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Comparison of wavefront aberration after cataract surgery with acrylic intraocular lens implantation

Choi, Jaewan MD; Kim, Tae-im MD; Tchah, Hungwon MD

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Journal of Cataract & Refractive Surgery: February 2005 - Volume 31 - Issue 2 - p 324-329
doi: 10.1016/j.jcrs.2004.06.021
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Today, various types of intraocular lenses (IOLs) are implanted as part of cataract surgery. These IOLs have different biological and optical properties, and several of the biological properties have been studied. For example, a decreased incidence of posterior capsule opacification and less inflammatory reaction on the IOL surface have been reported in foldable acrylic IOLs than in silicone IOLs.1,2 In some reports, hydrophobic acrylic IOLs have better capsule biocompatibility than hydrophilic acrylic IOLs.3 Not as much is known about the IOLs' optical characteristics as about their biological properties.

With the development of refractive surgery, the importance of higher-order aberration (HOA) change has become more important. Multiple studies of HOA changes in refractive surgery have been performed. We know that photorefractive surgery increases ocular and corneal aberrations and changes the relative distribution of coma- and spherical-like aberrations.4,5 But few studies of the aberration changes caused by cataract surgery have been done.

This study measured the HOAs in eyes with 3 different acrylic IOLs 1 month after cataract surgery and in normal phakic eyes using a Hartmann-Shack (HS)-type aberrometer. The HS-type aberrometer is a device that can measure local refractive change using lenslet arrays and can calculate HOAs by wavefront analysis of the eye.6–8 Higher-order aberrations up to the 4th order were calculated as Zernike polynomials. Differences in HOA aberrations between 3 different IOL groups and a normal control group were studied.

Patients and Methods

Initially, 20 eyes were enrolled for each group from those who had phacoemulsification with posterior chamber IOL implantation between June 2001 and July 2002 performed by the same surgeon (H.T.). Patients and controls were randomly selected from those older than 50 years. Patients who had a history of ocular surgery or ocular disease other than cataract (eg, diabetic retinopathy, age-related macular degeneration, or corneal opacity) were excluded. The IOLs were randomly assigned to patients. Three groups of 20 patients each received an Acrysof MA60BM IOL (Alcon), a Sensar AR40 IOL (Allergan Surgical), or a Corneal ACR6D IOL (Corneal Laboratoire). There was no correlation between demographic factors and the IOLs selected. Patients and the examiner were masked to the assigned group. Only the surgeon knew the group when he performed cataract surgery.

All patients received topical anesthesia by proparacaine hydrochloride 0.5% eyedrops (Alcaine) before surgery. A 2.75 mm wide self-sealing limbal incision was made on the temporal or nasal side. Sodium hyaluronate 1.4% (Healon GV) was used to reform and stabilize the surgical planes and protect the corneal endothelium. A 5.00 to 5.25 mm continuous curvilinear capsulorhexis was performed with a 26-gauge needle, and the nucleus was removed without intraoperative problems such as posterior capsule rupture. Phacoemulsification was performed using the Sovereign machine (Allergan Surgical). All IOLs were inserted in the posterior chamber with the injector system through the enlarged 3.2 mm wound. The limbal wound was not sutured.9

One month after surgery, the HOAs in the operated eyes were measured with a Zywave aberrometer (Bausch & Lomb Surgical). Eyes with visible abnormalities (eg, corneal edema, Descemet's fold) or with best corrected visual acuity (BCVA) less than 20/30 were excluded. Data from cases in which the automatic calculation was not performed by the aberrometer were excluded; eg, 4 eyes in the AcrySof group in which automatic calculation was not performed because of a small pupil.

The aberrations were measured in a nondilated pupil in dim light, and the central 4.00 mm diameter of the pupil was analyzed. In each eye, the measurements were repeated at least 3 times to obtain a well-focused, properly aligned image of the eye. Defocused images due to eye movement or the upper lid covering images were discarded. The aberrations in the normal control group were measured in the same way.

Individual Zernike coefficients of the 3rd and 4th orders (C3−1, C31, C3−3, C33, C40, C4−2, C42, C4−4, C44) were obtained by the Zywave automatic calculation program of the Zywave. The RMS value of the 3rd-order component of coma-like aberration (S3), the 4th-order component of spherical-like aberration (S4), and ocular HOAs (RMSH, RMS value of S3 and S4) were obtained.

The mean age, BCVA, spherical equivalent, and astigmatism in the 2 groups were compared using the Kruskall-Wallis test. Each of the Zernike coefficients and S3, S4, and RMSH were analyzed using the Kruskal-Wallis test. The C33, which showed a significant difference with the Kruskal-Wallis test, was reanalyzed using the Mann-Whitney U test. All statistical testing was done with SPSS version 11.5 (SPSS Inc.).

Results

There was no significant difference between groups in mean age, BCVA, spherical equivalent, and astigmatism (Table 1). There was no significant difference between groups in the Zernike coefficients except C33 (Figure 1 and Table 2). Reanalysis of C33 showed a significant negative shift in all IOL groups compared with the normal phakic group (P<.05) and a significant difference between the AcrySof MA60BM and the Corneal ACR6D groups (P<.05) (Table 3). In the RMS value of the 3rd and 4th orders (S3, S4) and the total RMS value of the higher order (RMSH), there was no significant difference between the IOL groups and the normal phakic group (Figure 2 and Table 4).

Table 1
Table 1:
Mean age and BCVA in each group.
Figure 1.
Figure 1.:
Mean Zernike coefficients in eyes in each group.
Table 2
Table 2:
Mean individual Zernike coefficient in each group.
Table 2
Table 2:
(cont.)
Table 3
Table 3:
Difference between groups in C3 3.
Figure 2.
Figure 2.:
Mean calculated aberrations of the 3rd- and 4th-orders and the RMS value of HOAs up to the 4th order (S3 = RMS value of the 3rd-order aberration; S4 = RMS value of the 4th-order aberration; RMSH = RMS value of the HOAs up to the 4th order).
Table 4
Table 4:
Mean HOA in each group.

