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Catalogue from Homo Faber 2007
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Mathur, Ankit and Atchison, David A. (2010)
Influence of spherical
intraocular lens implantation and conventional laser in situ keratomileusis
on peripheral ocular aberrations.
Journal of Cataract and Refractive Surgery,
36(7). pp. 1127-1134.
Manuscript Title: Influence of spherical intraocular lens implantation and conventional laser
in situ keratomileusis on peripheral ocular aberrations
Authors: Ankit Mathur, BSc (Optom) and David A. Atchison, DSc
Affiliation: Visual & Ophthalmic Optics Laboratory, School of Optometry & Institute of Health and Biomedical Innovation, Queensland University of Technology, Kelvin Grove, Q 4059 AUSTRALIA
Short Title: Refractive surgeries and peripheral aberrations
Financial Interest: Ankit Mathur – None; David A. Atchison – None Corresponding author: Ankit Mathur
Mailing Address:
5WS40, Institute of Health and Biomedical Innovation Q-Block, Queensland University of Technology 60 Musk Avenue,
Kelvin Grove
QLD 4059, Australia
Email: [email protected] Fax: 61 7 3138 6030
Address for reprints: Same as above
Funding Details: The study was supported by Australian Research Council Discovery grant DP0558209.
Abstract
Aim: To measure the influence of spherical intraocular lens implantation and conventional myopic laser in situ keratomileusis on peripheral ocular aberrations.
Setting: Visual & Ophthalmic Optics Laboratory, School of Optometry & Institute of Health and Biomedical Innovation, Queensland University of Technology, Brisbane, Australia. Methods: Peripheral aberrations were measured using a modified commercial Hartmann-Shack aberrometer across 42° x 32° of the central visual field in 6 subjects after spherical intraocular lens (IOL) implantation and in 6 subjects after conventional laser in situ keratomileusis (LASIK) for myopia. The results were compared with those of age matched emmetropic and myopic control groups.
Results: The IOL group showed a greater rate of quadratic change of spherical equivalent refraction across the visual field, higher spherical aberration, and greater rates of change of higher-order root-mean-square aberrations and total root-mean-square aberrations across the visual field than its emmetropic control group. However, coma trends were similar for the two groups. The LASIK group had a greater rate of quadratic change of spherical equivalent refraction across the visual field, higher spherical aberration, the opposite trend in coma across the field, and greater higher-order square aberrations and total root-mean-square aberrations than its myopic control group.
Conclusion: Spherical IOL implantation and conventional myopia LASIK increase ocular peripheral aberrations. They cause considerable increase in spherical aberration across the visual field. LASIK reverses the sign of the rate of change in coma across the field relative to that of the other groups.
Introduction
Refractive surgeries such as intraocular lens (IOL) implantation and laser in situ keratomileusis (LASIK) are the most commonly performed ophthalmic surgeries. IOL implantation is the only refractive treatment for cataract whereas LASIK is the most popular corneal correction for myopia. In LASIK, the corneal stroma is ablated with excimer laser, thereby altering the corneal shape to correct refractive errors.1 Improvements in instruments and surgical techniques have rendered both procedures safe with minimal post-operative complications.
A spherical surface IOL, no matter what its shape, has positive spherical aberration which adds to the generally positive spherical aberration of the cornea.2-4 The situation is somewhat similar to the older eye; in young eyes, the corneal spherical aberration tends to be partially compensated by negative spherical aberration of the lens, but with increasing age this balance is lost as the spherical aberration of the older lens tends to become positive.5, 6 The spherical aberrations of eyes with spherical IOLs are higher than those of age matched
phakic eyes.7-9 In theory, spherical aberration can be reduced by IOLs with appropriate
aspheric surfaces.10, 11 and this has indeed been found with the aspheric IOLs produced in
recent years.12 Aspheric IOLs provide some improvement in spatial vision over that obtained
with spherical IOLs.12
While the effects of IOL design and the nature of corneal ablations have been considered for axial aberrations and image quality, some attention is warranted for their effects on peripheral vision as this is important for movement13 and detection tasks, both of which are affected by peripheral refractive errors.14, 15 Also, there may be effects on
diagnostic or therapeutic procedures carried out in the periphery such as fundus imaging and photocoagulation.
