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

Figure 12-5: The annular hyperopic ablation profile. (VISX Inc., Santa Clara, CA, USA)

under clinical investigation. Hyperopic astigmatism may also be corrected by differential application of excimer laser along the annular ablation zone.

Summit Technology (Waltham, MA) corrected hyperopia by creating an annular ablation profile by using an erodible disc and axicon quartz lens system.(18) The erodible disc delivery system has the theoretical advantage of transferring any tridimensional shape on to the corneal surface by photoablation. The axicon quartz lens is used to gradually smooth the peripheral blend zone.

Patient Selection and Preoperative

Considerations

Typically, LASIK has been successfully used to correct low to moderate levels of hyperopia (+1.0 to +5.0 diopters). As with myopic LASIK, patients should be older than 21 years and demonstrate stability in the refractive error for at least 12 months. Absolute contraindications include eyes with active pathology in corneal shape, thickness, or inflammation. Eyes with systemic vasculitis, autoimmune diseases, collagen vascular disorders, unstable diabetes or other states with abnormal healing are also suboptimal candidates.

Figure 12-6: LADARVision uses a flying spot beam to scan an annular ablation zone in order to correct hyperopia. (SummitAutonomous Technology, Orlando, FL, USA)

As in the case of LASIK for myopia,the patient should undergo a complete history and examination. This includes a manifest refraction, cycloplegic refraction, corneal topography, slit lamp examination, pachymetry, and dilated fundus examination. Potential risk factors for a complicated procedure can be identified with a careful preoperative examination. The identification of corneal neovascularization is especially important since a large primary keratectomy is required to accommodate the large hyperopic annular photoablation. Cycloplegic refraction is also an integral part of the preoperative examination as it may unmask latent hyperopia in patients with vigorous accommodation ability. The mean central keratometry should be carefully evaluated. Eyes with low preoperative mean keratometry readings are more likely to have smaller diameter flaps which cannot accommodate a large diameter hyperopic ablation profile. Eyes with a postoperative mean central keratometry of >51 to 52 diopters may suffer from poor quality of vision, monocular diplopia and loss of best spectacle corrected visual acuity similar to patients with keratoconus. These eyes may also suffer from chronic apical dryness that may exacerbate the patient’s symptoms during the postoperative course. Thus, keratorefractive surgery should be avoided in eyes with a steep preoperative mean cen-

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Chapter 12

tral keratometry and a large hyperopic correction. In these eyes, other alternatives such as a hyperopic phakic intraocular lens may be a more viable option.

Patients with secondary hyperopia resulting from previous keratorefractive surgery (RK, PRK, LASIK) require special attention. Since overcorrections may regress significantly, the refractive stability should be evaluated prior to the retreatment procedure. In our experience, this requires a minimum of 6 months. In addition, the location and number of radial keratotomy incisions should be noted. LASIK on eyes with over 8 incisions may result in an unstable flap and wound dehiscence. It is also important to evaluate the RK incisions for epithelial plugging and/or wound gape. In these eyes, a thicker plate (i.e. 180 microns) will result in a thicker, more stable flap. The diameter and centration of the primary keratectomy of eyes that had undergone previous LASIK should be carefully noted. Significantly decentered flaps and small diameter flaps will require a new flap in order to accommodate the large diameter hyperopic ablation. Eyes with hyperopia from unintended overcorrection of myopia may require an adjustment in the hyperopic LASIK nomogram.(4) Since these eyes achieve more effective correction than eyes with primary hyperopia, we typically reduce our attempted correction by 20 to 30 %.

Technique

The fundamentals of performing a successful hyperopic LASIK are similar to performing a successful myopic LASIK with a few exceptions. Thus, a surgeon skilled in lamellar surgery should make the transition from myopic LASIK to hyperopic LASIK with little difficulty. After the instillation of a topical anesthetic and topical vasoconstictor, we isolate the eyelashes with a sterile drape and speculum. The corneal marking is performed and the pneumatic suction ring is positioned. A large suction ring >9.5 mm should be used in order to accommodate the large ablation profile of a hyperopic correction.

