Download Keratometric index in keratoconic eyes before and after intracorneal

ARTICLE
Keratometric index in keratoconic eyes before and
after intracorneal ring segment implantation
Victoria de Rojas, MD, PhD1; Antía Gestoso, MD1; Alejandra Gómez, MD1; Margarita de la Fuente, MD1;
María López, MD1; Renata Rodrígues, MD1; Isabel López, MD1; Josefina Pombo, MD1
OBJETIVE: To compare anterior and posterior corneal surface curvature in normal
(control) eyes, keratoconic (KC) eyes and keratoconic eyes after intracorneal ring segment
(KCICRS) implantation.
SETTING: Complexo Hospitalario Universitario, A Coruña (Spain).
METHOD: Retrospective study in which 30 eyes with KC before and after ICRS
implantation and 30 control eyes were analyzed. The mean radius of the anterior (antR) and
posterior (postR) corneal surface and true net power were determined using images from a
rotating Scheimpflug camera, from which the antR/postR ratio and the mean keratometric
index (KI) were calculated for each group.
RESULTS: Mean antR and postR were 7.82 ± 0.31 mm and 6.42 ± 0.37 mm in control eyes,
6.94 ± 0.46 mm and 5.51 ± 0.5 mm in KC eyes, and 7.18 ± 0.5 mm and 5.5 ± 0.49 mm
in KCICRS eyes, antR being significantly different between all groups (p = 0.0001) and
postR significantly different between control and KC eyes and between control and KCICRS
eyes (p = 0.0001). The antR/postR ratio between the three groups was significantly different
(p = 0.0001). Mean KI between control, KC and KCICRS eyes was statistically different
(p = 0.0001 KC vs. KCICRS, KCICRS vs. control; p = 0.012 KC vs.control).
CONCLUSION: Anterior and posterior corneal curvature is significantly greater in KC eyes
than in control eyes, with an increase in the antR/postR ratio. After ICRS implantation, the
anterior surface is flattened, while there are no significant changes in the posterior surface.
These findings suggest that the standard KI is not valid for calculating intraocular lens power
in KC or KCICRS eyes.
J Emmetropia 2014; 5: 9-13
Calculation of intraocular lens power is more
complex after corneal refractive surgery. Two causes
of error have been identified: error in calculating real
corneal power after surgery and error in calculating the
effective position of the lens after surgery. The first of
these errors, corneal power calculation, has a double
origin. On the one hand, the instruments cannot
Submitted: 11/19/2013
Accepted: 01/05/2014
Department of Ophthalmology. Complexo Hospitalario Universitario,
A Coruña (Spain).
1
Financial disclosure: The authors do not have any financial interest in
any of the products mentioned.
Corresponding Author: Victoria de Rojas Silva
Urbanización Aldea Nova, 68
15220 Ames, A Coruña, Spain.
E-mail: [email protected]
© 2014 SECOIR
Sociedad Española de Cirugía Ocular Implanto-Refractiva
accurately determine the power in the central area
modified by surgery, and on the other hand, an error
is introduced in the keratometric index when the ratio
between the anterior and posterior corneal surface radii
is changed. All these errors have been well documented
in the setting of laser corneal refractive surgery1,2,3.
It was thought for years that surgery by radial
keratotomy would be affected by the former errors but
not by the latter, as the anterior and posterior corneal
surfaces were modified simultaneously. Recently
however, Camellin et al.4 showed that after radial
keratotomy, both the anterior and the posterior corneal
surfaces are flattened, but the second to a greater extent
than the first, causing an decrease in the ratio between
the anterior and posterior surfaces, with the resulting
increase in the keratometric index.
