Article

HOW SAFE ARE SCLERAL LENSES ON GRAFTED EYES?

This retrospective study analyzed the clinical outcomes of scleral lens wear following penetrating keratoplasty.

Corneal ectatic disorders (CEDs), although complex, are quite common. These conditions result in an irregular corneal surface with impairment of vision due to irregular astigmatism. Most notable of these conditions is keratoconus, with a prevalence of 2,300 per 100,000 individuals in India.1 Visual acuity can be corrected in these conditions by replacing the irregular and uneven corneal surface with a more regular and even refractive medium, which reduces the irregular astigmatism.

Corneal transplantation or keratoplasty is well known to be an effective treatment option for visual rehabilitation in patients who have keratoconus.2 Indications for penetrating keratoplasty (PK) range from advanced corneal ectatic conditions to corneal lacerations as well as corneal diseases and infections for which other conventional options fail to provide satisfactory visual correction.3 Dandona et al reported that 6% of all PKs performed in India are on keratoconic eyes.4 Although PK is the last option of treatment for these highly irregular corneal conditions, patients more often fail to obtain a satisfactory post-surgical unaided visual acuity. High irregular astigmatism is common in grafted eyes, which require other forms of optical correction to obtain a satisfactory visual acuity.5

Patients who have a low amount of irregular astigmatism may be able to obtain satisfactory visual correction with spectacles. Corneal GP contact lenses are the first lens of choice in patients having high residual irregular astigmatism post-PK. Although there are many corneal GP lens designs available for fitting these eyes, obtaining an optimal lens-to-cornea fitting relationship is not always possible and often requires additional chair time compared to regular cornea fits. Complaints of lens intolerance are also common with corneal GP lenses.6

Scleral contact lenses are large-diameter, corneal vaulting devices that can restore vision, support healing, and provide ocular comfort in patients suffering from complex corneal conditions. The Scleral Lens Education Society classifies scleral lenses according to the bearing and clearance of the lenses compared to the corneal diameter.7 Scleral lenses rest on the sclera, vaulting over the cornea and submerging the corneal surface in a pool of clear fluid that neutralizes the corneal irregularity, thereby improving vision. Scleral contact lenses are generally well tolerated by patients and can be tailored according to the ocular surface contour of each patient to improve the fitting relationship.8 Scleral lenses can improve vision and comfort in grafted eyes, but not much is known about how these lenses affect the already fragile post-PK corneas.

Corneal edema that can lead to hypoxia is a possibility with scleral lenses. Due to the increased thickness of these lenses and a sealed fluid chamber that restricts the flow of oxygenated tears, scleral lens wear can result in hypoxic stress.9

In this retrospective case series, we report the clinical outcomes of a scleral lens device in the visual rehabilitation of patients after undergoing PK.

MATERIALS AND METHODS

This study followed the declaration of Helsinki and was approved by an Institutional Review Board (Medical Research Foundation, Chennai, India). This retrospective case series reviewed medical records of patients fitted with the PROSE (Prosthetic Replacement of the Ocular Surface Ecosystem, Boston Foundation for Sight) device in post-keratoplasty eyes from 2015 to 2016 for visual rehabilitation. Data collected included patient demographics, refractive status, anterior segment findings, corneal topographical readings, device parameters, fitting-related information, and corneal endothelial cell count. Corneal thickness was measured using an ultrasonic contact pachymeter at baseline and after six hours of device wear by a single and experienced optometrist. An average of 10 pachymetry readings were taken in all four quadrants for each patient. The study included 10 eyes of nine patients, one of whom was a bilateral PROSE wearer. The entire device fitting was performed by a single practitioner. The PROSE trial was carried out over a period of three days. The fitting was assessed after one hour, three hours, and six hours of continuous device wear on subsequent days. An optimal fit was confirmed if there was adequate vaulting of the device over the corneal surface (200 to 300 microns), no staining or impingement on the conjunctival bearing area (scleral landing zone), and satisfactory vision after six hours of continuous device wear.10

The data were entered in Microsoft Excel 2003 and were analyzed using SPSS version 17. Descriptive statistics for the data set were performed and represented as median and interquartile range (IQR). Wilcoxon signed rank test was performed to compare between the best-corrected visual acuity (BCVA) with habitual correction after keratoplasty and with the PROSE device. Corneal thickness at baseline and after six hours of device wear after removing the PROSE device was also compared using the Wilcoxon signed rank test. A “p” value of less that 0.05 was considered statistically significant.

PATIENT DEMOGRAPHICS

Of the nine patients, there were eight males and one female in the study. The median/IQR age of the study population was 34/17 years, ranging from 24 years to 50 years. Table 1 summarizes the demographic details.

