How does laser eye surgery work?
To answer the question 'How does laser eye surgery work?' we'll need to cover a few different topics:
- Understanding the anatomy of the cornea
- How does the laser reshape the cornea permanently?
- Changing the corneal shape to correct vision
- How does LASIK work?
- How does PRK / LASEK work?
- Misconceptions about how laser eye treatment works
We will look at why vision is blurred when you have short-sight, long-sight or astigmatism, as well as the anatomy of the cornea and how the lasers change its shape to correct eyesight.
Laser eye surgery has been performed for over 25 years, with the first surgery taking place in 1989. Technology has moved on a lot since then with the method of how laser eye surgery works evolving over time. Each procedure now takes no more than 5 minutes per eye whilst achieving an impressive level of accuracy.
Briefly, how laser eye surgery works is by changing the curvature of the cornea. Light entering at the front will then be correctly focused on the retina at the back of the eye. This results in a clear image without the need for glasses or contact lenses.
Vision correction is defined by two main types of laser surgery – 1) LASIK and 2) PRK (also known as LASEK).
LASIK is by far the most popular method, with more than 95% of procedures performed using this approach.
Both types of treatment work in the same way, through evaporating collagen and reshaping corneal curvature. The main difference lies in where the collagen is removed. It is either on the surface as in PRK, or from slightly deeper layers through creating a superficial flap in the LASIK approach.
Understanding the anatomy of the cornea
The cornea is the clear curved window at the front of the eye. It focuses light, converging light rays to help create a sharp image on the retina at the back of the eye.
The top 10% of the corneal thickness is transparent skin cells, only 50 microns thick (1/20th of a millimetre), known as ‘epithelium’. This skin layer is constantly being worn down as we blink and rub cells away. It is also constantly replaced by cells growing in from the edge of the cornea, so that every 7 days you get a brand new coating to the front of the eye.
Every seven days you get a brand new coating to the front of your eye
The lower 90% of the cornea is primarily made of collagen, found throughout the body e.g. in skin, ligaments and tendons, and is a strong, resilient tissue.
There is an important layer of cells on the inner surface of the cornea, known as the endothelium. This is responsible for removing water from the collagen stroma and keeping the cornea clear.
The diagram below illustrates the 3 main layers of the cornea:
- The topmost epithelium composed of several layers of transparent skin cells
- The main structural collagen layer known as the stroma, approximately 90% of the total corneal thickness
- The thin single-celled layer of endothelial cells lining the inner surface
How does the laser reshape the cornea permanently?
Of course, there would be no benefit to lasering and reshaping the topmost epithelial layer. Due to its constant turnover, it have been replaced after just one week and the effect lost.
Laser eye surgery works by reshaping the collagenous stromal layer of the cornea, found underneath the epithelium skin layer. The excimer laser evaporates collagen very efficiently and can be applied in such a way as to gently and very precisely sculpt the tissue into a different shape.
The evaporated collagen will not be replaced, and so the effect is permanent
Focus Clinic is so confident in the accuracy and performance of its distance vision treatment for short-sight and astigmatism that it includes a lifetime guarantee of the benefit.
Changing the corneal shape to correct vision
Understanding how and why images are blurred when you have a refractive error such as short-sight (myopia) helps clarify how laser correction can eliminate the need for glasses.
With short-sightedness, light is incorrectly focused in front of the retina, resulting in a blurred image. Light rays continue from this focal point to produce a blurred image at the back of the eye.
In myopia, typically during the teenage years, the eyeball grows longer from front to back
The cornea and lens at the front of the eye often focus light correctly, but the ‘movie screen’ (retina) has moved too far away for clear focus. The result is blurred distance vision but clear short-range close vision, hence the term ‘short-sight’.
Laser eye surgery works for myopia by flattening the curvature of the cornea. This effectively makes the cornea less powerful as a lens, and moves the focus backwards until it is correctly positioned on the retina. You can now see a sharply focused clear image.
With long-sight, or hyperopia, light is focused beyond the retina. Laser treatment works to correct eyesight by steeping the cornea, making it focus light more powerfully. This brings the image forwards to be correctly focused on the retina.
How does LASIK work?
With LASIK, after the checks and setup is complete, the surgeon uses a femtosecond laser to create a thin, circular “flap” in the uppermost layer of the cornea. Once created it can be folded aside, which then allows a second laser to correct the vision. Creating a replaceable hinged flap is the reason that LASIK works so quickly and effectively, by giving the surgeon access to collagen within the stromal layer.