Careful positioning of the pneumatic suction ring is important in hyperopic LASIK. While a slightly decentered primary keratectomy will accommodate even the largest myopic ablation profiles, it

may not accommodate a 9.5 mm hyperopic ablation profile. Slight decentration of the pneumatic ring towards the hinge is useful in placing the hinge out of the field of the hyperopic ablation. Positioning the pneumatic ring to place the hinge in areas of corneal vascularization will also limit the degree of hemorrhaging that may occur when making a large diameter flap. A moist methylcellulose sponge may be expanded and used to protect the hinge during the photoablation. Centration of the photoablation is mandatory since the relatively small optical zone created by hyperopic LASIK is less forgiving than the larger optical zone of a comparative level of myopic treatment. After the photoablation, the stromal bed is irrigated, the flap is refloated and the epithelial markings are re-aligned. A topical antibiotic, nonsteroidal anti-inflammatory, and steroid is administered. The postoperative medication regimen is identical to that following LASIK for myopia. We administer fluometholone 0.3% and ciprofloxacin 0.3% four times a day for four days.

Clinical Results

A general review of the current literature demonstrates effective reduction of low to moderate levels of spherical hyperopia, simple hyperopic astigmatism and compound hyperopic astigma- tism.(4-16) In these studies, approximately 70 to 90% of the hyperopia is corrected depending on the level of preoperative hyperopia and duration of follow-up. Predictability is also an important measure of refractive accuracy and is typically recorded as the percentage of eyes within +/- 1.0 diopter of attempted correction. In these studies, 60 to 100% of eyes were within +/- 1.0 diopter of attempted correction for low to moderate hyperopia. For higher levels of correction, the predictability within +/- 1.0 diopter of attempted correction decreases to approximately 50 to 80%. (5,7,16) Significant regression can occur following LASIK for hyperopia. (4) In our experience, eyes may be initially slightly overcorrected in the early postoperative period. Stability often requires 3 to 6 months before complete stabilization of the refractive error. After 6 months, eyes may be safely retreated.

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Uncorrected visual acuity of 20/40 or better was demonstrated in approximately 70 to 95% of eyes depending on the level of preoperative correction. (4-16) In these same studies, loss of best spectacle corrected visual acuity generally ranged from 0 to 7%. However, LASIK for hyperopia greater than +5.0 diopters is not recommended as it may result in a loss of best spectacle corrected visual acuity in a significant number of eyes (13 to 15%). (8)

The type and frequency of complications following hyperopic LASIK are similar to myopic LASIK with few exceptions. As noted previously, intraoperative hemorrhages may occur following the microkeratome pass for large diameter flaps. Common avoidable causes of loss of best spectacle corrected visual acuity include decentration and steepening of the central cornea >51 to 52 diopters. The comparatively small optical zone following hyperopic LASIK mandates meticulous attention to centration during the photoablation. There are anecdotal reports of an increased rate of epithelial ingrowth following hyperopic LASIK. This may be related to spill-over ablation outside margin of the primary keratectomy.

Secondary Hyperopia

Secondary hyperopia may be safely treated using hyperopic LASIK technology. (4,10) In these limited studies hyperopic LASIK for eyes previously treated with myopic LASIK or RK demonstrated effective reduction of hyperopia. 83% to 93% of these eyes demonstrated an uncorrected visual acuity of 20/40 or better and no eyes lost best spectacle corrected visual acuity. At Stanford University, we prospectively evaluated 19 eyes with secondary hyperopia resulting from PRK, LASIK, or RK. These eyes underwent hyperopic LASIK with theVISX S2 Smoothscan excimer laser for a mean spherical equivalent of +1.64+/-0.80 diopters (range, +1.5 to +2.75 D). In these eyes with secondary hyperopia, we reduced our nomogram by 20 to 30%. The procedure was performed as described in the technique section of this chapter. The Hansatome microkeratome (Bausch & Lomb, Rochester, NY) was used with the 9.5mm pneumatic suction ring in cases that required a new flap.

