Using Large-Diameter GPs to Improve Comfort
LARGE-DIAMETER GP LENSES
Using Large-Diameter GPs to Improve Comfort
These four cases demonstrate that larger diameters offer many benefits when fitting GP lenses.
By John Laurent, OD, PhD
Dr. Laurent received OD and MS degrees from The Ohio State University in 1976 and earned a PhD from the University of Alabama at Birmingham in 1996. He has practiced optometry in private practice, the U.S. Army, and the U.S. Navy. He served for four years as the Research Director for the U.S. Navy Refractive Surgery Center in San Diego. He recently completed a residency in Cornea and Contact Lenses at UABSO and currently practices with Schaeffer Eye Center, Birmingham, AL.
The first commercially available contact lenses were large scleral lenses originally made of glass and then of polymethyl methacrylate (PMMA). These materials were not gas permeable, and the large contact lenses made from them caused severe corneal edema.
Smaller corneal contact lenses made from PMMA caused much less edema because they covered only part of the cornea and they could be designed to provide tear exchange under the lens as it moved with each blink. Rigid PMMA corneal contact lenses became the predominant type of contact lens fit in the United States from the mid-1950s to the mid-1970s (Mandell, 1988). However, discomfort was a factor with these lenses, and it usually took a week or two for patients to adapt to them.
Hydrogel soft contact lenses became popular in the early 1970s, and one of the most important differences between these lenses and the rigid PMMA lenses was an improvement in comfort. In just a few years, soft lenses dominated the U.S. contact lens market. Today, soft lenses account for 87-to-88 percent of U.S. contact lens sales and for more than 90 percent of new contact lens fits (Bennett, 2008).
In spite of the improved physiology of today's rigid lenses that are made from gas permeable materials rather than from PMMA, patient discomfort with these lenses is essentially the same as it was with the PMMA lenses. The primary cause of this discomfort is the sensation of the eyelids coming into contact with the edges of the lens. GP corneal lenses, even those riding "piggyback" over a soft lens, will produce an unpleasant foreign body sensation for most patients.
Improving GP Lens Comfort
We can improve comfort with GP lenses by making them larger (Mandell, 1988). Intralimbal lenses (10.5mm to 12.5mm) will fit many patients so that the superior edge of the lens is under the superior eyelid. These lenses will feel more comfortable than would a smaller corneal lens because only the inferior eyelid feels the lens during a blink. These lenses also spread the corneal bearing pressure over a wider area, although this is likely a minor factor in terms of patient comfort.
As mentioned previously, when PMMA was the only material available for contact lenses, lens size was limited by the amount of lensinduced corneal edema and subsequent neovascularization. Large PMMA lenses with limited movement and extensive corneal coverage were simply not physiologically compatible with the human eye.
This situation changed with the development of GP materials that have greater oxygen permeability (Dk) values. Today it is possible to design GP lenses that extend beyond the limbus and yet are still physiologically compatible with the cornea. An important benefit of this increased size is an improvement in comfort, as the edges of the lenses frequently extend under both superior and inferior eyelids.
These large GP lenses are referred to as corneoscleral, semi-scleral, mini-scleral, or scleral lenses, depending on the size and manufacturer. Table 1 lists some of the lenses that are currently available.
Presented here are four case reports of patients who were uncomfortable with their corneal GP lenses or, in one case, with a toric soft lens. All of them were successfully refit with semi-scleral GP lenses.
Our first patient, a 58-year-old male, came to the clinic for a contact lens evaluation. He had a history of severe epithelial basement membrane dystrophy and reduced vision since childhood. He reported that his vision was "never correctable to 20/20," although it was better with rigid contact lenses compared to glasses or soft lenses. At the time of his clinic visit the patient was wearing a piggyback system with GP corneal contact lenses over Night & Day (CIBA Vision) soft lenses. He said the lenses were very uncomfortable and wearing time was limited to six or seven hours a day for approximately four days per week. In addition, he needed to keep his eyes in a down-gaze position to minimize discomfort, which resulted in an unnatural head position that was most noticeable when working on a computer.