TABLE 1 CLINICAL DETAILS OF THE STUDY POPULATION (10 EYES)
Median/IQR* Range
Age (years) 34/17 24 to 50
SIMULATED KERATOMETRY (Sim K)
Flat Sim K (mm) 45.21/6.70 35.82 to 54.24
Steep Sim K (mm) 52.50/8.16 46.50 to 61.43
BCVA** with habitual correction (logMAR) 0.20/0.55 0.00 to 1.00
BCVA with PROSE device (logMAR) 0.00/0.25 0.00 to 0.50
Endothelial cell count (cells/mm2) 1099 1024 to 1664
*Interquartile range, **Best-corrected visual acuity

Keratoconus was the major indication for keratoplasty in the study population (six eyes). Other indications included pellucid marginal degeneration (one eye), Fuchs’ endothelial dystrophy (one eye), and penetrating corneal injury (one eye). The reason for keratoplasty in one eye was unknown. Spectacles, which were dispensed with tolerable cylinder, were used by the majority of patients as habitual correction for vision post-keratoplasty (seven eyes). Corneal GP lenses (one eye), Rose K2 (Menicon) GP lenses (one eye), and fenestrated scleral lenses (one eye) were among other visual correction devices used by patients post-keratoplasty. Table 2 summarizes the reasons for selecting the PROSE device over the patients’ habitual correction apart from the primary aim of maintaining corneal health with the fluid reservoir.

TABLE 2 REASONS FOR CHOOSING THE PROSE DEVICE OVER PATIENTS’ HABITUAL VISION CORRECTION
REASON FOR CHOOSING PROSE DEVICE NUMBER OF EYES
Better visual acuity 6
Better fitting of the lens 3
Better comfort and lens fitting 1

Six out of the 10 eyes had a clear corneal graft, two eyes had developed scarring, one eye had a sutured graft, and one eye had a proud corneal graft (oblate profile) during the last visit. Four patients were referred by ophthalmologists, and five patients were referred by optometrists for the PROSE treatment.

DEVICE PARAMETERS AND FITTING

An optimal fitting was achieved on all of the eyes with the PROSE device. The median vault of the devices fitted was 5.3, ranging from 4.7 to 5.8, and the median diameter was 19.50mm, ranging from 16.50mm to 19.50mm. The median haptic curvature for proper landing on the scleral surface was 15.50mm, ranging from 14mm to 43mm. Five (50%) of the eyes required a toric haptic—two with the rule, one against the rule, and two quadrant specific as follows: four eyes needed the toricity at the 12 o’clock and 6 o’clock position, two eyes at the 9 o’clock position, and two eyes at the 3 o’clock position. The remaining five eyes achieved an optimal fitting with spherical haptic devices. Table 3 shows simulated keratometric values of the eyes requiring toric haptic and spherical haptic designs.

TABLE 3 SIM K* VALUES OF THE EYES REQUIRING TORIC-HAPTIC CONFIGURATIONS AND SPHERICAL-HAPTIC CONFIGURATIONS AT DIFFERENT POSITIONS
EYES REQUIRING TORIC-HAPTIC DEVICES EYES REQUIRING SPHERICAL-HAPTIC DEVICES
Sim K Haptic Positions Needing Toricity Amount of Corneal Cylinder Sim K Amount of Corneal Cylinder
Flat K: 41.77 @ 148
Steep K: 48.09
3 o’clock and
9 o’clock
6.32D Flat K: 47.87 @ 120
Steep K: 48.93
1.06D
Flat K: 54.24 @ 150
Steep K: 61.43
12 o’clock and
6 o’clock
7.19D Flat K: 45.00 @ 91
Steep K: 46.50
1.50D
Flat K: 46.50 @ 169
Steep K: 56.10
12 o’clock and
6 o’clock
9.60D Flat K: 48.81 @ 127
Steep K: 54.35
5.54D
Flat K: 40.44 @ 35
Steep K: 51.70
12 o’clock,
9 o’clock, and 6 o’clock
11.26D Flat K: 45.43 @ 59
Steep K: 52.21
6.78D
Flat K: 35.82 @ 143
Steep K: 59.20
12 o’clock,
3 o’clock, and 6 o’clock
23.39D Flat K: 44.10 @ 72
Steep K: 52.80
8.70D
*Simulated keratometric values

Six patients were comfortably wearing the device and had no device-related complaints; one patient reported handling difficulty with the device; one patient complained of redness, and one patient reported blurred vision after six hours of continuous device wear during the last follow-up visit. There were no other adverse events recorded with the device wear.