There are theoretical studies predicting peripheral image quality for IOLs of different shapes, 16-18 but as yet there are no experimental studies. Peripheral image quality is predicted to be considerably worse for pseudophakic eyes than for phakic eye, due to large amounts of
astigmatism.17 At 10° away from the centre of the field, IOL position relative to the pupil
should not influence the spherical aberration, but will have a considerable influence on
coma.16, 19 Coma increases linearly with the IOL shape factor when the IOL is positioned at
the pupil, and shows a quadratic change with the IOL shape factor when the IOL is moved away from the pupil. 16, 19
The ablation profile in LASIK and related corneal refractive surgeries is important for aberrations. Conventional LASIK leads to increases in higher-order aberrations (particularly spherical aberration) 20-22. The increase in spherical aberration is because the anterior cornea, generally prolate (negative asphericity) before surgery, becomes oblate (positive asphericity) following surgery. 23-25 The exact form of the surface, and hence the aberrations, is influenced by several factors including hinge position, flap, bed thickness, decentration, optical zone diameter, wound healing, stromal regression and corneal
biomechanics.26-34 “Wavefront guided” LASIK has been applied to customise corneal
ablation and minimize post operative aberrations.35, 36 While there have been promising
results,36, 37 there are many factors such as corneal hydration during the operation and post-surgical healing that influence outcomes.34, 38
Theoretical studies39, 40 indicate that peripheral image quality following corneal
refractive surgery is worse than that for emmetropic eyes. The image quality worsens
peripheral myopic shifts and greater astigmatism, along the horizontal meridian, in a conventional LASIK group than in an untreated myopic group. Peripheral higher-order aberrations were measured in two of the LASIK patients: the rates of change in spherical aberration were greater than for the control group, and the rates of change of horizontal coma were of the opposite sign in the central ±25° of the visual field than for the control group.24 The changes in coma and spherical aberration were predictable on the basis of simple eye models with positive corneal asphericity.
The success of IOL implantation and LASIK are ascertained by the levels of axial aberrations, visual acuity and contrast sensitivity function, without any consideration given to peripheral vision or peripheral optics. In order to better understand the effects of these surgical interventions, we have investigated their effects on peripheral aberrations.
Methods
The study complied with the tenets of Declaration of Helsinki and was approved by Queensland University of Technology’s human research ethics committee. Informed consent was obtained from each subject prior to experiment after receiving written and verbal explanation of the procedures and risks involved.
The subjects included patients from a private ophthalmology clinic in Brisbane and staff of Queensland University of Technology. Subjects were IOL implantation and myopic LASIK (laser in situ keratomileusis) patients. Only right eyes were tested. The IOL group consisted of 6 subjects with a mean age of 62 years (range: 55 to 68 years) and post-surgical mean spherical equivalent refraction of 0.12 D (range: −0.2 D to 0.9 D). Patients had undergone phacoemulsification and were implanted with Acrysof SN60AT spherical IOLs
(Alcon Inc., USA). These lenses are spherical biconvex lenses, for which the front surfaces are generally more curved than the back surfaces. The subjects had surgical scars at 11-12 o’clock on the right corneas with clear posterior capsules. The LASIK group consisted of 6 subjects with mean age of 32 years (range: 25 to 39 years). Details were as follows: Subject 1: pre-surgical refraction −6.50 D/−0.50 DC x 160, B & L Z100 platform, optic zone 6.5 mm, time since surgery 12 months, post-surgical refraction –0.25 D/−0.75 x 15; subject 2: –3.25/–
0.50 x 180, Carl Zeiss AG – MEL 80, 6.4 mm, 28 months, −0.25 D/−0.75 x 31; subject 3: –
2.25/–0.50 x 10, Nidek EC 5000, 5.0 mm, 43 months, +0.25 D/−0.25 x 120; subject 4: –
5.75/–2.75 x 180, Wavelight Allegretto 400Hz, 6.5 mm, 5 months, plano/–0.25 x 115; subject 5: –2.25/–0.25 x 90, platform not known, 6.5 mm, 48 months, +0.25/–0.50 x 155; subject 6: pre-surgical refraction and platform not known, 6.8 mm, 36 months, plano/–0.25 x 35. None of the subjects experienced any post surgical complications and all had high contrast visual acuity of at least 6/7.5 measured with a Bailey-Lovie chart.
