Presbyopia / SPO - 3.2.3.3. The use of spherical aberration in the correction of presbyopia

3.2.3.3. The use of spherical aberration in the correction of presbyopia

Víctor Sergio Eguiza Rubi, José Luis Güell Villanueva, Óscar Gris Castejón, Daniel Elies Amat, Mercè Morral Palau, Míriam Barbany Rodríguez , Francisco Bandeira e Silva, Spyridoula Souki y Felicidad Manero Vidal

IMO - Instituto Microcirugía Ocular de Barcelona, España

INTRODUCTION

Surgery for the correction of presbyopia is becoming one of the most requested interventions by patients to ophthalmologists, since being associated with age, presbyopia is universal and progressive from 40 years of age with symptoms that limit activities that require intermediate, close and, finally, distant vision1. The main problem we face with presbyopia is its multifactorial essence, due to the set of systems that cause it, or rather that stop working and as a result give presbyopia2. The theory of accommodation by Helmholtz is the most accepted one and argues that, during accommodation, the ciliary muscle contracts and causes a decrease in the diameter of the lens, causing relaxation of the zonular fibers, making the shape of the lens more spherical. Both the radius of curvature of the anterior and posterior surfaces of the lens decrease, causing an increase in its refractive power. On the contrary, in pharmacological cycloplegia, the crystalline lens is flattened by the tension of the zonular radial fibers, decreasing the refractive power3.

Broadly speaking we could say that presbyopia is a progressive dysfunction of this functional optical accommodation system to focus on nearby objects.

Presbyopia is affecting the quality of life in the elderly population around the world, and as the average age of life of the world population is increasing, a prevalence of 2.1 billion people affected worldwide is estimated towards 20202. Wanting to maintain an equally active lifestyle, and not wanting to carry out an external prosthetic correction method, could be one of the reasons for the increase in surgical treatment for presbyopia treatment4.

The depth of focus (DOF) could be defined simply as the dioptric interval through which we can have a vision precise enough to define the objects. This interval will depend on several external factors such as the type of task to be performed, the ambient light, the color of the lens and the contrast. The DOF can be measured through tests of contrast sensitivity and visual acuity5. But DOF will also depend on optical parameters, such as pupil size and optical aberrations (particularly spherical aberration), without forgetting that it may be affected by the neuro-retinal complex and psychophysical factors6 (Figure 1). Currently, the use of spherical aberration to treat presbyopia has been a topic of interest and debate4, since this concept can be applied in the different methods for the correction of presbyopia, but in the case of surgical correction it remains an important challenge. For this, there are different surgical strategies, extraocular (corneal or scleral) or intraocular (removal and replacement of the lens and perhaps in the future phakic lenses)7.

CONCEPT

Spherical aberration (SA) is one of the most important aberrations of the human eye, characterizing the lenses, where the peripheral rays of light upon impact on them are taken to a different focus than the rays that impact closer to the center8,9.

SA will affect vision, especially when it is associated with a large pupillary diameter. Consider that when the SA is positive (SA+) the peripheral rays will focus in front of the central rays, with the negative spherical aberration (SA-) the peripheral rays will focus behind the central rays9 (Figure 1).

The total SA in the human eye is a combination of the positive spherical aberration (SA+) of the cornea (which is more or less constant throughout life) and the negative aberration (SA-) of the lens. In young eyes, the numerical value of both is very similar and, by this compensation, they have a low total SA. With age, the lens's optical properties change, resulting in a more positive SA and therefore a decrease in optical performance, that is, a decrease in sharpness but a certain increase in depth of focus9.

As we mentioned before the depth of focus (DOF) of the optical system of the human eye is the margin of error, or the variation in the distance of the image, that can be tolerated to see the objective without noticing a lack of sharpness10,11. This dioptric interval defines the depth of "focus or field" of the eye. An important consequence of physiological optical aberrations is the existence of a significant depth of focus. In previous studies, researchers obtained controversial data on depth of focus with values from ±0.02 D (Oshima) to ±1.25 (von Bahr)11. The depth of focus is no longer considered a topic of theoretical speculation. It has become an important application, for example, in the evaluation of the efficacy of accommodative intraocular lenses11.

The DOF in the living human eye is not rigid or stable even under conditions of a constant opening of the pupil. The precorneal tear film, which is subject to changes practically every second, influences the amount and structure of the aberrations and in turn on the DOF. In an interesting study, some deviations in visual function were detected within the DOF. These minimal deviations can be explained by at least two causes:

a subjective character of the visual function examined by the lack of attention of the patients;
a changeable state of the tear film11.

