Ординатура / Офтальмология / Английские материалы / Phakic Intraocular Lenses_Hardten, Lindstrom, Davis_2004

Figure 7-4. Steep meridian is confirmed intraoperatively by keratoscopy. In this left eye viewed from the temporal side, the “short axis” of the corneal mire is seen to be at the 75-degree meridian.

The LRIs are placed at the most peripheral extent of clear corneal tissue, just inside of the true surgical limbus. This is done regardless of the presence of blood vessels or pannus. If bleeding occurs, it may be ignored and will stop spontaneously. Care must be taken not to place the incisions further out at the true surgical limbus because a significant reduction of effect is likely to occur; therefore, these incisions are really intralimbal in nature.

In the setting of concomitant cataract surgery, an empiric blade depth setting of 600 m is commonly used. For the younger patient population that would likely undergo phakic IOL implantation, adjusted blade depth settings based upon pachymetric measurements may be justified. If pachymetry is performed, the micrometer is set at 100% of the thinnest reading obtained over the entire arc length of the intended incision. Diamond blade style and configuration may influence results and require an adjustment in blade depth settings and/or the nomogram. The author’s personal preference is to utilize a diamond blade specifically designed for this technique (Mastel

Precision, Rapid City, SC) (Figure 7-2). Similar designs are available from Rhein Medical (Tampa, Fla), ASICO (Westmont, Ill), and other manufacturers.

The extent of arc to be incised may be demarcated in several different ways. The author’s preferred method makes use of a modified Fine-Thornton fixation ring (Mastel Precision, Rapid City, SC) that serves to both fixate the globe and delineate the extent of arc to be incised. One simply extrapolates visually from the limbus where the incision is being made to marks on the surface of the ring (Mastel Precision [Rapid City, SC], Rhein Medical [Tampa, Fla], and Storz [St. Louis, Mo]). Each incremental mark is 10 degrees apart, and bold hash marks (180 degrees) opposite to each other are used to align the steep axis. This approach avoids inking and marking of the cornea. If one desires, a two-cut radial keratotomy (RK) marker may be used to mark the exact extent of arc to be incised in conjunction with the fixation ring/gauge (Figure 7-3). Alternatively, various press-on markers are available, such as those made by Rhein Medical (NichaminKershner LRI markers or the Dell marker). Attention to detail is important in achieving a good result surgically (Figures 7-4 to 7-21).

Complications

LRIs are indeed proving to be a safer and more forgiving approach for reducing astigmatism but, as with any surgical procedure, the potential for complications will always exist, and several are listed in Table 7-2. Of those listed, placing the incisions upon the wrong axis is likely to be the most common error experienced. When this complication is encountered, it typically takes the form of a 90-degree mistake, with the incisions being placed upon the opposite, flat meridian. This, of course, leads to an increase and probable doubling of the patient’s cylinder. Compulsive attention is needed in this regard, and safety checks, such as clear writ-

Figure 7-5. The broad hash marks of the fixation ring/gauge are centered over the 75-degree meridian, using the 6:00 limbal mark for orientation. Alternatively, a Mendez gauge may be used.

Figure 7-7. The incision shown in Figure 7-6 is seen as it is completed 22.5 degrees to the opposite side.

Figure 7-6. The single footplate diamond blade is inserted perpendicular to the corneal surface and at the peripheral most extent of clear corneal tissue. In this case, the nomogram calls for arcuate incisions of 45 degrees. Therefore, the incision is begun approximately 22.5 degrees to one side of the broad hash mark.

Figure 7-8. In this left eye, the steep meridian is at the 120degree axis and has been delineated by opposing limbal marks. The upper left hand ink mark represents the 6:00 position for orientation.

Figure 7-9. The incision is begun 20 degrees to one side of the centering mark.

Figure 7-10. The incision is completed.

Figure 7-11. Total arc length equals 40 degrees.

Figure 7-13. The opposing incision is begun.

Figure 7-15. The temporal single-plane clear corneal incision is placed independent of the LRIs.

Figure 7-12. The starting point of the opposing incision is determined.

Figure 7-14. The incision is completed.

Figure 7-16. In this case, the steep meridian is at 90 degrees. The fixation ring/gauge is therefore centered with the 6:00 limbal mark.

Figure 7-17. The nomogram calls for arcuate incisions of between 45 and 50 degrees. Each incremental mark on the ring equals 10 degrees. The surgeon, therefore, counts over 25 degrees to one side of the centering hash mark.

