Ординатура / Офтальмология / Английские материалы / Master Techniques in Cataract and Refractive Surgery_Hampton Roy, Arzabe_2004

Figure 4-4. Viscoelastic is used to create a space for placement of the MCTR and viscodissect any remaining cortex so that it will not become entrapped by the ring.

Figure 4-6. The fixation hook of the MCTR model 1-L is displaced to the scleral wall with an Osher Y-hook to determine the best axis for suture fixation.

flap is fashioned at this site so that once the Prolene suture is tied the suture and knot can be covered. Viscoelastic is then used to create space between the undersurface of the iris and the anterior capsule in preparation for needle passage. The needles are placed through the incision, into the pupil, and behind the iris. The needle and suture should remain anterior to the anterior capsule at all times (Figure 4-7). The 2 needles should exit the scleral wall approximately 1.5 mm apart and 1.5 mm posterior to the corneal-scleral junction. This will position the fixation hook posterior enough to prevent postoperative iris chaffing. The sutures are then cinched to verify centration, and a temporary knot is tied (Figure 4-8). If a single-armed suture is used, the needle is passed through partial-thickness scleral beneath the scleral flap and then tied to itself. After suture fixation of the MCTR, any remaining cortex can be aspirated manually with a 24to 27-gauge cannula. Alternatively, one can use an automated irrigation/

Figure 4-5. A Cionni chopper (Duckworth and Kent, St. Louis, Mo) and Osher Y-hook (Duckworth and Kent) are used to place the MCTR, model 1-L into the capsular bag.

Figure 4-7. Each needle of the double-armed 9.0 Prolene suture is passed through the main incision, anterior to the anterior capsule, and out through the ciliary sulcus and scleral wall to exit beneath a partial thickness flap.

aspiration device, but vitreous prolapse may be more likely. The capsular bag is then reinflated with viscoelastic prior to PCIOL insertion.

We have found it easiest to insert a foldable-style PCIOL into the capsular bag in these cases. We currently favor the Alcon (Fort Worth, Tex) AcrySof SA 60 and Monarch II (Alcon) delivery system, which can be injected completely into the capsular bag through a 3.0-mm incision. This lens material is especially useful in younger patients because it

Figure 4-8. The 9.0 Prolene suture is tightened and a temporary knot tied, centering and stabilizing the capsular bag.

does not elicit a significant inflammatory response and because it has a low rate of posterior capsule opacific- ation.18-21

Once the PCIOL is in place, the temporary knot is released and a permanent knot is tied with just enough tension to center the IOL. The knot can then be rotated beneath the sclera, or simply buried beneath the scleral flap. If the 2- hook model (2-L) is used, the fixation site for each hook must be ascertained by displacing each hook to the scleral wall prior to suturing. Depending on the size of the capsular bag, the best centration may be obtained with the hooks less than 180 degrees apart. Viscoelastic is removed manually through the side-port incision or with an automated irrigation/aspiration handpiece. Miochol is instilled to ensure that the pupil rounds. Conjunctiva is reapproximated over the scleral flap and the corneal incision is hydrated and checked to be certain that it is watertight (Figure 4-9).

COMPLICATIONS

AND MANAGEMENT

If vitreous presents at any time during the procedure, it should be carefully and completely removed from the anterior chamber. We have developed a technique using Kenalog (Alcon) (triamcinolone suspension) to identify vitreous in the anterior chamber.22 Kenalog is injected into the anterior chamber where they become imbedded in the vitreous gel. The white suspension provides the surgeon better visualization of the location and the extent of prolapsed vitreous, allowing for a more thorough anterior vitrectomy (Figure 4- 10). It also allows for the surgeon to observe the vitreous behavior and avoid maneuvers that increase vitreous traction or prolapse.

Figure 4-9. The single-piece acrylic PCIOL is centered within the capsular bag at the end of the procedure.

Figure 4-10. Demonstrates Kenalog-assisted anterior vitrectomy. Note how the Kenalog particles highlight the location and extent of vitreous prolapse, as well as the movement of the vitreous to the vitrector.

