Movement and Centration

A rigid lens is smaller than a soft lens and, thus, will not cover the cornea. The old PMMA hard contact lenses were designed to have an interpalpebral fit. That is, the centered lens lies entirely between the lids. The more modern method of fitting gas permeable lenses (which are larger than PMMA contacts) is the alignment fit, where the upper third of the lens stays under the upper lid (Figure 9-7).

The movement of a rigid lens is much different from that of a soft lens. The rigid lens will move slightly upward with each blink (about 1.00 to 2.00 mm), then smoothly drift down and resettle. It should not "drop" down. Normally, the lens should not bump into the lower lid margin. When the patient looks left or right, the contact may lag 0.50 to 1.00 mm behind, but it should not move past the limbus. Adhesion of the lens to the cornea (usually in a decentered position) is a common occurrence (Figure 9-8). Movement is noted and described in the chart. (Examples: adequate movement; lens falls to lower lid after blink.)

Ideally, the lens will center so that the optical zone of the contact falls in front of the patient's visual axis. Make a note if you see that the lens is riding high (Figure 9-9A), low (Figure 9-9B), or laterally.

High Ride Rgp

Figure 9-9A. High-riding gas permeable lens. (Photo by Patrick Caroline, courtesy Bausch and Lomb/Polymer Technology.)

High Ride Rgp

Figure 9-9A. High-riding gas permeable lens. (Photo by Patrick Caroline, courtesy Bausch and Lomb/Polymer Technology.)

Figure 9-9B. Low-riding gas permeable lens. (Photo by Patrick Caroline, courtesy Bausch and Lomb/Polymer Technology.)

Rigid lenses can be left in place when fluorescein dye is instilled. In fact, observing the pattern of the dye under and around the lens provides the fitter with valuable information about the lens fit. If you, as a nonfitter, are asked to evaluate the fluorescein pattern, the general rule is to note areas where the dye pools and areas where the dye is absent. Be sure to use the cobalt blue filter. The appearance of the dye may be further enhanced by the use of a #12 (yellow) Wrattan filter. The Wrattan filter blocks out excess blue light, thus enhancing the green color of the dye. The background becomes dark. The filter is available at photography/camera shops. To use the filter with the slit lamp, hold or tape it on the patient side of the instrument.

If the fluorescein reflex is bright green and the tear layer is thick, there is clearance between the contact lens and the cornea. A faint green reflex and a thin tear layer indicate minimum clearance. If the reflex appears black and there is no tear layer, then the lens is touching the cornea. In a good fit, a thin film of dye will be evenly spread under the entire lens, with slight pooling at the periphery. If the patient has astigmatism, you might see a band of dye running horizontally or vertically. A loose (flat) lens will show a central absence of dye and a pooling around the edge (Figure 9-10). A tight (steep) lens will have pooling in the center and an absence of dye at the periphery (Figure 9-11). (There are exceptions.) For documentation, simply draw or describe what you see.

Figure 9-10. A loose (flat) gas permeable lens with pooling at lens edges. (Photo by Val Sanders.)

Figure 9-11. A tight (steep) gas permeable lens with no dye at lens edges. (Photo by Val Sanders.)

Figure 9-10. A loose (flat) gas permeable lens with pooling at lens edges. (Photo by Val Sanders.)

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