CORNEAL GRAFTS

Fitting Corneal Grafts

Custom multi-curve lenses are often required to achieve good fit and vision for these irregular corneas.

By Costas F. Katsoulos, Nick H. Vasileiou, Lefteris S. Karageorgiadis, OD, & Theodore T. Mousafeiropoulos, OD

Keratoplasty is sometimes the last resort for a variety of corneal conditions. Corneal scarring that limits best-corrected visual acuity, opacities on the cornea (congenital or acquired), dermoid cysts, chemical or thermal burns, trauma, bullous keratopathy, Terrien's marginal degeneration, recurrent pterygium or infected corneas that do not respond to treatment are but a few of the pathologies for which keratoplasty might be the last remaining remedy.

The keratoplasty procedure of choice depends on the underlying pathology and the area of the eye involved. If only the superficial cornea is affected, lamellar keratoplasty is usually the procedure of choice. If the defect is deep in the stroma, the surgeon can opt for deep lamellar keratoplasty; whereas in conditions affecting the whole corneal thickness, penetrating keratoplasty is performed. In cases of extreme peripheral thinning, techniques such as corneal–scleral keratoplasty can be attempted. If the pathology affects areas beyond the cornea, such as the limbus in cases of recurring pterygium, the only option might be corneal transplantation along with limbal stem cell transplantation.

More than 45,000 corneal transplantation procedures are performed each year in the United States, with a success rate of more than 90 percent. Rejection rate is also low due to the lack of blood vessels in the cornea, which limits immunologic reaction, and most grafts survive for about 20-to-30 years.

However, keratoplasty often results in largely irregular corneal surfaces that sometimes demonstrate immense amounts of irregular astigmatism. This makes contact lens fitting challenging, both in terms of finding the best fit and in restoring visual acuity.

Fitting Considerations and Vision Correction

The postoperative shape of the cornea is heavily dependent on the suture technique of the surgeon. Factors such as the number of sutures, insertion finesse, and time and order of removal all have an enormous impact on the corneal topography. These corneas typically exhibit large amounts of astigmatism, which is mainly irregular. The result is a higher-order aberration called coma, which is rotationally asymmetrical in nature. This can result from the possible tilt of the graft at one side. Spectacles cannot correct coma, and the correction of large amounts of astigmatism is also problematic because of increased thickness and poor aesthetics of the lens.

A rule of optical engineering states that aberrations should be corrected at the surface at which they originate. If a second surface in another position is employed to correct them, aberrations of the same type but of higher order may arise. In this regard, contact lenses may be a much better option than spectacle lenses because contact lenses are in contact with the cornea. Spectacle lenses are positioned at the vertex distance, and even though they may correct astigmatism, secondary astigmatism will appear.

GP lenses in particular are highly indicated, as the tear layer between the cornea and the lens optically neutralizes the abnormal surface of the graft, resulting in the best possible visual acuity and a less extreme over-refraction. Hydrogel contact lenses cannot correct higher-order aberrations as effectively, and they depend on increased thickness to mask them. Therefore, acuities of 20/20 are rarely obtained with hydrogel lenses on such corneas.

Post-graft contact lenses, both hydrogel and GP, are similar to keratoconic designs, with the exception of a larger diameter for GP designs to ensure that they cover the whole graft. GPs can be as large as 11mm, whereas soft lenses may have diameters of 15mm or even 16mm to improve stability.

Because of the highly irregular corneal surface, contact lenses with multiple peripheral curves are usually preferred to bicurve designs, with toric geometries or peripheries and possibly reverse curves if an area of the graft is greatly elevated (a very possible scenario). Lenses with decentered optical zones and piggyback fits with a silicone hydrogel to maximize oxygen supply to the cornea are other possible options.

If GP intolerance occurs because of lid sensitivity to the lens edge, a hybrid lens (GP center, hydrogel periphery) will usually solve the problem. However, use large-molecular-weight fluorescein during the fitting evaluation, as regular fluorescein will stain the soft skirt of such lenses and give the false impression of a flat lens edge curve. In highly ectatic grafts, a soft lens that drapes over the cornea might succeed where GPs fail.

