Centration of myopic refractive ablation: should we center treatment on the pupil or the visual axis?

Abstract

The purpose of this study is to compare pupil versus corneal vertex-centered ablation for myopic laser refractive surgery. This study is a retrospective case series of right eyes of consecutive myopic patients undergoing either photorefractive keratectomy (PRK) or laser-assisted in situ keratomileusis (LASIK) with pupil or corneal vertex-centered ablation from January 2018 to April 2018. Overall 258 eyes of 258 patients were included. Of the 104 that underwent LASIK, 52 were treated centered on the corneal vertex (50%), and of the 154 that underwent PRK, 77 were treated centered on the corneal vertex (50%). There were no significant differences in baseline age, gender, spherical equivalence, sphere, cylinder, or angle kappa between both groups in either LASIK or PRK. There were no significant differences between the corneal vertex-centered and pupil-centered groups in terms of efficacy index (LASIK: 1.02 ± 0.14 vs 1.01 ± 0.13, p = 0.86; PRK: 1.00 ± 0.13 vs 0.99 ± 0.15, p = 0.61), safety index (LASIK: 1.02 ± 0.12 vs 1.01 ± 0.13, p = 0.70; PRK:1.02 ± 0.12 vs 1.02 ± 0.09, p = 0.97), and residual astigmatism (LASIK: 0.26 ± 0.25 vs 0.23 ± 0.28, p = 0.65; PRK:0.37 ± 0.41 vs 0.39 ± 0.31, p = 0.78). In mixed effect models, there were no significant differences between the corneal vertex-centered and pupil-centered groups when accounting for angle kappa (p > 0.05). Patients with large angle kappa (> 300 μm) eyes yielded similar results (p > 0.05). For conclusion, in myopic refractive surgery, performing ablation centered on the corneal vertex or on the pupil leads to similar outcomes regardless of the amount of angle kappa.

Full text

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https://doi.org/10.1007/s10103-021-03358-2
ORIGINAL ARTICLE
Centration ofmyopic refractive ablation: should we center treatment
onthepupil orthevisual axis?
GiladRabina1,2 · MichaelMimouni3,4,5· JacquelineSlomovic6· NirSorkin1,2· AchiaNemet2· IgorKaiserman5,7,8
Received: 12 January 2021 / Accepted: 7 June 2021
© The Author(s), under exclusive licence to Springer-Verlag London Ltd., part of Springer Nature 2021
Abstract
The purpose of this study is to compare pupil versus corneal vertex-centered ablation for myopic laser refractive surgery.
This study is a retrospective caseseries of right eyes of consecutive myopic patients undergoing either photorefractive kera-
tectomy (PRK) or laser-assisted insitu keratomileusis (LASIK)with pupil or corneal vertex-centered ablation from January
2018 to April 2018. Overall 258 eyes of 258 patients were included. Of the 104 that underwentLASIK, 52 were treated
centered on the corneal vertex (50%), and of the 154 that underwent PRK, 77 were treated centered on the corneal vertex
(50%).There were no significant differences in baseline age, gender, spherical equivalence, sphere, cylinder, or angle kappa
between both groups in either LASIKor PRK. There were no significant differences between the corneal vertex-centered
and pupil-centered groups in terms of efficacy index (LASIK: 1.02 ±0.14 vs 1.01 ± 0.13, p = 0.86; PRK: 1.00 ± 0.13 vs
0.99 ± 0.15, p = 0.61), safety index (LASIK: 1.02 ± 0.12 vs 1.01 ± 0.13, p = 0.70; PRK:1.02 ± 0.12 vs 1.02± 0.09, p =
0.97), and residual astigmatism (LASIK: 0.26 ± 0.25 vs 0.23 ± 0.28, p = 0.65; PRK:0.37 ± 0.41 vs 0.39 ± 0.31, p = 0.78).
