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).