Despite these options, you will likely have to accept a compromised fit due to the irregularity of the cornea. This means that both you and the surgeon will need to closely monitor the patient during follow-up visits. Complications in such cases have the potential to result in graft rejection, which makes close scrutiny of the cornea in each follow-up visit essential.

Case Study #1

Patient #1, a 54-year-old male high school teacher, had received corneal grafts for both eyes about 20 years ago due to keratoconus. He had worn various types of GP lenses in the past. On his last visit, he reported good visual acuity with his contact lenses but increased sensation due to lack of lens stability. His topographies showed high amounts of irregular astigmatism with areas of extreme flatness and steepness very close to each other (Figures 1 and 2).

Figure 1. Topography OD for Patient #1.

Figure 2. Topography OS for Patient #1.

Based on the corneal topography, we tried various custom-made trial GP lenses until a satisfactory fit was achieved in both eyes with custom tetracurve corneal GP lenses from Eyeart Laboratories (www.eyeart.org). The details of these reverse geometry lenses were as follows: 7.4mm base curve radius, secondary curve radius/width (SCR/W): 7.1mm/0.25mm; intermediate curve radius/width (ICR/W): 8.9mm/1.5mm; peripheral curve radius/width (PCR/W) 10.2mm/0.4mm for the right eye and 7.4mm base curve radius, SCR/W 7.3mm/0.3mm; ICR/W: 8.9mm/1.45mm; PCR/W 10.2mm/0.4mm.

These lenses provided a good fit, and based on over-refraction were ordered with powers of –14.50D OD and OS with a front-surface optical zone of 7.50mm to cover the scotopic pupil. To reduce lid sensation, we ordered the lenses in the hybrid Harmony material (Vista optics, www.vista-optics.com), which has an 8mm GP central zone with a hydrogel material skirt. Figures 3 and 4 show the final fits, which achieved acuities of almost 20/20 in both eyes. The transition between the GP and hydrogel parts of the lens is visible in the right eye, and the excess of fluorescence at the lens edge is due to the use of low-molecular-weight fluorescein, which stained the hydrogel material.

Figure 3. Fitting relationship OD for Patient #1.

Figure 4. Fitting relationship OS for Patient #1.

Case Study #2

Patient #2 is a 30-year-old office clerk. Six years ago he underwent a corneal transplantation procedure in his right eye due to severe keratoconus. Due to cataract, he also had a monofocal IOL implanted in the same eye. Figure 5 shows the graft from his right eye, and Figure 6 shows the corneal topography in which the tilt of the graft is evident. He already wore GP lenses, but because of the monofocal intraocular lens and the lack of accommodation, his near vision was far from satisfactory.

Figure 5. Graft in right eye of Patient #2.

Figure 6. Topography OD for Patient #2.

We decided to design and fit a custom GP multifocal contact lens. After many diagnostic lenses, we achieved the best results with a custom-made inverse tricurve Omega Far Post PK Inv lens from Eyeart Laboratories. The details of this reverse geometry contact lens were as follows: OD 7.2mm base curve radius, SCR/W: 7.0mm/0.25mm; PCR/W 8.2mm/0.6mm, manufactured in Optimum Extreme (Contamac) material.

The optical zone of this lens was multifocal, with a central zone of 5.50mm with –1.50D power for distance vision and an aspheric transition up to 8.50mm, which provided an add of +2.00D. We selected the power of the add and the width of the progressive zone by measuring the photopic and scotopic pupil and observing the upward movement of the contact lens at downward gaze when the patient was reading. This lens provided 20/25 distance vision and, more importantly, J2 at near. Figure 7 shows the fitting relationship. The lens edge touches the cornea nasally, in areas which correlate with the tilt seen on the topography. Although vision is more than satisfactory, we need to closely monitor this patient to avoid complications. CLS

Figure 7. Fitting relationship OD for Patient #2.