TEXT
INTRODUCTION
The role of capsulotomy in the IOL final position has long been
established, 1, 2, 3 being consensually accepted that
an optimal (curvilinear, continuous, 360º IOL edge overlapping)
capsulotomy is required to obtain the best positioning of the IOL, by
ensuring long-term stability and centration, taken the IOL-capsular bag
interaction, namely, capsular adherence and time-related retraction.
Femtosecond laser-assisted capsulotomy has been regarded as an
improvement in this matter, as it is associated to higher levels of
effectiveness, predictability, and accuracy.4, 5Intraocular lens (IOL) centration is of paramount importance in
multifocal IOL implantation, since adequate IOL positioning is required
for attaining the goals of best performance and patient
satisfaction.6
Ideally, the IOL should centered on the visual axis, pursuing the
biometry goal (usually emmetropia) in purely refractive terms. In
practical terms, the vertex normal is used instead of the theoretical
visual axis, which correlates to the first Purkinje
reflex.7, 8 Additionally, in the case of multifocal
IOLs, pupil center must also be considered for adequate IOL positioning,
since pupillary diameter directly impacts on the different foci
(distance, intermediate and near, for trifocal lenses), influencing
these IOL’s performance.9 However, the optics of the
eye are not ideal, which renders it impossible to line up the optical
centers of all ocular elements. This evidence is translated, amongst
other concepts, into the angle alpha and angle kappa; whereas the former
reports to the angle formed between the visual axis and the optical
center, the latter, in particular, is of relevance when considering the
implantation of a multifocal IOL, as it relates to the angle formed
between the theoretical visual axis and the pupil center. For practical
purposes, the kappa angular measure has been substituted for a more
modern and functional equivalent, which is the chord mu, as described by
Chang and Waring;10 this is a two-dimensional vector
measured across the corneal surface from the center of the pupil to the
coaxially sighted, subject-fixated corneal light reflex (CSFCLR). The
CSFCLR can be described as a line-of-sight guided corneal reflex when
the patient fixates on the coaxial lights of the surgical microscope,
closest to the first Purkinje reflex.
In this line of thought, a large k angle may constitute a
contraindication for multifocal IOL implantation, as this condition
inhibits an efficient combination of the two reference centration
points; although a well-defined limit is not established, it is commonly
accepted for most authors that chord mu should be inferior to 500
micra.6 Regarding IOL implantation, it is currently
recommended that the lens should be manually centered halfway between
the pupil center and the visual axis (or first Purkinje
reflex),8, 10 thus conjugating both requirements.
However, this common gesture has two main pitfalls. Firstly, it assumes
that the IOL will stay in that position, not contemplating the
IOL-capsular bag interaction along time; this interaction depends on
several factors such as lens- and capsular bag-related conditions.
Secondly, pupil center changes with pharmacological pupillary dilatation
prior to surgery traduced by a significant increase of chord mu
length;11 therefore, intraoperative pupil center is
farther away from the ideal visual axis, which compromises the accuracy
of such positioning endeavor.
This study’s protocol proposed to manually center the IOL inner
diffractive circle in the alignment of along the first (CSFCLR) and
fourth Purkinje reflexes during surgery, as recommended by some
authors.8, 12
The methodology of IOL centration assessment varies in the literature.
Methods based on measuring the distance between the IOL optic edge to
the dilated pupil have the disadvantage of disregarding the fact that
dilation is often asymmetric and affects normal geometry, consequently
it does not reproduce the physiological status. Some sophisticated
methods of measuring IOL decentration using Scheimpflug imaging, and
anterior segment ocular coherence tomography (OCT) have been described.
These methods have their own limitations. Measurements from Scheimpflug
imaging can be inaccurate because of magnification and distortion
effects from the Scheimpflug camera and OCT imaging requires 3D
algorithmic analyses.13, 14
This study proposes to assess the influence of capsulotomy centration on
the long term IOL positioning, by comparing femtosecond laser-assisted
pupil-centered versus first Purkinje reflex-centered capsulotomy.
METHODS
This prospective randomized comparative 2-armed study was approved by
the Ethics Committee at Hospital
da Luz Arrabida (HLA), Porto, (ref: 04/2019/CES) and was conducted in
accordance with the ethical principles stated in the “Declaration of
Helsinki”, with signed informed consent given by each patient. All
patients were proposed to undergo femtosecond laser–assisted RLE in
both eyes for presbyopia correction. For each patient, capsulotomy
centration was randomly assigned to pupil centre in one eye and first
Purkinje reflex in the other. All patients had surgery between October
3, 2019, and March 2, 2020, at Hospital da Luz Arrabida (HLA). All
surgeries were performed at HLA by a single comprehensive surgeon (R.S.)
using the Centurion Vision (Alcon
Laboratories, Inc.) phacoemulsification system and the LDV Z8
femtosecond laser platform (Ziemer, Inc.). The IOL power was calculated
to achieve emmetropia in all cases.
Inclusion and Exclusion Criteria
Patients were excluded from the study if they had previous ocular
surgery, preoperative corneal astigmatism more than 0.75D, chord µ
> 500 µm, ocular pathology, corneal abnormalities, and an
endothelial cell count less than 2000 cells/ mm2. Also, posterior
capsule opacification above minimal was considered a bias for this
study’s conclusions and constituted an exclusion criterium.
Measurement Procedures and Study Devices
The Galilei G4 is a tomography system based on dual Scheimpflug imaging
and Placido-disc technology. Participants were asked to look at the
fixation point, blink, and open their eyes widely before each image
acquisition. Manual alignment of the red crosshair to the 4 Purkinje
dots, corresponding to the first Purkinje reflex (FPR) in the cornea, is
performed and mesopic (mean of 5.5 lux) pupil diameter and center (PC)
are acquired. The device calculates chord mu length, from FPR to PC;
this chord mu is apparent, as it measures the distance between the first
Purkinje reflex and the pupil center viewed through the cornea. Galilei
G4 displays several maps, enabling visualization of IOL central
diffractive rings, along with FPR (corneal vertex), pupil and optical
centers, and limbus-to-limbus imaging (Figure 1).