Ординатура / Офтальмология / Английские материалы / Refractive Lens Surgery_Fine, Packer, Hoffman_2005

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13.1The Nature of Presbyopia

In the human eye, multifocal vision is provided by the optical system comprised of the cornea and the natural crystalline lens, which in combination form a series of convex–con- cave lenses.Accommodation of vision at both infinity and near vision of 250 mm is provided by a peripheral muscular body extending about the capsular bag and connected to the equator thereof by the zonula of Zinn. While there are some differences of opinion regarding the exact mechanism, in general, tension and the relaxation of the ciliary muscles cause the capsular bag to lengthen or contract, which varies the focus of the eye.

Presbyopia is characterized as a reduction in both amplitude and speed of accommodation with age. The amplitude of accommodation decreases progressively with age from approximately 14 diopters in a child of 10 years to near zero at age 52. The exact explanation for the physiological phenomena is open to debate. However, it is observed that the curvatures of excised senile lenses are considerably less than those of juvenile ones. Failure could be due to a hardening of the lens material, sclerosis, decrease in the modulus of elasticity, a decrease in the thickness of the capsule or a combination of the above. Regardless of the cause, it is a recognized fact that beginning at about 40–45 years of age, correction for both near and far vision becomes necessary in most humans.

Many methods have been or are being explored to correct presbyopia, including monovision approaches, multifocal lenses, modification of the cornea, injectable intraocular lenses (IOLs) and single-optic IOLs that utilize the optic shift principle. All have experienced some limitation or have not yet provided a consistent solution. While new versions of bifocal contact lenses are constantly being developed, they are still limited in their range of accommodative correction. Monovision approaches with contact lenses seem to be suitable for a limited group of peo-

F. M. Sarfarazi

ple. Multifocal IOLs suffer from the fact that light is split, thereby reducing contrast sensitivity. Modification of the cornea using lasers, heat or chemicals to create multifocal patterns on the surface is still in an exploratory stage. Scleral expansion techniques have tended to experience regression over time.

Single-optic IOLs utilizing the optic shift principle are limited in the amount of accommodation they can provide. Injectable IOLs, where the capsular bag is filled with a flexible material, is an intriguing approach but appears to be far from developed and is not expected to be feasible for the foreseeable future. For this reason, a great deal of attention is focused on twin-optic IOLs.

13.2Twin-Optic Accommodative Lens Technology

The idea of using two or more lenses to create accommodation is not new. In 1989, Dr. Tsutomu Hara presented a twin lens system with spring action, which he called the spring IOL. The spring IOL consists of two 6-mm optics held 4.38 mm apart and four flexible loops [2, 3]. Early efforts to implant this lens were unsuccessful.

At approximately the same time, the author filed a patent for an accommodative lens with two optics and a closed haptic, which forms a membrane and connects the two optics to each other (US patent number 5,275,623).While the design most closely resembles the mechanics of a natural lens, the technology does not yet exist that can manufacture this lens.

13.3The Sarfarazi EAIOL

The elliptical accommodating IOL (EAIOL) is an accommodative lens system with dual optics that employs technologies that are novel in the ophthalmic field [1]. The anterior optic is a biconvex lens of 5.0-mm diameter (Fig. 13.1), the posterior lens is a concave–

Fig. 13.2. Insertion in the bag

convex lens with negative power and 5.0-mm diameter. The two lenses are connected to each other by three band-like haptics. Each haptic covers a 40-degree angle of the lens periphery, and the angle of separation between them is 80 degrees. A useful property of these optics is that the convex surface of the anteri-

Fig. 13.1. Lens assembly

or lens “nests” within the concave surface of the posterior optic,thereby simplifying insertion through the cornea and capsulorrhexis (Fig. 13.2). The overall diameter of the EAIOL lens assembly (including haptics) is 9 mm.

The haptic design is unique in that the haptics serve two critical roles. First, they position and center the EAIOL in the capsule in a fashion similar to that of the haptics for a standard IOL. Second, they provide the spring-like resistance that separates the two optics. It is called an elliptical accommodating IOL because it forms an elliptical shape, which resembles the shape of the natural lens (Fig. 13.3). When inserted in the bag after removal of natural lens material, the EAIOL occupies the entire capsular space. It uses the contraction and relaxation forces of the ciliary muscle against the spring-like tension of the haptics to emulate the accommodation of the natural lens (Fig. 13.4).

