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Refractive Surgery
 
Introduction

The term "refractive surgery" has come to be primarily associated by the public with one particular type of refractive surgery, that is Lasik eye surgery. I think we can understand how that would happen, given the flood of advertising that has accompanied this procedure. For our purposes, there are many types of refractive surgery, from radial keratotomy to the more recent advances in multi-focal intra-ocular lens implants. After a short history of the industry, we will examine each type.

Refractive surgery can be better understood with some basic knowledge of the optical properties of the eye. For more information, read Module 16. Refractive surgery is not usually performed on individuals under the age of 18. After age 18, the eye has, for the most part, stopped growing and the optics should be stable.

A Brief History of Refractive Surgery

The concept of refractive surgery is not new. As early as 1898 a Dutch ophthalmologist proposed how it might work. In the 1930"s a Japanese doctor named Sato performed some corneal incisions to alter the refractive properties of the eye. In the 1970"s a Russian doctor named Fyodorov began the practice of radial keratotomy after observing the refractive changes in one of his patients recovering from corneal lacerations. Russian ophthalmologists improved the corneal incision technique and proved that RK could be effective, relatively safe, and fairly predictable. Refractive surgery was off to the races... well, sort of. In the U.S., RK ran into the FDA, but the first procedure in the U.S. was performed in 1978.


Radial Keratotomy (RK)

Since 1978, over 2 million persons have had the RK procedure in the U.S., with a peak in the mid-90s. RK has since been displaced by PRK and Lasik procedures.

Radial keratotomy involves making a number of incisions in the cornea with a knife blade. The incisions were arranged in a radial, or spoke-like fashion, and the blade cut to a depth that was 90% of the corneal thickness.

As the cornea healed, the shape of the cornea would change in a predictable fashion, reducing the myopic correction. The number of incisions and depth of the incisions were varied according to formulae based upon the refractive properties of the eyes. Instruments were developed to precisely measure the thickness of the cornea, to gauge the depth of the incisions, and to improve the quality of the cut.

The PERK (Prospective Evaluation of Radial Keratotomy) Study published in 1994 showed RK to be an effective procedure for myopes ranging from -2.00 to -8.75 diopters, but with a significant number of eyes changing in the far-sighted direction up to 10 years after surgery:
  • 70% of patients did not require correction for distance vision after surgery.
  • 85% of patients achieved uncorrected vision of 20/40 or better.
  • 53% of patients achieved uncorrected vision of 20/20 or better.
  • 43% of eyes continued to change in a far-sighted direction after surgery.
A follow-up study with the PERK patients showed that diurnal (morning-to-evening) fluctuations in vision were a permanent side effect for a significant number of RK patients.
RK generally was performed on myopes up to -7.00 diopters, with astigmatism up to 4 D. RK procedure:
  • One eye was operated on at a time, generally with the none dominant eye done first. The other eye could be done one week later if all was well with the first procedure.
  • A mild sedative was given with topical anesthesia of the eye.
  • A lid speculum was applied and micro-slits were made in the cornea in a predetermined pattern.
  • Antibiotic and cycloplegic drops were instilled after the procedure. A patch was applied for 2 hours after the surgery. Antibiotic drops were used for 2 weeks after the surgery.
  • The patient could expect mild irritation to mild pain (treatable with over-the-counter pain relievers) for 24 hours.
  • The patient could expect good vision within one week post-op. Morning-to-evening vision fluctuation was common in the first 6 months post-op. Glare at night was a common complaint in the first 6 months.
  • About 1/3 of the eyes treated required at least one "enhancement" procedure to improve the visual outcome.
Photorefractive Keratectomy (PRK)

The PRK procedure was first performed in Germany in 1988. By 1994 over 1 million procedures had been performed, with the number of procedures rising greatly every year until the Lasik procedure arrived.

