Anterior Corneal Surface
Considering the intergroup comparison results, the total HOAs within the 4-mm zone appeared highest in the KC eyes, followed by the ALTK and PK groups, then DALK eyes, and lowest in controls. The total HOAs within the 6-mm zone were higher in the KC, ALTK, and PK eyes; intermediate in the DALK eyes; and the lowest in controls. Trefoil and tetrafoil were significantly higher in the PK eyes; intermediate in the KC, ALTK, and DALK eyes; and lowest in controls. Coma appeared significantly higher in the KC and ALTK eyes, intermediate in the DALK and PK eyes, and the lowest in controls. Spherical aberration and secondary astigmatism were significantly lower in controls than in other groups. The intragroup comparison revealed that the aberration components that were significantly higher included coma and spherical aberration in controls, coma in the KC and ALTK eyes, trefoil and coma in the DALK eyes, and trefoil in the PK eyes.
Posterior Corneal Surface
The intergroup comparison showed that the total HOAs, trefoil, spherical aberration, and secondary astigmatism appeared significantly lower in controls than in the other groups. Within the 4-mm zone, coma appeared highest in KC and ALTK eyes, followed by the DALK eyes, then less in the PK eyes, and lowest in controls. Within the 6-mm zone, coma was higher in the KC, ALTK, and DALK eyes; intermediate in PK eyes; and the lowest in controls. Tetrafoil was significantly lower in controls than in the other groups within the 4-mm zone; whereas, it appeared higher in the PK eyes, intermediate in the KC, ALTK, and DALK eyes, and the lowest in controls within the 6-mm zone. The intragroup comparison indicated that the aberration components that were significantly higher within the 4-mm zone were trefoil in the control, DALK, and PK eyes and trefoil and coma in the KC and ALTK groups. With regard to those within the 6-mm zone, they included trefoil in the control and PK eyes and trefoil and coma in the KC, ALTK, and DALK eyes.
Representative wavefront maps of anterior and posterior corneal surfaces in normal, KC, ALTK, DALK, and PK eyes are shown in Fig. 1. The correlations between the BSCVA and the magnitude of the HOAs of both corneal surfaces were not statistically significant in any of the five groups (p > 0.05).
Irregularities of the refractive surfaces of the eye cause irregular astigmatism, which is not correctable by conventional spectacles or soft contact lenses.20,22 Great amounts of HOAs have been shown to decrease the optical quality of the retinal image, thus reducing the optical performance of the eye by inducing halos, glare, monocular diplopia, and decreased contrast sensitivity and visual acuity,28,29 especially under mesopic or scotopic conditions.30 The cornea is the main contributor of HOAs in the eye, and regardless of cause, corneas with increased wavefront error show significant decreases in visual performances that is pupil size dependent.31,32
In agreement with previous authors,5 our results showed that, in normal eyes, the amount of the corneal HOAs within both 4-mm and 6-mm central zones was minimal, suggesting that only a small part of the variance in visual performance of normal eyes both under photopic and scotopic conditions can be attributed to wavefront error variations.29
Aberrometric studies conducted with both ocular or corneal wavefront sensors have demonstrated that KC eyes show a great amount of HOAs2–5 because of both corneal surfaces5,33 and that corneal HOAs can be useful for early detection,34 grading,35 and predicting visual performance of KC eyes.36 In agreement with previous authors,5,33 our results showed a higher amount of total HOAs from both corneal surfaces in the KC eyes as compared with normal eyes.
In comparison with the postkeratoplasty groups, the magnitude of the total HOAs from the anterior corneal surface was higher in the KC group within the 4-mm zone and comparable within the 6-mm zone; however, the magnitude of the total HOAs from the posterior corneal surfaces was comparable between KC and postkeratoplasty eyes. These data are indicative of higher visual impairment under photopic conditions in the KC eyes. These differences can obviously vary based on the severity of KC.
In accordance with our results, the most significant aberration component found by previous studies in the KC eyes was an increased coma from both anterior2–5 and posterior5,33 corneal surfaces, the cause of which can be due to the characteristic cone-shaped development on the KC cornea as it degenerates.
Considering the postkeratoplasty eyes, the results of our study showed that, in comparison with controls, the grafted eyes showed a significantly higher magnitude of the total and third- and fourth-order corneal HOAs within both 4-mm and 6-mm central zones. In agreement with our data, both PK2,4,6,8,10,23,37 and DALK6,8,10,23 for KC have been associated with considerable amounts of ocular and/or corneal HOAs. These results suggest the important role of irregular astigmatism after corneal grafting, which may cause visual distortions and limit the visual rehabilitation under both photopic and scotopic conditions in these postoperative patients.37
Previous studies have reported that ocular HOAs were either comparable6,10 or significantly higher8 after descemetic-DALK when compared with PK for KC patients. Our data regarding the corneal HOAs showed, however, that although the magnitude of the total HOAs from the posterior corneal surface was comparable among postkeratoplasty eyes, those arising from the anterior corneal surface within both 4-mm and 6-mm central zones were significantly lower in DALK eyes when compared with both the PK and ALTK groups, thus suggesting an overall better optical quality of the anterior surface in the DALK group. These results could be explained by a better recipient-donor match and a higher interface regularity in our DALK eyes that all underwent the “DLKP by intrastromal dissection” procedure, in which the donor button tends to neatly position into alignment over the minimal pre–descemetic layer.
