The Influence of Intraocular Pressure on Wavefront Aberrations in Patients Undergoing Laser-Assisted In Situ Keratomileusis

Abstract

Purpose.: To investigate the effect of intraocular pressure (IOP) on wavefront aberrations in the anterior cornea, the internal optics, and the whole eye for myopic patients undergoing laser-assisted in situ keratomileusis (LASIK) surgery.

Methods.: Fifty-seven myopic subjects were tested for wavefront aberrations in the anterior corneal surface and the whole eye using a corneal topographer and a wavefront analyzer, respectively, pre- and post-LASIK. The IOP and central corneal thickness (CCT) were measured with a noncontact tonometer and a pachymeter, respectively. Pre- and postoperative wavefront aberrations were compared, and the correlation between changes in the Zernike aberrations and the IOP was statistically tested.

Results.: The mean root mean square (RMS) values of the higher-order aberrations (HOAs) were significantly increased in the anterior cornea, the internal optics, and the whole eye. The mean RMS values for a majority of Zernike terms were significantly increased, and systematic increases in the spherical aberrations were found in both the cornea and the whole eye. The spherical aberrations after LASIK were significantly correlated to the IOP (r = 0.59, P < 0.01, for oculus dexter [OD, right eye] and r = 0.49, P < 0.01, for oculus sinister [OS, left eye] in the cornea; r = 0.38, P < 0.01, for OD and r = 0.46, P < 0.01, for OS in the whole eye).

Conclusions.: IOP contributes to LASIK-induced HOAs, particularly spherical aberrations. To control the HOAs after LASIK, a new algorithm should include the IOP as a variable for laser surgery.

Introduction

Currently, laser-assisted in situ keratomileusis (LASIK) is widely practiced in clinical settings worldwide to surgically correct refractive errors. With highly predictable outcomes, LASIK surgery reliably corrects myopic spherical error and astigmatism. However, for a certain patient segment, an increased level of higher-order aberrations (HOAs) have been produced by the surgery, ^1–6 although the visual impact of the HOAs was not clinically significant in most cases. ^7–9 Efforts have been made to control surgery-induced HOAs. ^10–14 Thus far, however, the factors that contribute to induction of HOAs from the biomechanical mechanism of the eye have not been well understood. 

The eye is a component optical system that mainly consists of the cornea, the crystalline lens, and the pupil. The optical property of the eye is determined by the geometrical shape of its optical elements and the alignment between their optical axes. However, the eye is a biological structure, and the geometric parameters of the cornea and the lens rely on a dynamic balance between its external forces (atmosphere and eyelid forces) and internal pressures (aqueous pressure, exactly the effort of the intraocular pressure [IOP]). Previous studies have demonstrated that eyelid force could change corneal astigmatism and HOAs in normal eyes. ^15–18 One of our previous studies has also revealed the dependence of HOAs on both the IOP and central corneal thickness (CCT) in normal young myopes. ¹⁹  

LASIK is an invasive refractive surgery that alters the original corneal shape, thereby modifying the dynamic balance between external forces and IOP. The change in biomechanical factors could influence the geometric structure of the eye and consequently change the HOA of the eye. Studies have been previously conducted to assess the effect of biomechanical change on the geometric property of the eyes, ^20–26 but the influence of biomechanical factors on the optical property of the eye has not been well explored. The purpose of this study was to investigate the influence of IOP and CCT on postoperative HOAs in the anterior cornea, internal optics (mainly the posterior cornea and/or the lens), and the whole eye in myopic patients who underwent LASIK surgery. 

