Corneal Shape Analysis and Biomechanical Assessment

Key Concepts

Advances in corneal imaging were boosted by the demands of refractive surgery, such as the diagnosis and characterization of ectasia susceptibility.
Enhanced refractive surgery screening and planning must go beyond corneal topography and single point ultrasound pachymetry.
The diagnosis of mild keratoconus should consider tomographic data, such as posterior elevation and thickness profile (pachymetric data from the thinnest point to the limbus).
The diagnosis of mild keratoconus evolved toward characterizing ectasia susceptibility and assessing biomechanical properties of the cornea.
Corneal convergence power, optical quality, and toricity characterization are fundamental for planning refractive procedures, including intraocular lens implantation.

The word “tomography” is derived from Greek tomos meaning “to cut or a section” and graphien meaning “to write.” In medicine, the classic term computed tomography scanning is used to refer to the radiographic technique for imaging a section of a solid internal organ, producing a three-dimensional image. As such, “corneal tomography” can be defined as the science of describing the front and back surfaces of the cornea along with pachymetry mapping. Herein lies the essential distinction between topography and tomography: the former is a two-dimensional representation of the anterior corneal surface and the latter a three-dimensional representation of the cornea and anterior segment.

Although Scheiner and Helmholtz introduced measurements of anterior corneal curvature and the concept of keratometry, respectively, it was Antonio Placido in 1880 with a disk of alternating white and black circles with a central perforation that enabled qualitative assessment of more than four points of the central corneal surface. A century later, the development of computer video-keratography represented the evolution of surface analysis with digital image processing and color-code mapping.

The first widely commercially available corneal tomographer was the Orbtek Orbscan slit-scanning device launched in 1995. In 2002 the Oculus Pentacam (2002) tomography device, and subsequently other similar tomography devices, have attracted considerable clinical interest. Using a digital rotating Scheimpflug camera and a monochromatic light source (i.e., blue light-emitting diode [LED] at 475 nm) that rotate together around the optical axis of the eye, multiple images are taken of the anterior segment from the anterior corneal surface to the posterior surface of the crystalline lens. Moreover, studies propose that a focal biomechanical abnormality initiates a cycle of pathology with secondary thinning and deformation. In this chapter, corneal tomography will be explored, and its applications will be discussed.

Scheimpflug Principle

The Scheimpflug principle is a geometric rule commonly used in photography. This innovative principle of photography was first described by Jules Carpentier in 1901, who was cited and credited in the original patent by Theodore Scheimpflug in 1904. In this technique, three imaginary planes—the film plane, the lens plane, and the plane of sharp focus—are arranged in a nonparallel manner. Scheimpflug alignment extends the depth of focus and provides more sharpness to points of the image located at different planes with minor distortion of the image.

Three-Dimensional Image—Cornea and Lens Densitometry (Backscatter)

High-resolution images and three-dimensional (3D) anterior chamber analysis are used to assess the anterior segment for abnormalities. Corneal densitometry is expressed in gray-scale units, ranging from a minimum light scatter of zero (maximum transparency) to a maximum light scatter of 100 (minimum transparency). The analysis of light backscatter has gained increasing relevance in corneal diagnosis. It has been described in postrefractive surgery, infectious keratitis, corneal dystrophies, and corneal graft surgery and for evaluating the crosslinking results in patients with keratoconus. In a keratoconic cornea, the densitometry is higher in more-advanced stages, and the light backscatter is higher in the corneal anterior central portion. This is consistent with quantified histopathologic changes in keratoconus pathology.

Elevation Maps—Enhanced Elevation

Elevation maps are typically viewed comparing the data to a standard reference surface. Elevation-based Scheimpflug imaging has advantages over Placido-based systems in that it allows for the measurement of both the anterior and posterior corneal surfaces. It has been suggested that changes in posterior corneal elevation might be the initial changes that can be detected in keratoconus. Mean posterior corneal elevation has been studied in an effort to distinguish keratoconic corneas from normal corneas. The posterior elevation shows as well circumscribed, clearly demarcated island of positive elevation representing the area of ectatic change. Commonly, the area of posterior ectasia and the thinnest area coincide.

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