POST-PK FITTING
Contact Lens Stability After Penetrating Keratoplasty
Understanding the factors that affect post-PK lens stability will help you better fit these transplant patients.
By Randall S. Collins, OD, FAAO, Anthony J. Jarecke, OD, and Ryan Traver, OD, FAAO

Post-surgical penetrating keratoplasty (PK) patients offer us one of our more challenging ­ and rewarding ­ contact lens fitting opportunities. There is plenty of information in texts, articles and guides about fitting options, and our opinions differ very little from what we read. However, in our setting as a teaching institution, we find that externs, residents and even staff doctors may know the options, but do not understand why certain options are better than others. We believe an illustrated discussion of factors affecting contact lens stability can help you cut chair time and achieve greater patient satisfaction in these post-surgical, irregular corneas.

Back to the Basics

Transplanted corneas frequently exhibit a certain degree of irregularity, and most clinicians agree that rigid lenses generally offer the best hope of good vision.

One of our contact lens mentors, Dr. Frank Graf, was known for his tongue-in-cheek statement that one of the biggest obstacles in fitting gas permeable lenses on a post-PK cornea is "lens retention." If you've managed an oblate cornea that drops off rapidly at the edge of the graft, you probably know what he's talking about. Lens retention becomes less ominous and the numerous PK-fitting recommendations make some sense once we understand three forces that influence lens position and stability: contour conformation, surface tension and lid pressure.

Figure 1. An intrapalpebral rigid lens naturally settles where it best conforms to corneal curvature.

Contour Conformation

A common example of contour conformation is a GP intrapalpebral fit on a normal cornea. Many times we have asked residents and externs why a well-fitting lens centers well on a normal cornea. One resident offered the best and simplest answer: "Because that's where it fits." A contact lens seeks the ocular surface area that most closely matches its base curve. The intrapalpebral lens moves with blinks but returns to the steeper apex, resisting displacement to the flatter peripheral cornea where the fit is too steep (Figure 1). This is basic contact lens science.

Advances in GP lens manufacturing today allow us to order GP lenses with little more than diameter, base curve and power. Nomograms and computer-driven lathes can compute the rest, including intermediate and peripheral curves. As a result, clinicians today seem to have less understanding of the relationship between lens curvature and normal corneal topography.

Unfortunately, normal topography is not present in most PK patients. The graft tissue "button" is usually cut slightly larger than the host tissue opening to prevent leaking at the graft/host junction during healing. Most corneal surgeons remove all sutures after one year. At that time, the disparity between the size of the graft and the opening in the host tissue often causes the graft periphery to be much steeper than the visual axis area. Most references refer to this shape as an oblate surface (Figure 2). A simple, spherical GP contact lens on that cornea often fails due to lens displacement or dislodging. Without a flatter peripheral contour present to help hold that contact lens in place, it could go anywhere (Figure 3). Three fitting options use contour conformation to help keep a lens on this eye.

Figure 2. An oblate cornea results when the graft buttons sits in a host opening of a smaller diameter.	Figure 3. Conventional GPs often show edge stand off. RIGHT: Without a flatter midperipheral cornea to hold the lens in place, a GP often displaces easily.

Large Diameter GP A large diameter GP contact lens bridges the graft/host junction and uses the flatter contour of the peripheral host cornea to help center the lens (Figure 4). This can leave a shallow space for potential bubble formation, but most clinicians agree this is of little concern. The lens needs to be large enough to bridge the "trench" and maintain contact with the host cornea, typically at least 10mm in diameter because the graft button is usually about 7.5mm. We usually start at 10.5mm.

Reverse Geometry Lens A reverse geometry lens (RGL) seeks to conform to the graft button's shape. When it closely matches the contour of the graft's steep peripheral slope, this lens usually does well (Figure 5).Choose a base curve similar to that determined with normal fitting criteria. The diameter of the optical zone should extend about 1.0mm beyond where you estimate the steeper part of the graft begins (usually about 5.5mm to 6.0mm) to prevent it from fitting like a tight cap. The reverse curves' radius and width depend on each graft. In our experience, RGLs that fail do so because the reverse curves were not steep enough to conform to the contour of the graft.

Figure 4. Bubbles retained in the "trench" of a large diameter GP contact lens are not usually a problem. RIGHT: A large-diameter GP can bridge the steep graft/host junction and use the peripheral host tissue for stabilization.

Figure 5. Reverse geometry lenses can conform to the steep periphery of the graft "button." RIGHT: A reverse geometry lens can be made to conform closely to the steep arc of the graft/host junction.

Back-surface Toric Lens Most conventional rigid lenses on an oblate cornea displace downward due to gravity, so it may be a touch of good luck to find the graft with approximately 2.50D or 3.00D of with-the-rule (WTR) astigmatism. A back-surface toric lens that saddles the astigmatic cornea improves contour conformation and enhances the effects of surface tension and lid attachment.