The PRK procedure uses an excimer laser to remove tissue from the cornea to change the shape and refractive properties of the cornea. The excimer is a non-thermal "cold beam" laser that breaks the carbon bonds at the molecular level, causing tissue ablation (vaporization). PRK can be used to treat myopia, hyperopia, and astigmatism. PRK procedure: The following describes the basic procedure. Modifications continue to be tried/studied to improve the procedure. Soft contact lenses must be discontinued at least two weeks prior to the procedure. Hard contact lenses must be discontinued at least 4 weeks prior to the procedure. The contact lens wearer must have a stable refraction prior to the procedure. After the initial evaluation, the contact lens patient is asked to return in 2 to 3 weeks for a repeat refraction. When the refraction and corneal topography on two consecutive visits are stable, the eye is ready for the procedure.
  • A cycloplegic refraction is performed to confirm manifest results.
  • Corneal topography is performed to rule out keratoconus, irregular astigmatism, and contact lens induced corneal warpage.
  • Only one eye can be done at a time because the patient will have blurry vision up to 2 to 3 weeks after the procedure.
  • Prior to the procedure, a topical anesthetic is instilled and a speculum is placed on the lids.
  • The corneal epithelial layer is removed.
  • The computer driven excimer laser reshaping of the cornea takes less than 1 minute and is aided by eye tracking on some instruments.
  • Immediately after the procedure, antibiotic and anti-inflammatory drops are instilled into the eye, and a bandage contact lens is placed on the eye to aid healing of the corneal epithelium.
  • The patient can expect mild to moderate discomfort that can be treated with over-the-counter pain meds.
  • The bandage contact lens is removed in 2 to 3 days. Vision gradually improves, but maximum acuity may not be achieved until 6 weeks to 6 months after surgery. PRK and Lasik visual acuity results are roughly equivalent. See the discussion below of Lasik acuity expectations.
  • Potential risks include post-operative glare at night, and rarely, an infection of the cornea.
Laser Assisted In-situ Keratomileusis (Lasik)

The Lasik procedure was FDA approved in 1996 for myopia up to -6.00 D. In 1997 the FDA approved Lasik treatment for up to 4.00 D of corneal astigmatism. Nearly 1 million people had Lasik in the U.S. in 1999 with the number per year increasing in subsequent years. Lasik can now also be used to treat hyperopia.

This procedure is similar to PRK in that the excimer laser is used to remove corneal tissue to reshape the cornea. The important difference is that the Lasik procedure uses a microkeratome instrument to remove a flap of the cornea before the laser is used. After laser ablation is performed, the flap of corneal tissue is replaced. The corneal surface is thus instantly restored to a normal refractive surface, unlike the PRK procedure which requires the regeneration of the removed corneal epithelial layer. With the Lasik procedure, good vision is achieved almost instantly and both eyes can have the procedure on the same day.

Lasik procedure: The following describes the basic procedure. Modifications continue to be tried/studied to improve the procedure. Soft contact lenses must be discontinued at least 2 weeks prior to surgery. Hard contact lenses must be discontinued at least 4 weeks prior to surgery. The contact lens wearer must have a stable refraction prior to the procedure. After the initial evaluation, the contact lens patient may be asked to return in 2 to 3 weeks for a repeat refraction and corneal topography. When the refraction and corneal topography on two consecutive visits are stable, the eye is ready for the procedure.
  • A cycloplegic refraction is performed to confirm manifest results.
  • Wavescan testing is also performed to confirm manifest results.
  • Corneal topography is performed to rule out keratoconus, irregular astigmatism, and contact lens induced warpage of the cornea.
  • Both eyes can have the procedure on the same day, in most cases.
  • Prior to the procedure, a topical anesthetic is instilled and a speculum is placed in the lids. The photos that follow are from a video of an actual Lasik procedure.
  • The cornea is "stamped" with a temporary gentian violet ink, making several circles that overlap. These markings will help make sure the corneal flap is placed back in exactly the original position.
  • The microkeratome suction ring is placed on the cornea. The blade device will fit inside the microkeratome that the surgeon is holding.
  • The blade device rotates on the post and the cornea is thinly sliced.
  • The microkeratome is removed and forceps are inserted underneath the flap to lift is aside. Notice the corneal hinge present.
  • The forceps are used to fold the corneal flap away, over the hinge.
  • The computer driven excimer laser reshaping of the cornea takes less than 1 minute and is aided by eye tracking on most devices. You can see the centering red mires in the photo. A bluish vapor rises from the cornea as the tissue undergoes photoablative disruption. This reshaping is much like sandblasting your deck clean.
  • Immediately after the procedure, the flap is repositioned back onto the central corneal surface, in exactly the original position.
  • The patient uses antibiotic and anti-inflammatory drops for about the first week, sometimes longer. An eye shield is used at night for a week or two, and the patient is instructed to avoid rubbing the eye. There is little to no discomfort during the procedure or afterwards. Artificial tears are used copiously for weeks or months.
  • Most patients can expect to see well enough to drive the next day, unless there is discomfort or light sensitivity. A study completed in 1999 showed that about 90% of eyes had 20/40 or better vision after 3 months and 50% of eyes had 20/20 or better vision after 6 months. This study included patients with nearsightedness greater than 7 diopters. Generally, better results are achieved if nearsightedness is less than 7 diopters and astigmatism is less than 4 diopters. A latter study showed that about 67% of eyes achieved 20/20 or better vision if the pre-op myopia was 6 diopters or less.
  • Potential risks include post-operative glare at night, and rarely, an eye infection. Dry eye symptoms are common after Lasik procedures. For a more complete discussion of potential risks, follow this link to the FDA webpage on Lasik risks. The link will open in a new window. Close the window to return to this page.
PRK vs Lasik

