PILSKALNS, BEN OD, MS; FINK, BARBARA A. OD, PhD, FAAO; HILL, RICHARD M. OD, PhD, FAAO
Keratoconus is a noninflammatory, asymmetric, progressive corneal ectasia characterized by increased irregular myopic astigmatism and deterioration of vision.1–3 Rigid gas permeable contact lenses are the preferred mode of vision correction for persons with keratoconus or other forms of corneal irregularity because they mask most of the irregular astigmatism and provide a more uniform optical surface. In studies comparing visual performance of rigid gas permeable contact lenses with spectacles,4 and with both spectacles and soft lenses,5 rigid gas permeable lenses provided the best visual performance.
As keratoconus becomes increasingly advanced, management solely with rigid contact lenses may fail because of poor contact lens centration, poor comfort, or excessive irregular astigmatism. An alternative contact lens fitting strategy to combat these difficulties and perhaps delay penetrating keratoplasty is the hybrid contact lens.6 Hybrid contact lenses consist of a rigid gas permeable center fused to a soft lens skirt in a one-piece construction. They provide the optics of a gas permeable lens, with improved comfort and better lens centration.
The SoftPerm hybrid lens (Ciba Vision Corp., Duluth, GA), previously called Saturn II, has a rigid lens component diameter of 8.0 mm, an optic zone diameter of 7.0 mm, and a permeability of 14.0 × 10−11 (cm2/sec) (mL O2/mL × mm Hg). The soft skirt extends to 14.3 mm, has a water content of 25% and a permeability of 5.5 × 10−11 (cm2/sec) (mL O2/mL × mm Hg).7, 8 The center thickness of the rigid center (−3.00 D) 0.08 mm, whereas the skirt thickness is >0.20 mm. The Softperm lens achieved limited success because of a number of potential problems. The low oxygen permeability of both the rigid and soft lens materials and tendency toward minimal movement and adhesion can lead to overwear complications, such as corneal edema and neovascularization.9–12 As a result, wear time is often restricted, which is an inconvenience to the patient. Also the central gas permeable portion can flex, resulting in induced astigmatism.12 The chord width of the rigid portion is fixed, as is the relationship between the radii of curvature of the rigid and flexible portions, thereby limiting practitioner options during the fitting process. The SoftPerm is available with a base curve radius as steep as 6.5 mm, which still may be too flat to fit a very steep keratoconus patient. The Softperm lens can be somewhat difficult to remove and tearing of the lens at the junction between the two lens materials can occur.10,11 The lens is expensive, which can be a problem for a patient that splits many lenses. During the fitting of SoftPerm lenses, it is necessary to place high molecular weight fluorescein into the concavity of the lens before lens placement on the cornea. Because of the limitations of its design, most contact lens practitioners use SoftPerm lenses as a last resort in patients hesitant to undergo penetrating keratoplasty.
In an effort to overcome many of the limitations with the SoftPerm contact lens, Quarter Lambda Technologies, Inc. of San Marcos, CA, commenced a focused research and development program in September 2001 to create an improved hybrid contact lens. In October, 2003, the company received IRB approval to begin with four of the SynergEyes products. These trials included the SynergEyes KC lens for keratoconus, which uses a rigid center lens material of high oxygen permeability with a nonionic soft skirt of fairly low oxygen permeability. The rigid lens component has a diameter of 8.4 mm, an optic zone diameter of 7.8 mm, and a permeability of 100 × 10−11 (cm2/sec) (mL O2/mL × mm Hg). The center thickness is 0.15 mm for the −3.00 D back vertex power. The soft skirt extends to 14.5 mm, has a water content of 27% and a permeability of 9.3 × 10−11 (cm2/sec) (mL O2/mL × mm Hg). The mean thickness of the skirt is 0.16 mm. The SynergEyes lens employs a continuous aspheric base curve from lens center to periphery and is available with base curve radii as steep as 5.7 mm. The process uses a patent pending technology to produce a lens that is more than 10 times stronger than the currently marketed low Dk hybrid lens. In addition, multiple skirt radii are available for each base curve radius, 0.7 to 1.6 mm flatter than the central radius, in 0.3 mm steps. The increase in oxygen transmissibility, durability, steeper base curve availability, and options of skirt radii of the SynergEyes lens (SynergEyes, Inc., Carlsbad, CA) should serve to provide contact lens practitioners a useful tool in the management of keratoconus.