In mixed effect models,there were no significant differences between the corneal vertex-centered and pupil-centered groups
when accounting for angle kappa (p > 0.05). Patientswith large angle kappa (> 300 μm) eyes yielded similar results (p >
0.05). For conclusion, in myopic refractive surgery, performing ablation centered on thecorneal vertex or on the pupil leads
to similar outcomes regardless of the amount of angle kappa.
Keywords Corneal vertex· Pupil centered· Ablation· Myopia
Introduction
Over the past few decades, laser-assisted insitu keratomi-
leusis (LASIK) and photorefractive keratectomy (PRK) have
become the mainstay of refractive surgery. Both procedures
address a wide range of refractive errors and effectively
reduce dependence on glasses and contact lenses [1, 2].
The effort to improve the outcomes of refractive surgery is
ongoing.
One critical aspect which can affect surgery outcome is
centration of treatment. Decentered photoablation may lead
to over- or under-correction; induction of higher-order aber-
rations, especially coma [3]; reduced visual acuity (both cor-
rected and uncorrected); induced astigmatism; and reduced
contrast sensitivity and night vision disturbances (such as
glare) [4].
The varying optical axes of the eye include the visual
axis/corneal vertex (a line connecting the fixation point with
the foveola, passing through the two nodal points of the eye),
pupillary axis (a line between center of pupil and the center
* Gilad Rabina
giladrabina@hotmail.com
1 Department ofOphthalmology, Tel Aviv Sourasky Medical
Center, 6 Weizmann Street, 64239Tel-Aviv, Israel
2 Sackler School ofMedicine, Tel-Aviv University, Tel-Aviv,
Israel
3 Department ofOphthalmology, Rambam Health Care
Campus, Haifa, Israel
4 Bruce andRuth Rappaport Faculty ofMedicine,
Technion-Israel Institute ofTechnology, Haifa, Israel
5 Care-Vision Laser Centers, Tel-Aviv, Israel
6 Michael G. DeGroote School ofMedicine, McMaster
University, Hamilton, Ontario, Canada
7 Department ofOphthalmology, Barzilai Medical Center,
Ashkelon, Israel
8 Faculty ofHealth Sciences, Ben-Gurion University
oftheNegev, BeerSheba, Israel
/ Published online: 29 June 2021
Lasers in Medical Science (2021) 36:1733–1739

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of curvature of the anterior corneal surface), line of sight
(a line from the fixation point reaching the foveola via the
center of pupil), and achromatic axis (axis that joining the
center of pupil and nodal points) [5]. Defining the optimum
axis for centration for laser ablation is challenging. Centra-
tion can be aligned with either the corneal light reflex (CLR)
(formed by the reflection of light from the anterior corneal
surface), line of sight (pupil centration), or visual axis (cor-
neal vertex centration).
There is yet a universal consensus as to the optimal axis
on which ablation should be centered. Reinstein etal. con-
cluded that pupil centration may not represent the patient’s
view and the treatment zone should preferably be centered
on the corneal vertex [6], while Okamoto etal. found that
centration on the corneal light reflex resulted in better safety,
efficacy, and contrast sensitivity than pupil centration [7]. In
contrast, Bueeler etal. found that pupil centration enabled
good comparability between preoperative and postoperative
measurements [8].
Given the inconclusive data in published literature regard-
ing the optimal ablation centration axis, the purpose of the
current study was to compare outcomes of pupil versus cor-
neal vertex-centered ablation in myopic refractive surgery
in order to determine which of the two axes are the optimal
centration target in this scenario.
Methods
This study was approved by the Internal Review Board
(IRB) of the Barzilay Medical Center and complied with
the principles outlined in the Declaration of Helsinki.