The advantages of Lasik over PRK are obvious. Both eyes can be done the same day and visual recovery is much faster with Lasik. That is why many more people have the Lasik procedure than PRK. Why would anyone want to have the PRK procedure? The patient might be a candidate for PRK if the cornea is too thin, too steep, or too flat to safely have Lasik. Lasik requires an additional surgical procedure (the flap) compared to PRK. Although the risks are low, there are still additional risks associated with the flap. Some folks like to reduce risks if possible.

Laser Assisted Sub-Epithelial Keratomileusis (Lasek) and Epi-Lasik

The Lasek technique was introduced by Italian Massimo Camellin, MD, in 1999. The technique lies somewhere between PRK and Lasik. Instead of creating a corneal flap, the corneal epithelium is weakened with an alcohol solution and then the epithelium is lifted and pulled back as an "epithelial flap". The laser treatment is applied over the corneal surface and the thin epithelial flap is replaced. As with PRK, the patient has to wear a bandage contact lens after treatment until the surface is fully healed. This can take up to a week in some instances. Recovery is similar to PRK, but is still significantly slower than with Lasik, which is very rapid by comparison. It is possible for the epithelial flap to be "lost" during the procedure, in which case the procedure turns into a conventional PRK procedure. The advantage of Lasek or PRK over Lasik would be the elimination of the possibility of flap complications and the retention of more untouched corneal thickness.

A modification of Lasek is Epi-Lasik, by which a microkeratome with a blunt blade is used to create the epithelial flap, instead of epithelial removal by alcohol.

Thin-Flap Lasik or Sub-Bowman Keratomileusis (SBK)

As the name suggests, the flap is thin as compared to the regular Lasik procedure. The flap is 60-70 microns thick and runs just under Bowman"s membrane. This procedure falls somewhere between Lasik and Lasek, but is performed like Lasik.

Intralase Lasik (LaserLasik)

Intralase Lasik is like conventional Lasik except that the corneal flap is created using a laser (made by Intralase) instead of being created mechanically with the microkeratome. The idea is that the flap should be more precise in position and thickness, thus reducing possible complications. Some patients have experienced a shorter period of corneal swelling after the Intralase Lasik procedure. The inherent cost of purchasing and utilizing the Intralase is what prohibits most surgeons and patients from using this technology.

