Centering a Decentered GP Lens
BY ANN LAURENZI-JONES, OD, FAAO
The ideal fit of a GP lens is an alignment fit that centers over the optic zone of the pupil, which is usually the apex of a normal cornea. In many cases the corneal topography or lid position and tension can alter this position.When a diagnostic or habitual lens decenters, there are several parameters that can be adjusted.
Parameters to Adjust
Base Curve Always start by checking the fluorescein pattern while the lens is in its resting position, looking for central bearing or excessive apical clearance. A lens that is too steep will typically decenter inferiorly (and sometimes nasally) and needs to be flattened. Lenses that decenter temporally and/or inferiorly are usually fit too flat, and the central curve is seeking flatter ground such as that in the corneal periphery or even the sclera to center on. This base curve will need to be steepened. Base curve (BC) adjustments of ±0.50D are the minimum needed to make a significant change in the fit.
Diameter Lens diameter selection depends on pupil size and fissure width; however, the effect of lid tension on centration is also important. Average corneal GP diameter is 9.4mm, with 8.5mm to 9.00mm usually used for small fissures and 9.5mm to 10.0mm for larger fissures/pupils or looser lids. Lenses that ride low tend to be too small with inadequate lid interaction, whereas lenses that ride high have too much lid interaction because the lens is too large. Keep in mind that this decentration can affect vision, especially if the edge of the lens or optic zone is bisecting the pupil. Alterations in the overall diameter (OAD) by 0.3mm can be significant, but recall that any changes in OAD may also impact the optic zone diameter (OZD), and this may affect the BC fitting relationship: every 0.5mm OZD change = 0.25D BC change. If you find that the OAD is optimal but decentration or flare still occurs, a change in just the optic zone can help—keeping in mind that you must maintain the BC relationship.
Thickness The lens power will dictate the central and edge thickness. For powers greater than ±5.00D, this can affect lens positioning. If the lens is too thick in the center, as in high plus powers, it is too heavy for the lid to support and will drop. Replace it with an ultra thin or lenticular design (minus carrier) to reduce the center thickness and to increase the edge thickness; this increases the upper lid influence. In high-minus lenses, the edge may be too thick and the lid will push it down: tapering and rolling the edge inward may remedy this. However, a thick edge on a tight lid may also get stuck under the lid and ride high. A plus lenticular carrier (thinner edges) or CN bevel will help these myopes get a more proper lid-attachment fit.
Lens Material The specific gravity of the lens material will increase or decrease the weight of the lens. Increasing the specific gravity will help drop a superiorly riding lens, while decreasing it will help to raise an inferiorly positioned lens. Changes of 0.1 or greater are significant.
Peripheral Curve System Also assess the peripheral curves (PCs) for excessive lift or tightness. Increasing the edge lift increases lid interaction and will move the lens to a higher position. Decreasing the edge lift allows less lid interaction and will allow the lens to move to a lower position.
Changes in PC radius of 1.0mm and 0.2mm in width are needed to modify the fit.
Design Highly toric corneas need toric lenses. Highly with-the-rule corneas will cause a lens to ride low, and using a bitoric or back toric BC will center it. Also, a lens will center over the steepest curve, especially in irregular eyes. Utilizing larger intralimbal designs, decentered BCs, or prism ballasting may aid centration.
Parameter changes are not exclusive of one another, and changing one may influence the other. However, by following these rules you can usually achieve the goal of a centered lens and a happy patient. CLS
Dr. Laurenzi-Jones currently has a staff position at NorthShore University Hospital in Glenbrook, Illinois. You can reach her at email@example.com.