Study participants
The electronic medical records of all patients undergoing
PRK or LASIK in a refractive surgery facility (Care Vision,
Tel Aviv, Israel) between January 2018 to April 2018 by a
single surgeon (I.K.) were reviewed. The data were routinely
collected and entered into the electronic medical records
database by the facility staff. In this study, only the right eye
of each patient was included in order to avoid biases resulting
from inter-eye correlation [9]. Inclusion criteria were age 18
to 40years, a stable refraction for at least 12months, preop-
erative spher ical equivalent between − 0.50D and − 12.00D,
preoperative subjective astigmatism up to − 3.00D, IOP less
than 21mm Hg, and a period without wearing contact lenses
(more than 2weeks for rigid contact lenses and more than
4days for soft contact lenses). Excluded were patients with
previous ocular surgery, ocular comorbidities, and those
undergoing monovision treatment. The choice between
LASIK and PRK was made by the operating surgeon, in
accordance to each patient’s parameters. In general, PRK
was recommended where central corneal thickness (CCT)
was less than 500µm.
Surgical technique
Patients underwent either microkeratome-assisted LASIK
[10] or alcohol-assisted PRK [11] as previously described by
our group. Briefly, one drop of a topical anesthetic (benoxi-
nate hydrochloride 0.4%) was instilled in the conjunctival
fornix of the eye before surgery, after which a lid speculum
was inserted. In the LASIK group, a microkeratome (Moria
SBK, France) with a thickness plate of 90mm was used
to create the flap with a nasal hinge. After flap creation,
a Wavelight EX500 (Wavelight AG, Erlangen, Germany)
wavefront-optimized treatment was used for stromal abla-
tion, and the flap was then placed back into position. In the
PRK group, epithelial removal was performed mechanically
after a 15-s exposure to 20% ethyl alcohol. Following epi-
thelial removal, a Wavelight EX500 wavefront-optimized
treatment was used for stromal ablation. After ablation, a
sponge soaked with mitomycin c (MMC) 0.02% was placed
on the stroma for 20–60s. The MMC was rinsed from the
ocular surface and a contact lens placed on the cornea. In
all cases, the optic zone size was 6.5mm. All eyes had a
mesopic pupil diameter ranging between 6.0 and 7.0mm.
Data collection
The following demographic and preoperative data were col-
lected: age, gender, sphere, cylinder, uncorrected distance
visual acuity (UDVA), and corrected distance visual acu-
ity (CDVA). The following intraoperative parameters were
collected: type of surgery (LASIK vs PRK) and pupillary to
corneal vertex distance and ablation centration (pupillary
vs corneal vertex). The following postoperative parameters
were collected: sphere, cylinder, UDVA, and CDVA. Effi-
cacy index (EI) was calculated as EI = postoperative UDVA/
preoperative CDVA and safety index (SI) as SI = postopera-
tive CDVA/preoperative CDVA [12].
Study groups
From January 2018 to February 2018, all ablations were
centered on the pupil (pupil-centered group), and from
March 2018 to April 2018, all ablations were centered on
the corneal vertex (corneal vertex-centered group). A com-
parison of pupil-centered versus corneal vertex-centered
was performed separately for LASIK and PRK cases. The
EX500 software allows the operator to position the centering
at 0%, 25%, 50%, 75%, or 100% of the distance between the
pupillary and corneal vertex center. In the pupillary centered
1734 Lasers in Medical Science (2021) 36:1733–1739

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group, it was set to 0%, and in the corneal vertex-centered
group, it was set to 100%.
Main outcome measures
The main outcome measures were postoperative refractive
error, UDVA, CDVA, efficacy index, and safety index at
30days for LASIK and 90days for PRK.
Postoperative follow‑up
Starting the day following their surgery, patients were given
moxifloxacin 0.5% QID, dexamethasone 0.1% QID, and non-
preserved artificial tears as needed. Patients were routinely
examined at 1day, 1week, and 1, 3, and 6months postop-
eratively and thereafter as necessary. In addition, they were
encouraged to return for examination if vision deteriorated
at any time after surgery and were offered additional treat-
ment free of charge.
Statistical analysis
Data were analyzed using Minitab software (version 18,
Minitab Inc., Paris, France).
