Penetrating keratoplasty (PKP) is a safe and effective procedure for restoration of sight in corneal opacifications. The maintenance of a clear graft is over 90% as a result of current developments in microsurgical techniques and therapeutic modalities.1 Refractive surgical techniques such as suture removal or adjustment, relaxing incisions, wedge resections, and photorefractive keratectomy (PRK) or laser in situ keratomileusis (LASIK) can dramatically reduce postoperative astigmatism after PKP and lead to improved, desirable visual acuity levels in clear grafts.2,3 Overcoming such an important problem has now directed the attention to the quality of vision in transplanted patients.
Most reports concerning the visual results after PKP include the measurement of the best corrected distance visual acuity by using high-contrast optotypes such as Snellen charts.4,5 But other parameters of visual function such as contrast sensitivity, central retinal light sensitivity, and wavefront deformations have not been completely studied in these patients. Recently, attention has been focused on these parameters, which are important indices of visual performance for a variety of real-world conditions.6–9
In this study, we aimed to evaluate the more global picture of the visual function following PKP. To this end, contrast sensitivity, foveal threshold value of visual field tests, and wavefront deformations of the eye were investigated in clinically successful PKP cases.
Patients and Methods
The study included 2 groups of patients. In the first, the patient group (PG), there were 9 patients (12 eyes) who had PKP in our department. The control group (CG) included 12 subjects (18 eyes).
In the PG, there were 4 men and 5 women; their mean age range was 32.55 years ± 9.25 (SD) (20 to 45 years).
Eyes included in this study had anatomically successful corneal grafts and best corrected or uncorrected visual acuity of 20/25 or better using Snellen charts. All operations were performed by 1 of the 2 surgeons (A.Y., S.E.). The graft size was 7.50 mm in all eyes. Donor cornea diameter was 0.25 mm larger than the recipient graft. Corneal graft was sutured to recipient cornea with 16 10-0 nylon sutures in interrupted fashion. At the final examination, all sutures had been removed at least 6 months previously.
Indications of PKP were corneal dystrophy in 4 eyes and keratoconus in 8 eyes. The minimum postoperative follow-up time was 18 months in all patients. Standard ophthalmic examination including visual acuity, manifest refraction, autokeratorefractometry, and intraocular pressure measurements were performed in all subjects. Anterior segment was normal; there was only a tiny scar tissue at the recipient–donor cornea junction. Eyes that had previous rejection attack or attacks, glaucoma, cataract, or any kind of anterior and posterior segment pathology were excluded. Subsequently, contrast sensitivity, central retinal light sensitivity, slit-scanning corneal topography, and ocular aberrometry were performed.
In PG, the mean cylinder degrees in plus cylinder form was 2.60 ± 1.25 diopter (D) (range +1.00 D to +4.75 D).
Results of subjective refraction were collected in standard diopter notation (sphere, cylinder, and axis) and converted to spherical equivalent values for analysis. This mean value was −3.66 D ± 3.57 D (range +2.00 D to −8.00 D) in the PG.
The CG included 12 subjects whose mean age was 36.75 ± 5.85 years (range 24 to 46 years); none had an ocular disease other than refractive errors. The best spectacle-corrected visual acuity was 20/25 or better. The mean cylinder degrees in plus cylinder form was 2.79 ± 2.51 D (range 0.00 to 7.00 D), and the mean spherical equivalent of refractive errors, −5.52 ± 3.37 D (range +1.38 to −12.00 D) in this group.
Contrast Sensitivity Measurement
Cambridge low-contrast gratings (CLCGs) and a computer-generated square-wave grating system were used for assessing contrast sensitivity. A commercially available CLCG (Clement Clark International Ltd.) consists of 12 pairs of plates. The density of strips, formed by multiple dots, diminshes slightly from 1 pair of plates to the next. These square-wave gratings were designed to measure the contrast sensitivity only at 4 cycles/degree (cpd) spatial frequency. The mean room illumination was 80 candela/m2 at a testing distance of 6.0 m.
The computer-generated contrast sensitivity test (S. Egrilmez, MD, and coauthors, “A New Contrast Sensitivity Testing Software for Personal Computers,” presented at the XVth Congress of the European Society of Cataract and Refractive Surgeons, Prague, Czech Republic, September 1997) provides testing at different spatial frequencies. This system presents “E” figures with 5 spatial frequencies (3, 6, 9, 12, and 18 cpd) and 17 contrasts on a computer monitor. Each spatial frequency decreases in contrast by a factor of Symbol or in steps of 0.15 log units, from 100% to 0.4%. Calibration of the monitor contrast and brightness levels are adjusted to light-meter (Lutron Lx-101 luxmeter) measurements. Test distance is 3.0 m, and mean room illumination is 80 candela/m2 for measurements.
