Article Date: 5/1/2004

Refractive Surgery: How to Choose What's Best for Your Patient
The options have multiplied. Here's what you need to know to help your patients.
BY BRIAN D. MARSHALL, O.D., F.A.A.O. AND JONATHAN J. STEIN, M.D., Fairfield, Conn.

ILLUSTRATION BY MARK ERICKSON

Today we have more surgical options available for correcting refractive conditions then ever before. New technologies and improvements of existing technologies have broadened our ability to treat higher degrees of myopia, hyperopia and even astigmatism. There's even hope for presbyopes. Now procedures are minimally invasive and potentially reversible for those who aren't risk takers.

In fact, refractive surgery has become so sophisticated that many niche procedures have recently been developed for addressing specific patient needs. The American Academy of Ophthalmology estimates that more than 63 million people in the United States are candidates for refractive surgery. And analysts predict 1.5 million procedures will be performed this year.

Optometrists co-manage all types of refractive surgery. In this article, I'll simplify the myriad of options so you can suggest the best and safest plan for each patient's needs and expectations.

AK still has its place

Over the past decade, radial keratectomy has been replaced by dramatically better technologies. However, the astigmatic keratectomy (AK), also referred to as a relaxing incision (performed in the peripheral cornea), is still effective and widely used today. For patients who have mixed astigmatism and a plano spherical equivalent refractive error, it can entirely eliminate their need for glasses.

For example, if a patient has a refractive error of +0.50 -1.00 x 180, AK can effectively reduce the eye to plano. It's also effective in reducing large amounts of astigmatism and is routinely used in combination with a laser refractive procedure.

Lasers, on the other hand, treat astigmatism by creating an oval ablation pattern on the patient's steep corneal meridian and require much more corneal tissue removal. If a patient has moderately large pupils, this may result in glare and halo symptoms as well as a deep ablation. AK can reduce the astigmatism enough in some cases to eliminate resultant glare. The depth and length of the incision in the axis of the patient's steep corneal meridian are directly related to the desired effect. The blades are available in depths of 500 µm and 600 µm presets; incisions up to three clock hours in length are effective. The procedure can be done in-office with a topical anesthesia.

Because the surgeon makes the incision at the limbus, the patient won't experience halos or glare at night. AK provides an instantaneous improvement in visual acuity and heals quickly with only slight soreness for several days. The O.D. follows the patient in one to five days. AK doesn't affect the central cornea and is minimally invasive, making it a good alternative for patients who are averse to risk. It's useful for fine tuning minimal astigmatic errors following photorefractive keratectomy (PRK) or LASIK.

The laser rules

Today, excimer laser photoablation is the gold standard for refractive ammetropia. This is a result of its incredible accuracy and substantial improvements in laser delivery systems. The excimer is capable of etching accuracy of within 0.25 µm. In addition, the new eye-tracking systems and custom ablations now provide better results than ever thought imaginable.

PRK was originally the treatment option of choice, but over the past five years or so, there's been a paradigm shift toward LASIK as the most widely used technique.

However, PRK isn't dead. For those who have refractive errors of around -1.00 or for those with thinner corneas, epithelial membrane dystrophy and several other ophthalmic conditions, it's arguably the best option. In fact, the military has adopted this technology for its service personnel, including some pilots.

Thanks to the use of mitomycin C intra-operatively, instances of haze are greatly reduced in high myopes. (Mitomycin is an agent used for chemotherapy patients and to reduce scarring during glaucoma filtering bleb surgery. It inhibits collagen production and fibrosis on the cornea, which prevents scarring and haze.)

PRK involves removing the epithelial surface down to Bowman's membrane. The laser then reshapes the eye according to the desired prescription. The surgeon places a bandage lens over the treated surface, which is left in place until new epithelium forms (usually about three days). Because the epithelium is removed, the biggest concern is an increased risk of infection. It also takes several days to a week to achieve functional vision.

On the other hand, the final visual outcome can often be titrated with the use of a topical steroid. For this reason, most patients achieve excellent results with PRK. The surgeon should give extra care for steroid responders or glaucoma patients. Because there's no surgical flap to create high-order aberrations, PRK may be capable of achieving even better refractive results than LASIK.


