Ординатура / Офтальмология / Английские материалы / LASIK and Beyond LASIK Wavefront Analysis and Customized Ablation_Boyd_2001

are trying to develop some techniques to reverse this state. Most of them are undergoing the investigative process.

SCLERAL TECHNIQUES

Anterior Ciliary Sclerotomy (ACS)

(Thornton’s Technique)

Anterior ciliary Sclerotomy (ACS) was inadvertently performed by the first radial keratotomy surgeons (RK), who continued their incisions past the limbus into the anterior sclera, reversing its affect and causing less than predicted hyperopic shifts. These incisions produced regression and undercorrection arose as complication.

The theory behind an incisional reversal of presbyopia involves placing incisions radially, beginning in the limbus and carried back to, but not over, the pars plana to avoid retinal complications. The ciliary body expands the globe and increases the space for the ciliary body zonular complex.

The underlying rationale of ACS is based on the observation that the lens is ectodermic in origin and constantly grows throughout life, gradually “crowding” the posterior chamber and eventually preventing full function of the ciliary body/zonular complex. Rather than a loss of elasticity of the lens with age, it is felt that this “crowded” state is the cause of the reduction of lens power change with attempted accommodation. If ciliary body is impaired in its ability to change the shape of the lens because it is “crowded” by the lens, then making more room for the ciliary body would allow more space for the lenszonule complex. By expanding the globe in the area of the ciliary body, this could be accomplished.

The ACS technique involves placing eight or more symmetrical, partial-thickness radial incisions into the sclera over the ciliary body to allow an expansion of the sclera and resulting increase in scleral diameter over the ciliary body. This allows an increased area for ciliary muscle action and consequent increased zonular effectiveness in changing the focal power of the lens.

Chapter 41

that is similar to a theory originally proposed by Tschering. They believe that the equatorial zonule inserts to the anterior aspect of the ciliary muscle at the root of the iris, and the posterior zonule inserts into the posterior ciliary body.

Schachar and Anderson contend that contraction of the ciliary muscle causes a posterior-outward movement of the anterior ciliary muscle toward the sclera at the iris root, increasing tension on the equatorial zonular fibers while releasing tension of the anterior and posterior zonular bundles. They believe that this provides a net outward-directed force at the lens equator through the equatorial zonular fibers. This force, would pull the lens equator toward the sclera during accommodation and, together with the concurrent relaxation of the anterior and posterior zonular bundles, would cause a flattening of the peripheral lens surface while increasing the central anterior and posterior lens surface curvatures.

From a theoretical standpoint, pulling on the lens equator could cause an increase in the central lens curvatures, depending on the viscoelastic properties of the lens. This theory, unlike that of Tschering, identifies no role for the vitreous. Schachar believes that the presbyopia is due to the continued growth of the lens throughout life, which results in an increased lens equatorial diameter, a crowding of the posterior chamber, and a reduction in tension of the zonular fibers at the lens equator with increasing age. On the basis of this theory, this would result in a failure of zonular tension to increase sufficiently during contraction of the ciliary muscle (Fig. 41-4).

Schachar´s surgical procedure for the reversal of presbyopia, is intended to increase the scleral diameter at the ciliary region, reintroducing zonular tension at the lens equator. He predicts that the lens equator will move toward the sclera and that the lens equator diameter will increase during accommodation.

The technique consists of inserting four polimethylmetacrilate implants into an scleral tunnel performed 6mm from the limbus. The sclerotomy is made with a 5 mm diamond knife in a size of 900 m, the implant is inserted leaving both free extremes outside the sclera. The implant over the ciliary body perform an outward traction, which theorically, would restore accommodation.

Figure 41-4: Schachar´s force diagram of the circular fibers at equilibrium. Faforce directed towards the effective attachment point of the equatorial zonules, Fcforce of the circular fibers, Fp-the force directed towards the posterior radial fibers, R- resultant force transmitted by the radial fibers that can be divided in Fa and Fc.

This technique is experimental, but results from some trial centers in Puerto Vallarta and Tijuana (Mexico) and Marseille (France), show an average dioptric change of 2.33D.

