Orbscan corneal mappingfor refractive surgery diagnostics

The Orbscan II, together with other diagnostic tools such as aberrometry (for Wavefront treatments), pupillometry and pachymetry, provides us with an unprecedented opportunity to select patients for an appropriate technique, minimising complications such as long-term posterior kerectasia. Posterior kerectasia has been identified after refractive surgery techniques such as Lasik, where insufficient corneal tissue has been left behind postoperatively.1 It has manifested itself as a forward 'bulging' of corneal tissue, developing from the posterior corneal surface. Indeed, research indicates that up to 90 per cent of kerataconus developing in the untreated eye stems firstly from the posterior surface.2 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 gives us additional information to predict these. It has become a globally accepted standard to leave at least 250µm residual stromal bed as a safety measure in Lasik,3 and the Orbscan II has played its part in bringing this into being. However, the true validity of this limit is in some doubt.4 The average cornea can range in thickness from 490 to 600µm, so it is not logical to leave a standard 250µm in all cases. A number of surgeons support the view that a percentage thickness of the cornea should remain instead,4 and/or that a minimum of at least 260µm is a more realistic standard. To facilitate this, intra-operative 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. In summary, the residual stromal bed is far from our only indicator for safe pre-operative screening. So how does the Orbscan II influence decisions on whether or not to treat? It is important for the reader 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 slit- scanning technology in addition to traditional placido-based techniques (Figure 1). The placido image gives us information on axial keratometric readings, by converting distortion of the rings 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 colours, and areas 'below' the surface in cool colours. 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. A 'quad map' can be produced, which gives four different maps, each portraying different information about the cornea (Figure 2). 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 must be well aligned with the patient's spectacle prescription, particularly where 2D or more astigmatism is present. Similarly to soft toric contact lens fitting, we cannot expect a good result unless the majority of the astigmatism is mapped onto the front surface of the cornea. It is also important that the 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 decentred infero-nasally. Details of the K readings can be found in the stats and data information in the centre of the quad map. The top left hand map of Figure 3 is the anterior elevation map, and as with the top right hand posterior elevation map, slit scanning provides the means of creating the information. I have mentioned that this 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, then the warmer colours are above sea level, or towards the viewer, and the cooler colours are below, or further away from the viewer. A 3D interpretation of both elevation maps can be seen in Figure 3. The meshwork affect 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 100µm; Rousch criteria6 (Figure 4). In addition, on the posterior map, the highest elevation value can again be interpreted as a kerataconus indicator, or at least as a screen for those patients who may be at risk of developing kerectasia 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 3 and 4, 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.9 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.10 Figure 5 shows two corneal cross-sections. This very simplistic diagram 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 at 55D, the cornea was of borderline thickness, and the preoperative prescription high. The final map to study is the pachymetry map. This is map four of our quad map in Figure 2. Traditionally, pachymetry has been measured using ultrasound, which provides a reading of corneal thickness from Bowman's membrane to Descemet's membrane. Through slit-scanning technology, Orbscan provides us with a pachymetry reading from the pre-corneal tear film to the endothelium, therefore slightly thicker readings can be expected.11 The Orbscan can, however, be calibrated to take this into consideration when comparing readings. The true advantage of the pachymetry map is that it provides us with thickness information across the cornea from limbus to limbus, not just in single points as with ultrasound. This once again gives the opportunity to detect areas of weakness, thinning or scarring. Auffarth et al12 state that the relationship between the highest point on anterior and posterior elevation maps, and the thinnest point (shown by a yellow dot) is an indicator of kerataconus. The relationship between pachymetry readings can be looked at, and it has been suggested that 100µm should be a cut-off criteria between thickness regions on the map. Figure 6 shows the relationship between the central reading in the white circle, and the four peripheral readings, indicated by the arrows. Once again these criteria would be used alongside other information, but alone would not exclude a patient. The readings within the circles are averages of measurements within the area, but the Orbscan also flags the thinnest point, indicated by a yellow dot. In conclusion, it can be seen that much information can be obtained from analysis of Orbscan maps, and this article does not have the scope to cover it all. The most important message is that the criteria does not stand alone, and by looking at all the maps together along with other information, an informed decision can be made as to whether it is safe to proceed to surgery. This is well summarised by Vukich's screening criteria:13 One abnormal map = concern Two abnormal map = caution Three abnormal maps = contra-indication. The Orbscan is also used alongside wavefront aberrometry (detects detailed deviations of light within the eye's optics) to help analyse a patient's suitability for wavefront Lasik. The software is combined with that from the Zywave aberrometer from Bausch & Lomb in order to complete the patient profile. At Advance VisionCare, wavefront treatment using this technology forms the majority of treatments offered to patients today. In the near future it maybe used to enhance aberrations remaining after standard treatment, to improve contrast and night vision for those patients. From this article, hopefully the reader will obtain an insight into the careful analysis and planning which should take place before any patient is selected for refractive surgery. With technology such as the Orbscan and wavefront available, the opportunity for safe and successful treatment in the hands of an experienced surgeon is better than ever before.