Minimum Corneal Thickness For LASIK: How Thin Is Ok?
A good candidate for LASIK will need to have a certain amount of thickness to their corneal. Before we take a look at what the minimum corneal thickness for Lasik is, let’s understand why it matters.
Why Minimum Corneal Thickness For Lasik Matters?
Minimum corneal thickness refers to the lowest amount of corneal tissue thickness required for a patient to be considered a good candidate for LASIK surgery. It is very important to make sure that a patient has adequate corneal thickness because this will help to minimize the risk of complications during and after the surgery.
LASIK Procedure And What It Does To The Cornea Thickness
LASIK surgery involves creating a corneal flap. Once the LASIK flap is created, laser ablation is done to change the shape of the cornea.
The ablation depth, or amount of laser required, depends on the degree of refractive error.
Higher refractive error (more nearsighted/farsighted)
These patients need a deeper ablation depth to reshape the cornea effectively.
Lower refractive error (less nearsighted/farsighted)
These patients usually need a shallower ablation depth.
There are formulas used to estimate the ablation depth based on the specific refractive error.
What Is Normal Corneal Thickness?
How thick or thin a patient’s cornea is varies from person to person. It is usually not something that changes over time unless the person has trauma, surgery, or a corneal disease process.
In the general population, corneal thickness ranges between 500 and 600 micrometers. This is about half a millimeter, so it is quite small.
What Is Considered To Be The Minimum Corneal Thickness For LASIK?
The minimum corneal thickness to be a good LASIK candidate will vary slightly from surgeon to surgeon. Generally, the minimum corneal thickness for LASIK is usually right around 500. Thick corneas are better than thinner ones.
However, each LASIK surgeon has their own preferences and protocols. For some LASIK surgeons, they may feel comfortable with a thickness that is slightly lower than this. The minimum for PRK surgery can be slightly lower by around 20 micrometers, but, again, this will depend on the surgeon and the specific patient.
Because LASIK involves reshaping the cornea using a laser, the surgeon needs to make sure there is enough tissue to safely remove without compromising the structural integrity of the cornea. If the cornea is too thin, LASIK may not be recommended due to an increased risk of complications.
What Is Residual Stromal Thickness (RST)
Residual stromal thickness (RST) is the remaining corneal tissue after LASIK surgery. Having enough tissue left after LASIK is important for corneal stability. LASIK surgeons will want this number to be 250 micrometers after the excimer laser.
How Does A LASIK Doctor Evaluate You For Surgery?
Before having LASIK surgery, a thorough evaluation of the patient’s eyes is done. This includes measuring the corneal thickness of both eyes as well as what your refractive error is.
Doctors who are performing any type of vision correction surgery, whether it is LASIK, PRK or SMILE eye surgery will want to make sure that your eye prescription in glasses and contact lenses has been stable for a year.. This information helps the surgeon determine the suitability of the patient for LASIK eye surgery.
What Is The Corneal Pachymetry Test
To figure out a patient’s central corneal thickness or CCT, which is the thickness of the central part of the cornea, a pachymetry test is done. This pachymetry test gives the surgeon with essential information about the patient’s corneal thickness, ensuring that they have enough corneal tissue for a safe and successful LASIK surgery.
Performing a corneal pachymetry in the office is easy and quick. A numbing drop is placed on the eye and then a device called a pachymeter is used. The corneal pachymeter gently touches the central cornea to get the reading. It is painless because the numbing drop prevents the patient from feeling anything. It takes less than 30 seconds.
Average Corneal Thickness
The average corneal thickness in the general population is between 540 and 550. This means that most people should have corneas that are thick enough to also be LASIK patients.
Corneal Topography
Corneal topography provides detailed information about the shape, curvature, and thickness of the cornea. This corneal mapping is typically done by projecting rings or patterns onto the cornea and analyzing the reflection. This data is then used to create a detailed, three-dimensional map of the corneal surface.
Risks Associated With Thinner Corneas
Thinner corneas may introduce additional risk factors for patients considering LASIK surgery. When the cornea is too thin, the stability of the eye can be compromised. This increases the possibility of postoperative complications like corneal ectasia.
Post-LASIK Ectasia
This is a rare complication of LASIK surgery. Corneal ectasia can happen after LASIK when too much tissue is removed from the cornea during surgery, and there is not enough remaining tissue. This is the residual stromal thickness that is mentioned above. Not having enough residual stromal thickness can weaken the cornea and making it more likely to bulge.
Symptoms of corneal ectasia may include:
- Blurry vision, especially in one eye
- Increased nearsightedness (myopia) or astigmatism
- Difficulty seeing at night
- Sensitivity to light and glare
- Seeing halos around lights
Is Corneal Ectasia From LASIK common?
Fortunately, corneal ectasia is a rare complication. It affects only 0.02% to 0.6% of people who under Laser-Assisted In Situ Keratomileusis or LASIK.
If someone suffers from this LASIK complication, the treatment options include Rigid Gas Permeable (RGP) Lenses, Scleral Lenses, Corneal Collagen Cross-linking (CXL), Intracorneal Ring Segments (ICRS), and in severe cases a corneal transplant.
Corneal Scarring
In addition to corneal ectasia, other long-term factors can impact vision quality after LASIK surgery. For instance, corneal scarring may be associated with lower corneal thickness and can contribute to vision problems.
Alternatives To LASIK For Patients With Thin Corneas
Patients with thinner corneas may not be ideal candidates for LASIK due to the corneal tissue removal involved in the procedure. However, there are alternative options for those looking to correct their vision. In this section, we will discuss other refractive surgery options and compare photorefractive keratectomy (PRK) and phakic intraocular lenses (IOLs) to help you make an informed decision.