Although many aspects of keratoconus have been studied, measurement of a basic physiological response of the cornea, corneal oxygen uptake, has not been performed on keratoconic eyes. Benjamin and Hill found that oxygen uptake rates increase almost linearly between oxygen tensions of 20.9 and 1.5%. Below 1.5% oxygen, the rate increases substantially.13 Contact lens wear reduces the oxygen supply to the cornea, and natural log equations describe the reduction in corneal oxygen uptake associated with increase in lens transmissibility.14–17
This study measures corneal oxygen uptake on keratoconic eyes across the temporal hemi-meridian, from central cornea to 1.0 mm temporal to the limbus, under normal open eye conditions and after wear of the SoftPerm and SynergEyes lenses. A comparison is made of the hybrid lens materials to determine if the higher transmissibility of the central rigid component of the SynergEyes lens results in lower oxygen uptake rates for the central or peripheral cornea.
The corneal oxygen uptake rates of the right eyes of 14 subjects and the left eye of 1 subject, each with keratoconus, were measured in this study. Subjects ranged in age from 39 to 65 years. The left eye of one male subject was used because the corneal curvature in his right eye was too great to be fitted with the SoftPerm design contact lens. Using his left eye eliminated this problem. There were nine males and six females; the subjects were white or black. All subjects read and signed a consent form, which had been approved by the Institutional Review Board of The Ohio State University.
For inclusion, the subjects were required to have keratoconus as demonstrated by irregular corneal surface in either eye, determined by distortion of keratometric mires, or the retinoscopic reflex, or of the red reflex; either Vogt's striae in the deep stroma, Fleischer's ring of at least 2 mm of arc, or corneal scarring characteristic of keratoconus in either eye. Contact lens wear, or lack thereof, was not an inclusion criterion. All subjects wore rigid gas permeable contact lenses, except for three subjects who were noncontact lens wearers. Because the subjects who wore contact lenses were dependent upon their lenses for clear vision, subjects were not asked to discontinue contact lens wear during the study. If they wore contact lenses, their lenses were removed during the measurement sessions just before data collection. The subjects underwent a thorough case history and ocular examination. The exam included entrance visual acuity, keratometry, slit lamp biomicroscopy, current contact lens evaluation, if applicable, and monocular Softperm and SynergEyes contact lens fitting.
The flattest corneal meridian, as determined by keratometry, was used as a starting point to fit the hybrid contact lenses. Base curve was adjusted accordingly until a near alignment high molecular weight fluorescein pattern was observed both under the rigid gas permeable component centrally and under the surrounding soft lens skirt peripherally. For all subjects, the same base curve radius was used for the SoftPerm and SynergEyes lenses. For both the SoftPerm and the SynergEyes lenses, the skirt radius was 1.3 mm flatter than the base curve radius.
The hybrid lenses used were Softperm (CIBA Vision) and SynergEyes (SyngergEyes, Inc.). The parameters for these lenses are displayed in Table 1.
Measurement of Corneal Oxygen Uptake
Corneal oxygen uptake rates were measured with a Clark-type polarographic electrode coupled to a pH/blood gas analyzer (PHM73 Radiometer Copenhagen). The electrode was kept in water baths containing sterile saline that were maintained at a temperature of 36°C, and a 12-μm thick polyethylene membrane covered the electrode. Calibration was obtained by alternately placing the electrode in water baths bubbled by nitrogen (0 mm Hg) and air (155 mm Hg). A time constant was determined by quickly transferring the electrode from the air to the nitrogen bath.