Calculating intraocular lens power in KC eyes is
complex and has been addressed in very few studies
in the literature5,6,7. Even rarer are those dealing with
the calculation after ICRS implantation8. As occurred
ISSN: 2171-4703
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KERATOMETRIC INDEX IN KERATOCONIC EYES WITH ICRS
in radial keratotomy, some authors assume that since
the procedure is additive, the change in curvature
would be similar in both the anterior and the posterior
surfaces8. In this study, we use a design similar to that of
Camellin et al.4 in radial keratotomy to evaluate firstly
the ratio between the anterior and posterior corneal
radii and the keratometric index in a group of patients
with keratoconus compared to a control group without
keratoconus, and secondly, to evaluate the change in
these parameters after ICRS implantation in the group
with keratoconic eyes. In this way, the keratometric
index was evaluated in keratoconic patients before and
after ICRS implantation, with the aim of estimating
if such a change would contribute to the error in the
calculation of intraocular lens power in these cases.
PATIENTS AND METHODS
A retrospective study was carried out of patients with
keratoconus undergoing ICRS implantation. Patients
were identified from the database of the Ophthalmology
Department of the Complexo Hospitalario Universitario
A Coruña and their clinical records were reviewed to
collect the following information: age, sex and time
since surgery. Only one eye per patient was included
and cases with a follow-up of less than 5 months were
excluded. Topographical data were obtained from the
examinations carried out using the anterior segment
image analyzer with a rotating Scheimpflug camera
(Pentacam, Oculus Optikgeräte GmbH, Wetzlar,
Germany) before and after surgery in keratoconic eyes.
A Pentacam examination was performed in a control
group without keratoconus. Subjects with previous or
current eye disease which might affect the measurements
were excluded and only one eye per participant was
included. They were informed about the study aim and
signed an informed consent for participation. In view of
the retrospective nature of the study, informed consent
from the keratoconic patients was not required.
Surgical procedure
Intracorneal ring segment implantation was carried
out with topical anesthesia in all cases, except for two
children who required general anesthesia. Tunnels were
created manually assisted by suction. Patients with
keratoconus with mean corneal power less than or equal
to 58 D and pachymetry in the area of the implant of
more than 450 microns with a transparent cornea were
included. In all cases, Ferrara (AJL) intracorneal ring
segments were implanted in the 5-6 mm zone following
the nomogram of Alfonso et al.9 The pupil center and
the path of the tunnels were marked at the beginning of
surgery. An incision was made in the curved axis with a
diamond knife calibrated to 70% of the thinnest corneal
depth in the tunnel area and the tunnel was created
using a curved trephine while the eye was stabilized with
a vacuum segment connected to a syringe maintaining
suction. After surgery, a contact lens was implanted and
ofloxacin 0.3% (Exocin®, Allergan S.A., Madrid, Spain)
eye drops and dexamethasone 0.1% (Maxidex®, Alcon
Cusi S.A., El Masnou, Barcelona) four times a day for
one week, along with preservative-free artificial tears,
were prescribed. The contact lens was removed between
2 and 4 days after surgery, if no epithelial defect was
observed.
Topographical analysis
Topographies were obtained using an anterior
segment imaging analysis system with a rotary
Scheimpflug camera. For study purposes, the following
data were recorded in a 3 mm central diameter: mean
anterior radius, mean posterior radius and true net
power in a 3 mm central diameter. The repeatability
and reproducibility of the Pentacam examination
was studied in both normal eyes10 and in keratoconic
eyes11.
Calculation of keratometric index
The study measurement was the fictitious
keratometric index. When the radii of the anterior and
posterior surface of the cornea and pachymetry had
been determined, using the Gauss formula, the true
total corneal power may be determined; the Pentacam
provides this data automatically as true net power. If
these data are entered into the fine lens formula for
paraxial imagery, the fictitious keratometric index for
each eye can be calculated4:
Keratometric Index = corneal power × anterior radius + 1.
