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Excimer Laser Vs. Phakic IOLs for Correction of Refractive Errors


Excimer Laser Vs. Phakic IOLs for Correction of Refractive Errors

Wouldn’t it be just lovely to be able to look at photos whenever we feel like it without having to trouble ourselves with glasses and contact lenses? Imagine attending a wedding and finally being able to see the bride and groom clearly and to have something to say the following day regarding her makeup and hair style!

The original problem can have a number of names ranging from myopia to hyperopia or astigmatism; all of which fall under the same category, refractive errors.


Necessary definitions:

Myopia:  Commonly known as nearsightedness, is a refractive defect of the eye in which the light produces an image focus that lies in front of the retina when the accommodation is relaxed.  It is usually corrected through corrective lenses either in the form of glasses or contact lenses, or refractive surgery.

Hyperopia:  Commonly known  as farsightedness, long-sightedness or hypermetropia, is a defect of vision caused by an imperfection in the eye (often when the eyeball is too short to a degree that every effort made in order to make the lens more round often  results in symptoms of eye fatigue and discomfort), causing difficulty focusing that is most pronounced when reading or trying to focus on near objects, and in extreme cases causing inability to focus on objects at any distance.  Simply, the image focus falls behind the retina instead of falling on the retina for clear vision, resulting in blurred images when the eye tries to see, mainly, a near object. It is corrected either by convex lenses in the form of glasses or contact lenses or by refractive surgery.

Presbyopia: Often confused by hyperopia, presbyopia is the condition where there is progressive disability focusing on near objects with age, very much like gray hair and wrinkles. The mechanism is not clear yet, although several hypotheses have  been postulated suggesting loss of elasticity of the crystalline lens along with loss of power of the ciliary muscles and changes in the lens’ curvature due to continuous growth.

Astigmatism: A visual defect in which the optical system of the eye is unable to focus a point object into a sharp focused image on the retina, resulting in blurred vision.
Astigmatism causes difficulties in seeing fine detail, and some patients complain that straight lines appear to be tilted and/or  out of focus. The astigmatic optics of the human eye can often be corrected by spectacles, hard contact lenses or contact lenses that have a compensating optic, cylindrical lens (i.e. a lens that has different radii of curvature in different planes), or refractive surgery.

Refractive surgery: Any surgery performed on the eye aiming to improve the refractive state of the eye, decrease dependency on glasses or contact lenses. The most common methods today use excimer lasers to reshape curvature of the cornea. Successful refractive eye surgery can reduce or correct  common vision disorders such as myopia, hyperopia and astigmatism, as previously mentioned.

According to surveys of members of the American Society of Cataract and Refractive Surgery, approximately 948,266 refractive surgery procedures were performed in the United States during 2004 and 928,737 in 2005.


Historical Background:


The Story of Refractive Surgery:

  • The first experimental study regarding refractive surgery was published in 1896 by Lendeer Jans Laus, a Dutch teacher of ophthalmology, who proposed the idea of performing penetrating corneal cuts to correct astigmatism.
  • In 1930 a Japanese ophthalmologist by the name of Tsutomo Sato carried out the first practical attempt of refractive surgery on military pilots. His theory consisted of performing internal radial cuts in the cornea hoping to correct defects up to 6 diopters. However, his work was later rejected due to the high rate of corneal degeneration.
  • In 1963 Jose Ignacio Barraquer in Colombia developed keratomileusis (cutting a circular flap of the cornea) allowing correction of both myopia and hyperopia.
  • Dr. Swinger further improved Barraquer’s technique in 1989.