Since the variance between eyes is usually less than that
between subjects, the overall variance of a sample of meas-
urements combined from both eyes is likely to underestimate
the true variance. Therefore, only right eyes were included
in this study [9]. Normality of the data was assessed by the
Kolmogorov–Smirnov test. For comparison between groups,
the Student T test was used for continuous variables, and
chi-square was used for categorical variables. Visual acuity
values are presented in decimal form but were converted to
logarithm of the minimum angle of resolution (logMAR)
equivalence for analyses. All analyses were two-tailed. A p
value of ≤ 0.05 was considered statistically significant. Data
are presented as means (± SD) or N (%).
Analysis ofastigmatism
For analysis of astigmatic treatment effect, vectoral analysis
was performed using the Alpins method [13], and the results
were presented using standard graphs for reporting outcomes
of astigmatism correction [14] which were produced using
the online Alpins Statistical System for Ophthalmic Refrac-
tive Surgery Techniques group analysis calculator [15].
Results
Overall, 258 eyes of 258 patients with a mean age of
25.8 ± 5.9years were included. We included 104 eyes that
underwent LASIK, of which 52 eyes (50%) were treated
centered on the corneal vertex, and 154 eyes that underwent
PRK, of which 77 eyes (50%) were treated centered on the
corneal vertex.
Baseline values
Table1 depicts a comparison of baseline values between the
pupil-centered and corneal vertex-centered treatment groups
(in both the LASIK and PRK cohorts). Briefly, there were
no significant differences in baseline age, gender, spheri-
cal equivalent, sphere, cylinder, or angle kappa between the
groups in either the LASIK or PRK cohorts.
Refractive andvisual outcomes
Table2 depicts a comparison of outcomes between the
pupil-centered and corneal vertex-centered treatment groups
in both the LASIK and PRK cohorts. Briefly, there were no
significant differences between the corneal vertex-centered
and pupil-centered groups in terms of final sphere, cylin-
der, spherical equivalent, UDVA, CDVA, efficacy index, or
safety index (p > 0.05 for all).
Pupil corneal vertex distance andoutcomes
The postoperative astigmatism, efficacy index, and safety
index were similar when comparing the pupil versus corneal
vertex-centered treatments across different ranges of pupil
corneal vertex distance in both the LASIK (Fig.1) and PRK
(Fig.2) cohorts (p > 0.05 for all). In mixed effect models,
there were no significant differences between the corneal
vertex-centered and pupil-centered groups when accounting
for the magnitude of angle kappa (p > 0.05).
Vector analysis
The target-induced astigmatism magnitude, surgical-induced
astigmatism magnitude, difference vector magnitude, angle
of error, correction index, and index of success were simi-
lar when comparing the pupil- versus vertex-centered treat-
ment in both the LASIK and PRK cohorts (p
>
0.05 for all,
Table2).
Discussion
This study demonstrated similar outcomes when compar-
ing pupil- versus corneal vertex-centered ablation for both
myopic PRK and myopic LASIK when using a wavefront-
optimized system. There were no significant differences in
distance from target (sphere or cylinder), efficacy index, or
safety index. Furthermore, these similar outcomes remained
consistent for different ranges of pupil corneal vertex
1735Lasers in Medical Science (2021) 36:1733–1739

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distance, and a mixed model effect found no difference
between treatment centration groups when accounting for
pupil corneal vertex distance. This study’s postsurgical out-
comes of spherical equivalent, visual acuity, efficacy index,
and safety index for both corneal vertex and pupil centration
when using a wavefront-optimized system LASIK or PRK
are considered good and correlated with previous studies
[16–19].
The human eye is an asymmetric optical system. Thus,
there are several optical axes that can be centered upon dur-
ing refractive surgery. The optimal axis for centration of the
laser ablation zone remains to be found despite previous
publications aimed at answering this question through the
testing of several possible centration foci such as the corneal
light reflex [20, 21], pupil [22, 23], or corneal vertex [24,
25]. None of these foci has been proven to be consistently
superior.
