Since the introduction of photokeratoscopy and, most recently, corneal modeling, much has been learned regarding the corneal topographical changes the cornea undergoes during the early stages of keratoconus.
|As this condition progresses, the curvature of the corneal apex becomes more elongated, the corneal surface bulges more creating increased irregular astigmatism. For these reasons, acceptable visual acuity with spectacles becomes increasingly impossible. Soft lenses, due to their flexible nature and limited ability to correct corneal astigmatism, do not provide the degree of visual acuity that these young, active patients require to function in their daily lives. By their physical nature, rigid contact lenses offer the best vehicle for providing very good visual acuity to these patients, until their condition reaches a point of remission or until surgical intervention is indicated. These patients typically become more dependent on their rigid contact lenses for day and night time vision correction. It is therefore imperative to maximize lens wearing comfort for the patient. Because of the prolonged wearing periods, it is also critical to protect the already compromised integrity of the corneal physiolog The use of gas permeable lens materials offering high oxygen transmissibility are extremely important.
Because of the limited and often inaccurate information provided by keratometry, keratoconus lens fitting dictates that diagnostic lenses are used to achieve the best possible fit.
Identification of Cone Type
The contact lens fitter can determine the type and size of the cone in one of several ways. Following dilation of the pupils, the "red reflex" can be used to retro illuminate the cornea and determine the shape and approximate size of the cone. A more direct approach is to place a flat-fitting lens on the eye and view the outline produced by the flat fitting relationship as viewed using sodium fluorescein.
|Rigid Lens Fitting
While keratoconus can present numerous lens fitting difficulties, these can be minimized if a disciplined approach to lens fitting is used. In the early stages of this condition, use of rigid spherical and aspheric lenses employing steep base curves, standard designs, and average lens diameters will function very well to provide good patient comfort and very good visual acuity.
As the disease progresses, fitting moderate to severe keratoconus requires more attention to identifying the size and shape of the corneal aberration. As stated previously, use of diagnostic lenses is the only sure method to achieve an optimal lens fitting relationship. Basic lens design requirements dictate that lenses provide steep base curves, relatively small posterior optical zones and flat peripheral curves. There are several design systems that meet these criteria.
McGuire Cone Lens Design
Attempts to vault the corneal apex to reduce inferior edge lift, will result in an excessively steep fitting relationship on the more normal superior portion of the cornea.
The fitting system consists of three separate diagnostic lens sets specifically designed for each type of cone configuration.. The design is predicated on the lens optical zone size as it relates to the size of the cone., varying form 6.0mm for nipple-type cones to 6.5mm for oval cones, and 7.0mm for globus cones. Each incorporates identical peripheral curve systems blended together to resemble an aspheric-like flattening in the posterior lens periphery. Four peripheral curves are utilized:
Soper Keratoconus Design
1) Mild - (A, B, C, D) less than 48.00 diopters in either corneal meridian
2) Moderate -(E, F, G) 48.00 to 54.00 diopters in either corneal meridian
3) Advanced - (H, I, J) >54.00 diopters in either corneal meridian
The difference in this fitting system is that overall lens diameter and base curve radius is changed to increase or decrease lens sagittal depth, thereby allowing this design to be used in fitting a range of corneal steepnesses. Lenses are identified by letters to signify which type of cone each would fit.
Aspheric Rigid Lens Designs
Several of these aspheric designs are also "junctionless". That is, the design has been mathema-tically calculated to allow the various aspheric curves to be joined in a manner in which no junctions are created. This helps to eliminate narrow bearing areas in the mid-periphery, allows lens mass to be distributed more evenly, creating a more uniform tear layer profile under the lens.
Other Contact Lens Options
Another option is the use of a hybrid lens such as the SoftPerm lens. The advantage of this system is that the soft lens and gas permeable lenses are polymerized together into one unit.
Consideration must be given to the physiological impact that these fitting systems may have on the cornea with regard to providing adequate oxygen.
Fowler, Craig W MD; Belin, Michael W. MD; Chambers, Wiley A. MD; "Contact Lenses in the Visual Correction of Keratoconus", presented at Contact Lens Association of America Mid-Winter Meeting, Las Vegas, 1988.
Weiner, Barry M. OD; Nirankari, Verinder S. MD; "A New Biaspheric Contact Lens for Severe Astigmatism Following Penetrating Keratoplasty"; The CLAO Journal, January 1992, Vol. 18, Number 1, Pages 29-33.
Bennet, Edward S. OD; Grohe, Robert M. OD; "Rigid Gas Permeable Contact Lenses" Professional Press Books, Fairchild Publications, New York, NY; 1986. Pages 297-344.
Further Information :
Author: John Dreyer Optometrist Bsc(Hons), MCOPTOM, DipCLP
Created: 24 Apr 2015, Last modified: 17 Jul 2019