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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