Subjects and Methods

Subjects

Fifty-seven myopic subjects (20 males; 37 females) undergoing refractive surgery at the Eye Hospital of Wenzhou Medical College were enrolled in this study. Patients with ocular pathology or best-corrected visual acuity < 20/20 were excluded. The subjects were young adults with a mean age of 25.0 ± 5.4 years (range, 18–38 years), and a mean spherical equivalent (SE) refractive error of −4.96 ± 1.74 diopters (D) (range, −1.50 to −8.75 D). Rigid gas-permeable contact lens and soft contact lens wearers were asked to discontinue wearing contact lenses before the study for at least 4 and 2 weeks, respectively. LASIK was performed using a commercial laser system (MEL 80 Excimer Laser System; Carl Zeiss Meditec, Dublin, CA). The research followed the tenets of the Declaration of Helsinki and was approved by the Committee of Ethics of Wenzhou Medical College. 

Instruments

The Humphrey Atlas corneal topography system (Carl Zeiss Meditec) was used to measure corneal aberrations. The system, a Placido-based videokeratographer (in front of a bullseye) with 24 rings, provided direct estimates of the pupil margins, the pupil center, vertex location, shape factor, curvatures of the anterior corneal surface, and keratometric measurements. Corneal curvature and corneal height data were used to analyze the corneal aberrations. Wavefront aberrations in the whole eye were measured using a wavefront aberration–supported corneal ablation wavefront analyzer (Complete Ophthalmic Analysis System; Carl Zeiss Meditec), which was a Hartmann–Shack wavefront sensor. The system provided information about refractive errors, the RMS of wavefront aberrations, and a series of Zernike aberrations, which were available in an Excel file (Microsoft Corp., Redmond, WA). IOP was measured using a noncontact tonometer (X-10; Canon U.S.A., Melville, NY), which has automatic alignment and focusing indicators on the screen for correcting the working distance. When the correct alignment is achieved, the instrument blows air puffs and automatically measures IOP. The CCT was measured with a pachymeter (SP-3000; Tomey Corp., Nagoya, Japan). After a proparacaine 0.5% drop was applied to anesthetize the cornea, the pachymeter probe was used to measure the CCT, and 10 individual CCT measurements were recorded for each eye. 

Experimental Procedure

For each subject, all of the measurements were performed pre- and post-LASIK. First, four corneal aberration measurements were performed, and then three whole-eye aberration measurements were obtained. During the whole-eye measurement, the pupil sizes were 6.0 mm or larger. After the aberration tests were completed, the IOP tests were repeated in each eye until three readings were obtained. A proparacaine 0.5% drop was then applied to the cornea for the CCT measurements. Both right and left eyes were tested, and the entire session lasted approximately 20 minutes in most cases. All of the measurements were performed between 2:00 PM and 5:00 PM. 

Statistical Analysis

Wavefront aberrations were calculated from the corneal heights exported from the Atlas system using a customized ray-tracing program (MATLAB; The MathWorks, Natick, MA) to derive Zernike aberrations up to the seventh order (35 terms). ²⁷ Data on corneal heights within a 6.0-mm diameter of the corneal area were calculated to match the pupil area (6.0 mm) in the whole-eye aberration measurement. To describe the corneal aberrations with respect to the visual axis of the eye, as with the whole-eye aberrations, a correction of the displacement between the corneal vertex and pupil center was performed in the customized program according to Atlas system measurements of the pupil center. ¹⁰ This corneal area covered approximately 13 Placido rings, although the number varied from 12 to 14 depending on the eye size. For either the anterior cornea or the whole eye, a mean of three measurements was used to estimate the Zernike aberrations for each subject. The Zernike aberrations in the internal optics of each subject were derived by subtracting the Zernike aberrations from the anterior cornea from those of the whole eye. The study used the single-index conversion for naming the Zernike aberrations; this process was recommended by the Optical Society of America (OSA)/Vision Science and Its Applications (VSIA) Standards Taskforces. 

The mean of three IOP measurements (or 10 CCT measurements) was taken to estimate the IOP (or CCT) in each individual eye. Correlations between the Zernike aberrations and IOP and/or CCT were tested. All the analyses were performed using a commercial analytical software program (SPSS, version 19.0; SPSS, Inc., Chicago, IL). 