To measure corneal oxygen uptake for the uncovered eye at three corneal locations and one scleral location, the subject was asked to fixate at either a penlight or at one of three decal targets mounted on a wooden stick. Fixation of each respective target resulted in either a corneal or conjunctival reflection of the penlight. Target decals were positioned such that a reflection was created at 2.0 and 4.5 mm temporal to the central cornea and 1.0 mm temporal to the limbus. The reflection served as a location guide for proper probe positioning.
Oxygen uptake rates were also measured at the four locations after 5 minutes of static (without blinking) wear of the each of the hybrid contact lenses. The hybrid contact lenses to be used were cleaned overnight, the day before use, with Clear Care (CIBA Vision) cleaning and disinfecting solution. The day of the measurements the hybrid lens was removed from the cleaning vial and then placed onto the subject's cornea. The subject held his/her upper lid against his/her bony orbit for the 5-minute test period. The subject was allowed to blot his/her tears throughout this period. Near the end of the 5 minutes, the subject reached around his/her head with his/her hand and regrasped his/her upper lid. When time had expired, the chart recorder was turned on and the electrode was removed from the investigators right hand. The investigator grasped the soft skirt component with the left hand and removed the hybrid lens. About 1 s after the hybrid lens was off of the cornea, the electrode was centered on the reflection and applanated perpendicularly to the cornea until the gas analyzer read 40 mm Hg. Therefore, corneal oxygen uptake rates were measured in random order under the following conditions:
1. Normal open eye: central cornea, 2.0 mm temporal-to-central cornea, 4.5 mm temporal-to-central cornea, 1 mm temporal to limbus.
2. After 5 minutes of static (without blinking) wear of SoftPerm contact lens: central cornea, 2.0 mm temporal-to-central cornea, 4.5 mm temporal-to-central cornea, 1 mm temporal to limbus.
3. After 5 minutes of static (without blinking) wear of SynergEyes contact lens: central cornea, 2.0 mm temporal-to-central cornea, 4.5 mm temporal-to-central cornea, 1 mm temporal to limbus.
A period of 5 minutes without contact lens wear followed each measurement, and both the SoftPerm and the SynergEyes contact lenses were inserted four times for corneal oxygen uptake measurements at each location.
To determine the differences among oxygen uptake rates relative to the uncovered eye at each measurement location for each hybrid lens, a two-way repeated measures analysis of variance (ANOVA) was used. This analysis included a factor for the lens type, a factor for measurement location, and the interaction of lens type and measurement location. A significant interaction effect would imply that the oxygen uptake changes more than can be explained from a change in lens type alone or location of measurement alone. The data were relativized to the normal open eye by dividing the oxygen uptake rate for each test condition by the oxygen uptake rate of the uncovered eye for that same measurement location. Multiple comparisons procedures were performed for the difference in mean relative rate between the four measurement locations. Such post hoc testing is useful to differentiate which group means differ from which others, after the overall F test has demonstrated that at least one difference exists. Regulation for type 1 error, rejecting the null hypothesis when it is true, was made using the Tukey-Kramer adjustment.
A one-way ANOVA was used to find out if differences in corneal oxygen uptake rate were present among measurement locations under normal open eye conditions. Corneal oxygen uptake rate was the dependent variable and measurement location was the independent variable. Multiple comparisons with Tukey-Kramer adjustment were used post hoc to determine which locations were significantly different.
