Corneal surgery has been characterized, since the advent of microsurgery in the middle of the last century, by a high degree of precision. Because the cornea is the main optical element of the eye, it is critical that its surgical manipulation be precise, either when it has an explicitly refractive purpose, or in "therapeutic" indications due to the inevitable collateral refractive effect. The rise of lamellar keratoplasty, in its different anterior and posterior variants, has only increased the need for precision. Although the biomicroscope with slit lamp is still the basic work tool for the clinician, the contributions of technology in the field of diagnostic imaging have radically changed the way in which the indications, preparation and monitoring of keratoplasty are performed, and its incorporation into surgical equipment is already changing the way to practice them.

CORNEAL TOPOGRAPHY

By topography (from the Greek "topos", place, and "grafía", writing) we understand the study of the form and features of the surface – normally of the Earth. Applied to the cornea, it has become the complementary exploration that provides more information about its morphological characteristics – which in turn depends on refraction –, and even on neighboring structures such as the anterior chamber and the lens. Currently, corneal topography can be studied by any method capable of integrating information on the shape of the corneal surfaces, both anterior and posterior. We will review the main types.

Placido disc-based topography

The first technology capable of generating topographic maps of the cornea was based on the Placido disc. It uses the images of a series of concentric circles reflected on the surface of the cornea to measure the geometric values of the slope at different points of the cornea (Figure 1). With these measurements the values of axial and tangential curvature are calculated, but only of the anterior surface of the cornea and it is not possible to know the values of its elevation with respect to a reference plane. It would therefore be debatable whether we can properly speak of "topography". To denote the exclusive measurement of curvatures without considering the elevations, we proposed some time ago as the most appropriate term the one of «kurtography» (from the Greek «kurtós», curved)¹.

Placido disc topography has been used for many years to determine the power of the cornea, by converting the values ​​of radius of curvature (r, in mm), into diopters (D). This requires assuming a certain refractive index. Usually the formula D = (1.3375 - 1) * (1000) / r is used. With the advent of corneal refractive surgery, some shortcomings of this technology became apparent. Since it infers the total power of the cornea from the values ​​of only the anterior surface – using the keratometric index – the results are only approximate, often without the sufficient precision required by the current refractive surgery and the calculations for intraocular lenses (IOL). It remains useful, however, to determine the amount and type of astigmatism, both preoperatively and, above all, in the postoperative controls after keratoplasty, since this remains one of the main problems after this surgery.

Scanning topography with slit

The appearance of the Orbscan system at the end of the last century revolutionized the study of the cornea and quickly became essential in corneal refractive surgery and in the early diagnosis of keratoconus². Its method of horizontal scanning by slits allows obtaining elevation maps³ that compare the anterior and posterior aspherical surfaces of the cornea with a reference sphere⁴ (Figure 2). This type of maps, genuinely topographic, provide us with a more real image of the corneal morphology. By knowing the heights of the anterior and posterior surfaces, the system deduces the thickness for each point and generates a pachymetric map. The Orbscan can obtain curvature/power measurements from elevation data, but still incorporates a Placido disc to measure keratometric data – which in fact continue to be more reliable for the anterior surface⁵.

Topography by Scheimpflug tomography

The Scheimpflug principle – an Austrian military engineer who developed it at the beginning of the 20th century – is a geometric rule that relates the orientations of the focal plane, the lens and the image in an optical system such as a camera. It was applied mostly to correct aerial photography images. By tilting the lens, it allows keeping in focus the objects that are not parallel to the plane of the film or sensor, such as the structures of the eye in the sagittal plane of the slit, with respect to a camera that is placed in the frontal position (Figure 3). Instruments that apply this principle to a monochromatic slit light, such as Pentacam (Oculus), Galilei (Ziemer) or Sirius (CSO), allow to obtain tomographic images of the cornea, anterior chamber and even the lens. By rotating the camera and integrating multiple tomographic images (two-dimensional cuts), they produce topographic maps of surfaces or three-dimensional models⁶.

