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

Chapter 9

of Calcualation

Optical Maximal (160 Zone

plate) um

ableT 3-1

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Table 1-4

Calculation of Maximal Optical Zone (180 micron plate)

and with the rule astigmatism. Conversely, a nasal hinged flap is more advantages when treating hyperopic astigmatism that is against the rule. Secondly, fluid at the hinge should be avoided because it may change the results. Larger diameter flaps have a greater potential for causing bleeding from the corneal pannus. If any blood or other fluid collects at the hinge, the ablation should be paused and continued after the fluid is removed.

Presbyopia

Patients in the presbyopic age range should be counseled about the need for reading glasses after surgery. This is especially important for presbyopes that have low myopia, because they are generally used to seeing well at near without glasses. They may be unaware that by having refractive surgery they will be exchanging distance correction for readers. In our experience this is particularly disturbing to women who discover that they can no longer apply eye makeup without a magnifying mirror. Some patients may find that even though they were able to read with their distance glasses preoperatively, they require reading glasses postoperatively. This is because there is an increased accommodative demand (for near work) when a myope’s correction is done at the corneal plane, such as contact lenses or refractive surgery.6

Monovision correction should be discussed and a trial of soft contact lenses or trail frame should be offered in order to simulate it. We generally begin by correcting the dominant eye for distance (unless the patient is successfully wearing monovision contact lenses with the distance correction in the nondominant eye). If the patient does not like monovision with the dominant eye for distance, then a trial using the non-dominant eye for distance is done. In general, we have found that patients involved with visually demanding work or play requirements do not usually like monovision correction. For example, golfers tend to prefer full distance correction in both eyes. Patients should be aware that monovision correction will not completely free them from glasses for all activities. For example, glasses may be needed for improved depth perception when driving at night. Methods for correcting presbyopia, such as scleral

flap integrity, resulting from dehiscence of the RK incisions. However, with careful manipulation of the flap we have avoided this problem. The incisions usually hold together well, although there may be some dehiscence at the flap periphery. If subsequent enhancements are necessary we recommend re-cutting rather than re-lifting the flap, due to possible dehiscence of old RK incisions.

The results of LASIK after RK have been very good at our refractive surgery center. Compared to PRK, recovery is quicker with less regression and a higher refractive correction is possible. However, we have found some variability in response to the laser. Also, prolonged refractive instability and hyperopic shifts due to the RK incisions have occurred. Epithelial ingrowth occurs more often in post RK patients. It is seen more commonly when there is gaping of the RK wound seen on preoperative exam.

Alternatives to LASIK

PRK

Although LASIK is overwhelmingly the most popular procedure performed at our center, PRK remains an excellent option for low to moderate myopia and low to moderate astigmatism.11 It remains a good alternative to LASIK and IntacsTM for this range of refractive error. In some cases PRK may be preferable to LASIK. For example, patients with inadequate corneal thickness for LASIK and low to moderate myopia will usually do well with PRK. Another instance when PRK may be preferable to LASIK is when the preoperative corneal curvature is less than 41 or greater than 46 diopters. Some patients may even prefer PRK because they are concerned about the possibility of flap complications. Counseling patients about the risks, benefits, alternatives and complications of PRK should include a discussion of early postoperative discomfort, corneal haze and infection.

Refractive Lensectomy

Refractive lensectomy and IOL implantation offers another alternative in both the presbyopic and high refractive error patients who are not appropri-

LIMITATIONS AND CONTRAINDICATIONS OF LASIK

ate candidates for corneal refractive surgery. It can be considered in cases with larger pupils, limited pachymetry, high corrections, or when lens opacities are present.12 In our practice any patient over 50 with lenticular changes is counseled about refractive lensectomy. Patients opting for refractive lensectomy should generally be older than 50 years of age because of the loss of accommodation. Significant lenticular changes include more than 2+ nuclear sclerosis, any posterior subcapsular cataract, or cortical changes. Refractive lensectomy may also be considered for near-presbyopes in select cases, for example, extreme hyperopia and myopia. A toric IOL is considered in patients with greater than 1.5 diopters of astigmatism.