Foveal Retinal Sensitivity
Central retinal threshold values were measured using a Humphrey field analyzer (model 750, Zeiss Inc.). All measurements were obtained monocularly with spectacle correction.
Slit-scanning corneal topography and pachymetry (Orbscan, Bausch & Lomb) were used for corneal topographic analysis and kappa intercept measurement, which is the distance (millimeters) between the visual axis and optical axis. Distance between the visual and optical axes was calculated by using x and y values of kappa intercept on topographical map in the following equation:
Aberrometric measurements were obtained under standardized scotopic light conditions without pupil dilation. Zywave® wavefront analyzer (Bausch & Lomb) was used to obtain wavefront aberrations. The device is based on Hartmann-Shack principle; it measures aberrations from lower to higher order and uses a wavelength of 780 nm. It takes measurements from approximately 70 to 75 locations within the pupil.
Each Zywave measurement was analyzed using the Zywave version 4.45ST1 software (Technolas GmbH). A total value of lower- and higher-order aberrations of the eye in the form of Zernike root mean squares (RMS) is available with this software. The magnitudes and the signs of each higher-order aberration are also available in this version of the software. Lower- and higher-order aberrations were measured for a pupil diameter of 6 mm and compared among the PG and CG.
The magnitude of kappa intercept was compared between the groups for detecting the subclinical graft decentration. Corneal topography and aberrometric analysis were performed in a private center.
The statistical analysis was performed using SPSS for Windows (version 11.0). A Mann-Whitney U test was used to compare means of the groups; a P value less than or equal to 0.05 was considered statistically significant.
There were no statistically significant differences in terms of age, cylinder power, and spherical equivalent of refractive errors between the PG and CG (P = .53, P = .88, P = .29, respectively).
The mean contrast sensitivity scores were 96.5 ± 41.1 in grafted eyes and 148 ± 27.7 in CG with CLCG (P = .004). Contrast sensitivity values measured using a computer-generated test revealed statistically significant (P<.001) lower values in grafted eyes at all tested spatial frequencies (Table 1). The difference in contrast sensitivity values exhibited a gradually increasing difference from lower to higher spatial frequencies (Figure 1).
The mean central retinal light sensitivity was 29.91 ± 2.39 db in PG and 33.08 ± 1.56 db in CG (P = .001).
In corneal topographic analysis, mean kappa intercept was 0.69 ± 0.37 mm in the PG and 0.55 ± 0.24 mm in the CG. Although the kappa intercept was higher in the PG, the difference was not statistically significant (P = .20).
At the time of aberrometric analysis, pupil diameters were 8.10 ± 1.02 mm and 7.52 ± 0.64 mm in the PG and CG, respectively; the difference was not statistically significant (P = .08, Mann-Whitney U test).
Lower-order Zernike RMS was 7.30 ± 3.89 μm2 for the PG and 8.58 ± 3.46 μm2 for the CG. This difference was not statistically significant (P = .37). However, the higher-order Zernike RMS was 2.15 ± 0.78 μm2 in the PG and 0.38 ± 0.10 μm2 in the CG; this difference was statistically significant (P<.001). All of the third- and fourth-order aberrations were significantly higher in the PG (Table 2). Figures 2 and 3 display an individual example of aberrometric analysis of patient and control, respectively.
