contact lens primer
We Have Edge Lift
BY TIMOTHY B. EDRINGTON, OD, MS, FAAO, & JOSEPH T. BARR, OD, MS, FAAO
Adequate edge lift and peripheral clearance are necessary for successful rigid contact lens wear. Edge lift is the distance between the imaginary arc formed by a continuation of the base curve radius and the peripheral curve. Most authors state that an axial edge lift of approximately 0.10 mm is ideal. Well, that's interesting but not very user-friendly. What is more important clinically is the amount of peripheral clearance observed upon fluorescein evaluation. Edge lift calculations do not take the corneal topography or eccentricity into account. Fluorescein interpretation of peripheral clearance does.
Too Little Clearance
If peripheral clearance is minimal or absent, tear and debris exchange may be insufficient. This could lead to central and peripheral corneal staining and decreased lens comfort. Minimal peripheral clearance is more common when lenses are fitted steep. It is also more common with spherical corneas. The slit lamp examination may reveal indentation rings or corneal chafing corresponding to the position of the lens edge.
Too Much Clearance
Excessive peripheral clearance may break apart the tear film surrounding the lens edge, forming bubbles or a gap. Excessive edge lift might also lead to increased lens movement and/or lens decentration. Lens discomfort may also result from excessive edge lift because the eyelids are more exposed to the lens edge.
Figure 1. Acceptable to minimal edge
Optimal peripheral clearance will allow for proper tear and debris exchange, capillary attraction of the tears and good comfort.
3 and 9 O'clock Staining
Holden (Tom, not Brien) et al reported on the effect of peripheral clearance on 3 and 9 o'clock staining. They fit 16 patients with an alignment base curve and varied the peripheral curve radii to obtain different amounts of edge lift. Secondary curve radii were calculated and changed to obtain axial edge lifts of 0.08mm, 0.10mm and 0.12mm. The overall diameter of the lenses was 9.0mm (intrapalpebral fitting relationship). One of the16 patients developed 3 and 9 staining with the 0.08mm edge lift design. Six of 14 patients developed 3 and 9 staining with the 0.10mm edge lift. The remaining eight patients developed 3 and 9 staining with the 0.12mm edge lift lens. After the 14 remaining patients were refitted into the 0.08mm design, 3 and 9 staining was eliminated.
Holden's study concluded that edge lift may be modified to decrease the incidence of 3 and 9 staining. If their results were due to better lid apposition to the peripheral cornea, the edge thickness of the lens would also contribute to lid lift. A thick or poorly formed edge could also inhibit proper blinking.
Contrary to the above study, Andrasko and Schnider recommend increasing edge lift by flattening the peripheral curve system or increasing peripheral curve width to decrease 3 and 9 staining.
Edge lift and peripheral clearance are also affected by parameters other than just the peripheral curve system. Overall diameter, optic zone diameter, peripheral curve radii and peripheral curve widths all affect edge lift.
Despite conflicting studies on the effect of edge lift on 3 and 9 staining, most practitioners agree that optimal edge clearance enhances tear exchange and patient comfort.
Dr. Edrington is a professor and in the contact lens service at the Southern California College of Optometry. E-mail him at email@example.com.
Dr. Barr is editor of Contact Lens Spectrum and assistant dean for clinical affairs at The Ohio State University.