Results

Mean Wavefront Aberrations, Pre- and Postoperative IOP and CCT

For the 57 myopic subjects, the mean root mean square (RMS) values of the higher-order aberrations (HOAs) in the anterior cornea, the internal optics and the whole eye are illustrated in Table 1, where the results at the baseline and at 3-month follow-up tests after LASIK were listed for comparison. Table 1 also shows the mean IOP and CCT at the baseline and the follow-up test. In Table 1, it is apparent that the RMS value of the anterior cornea at the postoperative test was significantly increased compared with the baseline level (t = 26.83, P < 0.01 for the oculus dexter [OD]; t = 29.91, P < 0.01 for oculus sinister [OS]), as were the RMS values of the internal optics (t = −7.54, P < 0.01 for OD; t = −2.69, P < 0.01, for OS) and the whole eye (t = 26.92, P < 0.01 for OD; t = 28.30, P < 0.01 for OS). Significant IOP changes were also observed in this subject group post-LASIK (t = 29.66, P < 0.01 for OD; t = 31.62, P < 0.01 for OS). As expected, the CCT was significantly decreased after the surgery (t = 100.70, P < 0.01 for OD; t = 98.06, P < 0.01 for OS). 

Table 1. 

 

Mean and SD of the RMS Values of the Zernike Aberrations From the Second to Fourth Orders in the Anterior Cornea, the Internal Optics, and the Whole Eye, and Other Parameters for 57 Myopes

	OD		OS
	Pre	Post	Pre	Post
Corneal aberration, μm	0.36 ± 0.10	0.78 ± 0.22	0.39 ± 0.11	0.77 ± 0.19
Internal aberration, μm	−0.08 ± 0.11	−0.31 ± 0.23	−0.11 ± 0.10	−0.21 ± 0.29
Whole-eye aberration, μm	0.28 ± 0.10	0.51 ± 0.14	0.28 ± 0.09	0.52 ± 0.14
IOP, mm Hg	13.57 ± 2.65	11.47 ± 2.92	13.34 ± 2.69	11.64 ± 2.78
CCT, μm	544.82 ± 29.43	466.81 ± 35.00	543.21 ± 27.55	470.28 ± 36.21
SE, D	−4.95 ± 1.77		−4.92 ± 1.67
AD, μm	87.64 ± 22.72		87.67 ± 23.04

 SE, spherical equivalent; AD, ablation depth.

Table 2 shows the mean absolute Zernike aberrations in the anterior corneal surface, the internal optics, and the whole eye at a 6.0-mm pupil. Only the Zernike terms in third and fourth orders (nine terms) are illustrated in Table 2 because aberration terms with orders higher than the fifth are usually small. Pre- and postoperative results are listed for comparison. Table 2 shows that the mean values for several terms, including Z7 to Z14 in the right eye and Z7, Z8, Z11 to Z14 in the left eye, were significantly increased after the cornea surgery (P < 0.05). For the internal optics, Z6 to Z13, Z24 in the right eye and Z6 to Z14, Z24 in the left eye were significantly increased after surgery (P < 0.05). For the whole eye, Z7 to Z9, Z11, Z12 in the right eye and Z6 to Z9, Z11 to Z14 in the left eye were significantly increased after surgery (P < 0.05). 

Table 2.

 

Mean and SD of the Absolute Zernike Aberrations in the Anterior Cornea, Internal Optics, and Whole Eye at 6.0-mm Pupil

