Lid Attachment or Interpalpebral Fitting?

BY EDWARD S. BENNETT, O.D., M.S. ED.
DEC. 1997

Should the RGP lenses you prescribe employ an interpalpebral or lid attachment fitting relationship? It's one of the ongoing debates among contact lens practitioners. Instead of trying to answer this question, be sure you understand the differences between each and know which fitting factors demand more emphasis.

LOOKING AT THE DIFFERENCES

The lid attachment concept was made popular by Korb. His Polycon 9.5mm diameter lens design included an edge contour that sloped toward the lid to enhance lid-lens contact, often resulting in a slightly superiorly decentered fitting relationship. This concept has been broadened to include the "tucked underneath the upper lid" fits.

An interpalpebral lens-to-cornea fitting relationship often refers to a lens that centers well and is positioned between the lids. It is often achieved with a slightly smaller overall diameter and steeper base curve radius than a lid attachment design.

Like many practitioners, I typically prefer the lid attachment approach, believing that the lens will be more comfortable if its edge is tucked underneath the upper lid. However, a recent study by Sorbara (1996) offers some interesting results. When 25 subjects were asked to choose between the two designs, 15 preferred the interpalpebral, six had no preference, and only four preferred the lid attachment design. The lid attachment design had a high axial edge lift (0.15-0.30mm) and a flat front surface carrier radius; the interpalpebral lens had a lower axial edge lift (0.11mm) and a thinner edge. The study also mentioned that using a flat base curve radius with an anterior edge apex lens design fit to achieve lid attachment may decenter inferiorally because of the high edge clearance. The interpalpebral lens induced fewer corneal topographical changes and was perceived to be more comfortable and result in better vision.

Certainly there are patients who would benefit from lid attachment designs, such as individuals with low upper lids or those requiring a large overall diameter (i.e., patients with a large pupil diameter or athletes). Likewise, patients who have an upper lid near the superior limbus or who have steep corneal curvatures would typically benefit from an interpalpebral fitting relationship.

MINIMIZE INFERIOR DECENTRATION

Aiming for either an interpalpebral or a lid attachment relationship is not as critical as minimizing inferior decentration. Assuming a good quality edge design, much of the initial lens awareness experienced by a first-time RGP wearer is the result of lens movement, notably if the lens decenters inferiorally after the blink. Flare and corneal desiccation may also result.

If a lens is decentering inferiorly, change the lens design by: reducing the diameter a minimum of 0.4mm; decreasing center thickness a minimum of 0.04mm; using a lenticular if indicated; or steepening the peripheral curve radius to decrease edge clearance.

Your goal is to optimize initial comfort, regardless of whether you're striving for lid attachment with a large diameter lens or an interpalpebral fit with a small diameter and a thin edge.

STRIVE FOR ALIGNMENT

It's also important to strive for an alignment fitting relationship. An alignment fit represents an even or uniform fluorescein pattern, not an on-K fit. Optical zone diameters in use today average 7.8 - 8.0mm, and the so-called corneal cap (the central cornea which flattens at a slower rate than the periphery) averages about 7.0mm, so an RGP lens must be fit about 0.50D flatter than K to achieve an alignment relationship. However, as the topography from cornea to cornea can vary considerably, a steeper-than-K base curve radius can sometimes yield an alignment fit. Diagnostic fitting plus fluorescein application is essential for achieving an alignment fit and for ruling out areas of harsh bearing and/or excessive clearance.

For assistance, the Contact Lens Manufacturers Association (CLMA) laboratories have trained consultants, and they can also advise on new lens designs. Contact the CLMA (800-344-9060) for laboratory locations as well as for instructional videotapes.

Don't become discouraged if every rigid lens fit isn't a perfect one. Instead, concentrate on avoiding inferior decentration while achieving as close to an alignment fitting relationship as possible. CLS

Dr. Bennett is an associate professor of optometry at the University of Missouri-St. Louis. He is executive director of the RGP Lens Institute.