The shape of the sclera may be symmetric or asymmetric, and each sclera requires a different customization in the periphery of a scleral lens (SL) to achieve a proper fit.1 By definition, a toroidal surface is considered rotationally symmetric when the axis of symmetry meets the axis of revolution through the vertex of the surface.2 Also, a rotationally symmetric surface is characterized as having a shape that looks the same when it is rotated; the number of times that the shape looks the same while it is being rotated indicates the order of the rotational symmetry, which must have a minimum order of 2. A shape that has an order of 1 means that it cannot be turned around to any other position and still look the same; therefore, it cannot be considered rotationally symmetric. Along these lines, a prolate ellipsoid, oblate ellipsoid, hyperbolic paraboloid, and toric surface may all be considered rotationally symmetric surfaces of order 2 (Figure 1).
An asymmetric sclera has different heights in different quadrants. The asymmetry may be considered regular when this variation is constant, even if it is significant in its shape, and irregular when there is no geometrical similarity in the different quadrants, such as in the presence of significant variations at least in the same quadrant.
TECHNIQUES FOR SCLERAL PROFILE ASSESSMENT
Clinically, a sclera may be considered spherical when it is rotationally symmetric in all 360º; perpendicular meridians varying up to 100µm may be fit optimally with a spherical SL.1,3,4 Consequently, a mostly spherical sclera may be fit with a spherical lens, a rotationally symmetric toric sclera with a toric SL, an asymmetric sclera with a quadrant-specific SL design, and an irregular sclera with a customized SL or with impression techniques. To choose the proper lens design, it is crucial to identify the scleral shape on which a scleral contact lens will be applied.
The scleral profile may be evaluated by using optical coherence tomography (OCT), anterior segment tomography, a corneo-scleral topographer or profiler, and via a slit lamp biomicroscope. The first three technologies provide an accurate and detailed evaluation and measurement of the ocular surface.
Anterior segment tomography and anterior segment OCT are not used extensively in worldwide scleral contact lens practice. They are generally used in combination with diagnostic fitting of a SL to evaluate the relationship between the lens and the ocular surface after lens application on the eye and for determining the ocular surface shape prior to SL fitting.
Scleral topography detects up to almost 22.00mm and provides a better understanding of the ocular surface shape, increasing the rate of fitting success without a diagnostic set. However, this relatively new technology is still not widespread. As these lenses continue to gain in popularity among practitioners, the use of corneoscleral topographers is presumably more likely to increase.
Consequently, the slit lamp remains the instrument with the greatest availability and may be a valuable tool for evaluating the anterior ocular surface shape by observing the fluorescein pattern of a SL on the eye.
SLIT LAMP ASSESSMENT
Practitioners who do not have access to new technological devices may assess the ocular surface shape during slit lamp evaluation of a diagnostic SL. The scleral shape can be determined by observing the following SL behavior and clinical signs:
- Presence of Sectorial Blanching On a highly toric or asymmetric sclera, the spherical SL will exhibit lens touch at the flattest meridian/quadrant and lift-off in the steepest one. Excessive bearing will compress the conjunctival vessels, resulting in sectorial blanching (Figure 2). Sectorial blanching in the horizontal meridian means that the sclera has with-the-rule toricity. It may also appear only in one quadrant, typically the flattest. At lens removal, there will be a rebound hyperemia in the area in which the landing zone restricted the blood vessels. The amount of blanching is directly proportional to the scleral toricity. When blanching occurs, a toric lens or a quadrant-specific design is indicated. Blanching may appear in different areas of the landing zone: in the inner area near the limbal zone (Figure 3) or in the outer area near the lens edge.
- Fluorescein Influx Fluorescein instillation in an eye fit with a spherical SL may help identify scleral toricity. When the sclera is significantly toric, the fluorescein may enter into the reservoir under the lens in a specific meridian where the lens is lifted off. The less time it takes for the fluorescein to enter into the reservoir—indicating a greater influx—the higher the scleral toricity. If it enters at the vertical meridian, the sclera has with-the-rule toricity.
A flat landing zone may cause discomfort and create a path for air bubbles and debris to enter into the reservoir, necessitating it to be steepened in the area where the lens is lifting-off (Figure 4).
- Lens Edge Lift-Off A lens edge that lifts off in a specific area also demonstrates scleral toricity or asymmetry. Following fluorescein instillation, lens edge lift-off in a specific area may appear as mild fluorescein pooling beneath the lifted edge. This pattern is detected by using white light at the slit lamp (Figure 5). Excessive lens edge lift will increase lens awareness and discomfort and will disrupt lens use.
- Removing the Lens Fluorescein instillation at lens removal may reveal sectorial conjunctival staining. This appears when the lens edge is too steep in a specific area, resulting in impingement into the conjunctiva (Figure 6). When the sclera has with-the-rule toricity, impingement will occur in the horizontal meridian. When the impingement appears only in one quadrant, this indicates that the sclera is asymmetric. In this case, flattening the lens edge in the area of impingement is needed. Sectorial impingement and blanching may occur together and are more likely caused by a lens that is steep in a specific zone (Figure 3).
- Fitting a Toric Lens A toric scleral shape may be confirmed by applying a toric SL, if available in the diagnostic fitting set. Toric SLs are labeled with a laser mark or black dot (Figure 7). To identify the scleral toricity, it is necessary to manually rotate the SL. If the lens recovers to its initial position, the sclera is toric. To determine the axis of the meridian, rotate the beam of the slit lamp to the laser marks. Generally, SLs are marked in the steepest meridian; however, it is necessary to always check with the laboratory as to which meridian is labeled, the steepest or the flattest one. Identifying the axis is also important when designing a lens that has a back-surface toric haptic that needs a front-surface toric power to correct residual astigmatism.
The newer diagnostic devices certainly provide an accurate evaluation of the anterior ocular shape; however, the slit lamp is valuable when these devices are not available. Observing the fluorescein pattern and clinical signs can be useful in identifying the scleral shape, thus modifying SL parameters where necessary when managing issues due to suboptimal lens landing on the ocular surface. CLS
- Fadel D. Landing zone: is it a neglected parameter? Presentation at the International Congress of Scleral Contacts. Miami, 2017 July 28.
- Jalie M. Ophthalmic lenses and dispensing. Page 16. Butterworth Heinemann, Oxford 1999.
- Ritzmann M, Caroline P, Walker M, et al. Understanding scleral shape with the Eaglet Eye Surface Profiler. Poster presented at the Global Specialty Lens Symposium. Las Vegas, Jan 2015.
- Ritzmann M, Morrison S, Caroline P, Kinoshita B, Lampa M, Kojima R. Scleral shape and asymmetry as measured by OCT in 78 normal eyes. Poster presented at the Global Specialty Lens Symposium. Las Vegas, Jan 2016.