Wavefront Guided or "Custom" Corneal Refractive Surgery

Wavefront technology is capable of measuring "high order" optical aberrations (optical imperfections) in the human eye. This information can be used by some refractive lasers to "customize" the laser treatment of the cornea using any of the conventional methods (e.g. PRK, Lasik). The common refractometry methods used in the typical eye exam only measure "lower order" optical aberrations. These are the sphere power needed to correct myopia and hyperopia, and the cylinder power and axis needed to correct for corneal astigmatism. Imagine being able to project an optical measuring grid into the eye, then being able to analyze the shape of that grid as it is reflected back through the optical structures of the eye. If there were no optical aberrations, the grid would have the same regular, symmetrical shape coming out of the eye as it had going into the eye. The grid has a distorted shape coming out, even for those eyes that are "plano". If the eye has a refractive error, spherical and cylindrical correction can be made to reduce the distorted shape of the grid, but some distortions commonly still remain. These are higher order aberrations.

Higher order aberrations can be analyzed mathematically with calculations called "Zernike polynomials". The aberrations have been given names like "spherical aberration", "coma", and "trefoil", to name a few. These three have more visual significance than some of the other aberrations.

It has been found that all refractive procedures that alter the cornea increase the baseline levels of higher order aberrations above what was naturally occurring before the procedure. The trick is that wavefront guided procedures increase the aberrations less than non-wavefront guided procedures. The result should be, and generally is, that the post-procedure vision is improved with wavefront guided corneal ablation. Eyes having cataracts cannot have wavefront mapping sucessfully performed.

This does not mean that every eye would benefit from wavefront guided refractive surgery. A pre-surgery wavefront examination may reveal that the eye would not significantly benefit from the wavefront guided procedure enough to justify the additional cost involved. The wavefront exam can also give the doctor information that would indicate that the patient would not benefit from corneal refractive surgery of any kind.


Intacs Corneal Implants

Intacs are small polymethylmethacrylate (PMMA) ring segments that are surgically implanted into the cornea. The rings change the shape of the cornea, thus changing the refractive properties of the cornea and reducing mild nearsightedness. They do not improve hyperopia or astigmatism. The advantage is that the procedure is reversible; the rings can be removed. The rings can also be replaced to adjust the correction if needed. A disadvantage is that the implants can only correct nearsightedness up to -3.00 diopters with astigmatism less than 1 diopter. However, more than 50% of myopes fall into this category.


Intacs procedure:
  • Soft contact lenses must be discontinued at least two weeks prior to the procedure. Hard contact lenses must be discontinued at least 4 weeks prior to the procedure. The contact lens wearer must have a stable refraction prior to the procedure. After the initial evaluation, the contact lens patient is asked to return in 2 to 3 weeks for a repeat refraction. When the refraction and corneal topography on two consecutive visits are stable, the eye is ready for the procedure.
  • A cycloplegic refraction is performed to confirm manifest results.
  • Corneal topography is performed to rule out keratoconus, irregular astigmatism, and contact lens induced corneal warpage.
  • Both eyes can have the procedure on the same day. The procedure takes about 20 minutes per eye.
  • Prior to the procedure, a topical anesthetic is instilled and a speculum is placed on the lids.
  • A centering guide is placed on the cornea for one to two minutes, during which two small tunnels are made in the corneal stroma.
  • The Intacs implants are placed inside the tunnels and the entry site is often times closed, possibly with a stitch that is removed later.
  • There may be some mild discomfort the day of surgery. Antibiotic and anti-inflammatory drops are used for about 2 weeks after the procedure.
  • In one study, 80% of patients had 20/40 or better vision the day after surgery and 74% had 20/20 or better vision a year after the procedure.
Conductive Keratoplasty (CK)

Conductive Keratoplasty is a technique that uses radio waves to change the shape of the cornea. In 2002, CK was approved for the treatment of hyperopia in the range of +0.75 D to +3.25 D, with astigmatism < 0.75 D. In 2004, CK was approved for the treatment of presbyopia, with an "add power" (myopia) of 1 to 2 D in the non-dominant eye. The technique produces a peripheral band of collagen shrinkage that serves to steepen the central cornea.

Treatment is applied with a pencil-like probe to points on the cornea as marked on the image at the right. Eight to thirty-two treatment spots can be applied. The more treatment spots, the greater the effect. A "light touch" technique is preferred by the surgeon.