	Figure 6. A steep lens like this will likely cause mechanical irritation to the graft and could even cause lens binding. Right: With a steep lens, the tear pool will try to flatten to a uniform depth of 4.0µm while maintaining surface tension. This draws the lens "tight" to the cornea

Surface Tension

Surface tension, as it relates to contact lenses, is the force the tear film exerts on a contact lens to keep it against the eye. The human tear film is usually about 4.0µm thick. A contact lens that fits parallel to the cornea is sandwiched between two tear layers, each of which is approximately 4.0µm thick. We all know that pooling results if a lens vaults the central cornea. A "tight" lens results when this tear pool obeys two laws of physics: the film maintains contact with the back surface of the lens while also attempting to flatten to the ideal homeostatic thickness of 4.0µm. This force pulls the contact lens tighter to the cornea, potentially resulting in peripheral lens binding (Figure 6). If a lens fits flat, the thicker tear film at the edge still tries to achieve its optimum depth of 4.0µm. Under this circumstance, surface tension draws the contact lens to bear on the central cornea while contributing to lens instability. (Figure 7a). The lens becomes considerably less stable when the tear film continuity breaks due to excessive edge lift, which allows air beneath the lens (Figure 7b). This surface tension force is far less difficult to manage with hydrogel materials because they allow the tear film to easily maintain homeostasis.

Figure 7a. The tear pooling at the edge of a flat lens draws the lens to bear on the central cornea.	Figure 7b. The break in the tear film at the inferior edge of the lens detracts from its stability. RIGHT: That same flat lens may show a break in the tear film at the inferior edge. Air beneath the lens makes that lens considerably less stable.

If we apply these principles to PK corneas, we can understand more of the suggested fitting options. If your goal is to maintain optimum surface tension, clearly soft contact lenses have the best chance. Soft lenses are an option if corneal irregularity is not excessive. If there is significant corneal surface distortion, hybrid lenses can maintain surface tension with the soft skirt while masking the corneal irregularity like a GP lens. Piggyback lenses do the same. Naturally we need to keep in mind the potential for hypoxia with these options and monitor patients properly. Once again, small bubbles that remain trapped in the "trench" at the graft/host junction appear to be of little concern as long as they do not migrate into the visual axis.

Most clinicians first try spherical GP contact lenses on oblate corneas. We do too. However, the most common reason for failure with spherical GPs is the break in tear continuity with excessive standoff and loss of surface tension at the graft/host junction. Therein lies the problem of "lens retention." Reverse geometry lenses bring that edge closer to the surface of the eye, giving a continuous tear film a chance.

In some literature we read that fitting a small diameter contact lens on these corneas has been widely abandoned. We do not discard the option. If a spherical GP lens of average diameter (9.0mm to 9.6mm) usually maintains a high position but displaces occasionally due to a small amount of inferior edge standoff, decreasing the lens size by a millimeter or so may help. As long as the lens maintains the high position, there is less material to hang off the graft's inferior edge and the break in the tear film continuity may disappear (Figure 8a and 8b). We have succeeded with contact lenses about 8.4mm to 8.8mm in diameter in these cases.

Figure 8b. No stand off with a lens 0.8mm smaller in diameter. RIGHT: A small-diameter GP may prevent the unacceptable inferior edge lift frequently seen on oblate grafts.

Figure 8a. Some inferior edge stand off. RIGHT: A small-diameter GP may prevent the unacceptable inferior edge lift frequently seen on oblate grafts.

Lid Pressure

Even though lid pressure is the simplest of the three concepts we're discussing, it is often overlooked. Lid pressure can work for you or against you. Lid attachment works with both contour conformation and surface tension to help certain contact lenses succeed. For example, the success of smaller GP lenses in avoiding edge standoff at the graft's inferior part depends heavily on consistent lid attachment. Naturally, minus and lenticulated contact lenses are held up better by lid pressure. Lid pressure can work against you in plus power and thin edge contact lenses by squeezing the lens from beneath the lid.

We previously mentioned that a back surface toric contact lens is likely to be more stable than a spherical GP lens on a graft with WTR astigmatism. This is due, at least in part, to the thicker superior edge of the lens. Lid pressure has an impact on refractive issues as well. For example, if there is a significant degree of against the rule corneal cylinder, lid pressure on spherical GPs or hybrid lenses is likely to result in more lens flexure and subsequent residual astigmatism than surface tension can produce alone. In such a case, less flexible materials or a back surface toric design may help counteract that flexure.

Understanding the Forces

None of the forces we've discussed here stand alone in the success or failure of any lens. Understanding these forces and the way they work in concert will help you make the most of your chair time with transplant patients. These patients are some of the most rewarding contact lens cases we encounter. Becoming proficient in managing them is worth the time and effort.

The authors wish to thank their medical illustrator, Justin Hanson.

References are available upon request to the editors of Contact Lens Spectrum. To receive references via fax, call (800) 239-4684 and request document #88. (Have a fax number ready.)