To my knowledge, no surgical technique other than intraoperative slit-beam observation has been described to visualize the depth of a corneal incision or a lamellar stromal dissection during surgery. To safely obtain deep dissections, a diamond knife equipped with a micrometer may be used to avoid corneal perforation, and the dissection may be extended from the bottom of a keratotomy wound made at planned depth.

During surgery, the depth of corneal incisions and lamellar dissections relative to the corneal thickness may be visualized by creating an optical interface at the posterior corneal surface. For this purpose, the anterior chamber may be filled with a liquid or gas of which the refractive index differs from the cornea, for example air. With a complete air fill of the anterior chamber, the interface between the air and the corneal endothelium, i.e. the posterior corneal surface, was found to be useable a reference plane in four ways.

Figure 2.1: Diagrammatic representation of three optical effects used to visualize the corneal thickness and the relative depth of corneal incisions and dissections during surgery. (a) Mirror effect. When the anterior chamber is filled with air, both the anterior and posterior corneal surfaces act as a convex mirror, and two virtual images are produced when a blade is held against the cornea. Note that the corneal thickness is half the distance between the tip of the blade and its reflection from the posterior corneal surface. Compare to Figure 2.2. (b) Indentation effect. At the air-to-endothelium interface, a specular light-reflex is created by the indentation of the stroma during the performance of an incision. The non-incised posterior corneal tissue is seen as a dark band between the blade tip and the light-reflex. Compare to Figure 2.3. (c) Folding effect. As the blade approaches the posterior corneal surface, small folds are produced in the non-incised posterior corneal tissue in front of the blade. Compare to Figure 2.4. (Source: Cornea 1999;18:80-86)

Figure 2.2: Mirror effect. In an eye that has the anterior chamber completely filled with air, two mirror images of the blade are visible, one reflection from the anterior corneal surface (1) and one from the posterior surface (2). The dotted line indicates the approximate location of the plane of reflection, i.e. the posterior surface. Thus, the estimated corneal thickness (arrows) is half the distance between the blade tip and its reflection from the posterior corneal surface (2). See Figure 2.1a (Source: Cornea 1999;18:80-86).

First, the air bubble in the anterior chamber acted as a convex mirror, so that a blade held against the anterior corneal surface was reflected at the posterior corneal surface (Mirror effect, Figures 2.1a and 2.2). Because the corneal thickness was half the distance between the tip of the blade and its virtual image from the posterior corneal surface, it could be estimated how deep the blade had to be inserted into the stroma to obtain the desired incision depth. Because the cornea is very thin relative to the surgical working distance, i.e. the surgical instrument was held very close to the reflective mirror plane, the minifying effect of the convex air bubble mirror may be negligible.

Second, at the air-to-endothelium interface a semi-circular, specular light-reflex was created near the tip of the blade, by the indentation of the tissue during the performance of an incision (Indentation effect, Figures 2.1b and 2.2a-f). The amount of non-incised corneal tissue between the tip of the blade and the light-reflex was seen as a dark band directly surrounding the blade tip, i.e. the non-reflective tissue between the blade tip and the air-to-endothelium interface.

The thickness of the dark band decreased with advancement of the blade into the deeper stromal layers, so that the achieved incision depth could be judged from comparing the thickness of the non-reflective dark band (the non-incised tissue underneath the tip of the blade) to the total corneal thickness. The distance between the blade tip and the light-reflex, i.e. the amount of non-incised corneal tissue relative to the corneal thickness was used as a measure of relative incision depth. Thus, the indentation effect could be used to monitor the depth of the blade while an incision or dissection was made.

Figure 2.3: Indentation effect. Clinical, macroscopic, and light microscopic pictures of lamellar dissections made at an intended depth of (a-c) 60%, (d-f) 80%, and (g-i) 99%. A dark band is visible between the blade tip (arrow) and the semi-circular light-reflex at the air-to-endothelium interface (open arrow). Note how the amount of non-incised tissue underneath the blade tip in a, d and g compares to the dissection depth in b/c, e/f and h/i. Thus, the achieved dissection depth (thick arrows) can be estimated by the thickness of the dark band adjacent to the blade tip. See Figure 2.1b (Source: Cornea 1999;18:80-86).