There are groups, like Glaser et al, who found no accommodation with SEB. They measured accommodation with objective infrared refractometry in three subjects examined by an independent observer. They proposed that this difference can be attributable to the fact that Schachar´s group used only subjective near vision as an assessment of accommodation. They conclude that, on the base of retinoscopy aberrations, that surgical expansion of the sclera may induce lenticular aberrations in a multifocal optical system, rather than true accommodation.

Mathew refers that there are reasons suggesting that further verification of the efficacy of SEB would be prudent, since Schachar´s results of improving accommodation are based on subjective recordings of near vision, utilizing same near card in the “push-up” testing.

Sing et al report as complication of SEB scleral thinning with resultant axial lengthening and

Contents

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Section 2

Section 3

Section 4

Section 5

Section 6

Section 7

Subjects Index

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INTRACORNEAL TECHNIQUE

Intracorneal Implants

The theoretical benefits of synthetic keratophakia over conventional cornea lamellar procedures are the elimination of donor concerns and superior refractive predictability. Additionally, synthetic material can be inspected for optical quality and power, and it can be sterilized. Several materiales have been used, as is the unfenestrated polysulfone intracorneal lens by Horgan’s group, which appears to be associated with color change and varying degree of stromal opacity in eyes evaluated 12 years postoperatively and this cannot be considered clinically acceptable.

Permalens hydrogel intracorneal lenses, highly glucose-permeable with an equilibrium water content of 68% (Lidofilcon A), seem to be well tolerated by the corneal stroma and can provide predictable refractive results. Limitations of the procedure are uneven microkeratome resections, loss of best-corrected visual acuity, and irregular astigmatism in some patients, as reported by Steinert. Although their data show good evidence of biocompatibility of the implant material, technical surgical progress is needed to advance this procedure into clinical therapeutic practice.

CxGelSix is a novel type VI collagen preparation studied by Leskull et al as a biomaterial in the rabbit cornea stroma. They found this disc does not alter the structure of the corneal epithelium above the implant, suggesting normal transport of nutrients trough the implant. This material is remarkably stable despite its exposure to endogenous enzymes during inflammation and wound healing. They conclude that the result of the study suggest that CxGelsix is potentially useful as a biomaterial.

In Colombia, Barraquer and his group implanted in humans intracorneal hydrogel lenses (Permalens) and reported good tolerability and their refractive results were stable.

The use of this intracorneal implants for presbyopia is still in an experimental phase.

1. Pseudoaccommodative IOL (A-45)

This is a silicon lens that contacts the vitreous and its movement is done by ciliary muscle contraction and the vitreous pressure from behind. For each millimeter of anterior movement, this lens gives 2D of accommodation. The results of 16 patients reached a visual capacity near J4 or better. This technique is also experimental and more trials are needed to start using this in the clinic as reported by Avitable and his group (Fig. 41-5).

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Subjects Index

Figure 41-5: Pseudoaccommodative IOL and its changes.

2. Multifocals (IOLsM)

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Conventional IOLs have one focal point, and after implantation, patients have clear vision at one distance only. Bifocal, distance, or reading glasses are required to provide a usable vision at an additional focal distance

Multifocal IOLs were recently developed in an effort to treat both surgical aphakia and loss of accommodation. Jacobi et al implanted in 29 patients bilateral multifocal IOL with asymmetrical light

Chapter 41

distribution for far and near focus. Each patient had a distant-dominant multifocal IOL implanted in one eye and a near-dominant multifocal IOL implanted in the fellow eye. They concluded that bilateral implantation of asymmetrical diffractive IOL is an effective alternative for restoring simultaneous distance and near vision with a potential for improved contrast sensitivity compared with conventional multifocal IOLs.

3M Multifocal IOL

This lens design is based on diffractive optics, whereby light is partitioned into two focal points, one for distance vision and one for near vision. With diffractive optics, the wave nature of light is manipulated to allow the wave fronts to combine constructively and destructively to place light at different focal points. Forty-one percent of the light entering the eye is focused at the distance focus point, and 41% is focused at the near focus point. The remaining 18% is focused at higher orders of diffraction, not contributing to the image that is perceived. Only one focus point is located directly on the fovea at one time, giving one clear image. The precise placement of light is accomplished by the microscopic diffractive structure (steps) embedded onto the posterior surface of the IOL in a multiple ring pattern. The spacing and height of each ring (20 to 40) determine the power of the near focal point or add power of the lens. One of the primary advantages of this lens is that the entire lens surface contributes to both focal points. As a result, lens performance should function independently of pupil size for both distance and near vision.