Pupil-centered ablation is currently the most common
centration method. Pupil boundaries are easily detected
by the eye-tracking systems, and the pupil can be well rep-
resented by a circular or oval aperture. Centering on the
pupil offers the opportunity to reduce the optical zone as
well as ablation depth. However, the optical zone should be
the same size or slightly larger than the functional pupil for
the patient to avoid postoperative higher-order aberrations
[26]. For a patient who fixates properly, pupillary centration
defines the line of sight; however, this is not necessarily the
patient’s actual visual axis. In addition, the center of one’s
pupil is dynamic and changes with the pupil size, especially
due to changes in lighting conditions during keratorefractive
surgery [22]. If the human eye’s optical system was truly
coaxial, then corneal vertex would represent the corneal
intercept of the optical axis. Despite the fact that the human
optical system is not truly coaxial, the cornea is the main
refractive surface. Thus, the corneal vertex is considered a
stable preferable morphologic reference [22].
Similar to this current study, Arbelaez at el. compared
the clinical outcomes of corneal vertex- and pupil-centered
LASIK. In their study, 88% of the eyes that underwent
corneal vertex centration achieved better than 20/20 uncor-
rected visual acuity 6months after surgery, compared with
97% of eyes where ablation was centered on the pupil.
However, the findings of this study were not statistically
significant (p = 0.25) [27]. Cheng at el. evaluated the clini-
cal efficacy of LASIK with ablation centration on the pupil
or corneal vertex. In their study, no significant difference
was found in postoperative UCVA or BCVA between the
two groups [28]. These finding were similar to ours with no
difference in visual acuity between both groups. Okamoto
etal. compared refractive outcomes of myopic LASIK with
centration on the coaxially sighted corneal light reflex ver-
sus centration on the pupil. Their findings were statistically
significant, indicating a better safety index and efficacy index
for the coaxially sighted corneal light reflex group. They
concluded that myopic LASIK centered on the coaxially
sighted corneal light reflex was significantly safer and more
effective than myopic LASIK centered on the pupil [29].
Bueeler etal. performed computer simulations on several
variants of the Gullstrand-Emsley schematic eye, which was
modified by an off-axis fovea. They found that the postopera-
tive line of sight was dependent least on the choice of the
preoperative centration axis for both myopic and hyperopic
treatments. They concluded that pupil centration enabled
a good correlation between preoperative and postoperative
measurements [8].
Arbelaez at el. found significant differences in higher-
order aberrations. Although the amount of induced coma
was small with both centration strategies, the difference in
induced coma between groups favored the corneal vertex
group (p = 0.01) [27]. Okamoto etal. found a statistically
significant greater induction of higher-order aberrations
(P = 0.04) and coma (p < 0.01) in the pupil centration group
postoperatively [29]. In the current study, we did not assess
higher-order aberrations.
This study has several limitations, the first of which is
the retrospective in nature. Second, there was a lack of ran-
domization between groups. However, as demonstrated, the
groups were quite similar at baseline in both the LASIK
Table 1 Comparison of baseline
parameters between the pupil
centered and vertex centered
treatment groups in both the
LASIK and PRK cohorts
LASIK laser in situ keratomileusis, PRK photorefractive keratectomy, D diopter, UDVA  uncorrected dis-