Advantages of refractive lensectomy are early refractive stability and rapid visual rehabilitation. It maintains the corneal integrity and there is no change in corneal asphericity. Additionally, refractive lensectomy will give an improved quality of vision in higher refractive corrections when compared to corneal refractive surgery. There is minimal loss of contrast sensitivity and no risk of decentered ablations or phakic IOL placement. Refractive lensectomy avoids the future need for cataract surgery in patients with lenticular changes.

Refractive lensectomy is an intraocular procedure, making it relatively more invasive than corneal reshaping surgery. It causes loss of accommodation and the long-term risks are still unknown. In a recent report by Gimbel, he evaluated the results of 212 hyperopic and 163 myopic lensectomies performed between in 1986-1998.13 The overall rate of retinal detachment in the myopic group was 0% and 0.47% in the hyperopic group. There were no reported cases of endophthalmitis or cystoid macular edema. Eighteen patients required Nd:YAG posterior capsulotomy.

Surgeons performing refractive lensectomy should be skilled in modern phacoemulsification and managing its complications. The intraoperative and postoperative complications of standard modern phacoemulsification are comparable to refractive lensectomy. Intraoperative complications should be discussed with the patient including capsular tears, iris prolapse, zonular dialysis, dropped nucleus, and vitreous prolapse. Postoperative complications should be addressed, including iritis, cystoid macu-

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Chapter 9

lar edema, endophthalmitis, wound leakage, posterior capsule opacification, and IOL decentration. In myopic eyes the risks of retinal detachment must also be carefully considered.

Patient expectations after refractive lensectomy should be addressed preoperatively. Patients should be warned that they will need reading glasses postoperatively if the targeted refraction is near plano. Monovision should be discussed. The possibility of an enhancement with PRK, LASIK, or AK should also be discussed. Patients with long or short axial lengths should be warned about the increased difficulty in predicting refractive outcome.

In summary, our experience indicates that refractive lensectomy is a safe and effective procedure. It is a good surgical option for patients with high refractive errors, and is most beneficial to patients in the presbyopic age group. Advantages of refractive lensectomy include rapid visual rehabilitation, refractive predictability, and early stability. It maintains the prolate cornea and there is minimal loss of contrast sensitivity. There is no risk of decentered ablation or phakic IOL placement. The disadvantages of refractive lensectomy revolve around the relative risks of any intraocular surgery, and the loss of accommodation. The long term risks of retinal detachment and other late complications need to be evaluated as do any quality of vision concerns.

Phakic Intraocular Lens

Although not FDA approved in the U.S., a phakic IOL can be considered internationally in patients younger than 50 years of age who are not good candidates for LASIK. Phakic IOLs may be a good option for patients with high refractive error when the corneal pachymetry is not sufficient to perform LASIK. Phakic IOLs are most beneficial to patients with moderate to high myopia and hyperopia. Patient selection criteria depend on lens type. For the Staar ICL, the anterior chamber depth needs to be greater than 3.0 mm in myopes and greater than 2.75 mm in hyperopes. It should be avoided in patients with large pupils because the optics range from 4.5 to 5.5 mm in diameter, predisposing to nocturnal halos.

Phakic IOL implantation requires additional preoperative testing. The corneal horizontal white- to-white, endothelial cell counts, and anterior chamber depth should be measured. Additionally, a YAG iridotomy is required to prevent papillary block in posterior chamber phakic implants.

Complications of phakic IOL implantation include iritis, early postoperative elevation of IOP, wound leakage, induced astigmatism, papillary irregularities, and implant decentration, dislocation or rotation. Other postoperative complications include nocturnal halos, pigmentary glaucoma, residual refractive error, corneal edema, endothelial cell loss, retinal detachment, and cataract formation. Although there have been no reports of cataract formation following anterior chamber phakic IOL implantation; posterior chamber phakic IOLs have been reported to cause cataracts.14

Thermokeratoplasty (LTK)

Seiler introduced laser thermokeratoplasty over a decade ago.15 The Hyperion LTK System is the first laser procedure designed specifically for farsightedness. Hyperion LTK technology represents an innovative alternative for a significant segment of this over-forty group, representing millions farsighted Americans.