RESULTS

A significant improvement in median/IQR visual acuity with the PROSE device was observed as compared to the patients’ habitual correction; the visual acuity improved from 0.2/0.55 logMAR units to 0.00/0.25 logMAR units (p < 0.05). Table 4 summarizes the change in corneal thickness after six hours of device wear compared to the baseline. There was no statistically significant difference observed in the corneal thickness in different zones post-PROSE device wear as compared to the baseline, as is evident from Table 4. The temporal corneal zone had decreased thickness post-PROSE device wear as compared to baseline.

TABLE 4 CHANGE IN CORNEAL THICKNESS IN DIFFERENT ZONES POST-SIX HOURS OF PROSE DEVICE WEAR AS COMPARED TO BASELINE
CORNEAL ZONES Median/IQR* Corneal Thickness (microns) at Baseline Median/IQR Corneal Thickness (microns) After Six Hours of Device Wear Difference in Median Corneal Thickness (microns) from Baseline p value**
CENTRAL 564.50/104 574.50/106 10 0.12
SUPERIOR 568.50/49 621.50/75 53 0.128
INFERIOR 591.50/88 630.50/105 39 0.116
NASAL 575.50/155 596.50/81 21 1
TEMPORAL 619.50/90 613.50/118 –6 0.225
*Interquartile range, **p value obtained through Wilcoxon signed rank test

DISCUSSION

Scleral lenses were among the first contact lenses to be fitted on the eye.7 Although the clinical benefits of these lenses have been well established, complications such as corneal hypoxia and difficulty in the manufacturing process initially restricted these lenses from gaining widespread acceptance among contact lens practitioners. Developments in contact lens materials and advanced technologies have made scleral lenses more popular in recent days.7

Device Parameters and Fitting A proper alignment of the scleral landing zone or the haptic over the underlying scleral surface is necessary to achieve an optimal lens-to-sclera relationship, comfort, and vision. Like all scleral lenses, the PROSE device is highly customizable and can be fitted successfully to almost all scleral contours. Scleral lens landing zones can be customized to closely match the underlying scleral contour using computer-aided design and manufacturing.

Recent studies on normal eyes have shown that most scleras are rotationally asymmetrical and can exhibit with-the-rule, against-the-rule, and quadrant-specific toricity.7 The degree of asymmetry increases with distance from the corneal limbus. Fitting lenses with a spherical landing zone on these toric surfaces may result in excess lift or compression depending on the underlying scleral profile as well as the corneal toricity. Table 3 summarizes the probable relationship between the increase in corneal toricity and the need for a toric haptic. Due to our smaller sample size, the exact relationship between the influence of corneal toricity and the scleral lens haptic shape cannot be determined from this data. Consejo et al has suggested that astigmatism is not restricted to the cornea, but should rather be considered a property of the entire eye globe.11 In the current study, the corneal condition before PK was not typically limited to astigmatism but also included other pathologies. The theory stated in the Consejo et al study11 might apply for pathologically irregular eyes also, but this needs further investigation and understanding.

Incorporating a toric haptic configuration on the landing zone of the lens can significantly improve the lens-to-sclera relationship, thereby improving the fitting. According to a study by Visser et al,12 patients reported significant improvement in comfort, visual acuity, and overall satisfaction with toric haptic designs as compared to spherical-haptic-design scleral lens. They also reported that the patients who wore toric haptic lenses exhibited less accumulation of trapped tear debris between the lens and the cornea. In the present study, almost 50% of the eyes required a toric haptic for better fitting; two eyes needed with-the-rule haptic toricity, one eye needed against-the-rule haptic toricity, and two eyes required quadrant-specific haptic toricity.

Visual Outcome with the PROSE Device Barnett et al published a retrospective review of 34 patients who had undergone PK and were visually rehabilitated with mini-scleral lenses.13 The primary indication for PK in their study was keratoconus. They reported significant improvement in visual acuity with the mini-scleral lenses as compared to the habitual correction; 91.7% of the patients achieved a visual acuity of 20/40 or better. Our study is in agreement with this; all of our patients reported an improvement in visual acuity with the PROSE device in comparison with their habitual correction. Although the study by Barnett et al used Jupiter mini-scleral lenses, the working principle of both mini-scleral and scleral lenses is essentially the same, i.e., to replace the irregular corneal surface with a fluid-ventilated, regular refractive medium. In the present study, most of the patients chose the scleral lens over their habitual correction for better vision quality (Table 2).

