Corneal Thickness for LASIK: How Thick Is Thick Enough?
Why does corneal thickness matter for LASIK?
LASIK removes tissue and cuts a flap, both of which weaken the cornea; enough strong tissue must remain to keep its shape for life.
If too little remains, the cornea can gradually bulge forward, a complication called ectasia that blurs vision. AAO EyeWiki cites reported rates between 0.04% and 0.6%, and onset can be delayed by years. Screening tries to identify at-risk eyes before surgery.
What is the residual stromal bed?
The residual stromal bed (RSB) is the untouched corneal tissue left beneath the flap after the laser treatment.
A commonly used minimum is 250 µm, and many surgeons prefer at least 300 µm. EyeWiki associates an RSB below 300 µm with higher ectasia risk. A 2024 Review of Ophthalmology article notes that LASIK generally requires at least 250 µm, one reason PRK is chosen for thinner corneas.
What is percent tissue altered (PTA)?
PTA is the share of the cornea that LASIK affects: flap thickness plus ablation depth, divided by central corneal thickness.
Santhiago and colleagues studied eyes with normal-looking topography and found PTA above 40% was the most significant factor linked to post-LASIK ectasia, more than RSB, thickness or age. Eyes that developed ectasia had a mean PTA of 45.1%, against 31.9% in eyes that did not. PTA helps because a fixed RSB alone can miss risk in eyes losing a large share of tissue.
Worked example: how surgeons check the numbers
This simplified example shows how the two measures are used together; real plans use your own scans.
| Measure | Eye A | Eye B |
|---|---|---|
| Central corneal thickness | 550 µm | 490 µm |
| Flap thickness | 110 µm | 110 µm |
| Ablation depth (illustrative) | 90 µm | 90 µm |
| Residual stromal bed | 350 µm | 290 µm |
| Percent tissue altered | 36% | 41% |
| Likely discussion | Within common limits for LASIK | PTA over 40%: PRK, SMILE or ICL may be safer |
Eye B still has an RSB above 250 µm, yet its PTA crosses 40%, which is why surgeons look at both.
Is ectasia risk only about thickness?
No. Thickness is one of several risk factors, and corneal shape is often the most important.
AAO EyeWiki lists abnormal preoperative topography, such as keratoconus, pellucid marginal degeneration or forme fruste keratoconus, as a major risk factor, alongside a low residual stromal bed, low preoperative thickness, high PTA, younger age and high myopia. It also notes that ectasia has been reported after low myopic and even hyperopic treatments. The Randleman Ectasia Risk Score combines several of these factors into a cumulative score, where 0–2 is low risk, 3 moderate and 4 or more high, though EyeWiki describes the system as controversial and PTA was later found to predict risk better in eyes with normal topography. For you, this means a "thick enough" reading is not a green light by itself; ask how your tomography looks too.
What is the minimum corneal thickness for LASIK?
There is no single universal cut-off; many surgeons avoid LASIK below about 470–500 µm.
The Randleman ectasia score adds risk points at thickness bands of 510, 480 and 450 µm. Shape matters as much as thickness: signs of keratoconus or forme fruste keratoconus on tomography can rule out LASIK even in a thick cornea. That is why the pre-op eye exam includes corneal mapping.
How is corneal thickness measured?
Thickness is measured at your pre-op exam with pachymetry and corneal tomography, which are quick and painless.
Pachymetry gives a thickness reading, while tomography maps thickness and shape across the whole cornea, front and back, and shows where the thinnest point is. Surgeons use the thinnest point, not just the centre, when planning. Contact lenses can temporarily change the shape of the cornea, which is why Korean clinics ask you to stop wearing them before the exam: commonly about one week for soft lenses and longer for hard or orthokeratology lenses. If your readings look borderline, it is reasonable to ask for a repeat measurement or a second opinion before deciding.
What happens if ectasia develops after surgery?
Ectasia is rare, but it can be managed, and the earlier it is found the more options there are.
AAO EyeWiki describes a stepwise approach. Corneal cross-linking may stop the cornea from progressing further; EyeWiki notes most of its effect occurs in the front 200 µm of the cornea. Specialty contact lenses can improve vision but do not stop progression. Intracorneal ring segments may improve vision in some ectatic corneas, and corneal transplantation is a last resort. Because ectasia can appear years after surgery, report any gradual blurring or new astigmatism to an eye doctor promptly, and mention your past surgery at every eye exam. See LASIK side effects.
What are my options if my corneas are thin?
Thin corneas rarely mean no option at all.
- PRK or LASEK: no flap, so more stroma is preserved.
- SMILE: no flap, though tissue is still removed and limits apply.
- ICL: removes no corneal tissue, often suited to thin corneas with high myopia.
Should I ask for my numbers?
Yes. Ask for your central and thinnest corneal thickness, planned flap thickness, ablation depth, RSB and PTA, in writing.
See questions to ask your LASIK surgeon and ICL vs LASIK. This page is general information; your ophthalmologist interprets your scans.
Sources
Key facts
- Average central thickness: about 540–550 µm
- Residual stromal bed (RSB): at least 250 µm; many surgeons target 300 µm
- Percent tissue altered (PTA): above about 40% linked to ectasia (Santhiago et al., 2014)
- Thin corneas: under about 470–500 µm often steered away from LASIK
- Alternatives: PRK, LASEK, SMILE or ICL
Related guides
Independent directory. Prices are typical ranges reported by Seoul clinics and change often. Always confirm a written quote and get a full eye exam before booking. This page is general information, not medical advice.
FAQ
Frequently asked questions
What is a normal corneal thickness?
Most central corneas measure about 540–550 µm. Thickness varies between people and is measured with pachymetry or tomography during your exam.
Is 500 microns enough for LASIK?
It can be for a small correction, but the answer depends on your planned flap, ablation depth, residual stromal bed and corneal shape. Many surgeons become cautious around 470–500 µm.
What is the 250-micron rule?
It is the widely used minimum residual stromal bed left under the flap after LASIK. Many surgeons prefer 300 µm, and percent tissue altered is also checked.
Is PRK safer than LASIK for thin corneas?
Often, yes. PRK cuts no flap, so more of the cornea's strength is preserved, and ectasia is less common after PRK.
Can corneas get thicker?
Not meaningfully. Thickness can read slightly differently with contact lens wear or swelling, which is one reason clinics ask you to stop lenses before the exam.
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