Third, when the blade approached the posterior surface, small folds in the posterior corneal tissue were seen (Folding effect, Figures 2.1c and 2.4). The number, width and motility of the folds increased with deeper blade depth, and could be used as indicators of how close the blade was to the posterior corneal surface. The folds were accentuated by the air in the anterior chamber, and the number, width and motility of the folds was found to indicate how close the blade tip was to Descemets membrane. Hence, the folding effect could be used to avoid perforation with deep incisions or dissections.

Fourth, when a dissection is made to approximately 95-98% corneal depth, the posterior layer obtains a moon-surface appearance (closely resembling abundant guttata, while guttata are actually absent), possibly resulting from a reflection of the endothelial cell layer.

To determine if these optical effects could be used to estimate the achieved corneal depth during surgery, lamellar dissections were made in fresh porcine eyes obtained less than three hours post mortem. Each globe was placed in an eye-holder to immobilize the posterior globe and to control the intraocular pressure. The epithelium was gently removed with a cellulose spounge. A self sealing side port was made at the 9 o clock limbus, and with a blunt canula the aqueous was aspirated and the anterior chamber was completely filled with air.

A custom made dissection blade (DORC, Zuidland, NL) was introduced just within the superior limbus. The tip of the blade was tilted slightly downward, to create a semi-circular, specular light-reflex at the air-to-endothelium interface (Figures 1b and 5a-i). When the tip of the blade appeared to have reached the desired stromal depth, the blade was positioned parallel to the posterior corneal surface, and a stromal dissection was made across the cornea. Using the optical effects described above, dissections were made at an intended depth of 60%, 80%, or 99% of corneal thickness (Table 2.1).

Relative lamellar stromal dissection depth averaged 58.3% (sd ± 9.4%) in eyes with lamellar dissections made at 60% intended depth; 81.1% (± 3.4%) in eyes with dissections made at 80% intended depth, and 94.4% (± 1.5%) in eyes with dissections made at 99% intended depth (Figures 2.5a-i). During surgery and in the light microscopy specimens, no microperforations were seen.

With the Student-t-test, only the intended dissection depth of 99% was not achieved (p=0.02). Because the variances were not homogeneous across the three intended lamellar dissections depths (p=0.02), the Kruskal-Wallis test was performed, which showed an overall significant difference between the three intended depths (p=0.002). For pairwise comparisons the Mann-Whitney test was performed, which showed a significant difference between 60% and 80% (p=0.008), and 80% and 99% (p=0.008) intended lamellar dissection depths.

Figure 2.4: Folding effect. Small folds (arrows) in the posterior corneal tissue are visible adjacent to the tip of a dissection blade that approaches the posterior corneal surface. See Figure 2c (Source: Cornea 1999;18:80-86).

These data show that using the four optical effects described, lamellar dissections can be made with a good predictability of achieved depth. No micro-perforations were noted in any of the stromal dissections. In a series of 25 human donor eyes, a micro-perforation rate of 12% was found with stromal dissections made at 99% intended depth.14 In a series of 68 patient eyes, seven microperforations were encountered (see Chapter 10).

Apparently, there is relatively low risk of perforation if the depth of the blade is monitored optically while the dissection is made, even if extreme deep dissections are attempted. Depth predictability of superficial dissections were not evaluated, since the indentation effect used to estimate the blade depth is virtually absent with dissections to an intended depth of less than 50%. The optical effects described may therefore be of limited use to monitor the depth of shallow corneal incisions.

In conclusion, an air-to-endothelium optical interface can be used to visualize the corneal thickness and the relative depth of a dissection knife within the stroma during surgery, and with the optical effects described dissections can be made to approximately the desired corneal depth.