Each subject underwent a detailed slit lamp examination and corneal topography measurement with a Medmont E300 corneal topographer (Medmont International Pty. Limited, Australia). Anterior corneal vertex radius of curvature R and asphericity Q were
estimated from corneal height data for the central 6 mm cornea as described previously42, 43
using the formula
X2 + Y2 + (1 + Q) Z2 – 2ZR = 0
where the Z axis is the visual axis.
IOL and LASIK group data were compared with those of control groups. The control group for IOL subjects consisted of 7 emmetropes (spherical equivalent: 0.1 D ± 0.6 D; mean age: 63 years; age range: 50-71 years) and the control group for LASIK subjects consisted of
10 myopes (spherical equivalent: –3.8 D ± 1.9 D; mean age: 27 years; age range: 22-35 years).
Peripheral aberrations were measured using a modified COAS-HD (Wavefront Sciences Inc. Albuquerque, USA) Hartmann-Shack aberrometer that uses a wavelength of 840 nm. Subjects placed their heads on the aberrometer’s chin rest and, through a 45° beamsplitter, fixated a 6 rows x 7 columns matrix of targets projected sequentially on a rear projection screen across the 42° x 32° central visual field. The screen was 1.2 m from the eye. The center of the target matrix was aligned with the internal fixation target of the aberrometer. The pupil centre was aligned with the aberrometer’s measurement axis, and the cornea (rather than the entrance pupil) was made conjugate with the lenslet array with the help of aberrometer’s pupil camera before taking measurements. Two measurements were taken at each visual field location and their aberration coefficients were averaged. A detailed description of the methods has been given previously.44
Axial aberrations were measured using the internal fixation target of the aberrometer as per the measurement instructions given by the manufacturer. The internal target was fogged by 1.5 D to limit the influence of accommodation on axial aberrations. Axial aberration coefficients for the refractive and control groups were compared using independent sample t-tests.
Aberrations were described using Zernike coefficients up to 6th order at 555 nm
wavelength for a 5 mm pupil in accordance with the ISO aberration standard.45 Zernike
coefficients for the elliptical pupils due to eye rotation relative to the aberrometer were estimated with the aberrometer software and a custom made Matlab based algorithm which compensated for the elliptical shape of the pupil by stretching it by the inverse of the cosine
equivalent M, regular astigmatism J180 and oblique astigmatism J45 from Zernike coefficients
as described previously.46, 47
Results
IOL group and control emmetropic group
Anterior corneal radius of curvature (p = 0.48) and asphericity (p = 0.30) were similar for the IOL group (R = 7.6 ± 0.2 mm, Q = –0.08 ± 0.15) and the emmetropic control group (R = 7.7 ± 0.3, Q = –0.16 ± 0.08).
Figure 1 shows mean axial higher-order aberration coefficients and higher-order root-mean-square aberrations (HORMS) for the IOL group and its emmetropic control group. The IOL
group had more positive spherical aberration coefficient 0
4
C (mean difference = +0.10 ± 0.04
µm, t = 3.1, p = 0.01) and greater HORMS (mean difference +0.18 ± 0.03 µm, t = 5.1, p < 0.001) than the control group. Other axial higher-order aberration coefficients were not significantly different between the groups.
Figure 2 shows mean higher-order wavefront maps across the pupil at each visual field location. The IOL group’s wavefront maps are dominated by spherical aberration, but those for the emmetropic control group are dominated by coma across most of the visual field (Figure 2). Coma increased away from the centre of the field in both cases so that its orientation approximately matched the visual field meridian.
Emmetropes IOL group Zernike coef ficient/RMS aberration ( mm) -0.2 0.0 0.2 0.4 CC33 C3 C4 HORMS 0 -3 -1 1 * *
Figure 1. Mean axial trefoil coefficient 3 3
C , vertical coma coefficient 1 3
C , horizontal coma(C13)coefficient, spherical aberration coefficientC40, higher-order root-mean-square (HORMS) and total root-mean-square aberrations (TotalRMS) in emmetropic control group and IOL group. The error bars represent standard deviations for the mean aberrations. Significant differences between groups are shown by asterisks above the results of the IOL group.
