Managing Complex Corneal Shapes with Corneal (R)GPs
Gustav Pöltner Dipl.-Ing-FH
Fitting contact lenses after a corneal transplant or after refracting surgery is one of the most delicate procedures in our profession. This case report describes the fitting of a corneal lens, but not just a standard lens: a four-curve-toric-oblong rigid corneal contact lens with reverse curves and ventilation holes is fit on a post-penetrating keratoplasty patient, with very satisfying results (Figure 1, below).
A Caucasian 35-year-old male with a history of penetrating keratoplasty in 1994 (OD) and 1997 (OS) due to keratoconus presented to our clinic. The transplants on both eyes were clear. General eye exams are performed every six months with an ophthalmologist and optometrist to monitor his condition.
OD: -1.75 –5.25 x 77° V=0.8
OD: -1.50 –1.00 x 108° V=1.0
Focusing on the right eye (Figure 2, below), the corneal geometry showed very different values in every half-meridian, with eccentricities between -0.68 and -1.20. The central against-the-rule astigmatism was 5.40D at 76°. The value of irregularity as shown by the aberration-coefficient ABR is 2.8. This shows the same magnitude of irregularity similar to a keratoconus grade II – III.
Figures 3 and 4 (below) shows the cornea from the 2-D Zernike analyses. This is the basis for the calculation of the simulated fluorescein picture. The main bearing area of the contact lens will be located in the purple area, at 300° to 330° on the cornea. The center of the transplant looks saddle-shaped. This means that we will see a central fluorescein pool with a (R)GP lens in place.
OCT thickness measurement
To illustrate the differences in height between different meridians, OCT measurement of the corneal and post-lens tear film thickness behind a corneal (R)GP lens was performed with the Zeiss-VisanteTM-OCT. In the center, the cornea had a normal thickness of 530µm after the transplantation.
But more interestingly from a contact lens perspective was the thickness of the tear film: more specifically, the tear film thickness behind the lens in the 90° meridian. From the edge of the graft junction, the tear film thickness increases from 127µm to 137µm and further on to 275µm at the edge of the contact lens (Figure 5, below left). The tear film at the 180° meridian was thinner: the thickness increased here from 72µm to 85µm, and on the edge of the contact lens it is was 222µm (Figure 6, below right). The higher tear film thickness value toward the periphery is essential to achieve dynamic movement of the contact lens. The result will be a continuous movement of the lens over the ocular surface, which is one of the main criteria for a comfortable and healthy fit on post-keratoplasty eyes.
The corneal geometry after a corneal transplant typically has a flat central zone with a steeper periphery. The ideal fluorecein pattern should align centrally with a soft touch in the periphery. The contact lens geometry should have a spheric or toric central curve, followed by an oblong shape and then a reverse curve before entering the landing zone area.
The corneal topographer used in this case report calculates the lens geometry to create a simulated fluorescein pattern (Figure 7, to right). It is important to understand that with an oblong corneal shape, the calculation software can make mistakes, based on false assumptions that it makes. In our example, the calculated ideal lens was supposed to have back surface toric radii of 7.85/7.05mm. The prescribed radii in the end were 7.80/7.45mm in this case. This shows that it is important to have good contact lens calculation software, but also that an experienced contact lens specialist is needed to evaluate the simulation for the best and most acceptable end result.
Contact lens fit:
The first diagnostic lens in this case was a spherical oblong lens with a reverse curve. The central radius was 7.8mm, and this rotationally symmetric lens showed an alignment fluorecein pattern in the flatter meridian. Using this information, a toric test lens was ordered with base curves of 7.8mm/7.05mm. The fluorescein pattern of this test lens was evaluated as too steep in the steeper meridian. The prescribed lens ordered was: OD 7.80/7.45 / -1.75D / -3.75D / ø 10.2mm (Figure 8, to right).
At dispensing, the ordered lens showed some air bubbles superiorly, very close to the graft junction, leaving some dimples in the corneal epithelium after wear. To counter this, ventilation holes were placed. You can drill these holes by hand or order them from the manufacturer.
With some manufacturers’ software, like the “Hecht-APEX-program“ that we used, the exactly coordinates for the ventilation holes can be determined (Figure 9, below). The ideal places for the four ventilation holes in this case were 110°, 100°, 90°, and 80°. After incorporating the ventilation holes, air bubbles or dimples were never seen again.
The fitted lens provided a visual acuity of 1.25. The lens was comfortable and could be worn 16 hours per day. Crucial for comfort was the continuous movement of the lens and a material with a good wettability (Optimum Extra, Contamac).
Corneal (R)GP lenses are still considered one of the best and healthiest options for the eye, even in more complex cases. However, the lens fit can be more challenging. This case report presents a contact lens fit on a post-keratoplasty cornea with a bitoric-oblong-four-curve lens with a reverse curve and ventilation holes. The case shows that ‘going the extra mile’ to achieve a good lens fit can result in excellent visual acuities, long wearing times (16 hours per day) and very satisfied patients. Keeping up the skills needed to fit this type of lens is crucial – and experienced eye care practitioners and schools together have an important role in teaching the new generation.
Gustav Pöltner (Dipl.-Ing-FH) is a renowned practitioner and educator in the field of contact lenses.
He works at his contact lens institute in Innsbruck, Austria, where he fits very challenging eyes. He teaches at the Fachhochsule-Gesundheit-Tirol in Innsbruck, the HTL-Kolleg-Optometrie in Tirol (Austria) and at the University of Applied Sciences in Velika Gorica in Croatia.
Gustav is on the Board of Examiners of the module Contact Lenses for the ECOO- European Diploma in Optometry.