The Hyperion LTK system (Sunrise Technologies) uses a non-contact delivery system that emits a 2.13 mm infrared laser radiation wavelength in a pulsed mode with a 0.25 millisecond pulse and repetition rat of 5 Hz. The benefits of LTK are that no corneal flap is necessary and the procedure takes only a few seconds to perform. As a result, the possibility of intraoperative complications, postoperative infections, or healing irregularities is minimized. The system is easy to learn and operate. In addition, the quickness of the procedure and the absence of cutting or ablation remove the fear factor for patients.

The benefits of LTK are that it is a quick “notouch” procedure with a good safety profile. However, some of the effects of LTK are known to regress with time, and it is not currently FDA approved to treat any significant degree of astigmatism. Thus, it is less desirable than LASIK for many patients. However, LTK may be an option for patients with

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little astigmatism who desire the “no-touch” technique. In the future, LTK may play a more significant role in refractive surgery if it is approved to treat significant degrees of astigmatism.

The safety record exhibited during U.S. clinical trials of the Hyperion LTK procedure has been exceptional. Close to 700 eyes were treated in the two-year trial, with patients generally showing improved vision immediately after the procedure and most resuming regular activities the day after treatment. It received FDA approval on June 30, 2000 for the temporary reduction of hyperopia in patients with +0.75 to + 2.50 diopters of MRSE with less than or equal to +/- 0.75 diopters of astigmatism. The FDA recently (December 13, 2000) approved a change to the company’s label reflecting that some of the effects of the sunrise LTKTM procedure can last for ten years and beyond. Patients should be aware of the possibility of regression. Regression appears to be less common in patients over 40 years of age. Poor response to LTK is more common in young patients, high intraocular pressure, and thicker pachymetry. We do not currently offer LTK at our center because of its limitations and the success of other available refractive surgery techniques such as excimer laser and refractive lensectomy.

Summary

LIMITATIONS AND CONTRAINDICATIONS OF LASIK

REFERENCES

1Gimbel (personal communication)

2Slade SG. LASIK complications and their management. Free cap, thin and perforated corneal flaps. In: Machat JJ, ed. Excimer Laser Refractive Surgery. Practice and Principles. Thorofare, NJ: SLACK Incorporated; 1996.

3Seiler T, Koufala K, Richter G. Iatrogenic keratectasia after laser in situ keratomileusis. J Refractive Surgery. 1998;14(3):312-317.

4Wang Z, Chen J, Yan B. Posterior corneal surface topographic changes after laser in situ keratomileusis are related to residual corneal bed thickness. Ophthalmology. 1999;106(2):406-409.

5Vinciguerra P, Sborgia M, Epstein D, Azzolini M, McCrae S. Photorefractive keratectomy to correct myopic or hyperopic astigmatism with a cross-cylinder ablation. J Refract Surg. 1999; 15 (suppl):S183-S185.

6Hunter, D., West C. Last Minute Optics

7 Stulting RD, Carr JD, Thompson HP, Wiley W, Waring III GO. Laser-in-situikeratomileusis for the correction of myopia after previous ocular surgery. (Abstract #144). In American Society of Cataract and Refractive Surgey. Symposium on Cataract, IOL, and Refractive Surgery, 1998, p.37.

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12 Chen R, Feinerman G, Penno E, Gimbel H. Refractive Lensectomy. In: Gimbel HV, ed. Refractive Surgery: A Manual of Principles and Practice. Thorofare, NJ: SLACK Incorporated; 2000.

13Gimbel H. Retinal detachment rate after high myopia lensectomy/IOL. Presented at the Hawaiian Eye Meeting, January 2001.