Figure 1: Note the the placement of the segments in the cornea.

LASIK is a modified form of PRK in which a surgeon uses a microkeratome to make a flap. The microkeratome is a 40-year-old technology that's improved dramatically over the last several years; today flaps are made more precisely then ever before. A new femtosecond laser technology called IntraLase (by IntraLase) can make the flap with a laser instead of the blade in the microkeratome. The major advancement with LASIK is that the flap provides a protective barrier to infection by covering the area of the cornea that was treated. A bandage lens is therefore not needed and visual rehabilitation is greatly enhanced.

On the other hand, making a flap reduces the stromal bed of the eye. In cases where the cornea is already thin, it could result in a cornea that becomes ectasic. The average cornea is around 535 µm thick. Most flaps are between 120 µm and 180 µm thick and it's risky to leave a residual bed thinner than 250 µm. Therefore if a patient has an average cornea of 535 µm and a 160 µm flap is made, the patient can safely have 125 µm removed. For patients with average-sized pupils of 5 mm to 7mm, most laser systems remove approximately 15 µm per diopter of myopia.

Complications can occur with making the flap. The most common is a wrinkle or stria. These can occur any time, but most happen within 30 minutes of surgery and are more common with higher myopic refractive errors. If caught early, the clinician can easily remove them with a cotton swab at the slit lamp. For larger striae, the surgeon will need to lift, reposition and smooth the flap. A free flap can occur where the flap is totally dissected from the corneal bed. These usually occur with flat corneas but can occur anytime. They're treated by replacing the flap and then applying a bandage lens.

A button-hole occurs when the surgeon makes the flap too thin centrally and results in a flap with a donut hole in the middle. These usually occur in patients who have steep corneas with large flaps or in cases where suction was broken during the microkeratome pass. In the latter case, the surgeon aborts the procedure, replaces the flap and recommends PRK with mitomycin after several months. See LASIK patients in 24 hours; in most cases they see 20/40 or better. Carefully inspect the flap position, clarity and interface for infection and diffuse lamellar keratitis.

Custom PRK/LASIK. Wavefront technology provides a customized treatment option for patients. Everyone has natural imperfections or aberrations in the visual system. These include spherical, coma, trefoil and other related aberrations. Most of these are magnified at night when the pupil gets its largest. Therefore, people who have naturally large pupils or large amounts of astigmatism typically benefit the most from this technology. An advanced aberrometer measures the imperfections and communicates electronically with the laser. The surgeon then removes the aberrations during the laser procedure. New advancements have resulted in more accurate aberrometer captures and improved outcomes for many patients.

Figure 2: Notice the placement of the probe in the cornea. 

It doesn't end with the laser

Researchers have come up with other devices to correct refractive error. These include:

Intacs. Approved by the FDA in 1999, Intacs are indicated in low myopia between -1.00 to -3.00 sphere with no more than -1.00 cyl. After instilling a topical anesthetic, a small vertical incision is made in the inferior cornea and widened at the level of the stroma. A trephine is then inserted and tunneled into the stroma mid-peripherally. Two small, crescent-shaped pieces of PMMA are then inserted into the space created by the trephine.

Figure 1 shows the placement of the segments in the cornea. This procedure is best done with a surgical microscope. The thickness of the rings is selected based on the amount of myopia. The thicker the rings, the more central flattening achieved. A suture is placed at the site of the original incision. The patient may feel slight discomfort for the next eight hours.

Intacs have several advantages. First, no tissue is removed from the cornea. The segments can be removed at any time and the patient will return to roughly the same amount of myopia as before. This may be advantageous for low myopes who may want their near vision back again in their 40s. Second, the central cornea isn't touched and a laser isn't used. For many who are risk-aversive, this procedure is an attractive alternative. However, it requires a moderate amount of surgical skill to perform and the results aren't as accurate as laser procedures. It works best for those who aren't visually demanding.

A modification was approved in Europe last year for treatment of keratoconus and is currently in trials in the United States. Researchers are also studying the rings for treating mild forms of hyperopia and astigmatism.

