Presbyopia / SPO - 3.2.3.5. The role of Femtosecond laser in presbyopia surgery

3.2.3.5. The role of Femtosecond laser in presbyopia surgery

Ramiro M. Salgado

Hospital Santo António, Porto, Portugal Hospital da Luz Arrábida, Porto, Portugal

INTRODUCTION

The femtosecond laser (FSL) for use in Ophthalmology dates back to 20011, initially for procedures within the cornea, such as the creation of a corneal flap in LASIK, replacing the mechanical microkeratome. The rationale underlying the use of this laser lies in the possibility of producing tissue cuts with high precision, minimizing collateral tissue effects.

The physical principle is the production of short-duration pulses (in the order of 10-15 s, that is, femtosecond-SI unit) by associating low pulse energy with high frequency, creating a plasma wave and photo-disruption in the target tissue.

The use of femtosecond laser in the scope of presbyopia surgery is relatively recent and involves two main aspects: refractive procedures assisted by FSL and refractive procedures produced (directly) by the FSL, as well as the target tissue: the corneal plane or the lenticular plane.

INDICATIONS

Corneal Techniques

Procedures Produced by the FSL

The femtosecond laser was proposed to produce directly on the cornea a multifocal profile. The IntraCOR2 technique (FSL Victus, B&L) consists of producing a series of concentric intra-stromal rings with no margin or interface with predictable lower incidence of related complications (infection, epithelial growth, among others). The published works with the highest follow-up show a loss of efficacy over time, requiring retreatment. These same studies report a decrease in corrected distance visual acuity, with altered quality of vision indexes, compromising patients' final satisfaction3,4.

FSL-assisted procedures

In the case of PresbyLasik, similarly to LASIK, the femtosecond laser is used to create a corneal flap, in the initial step of the technique5. The benefit of using FSL in relation to the microkeratome lies in the possibility of a more personalized cut (through the choice of several parameters, such as the thickness and dimensions of the flap) and greater precision of this initial step, associated to the lower incidence of stress imposed on the patient's eye, since the use of vacuum levels is substantially lower than that of the conventional microkeratome. At the level of complications, lower rates of incidence of events related to the lenticule, such as the button-hole and free cap, are described. Also reported in the literature, functional recovery is faster, and the incidence of symptoms of discomfort by the patients is also lower, both in relation to the operative procedure and the postoperative discomfort6,7.

The femtosecond laser is also used, at the corneal level, to create the pocket for placement of presbyopic corneal implants (inlays). Currently, there are three implants marketed (with different mechanisms to produce improvement of near visual acuity): Raindrop (hyper-prolaticity), Flexivue (multifocality) and Kamra (pinhole effect) (Figure 1). For all of them, it is necessary to create an intra-stromal pocket and respective access tunnel, making the femtosecond laser an added value, thanks to its greater precision, reproducibility and safety8.

Figure 1 - Kamra Implant

Lenticular Techniques

Procedures Produced by the FSL

Refractive lentotomy was initially proposed and described by Lubatschowski9 in 2009, using nonlinear photo-disruption to produce a pattern of microincisions within the lens. These 3-D structures allow the creation of intra-lenticular sliding planes with the objective of regenerating dynamic accommodation. In in vitro experiments performed on donor human eyes of various ages, an increase in central thickness (up to 100 µm) associated with a decrease in the equatorial diameter of the lens, and an increase in lenticular flexibility (mean value of 16% in the Fisher’s spinning test) was observed, whereas in in vivo rabbit eye experiments, no cataract onset was observed for up to 1 year follow-up.

Recently the IKARUS project developed a femtosecond laser to produce a specific intra-lenticular incision pattern with 2 axial concentric cylinders and 12 radial incisions. Following this project, the Laser Zentrum Hannover Institute (LZH, Germany) is developing a virtual model for monitoring the impact of incisions through ray tracing (RayFEye: Ray tracing in ophthalmic finite element models for predicting visual acuity enhancement) and wavefront measurements, in order to determine the predictability of the procedure and to establish its customization10.

