Purpose
To investigate the corneal higher-order aberrations (HOAs) of the anterior and posterior corneal surfaces in eyes that underwent penetrating keratoplasty (PK), deep anterior lamellar keratoplasty (DALK), and Descemet stripping automated endothelial keratoplasty (DSAEK).
Design
Retrospective, case-control study.
Methods
study population: Twenty-four eyes underwent PK, 28 eyes underwent DALK, and 19 eyes underwent DSAEK; 29 normal eyes served as controls. observation procedures: The anterior and posterior corneal heights and pachymetric data were obtained with a Scheimpflug-based corneal topographer. Corneal HOAs for 4-mm pupils were calculated from the height data and were expanded with normalized Zernike polynomials. The HOAs resulting from the anterior and posterior corneal surfaces were compared among the procedures. main outcome measures: Anterior and posterior corneal HOAs (root mean square).
Results
Control eyes had significantly lower total HOAs and Zernike vector terms of the anterior and posterior surfaces than the other groups, except for spherical aberration. The mean anterior corneal surface total HOAs in the PK, DALK, DSAEK, and control groups were 1.38 ± 0.67 μm, 1.19 ± 0.57 μm, 0.61 ± 0.33 μm, and 0.21 ± 0.07 μm, respectively. The anterior corneal HOAs in the DSAEK group were significantly less than those in the PK group ( P < .001) and DALK group ( P < .001). The mean posterior corneal surface total HOAs were, respectively, 0.20 ± 0.09 μm, 0.24 ± 0.11 μm, 0.27 ± 0.15 μm, and 0.07 ± 0.02 μm. There were no significant differences in the posterior corneal HOAs among the treatment groups.
Conclusions
Because the refractive indices between the anterior and the posterior surfaces differed greatly, eyes that undergo DSAEK have lower anterior corneal HOAs compared with PK or DALK eyes. However, the anterior and posterior corneal HOAs in DSAEK eyes still were greater than those in control eyes.
Penetrating keratoplasty (PK) has been the gold standard for treating corneal disorders for many years. With recent developments in corneal transplantation, lamellar keratoplasty or selective replacement of a diseased corneal layer is performed, instead of replacing all corneal layers. The recent trend is to perform deep anterior lamellar keratoplasty (DALK) for patients with healthy normal endothelium and Descemet stripping automated endothelial keratoplasty (DSAEK), which has became the most popular variation of posterior lamellar keratoplasty, for patients with only endothelial dysfunction.
Several recent studies have compared the optical quality among these surgeries by measuring the corneal or ocular higher-order aberrations (HOAs). While no significant differences in ocular HOAs between eyes treated with DALK or PK have been reported, one study reported that DALK is associated with higher ocular HOAs than PK. Although DSAEK induces less ocular HOAs than PK, the irregularities of the posterior corneal surface can be presumed to increase after DSAEK. Recent studies have calculated the anterior and posterior corneal higher-order irregularities in DSAEK eyes using the topographic height data obtained with a Scheimpflug-based corneal topographer. However, those studies did not evaluate the corneal irregular astigmatism as HOAs. Using a Scheimpflug-based corneal topographer, a recent study reported that posterior HOAs of the central 6 mm of the cornea were significantly higher in eyes that underwent DSAEK than in those that underwent PK, despite a nonsignificant difference in the central 4 mm. Thus far, little is known about the corneal HOAs from the posterior corneal surface after keratoplasty.
With the increasing popularity of corneal lamellar keratoplasty procedures such as DALK or DSAEK, it is important to characterize the optical differences among PK, DALK, and DSAEK. We reported that evaluation of HOAs resulting from the posterior corneal surface and Zernike vector analysis was useful in understanding the relationship between the anterior and posterior corneal aberrations. In the current study, the corneal aberrations caused by the anterior and posterior surfaces were evaluated separately to gain an understanding of the optical characteristics of the anterior and posterior corneal surfaces for the 3 keratoplasty procedures.
Methods
Patient Population
We retrospectively reviewed the clinical charts of patients who underwent PK, DALK, or DSAEK, and reliable anterior and posterior corneal topographic evaluations were possible after surgery. Patients were included who had no clinically relevant postoperative corneal edema and who had accurate, good-quality corneal topographic data (24 eyes of 24 patients who had undergone PK, 28 eyes of 28 patients who had undergone DALK, and 19 eyes of 19 patients who had undergone DSAEK). Twenty-nine eyes of 29 normal subjects with no ocular pathologic features except refractive errors served as controls. The characteristics of the subjects in each group are shown in Table 1 . All patients had been followed up for at least 3 months. Patients were excluded who had corneal edema or a stromal scar in the central 4 mm of the graft that could cause apparent scatter.
