People who look through keyholes are apt to get the idea that most things are keyhole shaped.
Corneal thickness or pachymetry has become a standard part of the ophthalmic examination. This routine, albeit vital, measurement is used by glaucoma specialists in assessing risk factors; by generalists in examining corneal health; and by refractive surgeons in screening, planning, and monitoring patients. Corneal thickness is a physical property that correlates to the general health of the cornea and endothelial function. Its measurement or pachymetric evaluation can be performed via different approaches including optical, ultrasonic, and cross-sectional imaging techniques. Although numerous studies have compared and contrasted different instrumentation, little has been written on different reconstruction methods.1–9
Optical pachymetry, popularized in the 1950s by Maurice and Giardini,10 measures corneal thickness by evaluating oblique sections of the cornea using split prisms and aligning the split images so the epithelial and endothelial layers coincide. The most common, although rarely used, is the Haig-Streit pachymeter. Optical pachymetry has been largely replaced by ultrasound (US) pachymetry, first introduced in the 1980s because of its ease of use, accuracy, and reproducibility.11–13
Ultrasound pachymetry uses high-frequency sound waves (20 to 50 MHz) and highly reflective surfaces to locate the epithelial and endothelial layers. Using a corneal velocity of 1641 m/sec, the distance between the 2 reflecting surfaces can be calculated by detecting the time lapse between reflected sound waves from the 2 surfaces. These distance measurements are based on reflections of interfaces with different densities. In the cornea, the exact points of reflection are not known. Ultrasound pachymetry is limited to a single point measurement and is sensitive to positioning and angulation. Despite the numerous limitations of US, many papers refer to it as a “gold standard,” a common misnomer. While US is probably the most commonly used thickness measurement, its inherent limitations should preclude it as serving as a baseline to judge other devices.14
Corneal cross-sectioning or tomography (eg, Pentacam, Orbscan II, Galilei, and anterior chamber optical coherence tomography [AS-OCT]) provides a 3-dimensional (3-D) reconstruction of the cornea through optical methods that enables evaluation of the anterior and posterior corneal surfaces via horizontal slit-scanning and Placido-based computerized videokeratography (Orbscan II, Bausch & Lomb), rotating Scheimpflug camera (Pentacam, Oculus GmbH; Galilei, Ziemer AG), and high-speed AS-OCT (Carl Zeiss Meditec AG). Full corneal cross-sectional analysis allows a full pachymetric map enabling characterization of the thickness profile of the cornea.
A review of recent studies reveals a multitude of articles that evaluate new technologies measuring corneal thickness.1–9 These pachymetric innovations are evaluated in terms of reliability and reproducibility, commonly against US pachymetry, which remains de rigueur at many ophthalmic practices due to its low cost, ease of use, and relative reproducibility.
Although the above methods differ in design, use, and methods of measuring corneal thickness, it is assumed that what they are measuring is comparable. Most ophthalmologists assume that what is measured is the distance from the surface normal to the endothelial surface (Figure 1). If we use the same approach, however, and measure the thickness of the cornea from the posterior surface at point B, one would assume that the thickness from point A measured to point B would be the same if measured from point B to point A. Figure 1 illustrates the conundrum using this logic as the 2 surfaces are not parallel. Using a point normal to the surface tangent at point B, we can see that the distance from point A to point B does not equal the thickness measured from point B.
Although most of us consider Figure 1 as the way to measure corneal thickness, thickness can be measured in multiple ways, each of which may have a clinical application. These other avenues are depicted in Figure 2. Normal to the surface tangent is how most of us think of corneal pachymetry (Figure 2, number 1). In refractive surgery, corneal thickness measured by this technique has its origins in radial keratotomy, in which the surgeon was taught to maintain the diamond blade perpendicular to the corneal surface. Here the clinical application and method of measurement coincide.
Other methods of measuring thickness exist and may be better suited to different clinical situations. One method, parallel vertical sections, is illustrated in Figure 2, number 2. In contrast to the normal to the surface tangent method, in which pachymetric determination is based on the incidental angle, in parallel vertical sections, the limiting factor is the required parallel relationship of each optical section to the prior section. This method emphasizes vertical distance throughout the cornea as opposed to incidental angle. In industry, thickness determined by parallel vertical measurements are used in such devices as cutting water jets, which move along the object’s surface and whose jet stream is always vertical regardless of the object’s surface architecture. In ophthalmology, this is analogous to a translational femtosecond laser (eg, Femto LDV, Ziemer AG) in which corneal thickness determined by parallel optical sections may better reflect the laser’s actual path through the cornea.
