Background

Corneal transplantation is the main approach to restoring the sight of patients with corneal blindness, [1,2,3] but the surgery usually induces irregularity of the front corneal surface, leading to poor postoperative visual acuity despite the correction with spectacles [4,5,6]. Although the implanted corneal graft becomes transparent, the poor visual performance may still affect the quality of life [7,8,9]. Rigid gas permeable contact lenses (RGP-CLs) have been reported to significantly improve visual acuity of patients undergoing keratoplasty for high permeability, safety, and correction of corneal astigmatism [7, 10, 11]. However, attention has just been paid to the vision under high contrast conditions. The reduced contrasts, which are more closely related to our daily activities, [12] have not been investigated. For keratoconic patients, wearing RGP-CLs could reduce second-order and higher-order aberrations in the cornea, and thereby improving the optical quality [13, 14]. But it is unclear whether different contrast visions could be increased for patients after keratoplasty by wearing the RGP-CLs, and whether there is a link to the reduced corneal aberrations. In this study, the visual and optical effects of RGP-CL wear on post-keratoplasty patients were evaluated by measuring contrast visual acuities (CVAs) at high, medium, and low levels and identifying the key corneal aberration change after lens wear, so as to provide an individualized further intervention for each patient with prior treatment of keratoplasty.

Subjects and methods

Patients

Nineteen post-keratoplasty patients (19 eyes) who were successfully fitted with RGP-CLs at our hospital from July 2017 to June 2018 were included. There were 14 males and 5 females, aged 16–48 years (mean, 30.45 ± 5.83 years). All of them had the sutures of penetrating keratoplasty removed at least 3 months before, with best spectacle-corrected visual acuity > 0.3 logMAR and corneal endothelial cell count ≥ 1000/mm2. They all expected better visual acuity, accepted RGP-CL wearing, and were not allergic to RGP-CL solutions. Patients who suffered active conjunctivitis or keratitis, allergic conjunctivitis, moderate or severe dry eye were excluded. Informed consent was obtained from all subjects or their guardians after the nature and possible consequences of the study had been explained to them. The study had an approval from the Shandong Eye Hospital Ethics Committee (SDSYKYY20170521) and complied with the tenets of the Declaration of Helsinki.

The primary diseases included keratoconus in 17 eyes, herpes simplex keratitis in 1 eye and fungal keratitis in 1 eye. The average time between initiation of RGP-CL wear and keratoplasty was 4.4 ± 2.0 years. Data of CVAs with spectacles and RGP-CLs as well as the wavefront aberrations at the anterior surface of the cornea before and 1 month after continuous wearing of RGP-CLs were collected. Before wearing RGP-CLs, corneal topography (Allegro Oculyzer, WaveLight GmbH, Erlangen, Germany) revealed significant irregular astigmatism with an index surface variance of 108.91 ± 46.26 and an index height asymmetry of 28.57 ± 18.73 [15]. The mean subjective spherical diameter was − 3.41 ± 3.50 D, and the column diameter was − 4.33 ± 2.02 D.

Measurement of contrast visual acuity

The included eyes were tested for the uncorrected visual acuity (UCVA), best spectacle-corrected visual acuity (BSCVA), and RGP-CL-corrected visual acuity at 5.5 m from a multifunctional visual acuity tester (MFVA-100, BriteEye Medical Tech Co. Ltd, Shenzhen, China), which is capable of measuring distant visual acuity using isolate letters at four contrast levels (100%, 25%, 10%, and 5%), [16] in a dark room, with the contralateral eyes completely covered. Data at the levels of 100%, 25% and 10% were statistically analyzed, since the UCVA at 5% was over 1.4 logMAR after keratoplasty, exceeding the measurement range of the instrument. Low contrast loss was defined as low contrast visual acuity minus high contrast visual acuity [17, 18].