Procedures assisted by FSL

The use of femtosecond laser in cataract (crystalline) surgery is based on the expectation of greater efficacy and safety in performing relevant steps inherent to the technique. Thus, capsulotomy, phaco-fragmentation and corneal incisions (steps traditionally associated with the standard phacoemulsification technique) have emerged sequentially from the historical point of view in the scenario of FSL use. Several FSL devices have been on the market for this purpose since the appearance of LenSx (Alcon) in 2008, with four more: Catalys (Abbott Medical Optics), LensAR (LensAR Inc, Orlando, Florida), Victus (Technolas Perfect Vision / Bausch & Lomb, Rochester, New York), and Femto LDV (Ziemer Ophthalmic Systems AG). These are solid-state lasers that have in common a docking device, an integrated imaging system and a method of laser application. They vary in versatility, docking type, speed of action, and other parameters such as portability (Table 1).

FSL platforms for cataract surgery

Distinctively, these devices are divided into two groups as to the energy level and frequency of the pulses: a group in which low energy (nJ scale) and high frequency (MHz) pulses are produced, to which the Ziemer LDV belongs, and another group where the pulses have higher energy (µJ) and lower frequency (KHz), in which all the other devices are included. The difference between the two groups in the production of the pulse lies in the fact that the Ziemer LDV uses a pulse amplification process (patented, not disclosed) different from that used by the platforms of the other group, which use a phenomenon called CPA11 (chirped pulse amplification ), in the creation of the final pulse to apply in the tissues.

Thus, the performance profile is different, in the sense that the effect of photo-disruption and associated shock wave on neighboring target tissues is lower in the first group (Ziemer LDV), based more on the effect of plasma ablation on the target tissue (associating higher frequency of pulses on this same tissue). In the other group, the mechanical effect of the photo-disruption (applied by the expanding blisters) is greater proportionally, although, in relation to the production of the corneal lenticule in Femto-LASIK, there are no significant differences between the two groups12.

As for the basic technique of using FSL in cataract (crystalline) surgery itself, and with particularities among the various devices, there are four sequential procedures: planning, coupling, visualization and personalization of treatment, being essential the possibility of observing the lens and other ocular structures in vivo (Figure 2).

Figure 2 - OCT image, courtesy of Ziemer

The use of FSL in cataract surgery (FLACS) dates back to 2008 (Nagy)13, and initially involved the production of capsulotomy, a fundamental step in the cataract removal technique associated with phacoemulsification. Many studies have demonstrated a better accuracy of FSL capsulotomy, favoring a better optical quality, with greater stability of the intraocular lens in the capsular bag, resulting in a lower incidence of tilting and lens offset. The centration is particularly relevant for premium lenses, such as in correction of presbyopia, with high-performance intraocular lenses: multifocal, EDOF (enhanced depth of focus), CTF (continuous transitional focus), among others14,15.

Customization, automation, and precision associated with FSL theoretically would increase the effectiveness, predictability, and security of this procedure. If, in relation to the first two indices, there is such evidence16, in relation to safety, several published studies point to an equal or higher number of complications related to the use of FSL when compared with the classic phacoemulsification technique (capsulorhexis performed manually)17,18. Studies to date do not include a casuistry of relevance with Ziemer LDV. This device, as previously mentioned, produces pulses of low energy and high frequency (on the contrary to the other FSL devices) and, in the specific case of capsulotomy, an apparent greater regularity of the capsulotomy margin is observable by electron microscopy. This may be associated with a considerably lower incidence of capsulotomy-related complications traditionally associated with most FSL devices, with a small number of complications related to this step being preliminarily reported by small groups of users of this device.

