Key Points of Refractive Surgery in Corneal Shape Analysis

Key Points at a Glance

1. Corneal Shape Analysis and Refractive Surgery

Corneal topography is a technique that measures and evaluates the geometric characteristics of the corneal surface. The term derives from the Greek words “topos” (place) and “graphein” (to draw).

Conventional keratometers measure only the curvature of the central 3–4 mm of the anterior cornea, which is insufficient for refractive surgery evaluation. Today, computerized corneal shape analysis has become the standard clinical examination.

Main Purposes of Corneal Shape Analysis

Detection of epithelial irregularities and stromal abnormalities
Evaluation of corneal astigmatism
Screening for subclinical keratoconus
Confirmation of refractive stability
Evaluation of undetected corneal disease

In preoperative evaluation for refractive surgery, topography and tomography are performed after a contact lens wear cessation period ⁴⁾. Irregular astigmatism and posterior corneal abnormalities are associated with unpredictable refractive outcomes and postoperative ectasia ⁴⁾.

Q Why is topography essential for refractive surgery?

Corneal refractive power accounts for about two-thirds of the eye’s total refractive power, and refractive surgery corrects vision by altering corneal shape. If latent keratoconus is missed preoperatively, it can lead to a serious complication called corneal ectasia after surgery. Topography evaluates the shape of the entire cornea and can detect subtle abnormalities that are difficult to find with routine examinations, making it indispensable for preoperative evaluation.

2. Main Symptoms and Clinical Findings

Basics of Topography Maps

In topography maps, diopter values are represented by colors. Warm colors indicate steep curvature (high D values), while cool colors indicate flat curvature (low D values). A normal cornea flattens toward the periphery and is shown in cool colors. Both eyes tend to have mirror-symmetric topography.

Topography Patterns

The following pattern classifications based on normal corneas have been proposed:

Round
Oval
Symmetric bowtie
Asymmetric bowtie
Irregular

Elevation Map

Displays the height deviation from the reference surface (best-fit sphere: BFS). Areas higher than the reference are shown in warm colors, and lower areas in cool colors.

Anterior Elevation Map

Normal: ≤+12 µm

Suspicious: +13 to +15 µm

At risk: >+15 µm

Posterior Elevation Map

Normal: ≤+17 µm

Suspicious: +18 to +20 µm

At risk: >+20 µm

Posterior elevation difference is an excellent predictor of keratoconus, with many studies reporting sensitivity and specificity over 90%.

Corneal Thickness Map

Displays the distribution of corneal thickness across the entire cornea. Central corneal thickness (CCT) less than 500 µm, together with topographic asymmetry, is a diagnostic criterion for early keratoconus.

Topography After LASIK

In myopic LASIK, the central anterior cornea flattens and the central corneal thickness decreases, but the posterior cornea remains unchanged. Postoperative corneal shape analysis is useful for evaluating the ablation bed and detecting ectasia.

3. Causes and Risk Factors

Risk Factors for Corneal Ectasia

When topographic or tomographic abnormalities suggest subclinical keratoconus, clinically significant ectasia may develop after refractive surgery ⁴⁾.

The main risk factors are as follows ⁴⁾:

Young age
High manifest refraction spherical equivalent
Decreased corneal thickness
Thin predicted residual stromal bed (RSB) thickness

Abnormal preoperative topography and insufficient RST are the most important contributing factors for corneal ectasia ⁵⁾. Overall, the risk of corneal ectasia is lower with PRK and SMILE compared to LASIK ⁴⁾. This is attributed to the thicker residual corneal stroma and the absence of a corneal flap in PRK ⁴⁾.

Risks of Enhancement Surgery

Enhancement surgery after LASIK (flap lift) has been reported to have a high incidence of epithelial ingrowth at 32%. Li & Gu reported a case of rapidly progressive epithelial ingrowth on the first day after enhancement surgery ³⁾. Corneal topography showed a progressive increase in anterior elevation difference in the inferonasal quadrant and corneal thickening in the same area, with irregular astigmatism increasing from 0.6 D on day 1 to 2.0 D on day 5 ³⁾.

4. Diagnosis and Examination Methods

Indices for Ectasia Risk Assessment

Indicator	Reference Value	Characteristics
BAD-D	<1.6: normal, >2.6: abnormal	Integrated assessment of elevation difference and corneal thickness
PTA	<40%: low risk	(Flap thickness + ablation depth) / central corneal thickness
KISA%	60–100% for forme fruste KC	Central K + I-S + SRAX

BAD-D (Belin-Ambrósio Ectasia Display)

A comprehensive screening tool developed by Belin and Ambrósio ⁴⁾. It evaluates using a “D” score that integrates five parameters (dp, db, df, dt, dy) based on anterior and posterior elevation differences and corneal thickness. A standard deviation of less than 1.6 for each parameter is normal (white), 1.6–2.6 is suspicious (yellow), and greater than 2.6 is abnormal (red).