Zernike Aberration, μm	Anterior Cornea				Internal Optics				Whole Eye
	OD		OS		OD		OS		OD		OS
	Pre	Post	Pre	Post	Pre	Post	Pre	Post	Pre	Post	Pre	Post
Z6	0.10 ± 0.09	0.13 ± 0.11	0.11 ± 0.09	0.11 ± 0.08	0.05 ± 0.04	0.16 ± 0.13	0.06 ± 0.04	0.17 ± 0.14	0.09 ± 0.08	0.12 ± 0.10	0.10 ± 0.07	0.13 ± 0.08
Z7	0.14 ± 0.09	0.23 ± 0.20	0.16 ± 0.12	0.24 ± 0.21	0.09 ± 0.07	0.22 ± 0.20	0.10 ± 0.06	0.26 ± 0.22	0.11 ± 0.08	0.21 ± 0.12	0.12 ± 0.08	0.18 ± 0.13
Z8	0.14 ± 0.09	0.31 ± 0.18	0.14 ± 0.10	0.23 ± 0.17	0.12 ± 0.07	0.35 ± 0.20	0.11 ± 0.07	0.30 ± 0.23	0.07 ± 0.05	0.15 ± 0.11	0.07 ± 0.06	0.16 ± 0.10
Z9	0.07 ± 0.06	0.11 ± 0.10	0.07 ± 0.06	0.10 ± 0.08	0.05 ± 0.04	0.15 ± 0.12	0.06 ± 0.05	0.15 ± 0.12	0.07 ± 0.05	0.09 ± 0.08	0.07 ± 0.06	0.11 ± 0.09
Z10	0.03 ± 0.02	0.05 ± 0.05	0.04 ± 0.04	0.05 ± 0.04	0.03 ± 0.02	0.08 ± 0.06	0.03 ± 0.03	0.07 ± 0.05	0.04 ± 0.03	0.04 ± 0.04	0.04 ± 0.02	0.05 ± 0.04
Z11	0.02 ± 0.02	0.05 ± 0.04	0.05 ± 0.03	0.06 ± 0.04	0.02 ± 0.02	0.07 ± 0.05	0.03 ± 0.02	0.07 ± 0.05	0.03 ± 0.02	0.05 ± 0.03	0.03 ± 0.02	0.05 ± 0.03
Z12	0.20 ± 0.08	0.56 ± 0.17	0.19 ± 0.08	0.57 ± 0.19	0.12 ± 0.08	0.29 ± 0.18	0.13 ± 0.08	0.27 ± 0.20	0.12 ± 0.10	0.32 ± 0.16	0.11 ± 0.09	0.32 ± 0.16
Z13	0.05 ± 0.04	0.07 ± 0.06	0.05 ± 0.05	0.07 ± 0.05	0.04 ± 0.04	0.09 ± 0.07	0.04 ± 0.04	0.08 ± 0.06	0.05 ± 0.04	0.06 ± 0.04	0.04 ± 0.03	0.06 ± 0.05
Z14	0.04 ± 0.04	0.07 ± 0.06	0.05 ± 0.05	0.07 ± 0.04	0.05 ± 0.05	0.08 ± 0.07	0.05 ± 0.04	0.09 ± 0.06	0.05 ± 0.05	0.04 ± 0.03	0.03 ± 0.03	0.05 ± 0.04

Table 3 shows the mean Zernike aberrations with the sign in the anterior corneal surface and the whole eye in a 6.0-mm pupil. As shown in Table 3, the term with a significant increase in Zernike value is the Z12 only for the cornea (t = −16.79, P < 0.01 for OD; and t = −17.00, P < 0.01 for OS). In the internal optics, Z12 was significantly increased after surgery (t = 7.08, P < 0.01 for OD; and t = 3.10, P < 0.01 for OS). In the whole eye, Z12 was significantly increased after surgery (t = −11.63, P < 0.01 for OD; and t = −12.93, P < 0.01 for OS). 

Table 3.

