How LASIK Works: The Flap, the Laser and What It Changes

What happens during LASIK

LASIK stands for laser-assisted in situ keratomileusis, which is the least useful sentence about it. What it describes is a procedure with three moving parts: a flap, an excimer laser, and the tissue underneath that gets reshaped.

1. 01

   A flap is created in the surface of the cornea

   The outermost layer of the cornea is cut in a curve, leaving a hinge. Most LASIK today uses a femtosecond laser for this; the older technique uses a mechanical blade. Either way the flap stays attached and is folded back, not removed.

   Why the flap exists

2. 02

   An excimer laser reshapes the tissue underneath

   With the flap folded back, the exposed corneal tissue is reshaped. The excimer laser removes tissue in a pattern calculated from your refraction, which is what changes how the eye focuses.

   The two lasers

3. 03

   The flap is laid back into place

   The flap is repositioned and holds itself in position without stitches. It is not glued or sewn; the surface layer adheres on its own as it settles, which is why the first instruction afterwards is not to rub the eye.

   The first days afterwards

4. 04

   The cornea heals, and vision keeps settling

   Vision is often usable quickly, because the surface of the eye has been lifted rather than removed. It still fluctuates for weeks while the cornea and the tear film settle, and the follow-up visits exist to track that.

   Recovery, in detail

The whole procedure is performed with the eye numbed by drops rather than by an injection. You are awake, and the eye is held steady by a suction ring for the part where the flap is created, which is the part most people find the strangest: a brief sensation of pressure and a darkening of vision. The surgeon looks through a microscope throughout, and the eye is treated one at a time.

The flap is the whole difference

Almost everything that distinguishes LASIK from the alternatives comes from one structural feature. Cutting a flap means cutting across a band of corneal nerves, and it leaves an interface between the flap and the tissue beneath it that heals but never re-bonds in the way intact tissue does.

Two of the standard instructions after LASIK exist because of that interface: do not rub the eye, and take care with contact sports and with anything that could catch the eye while the flap is settling. Complications involving the flap, such as it being displaced or epithelial cells growing under it, are recognised and uncommon. A clinic that does not mention them is not describing a safer procedure, only a shorter consent form.

Two lasers, and what each one does

The word LASIK covers two quite different instruments working in sequence, and the first one is where the variants people argue about come from.

Creating the flap

A femtosecond laser makes the flap by separating tissue beneath the surface with microscopic pulses, without heat, leaving a hinge. An older approach uses a mechanical microkeratome, a precision blade on a suction ring, and is still called LASIK by anyone who does not add the word femto. The laser-created flap is thinner, more predictable in thickness and less likely to have surface irregularities, which is why most clinics have moved to it; the blade technique is cheaper to run, and a clinic's reason for still using it is worth asking about.

Reshaping the cornea

The excimer laser is the second instrument, and it is the one that actually corrects your vision. It removes tissue in a pattern calculated from your refraction and from the shape of your cornea, with no heat and no cutting: it breaks molecular bonds at the surface. Removing tissue from the centre flattens the cornea and moves the focus towards short sightedness correction; removing it from the edge steepens it for long sightedness; a toric pattern levels an astigmatic eye.

What it can correct, and what it cannot

LASIK has the broadest established range of the corneal procedures: short sightedness, long sightedness, astigmatism, and mixed prescriptions, within limits set by how much tissue your cornea can spare. The limit is arithmetic rather than arbitrary. Every dioptre removed costs tissue, a flap costs tissue of its own, and what must remain behind is set by how thin a cornea can safely be. Two people with identical prescriptions can therefore get different answers from the same clinic, because their corneas are not identical.

What it cannot do is change the fact that your eyes will keep ageing. It does not prevent presbyopia, the reading vision change that affects almost everyone in middle age, and it does not prevent cataract. It also does not stop the eye changing if the prescription had not stabilised before treatment, which is why stability is checked rather than assumed.

The first days and weeks

Because the surface layer is lifted rather than removed, the eye feels close to normal quickly, and most people have usable vision within a day or two. That is the main practical advantage of a flap, and it is why LASIK is popular with people who cannot take a week of blurred vision. It is not the same as being finished: vision fluctuates while the cornea and tear film settle, and dryness and light sensitivity are common in the early weeks.

The recovery guide goes through this week by week

What can go wrong

The common side effects are dryness, glare and halos at night, and vision that is not quite as crisp as it was expected to be for a few weeks. All of them usually improve. Less common are a residual prescription that needs glasses for some tasks or a refinement, inflammation under the flap, and epithelial cells growing into the interface. Rare and serious complications include infection. Anyone who tells you the procedure carries no risk is describing a product.

The risks and side effects guide covers this in more detail

Questions people ask about LASIK

Each question below opens the page that answers it in full, where the long version of the answer is. The direct answer is printed here so you can tell whether it is the question you are asking.

• How does LASIK work?

  LASIK works by removing microscopic amounts of corneal tissue so that the cornea becomes flatter or steeper, which changes where light comes to focus inside the eye.

• How does laser eye surgery work in general?

  All laser vision correction procedures remove or reshape corneal tissue to change the eye’s focusing power, and they differ mainly in how the laser reaches the tissue it needs to reshape.

• What tests are needed before LASIK?

  Before LASIK, the assessment usually includes refraction, corneal topography and tomography, measurement of corneal thickness, an assessment of the tear film and ocular surface, eye pressure, and a retinal examination with dilated pupils.

• What does LASIK correct?

  LASIK is used to correct short sightedness, long sightedness and astigmatism, within limits that depend on your prescription, corneal thickness and corneal shape.

• Is LASIK permanent?

  The change LASIK makes to the cornea is permanent, but the eye continues to change with age, so vision later in life can differ from the vision you have in the months after surgery.

• How long does LASIK recovery take?

  Most people are back to ordinary daily activities within a day or two, while the eye continues to settle for several weeks and in some cases a few months.

• LASIK or SMILE: how does a surgeon actually choose?

  The choice is made from your measurements rather than from your preference or from a general claim about which is better. Corneal thickness, corneal shape, prescription, tear film and the planned correction all feed into it.

• Which procedure is suitable if I have dry eyes?

  All of the corneal procedures can affect the tear film, and the honest answer is that dryness is assessed and treated before any of them is planned. Where dryness is significant, a lens-based option may be discussed instead.

Questions worth asking

The answers to these say more about a clinic than any brochure, and none of them require you to understand optics.

• Which technique would you use to create my flap, and why that one?
• What would my cornea measure after treatment, and how much margin is left?
• How many procedures like mine do you perform in a month?
• What is your enhancement policy, and what does a refinement cost?
• Who do I see, and how quickly, if something worries me in the first week?