Designing Lenses for Advanced Keratoconus
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Designing Lenses for Advanced Keratoconus
BY LUCIANO BASTOS & MARCELO BITTENCOURT, MD
When you treat advanced keratoconus, there are certain cases in which you need to give up fitting specialty contact lenses and recommend a keratoplasty procedure. Sometimes a corneal transplant is the only viable alternative to restore a patient's vision and quality of life.
However, what is the limit? Where I practice in Brazil, the ophthalmologists who fit the GP Ultracone design (a modified Soper lens design from Brazilian manufacturer Ultralentes) can successfully fit patients who have corneas measuring up to or exceeding 60 diopters.
New Designs Push the Limits
Recent developments in the Ultracone design allowed us at the Instituto de Olhos Dr. Saul Bastos (IOSB) to fit keratoconus patients with base curve lenses as steep as 70+ diopters. We have developed a new version of the design called the Ultracone Mini-Scleral 3S GP lens. The overall diameter of this lens can be up to 16.0mm.
The posterior curves are generated through a computerized numeric control software program utilizing aspheric base curves with reverse aspheric zones. The lens is designed so that it does not touch the corneal apex, so we can achieve an apical clearance fit with no bubbles under the lens. The remaining lens zones follow the natural corneal curvature out to the edge. The software will also calculate the ideal minimum lens mass and volume to avoid excessive weight and help maintain lens stability and durability even in a hyper-Dk material.
This type of fitting is desirable in some cases and may be crucial to help patients have a normal quality of life until they can undergo a corneal transplant. In Brazil, a patient who needs a corneal graft may wait 12 months or more for a donor cornea to become available. The idea is to provide the patient with reasonable visual acuity and comfort and to maintain corneal physiological health during the waiting period.
On the other hand, it's somewhat important that the patient has no significant leucoma or corneal opacities. If so, you should evaluate the cost versus benefit aspect of this type of fit.
We will now present two cases of patients who had advanced keratoconus in which we had the fortune to successfully fit them with GP lenses. We'll also discuss initial difficulties we experienced and explain how we eventually achieved the best lens-to-cornea relationship possible. My father Saul Bastos, MD, who passed away in September 2004, used to say: "Sometimes the best fitting is not the ideal, but the best possible to obtain."
Patient #1 was a 46-year-old female with severely advanced keratoconus OS. She had undergone a previous corneal transplant OD. She had not worn her contact lens OS for three months because her keratoconus had progressed and she could no longer tolerate the lens.
Figure 1 shows our first diagnostic lens, which had a base curve of 69.50×48 diopters (4.85mm × 7.03mm), 11mm overall diameter (OAD) and –27.50D lens power. Note the apical touch over the central cornea with the presence of air bubbles at a diameter of approximately 6mm, indicating that the central curvature was not steep enough and that the optical zone was larger than it should be. The lens periphery was also quite steep and did not allow desirable lens movement, which could result in adherence.
Figure 1. The first diagnostic lens for Patient #1. Note the apical touch and presence of bubbles.
Figure 2 shows a 71×48×45 diopter base curve Ultracone Mini-Scleral GP lens, with 11mm OAD and –29.00D power. Visual acuity with this lens was 20/25. Note the excessive tear pooling around the geometric center, which we initially thought demonstrated central touch. However, we later found that the excessive pooling was masking a thin lacrimal layer in the upper central area of the lens. At this time, the patient was able to tolerate the lens with no complaints, lens movement was up to 0.25mm and lacrimal exchange was acceptable.
Figure 2. An Ultracone Mini-Scleral GP lens on Patient #1.
Using fluorescein solution may cause an inaccurate fluorescein pattern because of an overflow of the liquid, so we use only fluorescein strips to provide a more precise idea of the lens-to-cornea relationship. In this case, there was no apical touch despite the appearance of the fluorescein pattern in Figure 2. Figure 3 shows the lateral slit lamp view in which you can see the lacrimal layer under the lens. There is no touch at all; when the patient blinks it's clear that there are tears under the lens at the corneal apex.
We instructed the patient to use Vidisic Gel (cetrimide, carbomer, sorbitol, water; Bausch & Lomb International) every day prior to lens application. We also recommended that she wear the lens for only one hour the first day and then add an hour of wear each subsequent day to re-adapt herself to the lens and until it was enough wear time that she could accomplish her daily tasks. We further instructed her to return in four days so we could re-evaluate the fit.
Figure 3. The lateral slit lamp view shows the presence of tears over the entire corneal apex, which helps prevent scarring.
When the patient returned for the follow up, we examined her and then asked how the first few days in the new lens had gone. She said, "I had some discomfort after a period, but not enough to make me take the lens off." We asked her if the discomfort was during the first or second hour of wear, to which she responded: "No, it was after nine hours of wearing the lens." This symptom occurred on the second day when she was supposed to be wearing the lens for only two hours.