Before injecting Kenalog into the anterior chamber we exchange the preservative-containing vehicle of Kenalog for BSS. To remove the preservative, Kenalog suspension is prepared exactly as follows. Withdraw 0.2 ml of well shaken Kenalog (40mg/ml) into a tuberculin syringe. Replace the needle with a 5-µm syringe filter (Sherwood Medical). The plunger is then depressed forcing the suspension into the 5-µm filter, allowing the vehicle to pass through, yet capturing the Kenalog particles. The filter is then transferred to a 6-ml syringe containing 2 ml of BSS. The plunger is depressed causing the BSS to rinse through the filter and the Kenalog. Without removing the filter from the syringe, a 22gauge needle is placed on the distal end of the filter, and approximately 5 ml of BSS is drawn up into the syringe,

resuspending the Kenalog. The syringe is inverted several times to ensure thorough resuspension and washing of the Kenalog particles. The plunger is then depressed, expressing all of the fluid through the filter and re-capturing the Kenalog particles. Finally, 2 ml of BSS is drawn into the syringe through the filter to resuspend the Kenalog particles. The filter is removed and the washed Kenalog suspension is transferred to a sterile 3-ml syringe. Before injecting the washed Kenalog the syringe must be inverted several times to ensure thorough suspension. The Kenalog is then injected into the anterior chamber through a 27-gauge cannula.

Small amounts of vitreous can be removed by using a "dry" vitrectomy technique with an automated vitrector and an anterior chamber filled with viscoelastic.23 Significant vitreous prolapse is best handled by Kenalog-assisted bimanual vitrectomy. After injecting Kenalog into the anterior chamber, a side-port incision can be used for irrigation with a 25gauge cannula. The vitrectomy handpiece can be inserted through the initial incision or through a pars plana sclerotomy.24

CTRs and MCTRs of any model should not be used if a complete continuous capsulorrhexis is not attained or if a posterior capsule tear occurs since the expansile forces may cause the capsular bag to rupture. In such a case, the surgeon may choose to suture the IOL to the iris or to the scleral wall. Alternatively, an anterior chamber IOL can be implanted.

CONCLUSION

The CTR has been FDA approved; however, the MCTR has not yet been approved by the FDA. These devices afford us the possibility of saving the capsular bag, recentering the capsular bag and even placing a PCIOL within the capsular bag. This surgery can be performed through a 3.0-mm incision, giving the patient a rapid visual recovery. This is most important in young children, as prolonged visual deprivation could result in dense amblyopia.

At the time of this writing, the CTR and MCTR are not yet FDA approved for use in the United States despite their widespread availability throughout the rest of the world. While these newer devices have improved the operative management of zonular weakness, the surgeon must be familiar with each of these advanced surgical techniques since these cases represent some of the most difficult procedures that we encounter. Yet skilled and knowledgeable management will usually result in a satisfying outcome for both the patient and his surgeon.

REFERENCES

1.Jensen AD, Cross HE. Surgical treatment of dislocated lenses in Marfan's syndrome and homocystinuria. Trans Am Acad Ophthalmol Otolaryngol. 1972;76:1491-1499.

2.Varga B. The results of my operations improving visual acuity of ectopia lentis. Ophthalmologica. 1971;162:98-110.

3.Maumenee IH. The eye in Marfan's syndrome. Trans Am Acad Ophthalmol Soc. 1981;79: 684-733.

4.Straatsma BR, Allen RA, Pettit TH, Michael MO. Subluxation of the lens with iris photocoagulation. Am J Ophthalmol. 1966;61:1312-1324

5.Cionni RJ, Osher RH. Management of zonular dialysis with the endocapsular ring. J Cataract Refract Surg. 1995;21:245249.

6.Cionni RJ, Osher RH. Management of profound zonular dialysis or weakness with a new endocapsular ring designed for scleral fixation. J Cataract Refract Surg. 1998;24:1299-1306.

7.Zetterstrom C, Lundvall A, Weeber Jr H, Jeeves M. Sulcus fixation without capsular support in children. J Cataract Refract Surg. 1999;25:776-781.

8.Dick HB, Augustin AJ. Lens implant selection with absence of capsular support. Curr Opin Ophthalmol. 2001;12(1):47-57.

9.Menezo JL, Martinez MC, Cisneros AL. Iris-fixated Worst claw versus sulcus-fixated posterior chamber lenses in the absence of capsular support. J Cataract Refract Surg. 1996;22(10):1476-84.

