Preoperative corneal shape analysis (preoperative topography) is a computer-assisted corneal curvature mapping test performed before cataract surgery 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 the accuracy of intraocular lens power calculation, evaluate astigmatism, and screen for corneal shape abnormalities.
Modern cataract surgery is nearly synonymous with refractive surgery, and precise preoperative measurements are essential to achieve 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 error1)2).
The history of corneal shape analysis began with Placido disc-based keratoscopy. Subsequently, technological innovations such as videokeratoscopy, Scheimpflug cameras, and anterior segment OCT have enabled 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 shape analysis necessary for all cases?
A
Basic keratometry measurement is necessary for all cataract surgery cases. Furthermore, corneal topography/tomography is recommended when selecting toric or multifocal intraocular lenses, or in cases of keratoconus or a history of refractive surgery.
The symptoms reported by “target patients” in preoperative corneal topography analysis are primarily visual acuity loss, astigmatism symptoms, and photophobia due to the underlying disease.
Visual acuity loss and astigmatism: Caused by uncorrected corneal astigmatism or irregular astigmatism. Differentiation from cataracts is necessary.
Photophobia and glare: Manifest as visual function decline associated with corneal irregularity (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) 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 being 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) of the anterior and posterior surfaces is useful for detecting keratoconus or ectasia after refractive surgery.
Quantitative Shape Indices
SimK (Simulated Keratometry): Curvature values of the principal meridians obtained from corneal topography. Used for intraocular lens power calculation.
SAI and SRI: Indices indicating corneal symmetry and local uniformity. 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.
The main factors affecting the accuracy of corneal topography are listed below.
Dry eye (tear film instability): In reflection-based optical biometers, tear film instability may increase measurement variability of corneal astigmatism. In particular, eyes with high osmolarity (≥308 mOsmol/L) and shortened tear breakup time (positive NIKBUT) showed significantly increased variability in Lenstar measurements1).
Corneal deformation due to contact lenses: Long-term wear of hard contact lenses, in particular, can alter corneal shape. Discontinuation of 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 like 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. Some reports indicate no significant difference in absolute error or astigmatic prediction error between dry eye (treated and untreated) and non-dry eye groups postoperatively2). However, measurement variability may increase with some reflection-based devices, 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 tilt photography principle to acquire anterior segment tomographic images. Reconstructs three-dimensional shape through rotational scanning.
Features: Enables simultaneous evaluation of anterior and posterior corneal surfaces, corneal thickness, and anterior chamber depth. Slightly affected by opacities. GALILEI incorporates a Placido ring for high keratometry accuracy.
Anterior segment OCT (AS-OCT): SS-OCT (e.g., CASIA) uses long-wavelength light at 1,310 nm, allowing visualization of 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 in the inferotemporal region
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 the elevation map
Eccentricity of the thinnest point on the 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: In addition to standard preoperative evaluation, corneal topography and/or tomography should be performed. It is also important to use calculation formulas that include posterior corneal astigmatism and effective lens position.
Candidates for multifocal IOL or EDOFIOL: Exclusion of irregular astigmatism and evaluation of corneal shape are essential.
Eyes after refractive surgery: Manual keratometry is inaccurate because it overestimates the effective corneal refractive power. Topography-based calculations reflecting the flattening of the central cornea (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 before LASIK/PRK is mandatory. 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. Tear film instability and hyperosmolarity cause irregularities on the tear film surface, leading to measurement variability as distortion of the Meyer ring image.
Nilsen et al. (2024) conducted an RCT involving 131 patients scheduled for cataract surgery and reported 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 had a significantly higher proportion of variability exceeding 0.25 D in mean K value measured by Lenstar (p=0.048)1). No similar significant differences were observed with Anterion or Eyestar.
OCT-based devices (such as Anterion) directly detect backscattered light from tissue, so they do not rely 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 error1)2). Particularly 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 2 weeks of artificial tear treatment (Thealoz Duo, 6 times daily) did not significantly improve keratometry variability or postoperative refractive prediction error (absolute error and astigmatism prediction error)2). Other studies using anti-inflammatory drugs (cyclosporine, lifitegrast, etc.) have shown improvement, suggesting that higher-level treatment may be necessary.
Diagnostic criteria based on DEWS II may not be optimal in the context of cataract surgery. Investigations are ongoing into whether individualized dry eye treatment using hyperosmolarity or positive NIKBUT as indicators can improve preoperative measurement accuracy2).
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 efficacy of therapeutic interventions beyond standard artificial tears.
New-generation biometers integrating OCT and reflection technology (e.g., Eyestar, IOLMaster 700) may have higher resistance to keratometry fluctuations caused by tear film instability compared to conventional reflection-based devices 1). Long-term safety and accuracy profiles are being validated.
Nilsen C, Gundersen M, Graae Jensen P, Gundersen KG, Potvin R, Utheim ØA, et al. The Significance of Dry Eye Signs on Preoperative Keratometry Measurements in Patients Scheduled for Cataract Surgery. Clinical ophthalmology (Auckland, N.Z.). 2024;18:151-161. doi:10.2147/OPTH.S448168. PMID:38259819; PMCID:PMC10800283.
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. Clinical ophthalmology (Auckland, N.Z.). 2024;18:1503-1514. doi:10.2147/OPTH.S459282. PMID:38827772; PMCID:PMC11143984.
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.
Copy the article text and paste it into your preferred AI assistant.
Article copied to clipboard
Open an AI assistant below and paste the copied text into the chat box.