The results from the clinical evaluation of the Food and Drug Administration (FDA) study for the 3M diffractive multifocal IOL demonstrated that the overall uncorrected distance visual acuity after one year from the surgery showed 57% of patients with 20/40 or better acuity. In this same group, 78% achieved J3 or better. Measurements of contrast sensitivity consistently documented a small loss, which is considered clinically insignificant.

This multifocal lens appears to work very well for most patients, and the study showed several considerations that are important for optimizing clinical performance and patient satisfaction: patient

selection, realistic expectations, accurate biometry, and adequate control of surgical procedures.

AMO-ARRAY Multifocal IOL

The goals when using the ARRAY Multifocal IOL are: 1) Maintain distance acuity and function (20/20) distance, 2) Decrease dependence on glasses (J3 or better), 3) Increase range of focus (useful near and intermediate). The optical design of the lens has a progressive multifocal optic; the refractive design has a multifocal power on the anterior surface. Is a distance dominant lens with a 3.5 diopter add. The lens has five concentric zones, each zone repeats entire refractive sequence of variable and continuous power. The available range goes from 6 to 30 D, in 0.5D steps (Fig. 41-6).

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Figure 41-6: Optical design of multifocal AMO-ARRAY

IOL

Vaquero-Ruano and his group report that with appropriate patient selection, the AMO Array IOL provide distance acuity and contrast sensitivity similar to that of monofocal IOLs, and excellent intermedi-

ate vision. Near vision, without spectacles, with the multifocal IOL was functionally acceptable although short of refractive power for best acuity results (Table 1).

Chapter 41

LASER TECHNOLOGY

TECHNIQUES

Laser Presbyopia Reversal

(Lin´s Technique)

J.T. Lin invented laser presbyopia reversal (LPR), the first technique that uses an infrared laser to counteract the effects of ocular aging. The IR-3000 device incorporates an infrared laser, a beam-shap- ing delivery unit, and a scanning mechanism. This laser is a 3m range, cold laser that has a minimal thermal effect on ocular tissue.

In the LPR procedure, the laser produces changes in the zonular activity removing between 500 and 600 m of tissue from outside the optical zone. This is one of the major advantages of this procedure, so that the patient’s distance vision remains unaffected. Another advantage is that the procedure can be completed in 5 to 0 minutes, as stated by Lin.

The first international patients who underwent LPR in South America, have not had a regression on visual acuity after a follow-up of about 7 months. Clinical trials started in Venezuela in June 1999 showed data from 100 patients aged from 42 to 65 who had 1.5 to 3.5D of presbyopia. These data were presented in the fall of 2000. Clinical trials have not yet begun in United States, also this technique has not been approved by FDA.

Excimer Laser Photorefractive

Keratectomy

Using a specially designed mask, Vicinguerra et al developed a procedure for correcting presbyopia with EXCIMER laser photorefractive keratectomy (PRK). A mask consisting of a mobile diaphragm formed by two blunt blades was used to ablate a 10-17 m deep semilunar-shaped zone immediately below the pupillary center, steepening the corneal curvature in that area (videokeratography controlled), in three patients. After an initial regression of 1 D during the first 6 months, the presbyopic correction remained stable for the duration of one-

b) Presbyopia - Avalos and Rosakis Method (PARM )

With this LASIK technique the corneal curvature is modified, creating a bilateral bifocal cornea. The preoperatively corneal curvature is the main factor to consider in this type of corrective method. In order to have a high quality of vision, corneal keratometry should not be modified up to 48 D. Corneas

PRESBYOPIA

with more than 48 D produce undesired optical alterations like glare, halos, decreased visual acuity and decreased contrast sensitivity.

For each hypermetropic diopter corrected, the corneal curvature increases in 0.89 keratometric diopters as an average, that is why it is recommended to treat patients with keratometry in the range between 41 to 43 D to obtain postoperatively curves under 48 D (Fig. 41-7)

The corneal flap performed with the microkeratome must be between 8.5 to 9.5 mm in order to have an available corneal surface for treatment of at least 8 mm, in this way the laser beam does not touch the temporal edge of the flap. Pachymetry is not fundamental for this treatment.