 

Mean and SD of the Zernike Aberrations With the Sign in the Anterior Cornea, Internal Optics, and Whole Eye at 6.0-mm Pupil

Zernike Aberration, μm	Anterior Cornea				Internal Optics				Whole Eye
	OD		OS		OD		OS		OD		OS
	Pre	Post	Pre	Post	Pre	Post	Pre	Post	Pre	Post	Pre	Post
Z6	−0.02 ± 0.13	0.03 ± 0.16	−0.05 ± 0.13	0.02 ± 0.14	0.01 ± 0.06	0.01 ± 0.21	0.03 ± 0.07	0.01 ± 0.22	0.09 ± 0.08	0.12 ± 0.10	0.10 ± 0.07	0.13 ± 0.08
Z7	−0.03 ± 0.17	−0.13 ± 0.27	−0.04 ± 0.19	−0.14 ± 0.29	0.03 ± 0.12	−0.01 ± 0.30	0.05 ± 0.11	−0.05 ± 0.34	0.11 ± 0.08	0.21 ± 0.12	0.12 ± 0.08	0.18 ± 0.13
Z8	−0.12 ± 0.11	−0.30 ± 0.19	−0.13 ± 0.10	0.20 ± 0.20	0.10 ± 0.09	0.32 ± 0.26	−0.10 ± 0.08	−0.20 ± 0.32	0.07 ± 0.05	0.15 ± 0.11	0.07 ± 0.06	0.16 ± 0.10
Z9	−0.02 ± 0.09	−0.08 ± 0.12	0.01 ± 0.10	0.00 ± 0.13	0.03 ± 0.05	0.04 ± 0.19	−0.03 ± 0.07	−0.02 ± 0.20	0.07 ± 0.05	0.09 ± 0.08	0.07 ± 0.06	0.11 ± 0.09
Z10	0.02 ± 0.03	0.00 ± 0.08	−0.03 ± 0.04	−0.04 ± 0.06	0.01 ± 0.04	−0.02 ± 0.10	0.01 ± 0.04	0.03 ± 0.08	0.04 ± 0.03	0.04 ± 0.04	0.04 ± 0.02	0.05 ± 0.04
Z11	−0.01 ± 0.03	−0.01 ± 0.06	0.04 ± 0.04	0.06 ± 0.05	−0.01 ± 0.03	0.02 ± 0.09	−0.02 ± 0.03	−0.05 ± 0.07	0.03 ± 0.02	0.05 ± 0.03	0.03 ± 0.02	0.05 ± 0.03
Z12	0.19 ± 0.09	0.56 ± 0.17	0.19 ± 0.09	0.57 ± 0.19	−0.09 ± 0.11	−0.29 ± 0.19	−0.10 ± 0.11	−0.21 ± 0.26	0.12 ± 0.10	0.32 ± 0.16	0.11 ± 0.09	0.32 ± 0.16
Z13	−0.01 ± 0.06	−0.03 ± 0.09	−0.01 ± 0.07	−0.04 ± 0.07	0.00 ± 0.06	−0.01 ± 0.11	0.00 ± 0.06	0.02 ± 0.11	0.05 ± 0.04	0.06 ± 0.04	0.04 ± 0.03	0.06 ± 0.05
Z14	−0.02 ± 0.05	−0.03 ± 0.09	−0.04 ± 0.06	−0.04 ± 0.07	0.05 ± 0.05	0.04 ± 0.10	0.05 ± 0.04	0.04 ± 0.10	0.05 ± 0.05	0.04 ± 0.03	0.03 ± 0.03	0.05 ± 0.04

For fifth-order and higher Zernike aberrations, secondary spherical aberration (Z24 in the sixth order) was found to be significantly increased after surgery in both the right and left eyes for the cornea (t = −9.83, P < 0.01 for OD; and t = −11.34, P < 0.01 for OS), for the internal optics (t = 2.86, P < 0.01 for OS), and the whole eye (t = −12.86, P < 0.01 for OD; and t = −10.99, P < 0.01 for OS). 