Figure 7-19. The incision is completed an equal distance past the centering mark, visually extrapolating to the 10-degree marks on the surface of the ring.

Figure 7-18. The blade is inserted.

Figure 7-20. The blade is held above the starting point of the opposite incision.

derived settings. If encountered, these circumferential perforations, unlike radial microperforations, will not self-seal and do require placement of sutures.

Figure 7-21. The incision is completed. Note a small amount of blood within the incision.

ten plans, should be available within the surgical suite for reference. Incisions are always placed upon the plus (+) cylinder axis and opposite to the minus (-) cylinder axis.

Although rare, perforation may occur when utilizing an empiric blade depth setting of 600 m or pachymetry

Bioptics

Another option to manage pre-existing astigmatism is to employ a technique originally described by Zaldivar: “bioptics.” In this approach, excimer laser surgery is used to refine the refractive outcome following posterior chamber phakic IOL surgery for the high myope.20 Others have utilized a similar technique when utilizing other phakic IOL designs.21 Its initial conception came about due to the inherent difficulties and often unpredictable refractive outcomes encountered in this patient population; the preoperative refraction in the very high myope is hampered due in part to image minification and vertex distance considerations. If the selected phakic IOL power was off, LASIK would be performed to refine residual refractive error.

This same approach may be taken to treat residual astigmatism. Although LRIs are effective, it is difficult to compete with the exquisite accuracy achievable through excimer technology. One must, however, weigh these considerations against the additional risk, costs, and logistical concerns of this second operation. LASEK and other

surface ablation techniques are other options, particularly if wave-front technology is to be utilized.22,23 Similarly, other nonlaser technologies may be used in this bioptics approach. Although laser thermal keratoplasty seems to be falling out of favor at this time, conductive keratoplasty may hold promise as being a way to simply and safely treat residual hyperopia as well as mixed and hyperopic astigmatism.

CONCLUSION

Reduction or elimination of significant pre-existing astigmatism is an important element of phakic IOL surgery. Several different options exist, including the use of a toric implant. The availability and use of such devices is likely to increase markedly in the future. Currently, other approaches include tailoring of the implant incision to positively affect the pre-existing cylinder or using adjunctive relaxing incisions. This latter option has been made more facile and safer by moving their location to an intralimbal position. Additional keratorefractive modalities, such as the excimer laser, may be called upon to refine astigmatic error, often in a staged or bioptics approach.

REFERENCES

1.Abrams D. Ophthalmic optics and refraction. In: DukeElder SS, ed. System of Ophthalmology. St. Louis, Mo: Mosby; 1970:671-674.

2.Buzard K, Shearing S, Relyea R. Incidence of astigmatism in a cataract practice. J Refract Surg. 1988;4:173.

3.Lyle WA, Jin G. Prospective evaluation of early visual and refractive effects with small clear corneal incision for cataract surgery. J Cataract Refract Surg. 1996;22(10): 1456-1460.

4.Masket S, Tennen DG. Astigmatic stabilization of 3.0 mm temporal clear corneal cataract incisions. J Cataract Refract Surg. 1996;22(10):1451-1455.

5.Drews RC. Five year study of astigmatic stability after cataract surgery with intraocular lens implantation: comparison of wound sizes. J Cataract Refract Surg. 2000;26(2): 250-253.

6.Anonymous. Small incision surgery: wound construction and closure. J Cataract Refract Surg. 1991;17(Suppl):653748.

7.Ruhswurm I, Scholz U, Zehetmayer M, et al. Astigmatism correction with a foldable toric intraocular lens in cataract patients. J Cataract Refract Surg. 2000;26(7):1022-1027.

8.Sun XY, Vicary D, Montgomery P, et al. Toric intraocular lenses for correcting astigmatism in 130 eyes. Ophthalmology. 2000;107(9):1776-1781.

9.Dick HB, Alio JA, Bianchetti M, et al. Toric phakic intraocular lens European multicenter study. Ophthalmology. 2003;110(1):150-162.

10.Kohnen T, Koch DD. Methods to control astigmatism in cataract surgery. Curr Opin Ophthalmol. 1996;7(1):75-80.

11.Kershner RM. Refractive cataract surgery. Curr Opin Ophthalmol. 1998;9:46-54.

12.Gills JP. Treating astigmatism at the time of cataract surgery. Curr Opin Ophthalmol. 2002;13(1):2-6.