PARM has been used to treat patients with 20/20 distance vision and addition of +3.5D; for hypermetropic patients up to +3.5D in order to treat the addition for presbyopia, always taking into account the preoperatively keratometric measurements, as previously explained.

If astigmatism is present it is recommended to use as a limit 2.50D and in myopic patients the limit is –5.00D

Because of the corneal shape produced after the surgery , there is an induced astigmatism between 0.50 to 0.75 D, which could decrease one or two lines of visual acuity.

The usual LASIK re-treatment average for myopia, astigmatism and hyperopia is 7%, in cases treated for presbyopia it is increased up to 22% as seen in Dr. Avalos and Dr. Rozakis patients in the first two years of trial, after modifying the nomogram this retreatment percentage should diminish under the range of 12% (Fig. 41-8).

This group has not seen complications from the surgery, beside those already reported for regular LASIK technique.

Conclusion

Presbyopia treatment is one of the main goals for refractive surgery. Up to date and with the treatment techniques available one can consider that this is possible. As we have reviewed in this chapter, there are several treatment techniques for presbyopia, the new have been remarkable. This different techniques can be used for different types of patients and all of the surgical ones are in experimental phase, so they need to be improved. In a near future we will be finally say that there is a real solution for a refractive problem that affects to almost all the world population after the fourth decade of life.

REFERENCES

1.6th. Varilux presbyopia forum. www.presbyopia.org

2.Aguilar M, Mateos Felipe. Óptica Fisiológica Tomo 1. España: Universidad politécnica de Valencia, 1993:191-214

3.Atwood JD. Presbyopic contact lenses. Curr Opin Ophthalmol 2000;1:296-8

4.Avitabile T, Marano F. Multifocal intra-ocular lenses. Curr Opin Ophthalmol 200l;l2:l2-l6

5.Barraquer JI, Gomez ML. Permalens hydrogel intracorneal lenses for spherical ametropia. J Refract Surg 1997;13:342-8

6.Beers AP, Van Der Heijde GL. Age-related changes in the accommodation mechanism. Optom Vis Sci 1996;73:235-42

7.Beers AP, Van der Heijde GL. Presbyopia and velocity of sound in the lens. Optom Vis Sci 1994; 71:250-

8.Blystone PA. Relationship between age and presbyopic addition using a sample of 3,645 examinations from a single private practice. J Am Optom Assoc 1999;70:505-8

9.Boyd BF. Atlas de Cirugía Refractiva. Colombia: Highlights of Ophthalmology, 2000: Capítulo 9

10.Burd HJ, Judge SJ, Flavell MJ. Mechanics of accommodation of the human eye. Vision Res 1999;39:1591-5.

PRESBYOPIA

11.Chylack LT Jr: Aging changes in the crystalline, lens and zonules. En: Albert and Jakobiec ed. Principles and Practice of Ophthalmology (Clinical practice). Philadelphia: WB Saunders company, 1994: Chapter 54B.

12.Cuadernos de óptica oftálmica: Lentes de adición progresiva. Essilor Internacional

13.Dimitri TA, Leon Strauss. Principles of applied clinical optics. En: Albert and Jakobiec ed. Principles and Practice of Ophthalmology (Clinical practice). Philadelphia: WB Saunders company, 1994: Chapter 291

14.Gal et al. Image formation by bifocal lenses in a trilobite eye? Vision Research 2000;40:843-53

15.Gilmartin B. The aetiology of presbyopia: a summary of the role of lenticular and extralenticular structures. Ophthalmic Physiol Opt 1995; 15:431-7

16.Glasser A, Campbell MC. Presbyopia and the optical changes in the human crystalline lens with age. Vision Res 1998;38:209-29

17.Glasser A, Kaufman PL. The mechanism of accommodation in primates. Ophthalmology 1999;106:863-872

18.Herreman R. Manual de Refractometría clínica. 2ª. ed. España: Salvat Editores, 1994: 52-53

19.Hom MM. Monovision and LASIK. J Am Optom Assoc 1999;70:117-22

20.Horgan SE et al. Twelve years follow-up of unfenestrated polysulfone intracorneal lenses in human sighted eyes. J Cataract Refract Surg 1996;22:1045-51

21.Jakobi FK et al. Bilateral implantation of asymmetrical diffractive multifocal intraocular lenses. Arch Ophthalmol l999;ll7:l7-23

22.Kaufman P, Glasser A. The mechanism of Accommodation in primates. Ophthalmology 1999; 106:863-872.