Measurement of corneal wavefront aberrations

The wavefront aberrations of the anterior corneal surface before and 1 month after RGP-CL wear were examined in all included eyes. After the patient was acclimated to the dark room for 10 min, corneal topography was performed to measure the corneal surface in a naturally dilated state, 3–5 times for each eye. Three measurements with a good repeatability of 6 mm or more were selected to calculate the corneal aberrations in Zernike polynomials. The RMS values of the anterior surface aberrations in the central 8-mm area of the cornea, including spherical aberration Z40, vertical coma aberration Z3 − 1, horizontal coma aberration Z3 + 1, astigmatism term Z2 − 2, and Z22, were recorded in microns (µm) for statistical analysis.

RGP-CL fitting

FreshKon® Rose-K postoperative irregular lenses with a diameter of 10.0 to 10.6 mm were used in the study. The lens material is Boston XO, giving a nominal oxygen transmissibility of 100 × 10− 11(cm2/s) (mlO2/mL·mmHg). The RGP-CL was prepared according to the corneal parameters of each patient. The eye covered with the RGP-CL was stained with sodium fluorescein for observation of the distribution of tear fluids under the lens in a cobalt blue light of a slit lamp microscope. The fitting status was assessed and photographed. With staining, a tear fluid accumulation of 0.1 to 0.2 mm in the central cornea and a peripheral curve of 0.6 to 1.0 mm were detected. All lenses were well fitted with no obvious deviation, despite 1- to 2-mm movement (shown in Fig. 1).

Fig. 1
figure 1

A The graft is transparent with all the sutures removed after keratoplasty. B The RGP-CL fitting is good for the eye with the thickness of the central lacrimal layer being 0.1 to 0.2 mm and the peripheral curve being 0.6 mm

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Statistical analysis

SPSS 19.0 software was used for statistical analysis. The data followed a normal distribution through the Kolmogorov-Smirnov test. Different corrective modalities under varied contrast visual acuity conditions were compared using the Chi-square test, and the data before and after RGP-CL wearing was analyzed using the matched sample t-test. P < 0.05 was considered statistically significant.

Results

UCVA, BSCVA, and RGP-CL-corrected visual acuity at different contrast levels

At the 100% contrast, UCVA was below 0.3 logMAR in all patients, BSCVA was above 0.3 logMAR in 21.1% (4/19) of the patients, and RGP-CL-corrected vision was above 0.3 logMAR in 63.2% (12/19) of the patients. The UCVA, BSCVA, and RGP-CL-corrected visual acuity at the 100%, 25%, and 10% contrasts were found to be statistically significant (P < 0.001) (shown in Table 1).

Table 1 Comparison of UCVA, BSCVA, and RGP-CL-corrected visual acuity at different contrast levels
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The difference between BSCVA and RGP-CL-corrected visual acuity at different contrast levels

The differences between BSCVA and RGP-CL-corrected visual acuity at the 100%, 25%, and 10% contrast levels were 0.174 ± 0.074 logMAR, 0.230 ± 0.105 logMAR, and 0.247 ± 0.091 logMAR, respectively. With the decrease in the contrast level, RGP-CL-corrected visual acuity increased more significantly than BSCVA, and the increase at the 10% contrast was significantly better than that at the 100% contrast (P = 0.036) (shown in Fig. 2).

Fig. 2
figure 2

 A linear chart of the difference of the corrected vision between BSCVA and RGP-CL-corrected visual acuity at different contrast levels. Bars denote ± 1 standard error of the mean (SEM). * Significant differences (P = 0.036)

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Comparison of the losses of UCVA, BSCVA, and RGP-CL-corrected visual acuity at different contrast levels

The losses of UCVA, BSCVA, and RGP-CL-corrected visual acuity at the 10% contrast and the 25% contrast were significantly different (P < 0.05). The losses related to RGP-CLs were smallest. The mean values of the 10% and 25% contrast vision losses are shown in Table 2.