At day 4, day 10 and one month, she experienced no symptoms of pain, no scarring and no evidence of adherence or corneal abrasion. She is currently waiting for a corneal transplant. This patient is too young to continue depending on a lens of this complexity. However, she currently has better vision with the lenswearing left eye than with the post-PK right eye.
Patient #2 was a 21-year-old female diagnosed with bilateral keratoconus when she was 15. The condition had progressed quickly over the past few years.
In 2004 she was wearing the following Ultracone lenses: OD 59×45 diopter base curve, 9.6mm OAD, –19.00D power and OS 62×45 diopter base curve, 10.5mm OAD, –21.50D power. Despite achieving acuity of 20/25, the disease progression resulted in symptoms of reduced quality of vision.
She has clear corneas with no scars, no leucoma or any other opacities in both eyes (Figure 4).
Figure 4. Patient #2 had clear corneas with no scars, no leucoma or any other opacities.
Right Eye The patient developed a rapid, strong progression of the keratoconus in her right eye, and in August 2006 we refitted her with the following lens: 69×48 diopter base curve, 11.5mm OAD, –29.50D power (Figure 5). This lens properly vaulted the geometric center of the eye and the corneal apex and caused no corneal scarring.
Figure 5. The lens for Patient #2 OD in August 2006.
In June 2007, we found that her condition seemed to have regressed, as we found a 2.5-to-3.0 mm bubble over the central cornea. We are currently studying the possible reasons for this event. I will mention that the patient made a drastic change to her diet in the previous 12 months as a result of lessons in the Nutrition School, where she was taking classes. We all believe that keratoconus is a condition of progressive corneal deterioration; it does not regress, but remains stable or progresses. However, at our clinic we have another patient who has provided similar evidence, so we need more data and further research on this particular subject.
We refitted Patient #2 with a 66.50×45 diopter base curve, 10.2mm OAD, –26.50D Ultracone design (Figure 6). She now wears the lens for up to 14 hours a day with 20/30 VA.
Figure 6. The final lens for Patient #2 OD following regression of the topography.
Left Eye The patient had a more severe keratoconus condition OS. She was wearing an Ultracone design with 71×48 diopter base curve, 11.5mm OAD, –31.00D power, but we were not satisfied with the results. The patient could not tolerate the lens for more than six hours. She had some discomfort after this period.
Corneal pachymetry was thick enough to refit her with an Ultracone design; however, she underwent implantation of intrastromal corneal ring segments with a third surgeon. Average K readings indicated that her topography regressed from 71 diopters to 58 diopters. In this case, we first assumed that it would be a lot easier to fit a flatter curvature and that we could produce a GP lens that she could comfortably wear for a longer time.
We noticed that the temporal inferior portion of the segment provoked an elevation in this zone, making a contact lens fitting difficult. Figure 7 shows that even a specialty lens design caused some abrasion to the cornea, noticeable by the fluorescein stain. We designed another four lenses with different eccentricity values in which we modified the asphericity of the base curves and the geometric reverse transition, but none of them worked properly.
Figure 7. A standard Ultracone lens produced an abrasion (stained with fluorescein) over the temporal inferior segment zone of the cornea for Patient #2 OS.
We have other patients who have similar difficulties, so we designed a new version of the Ultracone, called Ultracone PCR (post-corneal rings). The corneal implant rings have a 5.5-to-6.5 mm diameter. We needed a lens that would prevent bearing on the elevated areas, with enough vaulting effect to protect the central cornea.
For the Ultracone PCR, we changed the posterior optical zone to a new value in which the asphericity of the reverse curve could prevent corneal touch for that particular portion of the cornea and could also produce an adequate fitting relationship with no apical touch and no bubbles.
Figure 8 shows the new Ultracone PCR lens fit, demonstrating an excellent fluorescein pattern and no corneal abrasion or staining. The patient wears this lens up to 16 hours a day with no physiological problems. CLS
Figure 8. The Ultracone PCR provided a better fitting relationship following corneal ring segment implantation for Patient #2 OS.
Mr. Luciano Bastos is the technological director of IOSB and Ultralentes. He is a pioneer specialist in designing and fitting specialty RGPs in extreme irregular corneas and an RGP lens consultant for contact lens ophthalmologists in Brazil. Dr. Bittencourt is Clinical Director IOSB, Consultant Opthalmologist for the Ambulatory of the Presidente Vargas Hospital in Porto Alegre, and RS Opthalmologist of the Hospital Pronto Socorro (HPS – Porto Alegre) Enfermary. Dr. Marcelo has no financial interest in any of the products mentioned in this article.
Contact Lens Spectrum, Issue: January 2009