LENS FITTING TECHNOLOGY
Using Advanced Technologies for Contact Lens Fitting
Today's diagnostic instruments and contact lens fitting software can help you increase fitting success in less chair time.
By S. Barry Eiden, OD, FAAO; Gregory W. DeNaeyer, OD, FAAO; Shana Brafman, OD; Jennifer Harthan, OD, FAAO; & Robert Davis, OD, FAAO
|Dr. Eiden is president and medical director of North Suburban Vision Consultants as well as co-founder of Eye-Vis Eye and Vision Research Institute. He is an adjunct faculty member at The University of Illinois Medical Center as well as at the Indiana, Illinois, Salus, and PCO colleges of Optometry. Dr. Eiden is a consultant or advisor to CooperVision, SynergEyes, Alcon, and SpecialEyes, and he has received research funds from Vistakon, Cooper-Vision, and B+L.|
|Dr. DeNaeyer is the clinical director for Arena Eye Surgeons in Columbus, Ohio, and a consultant to Visionary Optics, B+L, and Aciont. You can contact him at firstname.lastname@example.org.|
|Dr. Brafman is co-director of Contact Lens Specialty Services at North Suburban Vision Consultants, Ltd. and is an assistant clinical professor at the Illinois College of Optometry and Illinois Eye Institute.|
|Dr. Harthan is currently an assistant professor in the Cornea Center for Clinical Excellence of the Illinois Eye Institute. She also is chief of the Urgent Eyecare Service where she works with both students and residents, and she works part time in private practice. She is a consultant and lecturer for Teague Training Group.|
|Dr. Davis is co-founder of Eye-Vis Eye and Vision Research Institute. He practices in a suburb outside Chicago. He has received research funds from SynergEyes, CooperVision, and B+L and has a proprietary interest in SpecialEyes and Alternative Vision Solutions.|
In today's ever demanding practice environment, we are continually looking for methods to improve the efficiency and accuracy of contact lens fitting. Pressures to see greater numbers of patients and to provide optimal outcomes in shorter time periods suggest that if we had the ability to successfully fit our GP contact lens patients (both simple and complex cases) in a virtual environment, we might achieve both of these goals.
Advanced technologies that can measure corneal and anterior segment anatomy—such as computerized corneal topography, corneal Scheimpflug imaging tomography, and anterior segment optical coherence tomography (OCT)—allow us to achieve virtual contact lens fitting based upon numerous measures of lens-to-eye fitting relationships. Today's available technologies not only have the ability to measure anterior segment anatomy, they also include software that can create virtual contact lens fitting simulations and that can even suggest specific contact lens parameters and designs.
The clear advantages of incorporating such technologies into clinical contact lens practice include the ability to predict lens fitting outcomes without time-consuming diagnostic lens fitting, reduced costs and increased space resulting from fewer diagnostic lens fitting sets, and the ability to interact virtually with contact lens laboratory consultants in managing more challenging cases.
Imagine, for example, a patient who needs contact lenses to manage corneal irregularity. You could conduct your initial examination along with corneal shape analysis and then design a custom contact lens knowing there is a high probability that the lens will fit properly when the patient returns for dispensing. You could work with your software at a convenient time and manipulate the lensfitting simulations to achieve the optimal outcome. You would further have the ability to send the data to your laboratory and get input from lens designers and fitting consultants to assist you in achieving your goals. The ability to design GP contact lenses in such a way is available today, but only a small number of practitioners have started to use it. The future for growth and development in this area is bright, and we can expect to see this type of technology used more frequently as costs continue to decrease and more practitioners bring it into their practices.
Limitations of Traditional Contact Lens Fitting Methods
Traditionally, contact lens fitting has been based simply upon Ks and refraction. Today, we are more aware of the limitations of keratometric curvature findings. We know that Ks tell us only about the curvature of the steep and flat meridians at approximately 3mm from the apex of the cornea. They do not take into consideration the asphericity of the cornea nor the asymmetric nature of most corneal asphericity. Finally, we now know that the overall sagittal height of the cornea is the primary determinant of the lens-to-cornea fitting relationship and that multiple factors contribute to corneal sagittal height including apical corneal radius, corneal asphericity, and corneal diameter.
Diagnostic contact lens fitting specifically for GP lenses has relied on the analysis of fluorescein patterns. Unfortunately, the ability to visualize fluorescein is limited to approximately 20 microns. If the corneal clearance is below 20 microns, the area evaluated will appear to be aligned to—or bearing on—the corneal surface when this may not be the case.
