Toric and Multifocal Soft Lens Selection and Verification

Toric and Multifocal Soft Lens Selection and Verification

Soft contact lenses correcting astigmatism (toric lenses) and presbyopia (multifocal lenses) require additional fitting skills beyond those used for standard spherical lenses. This lab introduces the parameters, assessment techniques, and troubleshooting steps you will use daily as an optician.

Toric Soft Lens Parameters

A toric soft lens has three optical parameters: sphere power, cylinder power, and axis. It also has a stabilization system—usually prism ballast, peri-ballast, or accelerated stabilization design—that keeps the cylinder axis aligned with the patient's corneal astigmatism axis.

When selecting a trial lens, start with the prescription values. Most fitting guides instruct you to match sphere and cylinder power as closely as possible to the manifest refraction, then choose the base curve and diameter that best match corneal curvature. When the exact cylinder value is unavailable in the trial set, round to the nearest available step (commonly 0.25 D or 0.50 D increments, depending on the manufacturer).

The LARS Rule

After lens insertion and a short settling period (typically five to ten minutes), the lens may rotate from its intended position. Stabilization systems aim to hold the axis steady, but rotation of a few degrees is common. If the lens rests with its axis rotated from the target, you must compensate the ordered axis to account for it.

The LARS rule provides a quick way to make this adjustment:

• Left — Add: If the lens rotates to the patient's left (counterclockwise as you view the patient), add the number of degrees of rotation to the prescribed axis.
• Right — Subtract: If the lens rotates to the patient's right (clockwise as you view the patient), subtract the number of degrees of rotation from the prescribed axis.

Always keep the resulting axis within the 1–180° range. If your calculation falls below 1°, add 180°; if it exceeds 180°, subtract 180°.

Document the lens markings visible at the slit lamp. Most toric soft lenses have a scribe mark, dot, or line at the 6 o'clock position when the lens is correctly oriented. Deviation of this mark from 6 o'clock indicates rotation.

Slit-Lamp Evaluation of a Toric Lens

At the slit lamp:

1. Dim the room lights and use low illumination to avoid pupil constriction.

2. Ask the patient to blink naturally, then hold steady gaze.

3. Identify the orientation mark (dot, line, or laser engraving) on the lens.

4. Estimate the rotation in degrees from 6 o'clock. The clock face can help: each hour mark represents 30°.

5. Note the direction of rotation (left/counterclockwise or right/clockwise) from your perspective facing the patient.

6. Record findings on the fitting worksheet before adjusting the prescription.

A lens that rotates more than about 10–15 degrees consistently, or that fluctuates with each blink, suggests a fit problem (lens too flat, too steep, or wrong diameter). Consult the manufacturer fitting guide for specific tolerances before changing lens parameters.

Safety note: Never attempt to manually rotate a lens on the eye. If the fit appears problematic, remove the lens and reassess corneal measurements and lens specifications.

Multifocal Soft Lens Overview

Multifocal soft lenses correct presbyopia by providing distance and near power zones within the same lens, most commonly through concentric ring (center-near or center-distance) or aspheric designs. The add power is the positive difference between the distance zone and near zone of the lens.

Fitting multifocal lenses requires attention to:

• Pupil size, because the relative size of the near and distance zones interacts with ambient lighting.
• Add power selection, guided by the patient's near prescription needs and the manufacturer's fitting guide.
• Centration, because a decentered multifocal lens shifts the optical zones away from the visual axis, reducing clarity at one or more distances.

Verifying Add Power with the Lensmeter

After a multifocal lens has been removed (following proper disinfection protocol), you can verify its add power on the lensmeter:

1. Place the lens on the lensmeter stage with the front surface facing the lens stop, using the soft-lens attachment if available.

2. Focus the lensmeter on the distance power zone (center or periphery, depending on lens design).

3. Record the sphere power in that zone.

4. Move to the near power zone and record that sphere power.

5. The add power equals the near zone power minus the distance zone power.

6. Compare against the labeled power. The acceptable tolerance for soft lens add power verification is ±0.25 D.

Safety note: A lens removed for verification must be properly cleaned and disinfected before reinsertion. Never return a lens to the patient's eye without following your facility's disinfection protocol.

Troubleshooting a Toric or Multifocal Misfit

If visual acuity is unsatisfactory after axis compensation:

• Re-evaluate rotation—allow adequate settling time.
• Check base curve: a steep lens may ride high; a flat lens may rotate excessively.
• Consult the manufacturer fitting guide for the next recommended parameter change.
• Consider whether residual astigmatism or cylinder undercorrection is the cause.

For multifocal misfits, poor near vision often indicates insufficient add power or a lens riding low, while poor distance vision may indicate a lens riding high or excessive add interaction. Document all findings and steps taken before changing lenses.

Documentation and Scope of Practice

All trial lens selections, rotation findings, add power measurements, and parameter changes must be documented clearly on the patient fitting record. Opticians select and fit lenses according to a licensed prescriber's prescription and parameters established in the fitting guide. If a prescription change appears necessary, communicate findings to the supervising optometrist or ophthalmologist. Never alter the sphere, cylinder, or add values on the prescription without prescriber authorization.