Ablation Calculation and RSB

The ablation depth in LASIK is estimated using the Munnerlyn formula.

t = S²D / 3 (t: ablation depth [μm], S: optical zone diameter [mm], D: correction amount [spherical equivalent])

Residual stromal bed (RSB) = central corneal thickness − ablation depth − flap thickness, and a minimum of 250 μm (safe margin 300 μm) is ensured. The Japanese Ophthalmological Society guidelines also specify a residual corneal thickness of 250 μm or more, and myopia exceeding −10 D is considered the limit of corneal correction. Flaps created with femtosecond laser (100–120 μm) are more uniform and thinner than those with mechanical microkeratome (average 120 μm, with variability), which is advantageous for preserving RSB. The optical zone has traditionally been 6.5 mm, but setting it more than 15% larger than the pupil diameter can effectively suppress postoperative higher-order aberrations. It has been reported that with a 7 mm optical zone, there was almost no increase in higher-order aberrations for myopia correction of 3.50 D ⁵⁾. Enlarging the optical zone increases ablation volume, so refractive error, corneal thickness, and pupil diameter must be considered comprehensively.

In KLEx (SMILE), because the cap maintains biomechanical strength, applying the LASIK-style PTA calculation directly may overestimate the risk ⁵⁾. For KLEx, a planning protocol maintaining a minimum RST of 220 μm and total uncut stromal thickness of 300 μm has been reported ⁵⁾.

PTA (Percent Tissue Altered)

PTA = (flap thickness + ablation depth) / preoperative central corneal thickness

PTA of 40% or more is significantly associated with corneal ectasia even in eyes with normal preoperative corneal topography ⁷⁾. Because it captures risks not detectable by RSB alone, it is more sensitive than its individual components ⁵⁾.

Randleman Score

This is a risk factor stratification scale that includes age, corneal thickness, topography pattern, RSB thickness, and correction amount. Cumulative scores of 0–2 indicate low risk, 3 moderate risk, and 4 or more high risk.

Epithelial Thickness Mapping

Epithelial thickness mapping using OCT is useful for screening ectasia. In ectatic diseases, corneal steepening is accompanied by epithelial thinning, presenting an “epithelial doughnut pattern.” In contrast, contact lens warpage shows epithelial thickening and normal corneal thickness at the steepening site, allowing differentiation between the two.

Topography/tomography is also useful for evaluating irregular astigmatism due to corneal edema or scarring, and assessing the depth of corneal opacity aids in surgical planning ⁶⁾.

Q What is BAD-D?

The Belin-Ambrósio Enhanced Ectasia Total Deviation (BAD-D) is a comprehensive ectasia screening index that integrates information on anterior and posterior corneal elevation differences and corneal thickness. It is evaluated using a final “D” score based on the standard deviation of five parameters, with values below 1.6 considered normal and above 2.6 indicating possible corneal ectasia. It is widely used in preoperative screening for refractive surgery.

5. Standard Treatment

Topography-Guided LASIK (TG-LASIK)

TG-LASIK is a refractive surgery that performs customized ablation based on corneal topography data. It has the theoretical advantage of improving the natural shape of the cornea and reducing higher-order aberrations.

In a prospective study by Rush et al., TG-LASIK using Phorcides analysis software improved the overall visual satisfaction index on the PROWL questionnaire from 4.07 preoperatively to 5.00 (maximum) postoperatively ²⁾. 100% of patients reported maximum postoperative satisfaction ²⁾. At 26 weeks, binocular uncorrected visual acuity was 20/16 or better in 100% of patients and 20/12.5 or better in 87.0% ²⁾.

Significant postoperative improvements were observed in all measures of night vision, glare, halos, starbursts, and dry eye symptoms ²⁾. Corneal higher-order aberrations increased significantly in the 6 mm optical zone, but there was no significant change in total higher-order aberrations under scotopic pupil conditions ²⁾.

Postoperative Corneal Topography Analysis

Postoperative corneal topography analysis is useful for the following evaluations:

Assessment of ablation bed uniformity and surgical quality
Detection and progression assessment of ectasia
Analysis of residual astigmatism

Examination should be performed at least 1 week postoperatively. The minimum criterion for progression of corneal ectasia is documentation of at least two of the following: anterior steepening, posterior steepening, and thinning ⁴⁾.

Surgical Technique Selection and the Role of Topography

It is important to evaluate abnormally steep or flat corneal curvature. Steep corneas have a higher risk of buttonhole, while flat corneas have a higher risk of free cap. These complications have been reported with mechanical microkeratomes but are rare with femtosecond lasers.