Technologies to Evaluate Corneal and Anterior Segment Anatomy
Placido-Based Corneal Topography This is the most common method used to measure corneal shape in clinical practice today. It reflects multiple concentric illumination rings off of the anterior corneal surface, then the instrument's computer captures and analyzes the image of the reflected rings. Placido-based topography systems are very accurate in determining corneal curvature; however, they derive corneal elevation from curvature calculations, which may not be as accurate as true elevation-based corneal analysis systems. Regardless, many Placido-based topography systems have contact lens fitting software programs available and are able to simulate lens fittings quite well. Some examples include the E300 topographer by Medmont, TMS by Tomey, Keratograph by Oculus, Atlas 9000 by Carl Zeiss Meditec, and Keratron systems by Eyequip in the United States.
Elevation-Based Corneal Tomography A number of systems are available that directly measure corneal elevation. Such systems may provide more accurate assessment of true corneal shape as they provide 360-degree three-dimensional assessment of the anterior segment. As such, the term tomography is more accurate with these instruments rather than topography. They also have the advantage of measuring both the anterior and posterior corneal surfaces as well as providing global corneal thickness measurements via optical pachymetry, which are often very helpful in evaluating irregular corneas. These systems measure corneal elevation either by optical slit-scan imaging (e.g., Orbscan from Bausch + Lomb [B+L]) or by rotating Scheimpflug imaging (e.g., Pentacam from Oculus and Galilei from Zeimer). These systems also feature contact lens simulation fitting software that is based on the elevation data provided.
Anterior Segment OCT OCT of the anterior segment is becoming more clinically available. A number of systems primarily designed for posterior segment OCT have the ability to also produce OCT images of the cornea and anterior ocular segment (e.g., Cirrus HD-OCT by Carl Zeiss Meditec, Spectralis by Heidelberg Engineering, 3D OCT by Topcon, and RTVue by OptoVue), while others are exclusively designed for anterior segment OCT functions (e.g., Visante by Carl Zeiss Meditec). Anterior segment OCT systems provide exquisite and highly accurate imaging and measures of anterior segment anatomy.
Scleral contact lenses have become an important contact lens modality for successfully managing patients who have severe corneal irregularity or ocular surface disease. Currently scleral contact lenses are fit diagnostically. Technology has not been fully developed that would allow anterior segment measurements to be directly converted to data that manufacturers could use to empirically design a custommade scleral contact lens. However, OCT technology is available that allows you to take ultra-fine measurements of a scleral lens fit to enable more precise design and adjustments.
OCT measurement produces two-dimensional crosssectional images of the scleral contact lens and the anterior ocular surface. This provides a magnified view and accurate measurement of the scleral lens-to-ocular-surface fitting relationship for optimized design adjustments. Specifically, this can help to ensure that the scleral lens is fully vaulting over the corneal limbus, which can sometimes be difficult to assess when evaluating a fluorescein pattern. Also, OCT measurement can optimize adjustments for toric or quadrant-specific back-surface haptic designs for patients who have moderate-to-severe non-rotationally symmetrical scleral anatomy.
Sagittal depth values from OCT measurements can now be used to help make the initial diagnostic lens selection when using the ICD (Irregular Corneal Design) scleral lens design (ABB Concise/Valley Contax/Essilor). Gemoules (2008) was the first to report successfully fitting nine patients with scleral contact lenses utilizing sagittal depth and chord measurements obtained from a Visante OCT.
Software Options for Contact Lens Fitting
Most contact lens fitting and simulation software programs have a number of common properties.
They typically have a library of proprietary lens designs available and can either suggest a specific lens design for an individual topography map or allow you to select a lens design from the available choices and apply it to the topographic map. The software will suggest the most appropriate parameters of that lens design based on the topography; however, you can change and manipulate multiple lens parameters to achieve the desired fitting relationship. Alternatively, you can custom design a lens by determining all of the key lens parameters.
Typically, the software will provide a simulated fluorescein pattern that will change as lens parameters are modified in the program. The software will also calculate lens-to-cornea fitting relationships in terms of microns of clearance between the two surfaces. These measures can be calculated along any meridian of the cornea. Analysis of such data allows you to see central, peripheral, and edge clearance for 360 degrees. This graph is highly valuable in designing lenses with optimal lens-to-cornea relationships and is far more accurate compared to observing fluorescein patterns due to the previously discussed limitations of such observations.