Visual acuity (VA) with contact lenses was OD 20/50+1 and OS 20/40-1. The soft lenses appeared mildly tight-fitting with no movement, but the GP lenses moved and centered well with a light central touch. Because he was very uncomfortable even with the soft lenses covering the corneas, I assumed that his discomfort was related to eyelid interaction with the edges of the GP corneal lenses. This patient seemed to be a good candidate for semi-scleral contact lenses.
Slit lamp examination revealed epithelial basement membrane dystrophy with extensive corneal surface irregularity in both eyes. Figure 1 shows some of the pattern of his corneal dystrophy.
Manifest refraction was OD –1.75 –2.25 × 090 20/50-1 and OS –1.25 –2.00 × 090 20/40-1. Keratometry, with moderate distortion of the mires in both eyes, was OD 46.50/45.25 @ 095 and OS 45.87/44.62 @ 090.
Figure 1. Epithelial basement membrane dystrophy in patient in Case 1.
I fit the patient with semiscleral lenses made from Tyro-97 (Lagado Corp.), a GP material with a Dk of 97. The lenses vaulted most of the cornea in both eyes. Figure 2 shows the fluorescein pattern OS demonstrating the extent of corneal clearance. Lens parameters were OD 7.26mm base curve, 14.2mm diameter, –3.00D power, 20/40-1 and OS 7.26mm base curve, 14.2mm diameter, –4.25D power, 20/40+2.
Figure 2. Fluorescein pattern OS of patient in Case 1.
Visual acuity with the new lenses was only marginally better than that with glasses, but he reported a significant improvement in vision compared to glasses and a slight improvement compared to the previous contact lenses. Because the overall shape of his corneas was normal and the reduced vision was apparently due to small corneal surface irregularities, we could hypothesize that he had significant higher-order optical aberrations, which would account for the subjective improvement in vision without much improvement in visual acuity. However, most important to the patient was an improvement in comfort. He said these lenses were more comfortable than any he had previously worn.
At the first follow-up visit, this patient was very emotional about the positive benefits of his new lenses. He reported wearing the lenses comfortably for eight-to-11 hours daily without a need to look downward or hold his head in any particular position. He reported that acquaintances told his wife that he "looked much more natural because he was walking with his head upright and not looking at the floor."
After wearing his lenses full-time for more than a month, the patient had a corneal abrasion in his left eye that required him to discontinue lens wear for several days. I refit him with slightly larger diameter lenses, and he is doing well with them.
A 41-year-old male presented to the clinic for a contact lens fitting of his left eye. He had a history of keratoconus, primarily of the left eye, and had Intacs (Addition Technology) intrastromal rings in that eye. The rings had initially been implanted six months previously, but the inferior ring had been replaced with a thicker one two months prior to his clinic visit. He was referred by the ophthalmologist who implanted the Intacs.
The patient wore soft contact lenses part-time, but was not wearing them when he came to the clinic. He reported that he could see well with a spherical lens on the right eye, but the toric lens on the left eye was uncomfortable and did not provide satisfactory vision.
Manifest refraction and VA were as follows: OD –8.25D –0.75 × 105 20/20+2 and OS –1.50 –6.00 × 110 20/25-2 (slowly). Keratometric mires were clear OD, mildly distorted OS: OD 45.87/45.37 @ 060, OS 45.62/41.62 @ 112. Topography showed significant inferior steepening in the left eye, even with the intrastromal rings (Figure 3).
Figure 3. Topography shows that the patient in Case 2 had significant inferior corneal steepening even with implanted Intacs.