14Baikoff G. Phakic myopic intraocular lenses. In; Serdarevic ON, ed. Refractive Surgery: Current Techniques and Management. New York, NY: Igaku-Shoin Medical Publishers Inc; 1887:165-173.

15 Seiler T, Matallana M, Bende T. Laser thermokeratoplasty by means of a pulsed holmium:YAG laser for hyperopic correction. Refract Corneal Surg. 1990;6;335-339.

Gregg Allen Feinerman, M.D.

Medical Director

Feinerman Vision Institute

Long Beach Laser Center

Long Beach, California

Associate Clinical Professor

University of California,

Irvine, California

E-mail: lasiksurgeon@home.com

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Jaime R. Martiz, M.D., Stephen G. Slade, M.D.

Introduction

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Figure 10-3. Surgical instruments for LASIK procedure.

Patients should undergo an informed consent with their surgeon and be fully knowledgeable of:

-Risks -Benefits

-Reasonable expectations -Other surgical alternatives

They must comprehend that irreversible complications do happen and can result in loss of sight. Patient with more than 6 mm pupil size, high myopia and less than 250 microns residual bed should understand that surgeon might need to either decrease laser ablation diameter or patient treatment.

We perform bilateral surgery the same day in all our patients except:

>50 years old

>-7 diopters sphere or > 3 diopters of cylinder

Patient with nystagmus have the option for Laser with an eye tracker system.

PREOPERATIVE PREPARATION

The Patient

Patient’s preoperative preparation includes an oral sedative such as Valium (5 to 10 mg) prior to the procedure. Immediately before prepping, one drop of a topical anesthetic (Proparacaine) should be instilled and then one more drop before the keratectomy. No preoperative miotic is used. The pa-

Figure 10-4. A sample of color change in the fluence test from white to red.

and to achieve standard laser ablation rates. It is unacceptable for the environment to be turned off for night or weekends.

Laser beam calibration, homogeneity and alignment of the beam are achieved by fluence testing. In the case of the Bausch & Lomb Technolas 217 laser appropriate fluence is 65 pulses. The test is perform on polymethacrylate (PMMA) plate on which a subtle silver-plated foil is placed with the interposition of a layer of glue. The total number of spot necessary to obtain a complete exposure of the PMMA foil must be equal to 65 ± 2; in normal conditions, there is a color change from white to red in an interval of five to seven spots. A fine and dispersed white granularity can remain. (Figure 10-4)

The surgeon should also verify patient data inserted in the computer is appropriate for surgical operation and that the axis of astigmatism is correct and corresponds with that found topographically and by refraction. The minimum diameter of the ablation should be appropriate to the patient’s pupil diameter and the ablation depth should leave more than 250 microns residual stroma bed (Figure 10-5).

The Keratome

After inserting the blade into the microkeratome head, examine it very carefully under the operating microscope at maximum magnification to check the condition of the blade edge (Figure 10-6). All the parts of the keratome should be inspected

Figure 10-5. Ablation depth should leave more than 250 microns residual bed.

Figure 10-6. Always check the condition of the blade edge.

before proceeding with the cut. Discard a blade with notches or stains. Very small irregularities on the blade margin can be also detected by observing the reflection of the microscope’s light on the edge of the blade. By depressing the foot pedal, the head advances along the track for a test run on the suction ring. Although an alarm indicates when the microkeratome has reached the end of its pass, the surgeon should also understand and visually memorize the end point of the forward pass, so as not to rely solely on hearing the “beep-beep”. If during advancement, the speculum obstructs the microkeratome and interrupts the run, the computer interprets this interruption as the end of the pass. Once the head reaches the end of the pass, push the foot pedal once again to return the microkeratome to its starting point. At this point, rotating the head through half turn in the opposite direction to insertion raises the keratome head along the pin until it detaches from the suction

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Section 4

Section 5

Section 6

Section 7

Subjects Index

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