Discussion

With the development of commercially available wavefront aberrometers, ocular aberration can be easily measured and expressed in the form of Zernike polynomials. We know that photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK) increase HOAs of the oculus and cornea.4,5 It is also known that there is no correlation between HOAs and lower-order aberrations including astigmatism.10

Higher-order aberrations can be measured using a commercially available aberrometer. The Zywave aberrometer uses the Hartmann-Shack principle for calculating ocular aberration by measuring local refractive change and analyzing wavefront displacement from the reference points for an aberration-free eye. The aberrations are represented by Zernike polynomials.6–8

We assume there might be some change in HOAs as well as lower-order aberrations such as focus shift and astigmatic change after cataract surgery. But little is known about HOA change after cataract surgery. To our knowledge, there are few studies of this issue. In 10 postcataract surgery eyes, the mean HOA in the group was not significantly different from that in normal emmetropic eyes. But in an individual Zernike coefficient analysis, the assessed deviation of the coefficients C3−3, C42, and C44 was significantly larger than the standard deviation in the normal phakic eyes.11 This study comprised a small number of eyes and did not specifically describe the type of IOLs implanted. Today, acrylic IOLs are widely used in cataract surgery. We therefore attempted to evaluate HOAs in eyes with 3 acrylic IOLs and in normal phakic eyes.

The Acrysof MA60BM is a foldable, 3-piece IOL with an optic of 2-phenylethyl acrylate and 2-phenylethy methacrylate copolymers crosslinked with butanediol acrylate and haptics of poly(methyl methacrylate) (PMMA). It has a refractive index of 1.55. The biconvex optic has a diameter of 6.0 mm, and the overall length is 13.0 mm. The central optic thickness ranges from 0.61 to 0.84 mm. The IOL has a smooth surface and a sharp edge. The C-shaped haptics have an angulation of 5 degrees. The Sensar AR40 is a foldable, 3-piece IOL with an optic of phenylethyl acrylate and phenylethyl methacrylate and PMMA haptics. It has a refractive index of 1.47. The optic diameter is 6.0 mm and the overall length, 13.0 mm. The IOL has a smooth surface and a round edge. The haptics have a C design and an angulation of 5 degrees. The Corneal ACR6D is a foldable, 1-piece hydrophilic acrylic lens. The optic diameter is 6.0 mm and the overall length, 12.0 mm. It has a refractive index of 1.465. The optic has a round or square edge (SE model).

Despite the different properties of the IOLs, no difference was found in the Zernike coefficients except C33. It was significantly increased in all IOL groups compared with that in the normal phakic group. In eyes with the AcrySof MA60MB IOL, C33 showed a more significant negative shift than that in eyes with the Corneal ACR6D IOL. The C33 is a Zernike coefficient of the 3rd-order component of coma-like aberration (also called a trefoil), meaning triangular astigmatism based on the y-axis. An increase in the 3rd-order component of corneal aberration with accommodation in pseudophakic eyes has been reported.12 Change in the coma-like aberration of the cornea, along with corneal multifocality, was thought to contribute to apparent accommodation in pseudophakic eyes. But no comparison of HOAs between normal phakic eyes and eyes with IOLs was done in the study.

For the 4th-order spherical aberration, there was no difference between the IOL groups and the normal phakic group. It is generally known that ocular spherical aberration increases with age,13 while corneal spherical aberration does not.14 A more positive shift in the 4th-order spherical aberration is reported in eyes with IOLs.15,16 Our results did not coincide with those in the previous study. There was no significant difference in C40 between the groups. This dissociation from the previous result might be due to the small number of patients or to the relatively young age of the phakic group.

Despite the variability of C33 in the IOL group, the HOAs (S3, S4, and RMSH) did not differ between groups. Perhaps the overall HOAs were not affected by minor changes in this coefficient.

We do not yet fully understand how each Zernike coefficient correlates with clinical symptoms or other optical properties. There are few reports on this issue. Decreased contrast sensitivity after radial keratotomy could be related to the shift in distribution of aberrations from 3rd-order dominance (coma-like aberrations) to 4th-order dominance (spherical-like aberrations).17 Others suggest that change in coma-like aberration in accommodation could be beneficial in near vision acuity.12 We need to further investigate how local HOA change is associated with the clinical situation.

This study has some limitations. First, we did not apply the nonparametric test (Kruskal-Wallis test) for analysis due to the small sample size. Further analysis of larger samples are needed. Second, we could not separate corneal aberration from the overall ocular aberration. Corneal aberration change by cataract surgery should be considered in another study. Third, analysis was done for only the 3rd- and 4th-order aberrations with a single measurement of aberration. The reproducibility of wavefront aberrations is somewhat uncertain, but a recent paper reports that 2nd-, 3rd-, and 4th-order aberrations can be measured accurately and repeatedly in model eyes.18 Fourth, we measured aberration in undilated pupils only. It is universally known that aberration increases in dilated eyes.19 Aberration in dilated pseudophakic eyes might be much more increased due to exposure of the optic edge in the dilated state. Comparison of aberration in dilated eyes would give a different result.

In this pilot study, we did not find a statistically significant difference in HOAs between normal eyes and eyes with 3 types of acrylic IOLs. Consequently, AcrySof MA60BM, Sensar AR40, or Corneal ACR6D implantation in cataract surgery may not affect overall HOAs but may induce minor variation in coma-like aberrations.

References

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© 2005 by Lippincott Williams & Wilkins, Inc.