13.Budak K, Friedman NJ, Koch DD. Limbal relaxing incisions with cataract surgery. J Cataract Refract Surg. 1998;24(4): 503-508.

14.Nichamin LD. Changing approach to astigmatism management during phacoemulsification: peripheral arcuate astigmatic relaxing incisions. Paper presented at: Annual meeting of the American Society of Cataract and Refractive Surgery; May 20, 2000; Boston, Mass.

15.Nichamin LD. Reducing astigmatism. In: Wallace RB, ed.

Refractive Cataract Surgery and Multifocal IOLs. Thorofare, NJ: SLACK Incorporated; 2000:167-172.

16.Budak K, Friedman NJ, Koch DD. Limbal relaxing incisions with cataract surgery. J Cataract Refract Surg. 1998;24(4): 503-508.

17.Thornton SP. Radial and Astigmatic Keratotomy: The American System of Precise, Predictable Refractive Surgery.

Thorofare, NJ: SLACK Incorporated; 1994.

18.Swami AU, Steinert RF, Osborne WE, et al. Rotational malposition during laser in situ keratomileusis. Am J Ophthalmol. 2002;133(4):561-562.

19.Abrams D. Ophthalmic optics and refraction. In: DukeElder SS, ed. System of Ophthalmology. St. Louis, Mo: Mosby; 1970:671-674.

20.Zaldivar R, Davidorf JM, Oscherow S, et al. Combined posterior chamber phakic intraocular lens and laser in situ keratomileusis: bioptics for extreme myopia. J Refract Surg. 1999;15(3):299-308.

21.Guell JL, Vazquez M, Gris O, et al. Combined surgery to correct high myopia: iris-claw phakic intraocular lens and laser in situ keratomileusis. J Refract Surg. 1999;15(5):529537.

22.Durrie DS. Are stromal flaps necessary: patient acceptance and results of LASEK (talk). Presented at the American Academy of Ophthalmology Annual Meeting; October 18, 2002; Orlando, Fla.

23.Durrie DS. LASIK vs PRK/LASIK vs LASEK (talk). Presented at the American Academy of Ophthalmology Annual Meeting; October 18, 2002; Orlando, Fla.

INTRODUCTION

Both phakic intraocular lenses (IOLs) and corneal refractive surgery have received a large degree of interest for the correction of refractive errors. The most common corneal refractive surgery performed in most countries throughout the world is laser in-situ keratomileusis (LASIK).1 The predominant phakic IOLs that are used worldwide include the Artisan (Ophtec USA, Inc/Allergan, Boca Raton, Fla) and the Implantable Contact Lens (ICL) (STAAR Surgical, Monrovia, Calif), although several other types are available (Table 8-1). While both procedures can achieve excellent outcomes, they have very different ranges of correction, indications, contraindications, surgical techniques, results, and complications. It is important that the refractive surgeon understand all of the options available for each patient, as some patients will be excellent candidates for one procedure and poor candidates for another. The diagram for the options of refractive surgery (Figure 8- 1) describes all of the various refractive procedures that are currently available in 2003.

RANGE OF CORRECTION

The standard range of correction for LASIK extends from 1.00 to 10.00 diopters (D) of myopia with an extended range up to 12 D of myopia. The range for hyperopic LASIK extends from 1.00 to 3.00 D with an extended range up to 4.00 D of hyperopia. Standard astigmatic LASIK can correct up to 3.00 D. Cross cylinder LASIK can extend astigmatism correction up to 8.00 D.2

The range of correction of LASIK is limited by the corneal thickness and the keratometry of the cornea. At least 250 m of posterior stroma should be preserved after LASIK to prevent long-term ectasia of the cornea. If the flap is assumed to be 160 m, the total corneal thickness after LASIK should be at least 410 m. Because the average corneal thickness is approximately 540 m and the standard myopic correction removes about 15 m per diopter with most modern excimer lasers, the maximum average correction is limited to less than 9.00 D.3 While this range can be extended with thicker corneas or smaller optical zones, the optical zone used for LASIK should always account for the preoperative pupil size.4

The range of LASIK correction is also limited by the keratometry of the cornea.5 Corneal flattening to less than 35.00 D is associated with a higher incidence of a loss of best-corrected visual acuity (BCVA) and regression. Myopic LASIK flattens the cornea by approximately 70% of the spherical equivalent.6 Therefore, the maximum correction with a preoperative keratometry value of 40.00 D would be approximately 7.00 D. Corneal steepening greater than 50.00 D has been associated with a central superficial keratitis caused by ineffective tear film distribution over the steep cornea (Figure 8-2). Because the hyperopic steepening effect is 100%, the maximum correction of a cornea with preoperative keratometry of 47.00 D would be 3.00 D.

