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This article discusses the advantages and limitations for surgical compensation of presbyopia with the femtosecond laser using corneal inlays and the Intracor technique.
In short: This article discusses the advantages and limitations for surgical compensation of presbyopia with the femtosecond laser using corneal inlays and the Intracor technique.
By 2020, there will be around 2.1 billion presbyopes worldwide.1 It is unsurprising that there is a continuing interest in refractive surgical procedures that improve near vision.
Both surgical interventions for presbyopia, and their contact lens counterparts, may reduce distance acuity, quality of vision, stereopsis or contrast sensitivity. Many also consider clear lens surgery too invasive, particularly in the early stages of the condition. Thus, there is significant interest in corneal inlays as they do not remove tissue; they preserve future options for presbyopic correction; they may be used in pseudophakia and/or combined with laser refractive surgery; they are removable; and they are only implanted within the non-dominant eye.1
There are three different corneal inlay types currently available: refractive optic inlays, which change the eye’s refractive index, providing distance vision through a plano central zone surrounded by one or more rings of varying add power for near vision; corneal reshaping inlays, which reshape the anterior curvature of the cornea to enhance near and intermediate vision via a multifocal effect; and small-aperture inlays, which rely on pinhole optics to increase depth of focus by blocking unfocused light.
Only a small percentage of presbyopes are emmetropic; therefore, an additional LASIK might be performed in the case of a compound refractive error. Femtosecond lasers provide more predictable flap thickness, lower incidence of dry eye, faster visual recovery and better uncorrected distance visual acuity (UDVA) than mechanical microkeratomes.2-4 When a laser flap is created, many nerve-fibre bundles are cut; however, a pocket interface minimises the impact on the corneal nerves.
Corneal reshaping inlays
A clear, permeable hydrogel material makes up the Raindrop Near Vision Inlay (Revision Optics, Lake Forest, California, USA), which has around the same refractive index as the cornea (Figure 1).5 The inlay reshapes the central pupillary region of the non-dominant eye’s cornea to provide additional optical power relative to the unchanged peripheral region.
Twelve-month results of 30 presbyopic patients after inlay implantation combined with myopic LASIK demonstrated that mean binocular UDVA, uncorrected intermediate visual acuity (UIVA) and uncorrected near visual acuity (UNVA) were better than 0.1 logMAR (20/25), and 93% of patients had binocular visual acuities better than 0.1 logMAR across all visual ranges.6
Patient questionnaires also showed that, 1 year after surgery, visual symptoms were at preoperative levels, 98% of all visual tasks could be easily performed without correction and 90% of patients were at least ‘satisfied’ with their overall vision.
Twelve months after implantation combined with hyperopic LASIK, mean UNVA in the surgical eye (n=16) improved from 0.8 logMAR preoperatively to 0.0 logMAR (patients gained a mean of >7 lines of UNVA).8 The mean binocular gain in UNVA was also seven lines. The UDVA in the surgical eye significantly increased from 0.5 logMAR to 0.2 logMAR and performed even better binocularly. There were no reported visual symptoms.
Refractive optics inlay
The Flexivue Microlens (Presbia, Los Angeles, CA, USA), based on the InVue lens, is a transparent hydrophilic disk with a central 1.6 mm diameter plano disk and a peripheral zone that provides near addition power. The lens material’s refractive index is 1.4583 and it features a 0.51 mm central hole to facilitate the transfer of oxygen and nutrients to the cornea.10
The Icolens system (Neoptics AG, Huenenberg, Switzerland) comprises a microlens with a positive refractive power, a femtosecond laser (Femto LDV, Ziemer Ophthalmic Systems AG, Port, Switzerland) with pocket-cutting algorithm, preloaded deployment device and purpose-designed positioning instruments. The Icolens is based on a similar design to Flexivue, with a central hole. Its copolymer has hydrogel properties and a refractive index of 1.460 in hydrated conditions.
The Flexivue Microlens : Limnopoulou et al. 12 reported a significant increase in monocular and binocular UNVA (n=47). However, it should be noted that UDVA in operated eyes significantly worsened from 0.06 logMAR to 0.38 logMAR, maintaining stable binocular values. The mean refraction changed from 0.66 D to -1.95 D. No complications, removal or replacement occurred.
In a different study, postoperative slitlamp examinations showed clear corneas in 52 patients without evidence of thinning, scarring or vascularisation , with well-centered inlays at all timepoints.10
The Icolens system: Baily et al. 13 reported 12-month results after monocular implantation in emmetropic patients. Mean UNVA in the surgical eye (n=52) improved from 0.78 logMAR to 0.44 logMAR postoperatively, and patients gained a mean of 3.48 lines. The UDVA in the surgical eye significantly worsened (mean loss of 1.67 lines) but, binocularly, could be maintained. In total, 90% of patients reported being satisfied with the overall procedure.
The KAMRA inlay (AcuFocus Inc, Irvine, CA, US) is approved in >50 countries outside the USA, with >20,000 inlays implanted today worldwide. The current generation (model ACI7000PDT) is a 5 μm thin microperforated artificial aperture, with a total diameter of 3.8 mm and a central aperture of 1.6 mm, made of polyvinylidene fluoride with incorporated nanoparticles of carbon. The opaque permeable material has a light transmission of 6.7% and a pseudorandom microperforation pattern (consisting of 8,400 holes); it allows water and nutrient flow.