10.Hara T, Hara T, Yamada Y. "Equator ring" for maintenance of the completely circular contour of the capsular bag equator after cataract removal. Ophthalmic Surg. 1991;22:358-359

11.Legler U, Witschel B, et al. The capsular tension ring, a new device for complicated cataract surgery. Presented at the Amer Soc Cataract Refract Surg. Seattle, Wa: May 1993.

12.Osher RH. Video journal of cataract and refractive surgery. 1997;8:1.

13.Pfeifer V. Video presentation at the American Society of Cataract and Refractive Surgery. San Diego, Calif: April 1998.

14.Merriam J, Zheng L. Iris hooks for phacoemulsification of the subluxated lens. J Cataract Refract Surg. 1997;23:1295.

15.Maloney WF. Supracapsular phaco: achieving greater efficiency with phaco outside of the capsular bag: a 3-year experience. Presented at the American Society of Cataract and Refractive Surgery. Seattle, Wa: April 1999.

16.Osher RH. Slow motion phacoemulsification approach (Letter). J Cataract Refract Surg. 1993;19:667.

17.Cionni RJ, Osher RH. Complications of phacoemulsification. In: Weinstock FJ, ed, Management and care of the cataract patient. Cambridge, MA: Blackwell Scientific Publications Inc; 1992;209-210.

18.Hollick EJ, Spalton DJ, Ursell PG, Pande MV. Biocompatibility of poly(methylmethacrylate), silicone, and AcrySof intraocular lenses: randomized comparison of the cellular reaction on the anterior lens surface. J Cataract Refract Surg. 1998;24:361-366.

19.Ursell PG, Spalton DJ, et al. Relationship between intraocular lens biomaterials and posterior capsule opacification. J Cataract Refract Surg. 1998;24:352-360.

20.Linnola RJ, et al. Adhesion of fibronectin, vitronectin, laminin, and collagen type IV to intraocular lens materials in pseudophakic human autopsy eyes: part 1 histological sections. J Cataract Refract Surg. 2000;26:1792-1806.

21.Linnola RJ. Adhesion of fibronectin, vitronectin, laminin, and collagen type IV to intraocular lens materials in pseudophakic human autopsy eyes: part 2 explanted intraocular lenses. J Cataract Refract Surg. 2000;26:1807-1818.

22.Burk SE, Da Mata AP. Visualizing vitreous using Kenalog suspension. J Cataract Refract Surg. In press.

23.Osher RH. Dry vitrectomy. Video J Cataract Refract Surg. 1992;8(4).

24.Snyder ME, Cionni RJ, Osher RH. Management of intraoperative complications. In: Gills JP, ed, Cataract Surgery: The State of the Art. Thorofare, NJ: SLACK Incorporated; 1998: 149-152.

Over 2 million cataract surgery procedures are performed in the United States every year and approximately 98% are said to be successful. Although most papers suggest that the incidence of anterior vitrectomy in cataract surgery is 1%, there are approximately 5 anterior vitrectomy packs sold for every 100 phacoemulsification packs. This data suggest that at least 100,000 cases of capsule rupture and anterior vitrectomy in the United States occur every year. It is likely that this number is similar to occurrence rates outside the United States.1

Business publications today emphasize the need for focus to achieve better performance. It is the author’s contention that many surgeons place as much focus on operating time, incision size, and emmetropia as on prevention of capsular rupture. Even more critically, there appears to be more emphasis on prevention of posterior dislocation of lens material than on iatrogenic retinal detachment. There is no credible evidence that posterior dislocation of lens material causes any mechanical damage to the retina. This is even true for dense, “sharp,” nuclear fragments. Damage to the retina in these cases is caused by inappropriate approaches used to remove lens material and because of less than ideal vitrectomy methods. Clearly, retinal detachment is a more serious complication than posterior dislocation of lens material.

PREVENTION OF

CAPSULAR RUPTURE

Flow limiting using systems like the Alcon (Fort Worth, Tex) MicroFlare ABS as well as cortical cleaving hydrodissection decrease capsular rupture rates during phaco procedures. The new Alcon silicone-tipped I/A tools appear to significantly reduce capsular incursion during cortical cleanup.

