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Consider lid geometry when fitting RGP lenses
This aspect of fitting can influence lens performance more than the lens-to-cornea relationship.
When evaluating a rigid gas-permeable (RGP) contact lens on a patient’s eye, the preferred fluorescein pattern to obtain is the alignment fit. However, because lens performance is often heavily influenced by the patient’s lid configuration, geometry or tension, an alignment fit does not always guarantee good centration and comfort. Most of the computer topographic contact lens fitting programs available today do not consider lid geometry.
Ideally, fitters should strive for a well-centered lens combined with an alignment fluorescein pattern. Unfortunately, this cannot always be achieved because the lids can drastically alter centration. Lens parameters such as diameter and base curve must be adjusted to optimize performance.
The two most common fitting philosophies are “interpalpebral fitting” and “lid attachment.” The interpalpebral philosophy features a small diameter (e.g., 8.2 to 8.8 mm) lens, which is well centered with apical clearance (one-third steeper than flat K). For the interpalpebral fit, the lens diameter can equal the vertical lid aperture, less 0.20 mm.
The lid attachment philosophy features a larger diameter (e.g., 8.8 to 9.6 mm or larger) lens with a flatter base curve (0.25 D flatter than flat K). The fitter should choose the philosophy according to the lid geometry.
Evaluate upper, lower lid positions
---Four combinations: The eyelid geometry can be classified in four possible combinations: narrow, ideal, unusual and wide aperture.
To assess lid geometry, evaluate both the upper and lower lid positions with respect to the corneal limbus. When the upper lid covers the limbus slightly, the positioning is considered normal or low. If the superior lid is at the upper limbus or above it, it would be high.
For the inferior lid, a margin adjacent to or below the limbus is probably considered normal or low. If the inferior lid significantly covers the limbus, then the lower lid position is high. The eyelid geometry can be classified in four possible combinations: narrow, ideal, unusual and wide aperture.
Three forces usually determine lens centration: upper lid, gravitational and centering. Upper lid force is normally the strongest of the three. It comes into play when there is lid attachment. Lid attachment keeps the lens in place and overcomes the gravitational force. Upper lid interaction should be obtained where there is a low or normally positioned upper lid. If the lower lid is also in the normal or lower position, the largest diameter lens should be fit to maximize comfort and centration.
Centering forces
Centering forces come from the lens-to-cornea relationship. Steeper lenses have greater centering forces than flatter lenses. If the superior lid is high, then a lid attachment form of fitting may not be possible, and an interpalpebral fitting will more likely be successful. A smaller lens diameter is used for interpalpebral fitting.
Because the upper lid force is not used, centering forces are usually at work. Interpalpebral lenses are, therefore, fitted steep to take full advantage of the centering forces. This is why “small and steep” is another way to refer to interpalpebral fitting.
Narrow aperture
---Narrow aperture: This is characterized with a low upper lid and high lower lid. In this case, the lens diameter is 8.8 mm; it did not have to be fitted with the typically steep fit. An interpalpebral fit was used because there was great upper lid force in this patient and a large amount of lid tension. Unfortunately, a true lid attachment fit would cause this lens to ride superior. The diameter was decreased to reduce the amount of upper lid contact.
A narrow aperture is characterized with a low upper lid and high lower lid. The fit can be referred to as interpalpebral, but there is some upper lid overlap due to the geometry. Although the lens contacts with the upper lid, it resides primarily between the lids. For tight lids, such as the Asian eye, a reduced edge width or lift can be helpful.
Ideal aperture
---Ideal aperture: This is characterized with a low upper lid and low lower lid. The lower lid is located far below the lens, allowing the upper lid to hold it in place without interference.
An ideal aperture is characterized with a low upper lid and low lower lid. Because the upper lid is low, lid attachment can be achieved rather easily. A large, 10.0-mm diameter lens can be used because of its good corneal coverage. The lower lid is located far below the lens, allowing the upper lid to hold the lens in place without interference.
Donald Korb, OD, favored lid attachment because the lens becomes a part of the upper lid. The optimal contact lens is defined as an additional thin layer attached to the tear film. Because the tear film moves with the lid, the lens must move as if it were part of the upper lid. Attachment facilitates wetting, decreases staining and enhances blinking. RGP lenses allow the practitioner the option to fit with larger diameters.
In the past, with PMMA lenses, larger diameters were not advisable because of oxygen deprivation concerns.
Unusual aperture
---Unusual aperture: This is characterized with a high upper lid and high lower lid. In this case, a 9.6-mm diameter lens gave the patient good comfort and centration. The aperture was unusual because the upper lid is normally lower and the lower lid is normally higher.
An unusual aperture is characterized with a high upper lid and high lower lid. The aperture is unusual because the upper lid is normally lower and the lower lid is normally higher.
Wide aperture
---Wide aperture: This is characterized with a high upper lid and low lower lid. In this case, centration was achieved with a 9.5-mm diameter lens. Fortunately, the high upper lid was not needed to help center the lens. The centering force of the steep fit was enough to keep the lens centered. Normally, an alignment fit is more desirable and offers a better fit. In this case, where upper lid forces cannot be used easily, a steep fit was the best compromise.
A wide aperture is characterized with a high upper lid and low lower lid. Normally, an alignment fit is more desirable and offers a better fit. If the upper lid forces cannot be used easily, a steep fit may be the best compromise.
Lens edge-to-lid relationship
Another important consideration is the lens edge-to-lid relationship. Using lenticular designs is helpful in optimizing lens performance. For all plus lenses, increasing edge thickness with a minus lenticular is suggested. For higher minus lenses, such as -4 D and above, decreasing edge thickness with a plus lenticular is advantageous. Lenticular design also decreases the total mass in lenses.
When a lens does not center, pay special attention to the lid geometry. Most computer fitting programs based on topography do not take lid effects into consideration. As shown in previous examples, lid geometry can influence lens performance more than the lens-to-cornea relationship. Sometimes, we need to change the apical relationship and “fit the lids.”
Editor’s note:
This article was previously published in Primary Care Optometry News, a SLACK, Incorporated publication.
For Your Information:
- Adrian S. Bruce, BScOptom, PhD, FAAO, can be reached at Victorian College of Optometry, University of Melbourne, Corner Keppel and Cardigan Streets, Carlton VIC 3053, Australia; e-mail: abruce@mail.optometry.unimelb.edu.au.
- Manual of Contact Lens Prescribing and Fitting With CD-ROM. Revised edition. Woburn, MA: Butterworth-Heinemann; 2000.