The longest follow-up to date (5 years; n=32), showed that mean binocular uncorrected visual acuities improved as follows: UNVA from 0.4 logMAR to 0.1 logMAR and UIVA from 0.2 logMAR to 0.1 logMAR.14 The UDVA decreased from -0.2 logMAR to -0.1 logMAR. One inlay was removed after 36 months because the patient was dissatisfied with vision after a hyperopic shift in the surgical eye.
It has been suggested that the ability to perform common daily tasks without glasses may be a better indicator of functional success than actual visual acuity. After 12 months, mean reading distance changed from 46.7 cm to 42.8 cm, and the mean reading acuity “at best distance” improved from 0.33 logRAD (reading equivalent of logMAR) to 0.24 logRAD. Mean reading speed and the smallest readable print size also improved.15
Vilupuru et al. 17 reported that KAMRA inlay subjects demonstrated improved intermediate and near vision with minimal to no change to distance vision, better contrast sensitivity in the inlay eye when compared with the multifocal lenses, and better binocular contrast sensitivity when compared with accommodating and multifocal IOLs.
Visual simulations of the KAMRA corneal inlay suggest that the device extends depth of focus as effectively as traditional monovision in photopic light, in both cases at the cost of binocular summation.18
Tabernero21 suggested that the best depth of focus in KAMRA patients can be obtained with small residual myopia (-0.75 D to -1.0 D) in the inlay eye and a plano refraction in the fellow eye. The use of a small aperture significantly reduced the negative impact of monovision on stereopsis.
Effect of pupil size and retinal image brightness
Tomita et al. 19 reported that pupil size does not influence visual acuity after KAMRA inlay implantation (n=584).
The effect of the KAMRA corneal inlay on the retinal image brightness in the peripheral visual field has also been evaluated by ‘implanting’ a KAMRA inlay into a theoretical eye model in a corneal depth of 200 microns, varying pupil size from 2.0 to 5.0 mm and field angles from -70° to 70°. For large field angles, where the incident ray bundle is passing through the peripheral cornea, brightness is not affected. For combinations of small pupil size (2.0 and 2.5 mm) and field angles of 20–40°, up to 60% of light may be blocked with the KAMRA.20
An early increase in stromal cell death and inflammation was shown 48 hours after surgery in rabbit eyes that underwent a femtosecond laser pocket creation and KAMRA insertion compared with eyes with pocket formation only. The difference disappeared by 6 weeks after surgery.23
With the first-generation inlay (ACI7000), 18 (56%) eyes of 32 patients developed corneal iron deposition within 36 months after corneal inlay implantation.25 Alterations in tear film thickness, its composition and corneal epithelial basal cell storage, resulting from changes in corneal topography, may be contributing factors. Corneal iron deposition was only observed in 1 of 24 patients after 18 months with the new KAMRA inlay design (ACI7000PDT). Reduced inlay thickness and an increase in the number of nutritional pores as well as the modified implantation technique may contribute to this decrease.26
In a recent study by Tomita et al. 27 (n=277) all age-stratified groups achieved a mean UDVA of 0.0 logMAR after simultaneous LASIK and KAMRA implantation. Group 1 (40 to 49 years), Group 2 (50 to 59 years) and Group 3 (60 to 65 years) gained 1 line, 2 lines and 3 lines, respectively, suggesting that taking age into account might help achieve optimum outcomes.
The INTRACOR technique
The INTRACOR procedure is a minimally invasive intrastromal compensation for presbyopia using a femtosecond laser (FEMTEC Laser System; Bausch+Lomb/Technolas Perfect Vision, Munich Germany).35 The basic pattern for presbyopia is a series of femto-disruptive cylindrical rings that are delivered, starting within the posterior stroma, at a variable distance from Descemet’s membrane, and extending through the mid-stroma to an anterior location at a predetermined, fixed distance beneath Bowman’s layer. Laser delivery does not impact the endothelium, Descemet’s membrane, Bowman’s layer, or epithelium. The net effect is a central steepening of the anterior corneal surface in a multifocal hyperprolate corneal shape.35
The standard pattern consists of five rings,37,38 although it might also be modified by creating six rings or by placing eight additional radial intrastromal cuts in the midperipheral stroma.36,39
The longest follow-up reported 36 month results of 20 presbyopic patients with mild hyperopia (modified pattern with 6 rings).39 Patients were randomly divided into three subgroups to compare the effect of three different ring diameters of the additional placed sixth ring [1.8/2.0/2.2 mm (Groups A/B/C)]. Median UNVA increased from 0.7/0.7/0.7 logMAR (Groups A/B/C) to −0.1/0.1/0.1 logMAR 36 months after surgery. UDVA changed slightly from 0.1/0.2/0.1 logMAR to 0.2/0.3/0.1 logMAR. Loss of two lines of binocular CDVA was noted in 0/25/0% of eyes. Overall patient satisfaction with the procedure was 80%.
Combination with LASIK
Three case reports describe corneal ectasia after combining LASIK with INTRACOR.
The additive effects on stromal corneal stiffness of the LASIK lamellar incision in the frontal plane and INTRACOR incisions in the sagittal plane may disturb the corneal structural stability and lead to a progressive over-relaxing effect on the residual stromal bed. Therefore, the combination of both procedures can not be recommended.
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Dr Alois K. Dexl MD MSc
Dr Dexl is Associate Professor at the Department of Ophthalmology of the Paracelsus Medical University of Salzburg. He has no financial or proprietary interest in any material or method mentioned