Fitting Keratoconic Corneas with Peripheral Curve Bracketing
A new GP lens design may improve tear exchange and comfort for your keratoconus patients.
By Julie Jackson, RN, OD, William Miller, OD, PhD, Pat Segu, OD, Jon Darnell, BS, Norman Leach, OD, MS, and Jan Bergmanson, OD, PhD
Figure 1. Corneal topography of right eye.
Traditionally, typical gas permeable (GP) contact lens designs that practitioners use to fit keratoconic patients result in three-point touch or apical clearance. Even though these lens designs address the keratoconic apex, they often result in inadequate edge lift or mid-peripheral seal off, which causes poor patient comfort, inadequate tear exchange and difficult lens removal. These problems may limit wearing time and ultimately lead to lens intolerance.
Advances in GP lens manufacturing and design have produced more sophisticated edge lift profiles by altering peripheral curve (PC) asphericity to counter limitations of standard designs. The Metro Optics' ComfortKone is an FDA-approved keratoconic GP lens design that incorporates aspheric PC radii (A value) over the abnormal junction between the cone apex and peripheral cornea. The lens features a 4.0mm wide spherical base curve (BC), which is designed to fit the abnormal conical region, and two aspheric PCs to fit the more normal peripheral cornea.
Although clinical management and initial contact lens selection in keratoconus depends largely on the severity of the presentation, past literature suggests using GP contact lens fitting to delay or even avoid surgical intervention.
Keratoconus compounds normal GP adaptation events due to the progressive, non-inflammatory disease process itself. High amounts of irregular astigmatism, corneal ectasia, corneal scarring and possible atopy create additional challenges for both patient and practitioner concerning adequate fitting relationships and long-term comfort and health. Ironically, lens intolerance is not the primary reason why these patients cease lens wear.
Initial GP Considerations
GP contact lens visual correction provides the most satisfactory visual results for keratoconic patients. Although this statement sounds simplistic and definitive, eyecare practitioners recognize that keratoconic corneas are one of their most challenging in clinical practice to fit with GP lenses.
The Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study group found that despite higher than average keratometric readings in its keratoconic study population, keratometer readings were not singularly diagnostic of keratoconus. However, K readings serve as the most important piece of information for initial GP lens diagnostic fitting. Additionally, corneal topography is an especially valuable tool with this patient population. Corneal topography not only provides an appreciation for the corneal geography, but practitioners can frequently detect changes consistent with early keratoconus prior to clinical signs. Of special note, many topographers feature software packages (Maeda Klyce and Rabinowitz systems) that analyze certain topographical data points through sophisticated algorithms that highlight the likelihood of keratoconus.
Figure 2. A15/6.60mm BC/8.50mm OAD. Excessive apical vaulting with adequate edge lift.
GP Fitting in Keratoconus
You can often achieve adequate vision for early keratoconus patients with spectacles or perhaps soft contact lenses. But as the cornea thins and subsequently steepens, GP lens correction is indicated to maintain good vision. Although several marketed lenses are available specifically for keratoconus, only a few designs are FDA-approved for managing keratoconus. One such design is Metro Optics' ComfortKone, which comes in a wide range of base curve radii. A hallmark of this particular lens type is its A value, which denotes its particular peripheral curve designation. The A values range from 3 to 30 and signify the specific asphericity of the PC. Increasing the A value number increases the asphericity (or rate of flattening) of the PC, providing increasingly greater edge lift.
This case report demonstrates a PC radius bracketing technique we are currently using in a larger clinical study at the University of Houston College of Optometry. We developed this technique specifically for the ComfortKone to optimize edge lift and tear exchange. Although we successfully fit both of this patient's eyes with ComfortKone lenses, we will limit the discussion of our PC radius bracketing technique to the patient's right eye.
We refit a 24-year-old Caucasian male with keratoconus (diagnosed at age 16) from his current tri-curve spherical lens design (BVA 20/20 OD, 20/25 OS) to the aspheric ComfortKone using a PC radius bracketing technique. Findings included Munson's sign, prominent corneal nerves, a central corneal scar OS and corneal topography consistent with keratoconus.
Fitting the ComfortKone Corneal topography performed with the Orbscan revealed an inferior corneal steepening in both eyes. Figure 1 shows the corneal topography of the right eye. Additionally, pachometry readings over the right corneal apex of 460µm further confirmed the presence of corneal thinning.