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At 1 year, mean sphere was –0.12+/-0.67 diopters (range, -1.25 to +1.5 D), mean cylinder was +0.44+/-0.43 diopters (range, 0 to +2.0 D), and mean spherical equivalent was +0.13 +/-0.69 diopters (range, -1.0 to +1.75 diopters). 88% of eyes were within +/-1.0 diopter of attempted correction and 97% of eyes demonstrated an uncorrected visual acuity of 20/40 or better. Vector analysis demonstrated a mean magnitude of error of –0.20 D +/-0.67 D. The mean angle of error was 0.37 degrees +/- 18.9 degrees. 92% of eyes had a difference vector within +/-1.0 diopters.

There were no intraoperative flap complications, no significant decentrations, and no eyes lost 2 or more lines of best spectacle corrected visual acuity. There was a mean regression in the spherical equivalent of +0.4 diopters between first postoperative day and 6 months. Stability within +/-0.25 diopters occurred between 3 and 6 postoperative months. The average regression between 6 months and 1 year was +0.13 diopters.

(Note from the Editor in Chief: For the surgical management of hyperopia, Mahmoud M. Ismail, M.D., Ph.D, a distinguished ophthalmic surgeon from Egypt has reported highly positive results with the use of intracorneal lenses for the correction of hyperopia in albino rabbits. Dr. Ismail used a new hydrogel intracorneal contact lens (PermaVision, Anamed, Inc.), a product developed to address the limitations reported with the current relatively thick hydrogel lens implants. It is comprised of water content more than 70% and a refractive index that is substantially close to the refractive index of the corneal tissue (1.376). It is designed to correct hyperopia up to +6 diopters. All animals were followed up for at least 6 months by confocal microscopy.

The PermaVision lens intracorneal implant shows excellent compatibility according to the author. The new generations of soft intracorneal lenses may present a new alternative for the correction of hyperopia in the future. The procedure seems to be reproducible and implant removal is possible. This thin PermaVision lens which allows the passage of nutrients and fluid through the implant to nourish the corneal layers may offer a new scope for the application of intracorneal implants in hyperopia. We will need to see the results in humans in later years.)

17.Snibson GR, Carson CA, Aldred GF, Taylor HR. Oneyear evaluation of excimer alser photorefractive keratectomy for myopia and myopic astigmatism. Melbourne Excimer Laser Group. Arch Ophthalmol 1995; 113:994-1000.

18.Haw W., Manche E. Prospective study of photorefractive keratectomy for hyperopia using an axicon lens and erodible mask. Journal of Refractive Surgery 2000; 16:724-730.

Weldon Haw, M.D.

Cornea & Refractive Surgery

Department of Ophthalmology Stanford University School of Medicine 300 Pasteur Drive, Suite A157 Stanford, CA 94305

Phone:(650)-723-5517; Fax: (650)-723-7918 E-Mail: whawhaw@hotmail.com

LASIK (Laser In-Situ Keratomileusis) FOR HYPEROPIA

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Introduction

Irregular astigmatism represents one of the problems that are very difficult to manage and frustrating in results to refractive surgeons. It is also one of the worst sequelae of corneal injuries. It can also complicate certain corneal diseases as keratoconus. With the recent evolution of refractive surgery techniques and diagnostic tools, new types of irregular astigmatism are being observed 1,2.

The astigmatism is defined as irregular if the principal meridians are not 90 degrees apart, usually because of an irregularity of the corneal curvature. It cannot be completely corrected with a sphero-cylin- drical lens 3. Duke –Elder defines irregular astigmatism as a refractive state in which the refraction in different meridians conforms to no geometric plan and the refracted rays have no planes of symmetry 4.

The alternatives for correction of irregular astigmatism are very scarce and with very limited expectations. Spectacle correction is usually not useful in the correction of corneal irregular astigmatism as it is difficult to define principle meridians. Hard contact lenses represent a good alternative in which the tear fluid layer under the contact lens evens out the irregularity. We should consider that adaptation and stability of contact lenses is limited by irregularity corneal surface and the patient’s comfort. We also must remember that our patients consented to undergo refractive surgery because they did not want to use more the contact lens.

Lamelar and full thickness corneal grafting are surgical alternatives. The limited availability of corneal donor as well as the biological and refractive

Chapter 13

2)Traumatic

Corneal irregularity is caused commonly by corneal wounds (incision or excision) or burns (chemical, thermal or electrical) 1.

3)Postinfective

Postherpetic keratitits is the most common form of postkeratitic healing and scarring that may lead to an irregular surface 1.