Another recently introduced step is that of phaco-fragmentation (Figure 3). In this case, FSL, by custom-making lens incisions (depth, dimensions, pattern), reduces ultrasound usage (total energy and effective time). The biggest advantages are less endothelial damage, shorter intervention time, as well as reduced manipulation, relevant not only in cases of zonular instability, evolved cataracts, or endothelial pathologies, among others, but also in the case of refractive lens surgery in presbyopia, where the eminently elective character of the procedure requires the least possible damage.

Figure 3 – Phaco-fragmentation following a radial pattern

The contribution of FSL in cataract (crystalline) surgery also includes the performance of customizable corneal incisions associated with coaxial or micro-incisional phacoemulsification (MICS)22, with the benefit at the level of high-order aberrations23, the degree of induced astigmatism, and consequent improvement in refractive indices of safety and effectiveness in premium surgery.

Regarding other intraoperative complications described in the literature with the use of FSL, as the higher incidence of myosis24 described as a classic event associated with the use of FSL, it is not reported significantly with the use of the Ziemer LDV25.

Likewise, the time requirements for cortical mass aspiration, also described as slower after the application of FSL in the group that uses more energetic and lower frequency pulses26, do not change with Ziemer LDV (pulses with lower energy)25. Common to these two complications (myosis and “cortex blocking”) is that their incidence is statistically more significant in the more energetic and lower frequency pulse group.

Significantly, the fluidics indispensable for the phacoemulsification technique is reduced with the use of FSL, since the phaco-fragmentation step performed by the laser does not require the use of intraocular fluids. The effect of vitreous hydration as a consequence of phacoemulsification surgery (due to overhydration) is known, associating, according to some authors27, an independent risk of vitreous syneresis and even of reghmatogenous retinal detachment, with practically universal symptoms reported by some patients of entoptic phenomena, such as myodesopsies, often undervalued in clinical practice.

If we look at the age group of patients who are candidates for refractive surgery of the lens, and in particular the presbyopes, we find that the average age of the candidate is relatively lower than a few years ago, with the vitreous of these patients showing elastic and adhesion characteristics different to those of older patients, with a higher risk of retinal complications28.

Equally relevant, regardless of retinal risk, the presence of myodesopsies on the one hand interferes with the final optical quality of the procedure (elective, ab initio), and on the other hand is often a complaint by the patient of greater discomfort than photic phenomena, often associated with the implantation of presbyopic correction lenses, such as multifocal lenses.

Regarding refractive results, such as the best corrected visual acuity, the published studies point to similar results between CPE (phacoemulsification) and FLACS29,30, and the literature is silent about the comparison of phacoemulsification and lens surgery with FSL in the context of presbyopia correction specifically. This should be emphasized, since parameters of effectiveness (such as visual acuity at various distances) should also be evaluated. On the other hand, in several works, like those mentioned above between CPE and FLACS, there is a significant difference between optical quality and internal aberrations, with advantage for FLACS31-34.

FINAL COMMENTS

The use of femtosecond laser in presbyopia surgery, and in particular in the lens surgery with the implantation of premium lenses for this purpose (multifocal, EDOF, CTF, among others), currently presents controversial points.

Some uncertainty as to the financial and logistical implications (of space and time) must be included in the perspective of premium surgery, with the imperative of optimizing (either by increasing accuracy, safety and reproducibility) a procedure that is, ab initio, elective. The driving force that will determine the choice of FSL use in this procedure lies, in the first and last instance, in demonstrating the evidence that, in the various relevant parameters (efficacy and safety), the use of FSL offers advantages over traditional phacoemulsification, resulting from this clinical evidence also financial optimization (market law, similar to what happened with other techniques, such as Excimer laser and the transition to Femto-LASIK).

In this sense, it is of outmost importance, since it is a technology under development (as manifested by the different operating profiles of the devices, regarding the energy and pulse frequency of the FSL), to comprehensively and thoroughly evaluate all the devices available in the market. In this particular aspect, it appears that the various review publications do not include, for reasons unknown, probably related to lower casuistry, and possibly less impact or brand disclosure, results with low-frequency, high-frequency pulse energy devices such as the Ziemer LDV.

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