In addition to the physical fitting characteristics of a virtually fit lens, the software can also assist in determining its optical power properties when you enter refractive data and vertex distance. Changes in base curve radius resulting from the required optical power can immediately be calculated.
When considering using contact lens fitting software to fit an irregular cornea, data access, reliability, and accuracy are all issues that must be addressed. In the May 2011 Contact Lens Spectrum article “Evaluating Virtual Fitting for Keratoconus,” Sindt et al evaluated virtual fitting of 18 keratoconus patients (31 lens fits) utilizing the Medmont E300 Placido topography and contact lens fitting software system. They were able to achieve an accuracy rate of 95 percent in terms of the software's ability to predict the actual fit of lenses on the cornea.
Case #1: Medmont E300 Software A 25-year-old male presented for contact lens fitting for the right eye. His manifest refraction was OD −5.00 −2.00 x 180, 20/20 and OS −4.00 −0.50 x 170, 20/20. Medmont E300 Placido topography of the right eye showed about 2.2D of regular limbus-to-limbus with-the-rule astigmatism (Figure 1).
Figure 1. Case #1 Medmont E300 Placido-based axial curvature map showing significant limbus-to-limbus with-the-rule astigmatism.
Simulation of a spherical GP contact lens designed to achieve 20 microns of central clearance demonstrated excessive clearance along the steep meridian, which typically would result in poor physical fit, less-than-optimum comfort, and potentially unstable vision (Figure 2).
Figure 2. Case #1 simulated fluorescein pattern and clearance along the steep vertical meridian for a spherical GP fit to the toric cornea. Clearance along the vertical meridian in the intermediate and peripheral regions is excessive.
Simulation of a toric GP lens design on the eye resulted in appropriate central and peripheral clearance for 360 degrees (Figure 3). The software chose the toric base curve radii, intermediate curve radii, and peripheral curve radii based on the limbus-tolimbus nature of the regular with-the-rule astigmatism. The image of the actual lens on the eye based on the software-suggested parameters matches the simulated pattern (Figure 4).
Figure 3. Case #1 toric GP simulated lens fluorescein pattern and clearance along vertical meridian. Clearance along the vertical meridian is now appropriate in the intermediate and peripheral regions.
Figure 4. Case #1 actual toric GP lens on the right eye.
Case #2: Oculus Contact Lens Fitting Software Utilizing Keratograph Data A 49-year-old male presented wearing GP lenses that had been fit elsewhere six months ago. The patient complained of poor comfort and unstable vision and stated that the lenses often would decenter with the blink. Evaluation of the lens fit showed significant with-the-rule astigmatic fluorescein patterns in both eyes and instability of lens positioning with the blink. Manifest refraction was OD −4.00 −3.75 x 175, 20/20 and OS −3.25 −4.25 x 005, 20/20−. Keratograph Placido-based topography showed bilateral regular limbus-to-limbus with-the-rule astigmatism (Figure 5). Sim-Ks were OD 41.75/45.87 @ 95, OS 41.37/46.25 @ 95.
Figure 5. Case #2 Keratograph Placido image, axial curvature map OS.
Oculus contact lens fitting software was applied to the topography maps. Initial application of a spherical lens design showed the expected astigmatic simulated fluorescein pattern and unacceptable clearance values along the vertical meridian (Figure 6).
Figure 6. Case #2 Oculus contact lens fitting software and simulated fluorescein pattern of a spherical GP lens fit on a significantly toric cornea.
Application of a bitoric lens design resulted in the appropriate lens-to-cornea fitting relationship as demonstrated with both simulated (Figure 7) and actual fluorescein patterns. The lens was fit on-K along the flat meridian and approximately 1D flat along the steep meridian, which created a fit that is similar to that of a spherical lens fit on a low with-the-rule cornea. Visual acuities with lenses were 20/20 for each eye and were stable with the blink.
Figure 7. Case #2 Oculus contact lens fitting software and simulated fluorescein pattern of a toric GP lens fit to the significantly toric cornea.
Case #3: Visante OCT Utilized to Confirm Vault With Scleral Lens Fit A 23-year-old female presented for a contact lens fitting post-penetrating keratoplasty OD. The patient's ocular history was remarkable for keratoconus OD and OS and corneal hydrops OS. Entering VAs were 20/400 OD and 20/200 OS. Manifest refraction was +1.50 −6.25 x 080, 20/120 OD and +1.75 −3.50 x 110, 20/80 OS. Corneal topography maps exhibited 10D of irregular astigmatism OD and almost 4D OS (Figure 8).