Correlation of Zernike Aberrations With IOP and CCT

Zernike aberrations, including the third to fourth orders and the secondary spherical aberration Z24, correlated with the IOP for the 57 subjects, and the resulting correlation coefficients are summarized in Table 4. Significant correlations were always found for the spherical aberrations under postoperative conditions in both the cornea and the whole eye. For the preoperative condition, a significant correlation was found for the spherical aberration in the right cornea. Significant correlations were also found for postoperative secondary spherical aberration Z24, except for the cornea in the left eye. In the preoperative condition, a significant correlation for the secondary spherical aberration Z24 was found for the whole eye but not the cornea. For Zernike aberrations other than the spherical aberrations, the only term that was significantly correlated to the IOP was the corneal trefoil Z6 in the right eye under preoperative conditions, and the correlation was no longer significant postsurgery. 

Table 4.

 

Correlation Coefficients Between the Zernike Aberrations and IOP

Correlation Coefficient	Anterior Cornea				Internal Optics				Whole Eye
	OD		OS		OD		OS		OD		OS
	Pre	Post	Pre	Post	Pre	Post	Pre	Post	Pre	Post	Pre	Post
Z6	−0.28*	0.09	−0.23	0.02	0.21	−0.02	0.17	0.00	−0.21	−0.04	−0.15	0.05
Z7	0.07	−0.01	−0.06	0.00	−0.06	0.07	0.12	0.07	0.04	−0.04	0.02	−0.02
Z8	−0.06	−0.23	−0.10	−0.13	0.26	0.06	−0.03	−0.14	0.21	−0.11	−0.15	−0.01
Z9	−0.01	0.11	0.10	−0.08	−0.15	0.04	0.06	0.05	−0.11	0.04	0.14	0.12
Z10	0.12	0.12	−0.10	−0.26	−0.12	−0.30*	−0.05	0.14	−0.02	0.14	−0.17	−0.12
Z11	0.15	0.15	0.02	−0.05	−0.02	0.30*	0.16	−0.04	0.11	0.15	0.17	0.14
Z12	0.28*	0.59†	0.18	0.49†	−0.13	0.17	−0.11	0.19	0.08	0.38†	0.04	0.46†
Z13	−0.20	0.06	−0.03	−0.05	0.03	0.14	0.09	0.05	−0.17	0.07	0.07	0.02
Z14	0.02	0.18	0.05	0.09	−0.01	0.04	−0.23	−0.02	0.01	0.05	−0.16	0.00
Z24	0.12	0.39†	0.01	0.20	0.16	0.27*	0.34†	0.45†	0.28*	0.51†	0.34†	0.49†

*  Correlation is significant at the 0.05 level (two-tailed).

†  Correlation is significant at the 0.01 level (two-tailed).

Figure 1 illustrates the correlation between the postoperative spherical aberrations and the postoperative IOP for the cornea (Fig. 1A) and the whole eye (Fig. 1B) in both the right and left eyes. Figure 2 shows the correlation between the postoperative secondary spherical aberrations Z24, the postoperative IOP for the cornea (Fig. 2A), and the whole eye (Fig. 2B) in the right and left eyes. 

Figure 1

 

The correlation between the postoperative spherical aberrations and the postoperative IOP for the cornea (open circles) and the whole eye (solid circles) in the right eye (A) and left eye (B).

Figure 2

 

The correlation between the postoperative secondary spherical aberrations and the postoperative IOP for the cornea (open circles) and the whole eye (solid circles) in the right eye (A) and left eye (B).

Table 5 shows the correlation coefficients between the Zernike aberrations and CCT. Few conditions were found to be significantly correlated to the CCT, including the postoperative y-axis, x-axis comas, the primary and secondary spherical aberrations (Z12 and Z24) in the right eye cornea, and also secondary spherical aberration Z24 and astigmatism in the fourth order Z13 in the right internal optics. The postoperative corneal spherical aberration in the left eye and the whole eye x-axis coma in the right eye were significantly correlated to CCT. 

Table 5.