The mean rates of oxygen uptake for the uncovered eye, along with the standard error at each location, are presented in Table 2 and Fig. 1. The mean uptake rate under uncovered eye condition was highest at the 1.0 mm temporal to the limbus location and lowest centrally. One-way ANOVA revealed a statistically significant difference in mean corneal oxygen uptake rate with location (p = 0.0167). Table 3 shows the results of multiple comparisons using the Tukey-Kramer adjustment for differences in mean uptake rate among measurement locations. Mean uptake rate at 1.0 mm temporal to the limbus was found to be significantly greater than the central cornea (adj p = 0.0095), 2.5 mm temporal-to-central cornea (adj p = 0.0157), and 4.5 mm temporal-to-central cornea (adj p = 0.0203).
The mean relative rates of oxygen uptake for SoftPerm and SynergEyes lens along with the standard error are presented in Table 4, and a comparison of the mean relative rates for each lens and location can be seen in Fig. 2. The mean relative rate of uptake was less for SynergEyes at each measurement location. The standard error was greatest for both the SoftPerm and SynergEyes lens at the 1.0 mm temporal to the limbus location. Repeated measures two-way ANOVA found a significant effect for lens type (p < 0.0001), measurement location (p < 0.0001), and lens type and measurement location interaction (p < 0.011). This interaction is visible in Fig. 3. It shows that difference between lenses is similar at the three corneal locations, but becomes less at the 1.0 mm temporal to the limbus location.
To confirm the contribution of the 1.0 mm temporal to the limbus location, the two-way repeated measures analysis was repeated dropping this location. This resulted in a significant effect for lens type (p < 0.0001) and measurement location (p = 0.0001), but the lens type and measurement location interaction was no longer significant (p < 0.141).
Table 5 shows the results of multiple comparisons using the Tukey-Kramer adjustment for differences in mean relative uptake rate for the SoftPerm lens between measurement locations. Mean relative uptake rate at the central cornea was found to be significantly less than the mean relative rate at the 4.5 mm temporal-to-central cornea (adj p = 0.0009).
Table 6 conveys the results of multiple comparisons using the Tukey-Kramer adjustment for differences in mean relative uptake rate for the SynergEyes lens between measurement locations. Mean relative uptake rate at the central cornea and 2. 0 mm temporal-to-central cornea were found to be significantly less than the mean relative rate at 1.0 mm temporal to the limbus (adj p = 0.005 and 0.011, respectively).
Multiple comparisons results using the Tukey-Kramer adjustment for the difference in means of the relative rate between the Softperm and SynergEyes lens at each measurement location are shown in Table 7. The mean relative oxygen uptake rate was found to be significantly less in SynergEyes lens at the central cornea (adj p < 0.0001), 2.0 mm temporal-to-central cornea (adj p < 0.0001), and 4.5 mm temporal-to-central cornea (adj p < 0.0001).
As shown in Tables 2 and 3 and in Fig. 1, the mean oxygen uptake rate under normal open eye conditions was not significantly different among the three corneal locations measured (central, 2.0 mm temporal-to-central, and 4.5 mm temporal-to-central). This finding is consistent with previous studies showing that corneal oxygen uptake rate does not vary significantly with measurement location in the nasal or temporal direction.18,19 However, other studies have shown an increase in corneal oxygen uptake for the inferior cornea.20–22
The mean oxygen uptake rate 1.0 mm temporal to the limbus for the normal open eye was found to be significantly greater than at each of the three corneal locations. The measurement 1.0 mm temporal to the limbus is taken on the conjunctiva instead of the cornea and, thus, is measuring conjunctival oxygen uptake rate. An inherent difficulty in measuring conjunctival oxygen uptake rate is the presence of conjunctival blood vessels. During measurement, the polarographic electrode may inadvertently contact one or more conjunctival blood vessels thereby resulting in an artifactually high oxygen uptake reading. Although much effort was made to avoid these conjunctival blood vessels, this was not always possible. Thus, the higher mean oxygen uptake rate and larger standard error associated with measurements 1.0 mm temporal to the limbus are likely attributable to inadvertent contact with conjunctival vasculature during measurement. Because the cornea is avascular, blood vessels are not a factor in corneal oxygen uptake measurements.