My contact lens fitting goal for this patient's left eye was to completely vault the cornea, so I did not consider a corneal lens. The initial lens was a semi-scleral with a 6.90mm base curve, 13.5mm diameter, –10.50D power, made of Tyro-97. This lens was comfortable and provided 20/15 VA. However, on follow up the fluorescein pattern showed a light central touch on the cornea overlying the inferior ring. A slightly steeper and larger lens with a 6.78mm base curve, 14.2mm diameter, –11.75D power provided central clearance, good comfort, and 20/20+1 VA. Figure 4 shows the patient's left eye with this semiscleral lens in place.
Figure 4. The semi-scleral lens for the patient in Case 2 provided central clearance and good comfort and acuity.
I also fit this patient's right eye with a semi-scleral lens, but he preferred a soft lens for that eye so I refit him with an O2Optix (CIBA Vision) lens with an 8.6mm base curve, 14.2mm diameter, –8.00D power, which fit well and provided 20/20+2 VA. Although he was more aware of the semi-scleral lens on his left eye than of the soft lens on his right eye, he said the left lens was more comfortable than any other lens he had worn in that eye for many years. He wore both lenses for 15 hours daily.
A 56-year-old male was referred for a contact lens fitting of his left eye. He had a history of keratoconus and penetrating keratoplasty (PK) in both eyes. The right eye had undergone three surgeries, with the most recent occurring three months prior to his office visit. The corneal surgeon was worried about possible graft rejection in that eye and did not want the patient to wear a contact lens on it. The left eye had undergone PK 10 years earlier and was stable. He had worn a series of GP contact lenses on his left eye, but complained that they all had a tendency to decenter and frequently dislodge from the eye. This patient did not wear glasses and also had not worn a contact lens on his left eye since losing his lens five-to-six months prior to his clinic visit.
Uncorrected VA was 20/400 OD and OS. Manifest refraction OS was –4.25 –7.00 × 038 with 20/25-2 VA. Manual keratometry OS was 42.87/51.00 @ 128. Topography revealed an oblate cornea with inferior temporal steepening (Figure 5).
Figure 5. The post-PK patient in Case 3 had an oblate cornea and inferior temporal steepening OS.
The patient's record indicated that he had worn an aspheric GP lens with a 7.76mm base curve, 10.5mm diameter, –3.00D power in his left eye. I put a similar trial lens on his eye and found it to be very flat with heavy central touch and severe edge standoff, resulting in lens decentration after several blinks. I tried two different post-surgical GP lenses with reverse geometry designs, and both of them fit much better than a standard aspheric. However, both lenses still resulted in significant excess edge lift between 3 o'clock and 6 o'clock where the cornea was very steep. Small bubbles, created during blinks, collected under the center of the lenses.
I then fit this patient with a series of semi-scleral lenses, all of which were more comfortable than any of the previous contact lenses he had worn on his left eye. His final left lens had a 6.49mm base curve, 14.2mm diameter, –12.25D power, provided 20/20-2 VA, and could be worn comfortably for 12 hours daily. Figure 6 shows the left eye with the lens in place and visible corneal graft scars.
Figure 6. Final semi-scleral lens OS for patient in Case 3.
A 33-year-old female presented to the clinic for a contact lens refitting of her left eye. She had a history of keratoconus in both eyes and had undergone PK in her right eye, which had been fit with a SynergEyes (SynergEyes, Inc.) hybrid lens. The left eye had a corneal scar, and the patient reported discomfort and slowly decreasing vision in that eye with a GP corneal lens that was "about 15 years old." Measured lens parameters were 7.20mm base curve, 8.5mm diameter, –2.75D power. The patient expressed a desire for her left eye to be refit with a lens similar to the corneal lens she was already wearing on that eye.
Visual acuity of the left eye with the contact lens was 20/50+2 with a plano over-refraction. Slit lamp examination with fluorescein revealed 3-to-4mm of central bearing on the apex of an inferior-central cone. With the lens removed, the cornea showed a swirl pattern of 2+ SPK corresponding to the area of touch by the lens. There was a corneal scar approximately 1mm × 2mm in size in the same location.