While some of these devices continue to incorporate an additional Placido disc, the Pentacam HR represents a "pure" Scheimpflug system. It captures 50 meridional images in 2 seconds (rotating 180°) and evaluates 500 points in each of them (Figures 4 and 5), which supposes a total of 25,000 elevation data (Figure 6). A second camera detects and corrects the possible movements of the eye during capture. This makes these platforms more accurate than Orbscan⁷.

The anterior and posterior keratometric maps – be they refractive or of axial or tangential curvatures – and pachymetric are obtained from the measurements of elevation from limbus to limbus (Figure 7). They also allow measuring the depth and volume of the anterior chamber, the value of the angle in its 360° and the pupillary diameter, and perform optical densitometry of the cornea and the lens. All these data open new possibilities in the planning and monitoring of the different current modalities of corneal transplantation.

Optical coherence tomography

Optical coherence tomography (OCT) is an imaging technique that allows two-dimensional slices of translucent tissue to be obtained, with a resolution that resembles a non-invasive, live histology. Although its most widespread use in ophthalmology has been the study of the macula, it is increasingly used as a diagnostic tool in glaucoma and exploration of the cornea and the anterior chamber.

OCT is based on the principle of low coherence interferometry. A beam of laser light is emitted towards the structure under study and the delay of the reflection from it is compared in an interferometer with a reference beam (Figure 8).

In the first version of this technology, called «temporal domain», a moving reference mirror is used to measure the time it takes for light to reflect. This mechanical process is relatively slow and limits both the number of scans and the image quality. In a temporal domain OCT, about 400 axial scans (A-scan) per second are performed, which are integrated to create the two-dimensional image. More recently, the "spectral domain" or Fourier OCT technology uses only mathematical calculations, which avoids the mechanical process and makes it 100 times faster (40,000 A-scan/s). In addition, the spectral OCT simultaneously measures multiple wavelengths of the spectrum of the reflected light. More explorations result in a higher resolution and quality of the two-dimensional image, but also of surface maps (topographic, keratometric) and three-dimensional models (e.g., pachymetric) by integration of a large number of tomographic slices. The acquisition of images is combined with an eye tracking technology to compensate for possible movements during capture, as well as a simultaneous double image scan. This minimizes noise and allows the instrument to accurately track small changes over time.

In comparison with ultrasonic biomicroscopy (UBM), OCT is a non-contact method that does not require the patient's supine position and allows images to be obtained at a higher speed and resolution. Its disadvantage with respect to the UBM is the limitation by opaque structures to the light that the ultrasounds can cross – e.g., to visualize the ciliary body –. However, the near-infrared light that OCT usually uses has a capacity to penetrate semitransparent tissues – such as a leucomatous cornea – far superior to visible light.

The accuracy of the OCT has been imposed in applications such as measuring the depth of the anterior chamber to plan a surgery with phakic lens implant (Figure 9) and to measure its distance to the crystalline lens once implanted.

It also allows the study of the angle, with a resolution comparable or superior to the UBM. Apart from its interest in patients with glaucoma, this ability is very useful for the diagnosis, indication and planning of corneal transplant surgeries (Figure 10) – especially in complex cases where visibility is limited –⁸. It allows defining the depth of the graft according to the location of the corneal lesions and assessing the corneal thickness – in cases of edema, etc. – with a robustness that makes it remain effective in cases with a lot of opacity and irregularity, where visible light-based systems as the Scheimpflug tomography present many artifacts.

In the postoperative period after a transplant, the OCT allows us to assess the regularity of the cut, the thickness of the graft⁹ (Figures 11 and 12) or that of a residual lamellar bed; the correct apposition of endothelial grafts¹⁰ or of the recipient endothelium to a deep anterior lamellar graft, as well as the assessment of the structures of the anterior chamber, angle, iris, lens, intraocular lens¹¹, etc.