Several factors influence the corneal oxygen uptake rates of keratoconic corneas measured after no contact lens wear. History of contact lens wear is, in fact, a factor that affects corneal oxygen uptake rate. In this study, contact lens wear, or lack thereof, was not an entry criterion. Twelve subjects wore gas permeable contact lenses and three wore spectacles only. Most of the contact lens-wearing subjects reported wearing their contact lenses the entire day owing to poor refractive correction with spectacles. Therefore, it is possible that the oxygen uptake rates in these subjects may have been affected by contact lens wear before the study. It was, however, not practical to ask these subjects to stop wearing contact lenses as doing so would compromise their functionality and quality of life. Moreover, by not artificially ceasing contact lens wear, this study was able to examine oxygen uptake rates in a more realistic situation.
Carney and Brennan measured oxygen uptake rates for the first 30 weeks of daily wear hydrogel contact lenses and found that, after 19 weeks, rates were lower than the prefitting baseline values 60 minutes after contact lens removal.23 Holden and his colleagues measured oxygen uptake rates on both eyes of 27 long-term wearers of extended wear hydrogel contact lenses who had worn a contact lens in one eye only. When the lens was removed, a significant reduction in corneal oxygen uptake of 14.8% was found in the lens-wearing eye when compared with the control eye. The difference between eyes decreased over time with no contact lens wear, becoming insignificant by 33 days after contact lens removal.24 Therefore, oxygen uptake rates would be expected to be depressed in this sample of keratoconic subjects. It is not known how the profile of oxygen uptake rates across the cornea is influenced by contact lens wear.
In addition, several studies have shown that reduction in corneal integrity is associated with a reduction in corneal oxygen uptake.25–30 Frequently, the corneas of keratoconic patients exhibit fluorescein staining, edema, and scarring.31–33 One subject exhibited moderate scarring in the central corneal region. It is unclear how scarring influenced corneal oxygen uptake rates at the central location for this particular subject; however, values obtained were similar to those of other subjects without scarring.
Corneal thickness has also been shown to influence corneal oxygen uptake. Following periods of hypoxic stress, as the ratio of epithelial/stromal thickness rises, so does oxygen uptake (r = +0.92, p = 0.01).34 Horton found that thicker corneas tend to have lower oxygen flux.35 Using optical coherence tomography, Haque et al. found a reduction in both epithelial and total corneal thickness for the central corneas and at the cone apex of patients with keratoconus.36 Because both the thickness of the epithelium and stroma are reduced in keratoconus, and they influence oxygen uptake rates in opposite directions, the overall effect on corneal oxygen uptake needs to be further investigated.
To compare the oxygen uptake rates associated with the wear of the two hybrid contact lenses, the oxygen uptake rates associated with the test condition (wear of a SoftPerm or SynergEyes lens) were divided by the oxygen uptake rates for the uncovered eye for that subject at that location. This relative rate provides an estimate of the extent to which oxygen uptake rate is increased by the presence of a hybrid lens. Past studies have shown that this relativization renders differences because of the instrument or corneal instabilities insignificant, making the true difference between test conditions more apparent.37 As shown in Fig. 2 and Table 4, the mean relative oxygen uptake rate was the lowest centrally for both the SynergEyes and SoftPerm lenses. The values of 2.31 and 1.43 for the SoftPerm lens and SynergEyes lenses, respectively, indicate the number of times the oxygen uptake rate was elevated over that of the uncovered eye at the same central location. The largest standard error occurred at the location 1.0 mm temporal to the limbus for each lens type and can be attributed to the presence of limbal vasculature.
A two-way repeated measures analysis for the relative rates of oxygen uptake found a significant interaction effect between lens type and measurement location. This indicates that the material of SoftPerm vs. SynergEyes has a different effect on oxygen uptake rate depending on the measurement location. The presence of such an interaction is evident from Fig. 3. The relative rates are almost parallel for the three corneal locations, but the difference becomes significantly less at the conjunctival location. An interaction was expected because the material for the SoftPerm and SynergEyes lens changes from the center to the periphery. The change in lens transmissibility is more dramatic for the SynergEyes lens, as the material Dk changes from 100 centrally to 9.3 peripherally.