The aberration components greatly differed among postkeratoplasty groups. The most dominant aberration components from the anterior corneal surface, that is the most both within the 4-mm and 6-mm zones, were coma in the ALTK eyes, trefoil in the PK eyes, and both coma and trefoil in DALK eyes that were equally represented and appeared significantly lower when compared with those of the other postkeratoplasty groups.
The prevalence of trefoil after PK for KC has been already reported,4,37 which can be attributed to the wound malapposition caused by irregular wound incision profiles and differences in diameter, which can cause peripheral local deformations of the transplanted cornea. Coma and spherical aberration, also found in PK eyes,4 have been attributed to slight decentration of the donor cornea and mid-peripheral steepening induced by the wound, respectively.
Considering that coma is the dominant HOAs component in KC eyes,2–5,33 its prevalence after ALTK for KC could be caused by the standardized lamellar cut provided by the microkeratome on KC corneas, which are characterized by a wide range of corneal curvatures and stromal thickness irregularities. It is possible to speculate that the automatized cut is likely to leave an inhomogeneous recipient bed, which tends to be thinner at the cone apex and thicker in the periphery; thus, the apposition of the donor lamella on this irregular bed could partially reproduce the typical keratoconic anterior corneal profile postoperatively.
Experimental studies have demonstrated that different HOA components have a different impact on vision. Zernike terms having a greater effect on the central portion of the wavefront, such as coma, spherical aberration, and secondary astigmatism can adversely affect visual performance much more than terms near the edge of the Zernike pyramid, such as trefoil or tetrafoil, which play an important role in retinal image quality degradation only under mesopic or scotopic conditions.28,38–40 Penetrating keratoplasty and ALTK eyes showed a comparable amount of total HOAs from both corneal surfaces, indicating that these surgical procedure induce a similar total amount of corneal surfaces irregularity. The prevalence of trefoil in PK eyes and of coma in ALTK eyes suggests a greater central corneal configuration distortion, thus giving rise to a higher optical impairment, after ALTK compared with that after PK. These results are in agreement with the lower BSCVA found in the ALTK group in comparison with that of the PK group.
The surgical treatment of choice in KC is still not clear and remains a debatable issue. The results of our study are helpful in understanding the outcomes after penetrating and lamellar keratoplasty for KC. Our data showed an overall higher anterior corneal profile regularity after descemetic-DALK procedure than after conventional PK and ALTK and a greater central anterior corneal configuration distortion after ALTK than after PK and DALK, suggesting that DALK is superior to ALTK and PK in terms of providing a better anterior surface corneal optical quality. The ALTK procedure seems limiting with regard to reshaping the corneal profile in that an irregular corneal configuration remains after surgery, especially with regard to the anterior corneal profile, which is less pronounced but similar to that found in KC eyes. Although the correlations between the corneal wavefront data and patient BSCVA were not significant, the higher amount of coma from the anterior corneal surface within both 4-mm and 6-mm zones found in ALTK eyes, as compared with those found in DALK and PK eyes, can theoretically negatively affect the quality of vision and patient satisfaction postoperatively both under photopic and scotopic conditions. The lower BSCVA found in the ALTK eyes in comparison with the DALK and PK eyes supports this hypothesis even if other factors such as residual stromal scarring or graft-host interface irregularity can be regarded as causes of visual impairment after ALTK. Our visual results are in agreement with previous studies showing that the descemetic-DALK techniques are superior in terms of visual outcomes when compared with pre–descemetic-DALK techniques6,9,12,13,19,41 and that the visual performance after ALTK is worse than that obtained with PK and descemetic-DALK procedures.41
In disagreement with previous authors,4,36,37 significant correlations were not found between BSCVA and the magnitude of the total and third- and fourth-order corneal HOAs in normal, KC, and postkeratoplasty eyes; the hypothesis that the differences in wavefront errors may be responsible for the differences in BSCVA in these patients thus cannot be proven in this study. The lack of significant relationship between BSCVA and the HOAs magnitude can partially be explained by the fact that the high-luminance, high-contrast VA has shown to be relatively insensitive to variation in wavefront errors, especially when acuity is scored to line as opposed to the letter, as done in our study.20,30
Our study has several limitations, including that it was based on retrospective data, the number of eyes considered was relatively small, and the instrument reproducibility was not assessed. Moreover, the differences among the three postoperative groups because of patient age, graft/host diameter disparity, and surgeon and suture technique may have caused a bias that needs to be taken into consideration when evaluating the intergroup differences.
In conclusion, the results of our study seem to indicate that the DALK procedure can provide a higher postoperative corneal anterior surface optical quality when compared with the ALTK and PK techniques in patients affected by KC.
Further studies evaluating the contrast sensitivity and the low-contrast visual acuity under mesopic and scotopic conditions are needed to better assess the influence of the corneal wavefront error on the visual quality after PK and ALK surgery for KC.
Department of Ophthalmology
Santa Maria della Misericordia 15 33100
The authors have no financial or commercial interests to disclose. The study was performed without financial assistance, support, or funding.
Received August 2, 2012; accepted November 21, 2012.
The appendix is available online at http://links.lww.com/OPX/A113.
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Keywords:© 2013 American Academy of Optometry
keratoconus; deep anterior lamellar keratoplasty (DALK); automated lamellar therapeutic keratoplasty (ALTK); penetrating keratoplasty; corneal higher order aberrations