Preoperative corneal topography is a computer-assisted corneal curvature mapping test performed before cataract or refractive surgery. It quantitatively assesses the curvature of the anterior and posterior cornea, corneal thickness, and anterior segment shape, and is used to improve intraocular lens power calculation accuracy, evaluate astigmatism, and screen for corneal shape abnormalities.
Modern cataract surgery is nearly synonymous with refractive surgery, and precise preoperative measurements are essential for good postoperative refractive outcomes. Keratometry is a key input for intraocular lens power calculation, and its error has been reported to account for up to 22% of postoperative refractive error 1)2).
The history of corneal topography began with Placido disc-based keratoscopy. Subsequently, technological advances led to video keratoscopy, Scheimpflug cameras, and anterior segment OCT, enabling three-dimensional evaluation of the anterior and posterior cornea. Since the first report of intraocular surgery with refractive correction in 1956, the importance of preoperative topography has increased along with improvements in intraocular lens calculation accuracy.
QIs corneal topography necessary for all cases?
A
For cataract surgery, basic keratometry is necessary for all cases. Additionally, when selecting toric or multifocal intraocular lenses, or in patients with keratoconus or a history of refractive surgery, corneal topography/tomography is recommended.
The symptoms reported by “target patients” in preoperative corneal topography analysis are primarily visual impairment due to underlying disease, astigmatism symptoms, and photophobia.
Visual impairment and astigmatism: Caused by uncorrected corneal astigmatism or irregular astigmatism. Differentiation from cataracts is necessary.
Photophobia and glare: Manifest as visual dysfunction associated with irregular corneal shape (e.g., keratoconus) or dry eye.
Refractive instability: Often becomes apparent as postoperative refractive surprise.
Power map: Displays corneal refractive power in color. Warm colors (red) indicate steep areas, cool colors (blue) indicate flat areas. A normal cornea shows a central warm color with concentric ring pattern.
Astigmatism pattern: A bow-tie shape indicates regular astigmatism, with the vertical direction representing the astigmatism axis. Map asymmetry or localized steepening suggests keratoconus.
Elevation map: Displays deviation from a best-fit sphere in color. Localized protrusion (island-like elevation) on the anterior or posterior surface is useful for detecting keratoconus or ectasia after refractive surgery.
Quantitative Shape Indices
SimK (Simulated Keratometry): Curvature values of the principal and secondary meridians obtained from corneal topography. Used for intraocular lens power calculation.
SAI and SRI: Indices indicating corneal symmetry and local regularity. Used to quantify irregular astigmatism.
Corneal thickness map (Pachymetry): Identifies the thinnest point and confirms concentric ring pattern. Eccentricity of the thinnest point suggests keratoconus.
Dry eye (tear film instability): In reflection-based optical biometers, instability of the tear film may increase measurement variability of corneal astigmatism. Particularly in eyes with high osmolarity (≥308 mOsmol/L) and shortened tear break-up time (positive NIKBUT), variability of Lenstar measurements was significantly increased1).
Corneal deformation due to contact lenses: Long-term wear of hard contact lenses, in particular, deforms the corneal shape. Discontinuation of lens wear for a certain period before measurement is necessary.
Device type and measurement principle: Reflection-based devices (Lenstar, IOLMaster, etc.) are susceptible to the tear film. OCT-based devices such as Anterion and anterior segment OCT are less affected by the tear film and provide more stable measurements1)2).
Advanced age: Aging has been reported to independently influence keratometry measurements.
Not necessarily. One report found no significant difference in postoperative absolute error or astigmatic prediction error between dry eye (treated and untreated) and non-dry eye groups2). However, some reflection-based devices show increased measurement variability, so caution is needed regarding the reliability of preoperative measurements.
Principle: A ring-shaped light is projected onto the cornea, and the curvature of the precorneal tear film is calculated from the distortion of its reflected image (Meyer rings).
Representative devices: TMS, Atlas, etc.
Features: Excellent reproducibility, but cannot evaluate the posterior corneal surface or corneal thickness. Easily affected by the tear film. Covers only about 60% of the corneal surface.
Scheimpflug type
Principle: Uses a Scheimpflug camera based on the principle of tilted photography to obtain anterior segment tomographic images. Three-dimensional shape is reconstructed by rotational scanning.
Features: Allows simultaneous evaluation of anterior and posterior corneal surfaces, corneal thickness, and anterior chamber depth. Slightly affected by opacities. GALILEI has a built-in Placido ring for high keratometry accuracy.
Anterior segment OCT (AS-OCT): SS-OCT (e.g., CASIA) uses long-wavelength light of 1,310 nm to visualize the cornea, anterior chamber, iris, anterior lens surface, and angle in a single image. Not affected by the tear film, and enables high-precision shape analysis even in eyes with corneal opacity or edema. Also applied to intraocular lens power calculation using ray tracing methods such as OKULIX.
Combined biometers: Eyestar (combination of OCT and reflection), IOLMaster 700 (combination of SS-OCT and reflection), and other latest-generation devices integrate multiple technologies.