The Story of Excimer Laser:

  • The early experiments in 1970 using xenon dimer and in 1975 using the noble gas halides resulted in the invention of a new type of laser called the excimer laser.
  • In 1980, R.Srinivasan, a scientist in IBM discovered that excimer laser is capable of cutting organic tissue with high accuracy without significant thermal damage while he was performing trials of using excimer laser to make microscopic circuits in microchips.
  • In 1983, Srinivasan teamed along with Stephen Trokel of Colombia University to perform the very first Photorefractive Keratectomy (PRK) in Germany.
  • On June 20th 1989, the first patent for LASIK was granted to Gholam A.Peyman,MD by the US Patent Office under the name of “METHOD FOR MODIFYING CORNEAL CURVATURE”, describing the surgical procedure that consist of cutting a flap in the cornea and pulling it back in order to expose the corneal bed which is in turn ablated to the desired shape using an excimer laser, then rolling back the flap.
  • In 1991, Creta University and the Vardinoynnion Eye coined the name LASIK ( LASer In situ Keratomileusis ).

 

Types of Refractive Surgery:

In general, refractive surgery can be divided into:

1- Corneal surgery: Which can be further subdivided into:

  • Flap procedures.
  • Surface procedures.
  • Corneal incision procedures.

2- Scleral surgery.

3- Lens related surgery: That in turn includes phakic intraocular lens implantation, clear lens extraction, photophacoreduction and photophacomodulation for correction of presbyopia.

Much of the attention these days is directed towards corneal procedures, which shall be highlighted in this study.


Flap procedures

Where Excimer laser ablation is done under a partial-thickness lamellar corneal flap.

  • Automated Lamellar Keratoplasty (ALK): With the aid of an instrument called a Microkeratome, a thin flap of the corneal tissue is cut and pulled back like a hinged door. Then targeted tissue is removed from the corneal stroma, again with the microkeratome, and then the flap is replaced.
  • Laser Assisted In-Situ Keratomileusis (LASIK): In which  a microkeratome or femtosecond laser  is used to cut a flap of the corneal tissue (usually with a thickness of 100-180 micrometres). The flap is  again lifted like a hinged door, the targeted tissue is removed from the corneal stroma with an excimer laser, ensuring to preserve at least 250 microns of corneal bed thickness. The flap is subsequently replaced.  An additional method of creating this flap is by using the Intralase procedure where a Femtosecond laser creates the flap. Much talk has been heard regarding this method’s superiority over the traditional lasik but to this date more conclusive independent studies are needed to prove this statement true.

Surface Procedures

Since the excimer laser is able to ablate the most anterior portion of the corneal stroma, these procedures do not require any cut however partial in thickness into the corneal stroma. These methods differ only in the way the surgeon handles the epithelial layer.