PRK Or Photorefractive Keratectomy
Photorefractive Keratectomy (PRK) is a surface ablation procedure that removes the outer corneal layer to reshape the cornea without creating a flap like LASIK. This procedure is better suited for patients with thinner corneas.
- Pros:
- Better suited for patients with thinner corneas.
- No flap complications as the cornea is not cut.
- Slower recovery time than LASIK, but similar long-term results.
- Cons:
- Longer, more uncomfortable healing process.
- Possibility of corneal haze during healing.
- Less predictable outcome than LASIK due to possible healing variability.
Phakic IOLs
A phakic IOL is an artificial lens that is implanted in the eye to correct refractive errors without removing the natural lens or altering the corneal tissue in any way. A phakic IOL does not require the cornea to be lasered so the corneal thickness measurement is not as important in this procedure as it is in LASIK or PRK.
- Pros:
- Suitable for high myopic patients who are not candidates for corneal refractive surgery.
- No removal of corneal tissue; cornea remains intact.
- Reversible procedure.
- Cons:
- Requires a surgical incision, and thus carries risks of infection and intraocular lens complications.
- More expensive option than LASIK or PRK.
- Potential for increased risk of cataract development
Refractive Lens Exchange (RLE)
A procedure in which the natural lens is replaced with an artificial lens to correct refractive errors, which is particularly beneficial for presbyopic or high myopic patients. Here is a comparison of how RLE is different from a phakic IOL.
Refractive Lens Exchange (RLE):
- What it does: Replaces your natural crystalline lens with an artificial intraocular lens (IOL) to correct refractive errors (nearsightedness, farsightedness, or astigmatism).
- How it works: The natural lens is removed, similar to cataract surgery, and an IOL is implanted in its place to focus light properly onto the retina.
- Who it’s for: Suitable for adults over 50 who don’t have cataracts but want to improve their vision and potentially reduce their dependence on glasses or contacts.
Phakic IOL:
- What it does: Adds an artificial lens on top of your natural lens to correct refractive errors.
- How it works: A specialized IOL is inserted through a small incision into the eye, placed behind the iris (colored part) but in front of your natural lens, to improve focusing.
- Who it’s for: Ideal for younger patients (typically under 50) with moderate to high refractive errors who are not good candidates for LASIK due to corneal limitations or prefer a permanent vision correction solution.
SMILE Eye Surgery
SMILE is a fairly newer surgery in the United States. This vision correcting surgery was approved in 2016 in the US where as LASIK was first done in 1991 and PRK was first done in 1987.
This type of laser surgery may be considered a better option than LASIK for patients with thinner corneas. Here are the reasons that this may be the case:
Benefits
Flap Vs. No Flap
In LASIK, flap creation is necessary to access the inner tissue for reshaping. This flap creation requires a certain amount of corneal thickness for stability.
SMILE’s Minimally Invasive Approach
SMILE uses a femtosecond laser to create a lenticule (a disc-shaped piece of corneal tissue) within the cornea itself. It then removes this lenticule through a small incision, eliminating the need for a flap.
SMILE Surgery Preserves Cornea Strength:
Since SMILE doesn’t require a flap, it preserves the stronger anterior (front) layers of the cornea. This can be important because the anterior layers of the cornea play an important role in keeping the cornea strong.
Studies have shown that SMILE can be performed safely and effectively on patients with corneas that may be too thin for LASIK. For instance, research suggests SMILE might be a viable option for corneas as thin as 477 microns, while LASIK safety becomes a bit questionable at such a thicknesses.
Limitations
Not All Thin Corneas Are Ideal
While SMILE offers more flexibility for thinner corneas compared to LASIK, there’s still a limit. A thorough evaluation by an ophthalmologist is essential to determine if your specific corneal thickness is suitable for SMILE.
Doctor’s Expertise Matters
The success of SMILE depends heavily on the surgeon’s experience and expertise with the technology. Choose a qualified ophthalmologist who performs SMILE eye surgery regularly.
The best option for you may not be the best option for someone else. Each of these alternative options has their own advantages and potential complications that should be considered when deciding what is right for each patient.
Minimum Corneal Thickness For LASIK: Summary
Having a minimum corneal thickness is crucial for LASIK surgery, especially when using a femtosecond laser for flap creation. The thickness of your cornea, measured in microns of corneal tissue is an important factor. It determines the amount of corneal tissue available for correction. Adequate tissue ensures clear vision and optimal visual acuity, particularly for right eye or left eye conditions.
Preoperative corneal thickness, including the specific measurement of the flap thickness, is essential during lasik screening and preoperative evaluation. Thicker corneas generally provide a better safety margin for the correction of refractive errors. The residual stromal bed thickness, post-ablation, should be sufficient for stability and to minimize complications. Dr. Trattler emphasizes the importance of corneal thickness, considering it an independent risk factor.
Thin corneas may require alternative options. A normal preoperative corneal topography and corneal thickness test help determine suitability for laser eye surgery. Young patients and hyperopic patients may have different considerations due to corneal collagen and epithelial layer characteristics. The creation of a thinner flap is a good idea to preserve as much corneal tissue as possible. Intraocular pressure and corneal aberrations are also factors to consider. Refractive surgeons in the United States typically use ultrasound pachymetry to measure preoperative central corneal thickness. Overall, ensuring sufficient corneal thickness is one of the first things an eye doctor examines when assessing LASIK candidacy, as it directly impacts the success and safety of the refractive procedure.
