Refractive Surgery Calculator: Ablation Depth, Residual Stromal Bed and Percent Tissue Altered
Last updated: November 2026
Before laser refractive surgery you need to know how much cornea the treatment will remove and how much will be left. This calculator estimates the ablation depth with the Munnerlyn formula, then the residual stromal bed (RSB) and percent tissue altered (PTA) for LASIK, PRK or SMILE, and flags values linked to post-operative ectasia.
Munnerlyn formula: ablation depth (µm) ≈ optical zone (mm)² × correction (D) ÷ 3. For myopic astigmatism the deepest meridian decides the depth, so the correction used is the sphere plus the full minus cylinder. Modern aspheric and wavefront profiles, and the blend zone, usually remove about 10–25% more, so the calculator also shows a margin. Hyperopic ablation removes tissue mainly in the periphery and is not depth-limited in the same way.
Safety thresholds
| Measure | Commonly used limit |
|---|---|
| Residual stromal bed (LASIK and SMILE) | At least 250 µm is the traditional minimum; many surgeons aim for 300 µm or more |
| Percent tissue altered = (flap or cap + ablation) ÷ preoperative CCT | 40% or more is associated with ectasia even with normal topography (Santhiago) |
| Post-operative total CCT | Below about 400 µm is a common warning level |
| Preoperative CCT | Below about 480–500 µm needs extra caution; prefer surface ablation or a lens-based option |
Topography comes first. No thickness calculation makes an abnormal cornea safe. Forme fruste keratoconus on tomography (posterior elevation, Belin-Ambrósio display, thinnest-point displacement), young age and high myopia are the strongest ectasia risks. For high myopia or thin corneas, consider an implantable collamer lens (ICL).
For learning and quick checks only. Confirm any result that changes treatment against your own measurements, the device manufacturer's guidance and current local protocols.
Related: Refractive Surgery for Residents: PRK, LASIK, SMILE and ICL, Keratoconus: Signs, Indices, Staging, Progression and Treatment, Topography and Keratoconus Screening Card: Cut-offs and Staging, Post-Refractive Surgery IOL Calculation: A Practical Guide. See all eye calculators and tools.
Key sources
- Munnerlyn CR, Koons SJ, Marshall J. Photorefractive keratectomy: a technique for laser refractive surgery. J Cataract Refract Surg. 1988;14:46-52
- Santhiago MR, Smadja D, Gomes BF, et al. Association between the percent tissue altered and post-laser in situ keratomileusis ectasia in eyes with normal preoperative topography. Am J Ophthalmol. 2014;158:87-95
- Randleman JB, Woodward M, Lynn MJ, Stulting RD. Risk assessment for ectasia after corneal refractive surgery. Ophthalmology. 2008;115:37-50
- American Academy of Ophthalmology. Refractive Errors Preferred Practice Pattern (current edition)
📘 Want more exam-ready material? See Most Commons in Ophthalmology on Kindle, or browse all my books.
Looking for an eye specialist in Ahmedabad? LASIK at Sadbhaav or book a consultation with Dr. Dhaval Patel.