  • PhotoRefractive Keratectomy (PRK):  is an outpatient procedure generally performed with local anesthetic eye drops (as with LASIK/LASEK) . It reshapes the cornea by removing microscopic amounts of tissue from the corneal stroma, using a computer-controlled beam of light (excimer laser). The difference from LASIK is that the top layer of the epithelium is removed (and a bandage contact lens is used), in other words, no  flap has to be created. Recovery time is longer with PRK than with LASIK, though the final outcome (after 3 months) is about the same.
  • Laser Assisted Sub-Epithelium Keratomileusis (LASEK):  is a procedure that also changes the shape of the cornea using an excimer laser to ablate the tissue from the corneal stroma, while keeping the corneal epithelium intact so that it can serve as a natural bandage. The surgeon uses an alcohol solution to loosen then lift a thin layer of the epithelium with a trephine blade (usually with a thickness of 50 micrometres).  During the weeks following LASEK, the epithelium heals, leaving no permanent flap in the cornea, though the healing process can involve discomfort comparable to that with PRK.
  • EPI-LASIK:  a new technique similar to LASEK that uses an epi-keratome (rather than a trephine blade and alcohol), to remove the top layer of the epithelium (usually with thickness of 50 micrometres), which is subsequently replaced. Since it avoids the negative effects of alcohol, it can  provide better results than regular LASEK, and many have reported that the recovery involves less discomfort.  Refractive surgery in general may lead to the induction of important amounts of 3rd or higher order aberrations. The largest of which occurs for spherical aberrations. This is especially important when it comes to designing customized ablation algorithms supposed to cancel existing preoperative aberrations while avoiding generating new aberrations.
  • C-Ten (Customized TransEpithelial Non-contact ablation): It is the newest and the fastest Laser treatment, considered a refinement of Lasek, EPI-Lasik, and PRK. “C” for Customized refers to the individualization of the treatment for each patient, conforming to each individual’s requirements determined by the shape of the cornea and the topography of its surface, the extent of the correction, pupillary size and reaction, and the patient’s lifestyle requirements. “TEN” (Trans Epithelial, Non-Contact) means that the ablation of the epithelial layer, the regenerative surface of the eye, is accomplished with the laser alone, with no direct contact with the eye. This method of treatment is the only treatment technique done without actual contact with the eye. Before starting the procedure, bilateral eye examination is carried out using two instruments that have been specifically designed for laser treatments, called the Precisio and the Pupillometer. The “Precisio” measures the corneal topography, both its shape and thickness. The "Pupillometer” measures the size of the pupil under various light relationships. These measurements serve to ensure that the area undergoing treatment is neither too small (with the danger of ensuing halos or blinding) nor too large (which could cause ablation of excessive tissue). After treatment, the epithelium usually regenerates within a few days, under the protection of a contact lens. This method causes the least post-operative discomfort when compared with other laser methods. C-Ten also has a very low incidence of dry eye developing post treatment. But above all, the absence of any flap associated complications occurring after Lasik or even Femtosecond lasik is the major advantage. C-Ten isespecially suited to the treatment of myopia and irregular astigmatism. Up 12 diopters of myopia and over 6 diopters of corneal distortions can be corrected, while surface treatments are less suited for correction of hyperopia.

Corneal Incision Procedures: Are very rarely used nowadays, but for the fun of it we can have a look:

  • Radial Keratotomy (RK):  Uses spoke-shaped incisions (usually made with a diamond knife) and sparing the papillary area to alter the shape of the cornea and reduce myopia or astigmatism; this technique has now been largely replaced by the other methods (that use excimer laser).
  • Arcuate Keratotomy (AK): Similar to radial keratotomy, but the incisions on the cornea are done at the periphery of the cornea, used to correct astigmatism. AK is the only incisional procedure that still lives to this day since it can be used in special cases (Correcting residual astigmatism after keratoplasty or during cataract surgery).
  • Limbal Relaxing Incisions (LRI): In other words incisions placed near the outer edge of the iris, used to correct minor astigmatism (typically less than 2 diopters). This is often performed in conjunction with an Intraocular Lens implantation.


Candidates for LASIK treatment:

It has been mentioned earlier that the flap created for LASIK treatment is 100-180 microns in thickness, and that the corneal bed should be no less than 250 microns in thickness post treatment. Therefore:

Good LASIK candidates:

  1. Are over 18 years of age.
  2. Have stable prescriptions for glasses or contact lenses.
  3. Have healthy eyes with adequate corneal thickness.
  4. Suffer from a common refractive error (myopia, hyperopia, astigmatism).
  5. Understand the reality of LASIK treatment that provides the patient with the same visual results provided by prescription glasses or contact lenses.

Poor LASIK candidates:

  1. Have a long history of dry eyes.
  2. Are diagnosed with autoimmune eye disorders since the medications used in these cases can alter the healing process.
  3. Suffer from corneal abnormalities such as keratoconus or corneal scarring.
  4. Had an attack of ocular herpes within a year prior to having LASIK.
  5. Are pregnant or currently breast feeding.

 

Phakic Intraocular Lens:

Those who wish to correct their refractive errors but are not candidates for LASIK treatment are candidates for what is known as Phakic IOLs (intraocular lens).

This technology has been used around the world on thousands of patients and it has even been used in Europe 10 years prior to its approval in the United States in 2006. The approval in the States came after a huge FDA multicentral clinical trial.