Figure 2. Higher-order wavefront maps across the pupil (5 mm diameter) at each visual field location for (a) emmetropic control and (b) IOL group. I, S, N and T represent inferior, superior, nasal and temporal visual field.
Figure 3 shows various mean refraction and aberration terms as a function of visual
field position for the control group (A) and the IOL group (B). The astigmatic terms J45 and
meridian and 90°-270° meridians, respectively and decreased along the perpendicular meridians (Aa, Ac, Ba, Bc). The spherical equivalent M had corrections applied so that it appears as zero at the centre of the field for both groups. M decreased in a quadratic manner away from the centre of the field in both groups (Ab, Bb). The rates of change for J45, J180
and M across the field were greater for the IOL group than for the control group, particularly
so for M for which the change was approximately 0.0020 D/deg2 greater for the former
group. Trefoil coefficient 3
3
C was more negative for the IOL group than its control group
(Ad, Bd), but this was not significant (p = 0.47). Vertical coma coefficient 1 3
C and horizontal
coma coefficient 1
3
C showed similar magnitudes and linear dependencies along the vertical
and horizontal field meridians, respectively, for both groups (Ae, Af, Be, Bf). Spherical
aberration coefficient 0
4
C was significantly more positive (mean difference +0.15 ± 0.03 µm, t
= 5.8, p < 0.001) across the field for the IOL group than for the control group (Ag, Bg). Higher-order root-mean-square (HORMS) aberration was higher across the visual field for the IOL group (Ah, Bh), mainly because of the spherical aberration differences. Total root-mean-square aberrations excluding defocus (Total RMS) changed at a greater rate for the IOL group (Ai, Bi), mainly because of differences in the rates of change of the astigmatisms.
Horizontal (degrees) Vertical (degrees) I S (a) J 45 -15 -5 5 15 (b) M (c) J180 (e) C (3, -1) (f) C (3, 1) (d) C (3, -3) -15 -5 5 15 (g) C (4, 0) -15 -5 5 15 -20 -10 0 10 20 (h) HORMS -20 -10 0 10 20 (i) Total RMS (excluding defocus) -20 -10 0 10 20 (b) M (c) J180 (e) C (3, -1) (f) C (3, 1) (a) J45 -15 -5 5 15 (d) C (3, -3) -15 -5 5 15 -20 -10 0 10 20 (g) C (4, 0) -15 -5 5 15 (h) HORMS -20 -10 0 10 20 -20 -10 0 10 20 (i) Total RMS (excluding defocus) Horizontal (degrees) T N
A. Emmetropic control group B. IOL group
I S I S T N T N T N T N T N -1 -0.5 0 0.5 -1 -0.5 0 0.5 1 -0.15 -0.1 -0.05 0 0.05 0.1 -0.4 -0.2 0 0.2 -0.3 -0.2 -0.1 0 0.1 0.2 -0.05 0 0.05 0.1 0.15 0.2 0.25 0.2 0.3 0.4 0.5 0.6 0.7 0.6 1 1.4 1.8 2.2 -1 -0.5 0 0.5 1 -1 -0.5 0 0.5 -0.15 -0.1 -0.05 0 0.05 0.1 -0.3 -0.2 -0.1 0 0.1 0.2 -0.4 -0.2 0 0.2 -0.05 0 0.05 0.1 0.15 0.2 0.25 0.2 0.3 0.4 0.5 0.6 0.7 0.6 1 1.4 1.8 2.2 -2.5 - 2 -1.5 -1 -0.5 0 -2.5 - 2 -1.5 -1 -0.5 0
Figure 3. Mean refraction and aberration coefficients across the visual field for (A) emmetropic control group and (B) IOL group. (a) oblique astigmatism J45, (b) spherical equivalent M, (c) regular astigmatism coefficient J180 (d) trefoil 33
C , (e) vertical coma coefficientC31, (f) horizontal coma coefficient C31, (g) spherical aberration coefficient C40, (h) higher-order root-mean-square (HORMS), and (i) total root-mean-square excluding defocus (Total RMS). The spherical equivalent M has been shifted for each group so that it is zero at the centre of the field. The color scales represent the magnitude of aberration coefficients in μm and are common for a given aberration coefficient between two groups. I, S, N and T represent inferior, superior, nasal and temporal visual field. Pupil size is 5 mm. Dotted black lines are visual field meridians in 30° steps.