TIMING OF INTERVENTION

Lens material may cause inflammation and/or phacolytic glaucoma if the material is left in place for an extended period. This is not an argument for instant or emergency surgery to remove dislocated lens material. Timing can be nearly as important as technique with respect to surgical outcomes. Posterior vitrectomy requires excellent visualization to produce optimal outcomes. Striate keratopathy, corneal edema, and pupillary miosis are often present intraoperatively and for 1 to 2 weeks after complicated cataract surgery. Waiting until the cornea is clearer and the pupil is able to be dilated is a better approach than chasing lens material around the vitreous cavity at the time of cataract surgery. Similarly, it is better to wait for a few days or even weeks until the cornea

Figure 5-1. Cellulose sponges to remove vitreous means substantial pulling on the vitreous, which can cause retinal breaks and subsequent a retinal detachment.

clears to remove posteriorly dislocated lens material. If severe inflammation or significant phacolytic glaucoma presents the surgeon should proceed with vitrectomy.

ANTERIOR VITRECTOMY

Cellulose sponge vitrectomy was a major contribution when introduced over 30 years ago but has been overused for over 2 decades. Cellulose sponges remove vitreous via a wicklike mechanism, which means that substantial pulling on the vitreous occurs even before it is lifted to cut it with the scissors (Figure 5-1). Many surgeons use the term “simple anterior vitrectomy,” but the reality is that anterior vitreous fibers are firmly attached to the peripheral retina at the vitreous base. The peripheral retina has 1/100 the tensile strength of the posterior retina. There is nothing simple about a procedure that can cause retinal breaks and subsequent retinal detachment.

It is a common misconception that vitrectomy causes inflammation when, in fact, eyes undergoing total pars plana vitrectomy have virtually no postoperative inflammation. The inflammation associated with anterior vitrectomy is primarily caused by mechanical trauma to the iris from the hydrated sponge as it is withdrawn through the pupil. Cellulose fibers are often seen on the anterior vitreous after sponge vitrectomy raising the question of inflammation from retained material.

The so-called Charles sleeve is an obsolete device for anterior vitrectomy because it causes turbulent flow because of proximity between inflow and outflow. Using a sideport for injection of air during vitrectomy avoids turbulent flow. A soft eye can lead to suprachoroidal hemorrhage; therefore a viscoelastic, BSS, or air must be injected. Air is the best infusate choice because the interface between vitreous and

Figure 5-2. Vitreous cutters should always be used with the highest cutting rates to reduce vitreoretinal traction. Low suction and advancement of the probe while cutting works best.

viscoelastics is invisible and BSS causes hydration of the vitreous. Injecting air through a second incision keeps the anterior vitreous back via surface tension and prevents vitreous strands from being incarcerated in the wound.

Sweeping the wound for vitreous with a spatula causes unacceptable vitreoretinal traction and should be avoided. The focus should be on retinal detachment prevention; not just vitreous in the wound. Mechanized anterior vitrectomy using air infusion through the sideport is very effective at eliminating vitreous from the wound.

Vitreous cutters should always be used with the highest possible cutting rates to reduce vitreoretinal traction. High cutting rates reduce pressure variation per port open-close cycle and prevent uncut vitreous from being pulled through the port (Figure 5-2). Higher cutting rates also limit sudden flow through the port like the Alcon Legacy MicroFlare ABS phaco tip. Flow limiting is crucial when using the vitreous cutter to remove lens material or scar tissue. When dense tissue elastically deforms through the port, it is followed by a flow surge that can cause retinal breaks. Using the lowest vacuum or flow rate that will effectively remove vitreous reduces vitreoretinal traction and the likelihood of retinal breaks and detachment. Moving the port away from vitreous while vacuum is applied also creates traction; it is better to advance the port toward the tissue anytime aspiration is applied.

REMOVAL OF

DISLOCATED LENS MATERIAL

The phacoemulsification probe should never be used to remove lens material anywhere in the vitreous body or to

Figure 5-3. A jet of BSS can be used to wash the lens fragment into the anterior segment but it may cause a retinal detachment.

Figure 5-5. All of the vitreous is removed except for the most peripheral vitreous before removing any lens material. An Accurus fragmenter is used to remove the nuclear material.

perform vitrectomy. Ultrasonic energy breaks up hyaluronic acid, giving the appearance of vitrectomy but it cannot safely cut vitreous collagen fibers.2

Some surgeons suggest using a jet of BSS to move the lens or nucleus into the anterior chamber when it becomes dislocated during cataract surgery. A jet of fluid can create experimental retinal detachments. Forceful irrigation of the vitreous cavity causes vitreoretinal traction even if the jet of fluid does not penetrate the retina (Figure 5-3).