Minimal thickness is another alternative to normal to the surface tangent pachymetry. In the minimal thickness method, the minimal distance between the anterior and posterior surfaces of the cornea is searched for regardless of orientation (Figure 2, number 3). This method is particularly important in posterior ectasia in which focal thinning of the cornea may not be fully appreciated by applications that use the anterior surface only (normal to the surface tangent method) (Figure 3). These subtle changes can have major implications in refractive surgery, as in the surgical planning of the channel depth for intrastromal corneal ring segments (ICRS).
Thickness can also be measured from a fixed point (Figure 2, number 4). Here corneal thickness will vary depending on the distance of the fixed point to the cornea. Most current lasers operate in this manner. Pachymetric data collected using this method could guide surgeons in performing laser-assisted deep anterior lamellar keratoplasty (DALK) and laser-assisted Descemet-stripping endothelial keratoplasty (DSEK) donor preparation. At an infinite working distance, this method and the parallel sections method should be equivalent (Figure 4).
Corneal tomography allows 3-D reconstruction of the cornea and anterior segment. Figure 5 illustrates the 4 main methods of determining corneal thickness (normal to the surface tangent, parallel sections, minimal distance, and fixed point). In this example, central corneal thickness in all 4 maps are identical. This will be the case for the normal-to-the-surface tangent, the parallel sections, and the fixed-point method as long as the parallel section through the center is normal to the surface and the fixed point lies along the central measurement line. The minimal distance method, however, may vary depending on the underlying corneal shape.
Translational femtosecond lasers are used in the creation of laser in situ keratomileusis (LASIK) flaps, keratoplasties, DSEK donor preparations, and corneal incisions. The path of the laser is oriented so each application is vertical regardless of surface architecture and parallel to the prior application. The parallel-vertical-sections method of corneal pachymetry better approximates the path of a translational femtosecond laser. Figure 6 illustrates the difference in a simulated DSEK donor preparation in which the goal is a deep, consistent dissection. The circular overlay is the proposed graft size. The thinnest reading on the normal-to-the-surface tangent method at the proposed incision site is 658 μm at approximately the 10 o’clock position, while the thinnest site with parallel sections at the same location is 752 μm, a difference of 94 μm. Using the parallel section information could, in theory, allow a deeper ablation and a thinner, more consistent donor.
Intrastromal corneal rings are used to modify corneal curvature in patients with low myopia, keratoconus, and post-LASIK ectasia. These poly(methyl methacrylate) rings are implanted in the deep stroma after creation of 1 or 2 semicircular tunnels. Inadvertent perforation is a potential complication of overestimating corneal thickness. Unlike eyes with normal myopic corneas, in eyes with keratoconus or post-LASIK ectasia, there is an irregular relationship between the anterior and posterior corneal surfaces (Figure 7).
Posterior ectasia may not be thoroughly evaluated by the normal-to-the-surface tangent method used in US pachymetry and standard thickness maps leading to overestimation of corneal thickness. In theory, a more appropriate or safer pachymetric technique for determining channel depth in eyes with ectasia would be the minimal-distance method. Figure 8 demonstrates a comparison of the standard normal-to-the-surface tangent method and the minimal-distance method in the eye with post-LASIK ectasia shown in Figure 7. The map on the far right shows the difference between the 2 computations. Within the proposed ICRS channels, the traditional normal-to-the-surface tangent differs from the minimal-distance method by as much as 45 μm.
A similar situation occurs in keratoconic eyes. Figure 9 shows a typical case of moderately advanced keratoconus. Deep anterior lamellar keratoplasty is becoming a more commonly performed surgical procedure for correcting advanced keratoconic corneas while preserving native endothelium. Most DALK techniques require making an initial partial-thickness trephinization followed by a lamellar dissection. While a deep dissection is preferable, one must avoid perforating Descemet membrane. Figure 10 compares the standard corneal thickness method and the minimal distance in the keratoconic cornea shown in Figure 9. Differences of up to 50 μm exist. The minimal-distance method could allow a closer and safer approach to this technique.
Most lasers are not translational and rotate or angle their beam. This is how most refractive excimer lasers work. Corneal thickness measured from a fixed point will vary based on the working distance or focal length of the laser (Figure 11). Fixed-point pachymetric data may be applicable to some of the newer intrastromal ablation techniques or to lenticule extraction. Additionally, in collagen crosslinking, an understanding of the working distance of the delivery system and its area of penetration may better predict endothelial exposure and ultraviolet penetrance.
Tomographic imaging allows reconstructions of the anterior segment that were not possible with standard 2-D measurement techniques. The imaging can be used to modify the standard method of determining corneal thickness to more closely match the clinical situation and may have practical medical applications. The availability of the new reconstruction maps may be incorporated into future studies to determine clinical utility.
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