LASIK group and control myopic group
Anterior corneal radius of curvature (p = 0.005) and asphericity (p < 0.001) were significantly higher in the conventional LASIK group (R = 8.5 ± 0.4 mm; Q = +0.6 ± 0.1) than in its control myopic group (R = 7.7 ± 0.2 mm; Q = –0.2 ± 0.1).
Figure 4 shows mean axial higher-order aberration coefficients and HORMS aberrations for the LASIK group and its control group. The LASIK group had more positive spherical aberration coefficient 0
4
C (mean difference = +0.09 ± 0.04 µm, t = 3.5, p = 0.004)
and more negative horizontal coma (mean difference = −0.11 ± 0.04 µm, t = 2.8, p = 0.014) than the control group. Differences between the groups for other axial higher-order aberration coefficients were not significant.
Myopes LASIK group Zernike coef ficient/RMS aberration ( mm) -0.2 0.0 0.2 0.4 C3 C3 C3 C4 HORMS 0 -3 -1 1 * * *
Figure 4. Mean axial higher-order aberrations for myopic control and LASIK group. Other details same as figure 1. Significant differences between groups are shown by asterisks above the results of the LASIK group.
Figure 5 shows mean higher-order wavefront maps across the pupil at each visual field location. The LASIK group’s wavefront maps are dominated by spherical aberration near the centre of the field, but coma is the predominant aberration away from there. The myopic control group’s maps are dominated by coma across the whole field. The orientation of coma for the LASIK group matches the visual field meridian, but is opposite to the orientation for the control group (and also for the other two groups shown in Figure 2).
Figure 5. Higher-order wavefront maps across the pupil (5mm diameter) at each visual field location for (a) myopes and (b) LASIK group. Other details are the same as for Figure 2.
Figure 6 shows various mean refraction and higher-order aberration terms as a function of visual field position for the myopic control group (A) and the LASIK group (B). Astigmatic terms were similar for the two groups (Aa, Ab, Ac, Bc). The spherical equivalent
M had corrections applied so that it appears as zero at the centre of the field for both groups. M changed little across the field for the myopic group, but became more negative into the
periphery for the LASIK group (Ab, Bb) by a rate of about 0.0020 D/deg2. Trefoil
coefficient 3
3
C was more positive and showed greater rates of change for the LASIK group
than for the control group (Ad, Bd). However, the difference between the groups was not significant (p = 0.12). For the vertical coefficient 1
3
C and horizontal coma coefficientC31, the
slopes were in opposite directions for the LASIK and control groups (Ae, Af, Bd, Bf).
Spherical aberration coefficient 0
4
C was significantly higher across the field for the LASIK
group (mean difference +0.11 ± 0.03 µm, t = 3.7, p = 0.003) than for the control group (Ag, Bg) but not as high as for the IOL group (Figure 3Bg). HORMS and Total RMS changed at greater rates for the LASIK group than for the control group (Ah, Ai, Bh, Bi).
Horizontal (degrees) Vertical (degrees) I S (a) J 45 -15 -5 5 15 (b) M (c) J180 (e) C (3, -1) (f) C (3, 1) (d) C (3, -3) -15 -5 5 15 (g) C (4, 0) -15 -5 5 15 -20 -10 0 10 20 (h) HORMS -20 -10 0 10 20 (i) Total RMS (excluding defocus) -20 -10 0 10 20 (b) M (c) J180 (e) C (3, -1) (f) C (3, 1) (a) J45 -15 -5 5 15 (d) C (3, -3) -15 -5 5 15 -20 -10 0 10 20 (g) C (4, 0) -15 -5 5 15 (h) HORMS -20 -10 0 10 20 -20 -10 0 10 20 (i) Total RMS (excluding defocus) Horizontal (degrees) T N
A. Myopes B. LASIK subjects
I S I S T N T N T N T N T N -1 -0.5 0 0.5 1 -1 -0.5 0 0.5 -0.15 -0.1 -0.05 0 0.05 0.1 -0.4 -0.2 0 0.2 -0.3 -0.2 -0.1 0 0.1 0.2 -0.05 0 0.05 0.1 0.15 0.2 0.25 0.2 0.3 0.4 0.5 0.6 0.7 0.6 1 1.4 1.8 2.2 -1 -0.5 0 0.5 1 -1 -0.5 0 0.5 -0.15 -0.1 -0.05 0 0.05 0.1 -0.3 -0.2 -0.1 0 0.1 0.2 -0.4 -0.2 0 0.2 -0.05 0 0.05 0.1 0.15 0.2 0.25 0.2 0.3 0.4 0.5 0.6 0.7 0.6 1 1.4 1.8 2.2 -2.5 - 2 -1.5 -1 -0.5 0 -2.5 - 2 -1.5 -1 -0.5 0
Figure 6. Mean refraction and aberration terms across the visual field for (A) myopic control group and (B) LASIK group. The color scale for a term is the same as that used in Figure 3. Other details are the same as for Figure 3.
