Fig. 5 Correlation between internal aberrations (RMS) and
distance IOLC – FPR
FPR = first Purkinje reflex; IOLC: intraocular lens center; PC = pupil
center; RMS = root mean square
Quality of vision (QoV) and satisfaction were slightly better with FPR,
but still not statistically significant, with a mean difference of
0.03±0.56 (95% CI: -0.17 to 0.23; p=0.745) for QoV and a mean
difference of -0.07±0.45 (95% CI: -0.23 to 0.09; p=0.423) for the
satisfaction questionnaire.
DISCUSSION
Intraocular lens centration within the capsular bag is determined by
various factors on the long-term.15 Amongst them are
IOL and capsular bags characteristics which interact with each other;
IOL factors include the optic and haptic material, the overall IOL
length in relation to the capsular bag diameter, and the inherent
ability of the haptics to withstand compressive forces of a contracting
capsular bag; capsular bag related factors include integrity, geometric
features (such as size and morphology) and capsulotomy features. It is
widely consensual the importance of a circular, continuous capsulotomy,
as well as its centration, which impacts on the long-term positioning of
the IOL.16 Furthermore, the point of capsulotomy
centration has been a topic of discussion, concerning the profile of
multifocal diffractive IOLs. This profile demands an optimization of the
IOL centration, eliciting the search for the best match of the eye’s
several reference points and lines, along with the pupil’s paramount
role; some recent studies have proposed different capsulotomy centration
points other than the classical pupil center.17, 18
Also, postoperative changes to the capsular bag such as anterior
capsular contraction and posterior capsular opacification vary depending
on the IOL material and amount of anterior capsule overlap of the IOL’s
edge.17 In this study, posterior capsular
opacification was none to minimal for both groups, thus not presenting
itself as a major factor influencing IOL positioning. Capsular bag
contraction exerts a force on the IOL and may lead to decentration.
Improved centration is perceived as one the features of femtosecond
laser-assisted capsulotomy, with increased efficacy and
accuracy.6, 19 Moreover, the low-energy femtosecond
laser device used in this study is associated to an absence of
intraoperative capsular-related complications (tags or ruptures), as
several recent studies demonstrate.20, 21
Regarding the magnitude of IOL center – visual axis decentration, a
distance of less than 0.25 mm is seldom noticeable and a multifocal IOL
would typically be described by most clinicians as “well-centered”. A
decentration in the range of 0.25–0.50 mm is noticeable in a slit-lamp
examination, and it has been assumed that it is clinically
insignificant; however, this assumption lacks some clinical strong
evidence. Furthermore, not always is the visual axis (or its equivalent)
chosen as the reference point for centration assessment, but rather the
optical center or even the pupil center.22
The present study shows modification of the IOL positioning along time,
traduced by a lens displacement towards the optical center of the
cornea; this fact is not influenced by the performed intraoperative
manoeuvre of manually centration of the IOL aligned with the visual axis
(as commonly recommended). This feature has been reported in the
literature and it is has been mostly described as an auto centration of
the IOL in the capsular bag,23 with many factors being
involved in this phenomena, mostly geometric at first hand. Furthermore,
our results show evidence that the choice of the reference centration
point regarding capsulotomy, influences the long-term positioning of the
IOL. Therefore, beyond strengthening the fact that the IOL tends to
adjust itself to the capsular bag (more than intraoperative IOL manual
positioning), this study shows that capsulotomy centration on the first
Purkinje reflex is associated to better IOL centration towards the
visual axis, as compared to pupil centration. This fact is translated
into a significant impact on aberrometric values, notwithstanding the
fact that it did not influence the quality of vision or patient
satisfaction scores, in this study. Our results also shows that the
refractive outcomes do not seem to be influenced by capsulotomy
centration. A possible explanation for this apparent asynchrony could be
assigned to the sinusoidal technology of the Acriva Trinova, which may
allow increased tolerance to deviations from the ideal aberrometry
profile.
In this study, only IOL centration was evaluated regarding IOL
positioning, as a sole variable impacting on performance outcomes.
Several publications point out to a reduced influence of tilt on
multifocal IOLs performance,24, 25 as opposed to a
significant impact of decentration in this type of intraocular lenses.
Nevertheless, in this context, there was never intention to minorize the
role of tilt on MIOL; rather than an oversimplification, the authors’
choice of this single approach (isolating the centration factor in the
context of IOL positioning) related to the feasibility of the IOL
decentration assessment with respect to visual axis.
At last, the present study has some pitfalls: it contemplates only one
type (plate-haptic) of IOL, restricting the evidence to this specific
design; tilt is not measured in this paper, as mentioned above;
aberrometry measurements were performed under a single (mesopic)
illumination condition; and a larger series is recommendable to
strengthen the evidence so far. However, this study does enhance the
long-term relevance of capsulotomy centration of multifocal IOLs as
close as possible to the visual axis, as a first-choice procedure when
performing refractive lens surgery, and further introduces a novel
marking procedure for achieving such purpose.
ACKNOWLEDGEMENTS
João Duarte Reis, MSc, Biostatistics, Coimbra, Portugal, provided
statistical support.
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