Although the surgical and immunobiological aspects of PKP have been the subject of a significant body of research, little attention has been paid to the visual and functional aspects. A PKP performed for visual rehabilitation is considered successful if the graft remains optically clear and the visual acuity with or without correction reaches better levels than preoperative period with high-contrast optotypes.3–6 However, psychophysical tests such as contrast sensitivity and central retinal light sensitivity are more sensitive indicators of the quality of vision than the high-contrast visual acuity test. The development of videokeratoscopes capable of recording corneal shape in detail and aberrometers that measure the wavefront aberration of the entire eye have provided for the measurement of higher-order aberrations.6–10
In their case report, Munson and coauthors11 concluded that Shack-Hartmann aberrometry provides an objective, quantitative assessment of the optical outcome of vision following PKP and allows the clinician toctively measure retinal image quality. Viewing through an ideal spherocylindrical lens provides vision that is free from lower-order aberrations but not higher-order aberrations. The presence of a high level of higher-order aberrations impairs the quality of vision significantly.9 Nonacuity parameters that gain importance in everyday life and reflect the global visual performance of the eye have usually been the subject of studies following refractive surgeries.12–16
It is well known that there are many refractive surgery patients with minimal residual refractive error and excellent uncorrected high-contrast distance visual acuity who are dissatisfied with their postoperative quality of vision.8;9
Although we have not encountered such problems in keratoplasty patients who are seeking usable vision, it will be useful for us to know whether the graft patients who have visual acuity of 20/25 or higher can be considered totally normal. Studies evaluating the visual quality of patients who had PKP are mostly related to the measurement of contrast sensitivity. Similar to the study of Mannis and coauthors,7 we found the contrast sensitivity values significantly lower at all spatial frequencies in PG and the difference displayed gradual increment toward the higher spatial frequencies. Hess and Carney10 investigated the effect of induced corneal distortion on contrast sensitivity function, and they found a reduction in high spatial frequencies. They determined that light scattering that is attributed to corneal distortion and scarring decreases following PKP and helps the improvement of visual function.
Decentration might have an effect on lower visual quality in PKP patients.16,17 It has already been demonstrated that decentration of the treatment zone is associated with disturbing visual symptoms and decreased low-contrast visual acuity following refractive laser surgeriess.16 Mrochen and coauthors17 reported that decentrations as small as 0.2 mm become important following refractive laser surgeries. Also, Verdon and coauthors12 demonstrated a fair amount of low-contrast visual acuity because of decentrations of the ablation zone in the refractive surgery patients. Recently, Seiler and coauthors14 reported a similar correlation between the ocular aberrations and nonacuity visual functions. Although it did not reach a statistically significant level, decentration of the graft was analyzed in our transplanted grafts. In normal eyes, kappa angle, which is a physiological source of coma aberration, was found to be 0.15 mm larger in the PG than the CG, which might be one of the sources of lower visual quality in PKP. The effect of decentration on ocular aberrations denotes that there must be better solutions to the centration problem in keratoplasty operations, the same as in refractive surgeries.
Physiologically, and especially in refractive surgery, patients' visual performance becomes worse in lower light conditions because of the optical aberrations associated with larger pupil size.15,18–24 As done in refractive surgery patients, aberrometric analysis was performed in standardized scotopic light condition because none of our patients were symptomatic in photopic light condition and visual acuity levels were not less than 20/25 in this illumination level. Photopic spatial contrast sensitivity function measured with best spectacle correction is assumed as a psychophysical index of visual function related to higher-order aberrations (free from lower-order aberrations) in our study. On the basis of our findings of contrast sensitivity measurements, we can conclude that the higher-order aberrations might be responsible for the worse results in the PG than in CG under normal room illumination.
In PKP patients, circle-shaped opacified scar tissue at the interface zone of the graft and the recipient cornea represents the patients' newly formed limbus. The aspheric slope of paracentral cornea neutralizes the spherical aberration of the cornea in normal eyes. Changes in architecture and optical properties of the newly formed paracentral cornea might be one of the reasons for high spherical aberration that increases under lower light conditions.
Applegate and Howland25 evaluated the effect of pupil size on total wavefront aberrations in PRK patients, and they detected that dilation of the pupil diameter from 3 to 7 mm increased the total wavefront aberrations 14-fold preoperatively and 113-fold after PRK. If this finding is taken into consideration, the study evaluating the effect of the graft size on nonacuity parameters might provide important insight into the ablation mode of the transition zone in patients who have had LASIK.25–28
The following key findings of this study, in which the correlation between the visual performance and optical aberrations were evaluated, are as follows:
- Even though the PKP patients have high scores on Snellen charts, they may still have some optical problems because of the higher-order aberrations, which are the cause of reduced contrast sensitivity.
- These aberrations might be the result of reshaping the naturally prolate, centrally steeper, peripherally flatter profile of the cornea. The paracentral cornea, which acts as a neutralizing factor for corneal spherical aberration, deteriorated because of the surgical scar tissue located in this region.
- Even though the result is not statistically significant, decentration of the graft might have a role in higher-order aberrations, especially coma.
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