When a two-way repeated measures analysis was repeated using only the three corneal measurement locations, a significant interaction was not present between lens type and location. This shows that the location 1.0 mm temporal to the limbus was responsible for the interaction. The soft components of the SoftPerm and SynergEyes lens have similar transmissibilities and cover the 1.0 mm temporal-to-limbus location during wear. This explains the similar relative oxygen uptake rates between the two lenses observed at this location. The 4.5 mm temporal-to-central location is also located under the soft lens portion, but did not contribute the interaction. The lower oxygen uptake rate in the SynergEyes lens at this location may be attributable to lateral diffusion of oxygen from the central rigid component, which might occur through the contact lens, the tears, or the cornea. Oxygen flux depends on the transmissibility or these components and the concentration gradient of oxygen. Increased diffusion occurs with increased lens transmissibility.
However, Hill38 found increased oxygen uptake rates 1 mm from the edge of 10 mm polymethylmethacrylate (PMMA) contact lenses, indicating little passive diffusion of oxygen in the tears under a contact lens in the absence of tear movement. Fatt et al.39 calculated that there is little lateral diffusion of oxygen within optically powered hydrogel contact lenses. Lin40 used mathematical models to investigate lateral diffusion of oxygen in both hydrogel contact lenses and the cornea. The diameter of the hypoxic zone under plus lenses from + 3.00 to + 12.00 D corresponded with the optic zone of the lens. For minus lenses, the hypoxic zone was an annulus 4 mm from the center of the lens. No lateral spreading of oxygen from areas under thin portions of the lens to areas under thicker portions of the lens was found. Efron and Fitzgerald41 found that −6.00 D hydrogel lenses, Dk of 9.6 × 10−11 (cm2/sec) (mL O2/mL mm Hg), provided significantly more oxygen to the central cornea than to the peripheral cornea, suggesting little lateral oxygen diffusion in the postlens tears.
On the other hand, Rasson and Fatt42 found that the oxygen diffusivity of the corneal stroma is 0.6 cm2/sec, and that of the epithelium is 0.85 cm2/sec. The Dk of the tears, epithelium, and stroma are 78 × 10−11 (cm2/sec) (mL O2/mL mm Hg), 18.8 × 10−11 (cm2/sec) (mL O2/mL mm Hg), and 25.6 × 10−11 (cm2/sec) (mL O2/mL mm Hg), respectively.43 Bonanno et al. surmised that significant lateral diffusion of oxygen occurs in cornea from areas of high oxygen tension to areas of low oxygen tension.44 Increasing the minus power of hydrogel lenses results in a reduction of oxygen tension at the peripheral cornea, movement of oxygen from the central cornea to the periphery, and greater corneal swelling centrally than peripherally. In addition, Moezzi et al.45 used the Orbscan II corneal topopgrapher to determine contact lens-induced central and peripheral corneal swelling with PMMA (9.2–9.6 mm diameter) and hydrogel (14.0 mm diameter) lenses. Both lens types produced greater central than peripheral corneal swelling; however, the hydrogel lenses produced more swelling than the smaller PMMA lenses, indicating lateral diffusion of oxygen from the peripheral area of the cornea not covered by the PMMA contact lens toward the center.
