Ординатура / Офтальмология / Английские материалы / Jaypee Gold Standard Mini Atlas Series CORNEALTOPOGRAPHY_Agarwal, Jacob_2009

MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

240 data points per slit) are used to scan the cornea from limbus to limbus and to measure independently the x, y, and z locations of several thousand points on each surface. The images captured are then used to construct the anterior corneal surface, posterior corneal surface, and anterior iris and anterior lens surfaces. Data regarding the corneal pachymetry and anterior chamber depth are also displayed. In the newer version of the Orbscan® system, a placido disk has been mounted to this device in order to improve the accuracy of the curvature measurements.

An advantage of this device is that it measures all surfaces of the anterior segment. Scanning time (1.2-1.5 seconds) is required, this device uses a tracking system to track the eye movements in order to minimize the influence of involuntary eye movement.

By providing more comprehensive preoperative diagnostics and planning, exclusionary criteria such as keratoconus, pre-keratoconus, and corneal thinning can be identified to optimize outcomes in both primary treatments and enhancements. The Orbscan II may help explain decreased visual acuity postoperatively, and is designed to allow the surgeon to more accurately prescribe retreatments, if necessary. This technology is capable of detecting and analyzing posterior corneal abnormalities where corneal anomalies first appear.

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CHAPTER 5: ORBSCAN CORNEAL MAPPING IN REFRACTIVE SURGERY

The Orbscan II, together with other diagnostic tools such as aberrometry, pupillometry and pachymetry, provides us with an unprecedented opportunity to select patients for an appropriate technique (LASIK, LASEK, PRK, Phakic IOL, Intracorneal rings), minimising complications such as long-term corneal posterior ectasia.

Posterior ectasia has been identified after refractive surgery techniques such as Lasik, where insufficient corneal tissue has been left behind postoperatively.2 It has manifested itself as a forward ‘bulging’ of corneal tissue (Fig. 5.2), developing from the posterior corneal surface.

FIGURE 5.2: Three-dimensional representation of ‘bulging’ from posterior corneal tissue in a ectasia case after LASIK by Orbscan II

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

Indeed, several researchers indicates that up to 90 per cent of kerataconus developing in the untreated eye appear first from the posterior surface. This is thought to be related to leaving the cornea with too little tissue postoperatively; and indeed although this is established, there are other indicators which could also put a patient at risk. The Orbscan

II provide us additional information to predict these risks.

It has become a universally accepted the standard to leave at least 250 µm residual stromal bed as a safety measure in Lasik, and the Orbscan II has played its part to make it happend. However, the true validity of this limit is in some doubt .4 The average cornea can range in thickness from 490 to 600 µm, so it is not logical to leave a standard 250 µm in all cases.

A number of surgeons support the view that a percentage thickness of the cornea should remain instead, and/or that a minimum of at least 260 µm to 280 µm is a more realistic standard. To facilitate this, intraoperative pachymetry (corneal thickness measurement) can be performed to provide a more accurate idea of how much tissue will remain after a procedure.5 This in itself has limitations, but gives an additional safety criterion, rather than merely relying on manufacturers’ estimations of flap thickness.

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CHAPTER 5: ORBSCAN CORNEAL MAPPING IN REFRACTIVE SURGERY

In summary, the residual stromal bed is by far from our clinical perspective the only indicator for safe preoperative screening. So how does the Orbscan II influence decisions on whether or not to treat? It is important to understand that selection criteria for refractive surgery never stands alone, and it is the clinician’s responsibility to bring together all the information gathered in the screening process, before deciding whether it is safe to proceed. The Orbscan influences this decision in a number of ways.

Unlike other modern topography systems, the Orbscan is based on slits canning technology in addition to traditional placido-based techniques (Fig. 5.3).

The placido image gives us information on axial keratometric readings, by converting distortion of the rings

FIGURE 5.3: Orbscan is based on slit-scanning technology, in addition to placido-based techniques

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

into topographical data. A series of illuminated annular rings are projected onto the cornea. Using the corneal tear film as a mirror, the reflected image of the rings is captured by a digital video camera. The captured image is then subjected to an algorithm to detect and identify the position of the rings in relation to the video keratographic axis. Once these borders are detected, the digital image is ‘reconstructed’ to show anterior corneal curvature.

