There are different systems that compute the curvature of the cornea, the Orbscan combines optical sectioning with Placido reflection while the Pentacam and Galilei utilize Scheimpflug imaging to measure the corneal surface.
The major limitation of keratometry is the assumption that the cornea has an sphero-cylindrical surface, with a minor axis separated form its minor axis by 90 degrees.
With keratoscopy it was possible to evaluate 70% of the cornea surface, only gross abnormalities could be seen whit this technique; astigmatism less then 3 diopters is not detected.
In videokeratoscopy the information was digitalized from thousands of points of the corneal surface to produce detailed color-coded maps.
Placido-based videokeratoscopes: all systems contain a transilluminated cone acting as a modified Placido ring. It can be divided into near and distant design, near has greater corneal coverage but are more susceptible to focusing error, and distant design need more illumination. All Placido-generated maps are based on a 2 dimensional reflection from the corneal surface; elevation maps can be generated from this data.
Elevation-based topography: Pentacam, Orbscan and Galilei, these devices directly measures the x, y, and z coordinates. The Orbscan and Galilei employ a Placido disk to augment the anterior corneal measurements. An advantage of Pentacam is that the cross-sectional images of the cornea are meridional and have a central common point for image registration.
- To determine whether the patient´s corneal shape will allow surgery
- Operative assessment to determine surgical parameters
- Postoperative evaluation
- Aid in the calculation of IOLs for patients who have undergone refractive procedure.
The most common usage of corneal topography is in the routine evaluation of the potential refractive surgical patient.
First we need to look for contraindications to surgery, the average adult cornea has a central radius of curvature 7.8 mm and a central corneal power of 43.5. Preoperative pachymetry and preservation of an adequate residual stromal bed are important elements. In general, there is approximately .7 D of central corneal flattening for every 1.0 D of refractive effect.
Preoperative topography is mandatory before refractive surgery in order to identify conditions such as keratoconus, irregular astigmatism, contact lens induced warpage, and occult ectatic disorders.
Identification of patients with keratoconus is important before refractive surgery, also fruste keratoconus and early keratoconus are contraindications. Keratoconus symptoms are highly variable ranging from refractive error to severe distorted vision, so detecting early keratoconus may be difficult. Examination of curvature-based topography reveals an area of localized steepening usually in the inferotemporal quadrant. The following artifacts can also be confused with keratoconus, a prominent tear meniscus, misalignment when obtaining the topography, and accidental external pressure on the globe. Rabinowitz and McDonnell were the first to publish guidelines for the diagnosis of keratoconus that included the following: maximum simulated keratometry reading > 48.7 D, absolute simulated keratometry difference between the 2 eyes >.5 D, and the I-S value greater than 1.7 D. Also the creation of a pachymetric map allows for the identification of the true thinnest point and can contrast the thinnest point to the geometric center of the cornea.
Is useful for assessing the quality of the surgery and uniformity of laser. The best time to take this study is 30 days after PRK and at least 1 week after LASIK. Postoperative corneal ectasia may be related with the residual posterior stromal bed although the exact safe bed thickness is unknown. The topography may be useful not only for detecting ectasia but also the progression.
Photokeratoplasty suture removal and modification
By far the most common usage of corneal topography in a corneal practice is to assist with suture removal or adjustemt in corneal graft patients.
As more and more people undergo refractive surgery and the patients are getting older, cataract surgery is becoming more complex in these cases. Manual keratometry is known to be inaccurate in postrefractive surgical patients, since the keratometer overestimate the effective corneal power. Numerous methods to compute IOL power in these cases exist and include:
- Hard contact lens over-refraction.
- Modified IOL power computations based on derived keratometry.
- Refraction derived keratometry based on the prerefactive surgical keratometry and knowledge of the pre and postoperative refractions.
- Modified IOL power computations based on topographically derived corneal power stimates.
Many investigators and most corneal topography companies are working on determining the effective corneal power in the postrefractive surgical patient.
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- Rabinowitz YS, McDonnell PJ. Computer-assisted corneal topography in keratoconus. Refract Corneal Surg. 1989;5:400–408.
- Wang X, McCulley JP, Bowman RW, et al. Time to resolution of contact lens-induced corneal warpage prior to refractive surgery. CLAO J. 2002;28:169–171.
- Michael W. Belin, Stephen S. Khachikian Topographic Analysis in Keratorefractive Surgery In: In: Krachmer JH, Mannis MJ, Holland EJ, eds. Cornea fundamentals of cornea and external disease. St. Louis: Mosby; 2011 1781-1791