The second excimer laser will reshape the corneal collagen in a matter of seconds. The flap is then gently smoothed back into its original position
At Focus Clinic, most patients have 20/20 vision within 30 minutes of the end of the procedure. The eyes are comfortable and vision clear by approximately 4 hours after surgery.
The femtosecond laser has revolutionised accurate creation of the corneal flap and greatly improved safety. The older obsolete keratome (a bladed device formerly used to physically cut the flap) was not consistently reliable. The thickness of the keratome flap varied and could produce very thin or thick cuts, which could be more than 100-150 microns thicker than expected.
Femtosecond flaps are highly accurate, creating a flap usually within 2-3 microns of the intended thickness, and virtually eliminated flap-related complications at the time of the surgery.
The typical flap is 100 microns thick, composed of 50 microns of epithelium plus the first 50 microns of collagen directly beneath. This dual-layer flap is folded to the side, as it remains attached by a hinge at the top of the cornea.
The curvature of the cornea is then changed by a cool ultraviolet laser (no heat) by evaporating microscopic amounts of collagen tissue. The new shape changes the way that light enters the eye and therefore creates clear vision
To correct short-sight (myopia, nearsightedness), the cornea needs to be made a little flatter; more pulses are applied centrally over a 6-6.5mm diameter area, and feathering towards the edge of the total 9mm width treatment zone. More tissue is removed in the middle and less further out, resulting in a flatter cornea.
To correct long-sight (hyperopia, farsightedness), the central cornea is made steeper by removing tissue around the edge of the cornea, and feathering towards the centre. This leaves the central 6-7mm of the cornea more curved and able to act as a stronger magnifying lens.
Astigmatism is usually corrected simultaneously with either long- or short-sight, by applying excimer laser pulses in a more oval pattern. It is a common myth that LASIK cannot treat astigmatism
After the laser reshapes the cornea, the flap is then folded back into position, covering the area where the corneal tissue was removed. The cornea then heals naturally. Patient’s eyes may have a foreign body feeling for a few hours after surgery and anaesthetic drops are provided. Antibiotic and anti-inflammatory drops are used for the next few days following treatment.
The surface skin cells of the cornea re-seal the edge of the flap in just 4 hours. Sight is usually good immediately after surgery, with patients seeing 20/20 letters on the test chart within 30 minutes. Vision is typically better than 20/20 in 1-4 hours after LASIK.
How does PRK / LASEK work?
PRK does not have a corneal flap created. Recovery time for PRK / LASEK is significantly longer than LASIK – 4-7 days versus 4 hours for LASIK.
The surgeon still needs access to the collagen layer of the cornea. PRK works by first removing the topmost epithelial layer, most commonly with the help of a dilute alcohol solution.
The excimer laser can then work on the collagen at the surface of the stromal layer, reshaping it exactly as it does in the LASIK technique.
The gap in the epithelium, usually 7-8mm wide, needs to heal over which will take 3-4 days. A contact lens is placed over the cornea to act as a bandage while skin grows back across the surface. Once the epithelium has healed, pain will subside and the vision will become clear.
The exposed collagen layer explains why PRK is painful after treatment, as exposed nerve endings are not covered by the natural protective layer of epithelial skin cells. Anaesthetic eye drops are used in the postoperative period to reduce discomfort, until the skin layer has healed.
Misconceptions about how laser eye treatment works
It’s a common misunderstanding that the entire procedure is performed by lasers. In fact, the lasers make up just 10% of the total surgery. The rest of the time is taken up by your surgeon performing over 150 steps, requiring superb manual dexterity and skill. This is why you want the most experienced surgeon performing your operation, and with an excellent track record of results.
The precision of these 150 steps using our A-LASIK technique have helped improve surgery success rates, with Focus Clinic achieving a 100% success rate of 20/20 vision (or better) after LASIK for all common myopia prescriptions.
A-LASIK also helps keep recovery time to a minimum, with 20/20 vision achieved in less than 30 minutes in most A-LASIK cases.
Another common misconception is that laser eye surgery doesn’t work for astigmatism, and has in fact been available for 20 years. The very earliest lasers could not correct this refractive error. The idea that astigmatism isn’t treatable seems to have persisted ever since. Almost everyone has some degree of astigmatism, and so it forms part of almost every laser eye correction today.
Laser eye surgery has come a long way since its first treatments in 1989. The principle of how laser eye surgery works is still the same - changing the corneal curvature to place a clear image onto the retina. However, the accuracy of the modern procedure has achieved a great degree of precision. Wavefront optics and femtosecond laser technology caused a quantum jump in our expectations of the technique.
To learn more about laser eye surgery, download our information pack.