4)Postsurgical

Irregular corneal astigmatism can complicate any if the following refractive surgical procedures: keratoplasty, photorefractive keratectomy (PRK), laser in situ keratomileusis (LASIK), radial keratotomy (RK), arcuate keratotomy (AK), and cataract incisions. Scleral encirclement or external plombage may also contribute 1.

Diagnosis of Irregular Astigmatism

Clinically, irregular astigmatism will present with one of those typical retinoscopy patterns, the most common being spinning and scissoring of the red reflex. On attempting keratometry the mires will appear distorted. Corneal topography shows certain patterns for irregular astigmatism that will be discussed in detail later. The most recent and sophisticated technique is the application of wavefront analysis (aberrometers) 8. This emerging method measures the refractive status of the whole internal ocular light path at selected corneal intercepts of incident light pencils. By comparing the wavefront of a pattern of several small beams of coherent light projected through to the retina with the emerging reflected light wavefront, it is possible to measure the refractive path taken by each beam and to infer the specific spatial correction required on each path.

Clinical Classification of Irregular

Astigmatism Following Corneal

Refractive Surgery

In corneal refractive surgery using laser in situ keratomileusis (LASIK) the surgeon uses a microkeratome, whether automated or manual, to fashion a corneal flap and a stromal bed. Once the flap is fashioned and lifted, the excimer laser is used

optical zone of the cornea has one flat and one steep area (Figure 13-1a).

B.Decentered Steep: Shows a corneal hyperopic treatment decentered in more than 1.5 mm in relation to the center of the cornea (Figure 13-1b).

C.Central island: Shows an image with an increase in the central power of the ablation zone for myopic treatment ablation at least 3.00D and 1.5mm in diameter, surrounded by areas of lesser curvature (Figure 13-1c).

D.Central irregularity: Shows an irregular pattern with more than one area not larger than 1.0 mm and no more than 1.50D in relationship with the flattest radius, located into the area of the myopic ablation treatment (Figure 13-1d).

E.Peripheral Irregularity: It is a corneal topographic pattern, similar to Central Island, extending to the periphery. The myopic ablation is not homogeneous, there is a central zone measuring 1.5 mm in diameter and 3.00 D in relation to the flattest radius, connected with the periphery of the ablation zone in one meridian (Figure 13-1e).

2. Irregular astigmatism with undefined pattern

We consider irregular astigmatism with undefined pattern when the image shows a surface with multiples irregularities; big and small steep and flat areas, defined as more than one area measuring more than 3 mm in diameter in the central 6 mm (Figure 13-1f). The differential between flat and steep areas were not possible to calculate in the Profile Map and Dk showed an irregular line or a plane line. Normally, Dk is the difference between the steep k and the flat k, given in diopters at the cross of the profile map. A plane line is produced when the ∆k cannot recognize the difference between the steep k and the flat k in severe corneal surface irregularities.

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Topographic Patterns of Irregular Astigmatism

With Ray Tracing Study (Figs. 13-1 A-F)

Figure 13-1 A (left): Decentered ablation (myopic treatment more than 1.5 mm in relation to the center of the cornea. Note that although the peak distortion is minimal in the rat tracing study, the corneal surface quality outside the 3 mm reference pupil is markedly reduced, meaning that the patient will suffer glare and night vision troubles when this pupil dilates under scotopic conditions)

Figure 13-1 B (right): Decentered steep

(hyperopic treatment decentered in more than 1.5 mm in relation to the center of the cornea. Note the reduction in PCVA in spite of a uniform peak, and the irregular base diameter which correlates with the spherical aberrations this patient would suffer).

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Figure 13-1 D (right): Central irregularity

(irregular pattern with more than one area not larger than 1.0 mm and no more than 1.50D in relationship with the flattest radius, located into the area of the myopic ablation treatment. Note the distorted base diameter and the marked peak separation, and the irregular reduced SCSQ).

Figure 13-1 C (left): Central Island (increase in the central power of the ablation zone for myopic treatment ablation at least 3.00D and 1.5 mm in diameter, surrounded by areas of lesser curvature. Note again the reduction in the peripheral SCSQ, i.e. night vision problems).

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