Figure 8. Case #3 Placido corneal topography of patient's right eye.
Secondary to the patient's corneal irregularity and the need to maintain optimal health, we fit her in scleral lenses with the following parameters: OD 6.03mm base curve; 18.2mm diameter; peripheral curve of 12.75D/1.5D, 2D steep reverse, 1D flat periphery; and −13.00D power, 20/25, and OS 6.21mm base curve, 18.2mm diameter, 1D flat peripheral curve, −12.25D power, 20/20.
The patient was extremely happy with her lenses and was able to wear them comfortably all day. Fluorescein patterns of the lenses demonstrated central vault, limbal clearance, and no conjunctival blanching or impingement. Visante anterior segment OCT imaging over the lenses showed that the patient had adequate tear exchange beneath the lens, especially over the corneal graft (Figure 9).
Figure 9. Case #3 Visante anterior segment OCT Image over scleral lens demonstrating adequate vault (approximately 70 microns).
Case #4: Corneal Reshaping (Paragon CRT Fitting Software and Pentacam) An 11-year-old Asian female presented with a history of progressive myopia over the past three years. Visual acuity with current glasses was 20/30− for each eye. Manifest refraction was OD −3.50 −0.50 x 175, 20/20+ and OS −3.75 −0.50 x 180, 20/20+. This represented a −0.75D increase in myopia for each eye when compared to the habitual spectacle prescription, which was less than one year old. Oculus Pentacam corneal tomography revealed normal anterior and posterior corneal elevation, normal anterior corneal curvature, and normal global pachymetry. The anterior corneal asphericity was measured at e = 0.51 OD and e = 0.50 OS, and corneal diameter measured along the horizontal meridian from the Scheimpflug image was 11.7mm for each eye (Figure 10).
Figure 10. Case #4 Oculus Pentacam tomography of the patient's right eye.
Paragon CRT (Paragon Vision Sciences) fitting software was applied to the Pentacam elevation data. We will present outcomes from the right eye only for demonstration purposes. The suggested initial CRT lens had parameters of OD 8.80mm base curve radius, Return Zone Depth 500 microns, Landing Zone Angle 33 degrees. The simulated fluorescein patterns showed insufficient central apical clearance along with inadequate edge lift (Figure 11). We changed the Reverse Zone Depth in the software to 525 microns, but the resulting simulated fluorescein pattern demonstrated excessive central clearance yet inadequate edge lift (Figure 12). Changing the Landing Zone Angle to 32 degrees along with a 525 micron Reverse Zone Depth resulted in an optimal simulated fluorescein pattern (Figure 13).
Figure 11. Case #4 Paragon CRT fitting software simulated fluorescein pattern of the right lens using the software's suggested initial lens parameters, which showed insufficient central apical clearance along with inadequate edge lift.
Figure 12. Case #4 simulated fluorescein pattern of patient's right lens after changing the Reverse Zone Depth in the software to 525 microns, demonstrating excessive central clearance yet inadequate edge lift.
Figure 13. Case #4 simulated fluorescein pattern of patient's right lens. Changing the Landing Zone Angle to 32 degrees along with a 525 micron Reverse Zone Depth resulted in an optimal simulated fluorescein pattern.
The actual lens fit and fluorescein pattern closely matched the simulated pattern and resulted in appropriate corneal reshaping. Daytime vision following lens removal was 20/20 with a manifest refraction of plano −0.25 x 70 20/15−. The patient reported clear vision during the entire day (for more than 16 hours post-lens removal).
Access to advanced technologies such as corneal topography, elevationbased corneal and anterior segment tomography, and anterior segment OCT that can accurately measure corneal and anterior segment surfaces is becoming more widely available. With optional software, we can now fit our patients virtually with contact lenses. Accuracy in lens-to-cornea fitting relationships, reduced dependence on multiple diagnostic lens fitting sets, reduced chair time, and the ability to work virtually with lens laboratory design consultants are just a few of the many of advantages of utilizing such software and technologies. We expect to see contact lens fitting software utilization increase along with the percentage of successful contact lens fit outcomes. CLS
The authors thank Randy Kojima, FAAO, and Patrick Caroline, FAAO, for their assistance and for Medmont E300 case data.
For references, please visit www.clspectrum.com/references.asp and click on document #201.