tance visual acuity, CDVAcorrected distance visual acuity
LASIK PRK
Pupil Vertex P-Value Pupil Vertex P-Value
Age (years) 26.58 ± 5.93 26.38 ± 5.95 0.87 25.55 ± 6.09 25.42 ± 6.00 0.89
Gender (%male) 63.46% 59.62% 0.69 63.64% 61.04% 0.74
Sphere (D) -2.56±1.40 -2.65±1.37 0.75 -4.51±2.80 -4.57±2.88 0.90
Cylinder (D) -0.89±0.71 -0.88±0.65 0.96 -0.89±0.64 -0.88±0.65 0.95
UDVA (Decimal) 0.12±0.16 0.11±0.13 0.91 0.08±0.12 0.08±0.12 0.99
CDVA (Decimal) 0.97±0.06 0.96±0.07 0.89 0.95±0.08 0.95±0.08 0.76
1736 Lasers in Medical Science (2021) 36:1733–1739

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Table 2 Comparison of
outcomes between the pupil-
centered and vertex-centered
treatment groups in both the
LASIK and PRK cohorts
LASIK laser in situ keratomileusis, PRK photorefractive keratectomy, D diopter, UDVA uncorrected dis-
tance visual acuity, CDVA corrected distance visual acuity, TIA target-induced astigmatism, SIA surgical-
induced astigmatism, DV difference vector
* Calculated using the Alpins method (ASSORT® Group Analysis Calculator)
LASIK PRK
Pupil Vertex p value Pupil Vertex p value
Sphere (D) − 0.22 ± 0.69 − 0.28 ± 0.60 0.60 0.05 ± 0.75 − 0.09 ± 0.70 0.23
Cylinder (D) 0.23 ± 0.28 0.26 ± 0.25 0.65 0.39 ± 0.31 0.37 ± 0.41 0.78
Spherical equivalent (D) − 0.10 ± 0.75 − 0.15 ± 0.60 0.71 0.24 ± 0.79 0.10 ± 0.71 0.22
UDVA (decimal) 0.98 ± 0.13 0.98 ± 0.12 0.94 0.95 ± 0.10 0.95 ± 0.11 1.00
CDVA (decimal) 0.98 ± 0.12 0.98 ± 0.10 1.00 0.95 ± 0.14 0.96 ± 0.09 0.93
Efficacy index 1.01 ± 0.13 1.02 ± 0.14 0.86 0.99 ± 0.15 1.00 ± 0.13 0.61
Safety index 1.02 ± 0.13 1.02 ± 0.12 0.70 1.02 ± 0.09 1.02 ± 0.12 0.97
*TIA magnitude (D) 0.82 ± 0.68 0.83 ± 0.61 0.92 0.81 ± 0.58 0.80 ± 0.49 0.95
*SIA magnitude (D) 0.81 ± 0.65 0.77 ± 0.53 0.73 0.93 ± 0.59 0.97 ± 0.60 0.68
*DV magnitude (D) 0.23 ± 0.28 0.26 ± 0.25 0.55 0.39 ± 0.31 0.37 ± 0.40 0.74
*Angle of error (degrees) − 0.2 ± 11.5 − 1.3 ± 11.4 0.63 3.1 ± 15.3 3.5 ± 12.8 0.87
*Correction index (SIA/TIA) 1.05 ± 0.54 0.95 ± 0.43 0.36 1.14 ± 0.49 1.21 ± 0.52 0.44
*Index of success (DV/TIA) 0.37 ± 0.51 0.34 ± 0.44 0.77 0.50 ± 0.53 0.47 ± 0.56 0.76
Fig. 1 A comparison of astigmatism (top), efficacy index (middle),
and safety index (bottom) between the pupil-centered versus corneal
vertex-centered ablation groups. There were no significant different
between groups when treated with LASIK at different ranges of pupil
corneal vertex distances (<
200, 200 to 400, and
>
400μm)
Fig. 2 A comparison of astigmatism (top), efficacy index (middle),
and safety index (bottom) between the pupil-centered versus cor-
neal vertex-centered ablation groups. There were no significant dif-
ferences (p > 0.05 for all) between groups when treated with PRK at
different ranges of pupil corneal vertex distances (< 200, 200 to 400,
and > 400μm)
1737Lasers in Medical Science (2021) 36:1733–1739

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and the PRK cohorts. Third, additional outcomes includ-
ing contrast sensitivity, night vision disturbances (glare),
and higher-order aberrations were not assessed. Last, these
findings do not apply to patients with hyperopia or high
astigmatism (> 3D) undergoing refractive surgery. Future
prospective studies comparing a wider range of outcomes
in these groups of patients are warranted.
In summary, myopic patients undergoing wavefront-
optimized LASIK or PRK demonstrated similar outcomes
with both pupil-centered and corneal vertex-centered abla-
tion treatments. Insisting on centering treatment on corneal
vertex in this group of patients may not be necessary.
Declarations
Competing interest All authors declare no competing interests.
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1739Lasers in Medical Science (2021) 36:1733–1739