Refractive Surgery Pearls in Topography
Topography is derived from two words in Greek: topos (place) and graphein (to draw). Corneal topography relates to a representation of the geometrical properties of the corneal surface and it’s a technique for measuring and evaluating its shape and curvature (1,2). The advances in refractive surgery techniques have accentuated the requirement for precise analysis of the corneal surface. The keratometer only gives close estimations of the anterior corneal surface, but is insufficient in the assessment of patients for refractive surgery. In recent years, rapid advances in corneal topography have paralleled those of excimer laser refractive surgery, and computerized corneal topography has become a standard part of clinical practice (1).
In the seventeenth century, Christopher Scheiner compared pictures formed by the impression of marbles on the cornea (3). In 1880, Placido created a disk, involving the reflection of a series of concentric highly contrasting rings (4). In the late nineteenth century, Javal and Schoitz, created the keratometer. This device focuses on the 3 - 4 central millimeters and assumes that the rest of the cornea has a sphero-cylindrical surface. More recently, the photokeratoscopes have been developed to assess the rest of the corneal surface (1).
A fundamental component of the refractive surgery evaluation is the preoperative assessment of the cornea with topography. Its main goal is to diagnose epithelial and stromal abnormalities, evaluate corneal astigmatism, refractive stability or undetected corneal diseases (5). It is important that the patient maintains an adequate position and fixation in order to ensure a high-quality image (1).
Dioptric values in the topographic map are represented with colors: warmer colors illustrate steeper curvatures (higher D), and cooler colors exhibit flatter curvatures (lower D). Near the periphery, normal corneas tend to flatten, and it is represented with cooler colors. Both eyes tend to have mirror topographies (enantiomorphism) (6).
In 1996, Rabinowitz et al., created a database of videokeratography patterns and quantitative indices based on the normal corneas of 390 eyes. They proposed 10 different topographical patterns (7). They are:
Elevation maps illustrate the variation in height within the measured corneal surface and some reference surface. The most common reference is a sphere, referred to as “best-fit sphere” (BFS). Other shapes include the "best-fit asphere (BFA) or best-fit toric asphere (BFTA)". The values lying above the reference surface are represented with warmer colors, while cooler colors indicate areas below it (6). There are several patterns described for topography maps: “island, regular ridge, irregular ridge, incomplete ridge, and unclassified” (8). Tanabe proposed that 10 µm and 20 µm scales seem most suitable for the anterior and posterior elevation maps, respectively (6).
The distribution of corneal thicknesses is represented in pachymetric maps. It provides a global map of corneal thickness which eliminates errors due to probe decentration with the manual ultrasound. An accurate measurement is essential before refractive surgery (6).
The next indices are used to separate normal corneas from those with a pathology (keratoconus or forme fruste keratoconus) and are based on the pachymetry, anterior corneal curvature, and elevation patterns (6).
Randleman Ectasia Risk Score System
In 2008, Randleman et al. made a research of the epidemiologic features of ectasia after refractive surgery. There were included 171 ectasia cases. They evaluated the preoperative characteristics, including: “age, gender, spherical equivalent refraction, pachymetry, and topographic patterns, type of surgery performed, flap thickness, ablation depth, and residual stromal bed (RSB) thickness”; and postoperative features: time elapsed since the appearance of ectasia. They created a “risk factor stratification scale”, considering each characteristic. This is an important method to assess risk of ectasia after LASIK (9).
Santhiago et al., investigated the association between the percent tissue altered (PTA) and the development of ectasia in patients with normal topography, who underwent LASIK surgery. They included 30 eyes with normal topography that developed ectasia and another group of 174 eyes without this complication (10).
The equation for PTA is:
PTA = (FT + AD) / CCT
(PTA) Percent Of Tissue Altered
(FT) Flap Thickness
(AD) Ablation Depth
(CCT) Preoperative Central Corneal Thickness
They concluded that PTA values below 40.0% correlate with the low incidence of ectasia. This formula is a precise method for assessing ectasia risk, and it is more sensitive than the individual components that comprise it (10).
They described a new index for the assessment of keratoconus. Three criteria were considered:
- “Central K (central steepening)”: is used to distinguish central cones.
