Corneal Topography Findings
Inferior steepening: I/S ratio ≥1.2
Steepening of corneal refractive power: >46 D suggests ectasia
Asymmetric pattern: Skewed radial axes >21°
Ectasia After Refractive Surgery
1. What is ectasia after refractive surgery?
Section titled “1. What is ectasia after refractive surgery?”Ectasia after refractive surgery (iatrogenic keratectasia / post-refractive surgery ectasia) is a condition in which the corneal stroma progressively and eccentrically thins after refractive surgery such as LASIK, PRK, or SMILE, causing steepening of the anterior and posterior surfaces. It is considered one of the most serious complications of refractive surgery, and is clearly listed as postoperative complication ⑦ of excimer laser surgery in the refractive surgery guidelines (8th edition) 1).
It is irreversible and significantly reduces both uncorrected visual acuity and spectacle-corrected visual acuity. If progression is confirmed, early intervention affects the prognosis.
Incidence by surgical procedure
Section titled “Incidence by surgical procedure”The risk of ectasia varies greatly depending on the surgical procedure.
| Procedure | Prevalence (per 100,000 eyes) | Main features |
|---|---|---|
| LASIK | Approximately 90 | Flap creation reduces corneal biomechanics. Highest risk. |
| PRK | Approximately 20 | No flap. Risk about 1/4 of LASIK. |
| SMILE | Approximately 11 | The cap may contribute to corneal strength to some extent2) |
The incidence of LASIK is approximately 4.5 times that of PRK3). However, SMILE has a short follow-up period after approval, and there is a possibility of underestimation3). Additionally, it is estimated that up to 6% of refractive surgery candidates have some form of subclinical ectatic disease, and the accuracy of preoperative screening influences the incidence rate.
Q
Does SMILE prevent ectasia?
A
SMILE is reported to have a lower incidence of ectasia compared to LASIK (11 vs 90 per 100,000 eyes)3), but the risk is not eliminated. It has been suggested that the cap in SMILE may contribute to corneal strength to some extent2), but the follow-up period is still short, and long-term incidence may be underestimated. Corneal ectasia is also listed as a postoperative complication of SMILE1), and preoperative screening and adherence to safety thresholds are essential.
2. Risk Factors and Preoperative Screening
Section titled “2. Risk Factors and Preoperative Screening”The most important preventive measure for ectasia is thorough preoperative risk assessment.
Major Risk Factors
Section titled “Major Risk Factors”| Risk Factor | Details |
|---|---|
| Keratoconus (including forme fruste) | Most important factor. Listed as a contraindication in the 8th edition of the guidelines1) |
| Insufficient RST (Residual Stromal Bed) | Risk sharply increases when RST <280μm. RST <250μm is contraindicated2) |
| High LT index | Risk increases when the ratio of maximum ablation thickness to central corneal thickness (LT/CCT) exceeds 28%2) |
| High PTA | Percent tissue altered ≥40% is significantly associated with LASIK ectasia risk4) |
| Young age | Under 34 years old. Under 18, the cornea is unstable and progression is rapid. |
| High myopia | Requires a large amount of ablation, often resulting in insufficient residual stromal bed. |
| Eye rubbing habit | The only confirmed lifestyle factor associated with progression. |
Safety Threshold Criteria
Section titled “Safety Threshold Criteria”Evidence-based international guidelines for corneal cross-linking surgery recommend the following safety thresholds2).
| Parameter | Safe Criteria | Contraindication Criteria |
|---|---|---|
| RST (Residual Stromal Thickness) | ≥280 μm | <250 μm (not allowed even after considering measurement error) |
| LT index (LT/CCT ratio) | ≤28% | >28% |
| PTA (percent tissue altered) *LASIK | <40% | ≥40% |
In SMILE (KLEx), the interpretation of PTA calculation differs from LASIK. Since the cap contributes to corneal structural strength unlike the flap, there is debate about directly applying the LASIK-based PTA threshold 2).
Corneal Biomechanics Evaluation
Section titled “Corneal Biomechanics Evaluation”Corneal topography analysis or RST alone does not exceed 70% sensitivity in predicting postoperative ectasia 2). TBI (Tomographic and Biomechanical Index, SUCRA 96.2), CBI (Corvis Biomechanical Index, SUCRA 83.8), and CRF (SUCRA 66.4) are useful for early keratoconus detection 2). Comprehensive evaluation of corneal morphology and biomechanics is recommended.
3. Clinical Findings and Diagnosis
Section titled “3. Clinical Findings and Diagnosis”Subjective Symptoms
Section titled “Subjective Symptoms”The following symptoms appear several months to several years after surgery. They are all characterized by being progressive.