The procedure can be simplified into placing a lens made of 100% pure collagen copolymer material compatible with the body's natural chemistry, behind the iris and in front of the natural crystalline lens. The material of which this lens is made behaves much like the natural lens in terms of light transmittance.

Candidates for this type of treatment are:

  • Under 45-50 years of age.
  • Have moderate to high myopia and healthy eyes.
  • Have a clear crystalline lens that remains in the eye after the surgery (in other words, they don't have cataract).

To achieve the best results possible with this technique, a thorough eye examination should be carried out along with full medical history and correct lens measurements prior to the procedure. Also, a YAG peripheral iridotomy has to be performed 1-2 weeks before the procedure to each eye.

The procedure is performed on an outpatient basis, but unlike laser vision correction nothing is removed from the eye. The procedure takes around 30 minutes with very little pain.

 

Studies citing the Phakic IOLs technique:

  1. A study carried out by the American Academy of Ophthalmology on November 2009 by Huang D. , Schallhorn and colleagues under the title "Phakic intraocular lens implantation for the correction of myopia, a report by the American Academy of Ophthalmology", stated that, "Phakic IOL implantation is effective for the treatment of myopia and myopic astigmatism. In cases of high myopia of -8 diopters or more, P IOLs may provide a better visual outcome than keratorefractive surgeries and better safety than refractive lens exchange".
  2. A study was published on February 2010 by Jose F. Alfonso,MD. PHD.  under  the title of "Surgical pearls for TORIC ICL implantation". A Toric ICL is a posterior chamber phakic IOL with a haptic design similar to the spherical ICL in terms of size, thickness and shape. The study concluded that a posterior chamber phakic IOL provides safe, effective, predictable and stable visual and refractive outcomes with minimal intra and post operative complications when used for both myopia and hyperopia correction.
  3. The latest study in this field was published on May 2010 under the patronage of Moorfields Eye Hospital NHS Foundation Trust, London, by Barsam A. and Allan BD. under the title "Excimer laser refractive surgery versus phakic intraocular lenses for the correction of moderate to high myopia". The results of this review stated that phakic IOLs are superior to excimer laser surgical correction for moderate to high myopia (-6 to -20 D), and that further randomized controlled studies are needed in order to precisely identify the ideal range of myopia for phakic IOLs.

 

Conclusion:

The medical literature is almost unified when it comes to the result of the ongoing duel between excimer laser treatment and phakic IOLs, in that Phakic IOLs is a far more superior when it comes to correction of refractive errors both in terms of post operative comfort and results.

The big question that poses itself at this point is that this technique up till now is very expensive compared to excimer laser, so who will have access to this form of treatment?

Large studies are still needed to further clarify the debate regarding the use of phakic IOLs for LASIK candidates.

 

لقراءة هذه المقالة باللغة العربية اضغط هنا


اضغط هنا للقراءة باللغة العربية

Prepared by: Dr. Zaina Habrawi


Source :

1-Kanski manual of Ophthalmology, 6th edition.
2-"Excimer laser refractive surgery versus phakic IOLs for the correction of moderate to high myopia" Barsam A. and Allan BD. Moorefields Eye Hospital NHS Foundation Trust,London.12 May 2010.
3-Eye and contact lens: science and clinical practice. January 2011 Volume 37.
4-"Ocular aberration before and after myopic corneal refractive surgery: LASIK induced changes measured with Laser Ray Tracing" Moreni-Barriuso, Merayo and colleagues.  IOVS, May 2001, Vol 142.
5-"Phakic intraocular lens implantation for the correction of myopia: a report by the American Academy of Ophthalmology" Huang, Schallhorn and colleagues. November 2009.
6-"Surgical pearls for TORIC ICL implantation" Jose Alfonso, February 2010. http//bmctoday.net/crstodayeurope/2010/02/article






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