| Parameter | Control Group (29 Eyes) | PK Group (24 Eyes) | DALK (28 Eyes) | DSAEK (19 Eyes) | P Value |
|---|---|---|---|---|---|
| Age (years) | 49.5 ± 15.3 | 61.0 ± 18.6 | 62.1 ± 14.2 | 71.1 ± 7.1 | .062 |
| Follow-up time (months) | N/A | 45.5 ± 24.4 | 35.3 ± 22.1 | 9.9 ± 5.9 a | <.001 |
| Postoperative BCVA (logMAR) | −0.13 ± 0.0 | 0.27 ± 0.30 | 0.55 ± 0.40 b | 0.31 ± 0.39 | .018 |
a P < .05, comparison with PK and DALK by one-way analysis of variance.
b P < .05, comparisons with PK and DSAEK by one-way analysis of variance.
Surgical Technique
Twenty-four PK procedures were performed in a standardized fashion; the graft and host trephination sizes were 7.5 and 7.0 mm or 8.0 and 7.5 mm in all cases, and the donor cornea was sutured with combined 10-0 nylon. Twenty-eight DALK procedures were performed as described previously by Sugita and Kondo. Donor corneas were trephinated to a diameter 0.25 mm larger than the recipient bed (7.5 or 8.0 mm), and the donor cornea was sutured with combined 10-0 nylon suture. Nineteen DSAEK procedures included stripping Descemet membrane and endothelium from the recipient’s central cornea and transplantation of a 7.75- to 9.0-mm donor disc of donor endothelium and posterior stroma dissected with a microkeratome using the taco technique or the pull-through technique. Sutures were used only to close the sclerocorneal incisions.
In patients with clinically relevant cataracts, phacoemulsification and intraocular implantation were performed mostly before DSAEK. Seven patients who underwent PK and 2 who underwent DSAEK had undergone a combined cataract procedure. Only loose sutures were removed selectively for PK and DALK during the follow-up period.
Measurements
The corneal HOAs were calculated for 4-mm pupils from the difference between the height data and the best-fit sphere, using an original program for the anterior and posterior corneal surfaces according to a procedure used previously. Briefly, the anterior and posterior corneal heights and pachymetric data were obtained with a Scheimpflug-based corneal topographer (Pentacam HR; Oculus, Inc, Wetzlar, Germany). During one scan, 25 pictures were obtained to reconstruct a 3-dimensional model of the entire cornea. All subjects were examined at least twice to confirm the reproducibility of the data obtained. A custom program expanded the anterior and posterior height data to Zernike polynomials and extracted the components of the ideal wavefront of the best-fit sphere. The corneal HOAs were calculated by multiplying the residual components by the difference in the refractive indices at the anterior and posterior surfaces. The spherical aberrations included by the reference sphere itself were added to avoid underestimation of the spherical aberrations. The reference axes of the measurements were aligned along the primary line of sight.
The corneal HOA data were analyzed quantitatively in the central 4-mm diameter up to the sixth order by expanding the set of Zernike polynomials. For each pair of standard Zernike terms for trefoil, coma, tetrafoil, and secondary astigmatism, 1 value for the magnitude was calculated by Zernike vector analysis. Total HOAs were defined as the root mean square of the magnitudes for the third-, fourth-, fifth-, and sixth-order aberrations. The spherical aberration was expressed as a positive or negative value, not as an absolute value. The ratios of the anterior corneal HOAs to the posterior corneal HOAs (A/P ratio) for total HOAs and each Zernike vector term also were compared.
Statistical Analysis
Data were analyzed using statistical analysis software JMP version 9 (SAS Institute Inc, Cary, North Carolina, USA). The Kruskal-Wallis test was used to compare the total HOAs, trefoil, coma, tetrafoil, secondary astigmatism, and spherical aberration of the anterior and posterior corneal surfaces among the 4 groups. The Kruskal-Wallis test also was used to compare the A/P ratios for total HOAs and each Zernike vector term among the 4 groups. The Steel-Dwass method was used for multiple comparisons. P values less than .05 were considered significant for all analyses.
Results
Figure 1 shows the color-coded maps of the anterior and posterior corneal HOAs in representative cases from each group. No clinically relevant HOAs were seen in the control eye ( Figure 1 , First column, Top and Bottom). Color-coded maps of the anterior surface showed a marked increase of HOAs in PK and DALK eyes ( Figure 1 , Second column, Top, and Third column, Top), whereas fewer alterations in the posterior corneal map were found than in the anterior corneal map ( Figure 1 , Second column, Bottom, and Third column, Bottom). In a DSAEK eye, although no marked color changes were seen in the anterior corneal HOA map, the alterations in the posterior corneal map were greater than in a normal control eye ( Figure 1 , Fourth column, Top and Bottom).