- Progressive myopia and astigmatism
- Decreased uncorrected visual acuity
- Decreased best-corrected visual acuity (BCVA)
- Image distortion, halos, and glare due to irregular astigmatism
- Increased higher-order aberrations (vertical coma)
Clinical Findings
Section titled “Clinical Findings”Corneal Tomography Findings
Increased posterior elevation: Anterior protrusion of the posterior corneal surface is an early change
Eccentricity of the thinnest point: Eccentricity on the corneal thickness map
Increased BAD-D value: Suspect ectasia if >1.65
Slit-Lamp Microscopy Findings
Fleischer ring: Iron deposition in the epithelium at the base of the cone
Vogt’s striae: Folds in Descemet’s membrane
Corneal apex scar: Appears in advanced cases
Differential Diagnosis
Section titled “Differential Diagnosis”Postoperative ectasia and naturally occurring keratoconus have similar corneal shape and clinical findings, but the presence or absence of a history of refractive surgery is key to differentiation. Cases where surgery was performed with suspected keratoconus and ectasia became apparent postoperatively, and postoperative ectasia in normal corneas, are pathologically continuous in some aspects6).
For differentiation, it is important to review surgical records (flap thickness, ablation depth, postoperative RST).
Q
What is the difference between ectasia and keratoconus?
A
Corneal ectasia is caused by iatrogenic conditions after refractive surgery, while keratoconus is a spontaneously occurring degenerative disease, differing in etiology. However, the clinical features (corneal thinning, steepening, irregular astigmatism) and pathogenesis (breakdown of corneal biomechanics) are essentially similar, and many cases of forme fruste keratoconus are believed to become manifest after surgical trauma 6). Differentiation is based on history of surgery and review of preoperative data.
4. Treatment and Management
Section titled “4. Treatment and Management”The treatment strategy for ectasia consists of two pillars: ① halting progression and ② correcting and restoring visual function. Early intervention upon confirmation of progression is important for preserving visual function.
| Treatment | Indication | Purpose |
|---|---|---|
| Corneal cross-linking (CXL) | At confirmation of progression / first-line | Arrest progression (collagen cross-linking strengthening) |
| Rigid gas permeable (RGP) contact lenses | When irregular astigmatism is severe | Visual function correction |
| Intracorneal ring segments (ICRS) | Moderate ectasia | Reduction of irregular astigmatism |
| CXL + topography-guided PRK | Advanced ectasia | Simultaneous correction of irregular astigmatism and halting progression |
| CXL + ICRS | Moderate to severe ectasia | Combined approach |
| Penetrating keratoplasty (PKP) | Advanced cases with corneal opacity | Last resort |
| Deep anterior lamellar keratoplasty (DALK) | Cases with preserved endothelial function | PKP alternative (endothelial sparing) |
Corneal cross-linking (CXL)
Section titled “Corneal cross-linking (CXL)”CXL is the first-line treatment for confirmed progressive ectasia. After instillation of 0.1% riboflavin, ultraviolet A (3 mW/cm²) is irradiated to strengthen cross-links between corneal collagen fibers, thereby stabilizing the corneal structure. In addition to the standard Dresden protocol, accelerated and pocket methods are available. After CXL, progression stops in most cases, and some improvement in corneal steepening may be observed.
In high-risk patients (e.g., prepubertal or young individuals), early consideration of CXL is recommended without waiting for further visual deterioration.
Hard Contact Lenses (RGP)
Section titled “Hard Contact Lenses (RGP)”These are the mainstay for correcting reduced visual acuity due to irregular corneal astigmatism. By forming a tear lens on the posterior surface of the lens, irregular astigmatism is optically corrected. They are effective for moderate ectasia and allow many patients to maintain daily visual function. Good centration and movement fitting are important.
Corneal Transplantation
Section titled “Corneal Transplantation”This is considered in advanced cases with corneal opacity or scarring where contact lens correction is not possible. Full-thickness penetrating keratoplasty (PKP) has been the standard, but if the endothelium is normal, deep anterior lamellar keratoplasty (DALK), which preserves the endothelium, is an option.
Q
Can ectasia be cured?
A
With current treatments, “cure” (restoration of the original corneal shape) is difficult, but corneal cross-linking (CXL) can halt progression. After CXL, the corneal shape stabilizes in many cases, allowing continued visual correction with contact lenses or glasses. In advanced cases, combining CXL with topography-guided PRK or intracorneal ring segments (ICRS) may improve irregular astigmatism. As a last resort, corneal transplantation can restore visual function in some cases.
5. Pathophysiology and Detailed Mechanisms
Section titled “5. Pathophysiology and Detailed Mechanisms”Corneal Biomechanical Failure
Section titled “Corneal Biomechanical Failure”The essence of post-refractive surgery ectasia is the disruption of corneal biomechanics due to surgical trauma.
- Flap/Cap Influence: The LASIK flap is considered to contribute little to corneal biomechanics postoperatively 2). In contrast, the cap in SMILE, which preserves the anterior corneal stroma, may bear some strength 2).
- Anterior Stromal Cutting: The anterior corneal stroma is the layer that contributes most to biomechanical tensile strength, and its excision by excimer laser reduces this function 7).