Multiple comparison procedures are useful way to determine which group means differ from which others. These comparisons were made for differences between measurement locations for the SoftPerm lens alone, the SynergEyes lens alone, and the SoftPerm and SynergEyes lens. As shown in Table 5, for the Softperm lens, mean relative uptake rate was significantly less at the central cornea than at 4.5 mm temporal to the central cornea. The slightly higher Dk/t centrally (14.0) than peripherally (5.5) largely contributes to this difference. The 2.0 mm temporal-to-center location was likely not significantly different from the central value because this location was also covered by the rigid portion of the lens. The 4.5 mm temporal-to-center location is close to the junction between the rigid and soft component, but it is situated beneath the soft component. The lower Dk of the soft skirt was enough to result in a significantly higher oxygen uptake rate after lens removal. It is also worth noting that the 2.0 mm temporal-to-center location, which is covered by the rigid central portion, was not significantly different from the 4.5 mm temporal location. It is possible that some of the oxygen from the 2.0 mm temporal-to-center location diffused laterally to the 4.5 mm temporal-to-center location thereby making their respective oxygen uptake rates less different.
As shown in Table 6, the mean relative uptake rate of the SynergEyes lens was significantly less at the central cornea and 2.0 mm temporal to the central cornea than at 1.0 mm temporal to the limbus location. During wear of the SynergEyes lens, the central and 2.0 mm temporal-to-center location are covered by the rigid lens portion that has a Dk value of 100. The soft lens portion that covers the location 1.0 mm temporal to the limbus has a Dk of 9.3. The difference in Dk offers reasonable explanation for the difference in oxygen uptake rates observed. The location 4.5 mm temporal to the central cornea was not significantly different than the central or 2.0 mm temporal-to-center location. The 4.5 mm temporal-to-center location is situated under the soft lens portion and is adjacent to the junction between the rigid and soft lens component. It is plausible that some of the oxygen entering through the highly permeable rigid component diffuses laterally and partially supplies the 4.5 mm temporal location. This additional oxygen supply from the gas permeable portion could account for lower than expected oxygen uptake rate at this location.
As shown in Table 7, the SynergEyes lens was found to have a statistically significant lower mean relative oxygen uptake rate than the SoftPerm lens at all three corneal measurement locations. This difference is primarily attributable to the higher oxygen permeability of the central rigid component in SynergEyes lens. Oxygen reaching the central and 2.0 mm temporal-to-center location does so by passing through the rigid lens component. Because more oxygen can pass through the central portion of the SynergEyes lens than the Softperm lens, the oxygen uptake rate after lens removal is less at these locations. The overall and optic zone diameter of the rigid component is also larger in the SynergEyes lens than the SoftPerm lens. This allows for slightly greater tear pooling between the lens and cornea, which may also contribute to the lower oxygen uptake rate at the central and 2.0 mm temporal-to-center location.46–48 At the 4.5 mm location, oxygen reaching the cornea is most likely derived by a combination of oxygen passing through the soft lens component and by lateral flow of oxygen passing through the rigid lens component. The Dk of the soft component of the SynergEyes lens is only slightly greater than that of the Softperm lens (9.3 and 5.5, respectively). Thus, it does not appear that this offers an adequate explanation of the difference in oxygen uptake rate at these locations. Rather, it appears probable that the difference in oxygen uptake rate is primarily related to the higher Dk of the rigid component in the SynergEyes lens. Because more oxygen is reaching the central and 2.0 mm temporal-to-center location, there is more oxygen available to diffuse laterally to the 4.5 mm temporal-to-center location. With the Softperm lens, less oxygen is initially reaching the central and 2.0 mm temporal-to-center location, and, therefore, less oxygen is available to diffuse laterally. The additional oxygen provided by the SynergEyes lens lessens the corneal demand at the 4.5 mm location during wear and decreases uptake rate at this location after lens removal.
A statistically significant difference was not found between the SoftPerm and SynergEyes lens at the 1.0 mm temporal to the limbus location. Oxygen reaching this location must travel through the soft lens portion of the hybrid lens. The slightly higher Dk of the SynergEyes soft lens portion did not have a noteworthy effect on oxygen uptake at this location. Furthermore, it seems that any additional oxygen provided by the rigid portion of the SynergEyes lens was negligible at this measurement location. This suggests that although lateral oxygen flow from the central component occurs, its effect is localized to an area more adjacent to the rigid lens portion. Measurements at the 1.0 mm temporal the limbus location were also complicated by the presence of conjunctival blood vessels. This is evidenced by the larger standard error at this location.
