eye and laser

Laser eye surgery investigated

The pro and cons of laser eye surgery techniques including, LASIK, LASEK and PRK.

Are you one of the estimated 68 per cent of the UK population who wears glasses, contact lenses or uses some form of vision correction? Have you been dazzled by the array of options all claiming to offer the best result, cheapest price or least risk?

The first vision-correction surgery using a laser was performed in the late 1980s and is now the most popular elective surgery in the world with over 22 million procedures carried out. It is not without risk, but advancements in technique and technology mean that many people who were once considered unsuitable candidates can now have safe and effective laser eye surgery.

Laser surgery can correct various forms of refractive errors in the eye, such as short sight (myopia), long sight (hyperopia) and astigmatism or irregular curvature of the cornea.

When laser eye surgery was first developed it was considered a good result to achieve 20/20 vision, but over the last few years improvements in technology and diagnostic tests that can be programmed into the laser to enhance the treatment mean that vision as good as 20/15 or 20/10 can sometimes be achieved. 20/20 vision is considered normal vision, and means that you can read at 20 feet what 'normal' person could read at that distance. A best corrected vision of 20/40 is the minimal required to get a driving licence in many countries, and at 20/200 you are legally considered blind.

The cornea accounts for up to 80 per cent of the focusing power of the eye and gives a fixed focus. The cornea is the transparent front of the eye, which refracts the light. The remaining fine adjustments are made by muscles adjusting the curvature of the natural lens. The light rays entering the eye focus on the retina, the light receptor at the back of the eye.

In long-sight, the light rays entering the eye focus behind the retina, a condition caused by a short eyeball or flatness of the cornea. Long-sight often worsens with age. Many people over 40 need reading glasses as the eye muscles weaken and lose the ability to change the shape of the lens inside the eye, a condition known as age-related presbyopia. In short-sight the point of focus of light entering the eye is in front of the retina leaving images in the distance blurry. It is commonly a hereditary problem which often worsens in the teen years.

Over half of those with long or short sight also have astigmatism, where the curvature of the cornea is not even and it is shaped more like a rugby ball than a football. Parts of an image focus correctly on the retina, but others focus in different planes leaving images appearing blurry. Patients with astigmatism used to be advised not to have refractive surgery, but now with improved corneal mapping techniques and laser guidance, individual areas of the cornea can be accurately reshaped to improve the result.

Eximer laser

Laser eye surgery was made possible by the invention of the eximer laser, a form of ultraviolet laser originally developed for industrial applications. This is able to remove or ablate very precise amounts of tissue with a low risk of heat damage as the target material simply disintegrates into the air rather than burning or cutting.

When it comes to refractive or laser eye surgery, the treatment options fall into three main groups: flap procedures, surface procedures and corneal incision procedures. The treatment chosen generally depends upon a number of factors; the type and severity of vision defect needing correction, the thickness of the cornea and the size of the pupil. Also considered are age, general health and that of the eyes in particular. The correction of some vision problems is more suited to a particular technique.

Perhaps the best known procedure using the eximer laser is Laser Assisted In-Situ Keratomileusis (LASIK), a partial thickness flap procedure where the surgeon uses either a laser or a microkeratome oscillating surgical blade to cut an ultra-thin (less than 100μm) flap or door in the cornea, before removing or ablating the target stromal (mid-section) tissue to reshape the corneal bed and achieve better focus. The created corneal flap is then replaced and heals quickly with the protection of a bandage contact lens.

The procedure itself is remarkably quick, typically taking between 5 and 15 minutes for both eyes, depending upon the amount of correction required. Most patients require only anaesthetic eye drops, report only minimal discomfort and experience an almost instant improvement in vision.

The most common complication of LASIK surgery is dry eyes, which is thought to be related to the severing of the corneal nerves responsible for tear production when the flap is cut. Some studies indicate that up to 20 per cent of patients experience such problems, although this is generally mild and resolves over time. The greatest short- and long-term risk from LASIK is dislodging the flap, which never completely heals, although this is extremely rare. Patients who compete in contact sports are often advised not to have normal LASIK surgery because of this small but potentially serious risk.

Surface procedures that do not create a flap can also reshape the cornea. Photorefractive Keratectomy (PRK) uses the eximer laser to ablate the front of the stroma and remove microscopic amounts of the front layer of the epithelium - the thin tissue layer at the front of the eye kept moist with tears.