- Insufficient RST Feedback Loop: Insufficient residual stromal thickness (RST) leads to stress concentration on the remaining stroma, accelerating progressive thinning. When RST falls below 280 μm, corneal biomechanical stability deteriorates rapidly 2).
- LT index threshold effect: When the LT index exceeds 28%, the rate of change in CH (corneal hysteresis) and CRF increases significantly2)
Manifestation of Forme Fruste Keratoconus
Section titled “Manifestation of Forme Fruste Keratoconus”In many cases, subtle corneal biomechanical weakness (forme fruste or subclinical keratoconus) that could not be detected preoperatively becomes apparent after surgical trauma. The first reported case of ectasia after LASIK in forme fruste keratoconus was by Seiler et al. in 19986).
Significance of Posterior Corneal Elevation
Section titled “Significance of Posterior Corneal Elevation”Anterior shift of the posterior corneal elevation is recognized as an early sign of ectasia. In some cases, the posterior surface changes before the anterior surface, making tomography including posterior surface evaluation essential for early diagnosis.
Strength Comparison Using Mathematical Models
Section titled “Strength Comparison Using Mathematical Models”In Reinstein et al.’s mathematical model, the relative corneal tensile strength of PRK, LASIK, and SMILE was quantified 7). SMILE preserves more anterior stroma compared to LASIK, thus retaining greater corneal strength for the same amount of correction. This structural difference is thought to be associated with the lower incidence of ectasia in SMILE.
6. Latest Research and Future Perspectives
Section titled “6. Latest Research and Future Perspectives”- Advances in corneal biomechanics measurement: Real-time assessment using Corvis ST and ORA is improving the accuracy of preoperative screening 2). Comprehensive evaluation using TBI and CBI is expanding.
- AI and machine learning: Development of AI prediction models integrating corneal morphology, biomechanics, and genetic data is progressing. Corneal shape analysis alone does not exceed 70% sensitivity for predicting postoperative ectasia 2), but multimodal AI evaluation is expected to improve this.
- Prophylactic CXL: The concept of performing CXL simultaneously with refractive surgery in cases at high risk of ectasia is being studied, but evidence is not yet established.
- Molecular-level research on corneal collagen: Drug interventions targeting the regulation of MMP and TIMP systems, and individualized risk assessment through gene expression analysis of collagen fibers.
- New corneal strengthening materials: Research is progressing on corneal strengthening approaches other than CXL, such as topical administration of collagen cross-linking agents and reinforcement with new biomaterials.
7. References
Section titled “7. References”- 日本眼科学会屈折矯正委員会. 屈折矯正手術のガイドライン（第8版）. 日眼会誌. 2024;128(2):135-138.
- Wang Y, Xie L, Yao K, Sekundo W, Alió JL, Mehta JS, Goel S, Elmassry A, Schallhorn J, Shilova T, Cao H, Xu L, Chen X, Zhang F, Bai J, Zhang W, Liu Q, Zhou X, Chen Y, Wang Z, Jhanji V, Yang K, Writing Committee for the Guideline Working Group. Evidence-Based Guidelines for Keratorefractive Lenticule Extraction Surgery. Ophthalmology. 2025;132(4):397-419. doi:10.1016/j.ophtha.2024.11.016. PMID:39577672.
- Moshirfar M, Tukan AN, Bundogji N, Liu HY, McCabe SE, Ronquillo YC, et al. Ectasia After Corneal Refractive Surgery: A Systematic Review. Ophthalmology and therapy. 2021;10(4):753-776. doi:10.1007/s40123-021-00383-w. PMID:34417707; PMCID:PMC8589911.
- Santhiago MR, Smadja D, Gomes BF, et al. Association between the percent tissue altered and post-LASIK ectasia in eyes with normal preoperative topography. Am J Ophthalmol. 2014;158:87-95.e1. doi:10.1016/j.ajo.2014.04.002. PMID:24727263.
- Gomes JA, Tan D, Rapuano CJ, Belin MW, Ambrósio R, Guell JL, Malecaze F, Nishida K, Sangwan VS, Group of Panelists for the Global Delphi Panel of Keratoconus and Ectatic Diseases. Global consensus on keratoconus and ectatic diseases. Cornea. 2015;34(4):359-369. doi:10.1097/ico.0000000000000408. PMID:25738235.
- Theo Seiler, Andreas W. Quurke. Iatrogenic keratectasia after LASIK in a case of forme fruste keratoconus. Journal of Cataract and Refractive Surgery. 1998;24(7):1007-1009. doi:10.1016/s0886-3350(98)80057-6.
- Reinstein DZ, Archer TJ, Randleman JB. Mathematical model to compare the relative tensile strength of the cornea after PRK, LASIK, and small incision lenticule extraction. J Refract Surg. 2013;29:454-460. doi:10.3928/1081597X-20130617-03. PMID:23820227.