Statistical analysis
The data were entered in an Excel table (Microsoft
Corp) and the statistical analysis was performed using
an SPSS (IBM, NY, USA) package. Data were compared
between the control group and the keratoconus group
using the Mann-Whitney test, while for the comparison
of pre- and post-operative data of keratoconus patients,
the Wilcoxon test was used. Differences were considered
statistically significant in case of p ≤ 0.05.
RESULTS
In this study, 30 eyes from 30 patients (20 males and
10 females) with keratoconus undergoing intracorneal
ring segment implantation and 30 normal control eyes
from 30 patients (7 males and 23 females) with mean
ages of 33 ± 14 and 34.16 ± 9.07 years, respectively, were
included in this study. Of patients with keratoconus,
24 received 6 mm optical zone ring segment implants,
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KERATOMETRIC INDEX IN KERATOCONIC EYES WITH ICRS
while six received 5 mm optical zone implants. Only
one segment was implanted in 19 of the cases and two
segments in 11 cases. The mean follow-up of patients
with keratoconus was 8 ± 3 months (range 5 to 18
months).
The study results are shown in Table 1. Both the
anterior radius and the posterior radius are significantly
lower in keratoconus patients than in normal subjects,
i.e., both the anterior and posterior surfaces were
significantly more curved in patients with keratoconus.
The ratio between the anterior and the posterior radii,
the keratometric index and the corneal power are all
significantly higher in patients with keratoconus than in
normal subjects. In patients who received intracorneal
ring segments, there was a significant reduction in total
corneal power with significant flattening of the anterior
corneal surface, but not of the posterior surface. A
significant increase in the ratio between the anterior
and posterior corneal surface and a significant decrease
in the keratometric index were observed for both the
pre-surgical measurement and in comparison with the
index in normal patients.
DISCUSSION
The keratometric index is a fictitious number used
by instruments to convert the curvature of the anterior
corneal surface into the dioptric power of the whole
cornea (modeled as a single refractive surface). This
index, which provides excellent results for intraocular
lens power calculation in unoperated eyes, is unrelated
to the anatomy or other physical properties of the eye
and depends on a fixed ratio between the anterior and
posterior corneal curvature4. If the ratio between these
is changed by a surgical or pathological process, an error
in the index is produced which affects the calculation of
the corneal power.
The standard keratometric index has been set at
1.3375, which was selected for convenience rather than
for optical reasons, since it allowed concordance of the
important values (7.5 mm and 45 D)12. Nevertheless,
more recent studies based on the posterior corneal
curvature have calculated it to be between 1.3273
and 1.329013,14. In line with this data, the mean
keratometric index in the control group of our study
was 1.3282. This index is not only changed by surgery
(refractive laser, radial keratotomy, intracorneal ring
segment implantation), but it may also be altered by
disease processes, as shown by our study results in
the keratoconus patient group, which already before
segment implantation have a significantly greater
keratometric index than normal patients. Few studies
are available on the calculation of intraocular lens
power in keratoconus patients, but the error obtained
is greater than in normal corneas5,6,7. Our study shows
that the index error can contribute to a less predictable
calculation in these cases. In our series, both the anterior
and the posterior surfaces were significantly curved in
patients with keratoconus compared to normal patients,
as has been shown in other papers in which it has been
observed that curvature changes affect both corneal
surfaces15 and are correlated16.