Differences in contact lens centration can influence corneal oxygen uptake rates. This is especially true with hybrid contact lenses, where the Dk of the central component is different than that of the peripheral component. Furthermore, in keratoconus, an inferiorly displaced corneal apex may promote inferior contact lens decentration. For this study, each of the hybrid lenses used centered quite well. There was no visible difference in centration between the SoftPerm and SynergEyes lens on each subject.
Differences in keratoconus severity could potentially impact the measured corneal oxygen uptake rates. As keratoconus progresses, the cornea steepens and becomes more irregular. Because of a steep and sometimes decentered corneal apex, an uneven tear film may form between the spherical rigid component of the hybrid lens and the cornea. This can result in variable areas of corneal touch as well as tear lens pooling. These areas of touch and pooling can vary in size between subjects and when present could potentially influence the oxygen uptake rate.
The SoftPerm lens has achieved limited success due to hypoxia-related complications, such as corneal edema and neovascularization. Corneal edema occurs following insufficient oxygen supply to the cornea. The SynergEyes contact lens has been shown to allow significantly more oxygen than the SoftPerm lens to the central, 2.0 mm temporal-to-center, and 4.5 mm temporal-to-center locations during wear in keratoconus. Therefore, it is expected that the additional oxygen reaching the cornea with the SynergEyes lens will decrease the incidence of corneal edema.
Corneal neovascularization is thought to be related to chronic hypoxia of the limbus.49–51 The limbus is located approximately between the 4.5 mm temporal-to-central and 1.0 mm temporal to the limbus location. With the SynergEyes lens, lateral diffusion of oxygen from the high Dk central rigid component seems to help supply the 4.5 mm temporal-to-central location. This oxygen, however, does not appear to reach the more peripheral location 1.0 mm temporal to the limbus. This is evidenced by the lack of significant difference found between lenses at the 1.0 mm temporal to the limbus location. Because the limbus is situated between the 4.5 mm temporal-to-center and 1 mm temporal to the limbus locations, it seems plausible that some oxygen will laterally diffuse from the central rigid component of the SynergEyes lens and reach the limbus. The amount of oxygen reaching the limbus will likely be less than that reaching the 4.5 mm temporal-to-center location, but should help to lessen the hypoxic stimulus for corneal neovascularization. In addition, under normal wearing conditions, patients will be blinking. Although blinking does not ordinarily contribute significantly to oxygenation of the central cornea during hydrogel contact lens wear,17,52, 53 stirring of tears beneath the edge of the contact lens might be a source of oxygen to the peripheral cornea.54
In summary, for the normal open eye, there were no significant differences among oxygen uptake rates among the three corneal locations measured. The conjunctival location was found to have a significantly higher oxygen uptake rate than each of the three corneal locations, possibly due to proximal conjunctival blood vessels. The only significant difference among locations for the relativized SoftPerm data was between the central and 4.5 mm temporal-to-center location. Differences among all other locations were not significant. A significant difference for the relativized SynergEyes data was found between the central and 1.0 mm temporal-to-center locations. A significant difference was also determined between the 2.0 mm temporal-to-center and 1.0 mm temporal to the limbus locations. Differences among all other locations were not significant. Significant differences between the relativized SoftPerm and SynergEyes data were found for each of the three measured corneal locations. The conjunctival location was not significantly different. These findings suggest that the SynergEyes lens offers promise as a healthy alternative for the management of keratoconus or other corneal irregularities.
The contact lenses used in the study were supplied by SynergEyes.
Barbara A. Fink
The Ohio State University, College of Optometry
338 West Tenth Avenue
Columbus, Ohio 43210-1240
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