Q What are the advantages of TG-LASIK?

Topography-guided LASIK applies an individualized ablation pattern based on corneal shape data. It can reduce irregular astigmatism and higher-order aberrations, and higher patient satisfaction has been reported compared to conventional wavefront-guided/optimized LASIK ²⁾. The Phorcides analysis software enables objective determination of treatment parameters and improves reproducibility among surgeons.

6. Pathophysiology and Detailed Mechanisms

Optical Properties of the Cornea

Corneal refractive power accounts for about two-thirds of the eye’s total refractive power. In with-the-rule astigmatism, the elevation maps of the anterior and posterior surfaces show a horizontal ridge pattern, while the axial power map shows a vertical bow-tie pattern. In against-the-rule astigmatism, the anterior elevation map shows a vertical ridge pattern, but the anterior and posterior patterns are asymmetric.

Corneal Shape Changes in Keratoconus

In keratoconus, the central to inferior cornea becomes thinner, and both the anterior and posterior surfaces protrude forward. As a result, localized steepening occurs from the center to the inferior cornea.

Epithelial Remodeling

In keratoconus, the corneal epithelium becomes thinner at the protrusion and forms a ring of thicker epithelium (epithelial doughnut pattern) around it. The thinnest point of the epithelium is displaced temporally and inferiorly relative to the stromal protrusion. This epithelial remodeling may lead to underestimation of the degree of ectasia when using topography alone.

7. Latest Research and Future Perspectives

Artificial Intelligence (AI) Image Analysis

AI has been shown to potentially augment existing tomographic and biomechanical assessments and improve detection of corneal ectasia. Reports indicate that machine learning algorithms approach the accuracy of corneal specialists in distinguishing normal corneas, suspicious irregular corneas, and keratoconus.

Corneal Densitometry

Corneal densitometry using a Scheimpflug camera is an objective method for measuring corneal transparency ¹⁾. In a prospective study by Balparda et al. (110 eyes), it showed excellent reproducibility in the area within 10 mm, and a change of 1.0 GSU or more could be considered a true change in transparency ¹⁾. The 10–12 mm zone had large variability and insufficient reliability ¹⁾. It may be useful for quantitative evaluation of corneal haze after PRK ¹⁾.

Corneal Biomechanics

Patients with low corneal stiffness have been reported to have a 2–3 times higher risk of residual refractive error after KLEx (keratorefractive lenticule extraction) ⁵⁾. Measurement of biomechanics may have significant value in improving surgical accuracy ⁵⁾. Combining corneal biomechanical indices with topographic parameters has been reported to improve the prediction accuracy of KLEx by more than 25% ⁵⁾.

Nomogram Adjustment

Nomogram adjustment is directly linked to the accuracy and predictability of laser surgery ⁵⁾. Preoperative spherical equivalent is the most important factor, and age, eye laterality, corneal curvature, corneal diameter, and corneal biomechanical properties are also relevant. Adjustment strategies include simple spherical and cylindrical correction, multivariate regression analysis, and personalized adjustment using artificial intelligence ⁵⁾.

Q What is the clinical significance of corneal densitometry?

Corneal densitometry measures the backscattered light from the cornea using a Scheimpflug camera and objectively represents transparency as a numerical value from 0 to 100 GSU ¹⁾. It can quantitatively evaluate temporal changes in corneal haze after PRK and the response after corneal cross-linking. In the area within 10 mm, a change of 1.0 GSU or more is considered clinically significant ¹⁾.

8. References

Balparda K, MesaMesa S, MayaNaranjo MI, et al. Determination of the repeatability of corneal densitometry as measured with a Scheimpflug camera device in refractive surgery candidates. Indian J Ophthalmol. 2023;71:63-68.
Rush SW, Pickett CJ, Wilson BJ, Rush RB. Topography-guided LASIK: a prospective study evaluating patient-reported outcomes. Clin Ophthalmol. 2023;17:2815-2824.
Li X, Gu Y. Unusual visual impairment after enhancement refractive surgery. J Surg Case Rep. 2024;2:rjae074.
American Academy of Ophthalmology Corneal Ectasia PPP Panel. Corneal Ectasia Preferred Practice Pattern. Ophthalmology. 2024.
Ang M, Gatinel D, Reinstein DZ, et al. Evidence-based guidelines for keratorefractive lenticule extraction. Ophthalmology. 2025;132(4):404-418.
American Academy of Ophthalmology Corneal/External Disease PPP Panel. Corneal Edema and Opacification Preferred Practice Pattern. Ophthalmology. 2019;126(1):P216-P285.
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(1):87-95.

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