LASIK is a type of refractive eye surgery

lasik

1. LASIK
Laser-Assisted In Situ Keratomileusis
Lasers and Photonics in Medicine

2. Introduction
LASIK (laser in situ keratomileusis) is a surgical procedure designed to correct refractive errors. LASIK involves
creating a corneal flap using a microkeratome or a femtosecond laser, reshaping the cornea using an excimer
laser to remove tissue from the underlying stromal bed and then replacing the flap.

3. History of LASIK
LASIK evolved from a variety of techniques in refractive surgery. This procedure was followed by anterior lamellar
keratoplasty (ALK). Tissue from the corneal bed was removed to alter the refractive error and the flap was replaced.
Keratomileusis and ALK were relatively imprecise mechanical techniques. After its development, the excimer (excited
dimer) laser was used to reshape the cornea in a technique called photorefractive keratectomy (PRK). LASIK combines
the technique of creating a hinged corneal flap from ALK with excimer laser ablation from PRK.
Potential advantages of LASIK over PRK include earlier postoperative stabilization and faster improvement of visual
acuity; less patient discomfort; shorter duration of post medication use; and an easier enhancement procedure.

4. In order to understand LASIK surgery, one must understand the basic anatomy and physiology of the eye.
Light enters through the cornea, the outer lining of the eye, into the aqueous humor fluid.
Light next travels through the pupil, the opening of the colored iris, to the crystalline lens.
Unlike a camera lens, this lens has ciliary muscles which contract and relax in order for light to be correctly oriented to
focus onto an image on the retina.
The retina of the eye has rods and cones which determine intensity and frequency of light and transfer the information to
the optic nerve to the brain. Essentially we see (perceive vision) with our brain, not our eyes.

5. The crystalline lens essentially acts as a convergent lens with a focal length where the retina is. Therefore, light focuses on the
retina. We can see the ray model of light at work in the picture of the flower.

6. The crystalline lens has a variable index on its surface between 1.386 and 1.406. The ciliary muscles shape the lens
accordingly and wave focuses at the focal point on the retina to be interpreted. The darkening of this lens is referred to as a
cataract.

7. LASIK is used to treat three types of refractive errors: myopia, hyperopia, and astigmatism.
Myopia is also known as nearsightedness and occurs when the light focuses an image in the vitreous humor rather than the
retina. They can see near objects well but cannot see far objects.
Hyperopia, or farsightedness, occurs when the light hits the retina but does not focus. This is the opposite of myopia.
Astigmatism is when the cornea is not curved perfectly like a basketball but more like an egg. People with astigmatism
generally experience blurred vision and possibly distortion of images. There is an unequal bending of the rays of light so it
causes both myopia and hyperopia.

8. LASIK is used to treat a few major refractive errors causes by irregularities in the curving of the cornea. Corneas that are
steeper than they should be often lead to nearsightedness, myopia.
Diopter= 1/ focal length, the more flat would cause hyperopia or farsightedness which is in the negative diopter range.
Astigmatism, the third major refractive error that can be fixed by LASIK, is caused by an irregularly shaped cornea. An
astigmatism is less than 3 diopters, positive or negative.
Essentially, LASIK avoids contact lenses by actually changing the shape of the cornea to focus on the retina.

9. Excimer is short for "excited dimer." The excimer laser almost exclusively operates in the ultraviolet spectrum.
The laser medium is formed by the combination of inert noble gases Argon, Krypton, and Xenon and a reactive gas such
as Fluorine or Chlorine.
Because these excited atoms are generally complexes, the correct term for an excimer laser is an exciplex laser (excited
complex.)
These complexes exist in the upper laser state. When a photon passes through the medium, the complexes give off an
identical photon each and then return to their ground state giving LASER.
The reason these excimer lasers are so important for laser eye surgery is because they are in the ultraviolet.
Rather than burn the eye, the laser disrupts the bonds between atoms that compose the surface tissue. The bonds
break, and the layer soon disintegrates without changing the surrounding material at all. This process is called
photoablation or ablation, NOT BURNING!
LASER IN LASIK

10. The general steps involved in the LASIK eye surgery process, which can be broken down as follows:
1. Incision (I)
•A thin, circular flap is created on the cornea using a microkeratome blade or a femtosecond laser.
•The femtosecond laser focuses energy at a specific depth within the cornea to create thousands of microscopic bubbles,
separating the corneal layers.
•The flap is lifted to access the underlying corneal tissue.
2. Corneal Reshaping (C)
•An excimer laser is used to reshape the cornea by removing small amounts of tissue.
•This reshaping allows light to properly focus on the retina, improving vision.

11. Femtosecond LASIK
The femtosecond laser creates a corneal incision by delivering laser pulses at a predetermined depth in the cornea.
These pulses cause microphotodisruption, an expanding bubble of gas (CO2) and water that cleave the tissue and create a
plane of separation. Femtosecond lasers create LASIK flaps with more predictable and uniform thickness, and fewer
complications.
Femtosecond laser aided LASIK has been shown to provide better predictability of refractive outcomes and lower
enhancement rates than LASIK performed using a microkeratome. (Increased Precision, Reduced Risk of Flap Complications,
customizable Flap Thickness, faster Healing, Safer for High-Risk Patients)
Fig: Illustration of a femtosecond laser cutting a LASIK flap.