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Fig. 13.4. Lens in the bag

F. M. Sarfarazi

Fig. 13.3. Lens configuration

13.4Design Considerations for the EAIOL

The accommodation process in a twin-optic lens depends on increasing and decreasing the lens diameters (i.e., the lens diameter along the optical path). According to Wilson [4], during accommodation the lens diameter of the natural lens is consistently reduced and enlarged during non-accommodation.A finite element analysis for the EAIOL shows similar changes. The diameter of the EAIOL reduces from 9.0 to 8.5 mm during accommodation.

According to Koretz [5], the rate of change per diopter of accommodation is independent of age for the entire adult age range. With increasing accommodation, the lens becomes thicker and the anterior chamber shallower along the polar axis. This increase in sagittal lens thickness is entirely because of an in-

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crease in the thickness of the lens nucleus. In the EAIOL, during the accommodation process the lenses move further apart from one another (2.5 mm), decreasing the anterior chamber depth. The amount of distance between two lenses is reduced during the non-accommodative process.

Beauchamp suggested that about 30% of the lens thickening during accommodation is accounted for by posterior lens surface displacement [6]. If the crystalline lens power is calculated on the basis of an equivalent refractive index, changes in the posterior surface of the lens contribute around one-third of the increase in the lens power associated with 8.0 D of ocular accommodation [7, 8]. In the EAIOL, the posterior lens is a negative lens and it sits on the posterior capsule and experiences minimal movement. It could, however, use this posterior vitreous pressure to move forward.

Non-invasive biometry of the anterior structures of the human eye with a dual-beam partial coherence interferometer showed that the forward movement of the anterior pole of the lens measured approximately three times more than the backward movement of the posterior pole during fixation from the far point to the near point [9]. In the EAIOL, the haptics were designed according to this principle. The anterior lens moves forward in the accommodation phase and backward during the non-accommodative process.

Total anterior segment length (defined as the distance between the anterior corneal and posterior lens surfaces), vitreous cavity length (distance between the posterior lens and anterior retinal surfaces), and total globe length were each independent of age. This constellation of findings indicates that the human lens grows throughout adult life, while the globe does not, that thickening of the lens completely accounts for reduction of depth of the anterior chamber with age, and that the posterior surface of the lens remains fixed in position relative to the cornea and retina [10].

terior lens sits on the posterior surface of the capsule and has minimal movement during the accommodation process. Because of the stability of the globe during the aging process, the EAIOL could be a suitable lens for children as well as adults.

13.5Optical

and Mechanical Design

The design of the EAIOL evolved from its original concept through an extensive series of mechanical (Fig. 13.5) and optical (Fig. 13.6) engineering studies. Many variations on the basic system were investigated to determine an acceptable design for the lens that would result in the desired amount of accommodation. The configuration of the Zonula of Zinn was included in these representations to determine their effect as they pull outwardly on the lens.

Among the attributes studied were the shape and stresses that the implant would encounter during use. Color-coded plots were used to represent various magnitudes of deformation. Comparative stress studies at maximum deformation indicated that the lens material would not fail in this application.

Chief among the optical design factors determining the amount of accommodation and visual acuity was the available motion of the anterior lens. A high degree of motion allows for the lowest possible powers on the two lenses. The posterior lens is a negative lens and, in the recommended optical design, the anterior lens moves 1.9 mm to achieve a minimum of 4 diopters of accommodation. Ray aberration diagrams indicated excellent image performance and sufficient power in the lenses for this amount of accommodation. The curves for the candidate designs, distant (infinity) and near vision (250 mm) were evaluated with respect to such variables as:

(a) number of powered lenses, (b) use of as-

pheric surfaces, (c) pupil size, (d) lens placement and dimensions,(e) field of view and (f) wave length. Results were provided on image performance as a function of field position, and there was no difference between these two images. The letter E was clear during the entire accommodation process.