Table 2 Comparison of the losses of UCVA, BSCVA, and RGP-CL-corrected visual acuity at different contrast levels
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Comparison of wavefront aberrations in the anterior corneal surface and the anterior surface of RGP-CLs

The results of wavefront aberrations at the anterior corneal surface before and 1 month after RGP-CL wear are summarized in Table 3. The stigmatism Z− 22 and Z22 and spherical aberration Z04 were significantly improved after wearing RGP-CLs as compared to those before the lens wear (P < 0.05), while the coma terms Z− 13 and Z13 were not significantly improved (P > 0.05).

Table 3 Comparison of wavefront aberrations of the anterior surface of cornea before and after RGP-CL wear
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Discussion

After the treatment by keratoplasty, patients often recover corneal transparency successfully, but the visual acuity at the high contrast condition remains low, [19, 20] which may be correlated with corneal wavefront aberrations [21,22,23]. In this study, a gradual decrease in UCVA was found in post-keratoplasty patients when the contrast was reduced. The improvement of BCVA with spectacles also depended on the contrast levels. When the RGP-CLs were applied to correct the refractive errors, the improvement in visual acuity at all the contrast levels was more significant compared to the use of spectacles, particularly at the lower contrast levels. Therefore, it is necessary to identify the change of wavefront aberrations after RGP-CL wear in post-keratoplasty patients for better improvement of visual performance.

The effect of Zernike aberrations on the optical quality of human eyes varies greatly, with spherical aberration, defocus, astigmatism, and coma in a descending order [24]. The wavefront aberrations have been reported to be reduced with RGP-CL wear in keratoconic patients, including both lower and higher order aberrations, [25, 26] but Romero-Jiménez et al. [27] disclosed that patients with keratoconus can improve high-order aberrations of the anterior corneal surface by wearing RGP-CLs. Greg et al. [28] showed that patients who received keratoplasty had an average reduction of 66% in total higher-order aberrations, 83% in spherical aberration, and 71% in coma by wearing RGP-CLs. In this study, it was found that astigmatism and spherical aberration were significantly reduced with the RGP-CLs for post-keratoplasty patients. Therefore, we believe that the aberration control of the RGP-CLs plays an important role in the improvement of the visual performance for patients after keratoplasty, especially the low contrast viual acuity.

Previous studies have revealed significant irregular astigmatisms in 62.9–77.7% of patients after keratoplasty, and these refractive errors could not be corrected with the spectacles [29, 30]. Uncorrected higher-order aberrations have also been reported to be reduced by wearing RGP-CLs in keratoconic patients, and the contrast sensitivity at the low to middle spatial frequencies can be improved [31, 32]. In our series, the UCVA was worse than 0.3 logMAR for all patients after keratoplasty, and 78.9% of the patients remained the visual acuity of worse than 0.3 logMAR after wearing spectacles. Moreover, anisometropia made it difficult for monocular patients to adapt to spectacles. With the correction by the RGP-CL, 100% of the patients were able to get out of low vision, and 63.2% of the patients obtained corrected vision of above 0.3 logMAR, which was more conducive to achieving simultaneous vision of both eyes [33]. The RGP-CL-corrected visual acuity was better than BSCVA at both high and low contrast levels, especially at the low contrasts. The RGP-CLs may significantly improve patients’ night vision and visual-spatial discrimination, enhancing the quality of life.

The limitations of our study are the small sample size and the use of a single series of postoperative irregular lenses. Besides the Rose-k series, the scleral contact lenses are gradually applied in patients after keratoplasty [34, 35]. A larger cohort of patients with more kinds of contact lenses are needed for further investigation.

Conclusion

In conclusion, RGP-CL wear effectively improves visual performance of post-keratoplasty patients, particularly visual acuity at lower contrast levels. The reduction of corneal aberrations, mainly the spherical and astigmatism aberrations, with RGP-CLs contributes to the visual improvement at varying contrast conditions.