Another 'no flap' procedure is Laser Assisted Sub-Epithelium Keratomileusis (LASEK) which uses alcohol to loosen the outermost corneal layer which is then lifted with a trephine blade before stromal ablation. Epi-LASEK is similar but uses an epi-keratome to remove the top layer of epithelium. The most recent surface treatment is called Trans-Epi LASIK, of which more later.

Because of the sensitive nature of the eye, surface procedures tend to result in greater discomfort than flap procedures, although once recovery has taken place the results are similar.

The final group of procedures generally involve incisions into the cornea to alter its shape, and have largely been replaced by eximer laser techniques or used in conjunction with other procedures such as cataract surgery or intraocular lens implantation.

LASIK and LASEK surgery

One of the most important innovations in laser eye surgery has been the use of custom or wavefront LASIK and LASEK surgery. This improvement allows the surgeon to customise the treatment to correct even tiny imperfections which could not be improved by glasses or contact lenses.

Wavefront technology was originally invented by Nasa to reduce distortion in the high-powered telescopes used to view distant planets. Prior to its use in custom LASIK and LASEK surgery, only lower-order aberrations such as short-sight, long-sight and astigmatism could be treated. Higher-order aberrations or distortions causing glare, halos, poor contrast sensitivity and difficulty with night vision often remained. Now using the new wavefront procedures over 98 per cent of patients achieve 20/20 vision or better, and fewer lose visual acuity or experience problems.

Every patient's eyes differ in corneal shape, thickness and pupil size. Wavefront diagnostics analyse the entire visual system digitally. It does not rely on conventional diagnostics and ophthalmic tests to ensure an accurate result. The wavefront analyser sends a grid pattern of rays of light into the eye which is reflected off the back of the retina to produce a 3D map showing any distortion from the ideal pattern. The reflected light or wavefront is analysed by computer software and that information is then used to generate a perfectly customised treatment plan. The resulting data map is used to guide the laser to ensure an optimised surgical result.

Surgical manufacturers

Manufacturers of specialist eye surgery lasers and analysers include VISX, Baush and Lomb and WaveLight. In the UK Accuvision Laser Eye Clinics use the WaveLight brand. It is the only company to use the newest Allegretto Wave Eye-Q 400Hz custom eximer laser for treatment. The WaveLight technology was originally developed in Germany, but has recently been acquired by Alcon, based in Switzerland.

The WaveLight Allegretto Eye-Q is able to use the high-resolution custom data compiled during the wavefront consultation. This is a very accurate, computer-controlled mapping of the eye. Not only does this improve accuracy, but it takes 10-20 per cent off the time many other lasers need to complete treatment.

According to John Andrews at Accuvision, one of the UK's top laser eye clinics, the WaveLight Allegretto technology allows them to perform a unique surface procedure called Trans-Epithelial Laser Eye, which is safer and more accurate than many other forms of laser eye surgery. 'Unlike LASEK or PRK it allows us to remove the endothelial layer using only the Allegretto laser. We can remove as little as 1μm of tissue and we are left with a corneal surface which is very smooth as it hasn't had alcohol paste on it or been scraped; a very clean overall procedure.'

The precision and speed is achieved through a technology that WaveLight calls PerfectPulse. This ensures every single laser pulse is controlled from its generation until it meets the cornea. Once it has been created, the ultra thin 0.95mm laser beam goes through three smart energy control checkpoints on the way to the eye, and is adjusted to ensure its strength and accuracy.

Once the surface of the cornea is treated with the prescription derived from the diagnostics, a bandage contact lens is placed over the top. Because the treated cornea is so smooth the epithelium will grow back much quicker than it would if it was replaced, as is normally done in LASEK where the epithelium is dead or has been damaged by the alcohol.

According to Andrews, 'Either process can be uncomfortable, but with the Accuvision treatment you are left with a brand new epithelial tissue which will grow back very quickly on the front of the eye. The outcome is the same but the beauty of this procedure is that it also avoids any of the complications of the corneal flap created during LASIK. This procedure is unique to Accuvision and is something which no other clinic in the UK does.'

Many patients are concerned about what will happen if they move their eye during treatment. The WaveLight Allegretto has built in eye tracking technology which measures the position of the eye every 5ms. If the eye moves, the laser will stop and wait until it returns to the correct position before continuing with treatment.

