Corneal topography utilizes computer assisted technology to create a three-dimensional map of the surface curvature of the cornea. There are different systems that compute the curvature of the cornea: manual keratometry, keratoscopy, and videokeratoscopy.
Manual keratometry has been the staple of corneal keratometry measurements for many years. The major limitation of manual keratometry is the assumption that the cornea has an sphero-cylindrical surface, with a minor axis separated from its minor axis by 90 degrees.
A keratoscope, sometimes known as Placido's disk, is an ophthalmic instrument used to assess the shape of the anterior surface of the cornea. With keratoscopy, it is possible to evaluate 70% of the cornea surface; however, only gross abnormalities can be seen with this technique. Astigmatism less then 3 diopters is not detected.
In videokeratoscopy the information is digitalized from thousands of points of the corneal surface to produce detailed color-coded maps. This is essentially what is known as modern corneal topography.
Placido-based videokeratoscopes: all systems contain a transilluminated cone acting as a modified Placido ring. These can be divided into near and distant designs, with near having greater corneal coverage but more susceptibility to focusing error, and distant design requiring 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 Galile) 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. The Orbscan combines optical sectioning with Placido reflection and the Atlas uses Placido reflection. The Pentacam utilizes Scheimpflug imaging and the Galilei uses a combinataion of dual Scheimpflug imaging and Placido ring reflection.
- To determine whether the patient´s corneal shape will allow surgery
- Operative assessment to determine surgical parameters
- Aid in the selection of IOLs for patients with significant corneal astigmatism
- 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.
Preoperative topography for corneal refractive procedures
Preoperative topography is routinely obtained before refractive or cataract surgery in order to identify conditions such as keratoconus, irregular astigmatism, contact lens induced warpage, and occult ectatic disorders.
- Keratoconus- Identification of patients with keratoconus is important before refractive surgery or cataract surgery to manage patient expectations, safely choose candidates for surgery, and aid in intraocular lens calculations if indicated. Forme fruste keratoconus and early keratoconus are contraindications to corneal refractive surgery. 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.
- Postoperative evaluations are useful for assessing the quality of the surgery and uniformity of laser in corneal refractive procedures. This may be useful as soon as 30 days after PRK or 1 week after LASIK. Postoperative corneal ectasia may also be detected, but this often occurs later in the postoperative period. Topography may be useful not only for detecting ectasia but monitoring progression.
Post-keratoplasty suture removal and modification
The most common usage of corneal topography post-kereatoplasty in a corneal practice is to assist with suture removal or adjustment in corneal graft patients.
Post-refractive surgery patients are increasingly reaching cataract surgery age. In these patients, cornea topography is important to get good refractive results. Manual keratometry is known to be inaccurate in postrefractive surgical patients, since the keratometer may 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 estimations.
Corneal topography is also very important in assessing patients for potential toric intraocular lenses and assessing candidates for multifocal or extended depth of focus lenses.
- Miller D, Greiner JV. Corneal measurements and tests. In: Albert DM, Jakobiec FA, eds. Principles and practice of ophthalmology. Philadelphia: WB Saunders; 1994:7.
- Wilson SE, Klyce SD. Advances in the analysis of corneal topography. Surv Ophthalmol. 1991;35:269–277.
- Maquire LJ. Keratometry, photokeratoscopy and computer-assisted topographic analysis. In: Krachmer JH, Mannis MJ, Holland EJ, eds. Cornea – fundamentals of cornea and external disease. St. Louis: Mosby; 1997:223–235.
- 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