Figure 2 shows the averages of the total HOAs and each Zernike vector term for the 4-mm cornea. Control eyes had significantly ( P < .001) lower total HOAs and Zernike vector terms of the anterior and posterior surfaces than the other 3 groups, except for the spherical aberrations of both the anterior and posterior surfaces. The mean anterior corneal total HOAs in the PK, DALK, and DSAEK groups were 1.38 μm, 1.20 μm, and 0.61 μm, respectively ( Figure 2 , Top; Table 2 ). The anterior corneal total HOAs of DSAEK eyes were significantly lower than those of the PK eyes ( P < .001) and DALK eyes ( P < .001) and significantly ( P < .001) greater than those of the control eyes. The mean trefoil, coma, and secondary astigmatism from the anterior surfaces of the DSAEK eyes were significantly lower than those in the PK eyes ( P = .010, P = .005, and P = .001) and DALK eyes ( P = .029, P = .005, and P = .013), respectively. The mean posterior corneal total HOAs in the PK, DALK, and DSAEK groups were 0.20 μm, 0.24 μm, and 0.27 μm, respectively ( Figure 2 , Bottom; Table 2 ). There were no significant differences in the posterior corneal total HOAs among the PK, DALK, and DSAEK groups, although the DSAEK group had larger posterior corneal HOAs followed by DALK and PK. There were no significant differences in specific Zernike vector terms from the posterior surface, except for spherical aberration, among the 3 groups. DSAEK had the highest coma value from the posterior surfaces, followed by DALK and PK. The spherical aberration of the posterior surfaces in DSAEK eyes differed significantly from that of PK eyes ( P = .008) and DALK eyes ( P = .003); there were no significant differences among the groups in spherical aberration resulting from the anterior surface.
| Total HOAs | Trefoil | Coma | Tetrafoil | Secondary Astigmatism | Spherical Aberration | |
|---|---|---|---|---|---|---|
| Control | ||||||
| Anterior surface | 0.21 ± 0.07 | 0.10 ± 0.06 | 0.10 ± 0.07 | 0.07 ± 0.04 | 0.04 ± 0.02 | 0.10 ± 0.05 |
| Posterior surface | 0.07 ± 0.02 | 0.03 ± 0.02 | 0.04 ± 0.02 | 0.02 ± 0.02 | 0.01 ± 0.01 | −0.03 ± 0.02 |
| A/P ratio | 3.3 ± 1.3 | 3.8 ± 2.3 | 4.6 ± 4.5 | 3.8 ± 3.9 | 2.8 ± 0.7 | N/A |
| PK | ||||||
| Anterior surface | 1.38 ± 0.67 | 0.60 ± 0.36 | 0.88 ± 0.63 | 0.44 ± 0.27 | 0.35 ± 0.22 | 0.00 ± 0.47 |
| Posterior surface | 0.20 ± 0.09 | 0.10 ± 0.06 | 0.10 ± 0.08 | 0.08 ± 0.06 | 0.05 ± 0.04 | 0.00 ± −0.05 |
| A/P ratio | 7.8 ± 4.1 a | 7.9 ± 6.1 | 12.2 ± 11.0 b | 7.9 ± 5.9 c | 19.6 ± 36.5 d | N/A |
| DALK | ||||||
| Anterior surface | 1.19 ± 0.57 | 0.53 ± 0.38 | 0.76 ± 0.50 | 0.33 ± 0.25 | 0.28 ± 0.21 | 0.19 ± 0.36 |
| Posterior surface | 0.24 ± 0.11 | 0.13 ± 0.09 | 0.12 ± 0.09 | 0.09 ± 0.06 | 0.07 ± 0.03 | 0.01 ± 0.07 |
| A/P ratio | 5.3 ± 2.3 e | 5.8 ± 5.9 | 11.0 ± 12.2 f | 5.1 ± 1.3 | 4.6 ± 2.9 | N/A |
| DSAEK | ||||||
| Anterior surface | 0.61 ± 0.33 | 0.29 ± 0.22 | 0.34 ± 0.24 | 0.26 ± 0.20 | 0.14 ± 0.07 | 0.10 ± 0.13 |
| Posterior surface | 0.27 ± 0.15 | 0.11 ± 0.11 | 0.17 ± 0.14 | 0.08 ± 0.06 | 0.06 ± 0.03 | −0.05 ± 0.05 |
| A/P ratio | 2.7 ± 1.7 | 4.5 ± 4.8 | 2.7 ± 2.1 | 6.8 ± 10.5 | 3.4 ± 3.5 | N/A |
| P value for A/P ratio comparison | <.001 | .066 | <.001 | .031 | <.001 | N/A |
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