Beam energy density

The energy density of the beam is tightly controlled to ensure that it is in a Gaussian profile, with more energy in the centre and less in the periphery, thus ensuring a narrow beam and smooth ablation profile. Wider beams can produce deep groves and ridges in the corneal surface, reducing the effectiveness of treatment. Allegretto Wave laser systems are the only ones currently on the market which adjust the beam profile and energy distribution during preventative maintenance, thus ensuring uniformity of the beam.

The natural shape of the cornea is aspheric, ensuring that all light entering the eye is focused on a sharp point. It is important that this curvature is maintained during any ablation to avoid blurriness and vision disturbances. When laser light enters the centre of the cornea it is fully absorbed, but around the edges the curvature can cause energy reflections and sub-optimal treatment. 'Our laser tracks the curvature of the cornea - unlike other lasers, which oscillate from side to side, so as the beam goes to the left and right it is losing energy and precision,' says Andrews. Allegretto Wave laser systems give additional pulses to these peripheral areas to compensate for the energy loss to keep the optical zone as large as possible.

The treatment zone is the size of the area of the eye that is modified. Some systems and clinics are only able to address a small treatment zone, in some cases only 8-12mm compared to the Allegretto's 20mm. Andrews notes that 'this is especially important for somebody who has large pupils because as their pupils enlarge they are taking in the periphery of their vision. If there is a large transition zone - which is where the laser has stopped and the normal tissue starts - and you are looking through that in your peripheral vision, you might get halos, starbursts and other visual disturbances. If the treatment zone is much bigger you are not going to get the transition zone in your line of sight, even if you have a very large pupil'.

In patients with severe visual irregularities in whom standard wavefront measurement would be ineffective, another instrument called the Allegro Topolyser can provide high resolution data on the overall topography of the cornea, especially at its periphery. This data can be augmented by the Pentacam, which has become the gold standard piece of equipment for looking at the topography of the central cornea. The Pentacam uses a Scheipmflug camera to look at the front of the eye (cornea, iris and lens) and measures up to 25,000 data points in its two-second scan. This is compiled into a 3D model which can be viewed by the surgeon and used to give recommendations regarding the safety and likely effectiveness of the suggested procedures. The Allegretto Eye-Q is the only laser currently able to accept data direct from a Pentacam device, which further ensures the speed and accuracy of treatment.

The use of the WaveLight Allegretto and Topography-guided Custom Ablation Treatment (T-CAT) can tackle some previously difficult-to-correct problems. It is particularly important in the treatment of an eye condition called Keratoconus, and uncommon congenital condition affecting one in 2,000 of the population. In Keratoconus the structure of the cornea weakens, and the intra-ocular pressure then causes a cone-shaped bulge and poor visual acuity. The Allegretto laser can be used to ablate or reshape the cornea before the use of Corneal Collagen Cross-Linking with Riboflavin (C3-R) to improve the stiffness and rigidity of the cornea.

'Not only does the treatment reduce the cone-like shape of the cornea and vastly improve the vision, but the patient is then able to use a less rigid contact lens, which, instead of being highly domed, can be softer and more comfortable. It is something that Accuvision is doing on a very regular basis as one of only five or six clinics around the world offering the treatment,' says Andrews.

'This is where technology really changes people's lives. It is that emotional blend when a patient comes into Accuvision and wants help, often having been turned away from other clinics. The reality is that we are so confident about what we do because we have done so many difficult cases, and know we can treat people with even the most complicated prescriptions. It is that confidence in the technology, clinical expertise and patient care which makes Accuvision so special.'

The risks from laser eye surgery are very small, but a small percentage of patients do not get the result or outcome they were hoping for. In general, these patients are able to have another procedure to further improve their vision if the initial results are not optimal. Each type of procedure has its relative benefits and risks.

If you are thinking of getting corrective eye surgery, you need to build a relationship with a surgeon you trust who can help you choose the right procedure for you. It is vital to have a detailed consultation with a highly experienced laser optometrist or your prospective surgeon, a process which will probably take more time than the actual procedure.

If you are fed up with spending money on disposable contact lenses, trying to remember what you did with your reading glasses, or holding this page at arms length to get it in focus, there has never been a better time to consider having laser eye surgery. It is not without a small risk, but the recent advances in technology mean the benefits are there for all to see.

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