Handling Most of the patients in our study were comfortable PROSE device wearers and had no scleral-lens-related complaints; however, one patient did report handling-related problems as a result of decreased manual dexterity, which caused difficulty with device application and removal. With handling instructions and additional in-office training, we were able to encourage the patient to successfully use the device.

Corneal Swelling Not much is known about the effect of scleral lens wear on grafted eyes. In the present study, the corneal response was recorded in the form of change in corneal thickness, or the corneal edema induced with the scleral lenses after six hours of continuous wear. The corneal endothelial cells maintain the balance of the corneal hydration. Post-PK, endothelial cell function is reported to be compromised. The density of the corneal endothelial cells is reported to decrease at a rate of 7.8% from three years to five years after keratoplasty.14 Although scleral lenses are fabricated with high-oxygen-permeable polymers, they induce a certain amount of hypoxic stress and corneal edema.15 This can result in excess stress on the already compromised corneal endothelial cells. The prolonged hypoxic stress can ultimately lead to corneal graft rejection; Severinsky et al found that 10 out of 33 (30%) eyes fitted with scleral lenses had developed graft rejection.9

In our study, the lens material used to fabricate the PROSE device was Boston Equalens II (Bausch + Lomb), which is a GP lens material with a Dk of 85.16 But even with such a high-Dk lens material, an increase in corneal thickness was observed after six hours of continuous lens wear, suggesting hypoxic stress. The superior quadrant showed the greatest increase in corneal thickness after six hours of lens wear as a result of the upper eyelid overlapping this area, thereby reducing the exposure of the lens system to the atmospheric oxygen; conversely, the central region of the cornea received a consistent supply of atmospheric oxygen through the lens because of maximum exposure and therefore showed the least increase (Table 4). The temporal quadrant demonstrated a decrease in corneal thickness from baseline with scleral lens wear. The reason is not quite clear and requires further investigation. According to Holden and Mertz, the physiological corneal swelling post-sleep is approximately 4%.17 The average corneal thickness in Indian eyes is reported to be 533.3 microns.18 Hence, any increase in corneal thickness of more than 21.33 microns is a clinically significant change and needs to be addressed. Although there was no statistically significant difference in corneal thickness pre- and post-scleral lens wear, the change was clinically significant in two quadrants (i.e., the superior and inferior quadrants), and it was borderline in the nasal quadrant. Therefore, determination of corneal thickness before and after six hours of trial scleral lens wear is recommended to avoid any incidental corneal hypoxic stress.

Two patients complained of redness and blurred vision, respectively, after the six hours of continuous device wear, which we found to be associated with increased corneal swelling or corneal edema during wear of the device. The lens material was changed to Boston XO (Dk 100, Bausch + Lomb), a higher-Dk material as compared to the Boston Equalens II. The patients were also instructed to limit the continuous wearing period to five hours and to take a one-hour break before reapplying the device. A total of 10 hours/day wearing duration was suggested.

A higher-Dk lens material can be used to fabricate the final lens—for example, Boston XO2 (Dk 150) (Bausch + Lomb)—to prevent corneal hypoxia. Reducing the lens thickness and corneal clearance (in this case, the vault) can be beneficial and can reduce the risk for hypoxic stress.10 Patients can also be advised to reduce the number of continuous wearing hours. Instruct patients to monitor for colored halos and to remove the scleral lens if colored halos are observed around a light source, which is an indication of corneal hypoxia.

CONCLUSION

Being a retrospective case series, our study has a few limitations. The primary limitation was the small sample size. Being a tertiary care hospital, the patients treated here are referred from different eyecare centers across the country, where they either had already been treated with scleral lenses and were not successful, or they were already exhibiting signs of graft failure and scleral lenses were contraindicated. Therefore, obtaining a larger sample size was difficult. Measuring lens-to-cornea clearance in all quadrants with an anterior segment optical coherence tomographer (AS-OCT) would have been helpful in further explaining the corneal thickness change in different quadrants. Pachymetry was performed to monitor corneal edema because it is a faster and more patient-friendly procedure compared to AS-OCT.

In conclusion, high irregular astigmatism, poor fitting, and GP lens intolerance in post-PK patients can make management for this group tedious and more time-consuming. Scleral lenses provide a promising solution to these problems. Improved visual outcomes and comfort can be obtained with scleral lenses in these patients. Although most scleral lenses are made from high-oxygen-permeable materials, there always remains the possibility of corneal hypoxia with these lenses. As post-PK corneas are already compromised in different ways, it is wise to avoid any corneal hypoxic stress resulting from the lens. Choosing a higher-Dk lens material and instructing patients to periodically remove the lens may prevent these hypoxic changes and thereby be safer for the eye. CLS

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