Introduction
Presbyopia is the age-related loss of the eye’s ability to focus at near distances. The symptoms of presbyopia generally become perceptible at the age of around 40 years; by the age of 50 years, virtually everyone needs optical aids for reading or other activities that require near visual acuity.
The enormity of the presbyopic population (according to current estimates, there are currently more than 1.3 billion presbyopes worldwide) and the associated size of the market for presbyopia-correction products are major driving forces for developments in this area. Additionally, now that laser in situ keratomileusis (LASIK) and other surgical approaches for the correction of myopia and hyperopia approach their maturity, the direction of refractive surgery is rapidly shifting toward presbyopia, which is considered by many to be the “final frontier” of refractive surgery. This new focus on refractive surgery is further motivated by continued refinements of cataract surgery techniques and intraocular lens (IOL) designs.
The combination of these three factors (market potential, new focus on refractive surgery, and refinement of cataract surgery) has led to an explosion of research and development projects in the area of presbyopia correction. In this chapter, we will briefly discuss current approaches for the correction of presbyopia and present our work on the development of Phaco-Ersatz, a lens-refilling technique designed to restore accommodation.
Accommodation: The Helmholtz Theory
Ocular accommodation is the ability of the eye to change its focus in response to a stimulus and thus provide sharp vision over a continuous range of distances, from a near point to a far point. The physiologic response to the accommodation stimulus is a complex optomechanical process that involves primarily the lens, lens capsule, zonules, and ciliary muscle, but is also affected by the biomechanical forces exerted by the ciliary body, choroid, vitreous, aqueous, and the corneoscleral shell.
Our current understanding of the mechanism of accommodation relies on the principles set forth by Helmholtz in the mid-1800s. According to Helmholtz’s theory, the change in power of the eye during accommodation results from a change in the curvature of the lens surfaces produced when the ciliary muscle contracts or relaxes. When the ciliary muscle is fully relaxed, the zonules are under tension and pull on the lens capsule. The resulting force produces a flattening of the lens surfaces, which brings the focus of the eye to the far point. When the ciliary muscle contracts, the tension on the zonules and capsule is relaxed. The lens takes on a more curved shape, which increases its optical power and shifts the focus to the near point ( Fig. 40.1 ).
Even though most of the scientific evidence available today supports the general principles of the Helmholtz theory, many aspects of the optical and mechanical response to the accommodation stimulus are still only partially understood. Examples of topics that are currently under active investigation include quantification of the dynamics of accommodation and disaccommodation, quantification of the optical structure of the lens and its changes during accommodation, and quantification of the forces involved in accommodation and their relation to the optical and biometric changes in the lens. The lack of a definite proof of Helmholtz’s theory has led to the formation of a number of alternate or complementary theories. Excellent reviews of the development of early theories of accommodation can be found in the classic textbooks of physiologic optics. Currently, the most notable of these include Coleman’s catenary theory, which attributes an important role to the vitreous and choroid, and the controversial theory of Schachar, which finds very few supporters in the ophthalmic research community.
Presbyopia: The Loss of Accommodation With Age
Presbyopia is the age-related loss of accommodation ( Fig. 40.2 ). The cause of presbyopia is most likely a combination of age-related physical (optical, mechanical, and anatomic) and physiologic changes in the lens and the other anatomic structures of the accommodation system. However, there is strong evidence to support the hypothesis that presbyopia is due, for the most part, to a progressive loss of the ability of the lens to change shape. The aging crystalline lens is unable to change shape when the ciliary muscle contracts to relax the tension on the zonules and lens capsule. The focus of the eye (the optical conjugate of the retina) remains locked at the far point. This theory is strongly supported by anatomic and physiologic studies demonstrating that the ciliary muscle and zonules remain functional in normal subjects even in old age. The reason why the lens loses its ability to change its shape is still being debated. There is evidence that the lens becomes less compliant with age, but some argue that this loss of lens compliance could be a consequence rather than a cause of presbyopia. A number of alternative theories have been proposed over the years to explain presbyopia, including the current controversial description proposed by Schachar, based on his theory of accommodation. According to Schachar, presbyopia is due to the continuous growth of the lens with age, which progressively leads to crowding of the space between the ciliary muscle and the lens equator. Eventually, the distance between the zonular attachments at the ciliary body and at the lens equator becomes insufficient to allow a change of zonular tension when the muscle contracts or relaxes. Because of the lack of convincing scientific evidence to support this theory, it has very few proponents in the ophthalmic community.
