Advancements in contact lens technology makes a case for piggyback fitting.
Piggybacking lenses has been around for decades. Articles and lectures on the topic of slipping a soft lens behind a GP lens were being presented by CLSA members more than 30 years ago. The idea was another tool in the toolbox for fitting irregular corneas, but it really never went mainstream.
Now we have high DK soft lenses that can be fitted with greater central thickness (even custom) to improve the comfort, fit and vision that a corneal GP lens offers”¦when fitting those irregular corneal challenges. With the availability of customized lenses in silicone hydrogel (higher DK materials), piggyback fitting might just become a modality that is selected more often by the practitioner.
FITTING CORNEAL GP LENSES
Think larger diameter GP lenses for irregular corneas when fitting corneal GP lenses. Lens centration and preventing lens tilt are essential when trying to provide the best fit, the best comfort and the best visual acuity possible for the patient. When a GP lens is tilted on the cornea (“œZ” axis tilt), uneven tears between the back surface of the lens and the cornea occur. This can induce a significant amount of residual cylinder and/or aberrations that result in less than optimal visual acuity. Larger diameter lenses can center better, and when properly designed, can reduce or eliminate lens tilt on the cornea.
Choosing 10.0mm to 11.5mm diameters in a corneal GP lens for irregular cornea fits can be a good starting point. Be careful not to make the optic zone too large in these large diameter lenses. This results in more chances for an incorrect fit and can create an uneven tear layer behind the lens if the OZ is too large. There can be as much as 5.00D, 10.00D, 20.00D or more difference in the steep portions and flat portions of the corneal shape within a 9mm area, and making the OZ in sizes of 8mm, 9mm or greater can cause real fit and uneven tear issues
The topography in Figure 1 (above) shows how difficult it would be to make an 8mm to 9mm spherical OZ fit evenly on this very irregular cornea. Keep in mind that the total amount of cornea being viewed in this topography is about 9mm. Uneven tear and lens tilt are less likely if the OZ is kept smaller. The standard post-surgical OZ for LASIK is 6mm, and very few patients complain of ghosting or halos with this smaller OZ size. The secret in making certain the 6mm to 7mm OZ works well is to make certain that the lens positions perfectly centered over the pupil and the lens is not tilted on the cornea.
The use of reverse geometry in GP lenses has been a very useful tool when fitting irregular corneas. With the secondary curve being made steeper than the base curve, lens tilt is reduced and lens centration is improved. This will produce some additional central vault (additional tear volume) at the optic center of the lens, which should not create an issue with the patient”™s vision. The reverse curve prevents lens tilt, which in turn allows for good visual correction. The amount of tear volume that is acceptable for best fit and vision is between 20 and 40 microns. We know that scleral lenses produce great vision with between 150 to 300 microns of tear volume centrally, so some additional tears doesn”™t prevent good visual results as long as the tear volume is uniform.
Figure 2 (below) is an example of a good fit and a well centered GP lens with no lens tilt in a reverse geometry design on a patient with keratoconus.
Whenever using NaFl to evaluate a GP, all darkness doesn”™t indicate corneal “œtouch” as we often refer to it. Darkness indicates that less than 20 microns of fluorescent tears are present for us to view anything less than 20 microns, which just appears to be dark to our human eye. If the contact lens truly touched the cornea, then there would be evidence of desiccation where the touch occurred.
WHEN PIGGYBACKING IS NEEDED
When everything you have done doesn”™t result in an acceptable GP corneal lens fit and there are comfort issues, reduced wearing time and possible corneal insult, consider a piggyback modality. No need to redesign the GP lens you have ordered; first try your current GP lens over the top of a soft lens and see if the fit, comfort and vision are improved. Any needed adjustments in the GP fit and/or power changes can be accomplished when evaluating this new piggyback modality.
Figure 3 (above) is a patient”™s topography where a corneal lens fit never was found to work by itself. Small diameter, large diameter and large diameter reverse geometry GP lenses all wanted to position inferiorly, move loosely and tilt forward on this cornea. Figure 4 (below) is an attempted fit with this patient shown in the topography using a large diameter reverse geometry lens without a soft lens behind it.
Fig. 5 (below) shows a good piggyback fit, well centered after placing a +5.00 high modulus soft lens behind a reverse geometry GP lens. The patient is now 20/20 and is wearing this modality 14 hours a day very comfortably.
PIGGYBACK FITTING TODAY
Today we have soft and GP lenses with high DK values of 60, 80, and 100+. Frequent replacement of quarterly, monthly and even daily soft lenses has become a routine item in soft lens delivery systems.
It goes without saying that the higher delivery of oxygen to the cornea is the healthiest approach when using the piggyback
Frequent replacement means a minimum of quarterly replacement and preferably even more often for keeping the cornea as healthy as possible.
Why keep the center thickness as thick as what a high plus power soft lens might have? High plus powers (+5.00D or so) are thicker and the GP lens neutralizes almost all of the plus power (80% or so) when placed on top of the +5.00D soft lens, so minimal power adjustments are needed in the GP lens.
With a new improved surface that the soft lens provides for the GP lens to ride on, the GP lens can move more freely; the fit should become better centered, the lens will be more comfortable and vision is almost always improved.
Sometimes a trial soft lens will not have the parameters needed to fit the patient when the cornea is extremely irregular or extremely steep, so custom soft lenses are required to be the carrier for the GP lens. These lenses are available and can be custom made in every respect. Base curve, center thickness, power, diameter, peripheral curve design and lens material are all options that can be tailored to fit some very challenging corneal shapes.
Jim Slightom, ABOM, FCLSA has been an optician and contact lens technician for over 38 years. He is currently a contact lens consultant at ABB Optical Group.
This article was adapted with permission by the Contact Lens Society of America. It originally appeared in the Spring 2016 issue of EyeWitness magazine.