13.6Prototype Development

Initially, several prototypes from different materials such as PMMA, polypropylene, polyimide acetyl (used in heart valves) and Flexeon materials were made using different techniques such as etching and assembling. The PMMA lenses were used with varying haptic materials and configurations. Although the tests of these designs for mechan-

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ical and optical properties were satisfactory, insertion in the eye through a 3-mm corneal incision was difficult and caused permanent deformation and/or shear cracks in the haptics. Implantation of two versions of this unit in human cadaver eyes, using an open sky technique, showed that the EAIOL fitted in the capsular bag and occupied the entire bag space. A Miyake technique examination showed that it centered well. Pushing on the anterior lens transferred the force to the posterior lens and the haptics responded to the pressure.

These initial tests indicated that a PMMA version of the EAIOL would not be suitable for small-incision surgery and that a more flexible material was desirable. As a result, the development effort shifted to developing complete EAIOL designs from silicone and acrylic materials, both of which are already approved for use in human implantation. In both cases, the model analyses indicated that the finished EAIOL units would be more pliable and therefore more suitable for small-incision insertion. This was especially critical for implantation in the smaller eye of a monkey,which was to be the next phase of testing.

13.7Primate Testing

The goal of the next phase of the program was to design a lens that could be implanted in the eye of a monkey. This work was time consuming and costly due to a lack of information regarding the exact parameters of a monkey eye. Measurements of monkey vision were performed in vivo and in vitro to characterize a monkey’s vision and the lens requirements needed for the study. There had previously been no reported research on such parameters at the depth needed for molding and designing the lens. Further, there was no lens available on the market to fit the monkey capsular bag. Previous studies had been focused primarily on the ciliary muscle structure and the nature of accommodation.

Fig. 13.7. Flexible, foldable lens prototype

Fig. 13.8. First phase testing: Miyake technique (Dr. Mamalis, University of Utah)

Once a flexible,foldable lens prototype was developed (Fig. 13.7), the following tests were conducted.

Initial testing of the EAIOL was performed using the Miyake technique in a human cadaver eye. The test indicated that the lens centered well and gave an initial indication that the lens design would function successfully in a monkey eye (Fig. 13.8).

A second phase of testing was further proof of concept work on a monkey eye. Using Dr. Glasser’s stretching device to simulate

Fig. 13.9. Second phase testing: Dr. Glasser’s device, University of Houston

allow for better visualization of the surfaces of the lens. In each case, several tests were performed before and after accommodation, which was induced by a supramaximal dose of carbachol. It was observed that the anterior chamber shallowed and the lens thickened in response to the carbachol before IOL implantation. UBM examinations showed a strong contraction of the ciliary muscle (Fig. 13.11), as did goniovideography images. Baseline refraction was measured with the natural lens in place before and after carbachol injection.

The eye was prepared with the same standard procedures used for implanting conventional IOLs. The diameter of the optic on this lens was 3.7 mm. The corneal incision was approximately 4 mm, and the capsulorrhexis was between 3.5 mm and 3.7 mm. A normal phacoemulsification procedure was performed to remove the natural lens material.

Fig. 13.12. Implantation of lens into capsular bag (1)

13.8Implantation Results

Several days after IOL insertion, experiments were conducted to determine initial results. The Scheimpflug imaging technique was used in repeated tests to observe the lens in open and closed positions (Figs. 13.14 and 13.15).A slit lamp examination showed the cross-sec- tion of the cornea and the two optics in the closed position. After carbachol was added, the two optics separated and a simulated accommodation of 7–8 diopters was measured using a Hartinger coincidence refractometer and retinoscopy. Goniovideography images clearly showed the placement of the haptics adjacent to the zonules in the equator of the capsular bag (Fig. 13.16).

Fig. 13.13. Implantation of lens into capsular bag (2)

Ultrasound biomicroscopy confirmed that the two optics move properly in relation to the cornea (Fig. 13.17). Again, the haptics were clearly seen close to the equator of the capsule working in conjunction with the zonules and ciliary muscle (Figs. 13.18, 13.19 and 13.20).

Anterior chamber depth was obtained from A-scan, Scheimpflug, and UBM measurements. Anterior chamber depth was decreased due to the forward movement of the anterior optic of the EAIOL in the same manner as with the natural lens (Fig. 13.21). The thickness of the EAIOL (degree of separation of the optics) was increased after carbachol was added according to both A-scan and Scheimpflug measurements.