We fit the patient using a ComfortKone diagnostic lens fitting set and equivalent keratometric readings from topography. We derived an initial base curve for the right eye by splitting (or averaging) the flat and steep keratometric readings. An initial base curve that equals the steep keratometric reading is also acceptable. When fitting the ComfortKone design, your goal is to provide slight apical vaulting (First Definite Apical Clearance Lens).
Once you select the initial diagnostic base curve, the ComfortKone fitting guide recommends that you choose an A value by subtracting the millimetric values of the flat and steep K readings and using the resultant number as the initial A value. However, for simplicity, we chose A values by consistently starting with an A value of A7 or A10. After allowing an appropriate settling time for the diagnostic lens (15 minutes, as the fitting guide recommended), we evaluated the fluorescein pattern with a Wrattan filter. We concentrated on base curve selection with the ComfortKone lens. If we found an unacceptable fluorescein pattern, we selected another diagnostic lens with a different base curve, but left the A value unchanged (Figures 2 and 3). Changing a specific, solitary parameter greatly simplified the fitting process, as base curve changes can result in subtle changes in the fluorescein pattern of the peripheral curves.
After selecting the appropriate base curve, we focused on bracketing the A value. The peripheral curve asphericity of the ComfortKone more closely aligns with the mid-peripheral conical slope seen in keratoconic patients. We bracketed the peripheral curve radii by incremental steps of 5 until we achieved the best mid-peripheral relationship and edge profile (Figures 4, 5 and 6). Of special note, as Figures 3 and 5 show, once the A values approach the desired fit, small incremental steps of less than 5 can greatly enhance the mid-peripheral pattern and edge lift profile.
Final Fit Maintaining an apical vault, we bracketed the ComfortKone PC (A value) and photodocumented it to show how subtle A value changes affect the overall fit. The successful fit for this patient reveals apical pooling, diffuse mid-peripheral bearing and optimal edge lift, promoting good tear exchange (Figure 3). Snellen visual acuity was 20/20 and the lens demonstrated good movement with adequate centration. The patient reported excellent comfort with the ComfortKone.
Figure 4. A5/6.80mm BC/8.50mm
OAD. Poor edge lift with the presence of an air
Figure 5. A12/6.80mm BC/8.50 OAD. Small incremental steps of less than 5 can greatly enhance the mid-peripheral pattern and edge lift
Tips for Fitting the ComfortKone
- Select the initial diagnostic lens based on the K readings (practitioner's preference of beginning with the steep K or splitting the Ks).
- Choose an A value of A7 or A10 for the initial diagnostic lens.
- Vary only one parameter at a time. We suggest manipulating the base curve first rather than complicating the process by also varying the PC.
- Once you achieve an acceptable fluorescein pattern centrally, bracket the A values by increments of 5. You can achieve further enhancement of the PC by breaking the A value intervals into smaller steps.
- Choose overall diameter based on coverage. If the lens decenters, but comfort and visual acuity are good, you don't need to change the lens parameters. As with any GP lens fit, changes in overall diameter may influence the fluorescein pattern and lens fit.
Figure 6. A20/6.80mm BC/8.50
OAD. Excessive edge lift with this lens.
Due to the unusual corneal topography of keratoconus, it is difficult to achieve an adequate lens fit and fluorescein pattern over the apex and peripheral cornea. In this case we used a peripheral curve radius bracketing technique with the Metro Optics' ComfortKone lens design to successfully improve tear exchange and reduce physical pressure over the ectactic cornea.
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Dr. Jackson is the director of Student Affairs and a clinical associate professor at the University of Houston College of Optometry.
Dr. Miller is an assistant professor at the University of Houston College of Optometry.
Dr. Segu is a clinical associate professor at the University of Houston College of Optometry and director of optometry services at the Good Neighbor Healthcare Center.
Jon Darnell is currently a third year optometry student at the University of Houston College of Optometry
Dr. Norman E. Leach is a clinical professor and director of the Cornea and Contact Lens Service at the University of Houston College of Optometry.
Dr. Bergmanson is a professor of optometry at the University of Houston College of Optometry where he is the founding director of the Texas Eye Research and Technology Center (TERTC).
TABLE 1 ComfortKone Diagnostic Set Lens Parameters
|A VALUES||BASE CURVE RANGES|
|A5||6.7mm to 7.7mm|
|A7||6.5mm to 7.5mm|
|A10||6.2mm to 6.4mm|
|A12||6.2mm to 7.0mm|
|A15||5.9mm to 6.7mm|
|A20||5.6mm to 6.6mm|