Lens loops have been used by some surgeons to “fish” the nucleus or lens out of the vitreous. It can be readily seen that IOL haptics cause significant vitreoretinal traction when an attempt is made to reposition dislocated IOLs making it obvious that a lens loop will do the same (Figure 5-4).

Figure 5-4. A lens loop can be used to “fish” the nucleus out of the vitreous but it causes significant vitreoretinal detachment.

Pars plana vitrectomy is the safest and most efficient method to remove dislocated lens material. It is essential to use the best possible visualization for this procedure. A fundus contact lens (hand-held or sew-on) or a wide angle system is absolutely necessary; an operating microscope can only focus to a point just behind the lens without an additional optical system. An indirect ophthalmoscope leaves only 1 hand free; therefore, modern day 3-port vitrectomy technique cannot be used. In addition, an inverted view is very difficult to learn which may lead to retinal damage. A fiberoptic endoilluminator is essential as it provides illumination without glare from light scattered and reflected light by the cornea and IOL.

It is essential to remove all except the most peripheral vitreous before removing any lens material. Trying to use vitreous to “support” lens material is unwise and increases the likelihood of vitreoretinal traction. It is better to allow lens material to fall posteriorly as the vitreous is removed. The Alcon (Fort Worth, Tex) Accurus fragmenter uses the same ultrasonics as the Legacy and is ideal to remove nuclear material after vitreous removal (Figure 5-5). Twenty-gauge fragmenters are best used with continuous aspiration to cool the needle, which is far more thermally efficient than trying to irrigate the needle externally. Continuous sonification coupled with continuous, proportionally controlled ultrasound energy is the most efficient and safe approach for removing lens material. A vacuum-only technique is used to pick up the lens material and then fragmenter power is applied and increased until lens material begins entering the port. If the fragmenter needle penetrates the lens, the endoilluminator can be used to push the lens material off the needle. This technique resembles 2-handed phacoemulsification. Alternatively, the endoilluminator can be used to crush a “speared” lens fragment in order to split the fragment or push it in the fragmenter port. Some surgeons have advocat-

Figure 5-6. Perfluorocarbon liquid is used to float the lens material away from the retina so that the lens can be removed through the cataract wound.

ed using a phacoemulsification probe in the vitreous cavity but this requires a larger wound, causes greater turbulence, and has no advantages over the fragmenter.3

Perfluorocarbon liquids can be used to float lens material away from the retina but this step is unnecessary in most cases. A very dense nucleus can be floated up to the anterior chamber and removed through a cataract wound with a lens loop, cohesive viscoelastic, or vectis (Figure 5-6).

SUBLUXATED LENSES

In most instances it is safer to remove all vitreous except that in close proximity to the vitreous base before removing a subluxated lens. Often the remaining zonules will create a hinge, causing the lens to swing down against the peripheral

CONCLUSION

The goal of this chapter is to emphasize safe vitrectomy techniques and methods to remove lens material that becomes dislocated posteriorly during cataract surgery. Retinal detachment prevention should be the focus of these efforts rather than rapid removal of lens material. The time is now to eliminate unsafe, cellulose sponge vitrectomy in this world of high-tech, sophisticated small incision cataract surgery.

REFERENCES

1.Al-Khaier A, Wong D. Determinants of visual outcome after pars plana vitrectomy for posteriorly dislocated lens fragments in phacoemulsification. J Cataract Refract Surg. 2001;27(8):1199-206.

2.Hutton WL, Snyder WB. Management of surgically dislocated intravitreal lens fragments by pars plana vitrectomy. Ophthalmology. 1978;85(2):176-89.

3.Mittra RA, Connor TB. Removal of dislocated intraocular lenses using pars plana vitrectomy with placement of an openloop, flexible anterior chamber lens. Ophthalmology. 1998;105(6):1011-4.