The DOF can be used optimally in a corrective device, but we must consider that it is affected by the various changes in the optical system, such as pupillary size10.

The DOF is going to be inverse to the spatial frequency, that is, the objects of lower spatial frequency will produce a retinal contrast more tolerable to the defocus, and also the DOF can vary with the numerical aperture and increase of the objective, the higher the power of increase the deeper the DOF10.

Monochromatic high-order aberrations such as spherical aberration (SA), will produce a blur in the optical system, making it more tolerable to chromatic aberration, and increasing DOF produces a pseudo-accommodation. Clinically this is used as a passive approach to compensate for the symptoms of presbyopia (Figure 2). Interestingly, in other studies it has also been shown that when we have a complete correction of spherical aberration, there is an improvement in spatial vision without compromising the tolerance of subjective blur. So this leaves in debate how much residual SA is necessary for the benefit of DOF10.

Figure 2.

Defocusing caused by a positive spherical aberration causes greater tolerance to blur and chromatic aberrations, while negative spherical aberration will provide greater DOF than a positive spherical aberration of the same magnitude, enough to mitigate the symptoms of presbyopia. With this concept, therapeutic compensation designs of presbyopia are made, ranging from corrective devices (multifocal glasses, contact lenses), acting on the cornea as are laser ablations, or, perhaps, more invasively with intraocular lenses6.

TECHNIQUES:

CORNEA

Traditionally, the same principles that were used for monovision with contact lenses, were applied in corneal refractive surgery with laser techniques, such as in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK). However, the limitations found with the classic monovision system with contact lenses were also applied to monovision induced by refractive surgery, including loss of fusion and stereopsis. Therefore, several multifocal corneal ablation profiles have been tried, obtaining a slight improvement but little efficacy. The most recent techniques of LASIK to correct presbyopia are based on the manipulation of the corneal asphericity that cause an objective spherical aberration, in order to increase the depth of focus12-16.

Blurring and a negative SA in both eyes are caused in the non-dominant eyes by molding the cornea in a prolate form, thus exaggerating its spherical shape in order to induce a negative SA. With the sphericity generated, the monovision is reinforced and the necessary residual myopia in the non-dominant eye is reduced, achieving a more tolerable monovision17,18.

We must take into account the pupillary size factor as observed by Zheleznyak: in the 4-mm pupils the myopia associated with a positive SA was better for intermediate vision, and instead a negative spherical aberration was better for near vision19. On the other hand, Rocha observed that the depth of focus increased independently of the sign of the spherical aberration when the pupils were 6 mm20.

Reinstein has created the profile of corneal ablation in a method called ¨laser blended vision” that is optimized to increase the depth of field in each eye without altering the quality of vision, contrast sensitivity or night vision. With such manipulation, up to 1.5 accommodative diopters can be safely increased for any refractive error. Based on this and thus treating both eyes, the non-dominant eye is also programmed to be slightly myopic and thus the depth of focus of the dominant eye would be for far and intermediate vision and the non-dominant eye for near and intermediate vision. In intermediate vision, both eyes would have the same acuity. Micro-monovision depends on the processes of neuronal mechanism and suppression of blurring. Laser blended vision increases the depth of field provided by the contraction of the pupils during accommodation. Currently we use and/or benefit from 5 mechanisms:

Controlled increase in spherical aberrations
Pupillary contraction during accommodation that provides greater depth of field increase on the retinal image
Retinal and cortical processing to increase the contrast of the monocular-shaped retinal image
Combined vision zone to provide continuity of distance, intermediate and near vision, between the 2 eyes
Cortical central processing21,22

INTRAOCULAR LENSES

Several studies have shown that after the implantation of an intraocular lens (IOL), people achieve a certain depth of focus due to the multifocality of the cornea, the pupillary diameter, the corneal astigmatism and the small movement of the IOL within the sac. It seems that AE-, as we had mentioned, can increase tolerance to DOF, and seems to be even greater with neutral spherical or spherical lenses than with aspherical ones23.

The depth of field, as well as the DOF, do not match the breadth of accommodation. Langenbucher used the term "pseudophakic accommodation" for a dynamic change in the refractive state caused by the interactions between the ciliary muscle and the zonule-capsular complex of the IOL sac, leading to changes in refraction11.

The term "pseudophakic pseudo-accommodation" is used to highlight the static optical properties of the pseudophakic eye.