- “I-S values (reflecting inferior-superior dioptric asymmetry)”: Is the refractive power difference between the five inferior points and the superior points. It is used to detect inferior corneal steepening.
- “SRAX (relative skewing of the steepest radial axes above and below the horizontal meridian)”.
They found, that a central K greater than 47.20 D, I-S higher than 1.2 and SRAX index above 21°, identified 98% of the patients with keratoconus (11).
Corneal topography is an essential tool to perform the preoperative assessment in refractive surgery (5). It is important to evaluate risk factors that would lead to poor visual outcomes, such as keratoconus, forme fruste keratoconus, and pellucid marginal degeneration (12). Also, we need to determine the presence of astigmatism and the refractive stability. It is especially relevant to screen patients carefully and to diagnose those who have an elevated risk of ectasia. Some artifacts can be confused with keratoconus: "Displaced apex syndrome, CL-induced warpage, a prominent tear meniscus, misalignment when obtaining the topography, dry eye disease with poor precorneal tear film and accidental external pressure on the globe"(5).
In LASIK, PRK, and LASEK, it is relevant to assess the refractive error and corneal topography and determine whether it will lead to an abnormally steep or flat corneal curvature. The risk for buttonholes is bigger for steep corneas, whereas the risk for free caps is higher for flatter corneas. These complications are reported mainly with the use of mechanical microkeratomes; on the other hand, femtosecond LASIK flaps do not seem to have these complications (12). Postoperative corneal topography is helpful in determining the ablation bed, the quality of the surgery and the uniformity of laser. The study should be performed at least 1-week after surgery. It is also useful for detecting ectasia and evaluating progression. "Corneal topography-guided custom ablation patterns" is a helpful technique to prevent undesirable postoperative results (5).
1. Gatinel D., Corneal Topography and Wave Front Analysis :In Albert & Jakobiec's Principles & Practice of Ophthalmology (Chapter 70, Section History of corneal topography) Retrived from https://www.clinicalkey.com/#!/content/book/3-s2.0-B9781416000167500734 , accessed on 08.01.2017
2. Michael J. Taravella, Richard S. Davidson., Corneal Topography and Wavefront Imaging: In Ophthalmology, Fourth Edition (Chapter 4, Cornea And Ocular Surface Diseases) Retrived from https://www.clinicalkey.com/service/content/pdf/watermarked/3-s2.0-B9781455739844001111.pdf?locale=en_US, accessed on 08.01.2017
3. Naroo SA, Cervino A: Corneal topography and its role in refractive surgery. In: Narco SA, Butterworth H, eds. Refractive surgery: a guide to assessment and management.London: Elsevier;2004:9–16.
4. Placido A: Novo instrumento per analyse immediate das irregularitades de curvatura da cornea. Periodico Oftalmol Practica 1880; 6:44–49.
5. Elizabeth Yeu, Michael W. Belin, Stephen S. Khachikian. Topographic Analysis in Keratorefractive Surgery Cornea (Chapter 163)
6. Eric J. Kim, Mitchell P. Weikert, Carlos E. Martinez, Stephen D. Klyce Keratometry and Topography Cornea, (Chapter 12), 144-153.
7. Rabinowitz, Yaron S., et al. "Videokeratography database of normal human corneas." British journal of ophthalmology 80.7 (1996): 610-616.
8. Gatinel, Damien, et al. "Corneal elevation topography: best fit sphere, elevation distance, asphericity, toricity and clinical implications." Cornea 30.5 (2011): 508.
9. Randleman JB, Woodward M, Lynn MJ, et al. Risk assessment for ectasia after corneal refractive surgery. Ophthalmology 2008;115:37–50.
10. Santhiago, Marcony R., et al. "Association Between the Percent Tissue Altered and Post–Laser In Situ Keratomileusis Ectasia in Eyes With Normal Preoperative Topography." American journal of ophthalmology 158.1 (2014): 87-95.
11. Rabinowitz, Yaron S. "Videokeratographic indices to aid in screening for keratoconus." Journal of Refractive Surgery 11.5 (1995): 371-406.
12. James J. Salz, William Trattler Patient Evaluation and Selection in Refractive Surgery Cornea (Chapter 162).