C

C

ITH

INTRODUCTION

As anterior segment ophthalmic surgeons, we have come to realize—as we exit the 20th century and enter the 21st century (and a new millennium)—that any surgery we perform on the anterior segment will now be judged by doctor and patient alike according to the refractive outcome. Be it corneal, lenticular, or glaucoma surgery, we now include in our surgical planning the refractive consequences of our procedures and the refractive options available to us. Thus, a glaucoma surgeon who performs combined phacotrabeculectomy through a superior incision and consistently induces against-the-rule cylinder change may now consider the options of LRIs, astigmatic keratotomy, or toric IOL with these procedures. Increasingly, cataract and refractive surgeons are having to remove lenses in eyes that have had previous keratorefractive surgery. Not only is implant power calculation technology evolving for these eyes, but correction of residual or induced astigmatism must now be considered, as well as surgical planning so as not to induce new astigmatism in an eye that was previously made emmetropic by a keratorefractive procedure. The only area of concern that may require secondary (or tertiary) surgical intervention is that of induced irregular astigmatism. Jorge Alio reported an incidence of up to 5% of induced irregular astigmatism following LASIK1; other studies have reported up to 13% inci-

dence. These eyes may come to wavefront-guided custom corneal ablation following a phaco-IOL procedure. In this chapter, I will address methods and techniques for achieving astigmatism reduction as it relates to lens replacement surgery, either cataract or refractive clear lens exchange.

ASTIGMATISM

Astigmatism may be classified as either corneal or lenticular. Posterior (scleral-choroidal-retinal) astigmatism has never been classified, but may be a consideration in the future. An eye with a spherical cornea that demonstrates refractive cylinder has been believed to have “lenticular astigmatism,” but is never known for certain until the lens is removed and replaced with a spherical IOL. Eyes with posterior staphylomata or retinal elevation from some pathologic process may have posterior astigmatism, which may or may not be refractively correctable by an anterior technique. This form of astigmatism still remains elusive to diagnosis; however, it may be suspected in eyes with spherical corners and spherical IOLs if demonstrated by wavefront technology. Custom laser keratorefractive surgery may be able to neutralize these astigmatic aberrations.

Lenticular astigmatism may, therefore, be defined as refractive astigmatism with a spherical cornea and without

Figure 6-1. A topographic example of irregular corneal astigmatism. (Courtesy of EyeSys Vision, Inc.)

Figure 6-3. A topographic example of asymmetric corneal astigmatism. (Courtesy of EyeSys Vision, Inc.)

posterior pathology. In this situation, simple lens removal and replacement with a spherical IOL, assuming an astigmatically neutral incision, will correct the pre-existing astigmatism. Corneal astigmatism is further classified as regular versus irregular (Figure 6-1); and regular astigmatism is further subclassified as symmetric (Figure 6-2) versus asymmetric (Figure 6-3) and orthogonal versus nonorthogonal (Figure 6-4). These classifications may only become apparent following corneal topography. Keratometry alone, usually measuring the central 3 mm of the corneal apex, cannot detect or demonstrate asymmetric or nonorthogonal astigmatism, and this can affect the refractive outcome of anterior segment surgery. Therefore, if corneal modulation is to be included in anterior segment surgical planning, corneal topography must be available to the surgeon. Newer technology, such as Orbscan (Bausch & Lomb, Rochester, NY) evaluation of corneal thickness and the posterior corneal surface can provide even more valuable information. The value of wavefront aberrometry, in addition, may provide even more detailed information, which, regarding lenticular surgery, is just beginning to be studied.

Figure 6-2. A topographic example of symmetric corneal astigmatism. (Courtesy of EyeSys Vision, Inc.)

Figure 6-4. A topographic example of non-orthogonal corneal astigmatism. (Courtesy of EyeSys Vision, Inc.)

INCISIONAL CORRECTION

OF ASTIGMATISM

Astigmatic keratotomy (AK) was first successfully performed in Moscow by Svyatoslav Fyodorov2 (Figure 6-5) in the 1970s, originally combined with radial keratotomy for the correction of myopia (Figure 6-6). AK was further developed in the United States in the 1980s as straight tangential incisions, T cuts, by Spencer Thornton (Figure 6-7) and as arcuate incisions by Thornton,3 Lindstrom,4,5 and others6,7 (Figure 6-8). The CRIs were then moved to the limbus8 in the 1990’s, and nomograms were developed for these by Gills (Figure 6-9) and Nichamin9 (Figure 6-10). An elaborate system of AK was developed by Oliveira of Brazil10 to reduce the incidence of a phenomenon he called keratopyramis (Figure 6-11) and attempt to produce a more spherical cornea.

When planning incisional correction of astigmatism, the surgeon must determine how many incisions are necessary, at what axis (or axes), at what optical zone(s) (distance from