"Pseudophakic accommodation" is a function of accommodation generated by the active control of the brain that alters the optic system of the eye. A clinical use is produced by the artificial design (accommodative IOL) together with remains of the natural accommodative system of the eye.

Thornton24 reasonably argues that the natural accommodative mechanism includes the increase in the sphericity of the lens and its anterior displacement11.

The ultimate goal of the cataract and transparent lens extraction is to replace the lens with an intraocular lens that simulates the original function of the lens and provides patients with a full range of functional vision for all distances. Here you can also apply the same concept of monovision, causing some myopia of the non-dominant eye, or put multifocal lenses that try to replace the function of the lens to be able to focus on different foci.

Currently, available IOLs can be grouped into accommodative IOLs (AIOLs) or pseudo-accommodative IOLs (although the mechanism of action of some 'accommodative lenses' can be naturally pseudo-accommodative). With pseudo-accommodative multifocal IOLs (MLIO), the patient has two or three focal points of vision at different distances but will mainly perceive only the focused image7.

Aspheric IOLs

Until recently, IOLs had a spherical surface, inducing a positive aberration, causing a significant decrease in the quality of vision compared to the natural vision where positive aberration is compensated with the crystalline lens. However, in many cases depth of focus improved after surgical intervention in some patients for the same reason explained above. That is why aspherical lenses are currently used to compensate for that positive corneal aberration and thus improve the quality of vision.

It has been shown that with these lenses the aberrations are reduced and with this a better contrast sensitivity is achieved and without compromising the depth of focus, since in older people it will depend mainly on the pupil size rather than on the optical aberrations25. At the same time and for similar reasons, various studies with visual simulators have shown that manipulating the final total SA can allow a clear increase in DOF individually (Figure 3)26-28.

Figure 3.

Accommodative IOLs (AIOLs)

There are many different concepts and designs for AIOLs, including moldable gels, fluid displacement, and flexible haptics. These IOLs are designed to use contraction of the ciliary muscle, the elasticity of the capsular bag and changes in the pressure of the vitreous cavity to induce a change or movement in the shape of the IOL in order to produce an optical change in the eye based on the concept of axial optical change resulting from the action of the ciliary muscle. A hinge between the optic and the haptic allows the lens to move forward while the eye focuses on nearby objects, and backwards when the eye focuses on distant objects, thus increasing the dioptric power of the pseudophakic eye1,7,29. Actually with this movement we would theoretically modify the spherical power of the lens as well as the SA of it, being able to improve both the DOF. Unfortunately, clinical experience with these lenses has not been very encouraging so far.

Light-adjustable IOLs

This lens allows to create high-order aberrations, offering the possibility of inducing controlled amounts of SA to increase the DOF in patients, and thus allowing to improve near vision. These lenses contain photosensitive silicone molecules that allow a change of their shape after implantation according to the patient's needs through ultraviolet light irradiation in a non-invasive manner. The posterior surface of each lens is molded with a UV absorbing layer to impart additional UV protection for the retina during the irradiation procedure30,31.

Aspheric profiles with asphericity amounts were used to extend the DOF. In addition, the light-adjustable lenses allow to personalize the spherical aberration values depending on the specific needs of each patient. We can improve near vision by increasing the amount of SA-, although at the cost of a decrease in visual acuity for far. This is a better solution compared to pure standard monovision30,32. Most studies in this field were performed using adaptive optical simulators such as those presented above.

MULTIFOCAL IOLs

The multifocal intraocular lens (MIOL) implant after cataract surgery is an alternative to improve the performance of near vision and the quality of life of some patients. Currently, all available trifocal intraocular lenses have a diffractive design that has no relation to asphericity. However, the Mini Well Ready is a progressive multifocal aspheric IOL with an equivalent addition of 3.00 D. Its optical design is based on the introduction of an appropriate spherical aberration in the center of the pupil and the control of high-order aberration in the periphery of the pupil to increase the depth of focus and thus generate a progressive multifocality33. The results are very encouraging, including in the group of myopic patients.

CONCLUSION

Treating presbyopia through the manipulation of SA- to increase DOF is a safe method with good refractive results and tolerance for the patient, which can be adapted to the different means that we currently have for the treatment of this refractive problem. With the new technologies and adaptive optics simulators we could induce an individualized SA- according to the patient's need, which in many cases could be evaluated and measured before the intervention (especially in young, not very opaque crystalline lenses) and thus have more satisfactory results.

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