 

Correlation Coefficients Between the Zernike Aberrations and CCT

Correlation Coefficient	Anterior Cornea				Internal Optics				Whole Eye
	OD		OS		OD		OS		OD		OS
	Pre	Post	Pre	Post	Pre	Post	Pre	Post	Pre	Post	Pre	Post
Z6	0.00	−0.11	0.02	−0.03	0.08	0.28*	0.19	−0.20	0.05	−0.02	0.13	−0.10
Z7	0.15	0.26*	0.03	0.3	−0.07	−0.10	−0.05	−0.05	0.12	0.22	0.00	0.08
Z8	0.11	0.46†	−0.01	0.05	0.07	0.22	−0.17	0.16	0.23	0.32*	−0.16	0.05
Z9	0.00	−0.06	0.02	−0.18	−0.06	−0.13	0.02	0.04	−0.04	−0.09	0.03	−0.11
Z10	0.17	0.12	−0.23	0.07	0.00	0.06	0.12	0.13	0.14	0.07	−0.16	−0.05
Z11	0.12	−0.07	−0.07	0.05	0.08	0.16	−0.02	−0.06	0.17	0.08	−0.11	−0.13
Z12	−0.01	−0.29*	0.01	−0.31*	−0.03	0.01	0.02	0.05	−0.04	−0.10	0.03	−0.18
Z13	−0.05	−0.02	0.10	0.11	0.03	0.27*	−0.16	−0.03	−0.03	−0.01	−0.06	0.07
Z14	0.02	0.10	−0.11	0.18	−0.15	0.14	0.11	−0.03	−0.11	0.02	−0.04	0.10
Z24	0.00	−0.31*	−0.06	−0.23	0.07	0.29*	0.19	−0.01	0.07	−0.18	0.15	−0.13

*  Correlation is significant at the 0.05 level (two-tailed).

†  Correlation is significant at the 0.01 level (two-tailed).

Because both IOP and CCT were significantly correlated to spherical aberration, particularly in the cornea and whole eye, a partial regression was performed to test the relationship between the spherical aberrations in the cornea and the whole eye and the IOP and CCT. Table 6 shows the correlation coefficients found using model 1 for IOP and model 2 for IOP and CCT together. As shown in Table 6, the standard coefficients for IOP in model 2 closely resembled those in model 1 and were always greater than the CCT coefficients, which indicated that IOP plays a more important role in contributing to postoperative spherical aberration. 

Table 6.

 

Regression Analysis Between the Spherical Aberration, IOP, and CCT for the Anterior Cornea and Whole Eye in Right Eyes and Left Eyes After the Operations

Model	Anterior Cornea				Whole Eye
	OD		OS		OD		OS
	R	β	R	β	R	β	R	β
IOP	0.59	0.59*	0.49	0.49*	0.38	0.38*	0.46	0.46*
IOP	0.61	0.55*	0.54	0.45*	0.38	0.38*	0.47	0.45*
CCT		−0.16		−0.23		−0.01		−0.10

  R, correlation coefficient; β, standardized coefficient.

*   P < 0.05.

Discussion

Laser-assisted in situ keratomileusis is a reliable refractive surgery for correcting spherical error and astigmatism, but it induces HOAs during the surgery. Consistent with previous studies, ^1–4 the RMS values of HOAs for our 57 myopic patients in the current study were significantly increased in the anterior cornea, the internal optics, and the whole eye (Table 1). The HOA changes in our subjects were greater in the anterior cornea compared with the whole eye (for both the right and left eyes). The results thus indicated that some of the induced HOAs in the anterior cornea were compensated by the internal optics (the posterior cornea and/or the lens); therefore, the level of wavefront aberrations in the whole eye was relatively reduced. Thus, it would be interesting to further investigate the mechanism underlying compensation of the corneal aberrations after LASIK in a future study. 

