2 - soft toric lenses

What are the types of astigmatism?

1. corneal astigmatism

2. lenticular astigmatism

3. refractive astigmatism

How is corneal astigmatism measured?

1. keratometry (~2mm)

2. auto-k

3. topography (~8mm)

What is the average K value? range?

43.25D

range: 35.50D-48.25D

What type of astigmatism do spherical GPs correct? How?

corrects corneal cyl via lacrimal lens

What is refractive astigmatism?

aka total astigmatism = corneal cyl + lenticular cyl

What patient characteristics are indications for soft toric CLs?

1. refractive astigmatism ≥ 0.75D

2. corneal cyl ≠ refractive cyl

Who are the better candidates for soft toric lenses?

1. poor VA with spherical soft lenses

2. GPs difficult/not tolerate

3. higher sphere than cyl (cyl < 25% of sphere)

what are reasons for why GPs are difficult to fit or not tolerated?

1. ATR astigmatism (3-9 staining)

2. discomfort

What are the poor candidate for soft toric lenses?

1. cyl > sphere

2. oblique cylinder axes

How does the diameter of a soft toric lens compare to a soft spherical lens? Why?

slightly larger → more stable and centers better

what are the common diameters of soft toric lenses?

~14.5 - 15 mm

How does the BC of a soft toric lens compare to a soft spherical lens?

typically flatter than sphere d/t larger diameter → keeps same sag depth

how many options do you have for soft toric BC?

typically no choice

What parameters are most important when choosing a soft toric lens?

figure out what patient needs → choose power

what powers is cyl available in?

starting at -0.75 → 0.50 steps

what do you do if cyl power is not available?

select lower cyl power

what axis does soft toric lenses come in?

every 10°

What does a 10º axis misalignment in soft toric lens cause?

1/3 of the CL cyl power uncorrected (residual astigmatism) d/t cross cyl

-2.75 cyl off 10° → -0.92 uncorrected

How do we stabilize soft toric lenses (to minimize rotation)?

1. larger diameter

2. thickness difference across lens → thickest inferiorly

3. blink forces

4. watermelon seed effect

what does the rotation marking indicate?

rotational position of lens

What is the watermelon seed effect?

prism ballast stabilizes lens by squeezing action of the upper lid which forces the thicker part of lens downward

What factors affect the fit of soft toric lenses?

1. surface tension/tear film quality

2. lids/lens interaction → loose lids won't stabilize

What is prism ballast?

0.75-2 pd of BD prism to stabilize lens via thickness difference and weight (minimal)

What are the disadvantages of prism ballast?

1. decreased O₂ transmissibility

2. discomfort lid-lens interaction

3. visual problems if unilateral → prism in optic zone

What is peri-ballast?

minus carrier over entire lens, then thin superiorly and increased thickness inferiorly (stabilizing points at 4 and 8 o'clock)

no prism in optic zone

What is a dual thin zone (dual slab-off) lens?

thinning a top and bottom of lens, but nasal + temporal are thick (blink stabilized)

often 2 sets of orientation marks (J&J)

What is truncation in toric lenses?

cutting the bottom portion of lens off so it lines up with lower lid

what are physiological considerations for soft toric CL?

corneal vascularization that would match transmissibility profile of lens

how long does it take a soft toric CL to settle?

10-15 mins

what first trial lens do you choose for a soft toric CL?

spec Rx (vertexed) → assume will have no rotation

what must you make sure of before assessing orientation of soft toric CL?

rotation must be stable

how to manage rotation of soft toric CL?

LARS from spec cyl axis

Left

Add

Right

Subtract

How should you troubleshoot poor VA in a soft toric lens?

1. look for rotation:

-compensate if stable → LARS

-if unstable change lens parameters

2. do SOR [spherical overrefraction] (or SCOR to see if lens is inducing extra power)

3. recheck baseline data (manifest)

4. try new lens

how does steepening BC affect soft toric CL?

less movement

resist rotation → can prevent lens from orientating correctly

how does flattening BC affect soft toric CL?

more movement

better rotation → unstable or excessive rotation

how does increasing the diameter affect soft toric cL?

stabilizes lens

What type of SCOR will you get if a soft toric lens has rotated?

SCOR with a spherical equivalent of plano at a different axis than the patients

what factors can affect rotation?

1. lid position

2. blink force

3. gravity