In laser corneal refractive surgery in which the
anterior corneal curvature is modified without
changing the posterior surface, the relation between the
anterior and posterior surface radii are changed, so the
keratometric index changes, producing the referenced
index error2,3. It has recently been shown that corneas
Table 1. Results of parameters studied in normal eyes, keratoconic eyes and the same keratoconic eyes after
intracorneal ring segment (KCICRS) implantation
Control
Keratoconus
KCICRS
p-value
n
30
30
30
Mean anterior radius (mm)
7.82 ± 0.31
6.94 ± 0.46
7.18 ± 0.5
> 0.001
Mean posterior radius
6.42 ± 0.37
5.51 ± 0.5
5.50 ± 0.49
C vs KC y C vs KCICRS
0.0001;
KC vs KCICRS 0.846
Ratio antR/postR
1.22 ± 0.02
1.26 ± 0.04
1.31 ± 0.06
> 0.001
True net power (D)
41.99 ± 1.43
47.67 ± 2.95
45.15 ± 3.3
> 0.001
Keratometric index
0.0001 KC vs KCICRS
1.3282 ± 0.0074 1.3300 ± 0.0052 1.3228 ± 0.0039 and KCICRS vs C;
0.012 KC vs C
n, number; KCICRS, keratoconus treated with ICRS; antR, anterior radius; postR, posterior radius
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undergoing radial keratotomy show a change in the
ratio between the posterior and anterior corneal surface,
with greater flattening of the posterior surface compared
to the anterior surface, thus producing a different
keratometric index from the standard, which may also
cause index error in these cases4.
In contrast to the assumptions of some authors8,
and despite the technique being additive, intracorneal
ring segment implantation may also lead to greater
flattening of the anterior surface compared to the
posterior surface, which does not change significantly,
and to a corresponding change in the keratometric
index. Thus, lens positioning error, radius error and
index error would come into play in the intraocular
lens power calculation error, in the same way as in eyes
undergoing refractive surgery.
In the case of intracorneal ring segment
implantation, the results of our study show a reduction
in total corneal power due to the corneal flattening of
the anterior surface, since the posterior radius of the
cornea is not significantly modified. These results do
not agree with those of a previous study carried out
by Sogütlü et al.17 in which flattening was found on
both corneal surfaces, with the anterior surface being
affected more than the posterior. In this case, although
the study was also carried out with a Pentacam, the
measurements used for evaluating the topographical
changes in the cornea were different from ours; namely,
the anterior and posterior best-fit sphere and the
maximum elevation. Moreover, the exact mechanism
of action of the intracorneal ring segments is not well
defined. It has been suggested that the additive effect
on the middle of the corneal lamellae make these
increase their trajectory to surround the implant,
producing the compensatory effect of shortening the
arc, thus causing the flattening. This, however, does not
explain the increased curvature in the midpoint of the
segment and the flattening that appears at the ends.
Other theories, such as the twisting effect produced in
the lamellae have been proposed, which could explain a
flattening effect in the anterior surface with hardly any
changes in the posterior surface18.
Olsen has calculated that a change in the keratometric
index of 0.001 induces a change of approximately
0.13 D in corneal power12, and in keratometry an
error of 1.00 D causes a 1 D error in refraction after
intraocular lens implantation19. Accordingly, the
differences found in our study, together with the other
causes of error already described in these cases can lead
to significant errors in the calculation of intraocular
lens power.
The keratometric index used in the third-generation
formulae is the conventional value of 1.3375 or
1.3315, while that used for normal corneas measured
with topographs which analyze the posterior surface
has been calculated at between 1.3273 and 1.329013,14.
The use of a different keratometric index both in normal
corneas and in those modified by surgery or eye disease,
such as keratoconus, will therefore produce a significant
error. For normal eyes, the error produced by the current
third-generation formulae has been compensated by the
optimization of the constants. Thus, the change in the
keratometric index without a new optimization of the
constants would lead to errors, both by applying the
physiological keratometric index of normal corneas, or
other indexes from modified corneas.
To correct this type of error, the total corneal power
would have to be calculated, using either the Gauss
formula or ray-tracing4.
The results of this study show that both keratoconic
eyes and keratoconic eyes operated with intracorneal
ring segments have a different ratio between the
anterior and posterior corneal surfaces than normal
eyes, and thus, a different keratometric index from the
standard value, which may lead to errors in intraocular
lens calculation.
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JOURNAL OF EMMETROPIA - VOL 5, JANUARY-MARCH
First author:
Victoria de Rojas Silva
Complexo Hospitalario Universitario,
A Coruña, Spain