Orbscan goes much further than this; slit-beam scanners and triangulation are used to derive the actual spatial location of thousands of points on the surface. Each beam sweep across the cornea gives information on corneal elevation, or height, from the anterior corneal surface, posterior surface and iris. To represent the corneal surface data in a way that is easily understood, the computer calculates a hypothetical sphere that matches as close as possible to the actual corneal shape being measured. This is called the best fit sphere (BFS). It then compares the real surface to the hypothetical sphere, showing areas ‘above’ the surface of the sphere in warm colors, and areas ‘below’ the surface in cool colors.

This has many uses, but for the purposes of refractive surgery selection, ‘bulges’ in both the posterior and anterior surface can indicate patients who may be at risk of ectasia development. This enables the surgeon to screen them out early in the selection process.

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CHAPTER 5: ORBSCAN CORNEAL MAPPING IN REFRACTIVE SURGERY

A ‘quad map’ can be produced, which gives four different maps each portraying different information about the cornea (Fig. 5.4). The bottom left hand map is the axial keratometry map, based on placido technology. This is similar to maps produced from the majority of commercially available topography systems, and provides detailed keratometric information across the diameter of the cornea.

For Lasik selection, this information is important for a number of reasons. The ‘K’ readings it is important, because limits of K-readings are between certain values; the cornea must be neither too steep nor too flat. It is difficult for the microkeratome (blade designed for flap cutting), to create a good quality corneal flap in Lasik if either of these extremes is the case, as this can lead to surgical flap complications.

In addition, K-readings of more than 48D are an indication of potential kerataconus, particularly where this is decentered inferonasally. Details of the K-readings can be found in the stats and data information in the center of the quad map.

The top left hand map of Figure 5.5 is the anterior elevation map, and as with the top right hand posterior elevation map, slit scanning provides the means of creating the information. As mentioned before, slit scanning provides elevation data, and this also can create a 3D interpretation of the cornea. Looking at both elevation maps, if it is imagined that the green tissue is at sea level,

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CHAPTER 5: ORBSCAN CORNEAL MAPPING IN REFRACTIVE SURGERY

FIGURES 5.5A and B: (A) Anterior elevation map – 3D view

(B) Posterior elevation map – 3D view

then the warmer colors are above sea level, or towards the viewer, and the cooler colors are below, or further away from the viewer.

A 3D interpretation of both elevation maps can be seen in Figure 5.5. The meshwork effect indicates how the cornea would appear if it were entirely spherical and is referred to as the reference sphere.

This elevation data can be interpreted usefully in a number of ways. First the difference between the highest and lowest points is a potential kerataconus indicator, if over 100 µm; Rousch criteria 6 (Fig. 5.6).

In addition, on the posterior map, the highest elevation value can again be interpreted as a kerataconus indicator,

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CHAPTER 5: ORBSCAN CORNEAL MAPPING IN REFRACTIVE SURGERY

or at least as a screen for those patients who may be at risk of developing keratectaria postoperatively. This provides safety criteria to avoid treating patients at risk. From the work of Vukich7 and Potgeiter,8 55D elevation has been recommended as an absolute cut off. As can be seen from Figures 5.5 and 5.6, the elevation on the right hand side (posterior elevation) is more advanced than that on the left (anterior elevation), indicating that ‘bulges’ develop from the posterior surface of the cornea in the first instance.

From studying the relationship between the two elevation maps, further information can be gleaned. A ratio can be calculated between the posterior and anterior surfaces, which gives an indication of the relative difference in curvature between the two maps.9 Figure 5.7 shows two corneal cross-sections.

This very simplistic diagram (Fig. 5.7) shows us that the same elevation data for the posterior surface can have a different impact on the stability of the cornea. In diagram B where the ratio is high at 1.27, it can be seen that a weak area (indicated by the arrow) develops which is not apparent in A, even though posterior elevation data is the same for both. This information on elevation and ratio would rarely be used as exclusion criteria alone, but by considering these together, more conclusive information can be obtained. For example, a high ratio of say 1.26 would be far more concerning if the posterior elevation was high

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