VERIFICATION AND INSPECTION OF HYDROGEL LENSES

University of Missouri- St. Louis

School of Optometry

Optometry 553 Laboratory

Verification of hydrogel lenses is difficult; however, there are times when it is important to determine as accurately as possible the parameters of a hydrogel lens, such as when refitting a previous wearer for the first time, determining if the patient has lenses in the wrong eye, etc. In most cases, the patient may be fit without knowledge of the previous lens parameters; however, there will be times when the previous parameter information is of value. When hydrogel lenses are initially dispensed it is best to verify the parameters straight off the manufacturer's vial. The parameters which should be verified are:

    1. Back vertex power
    2. Overall diameter (OAD)
    3. Surface and edge inspection
    4. Base curve radius (concave surface)
    5. Center thickness

The equipment that can be used to verify hydrogel contact lenses are as follows: lensometer, measuring magnifier, soft lens tweezers, wet cell, projection magnifier, and/or biomicroscope.

    1. Blotting technique

      2. The lens should be blotted dry on a lint free tissue, such as Kimwipes. The semi-dry lens is placed concave side against the lens stop of the lensometer. The power of the lens is read off the measuring drum just as spectacles. The mire image will not be as distinct as spectacles or rigid contact lenses.

    2. Wet cell method

Fill the wet cell with saline and place the lens in the wet cell concave side down. A slide is pushed gently across the top of the wet cell. Best results will be achieved when no bubbles are left between the saline and the slide. The lens is placed on the lensometer with the back of the wet cell against the lens stop. Once again the concave side of the lens is closest to the lens stop. The power that is read off the measuring drum is multiplied by the appropriate factor to achieve the right power.

 

Power

Factor*

Minus

0 to -10.00 D

4
 

Greater than -10.00 D

3.5

Plus

0 to +10.00 D

4
 

+10.00 to +15.00 D

4.5
 

Greater than +15.00 D

4.7

* Determined for medium water content range - approximately 40%

Just as power, the overall diameter can be determined via the wet cell or by blotting the lens dry. The simplest method is the use of the measuring magnifier (or reticule).
    1. Blotting technique

      2. The semi-dry lens is placed concave side down on the measuring magnifier. The lens will need to be aligned with the scale and gently held up to a light source to determine the actual diameter. There is a scale of 0-20mm on the back of the measuring magnifier. A PD stick can also be used to determine the diameter.

    2. Wet cell technique

The lens should be placed in the wet cell as mentioned previously. The wet cell is placed on the measuring magnifier with the concave side of the lens down. The diameter is read directly off the scale.

An accurate method of determining hydrogel lenses is not available; however, a method similar to that used by many manufacturers will be demonstrated in the laboratory.

Soft Lens Analyzer (Hydrovue, Inc.) - This instrument allows the base curve to be determined by matching the lens curvature to the template submerged in saline. The lens is projected on a screen. The lens is simply moved from template to template until the lens matches. If a lens is too flat the lens will bear on the template centrally and lift off in the periphery. If the lens is too steep, the lens will bear peripherally and have a gap of saline between the center of the lens and the template. The correct template will be closely aligned with the lens throughout.

Center thickness of a hydrogel lens can be determined with a radiuscope. A microscope slide is placed on the lens mount used to determine base curve radius. The instrument is focused and the reading noted. The lens is blotted dry and placed on the slide in the area where the initial reading was taken. Another reading is taken when the instrument is focused on the upper concave surface of the lens. The difference between the two readings is the thickness of the lens. Care should be taken not to let the lens dehydrate too much or an inaccurate reading will result.

A measuring magnifier can be used to inspect the surface or edges; however, the simplest and probably the most accurate methods of inspecting the surface and edges are the projection magnifier, the biomicroscope and/or a phase contrast microscope.
    1. Projection magnifier - The lens can be held with soft lens tweezers in a position to project the lens image on the screen. Any defects on the surface or edge should be visible in the magnified view.
    2. Biomicroscope - The lens can be held with tweezers behind the biomicroscope to enable the observer to view any defects; however, on eye inspection is very simple with the biomicroscope. This will provide the observer with information about tears, edge chips, deposits, film, jell bumps, etc.
    3. Phase contrast microscope - The lens is placed on a microscope slide and flattened with a cover slide. This can be done without harming the lens. This method is excellent for viewing deposits, films, etc.

Reference: Janoff L. Hydrogel Lens Verification. In: Bennett ES, Weissman BA (eds.) Clinical Contact Lens Practice, Lippincott Publ., Philadelphia, PA. In press.

 

(Revised 1990 by VAH)

DATA SHEET

 

After the previous techniques have be explained, each student and a partner will be required to check the following parameters on at least one lens and record these parameters on the data sheet to be turned in 1 week after the laboratory sessions are completed.

 

Base Curve Radius

  

Back Vertex Power (blotted/wet cell)

/

Overall Lens Diameter (blotted/wet cell)

/

Center Thickness (optional)

  

Surface & Edge Inspection