ANTERIOR SEGMENT OCT
Benefits of OCT When Fitting Specialty Lenses
Anterior segment optical coherence tomography can be used for many pre- and post-contact lens assessments.
By Randy Kojima, FAAO; Patrick Caroline, FAAO; Maria Walker, OD; Beth Kinoshita, OD, FAAO; Mark André, FAAO; & Matthew Lampa, OD, FAAO
The optical coherence tomographer (OCT) was not intentionally built for contact lens evaluation, but many practitioners are adapting this versatile instrument for use in this area of care. At the Pacific University College of Optometry, we rely on this instrument for a variety of pre- and post-contact lens assessments (Table 1). This article will explore some of the valuable imaging capabilities that the OCT provides to our contact lens service and research center.
|•||Evaluating pachymetry and corneal edema|
|•||Measuring soft and GP lens thickness—centrally and peripherally|
|•||Examining lens edges for flaws and causes of discomfort|
|•||Measuring posterior tear film thickness in scleral lenses|
|•||Assessing points of lens bearing to determine the need for design modification|
Prior to fitting a diseased, post-surgical, or asymmetrical eye with a scleral or extra-limbal contact lens, an understanding of the scleral shape is imperative. Corneal topography is the tool that many practitioners rely on to understand the shape and elevation of the eye. Unfortunately, the vast majority of these instruments measure the cornea alone and not the limbus or sclera (Mandell, 1996; Mountford et al, 2004; Wang, 2012).
The OCT system contains linear calipers to allow precise analysis of the anterior segment at various sagittal depths, and it has angular calipers that allow a practitioner to measure the corneal, limbal, and scleral angles (Figure 1) (van der Worp, 2010; Sorbara et al, 2010; Kojima et al, 2013; Hall et al, 2013; and others. Full list available at www.clspectrum.com/references.). This is highly beneficial when fitting scleral contact lenses or any specialty lens whose diameter falls beyond the limbus.
Figure 1. The OCT system contains linear calipers to allow precise analysis of the anterior segment at various sagittal depths, and it has angular calipers that allow a practitioner to measure the corneal, limbal, and scleral angles.
Measuring with OCT
Sagittal Depth One of the first tests that we perform prior to fitting any scleral contact lens is to measure the sagittal depth of the anterior segment. Our OCT allows for the imaging of the cornea, limbus, and sclera to a diameter of approximately 16mm (Sorbara et al, 2010; Kojima et al, 2013). We ascertain the sagittal depth from a chord of 15mm across the anterior segment to the apex of the cornea (Figure 2). Considering that most soft lens diameters fall just inside of 15mm and that the majority of scleral lenses land near to this diameter, it provides a reference point to work from. At this chord, the mean sagittal depth of normal eyes is approximately 3,800 microns (Kojima et al, 2013). We are alerted when an eye measures ≤3,500 or ≥4,100 microns, in which case a specialized fitting approach may be required.
Figure 2. The sagittal depth from a chord of 15mm across the anterior segment to the apex of the cornea.
Angle of the Peripheral Cornea, Limbus, and Sclera An additional OCT measurement that is useful in a scleral contact lens fitting is the angle of the peripheral cornea, limbus, and sclera. While the sagittal depth determines the height of lens required, the angle tells us when we need to adjust the limbal or scleral landing zones. To measure these angles, calipers are placed across both a 10mm and 15mm chord of the anterior segment. The angle between these two chord depths can then be measured using angle calipers (Figure 3).
Figure 3. OCT measurement of the angle of the peripheral cornea, limbus, and sclera. The angle between the two chord depths (10mm and 15mm) can be measured using angle calipers.
The average peripheral corneal, limbal, and scleral angle is approximately 38.4° in normal eyes (Kojima et al, 2013). If a patient’s angle measures >41°, the diagnostic scleral lens is far more likely to have peripheral corneal or limbal bearing and to require additional vault through this zone. Conversely, if a patient has a calculated angle of ≤36°, the initial scleral lens is more likely to impinge at the edge.
Corneal Thickness Another valuable OCT function that we utilize during a contact lens fit is global pachymetry. When fitting specialty contact lenses, we are especially concerned about corneas that are prone to edema, such as post-penetrating keratoplasty cases, especially when the endothelium cell count is low.
Further, it is important to monitor corneal thickness changes with the difference display (Figure 4) in patients wearing thick custom soft keratoconus lenses, piggyback lenses, hybrid lenses, and scleral lenses. Comparing corneal thickness pre- and post-wear of specialty contact lenses can help monitor corneal swelling in some of our most complex pathologic cases.
Figure 4. Comparing corneal thickness pre- and post-wear can help monitor corneal swelling.
Once a lens is placed on the eye, the OCT can perform a wide range of analyses to help us fit corneal GP, scleral, and even soft contact lenses. We can control the location of image capture by instructing patients to focus on the fixation target while adjusting the location of the target to image the periphery and edge of the lens (Figure 5).
Figure 5. This image shows a peripheral scleral lens thickness of 1.47mm (1,470 microns).