Presbyopia Correction: an Overview
The current approaches for presbyopia correction can be divided into three broad categories:
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“traditional” optical approaches
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pseudo-accommodating IOLs
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techniques to restore accommodation
Traditional Techniques
The first category includes traditional techniques relying on bifocal, multifocal, or progressive optical designs as well as monovision treatments. These corrections are achieved conventionally with spectacles or contact lenses, but multifocal IOLs and laser surgery of the cornea have also been employed. These traditional techniques, however, achieve their near power through sacrificing other aspects of optical performance. For example, bifocal spectacles sacrifice field of view for near power while monovision corrections trade binocular/stereoscopic performance for reading ability. While these strategies are the primary form of presbyopia correction, they do not replicate the full-field, full-aperture, continuously variable focus of the young crystalline lens.
Pseudo-Accommodating IOLs
The second category includes pseudo-accommodating IOLs, a new type of IOL specially designed to effect a change of ocular power when the ciliary muscle contracts or relaxes. As suggested by the prefix pseudo , these IOLs do not restore the normal accommodative function of the eye. Instead, they rely on an artificial optomechanical mechanism to increase the power of the eye and provide near vision. These IOLs can be implanted after extracapsular cataract extraction (ECCE).
In principle, the most obvious optical design for a pseudo-accommodating IOL is an implant that would change curvature when the ciliary muscle contracts or relaxes. The feasibility of incorporating this concept into a practical working design remains to be demonstrated. Instead, most current accommodating IOLs rely on an axial displacement of the implant to produce the desired change in power. The basic optical principle ( Fig. 40.3 ) is the same as for a system of two lenses separated by a variable distance. In the case of a pseudo-accommodating IOL, the two lenses are the cornea and the implant. If we assume that the cornea (power P K ) and accommodating IOL (power P IOL ) are thin lenses separated by a distance d , then the paraxial power of the equivalent thick lens, which is the eye in this case, is given by
Peq=Peye=PK+PIOL−d⋅PK⋅PIOL.
ΔPeye=−PK⋅PIOL⋅Δd.
Restoration of Accommodation
The third category of techniques to correct presbyopia includes all approaches that aim at restoring the ability of the crystalline lens to change shape when the ciliary muscle contracts or relaxes. There are currently at least three different concepts being investigated, which are at different levels of development:
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scleral expansion surgery
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lens refilling
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lens softening
Softening the lens nucleus by photodisruption with a high-power pulsed neodymium:yttrium aluminum garnet (Nd:YAG) laser (photophako modulation) is an intriguing concept that was proposed by Myers and Krueger. Proof-of-concept experiments conducted on explanted cadaver lenses showed that application of laser pulses at energies above the threshold for bubble formation increased the elasticity of lenses from old donors. A team at the University of Hannover in Germany is currently further investigating this approach using femtosecond lasers.
Unlike scleral expansion or lens softening, which leave the natural crystalline lens in place, the goal of lens refilling is to replace the contents (cortex and nucleus) of the presbyopic lens with a flexible material or implant. The first step of lens refilling is a modified ECCE technique designed to allow removal of the presbyopic lens content through a miniature opening in the lens capsule, on the order of 1 mm in diameter (mini-capsulorhexis). The intact empty capsular bag is then refilled with an artificial lens that ideally possesses the same biomechanical and optical properties as the natural young accommodating lens. This technique is discussed in further detail in the following sections of this article.
Traditional Techniques
The first category includes traditional techniques relying on bifocal, multifocal, or progressive optical designs as well as monovision treatments. These corrections are achieved conventionally with spectacles or contact lenses, but multifocal IOLs and laser surgery of the cornea have also been employed. These traditional techniques, however, achieve their near power through sacrificing other aspects of optical performance. For example, bifocal spectacles sacrifice field of view for near power while monovision corrections trade binocular/stereoscopic performance for reading ability. While these strategies are the primary form of presbyopia correction, they do not replicate the full-field, full-aperture, continuously variable focus of the young crystalline lens.
Pseudo-Accommodating IOLs
The second category includes pseudo-accommodating IOLs, a new type of IOL specially designed to effect a change of ocular power when the ciliary muscle contracts or relaxes. As suggested by the prefix pseudo , these IOLs do not restore the normal accommodative function of the eye. Instead, they rely on an artificial optomechanical mechanism to increase the power of the eye and provide near vision. These IOLs can be implanted after extracapsular cataract extraction (ECCE).
In principle, the most obvious optical design for a pseudo-accommodating IOL is an implant that would change curvature when the ciliary muscle contracts or relaxes. The feasibility of incorporating this concept into a practical working design remains to be demonstrated. Instead, most current accommodating IOLs rely on an axial displacement of the implant to produce the desired change in power. The basic optical principle ( Fig. 40.3 ) is the same as for a system of two lenses separated by a variable distance. In the case of a pseudo-accommodating IOL, the two lenses are the cornea and the implant. If we assume that the cornea (power P K ) and accommodating IOL (power P IOL ) are thin lenses separated by a distance d , then the paraxial power of the equivalent thick lens, which is the eye in this case, is given by
Peq=Peye=PK+PIOL−d⋅PK⋅PIOL.
ΔPeye=−PK⋅PIOL⋅Δd.