Manifest refraction OS was –6.00 –1.25 × 110 with 20/40+1 VA. Keratometry measurement was 53.25DS with distorted mires.
I refit this patient with an ABBA Kone (ABBA Optical) lens, 6.31mm base curve, 8.6mm diameter, –9.50D power, manufactured in Paragon HDS (Paragon Vision Sciences) with a Dk of 58. Although VA with the new lens was acceptable at 20/25-2, there was excessive movement and the comfort was not much better than with the original lens. After several corneal diagnostic lenses did not provide an acceptable level of comfort, the patient consented to trying a GP semi-scleral.
A series of diagnostic GP semi-scleral lenses, all manufactured in Tyro-97, were all very comfortable. However, each new lens would show central clearance at the time of dispensing but central touch after a few days of wear. It seems likely that the cornea was steepening after removal of the flat-fitting lens and the pressure it exerted on the cone. A second complication was that this patient apparently had a flatter than average sclera on her left eye, with lenses showing a band of peripheral conjunctival blanching at the time of progress checks. Figure 7 shows the edge of one such lens on the left eye.
Figure 7. Initial semi-scleral diagnostic lenses resulted in peripheral conjunctival blanching in patient from Case 4.
The central cornea eventually stabilized, and it was possible to determine a correctly fitting base curve. Flatter peripheral curves solved the problem of peripheral conjunctival blanching. The patient's final lens on her left eye, with a 6.62mm base curve, 14.4mm diameter, –8.00D power, and manufactured in Tyro-97, fit very well with good central and midperipheral clearance and no peripheral blanching of the conjunctiva. The cornea exhibited no staining after lens removal. The patient was comfortably wearing the lens 14 hours daily with VA of 20/25+2, better than she had been able to see with that eye in several years.
Large-Diameter GP Benefits
The semi-scleral lenses I fit on these patients were more comfortable than their previous corneal lenses because of the reduced interaction between the eyelid and the contact lens. The comfort of a large-diameter GP lens that fits under the eyelids can approach the comfort achieved with soft lenses.
Contact lenses made from GP materials offer a number of advantages over soft lenses. The rigid optics often provide superior visual acuity. They are easier to take care of and are not prone to contamination, so corneal infections are extremely unlikely. They can be stored wet or dry, and they are more durable, usually lasting for a year or more. Scleral and semi-scleral GP lenses offer an additional advantage to patients who have irregular corneas because most of the pressure of the lens on the eye will fall on the conjunctiva/sclera rather than the cornea. It may be that vaulting the corneas of keratoconic patients will result in less lens-induced corneal scarring for some of these patients.
Scleral and semi-scleral lenses have been primarily marketed as "problem solving" lenses for keratoconus and other irregular corneas. However, other types of patients could also benefit from these lenses: patients who find their corneal GP lenses uncomfortable, toric soft lens wearers who experience unstable visual acuity, patients who have dry eye, those prone to giant papillary conjunctivitis, or anyone for whom lens care compliance is an issue. Because these lenses do not decenter or dislodge easily, they could also be used for active children or athletes.
There are some disadvantages to these types of lenses. Application and removal are more difficult than with the smaller corneal lenses. On application, the lens must be filled with saline or an artificial tear solution to avoid air bubbles under the lens. These bubbles can reduce visual acuity and take a long time to resolve. Removal of these lenses can be accomplished by manipulating the lid margins against the edges of the lens; but most patients use a suction device like the DMV Lens Remover.
Currently, most scleral and semi-scleral lenses are more expensive compared to smaller corneal lenses. However, the current cost of less expensive options is similar to a one-year supply of toric soft lenses. Considering the comfort, durability, easy care, and low risk of infection of larger-diameter GP lenses, it seems possible that they could become a viable alternative to soft contact lenses for many patients. CLS
All patients were fit with Essilor Perimeter contact lenses generously provided by Essilor Contact Lenses.
Contact Lens Spectrum, Issue: May 2009