One of the simplest and most obvious assessments of a contact lens with the OCT is to observe the material thickness from the apex to the edge. Many contact lens manufacturers provide us with the center thicknesses of the lenses that we order; however, do we know the thickness toward the edge? Has the construction of the lens been optimized to allow for the maximum oxygen permeability? Figure 5 shows a scleral lens on eye with a center thickness of approximately 0.41mm (410 microns); however, the measurement at the limbus is 1.47mm (1,470 microns)! This is an important finding to be aware of when considering the oxygen transmission throughout the entire lens surface area (Michaud et al, 2012).
We often perform this same assessment on custom soft lenses for keratoconus to understand the cross sectional lens thickness from center to edge. In corneal thinning disorders, we hope to fit a lens that is just thick enough to mask the irregular astigmatism, but thin enough to meet the minimum oxygen requirements. Figure 6 shows a custom soft lens on a keratoconic eye with an intracorneal ring segment visible within the stroma. Note the 0.5mm/500-micron thickness of the soft lens, which is designed to mask the patient’s keratoconus-induced irregular astigmatism. Figure 7 shows the same patient with a change of fixation to assess the edge thickness and shape. Note the tapered periphery of the lens designed to improve comfort and oxygen permeability.
Figure 6. A soft custom lens on a keratoconic eye with an intracorneal ring segment visible within the stroma.
Figure 7. Same patient as in Figure 6 with a change of fixation to assess the edge thickness and shape.
When patients have complaints of contact lens discomfort and the slit lamp findings are inconclusive, we often question the quality of the lens edge. Having the patient fixate off-center during OCT image capture allows us to view the edge profile in cross section. Figure 8 illustrates a poorly manufactured lens edge, resulting in a blunt tip with acute points that could affect patient comfort. By comparison, Figure 9 shows an optimally finished edge with a well-rounded tip that is conducive to a smooth landing and comfortable fitting relationship.
Figure 8. A poorly manufactured lens edge, resulting in a blunt tip with acute points.
Figure 9. An optimally finished edge with a well-rounded tip.
Lens Fitting Considerations
When fitting scleral lenses, an important measurement is the posterior tear film thickness. Practitioners must check the volume of fluid between the lens and cornea immediately upon application to determine whether any changes in diagnostic or custom lens parameters are required (Figure 10). Then, post settling, it is important to measure the amount of sinking into the conjunctival tissue that’s occurred and whether a healthy volume of fluid still exists at the apex (Caroline and André, 2012; Kaufmann et al, 2013).
Figure 10. The posterior tear film thickness can be measured in scleral lenses to determine the apical clearance.
Measuring the apical clearance of our scleral lenses with the OCT is a relatively efficient and accurate assessment, for which the slit lamp’s optical section can have a high standard deviation of error (Yeung and Sorbara, 2014).
Another valuable OCT application related to specialty contact lens fitting is the assessment of lens landing and bearing. The slit lamp can explain gross problems with the fit of our lenses relatively easily. However, it can be ambiguous in its determination of finite imperfections in the lens.
As an example, Figure 11 shows a scleral lens edge sinking into the conjunctiva at the tip, causing a constriction of vessels visible with the slit lamp. The OCT image shows us that the shape of the alignment zone and edge are without defect; however, the angle of the landing zone should be increased to create more edge lift and to better accommodate the flatter scleral surface angle.
Figure 11. A scleral lens edge sinking into the conjunctiva at the tip, causing a constriction of the vessels visible with a slit lamp.
Figure 12 is an example of the optimal edge relationship between lens and bulbar conjunctiva. Note the shape of the scleral landing zone and edge are optimally finished and free of defects. But just as importantly, the edge points along the surface of the conjunctiva rather than digging in or excessively lifting. Additionally, the edge apex or tip of the lens exhibits what we call a 50/50 scleral edge, in which 50% of the edge apex has sunk gently into the conjunctiva, while 50% is above the surface.
Figure 12. An optimal edge relationship between lens and bulbar conjunctiva.
When more than half of the edge apex has sunk itself into the conjunctiva, vessel restriction is far more likely. Conversely, when more than half of the edge apex is above the conjunctival surface, discomfort is far more likely.
We are always trying to find ways to improve the efficiency of our contact lens practice and, more importantly, to provide our patients with the best fits possible. The anterior segment OCT has become a valuable tool in our pre-fitting assessment of these very difficult cases. But, it has similarly been an indispensable analysis tool once the contact lens is placed on eye.
For patient evaluation, research, and as a teaching tool, this instrument is now considered an integral part of our contact lens department, with a wide variety of applications for every type of lens imaginable. And, we haven’t yet explored everything that it can do. CLS
For references, please visit www.clspectrum.com/references and click on document #227.
Randy Kojima is a research scientist and clinical instructor at the Pacific University College of Optometry. He is also a consultant to Precision Technology Services and Medmont Instruments, Australia.
Patrick Caroline is an associate professor of optometry at Pacific University. He is also a consultant to Contamac.
Dr. Walker is an assistant professor of optometry at The University of Houston College of Optometry.
Dr. Kinoshita is currently an assistant professor and the director of the Pacific Eye Care Clinic at Pacific University College of Optometry.
Mark André is an associate professor of optometry at Pacific University. He is also a consultant to CooperVision.
Dr. Lampa is an associate professor at Pacific University College of Optometry and is in private practice in Silverton, Ore. He has been a consultant to SpecialEyes, Alcon, Vistakon, and B+L.