The Eye Foundation of Kansas City recently did a retrospective study of 63 patients treated on the non-dominant eye for presbyopia, with the following results:
  • 78% of patients were within 0.50 D of the intended goal after treatment.
  • 21% experienced an induced astigmatism > 0.75 D.
  • Patients lost an average of 3 lines of distance vision in the treated eye.
  • All patients retained binocular distance vision (without correction) of 20/30 or better.
  • No complication occurred during or after the procedures
  • There was a 34% regression in the first 3 months post-procedure and a 28% regression from months 3 to 12. This means that the treatment lost effectiveness over time. For example, the eye might go from a -1.75 power at one month to a -1.00 power at 12 months post-treatment.
We can conclude from this study that CK is a safe, effective treatment for presbyopia in patients with low refractive errors, but the treatment loses effectiveness over time and retreatment may be necessary to regain reading ability. There is also the possibility of inducing unwanted corneal astigmatism postoperatively. A study from another eye center looked at using the CK procedure in the non-dominant eye of presbyopic patients who had previously had Lasik surgery. After 6 months, 87% had retained J3 or better near vision and 91% of the participants were "very satisfied" with the results.


Limbal Relaxing Incisions (LRI)

Limbal Relaxing Incisions are a cousin of astigmatic keratotomy, which are procedures used to reduce corneal astigmatism. Limbal relaxing incisions are cuts placed in the cornea at the limbal junction. One to two incisions are placed at opposite sides of the cornea at positions determined by the meridian of the corneal astigmatism. Thus the shape of the cornea is only altered along that meridian and astigmatism is reduced. LRIs can be used alone or in conjunction with other procedures. They are most often used to reduce pre-operative astigmatism in conjunction with cataract surgery. The procedure is very safe and it does not produce the glare that central cornea treatments can cause.


Phakic IOL

A phakic IOL is essentially a contact lens that is implanted inside the eye instead of being fit to the outside of the eye. The natural lens of the patient is not removed from eye, therefore the eye retains it"s natural ability to accommodate. One type of phakic IOL fits in front of the iris. The other type fits behind the iris, but in front of the natural lens. The lenses come in powers from -3.00 or -5.00 diopters up to -20.00 diopters. As you might guess, they are particularly suited to treating a patient with a high degree of myopia. This is not a good option, in general, for the treatment of hyperopia. Because this is an intra-ocular procedure, there are greater potential risks, particularly of developing acute glaucoma, retinal detachment, cataract, and corneal endothelial damage.


Clear Lens Replacement or Refractive Lens Exchange (RLE)

Clear Lens Replacement is essentially cataract surgery on someone who does not have a cataract. Cataract surgery is a type of intraocular refractive surgery. The natural lens of the eye is removed and it is replaced by a synthetic intraocular lens implant. The intraocular lens (IOL) comes in a variety of powers that can be pre-calculated to adjust for the pre-surgical refractive error of the eye. Advances in axial length measurements and refinements in IOL power calculation formulas have made it possible to accurately predict the post-operative refractive error. The downside is that with the removal of the natural lens, the eye can no longer accommodate. This factor may make this procedure only practical for the person who is over 40 years old. After surgery, the patient would have to wear reading glasses, or she could be adjusted for monovision (one eye for reading, the other for distance). Another possibility would be to have one of the newer multi-focal IOLs implanted (discussed in another section), thus allowing distance and near vision capabilities. Another disadvantage of this procedure is that it does not correct for astigmatism, although limbal relaxing incisions, Lasik, or PRK could be used in conjunction with RLE.

This procedure might be an option for the patient who is not a candidate for other types of refractive surgery because of a thin cornea or very high myopia.

This is an intra-ocular procedure, so there are additional risks (e.g. endophthalmitis, retinal detachment) as compared to traditional refractive surgery.


Aspheric Intra-ocular Lenses

The cornea is not spherical, it is aspherical (or "prolate"), meaning it has a steep curvature centrally that flattens out toward the edge. Asphericity causes spherical aberrations, which can cause distorted vision if not neutralized. If a spherical intra-ocular lens is implanted after cataract surgery, then additional spherical aberrations can occur, possibly resulting in decreased contrast sensitivity and reduced visual acuity.