The differences between coma slopes for the LASIK and other groups were
considered further. Figure 7 shows the vertical coma coefficient 1
3
C along the vertical visual
field and the horizontal coma coefficient 1
3
C along the horizontal visual field, for the LASIK
and myopic control groups. The mean slopes for the LASIK group were about 50% higher and of the opposite sign than those for the control group (p < 0.001). Coma slopes for the IOL and emmetropic control groups were comparable to those for the myopic control group (17% to 32% greater).
-0.010
+0.016 +0.012-0.008
Figure 7. Coma coefficients for myopic control group and LASIK group with 5 mm pupil: a) vertical coma coefficient along vertical visual field; b) horizontal coma coefficient along horizontal visual field. Coma slopes were estimated by least squares linear fits. The horizontal coma coefficient for a given horizontal field angle was the mean of horizontal coma coefficients with the same horizontal field angle but with vertical field angles of ±3°. The error bars represent the standard deviations of the coma coefficients.
Discussion
Similar to earlier reports on aberrations following IOL implantation48 and LASIK20-22, our
IOL and LASIK groups showed greater axial higher-order root-mean-square aberrations than control groups. Spherical aberration was more positive across the field for the IOL group than for the other groups, and was more positive for the LASIK group than for its control group.
The slopes for coma coefficients had opposite signs for the LASIK group than for other groups.
Our results with LASIK subjects support a previous study by Atchison24, in which the
LASIK subjects had horizontal coma slopes with signs opposite to those for untreated subjects within about 25° of fixation. Beyond this angle, he found that the slopes changed sign. We did not measure out far enough to test this. Atchison found that spherical aberration decreased into the peripheral visual field; we may not have measured far enough to confirm this.
For the IOL group and its emmetropic control group, anterior corneal radius of curvature (mean difference: 0.10 ± 0.14 mm) and asphericity (mean difference: 0.07 ± 0.07) were similar whereas the anterior corneal radius of curvature and asphericity were higher in the conventional LASIK group than in the myopic control group by 0.8 ± 0.2 mm and +0.71 ± 0.05, respectively. The changes in peripheral spherical refraction equivalent, in spherical aberration and in peripheral coma following LASIK can be largely explained by these differences.24, 41 Similar effects have been found for myopic orthokeratology.43, 49 One of the orthokeratology studies showed the influence of treatment zone, with the subject with the smaller zone having reversal in coma slope only out to about ±15°.43
Zhou et al.37 found that increases in corneal asphericity, on-axis coma and on-axis
spherical aberration were significantly smaller in a wavefront guided LASIK group than in a conventional LASIK group. Because of the way in which corneal asphericity influence aberrations, we predict that peripheral myopic refraction will be smaller with wavefront guided LASIK than with conventional LASIK, and that differences in coma slope and in spherical aberration across the field will be smaller compared with control subjects.
In conclusion, spherical IOL implantation and conventional LASIK produce considerable changes in peripheral ocular aberrations. Both procedures cause increases in rate of change of higher-order root-mean-square aberrations towards the periphery of the field. The spherical aberration across the field increases with IOL implantation and LASIK surgery, but the increase was higher for the former in this study. LASIK reverses the direction of change in coma across the field.
Acknowledgements
The study was supported by Australian Research Council Discovery grant DP0558209. We thank Dr. Lee Lenton, Dr. Frank Howes, Dr. Julie Albietz and Mr. Andrew McCormack for helping recruit IOL and LASIK subjects.
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