The overall increase of RMS values of the HOAs after LASIK represents an integrative change in wavefront aberrations for all Zernike terms, but it does not directly reflect the changes of each Zernike aberration. Although a number of the Zernike terms in the cornea, the internal optics, and the whole eye were increased after surgery (Table 2), the amplitude of the increase for each Zernike term was not equal. Apparently, the comas and spherical aberration made larger contributions to the overall increase in wavefront aberrations for the postoperative eyes, as compared with the other Zernike terms. The mean Zernike aberrations (as shown in Table 3) also imply that the aberration induction for some Zernike terms was at a random direction for an individual eye because the mean values were not significantly increased relative to the baseline values for our subjects. The results, therefore, might suggest that the influence of surgery processes on aberration induction is complex, and thus it is worth further investigating the dependence of Zernike aberrations on the surgery procedure. 

Both spherical aberration and secondary spherical aberration are rotationally symmetrical to the optical axis. Systematic increases in spherical aberration after LASIK have been found in previous studies ^3,4,28,29 and also in this study. When the change of spherical aberration in the anterior cornea was compared with that in the whole eye, however, the change in spherical aberration for our subjects was greater in the anterior cornea than that in the whole eye in both the right and left eyes. This result was not in agreement with the results reported in the study by Benito et al., ⁴ in which 15 myopes showed approximately the same amount of induction of spherical aberration in both the anterior cornea and the whole eye. The difference between the two studies might be caused by a higher level of mean refractive error corrected (approximately −5.0 vs. −4.0 D) in this study compared with that reported by Benito et al. ⁴ The difference in sample size might be another factor. 

Efforts have been made in previous studies ^28–32 to investigate the sources responsible for the LASIK-induced spherical aberration. The factors could include the decreased laser efficiency at the periphery cornea and corneal asphericity. In this study, we found that for our patients the postoperative spherical aberration and secondary spherical aberration were significantly correlated with the IOP (Table 4; Figs. 1, 2). The results indicated that, in addition to the factors revealed in previous studies, IOP is also a contributing factor responsible for the induction of symmetric aberrations after LASIK. Therefore, a new algorithm with IOP should be included as a variable for controlling postoperative HOAs in laser surgery. 

Our results also showed a significant correlation between the postoperative CCT and coma or spherical aberration (Table 5). Because the change in CCT was determined by corrected refractive error, the results might imply that the induction of the coma and spherical aberration could depend on the level of refractive errors. 

It would be interesting to understand how the IOP could manifest its contribution to spherical aberration induction after the CCT was reduced by laser ablation. In terms of the biomechanical interaction, as described by Roberts, ³³ the IOP could push the periphery corneal area to induce central flattening of the cornea after an amount of corneal tissue was removed by the surgery. It was predicted that the corneal positive spherical aberration would increase along with the myopic ablation depth (AD) because the central cornea becomes flatter. It is, therefore, easy to understand that the higher the IOP becomes, the flatter the central cornea becomes; thus, corneal spherical aberration would increase as well. Given the complex relationship between the induced spherical aberration and several contribution factors, it might be interesting to further investigate the relationship and to derive a new algorithm for optimizing the LASIK surgery. 

In summary, increased levels of higher-order wavefront aberrations were observed for our patients after LASIK surgery, which was consistent with previous studies. By comparing the changes in HOAs between the anterior cornea and the whole eye, it is likely that some compensation mechanisms are operated to compensate the corneal aberrations in the operated eyes to reduce the aberration levels in the whole eye. Postoperative spherical aberration depends on IOP, which should be taken into account when the control of postoperative wavefront aberration is considered in laser surgery. 

Acknowledgments

Supported by the National Nature Science Foundation of China Grant 81170869 (FL); the Program for Key Science and Technology Innovation Team of Zhejiang Province Grant 2011R09039-09 (LH); Wenzhou's Science and Technology Program Grant Y20100199 (LH); and a grant from Zhejiang Provincial Program for the Cultivation of High-Level Innovative Health Talents (FL). The authors alone are responsible for the content and writing of the paper. 

Disclosure: L. Hu, None; Q. Wang, None; P. Yu, None; Y. Yu, None; D. Zhang, None; J.C. He, None; F. Lu, None