Ultrasonic biomicroscopy

Ultrasonic biomicroscopy (UBM) is a high-resolution ultrasound technique that allows detailed visualization of anterior segment structures, including opaque ones¹². It is a more laborious study since it requires an immersion technique, but it allows to visualize the places where explorations based on light cannot access, such as the ciliary body. It is essential to determine if a tumor mass is solid or cystic (Figure 13) or when there are very dense corneal opacities that prevent the passage of OCT or create artifacts by shadows.

The ultrasonic waves are emitted by a transducer and propagated longitudinally through the ocular media, the echoes reflected by the different structures are picked up by the transducer¹³, transformed into electrical signals, and presented on the monitor as two-dimensional images of high resolution and dynamic – although measurements can be made on a given table –¹⁴. The frequency of emission, normally between 30 and 50 MHz for the anterior segment, depends on the type of probe used: the higher the frequency, the higher the resolution but the lower the penetration.

In cases of corneal opacification, the information provided by the UBM is very useful to plan the intervention and control the results¹⁵. The most commonly used applications include the study of the angle and discard anterior synechiae that could interfere with surgery or be the cause of postoperative ocular hypertension. With the UBM it has been possible to demonstrate the appearance of a cyclodialysis as a cause of hypotonia after endothelial keratoplasty (see chapter 6.9.5). The detection of problems in the lens such as cataract (Figure 14), subluxation or the presence of vitreous in the anterior chamber is also possible in the presence of corneal opacity.

Confocal microscopy

Confocal microscopy is an imaging technique using laser scanning that increases the resolution and contrast of the micrography thanks to a pinhole that is placed in the confocal plane of the lens and eliminates out-of-focus light¹⁶. It generates tomographic slices that represent a true histology "in vivo" in the frontal plane and can be made serial to study a volume plane by plane – e.g., the different layers of the cornea –¹⁷ (Figures 15, 16 and 17).

Confocal microscopy allows the detailed study of the epithelium, the stroma and its cicatricial processes, the endothelium and the corneal innervation. We can determine with great precision the corneal thickness and that of each of its layers. It also allows to follow the evolution of the histological changes that occur in the pathological cornea.

In recent years confocal microscopy has proved to be an especially useful tool for the diagnosis of certain corneal infections, such as those caused by Acanthamoeba or by fungi, where it allows non-invasive visualization of cysts or hyphae. Some corneal dystrophies can be diagnosed early with this technique, apart from its usefulness for monitoring them¹⁸, or that of the scarring phenomena (haze) after laser ablation¹⁹. In keratoplasties, the endothelium can be studied even in the presence of opacities or edema that would prevent obtaining specular microscopy images. In the postoperative follow-up it is useful to evaluate the healing process – e.g. in a lamellar interphase – and differentiate the causes of graft edema²⁰.

Specular microscopy

Specular microscopy is a non-invasive test that allows the qualitative and quantitative study of corneal endothelial cells. It takes advantage of the intense reflection caused by the endothelium-aqueous humor interphase when it is illuminated with a certain angle, to generate an image that will be captured by an optical system. This magnified image is generally subjected to analysis programs that allow quantifying the morphological parameters of the endothelium. If the distribution of the endothelial cells is assumed to be almost uniform, from a sample of a certain area we can know the endothelial cell density and the morphological characteristics of the cells through the percentage of hexagonality and the coefficient of variation of the cell area²¹ (Figure 18).

With specular microscopy it is possible to detect and monitor endothelial primary²² (Figure 19) or acquired diseases or the evaluation of possible alterations secondary to surgical procedures. To obtain clear images, the cornea must have sufficient transparency. This can be improved, in the presence of edema, momentarily by hyperosmolar agents such as glycerin.

In corneal transplants that include endothelium, specular microscopy is essential both for the selection and viability of the donor tissue and for the indication, evaluation and long-term follow-up of the recipient, ²³ and allows therapeutic decisions to be made in the face of circumstances such as accelerated cell loss. without apparent cause²⁴.