEGATIVE (lower minus) than the glasses prescription.

When do we carry out over-refraction

• only over-refract a lens that has less than 5 degrees rotation

• use loose sense in trial Frame → check sphere, axis and cycle power

• ensure if change that it can be accommodated for in a new lens

• -0.50 jumps in soft lenses

• 10 degree jumps in soft lenses