Aspheric IOLs are now being made to counteract spherical aberrations. The AcrySof (Alcon) and Tecnis (AMO Inc.) IOLs have "negative" spherical aberrations, meaning they can neutralize the "positive" spherical aberrations of the cornea. However, there seems to be an optical "trade-off" when eliminating positive spherical aberration; that is the depth-of-focus gets shallower. The SofPort AO (Bausch & Lomb) aspheric IOL is "aberration free" and is not a "negative" lens in terms of spherical aberration. This means that this lens would not add to any spherical aberrations, but the eye"s "natural" aberration could be maintained and depth-of-focus would not be reduced. It is also possible that the pre-operative spherical aberrations of the cornea could be "negative", perhaps after hyperopic laser surgery, or Conductive Keratoplasty. In this case, the eye may benefit from the "positive" spherical aberrations produced by the standard spherical IOL.

As you see, this business can get very complicated. A corneal topography measurement can be performed to arrive at a measure of corneal asphericity (the "Q value"). With this information, and with advanced formulas, the doctor can theoretically choose an intra-ocular lens with a design that will optimize the patient"s vision. This is one step closer to being able to design a "custom" intra-ocular lens for each eye.

Aside from the depth-of-focus issue, there are some other potential negatives with an aspheric lens design:
  • The lenses have to be placed without significant tilt and must be well centered to work effectively. There is significant potential for glare and halos around point sources of light at night. This seems to be reduced if the lenses are implanted in both eyes, and over time.
  • These lenses may be most beneficial to younger patients because changes in the aging retina may reduce any benefit from the increased visual contrast provided by the aspheric design.

Multi-focal Intra-ocular Lenses

The standard monofocal IOL has a single focal point. If the patient is set post-operatively for good distance vision with a standard IOL, then readers are needed for near vision, and visa-versa. An accommodative IOL, or a multi-focal IOL produces at least 2 focal points, which function similarly to the accommodative property of the natural lens. In other words, with this type of lens it is possible for the patient to have both distance and near vision without having to use glasses. As of the writing of this Module, there is one FDA approved accommodative IOL, and there are two FDA approved multi-focal IOLs, each with a different design:

Eyeonics Crystalens: This is an accommodating IOL that shifts position in the bag to alter the point of focus.
Alcon Restor: This multi-focal lens has a central diffractive zone that improves near vision when the pupil gets smaller during the synkinetic near response. This lens works best for distance/near (reading distance).
AMO Rezoom: This multi-focal lens has 5 alternating distance and near refractive zones. This lens works best for distance/intermediate (computer distance).
We see from the table above that near vision is improved with a multi-focal lens, and that the chances of not having to wear glasses at all are significantly improved over a standard mono-focal IOL.

 

 

ReZoom Multifocal Lens

 

 

Results of a more recent study demonstrated that patients with a Rezoom in one eye and a Restor in the other had better intermediate and near vision than patients with the same lens in both eyes.
On the horizon are new intra-ocular lens designs that will add to the options for refractive/cataract surgeons.

  • The AMO Tecnis Rezoom is an aspheric, foldable, diffractive multi-focal design.
  • The Synchrony is a dual-optic, accommodative multi-focal IOL. (pictured above)
  • A "deformable" IOL can be injected into the eye and then form the shape of the lens when warmed to body temperature. The lens material can change shape with ciliary body contractions (accommodation).
  • The "light adjustable" intra-ocular lens is designed to be power adjustable after implantation with exposure to a specific wavelength of light.
  • Cataract Surgery and Clear Lens Replacement/ refractive procedure combinations There are several options/techniques now available for the cataract/refractive surgeon when treating the cataract patient or the presbyope having Clear Lens Replacement:
  • Multi-focal IOLs are available to eliminate the need for reading glasses after surgery.
  • The surgical incision can be placed on steep axis so that post-op astigmatism is reduced.
  • Limbal Relaxing Incisions can be used at the time of surgery, pre-operatively, or post-operatively to reduce astigmatism.
  • A Lasik or PRK procedure can be performed after the IOL procedure to adjust the refractive result, once stabilization has occurred following phaco
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