Phakic IOLs are not associated with the limitations of LASIK, as they do not alter the thickness or curvature of the cornea. While phakic IOLs could be used to correct the full range of refractive errors, they are generally used for the correction of higher levels of myopia and hyper-

opia. This is because LASIK offers a high degree of accuracy for the correction of the smaller refractive errors with a higher level of convenience for the patients. Therefore, phakic IOLs are generally used for those patients that are not LASIK candidates.

Phakic IOLs have a standard range between 10.00 to 22.00 D of myopia with an extended lower range of myopia down to 5.00 D. For hyperopia, phakic IOLs are used for +4.00 to +10.00 D of hyperopia but can be extended down

into the +3.00 D range. Toric versions of the anterior chamber phakic IOL (ie, Vivarte), the ICL, and the Artisan phakic IOL have recently become available internationally, allowing the correction of up to 3.00 D of astigmatism associated with the spherical refractive errors. Most patients with residual astigmatism after nontoric phakic IOL implantation have LASIK as an enhancement procedure in a twostep procedure that Zaldivar named bioptics (Figure 8-3).7

Figure 8-2. Central superficial keratitis associated with a loss of two lines of BCVA 2 months after +4.00 hyperopic LASIK correction.

Figure 8-4. LASIK is the preferred option for the correction of residual refractive errors after radial keratotomy.

INDICATIONS

Both phakic IOLs and LASIK are elective refractive procedures; therefore, they are only indicated in patients that have been fully informed of their options for the correction of their refractive error. A thorough preoperative consent form should be completed by all patients prior to the procedure. Patients should understand the advantages as well as the limitations of refractive surgery. Realistic expectations are an important prerequisite for refractive surgery. The minimum age for refractive procedures is 18 years of age. However, refractive surgery is often done after 21 years of age to ensure refractive stability over the preceding 12 months. Both LASIK and phakic IOLs are generally performed on normal, healthy eyes.

LASIK is indicated for low to moderate levels of myopia, hyperopia, and astigmatism correction because of the relatively high accuracy and the low risk of surgical complications. Standard excimer laser ablations for LASIK can only treat regular astigmatism; however, customized wavefront LASIK corrections may allow irregular patterns to be treated in the near future. Successful LASIK also requires a stable corneal epithelium and normal corneal thickness.

LASIK is the preferred procedure for the correction of residual refractive errors after radial keratotomy (RK) (Figure 8-4), cataract surgery,8 penetrating keratoplasty,9

Figure 8-3. Bioptics is the combination of an intraocular lens procedure and a corneal refractive procedure, such as LASIK.

refractive lensectomy,10 and phakic IOL implantation,11 as these corrections are generally small. Photorefractive keratectomy (PRK) after previous corneal surgery has been associated with a high incidence of corneal haze.12,13

Alternatively, phakic IOLs are best suited for patients with high levels of myopia or hyperopia, as they are not restricted by the limitations of corneal correction. While they certainly can be used to correct lower levels of myopia, they do not have the precision of the LASIK correction. They are, therefore, likely to require a LASIK enhancement procedure afterward in order to refine the residual prescription to achieve the best uncorrected visual acuity (UCVA). They are also associated with a slightly higher surgical risk because they are intraocular procedures.

Phakic IOLs may be the preferred procedure for the correction of even smaller refractive errors if the risks for LASIK are found to be unusually high, such as in the case of preoperative forme fruste keratoconus or severe dry eyes. Patients with unusually thin corneas without any other pathology may benefit from phakic IOL correction of low refractive errors. Phakic IOLs can be removed and are, therefore, a potentially reversible procedure as compared to LASIK, which ablates corneal tissue that cannot be replaced. However, since the removal of phakic IOLs is considerably more difficult than their insertion, it should never be presented to patients as a readily reversible procedure.

CONTRAINDICATIONS

There are a number of contraindications for LASIK and phakic IOL surgery. Unrealistic expectations for the outcome of the procedure are extremely important to ascertain. Patients that expect a guarantee of 20/20 “perfect vision” postoperatively will generally be disappointed by the outcome. An unstable refraction, defined as a change in the sphere or astigmatism by more than 0.5 D over