23.Kaufman P. Acomodación y presbiopía: Aspectos neuromusculares y biofísicos. En: Hart W, ed. Adler Fisiología del ojo. Aplicación clínica. España: Mosby, División de Times Mirror de España,1994: 393-404

24.Kaufman P. Acomodación y presbiopía: Envejecimiento de los mecanismos de acomodación. En: Hart W, ed. Adler Fisiología del ojo (Aplicación clínica). España: Mosby, División de Times Mirror de España, 1994:404-

25.Key JE, Morris K, Mobley CL. Prospective clinical evaluation of Sunsoft Multifocal contact lens. CLAO J 1996;22:179-84

26.Koretz F. Accommodation and presbyopia. En: Albert and Jakobiec ed. Principles and Practice of Ophthalmology Clinical practice). Philadelphia: WB Saunders company, 1994: Chapter 16

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Section 7

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27.Koretz JF, Cook CA, Kaufman PL. Accommodation and presbyopia in the human eye. Changes in the anterior segment and crystalline lens with focus. Invest Ophthalmol Vis Sci 1997;38:569-78

28.Leskul M et al. CxGELSIX: a novel preparation of type VI collagen with possible use as a biomaterial. Cornea 2000;19:194-203

29.Lindstrom RL. Food and drug administration Study update. One-year results from 67l patients with the 3M multifocal intraocular lens. Ophthalmology l993;l00:9l-

30.Mathews S. Scleral expansion surgery does not restore accommodation in human presbyopia. Ophthalmology 1999; 106:873-877.

31.Michaels DD.: Visual optics and refraction a clinical approach, Saint Louis, 1975, Mosby Company, Chapter 16:268-314

32.Oates DC, Belcher CD. Aging changes in trabecular meshwork, iris and ciliary body. En: Albert and Jakobiec ed. Principles and Practice of Ophthalmology (Clinical practice). Philadelphia: WB Saunders company, 1994: Chapter 53B

33.Rosenthal P, Cotter JM. Contact lenses. En: Albert and Jakobiec ed. Principles and Practice of Ophthalmology (Clinical practice). Philadelphia: WB Saunders company, 1994 : Chapter 292.

34.Salud visual España.www.Essilor.es

35.Schachar RA y col. In vivo increase of the human lens equatorial diameter during accommodation. Am J Physiol 1996; 271:R670-6

36.Schachar RA, Anderson DA. The mechanism of ciliary muscle function. Ann Ophthalmol 1995;27:126-

37.Schachar RA, Huang T, Huang X. Mathematic proof of Schachar´s hypothesis of accommodation. Ann Ophthalmol 1993;25:5-9

38.Schachar Ra. Cause and treatment of presbyopia with a method for increasing the amplitude of accommodation. Ann Ophthalmol 1992;24:445-7

39.Schachar RA. http://www.presbycorp.com

40.Schachar RA. Pathophysiology of accommodation and presbyopia. Understanding the clinical implications. J. Florida M.A 1994;81:4

41.Singh G, Chalfin S. A complication of scleral expansion surgery for treatment of presbyopia. Am J Ophthalmol 2000;130:521-33

42.Steinert RF et al. Hydrogel intracorneal lenses in aphakic eyes. Arch Ophthalmol 1996;114:135-41

43.Strenk SA. Ages-related changes in human ciliary muscle and lens: a magnetic resonance imaging study. Invest Ophthalmol Vis Sci 1999; 40:1162-9

44.Sunsoft. www.sunsoftlenses.com

45.Thornton SP. Presbyopia: The new frontier. A Report on a procedure to reverse presbyopia: anterior ciliary sclerotomy. http://www.slackinc.com/eye/osn/ 1996a/presby.asp

46.Vaquero-Ruano M et al. AMO Array multifocal versus monofocal intraocular lenses: long-term follow-up. J Cataract Refract Surg l998;24:ll8-23

47.Vicinguerra P et al. EXCIMER laser photorefractive keratectomy for presbyopia: 24-month follow-up in three eyes. J Refract Surg 1998;14:31-37

48.Wahl HW et al. Deteriorating vision in the elderly: double stress?. Ophthalmologe 1998;95:389-99

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