Avoiding the Low Riding Lens
BY THOMAS G. QUINN, OD, MS
A decentered gas permeable lens can wreak havoc on the eye. The most threatening situation is a low riding lens, which often does not move sufficiently. Most patients with chronically low riding riding lenses display a partial blink. The inferior aspect of the lens tips in as the lens aligns with the lower cornea surface, which in turn tips out the top portion of the lens (Figure 1).
Figure 1: The tipped-out top of a low riding lens interferes with the blink.
A low riding lens promotes a partial blink and inhibits lens movement, which promotes 3 o'clock and 9 o'clock staining, conjunctival injection and dryness. Throw in some visual flare from the mal-positioned lens, and just about every aspect of the eye/lens system is compromised.
Never fear. You can take steps to promote good lens positioning and provide enhanced comfort, physiological response and visual performance.
Focus on three primary factors to properly position a lens on the corneal surface: lens mass, eyelid forces and the lens-to-cornea fitting relationship.
Lens Mass. A heavy lens has a greater potential to drop to a low corneal position. Order the thin-nest lens possible that does not compromise durability or flexural resistance.
Lenticular designs can reduce lens mass. Diminish the weight contributed by the peripheral portion of high minus lenses (greater than -4.00D) by a plus carrier lenticular design (edge thickness thinner than junction thickness). Plus lenses and, to a somewhat lesser degree, low minus lenses, have the greatest risk of dropping due to the location of their center of gravity. A regular carrier lenticular design (edge thickness equal to junction thickness) may work for low minus (-1.50D and below) and low plus lenses. On high plus lenses, try a minus carrier lenticular design (edge thickness greater than junction thickness). It not only reduces center mass, it provides a peripheral wedge for the upper lid to grasp and "attach" to, which assists in holding the lens up. Remember it this way: when increasing the center thickness of a lens, increase the edge thickness of a lenticular flange to help counter the effect.
Materials with low specific gravity also help reduce mass and may prevent a low riding lens.
Eyelid Forces. The dominant player in lens centration is the upper lid. The upper lid must cover the superior limbus to reach the lens and assist in achieving proper lens positioning. Fortunately, this lid-to-cornea relationship is present in approximately 80 percent of eyes.
Lens-to-cornea Relationship. The final player in determining lens position is the corneal curvature's relationship to the base curve of the lens. The steeper a lens is fit, the greater the role the base curve/cornea relationship plays in lens centration. Flattening this relationship accentuates the role of the upper lid.
In cases where the upper lid covers the superior limbus, design a lens large enough to reach up under the lid and flat enough to promote lid attachment. In the less common situation when the upper lid lies at or above the superior limbus, choose a diameter that fits within the palpebral aperture and is steep enough to allow the centering forces of the cornea to reign supreme.
Dr. Quinn is in group practice in Athens, Ohio, and has served as a faculty member at The Ohio State University College of Optometry.