12. 3. Adjustment and Flap Repositioning (A)
•After the corneal reshaping, the surgeon gently repositions the corneal flap back into place.
•The flap naturally adheres without the need for stitches.
4. Recovery (R)
•The eye begins healing almost immediately after the procedure.
•Post-operative care includes using eye drops, avoiding rubbing the eyes, and follow-up visits with the eye doctor to ensure
proper healing.
The ICAR steps ensure a quick and effective procedure, usually completed within 10-15 minutes per eye, with minimal
discomfort for the patient.

13. Fig: Illustration of excimer ablation of the stromal bed in myopic LASIK

14. LASIK Indications for the Correction of Myopia and Astigmatism


LASIK is indicated for the correction of low, moderate, and high myopia with and without astigmatism. The specific dioptric
limits depends on the specific laser system and the regulatory agency of each country.
The correction of high myopia may present a greater risk of post-LASIK ectasia and decreased quality of vision in some
patients. The surgeon and patient should decide whether LASIK is indicated based on a full preoperative evaluation and
consideration of goals and alternatives, including spectacles, contact lens, and lens implantation.

15. Outcomes of LASIK for Myopia and Astigmatism
For the correction of low to moderate myopia of less than -6D and low to moderate astigmatism of less than 2D,
results have shown that LASIK is effective and predictable in terms of obtaining very good to excellent
uncorrected visual acuity and that it is safe in terms of minimal loss of visual acuity.
In high myopia, showed that postoperative spheriqual equivalent was
±0.50 D in 55% and ±1.00 D in 83% of eyes after primary treatment.
After re-treatment, 69% of eyes were within ±0.50 D and 95% were within ±1.00 D.

16. There are several variations of LASIK surgery, each tailored to specific vision needs and corneal conditions. Here’s a
breakdown of the different types of LASIK and related laser eye surgeries:
1. Traditional LASIK (Laser-Assisted In Situ Keratomileusis)
•The standard procedure involves creating a corneal flap, reshaping the cornea with an excimer laser, and
repositioning the flap.
•Suitable for treating myopia, hyperopia, and astigmatism.
2. LASEK (Laser-Assisted Sub-Epithelial Keratectomy)
•Instead of creating a corneal flap, the outer layer of the cornea (epithelium) is loosened and moved aside.
•After corneal reshaping with a laser, the epithelium is repositioned.
•Ideal for patients with thin corneas or those at higher risk of complications from flap creation.
3. PRK (Photorefractive Keratectomy)
•Similar to LASEK but the outer corneal layer is completely removed, not replaced, allowing it to regrow naturally.
•Suitable for patients with thin corneas or dry eyes.
•Has a longer recovery time compared to LASIK, as the epithelium needs time to heal.

17. 4. IntraLASIK (Bladeless LASIK)
•Uses a femtosecond laser instead of a blade (microkeratome) to create the corneal flap.
•Known for increased precision and fewer flap-related complications.
•Often referred to as bladeless LASIK or all-laser LASIK.
5. Wavefront-Guided LASIK (Custom LASIK)
•Utilizes wavefront technology to create a detailed 3D map of the eye, allowing for a highly personalized
treatment.
•Corrects higher-order aberrations that traditional LASIK may not address, such as night vision problems.
6. SMILE (Small Incision Lenticule Extraction)
•A minimally invasive procedure where a small incision is made in the cornea, and a lens-shaped piece of
tissue is removed to reshape the cornea.
•Suitable for treating myopia and astigmatism.
•No flap is created, reducing some risks associated with LASIK.

18. LASIK Complications
Common Complications:
•Dry Eyes, Nighttime Issues, Reduced Contrast Sensitivity
•LASIK Flap Complications (Free, Incomplete, or Buttonholed Flaps) [In cases of irregular or incomplete flaps, LASIK
or PRK may be done later after healing]
•Flap complications are extremely rare when using a femtosecond laser.
•Inflammation in the interface layer.
•Infection: Rare but possible risk
•Epithelial Ingrowth (Cells growing beneath the flap, potentially affecting vision)
•Optic Neuropathy (A rare condition affecting the optic nerve, which can occur after LASIK)

19. Conclusions
LASIK is an excellent procedure for most patients with myopia and astigmatism. Proper pre-operative screening is
essential. While surgical complications are rare, good postoperative care is important.
It offers significant benefits, such as freedom from glasses and contact lenses, a quick recovery time, and a high
success rate. While there are some potential risks and side effects, such as dry eyes or night vision issues, serious
complications are rare, especially with advancements in technology like bladeless and wavefront-guided LASIK.

20. References
Laser In Situ Keratomileusis for Myopia and Astigmatism: Safety and Efficacy: Report by the
American Academy of Ophthalmology. Ophthalmology
https://www.physics.rutgers.edu/~pchandra/physics397/Project_Paper.pdf
https://eyewiki.org/LASIK_for_Myopia_and_Astigmatism:_Safety_and_Efficacy
Advances in refractive surgery: microkeratome and femtosecond laser flap creation in relation to
safety, efficacy, predictability, and biomechanical stability
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