Keratoconus screening: One of the most important screenings performed before refractive surgery and cataract surgery. The following patterns are suggestive:
Inferior steepening
I/S value (inferior-superior refractive power ratio) > 1.7 D
Maximum SimK > 48.7 D
Interocular maximum SimK difference > 0.5 D
Island-like anterior protrusion on elevation map
Eccentricity of the thinnest point on corneal thickness map
Evaluation of posterior curvature: Posterior corneal astigmatism is not always proportional to anterior astigmatism. Using methods that include posterior curvature (e.g., Barrett Toric formula) in toric intraocular lens calculations can significantly reduce residual astigmatism.
5. Standard Clinical Applications (Role in Preoperative Evaluation)
When implanting a toric intraocular lens: The ESCRS Cataract Guideline recommends that when planning toric IOL implantation, corneal topography and/or tomography should be performed in addition to standard preoperative evaluation (GRADE+). It also recommends using formulas that include posterior corneal astigmatism and effective lens position (GRADE+).
Candidates for multifocal IOL or EDOFIOL: Exclusion of irregular astigmatism and evaluation of corneal shape are essential.
Post-refractive surgery eyes: Manual keratometry is inaccurate because it overestimates corneal effective refractive power. Topography-based calculations reflecting central corneal flattening (3.0 mm zone) or special formulas are required.
Eyes with corneal disease: Shape evaluation in eyes with endothelial dystrophy, pterygium, or corneal opacity.
Preoperative screening: Exclusion of keratoconus, irregular astigmatism, and contact lens-induced corneal deformation is mandatory before LASIK/PRK. Forme fruste keratoconus and early keratoconus are contraindications for LASIK.
Postoperative evaluation: Assessment of laser ablation uniformity. Useful from 30 days after PRK and 1 week after LASIK. Detection and monitoring of postoperative ectasia.
6. Pathophysiology and Mechanisms of Measurement Error
Reflection-based keratometers analyze the reflected image of the precorneal tear film. Instability and hyperosmolarity of the tear film cause irregularities on the tear surface, leading to measurement variability as distortion of the Meyer ring image.
Nilsen et al. (2024) reported in an RCT of 131 patients scheduled for cataract surgery that although there was no significant difference in keratometry variability according to the comprehensive diagnostic criteria for dry eye (DEWS II signs), eyes with hyperosmolarity (≥308 mOsmol/L) showed significantly higher astigmatism variability measured by Lenstar (p=0.01), and eyes with positive NIKBUT showed a significantly higher proportion of variability in mean K exceeding 0.25 D with Lenstar (p=0.048) 1). No similar significant differences were found with Anterion or Eyestar.
OCT-based devices (such as Anterion) directly detect tissue backscattered light, so they do not depend on tear film reflection, maintaining accuracy even in corneas with opacity, edema, or irregular shape.
Keratometry is a key input for intraocular lens power calculation, and its error can account for up to 22% of postoperative refractive error 1)2). Especially in eyes after refractive surgery, overestimation of corneal effective refractive power (keratometric index error) tends to cause myopic refractive surprise.
Nilsen et al. (2024) reported in a prospective RCT of 131 patients that two weeks of artificial tear treatment (Thealoz Duo, 6 times daily) did not significantly improve keratometry variability or postoperative refractive prediction error (absolute error, astigmatism prediction error) 2). Other studies using anti-inflammatory drugs (cyclosporine, lifitegrast, etc.) have shown improvement, suggesting that more advanced treatment may be necessary.
The diagnostic criteria based on DEWS II may not be optimal in the context of cataract surgery. Studies are underway to determine whether individualized dry eye treatment using hyperosmolarity or positive NIKBUT as indicators can improve preoperative measurement accuracy 2).
There are reports that 28-day treatment with anti-inflammatory drugs (cyclosporine 0.09% or lifitegrast) improved preoperative biometric measurements and significantly reduced postoperative refractive prediction error, suggesting the effectiveness of therapeutic interventions beyond standard artificial tears.
New-generation biometers that integrate OCT and reflection technology (e.g., Eyestar, IOLMaster 700) may have higher tolerance to keratometry fluctuations caused by tear film instability compared to conventional reflection-based devices 1). Long-term safety and accuracy profiles are being evaluated.
Nilsen C, Gundersen M, Jensen PG, Gundersen KG, Potvin R, Utheim ØA, et al. The significance of dry eye signs on preoperative keratometry measurements in patients scheduled for cataract surgery. Clin Ophthalmol. 2024;18:151-161.
Nilsen C, Gundersen M, Jensen PG, Gundersen KG, Potvin R, Utheim ØA, et al. Effect of artificial tears on preoperative keratometry and refractive precision in cataract surgery. Clin Ophthalmol. 2024;18:1503-1514.
Shah Z, Hussain I, Borroni D, Khan BS, Wahab S, Mahar PS. Bowman’s layer transplantation in advanced keratoconus; 18-months outcomes. Int Ophthalmol. 2022;42(4):1161-1173. PMID: 34767125.
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