Conductive keratoplasty (CK). This procedure treats hyperopia and presbyopia in people over the age of 40. CK uses controlled release of radio frequency energy that's concentrated at the end of a 450 µm cylindrical probe placed into the cornea. The resulting heat shrinks the surrounding tissue.

By applying these probes in a circular pattern around the mid-periphery of the cornea, a steepening effect is achieved by the tightening or shrinking of the collagen. Figure 2 shows the probe being applied to the cornea. CK is minimally invasive and can be done in-office under topical anesthesia. It takes only five minutes and the effects are immediate: Within several minutes, most patients are reading and seeing well. It's approved for hyperopia up to +4.00 with no more than -0.75 astigmatism. However, it works best on low hyperopes or emmetropes who are interested in monovision.

But the affects diminish over time. Most patients will need to be re-treated every two to three years, or at about the same frequency that they would have their reading prescription changed. Because it doesn't involve the central cornea, CK is a great alternative for those patients who have been waiting for a less invasive alternative to correct their hyperopia. Researchers are currently investigating a technique for correcting astigmatism.

Phakic IOLS. Right now, two different surgical approaches correct refractive errors. Up until recently, much of the focus has been on reshaping the surface of the cornea. However, intraocular procedures are becoming popular alternatives. Instead of removing the crystalline lens and losing accommodation, the surgeon introduces a lens into the eye. Two types of phakic intraocular lenses (PIOLs) exist and include the posterior chamber lens (Implantable Contact Lens [ICL] by STAAR Surgical) and the iris-fixated lens (Artisan/Verisyse).

The Implantable Contact Lens is a foldable plate haptic lens made of collamer. A 3 mm clear cornea incision is formed, the lens injected and carefully placed behind the iris and in front of the natural crystalline lens. The surgeon must be careful not to come into contact with the natural lens.

Post-op care is similar to that of cataract surgery. Visual acuity accuracy and complication rates depend somewhat on surgical skill and dexterity, but to a greater degree on limitations of the material and power selection. STAAR uses a proprietary formula, which includes refraction, keratometric readings, AC chamber depth and corneal thickness in determining the proper power. It's available in a range from -3.00D to -20.00D for myopia.

AMO's Artisan lens is a one-piece design made of PMMA and uses a distinctly different approach. The surgeon makes a clear corneal incision and the lens is placed into the anterior chamber in front of the lens. The lens has two claws, which attach to the iris for stability. As with the ICL, success depends on proper placement and power selection.

The biggest advantage that PIOLs have are their ability to accurately correct high degrees of both myopia and hyperopia without altering the cornea and preserving accommodation in situations where LASIK isn't appropriate. They also offer better functional vision compared with spectacles for large ammetropias. Even though intraocular surgery is required, the technique is conceptually similar to cataract surgery, which is successfully performed several million times every year. The visual rehabilitation is quick with improved vision in one day. Another plus is reversibility: The lenses can be removed at any time and the pre-existing ammetropia returns. The disadvantages include risk of cataractogensesis, iris pigment dispersion and narrow angle glaucoma.

The last frontier?

Baby Boomers now make up 30% of the U.S. population. Because this group has the highest income, it's easy to see why so much research has been done to reverse the affects of presbyopia. The Refocus Group, which makes the PresVIEW Scleral Incision System, has received approval in Europe and is in Phase II trials in the United States.

The concept is easy. As the crystalline lens ages, it adds on new layers like the skin of an onion. By the early 40s, the lens begins to run out of physical space to effectively accommodate. Therefore the Scleral Spacing Procedure involves placement of several PMMA segments into the sclera of the eye. These segments will stretch or widen the sclera and provide more room for the lens to flex. It's still too early to tell how well this system will work. It's an invasive procedure requiring moderate surgical skill and will alter the physical appearance of the eye.

Weigh your options

It's easy to become confused with so many options currently available to treat ammetropia. Each has its own strengths and weaknesses. Table 1 summarizes the important characteristics of each and is intended for use as a quick reference. It's important for you to weigh the risks and benefits of each based on the patient's expectations and motivations for the patient to make the right decision.

Dr. Marshall is in private practice at the TLC Laser Center in Fairfield, Conn. You can contact him by phone at (203) 334-2020.

 


Optometric Management, Issue: May 2004