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Gas-permeable lenses a good choice or post-laser vision correction patients
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Occasionally, patients who have undergone PRK or LASIK procedures require postoperative vision correction with contact lenses. However, the new cornea, which is now oblate in shape, requires a complete rethinking of traditional lens designs and fitting techniques.
The major reason for contact lenses following refractive surgery is to provide binocular function, which can be compromised due to anisometropia. With today’s laser technology, over- and undercorrections remain the most frequently encountered problems affecting binocular function. A more serious problem arises for patients with visual complaints secondary to decentered ablations or irregular astigmatism secondary to flap complications.
With both PRK and LASIK, gas-permeable (GP) contact lens fitting is best delayed until approximately 8 to 12 weeks after surgery. At this point, the refraction and topography have stabilized, and the integrity of the flap is usually sufficient to withstand lens insertion and removal as well as the normal on-eye lens movement that occurs with blinking.
The postoperative cornea
PRK and LASIK are what we refer to as “tissue-subtraction” procedures. This is important to keep in mind because the amount of corneal tissue removed in the laser procedure plays an important role in the postsurgical management of the patient with contact lenses.
With myopic ablations, the mid-peripheral cornea (beyond the central 6.0 to 7.0 mm) remains unchanged. Therefore, the major concern when fitting contact lenses is the relative difference between the flatter central cornea and the steeper (normal) mid-peripheral cornea. This relative difference between the central and peripheral cornea is predicated on the amount of tissue removed. In other words, the greater the preoperative refractive error, the greater the disparity across the cornea from center to periphery.
Fitting after moderate corrections
For patients who were low to moderate myopes prior to surgery, the correction required the removal of very small amounts of tissue. Fortunately, this modest amount of tissue removal does not dramatically affect the way a contact lens will fit, or the on-eye lens dynamics. For example, a patient with a preoperative refractive error of –4 D might end up postoperatively 1 D undercorrected. The 3-D myopic reduction was produced by an ablation of only 36 µm of tissue (less than the thickness of the corneal epithelium).
These individuals are often easily fitted with a GP lens that has a base curve radius selected to align the mid-peripheral cornea 4 mm from center. The base curve can be selected by inspecting the corneal map and by choosing a lens with a radius of curvature equal to the corneal curvature 4 mm temporally. Clinical experience has shown that this is an excellent starting point in fitting postoperative corneas because the fit of any rigid contact lens is established by the relationship of the posterior mid-peripheral lens surface to the mid-peripheral cornea.
This fitting relationship is most critical along the flattest corneal meridian where the lens will be in the closest apposition to the cornea. The mid-peripheral bearing essentially locks the lens along the horizontal meridian to prevent nasal or temporal decentration. The lens is therefore allowed to move freely in the direction of the least mechanical resistance. The temporal cornea is used rather than the nasal quadrant due to shifting of the corneal map as a result of patient fixation in the photokeratoscope. Patient fixation results in an eccentric position (nasal displacement) of the corneal map, which significantly affects the mid-peripheral radius of curvature nasally and temporally.
This alignment error is related to the differences between the pupillary axis of the eye and the patient’s line of sight (angle lambda). As a result of patient fixation, the nasal cornea is often significantly flatter than the temporal cornea. When selecting a base curve for a post-PRK or post-LASIK cornea, the radius of curvature 4 mm temporal is evaluated because it is outside the ablation zone.
Fitting after large corrections
A patient with higher preoperative refractive error, for example –10 D, might end up postoperatively –1 D. The 9-D myopic reduction requires the removal of 110 µm of corneal tissue. Now, the disparity between the central and mid-peripheral cornea is such that a traditional GP lens design would result in excessive apical clearance, unstable optics and bubble entrapment beneath the center of the lens. In this situation, a reverse-geometry lens design is indicated because it more closely parallels the postrefractive surgery topography by incorporating a flat central radius of curvature with a steeper mid-peripheral design.
Numerous commercially available reverse geometry lenses are marketed for postrefractive surgery, as well as orthokeratology designs, and most GP finishing laboratories can custom manufacture a lens based on the practitioner’s specifications.
Generally, the main question in the practice is which central base curve to select. Clinical experience has taught us that an excellent starting point is a base curve radius approximately 0.75 to 1.00 D steeper-than-flat K. This should provide a fitting relationship that allows some central clearance without the presence of bubble entrapment.
The next step is to determine the mid-peripheral alignment curve radius, keeping in mind that this parameter is responsible for providing optimum centration and movement of the lens. This radius is determined by corneal mapping or peripheral keratometry and selecting a curve equal to the radius of the mid-peripheral temporal cornea 4.0 mm from center. This alignment curve can also be selected by evaluating a series of traditional diagnostic GP lenses on the eye (ignoring any central bubbles) until the desired mid-peripheral alignment is observed.
The final step is to determine the reverse curve, which joins the central base curve radius with the alignment curve radius. This can be calculated by the practitioner with commercially available software programs such as the Ortho-tools by Eye Deal or by simply having the laboratory determine this parameter.
Decentered ablations
Nearly all patients with decentered ablations are left with varying degrees of undercorrection. This is created by the eccentric location of the ablation zone as it positions away from the visual axis. GP lenses typically provide the best visual correction in their ability to mask the corneal irregularities and create a new artificial, spherical surface to the cornea. However, the fitter is faced with the challenge of significant asymmetry in the central and peripheral corneal curvature that typically bisects the cornea nasally and temporally and extends vertically throughout the entire ablation zone.
In this case, the nasal cornea is significantly flatter than the temporal corneal radius of curvature. Finding a base curve radius that is flat enough to accommodate the nasal cornea would most likely cause significant temporal lift-off and erratic decentration.
Over- and undercorrections with centered ablation zones can be managed with reverse geometry designs; however, in the case of the decentered ablation, the fitting challenge of aligning the mid-peripheral fitting curve of the lens on the asymmetrical cornea is the same as a traditional GP design.
In cases of extremely decentered ablations or highly irregular post-surgical corneas, semi-scleral lens designs are often the best choice. These lenses are available in diameters of 13.9 to 15.0 mm: the MacroLens from C & H Contact Lens and the Jupiter Lens from Correctech. These designs have many advantages over traditional GP and soft lens designs, including improved centration with stable optics over the pupil, total masking of all corneal cylinders and greater comfort due to scleral support.
The bottom line
The goal in fitting any GP corneal or scleral lens postrefractive surgery is to achieve relative alignment or slight clearance across the central cornea with mid-peripheral corneal alignment for optimum lens centration, movement and comfort.
For Your Information:
- Patrick Caroline, COT, FAAO, is associate professor of optometry at Pacific University College of Optometry in Forest Grove Oregon and assistant professor of ophthalmology at the Oregon Health University in Portland. He can be reached at Oregon Health Sciences University, Casey Eye Institute, 3375 SW Terwilliger Blvd., Portland, OR 97201; fax: (503) 352-2929.