Modifying RGP Multifocals

BY DAVID W. HANSEN, O.D.
APR. 1997

Success or failure with multifocal contact lenses isn't determined at the diagnostic appointment. Even though trial lenses provide a good diagnostic prediction of final parameters, you may need to modify the final design at or after the dispensing visit.

Some changes will necessitate designing a new lens. These include: base curve changes; adding or subtracting prism or changing orientation of the prism axis in translating lenses; increasing diameter; adding a toric base or a front toric design; and changing thickness. Other modifications can be made in the office.

Almost any contact lens material can be modified, including the super-permeables if care is taken. Consider in-office modification for: superior and inferior lid capture; edge problems; lagging contact lenses; laterally displaced contact lenses; and nasal or temporal rotation. Make simple design changes first, such as thinning or polishing edges and modifying diameter. If this is not effective, investigate the fluorescein pattern further to determine the appropriate modifications.

DIAMETER REDUCTION

The best tool to reduce lens diameter is a 60- or 90-degree cut-down tool with a hollow cone made of either crystalline, emery, carborundum or brass impregnated with diamond dust. Hold the lens on a suction cup and move it up and down in the spinning cut-down tool. Lubricate the cone with cool water to prevent heat.

EDGE MODIFICATION

This is one of the most critical procedures that can be performed in the office. A high percentage of comfort problems are attributed to edge design. Rolling and polishing the edge, using either a sponge tool with a central hole, a velveteen pad for polishing or a suction cup for fingerlishing, will greatly enhance comfort (Fig. 1).

FIG. 1: FINGERLISHING -- THE ROCKING MOTION OF THE FINGER AROUND THE EDGE PRODUCES A MORE COMFORTABLE EDGE PROFILE.

To reduce superior or inferior lid capture in higher minus prescriptions, it may be necessary to create a CN bevel or a thinner edge using a 90-degree cone tool.

PERIPHERAL CURVES

Peripheral curves can be flattened with the classic modification techniques by changing the radii of the tools and using a velveteen pad with polishing compound.

POWER CHANGES

Adding plus and minus to bifocal contact lenses is sometimes difficult and is more appropriately done at the laboratory. However, when a small amount (up to 0.75D) of minus or plus is needed, you can add it by using a standard spinner and positioning the lens against a toe pad modification tool or a drum tool (Figs. 2 & 3).

FIG. 2: ADDING PLUS POWER USING A TOE PAD.

FIG. 3: ADDING MINUS POWER USING A TOE PAD.

On front or posterior aspheric lenses, some labs can to add the power for distance or near individually. CLS

Dr. Hansen, a cornea and contact lens diplomate and fellow of the American Academy of Optometry, is in private practice in Des Moines, Iowa.