Contact Lens Management After Refractive Surgery

BY MICHAEL A. WARD, F.C.L.S.A.
OCT. 1996

Minimize the challenge of postsurgical contact lens fitting by following this step-by-step guide. Indications for contact lenses after refractive surgery include undercorrection, overcorrection, residual astigmatism, anisometropia and, most often, irregular astigmatism. Although contact lens management after refractive surgery can be quite challenging, our objectives remain the same as with all contact lens fitting: to maximize vision, maintain comfort and minimize interference with corneal function and structure.

RADIAL KERATOTOMY

Radial keratotomy is the most common refractive procedure performed for myopia. As the name implies, equally spaced radial incisions are made deeply into the corneal stroma with a resultant flattening of the central portion of the cornea, thereby decreasing its refractive power (Fig. 1). Transverse or arcuate incisions are used to reduce astigmatism, while hexagonal cuts to are used to treat hyperopia. The eye in Figure 2 has a trapezoidal incision to correct astigmatism; figure 3 shows hexagonal incisions with additional incisions at the corners. Contact lens management following failed hexagonal keratotomy is difficult at best due to the anterior protrusion of the central cornea.

FIG. 1: IN RADIAL KERATOTOMY, EQUALLY SPACED RADIAL INCISIONS ARE MADE DEEPLY INTO THE CORNEAL STROMA WITH A RESULTANT FLATTENING OF THE CENTRAL PORTION OF THE CORNEA.

FIG. 2: A TRAPEZOIDAL INCISION IS USED TO CORRECT ASTIGMATISM.

FIG. 3: HEXAGONAL INCISIONS WITH ADDITIONAL INCISIONS AT THE CORNERS.

Use keratometry, corneascopy or videokeratography to obtain corneal topographical measurements. The normal corneal contour is steepest centrally with a non-linear, aspheric flattening toward the limbus. After radial keratotomy, however, this shape is essentially reversed. Topography measurements reveal a flat central portion and a steeper mid-periphery.

Although postoperative central keratometry is of limited value, preoperative keratometry yields useful information. Corneascopy rings offer a near intuitive interpretation of topographical contours. Computerized videokeratography allows color enhanced analysis for quick evaluation. If none of these instruments is available, obtain preoperative K readings or use the curvature of the unoperated fellow eye to determine a starting point. If preoperative keratometry is unavailable, estimate it by adding the amount of surgical correction to the postoperative K readings.

Following radial keratotomy, the center of the cornea is flat and the mid-periphery is irregular and steep (Figs. 4 & 5). This is the 'elbow' or bend from flatter middle to steeper periphery, which makes it difficult to center a contact lens.

FIG. 4: BEFORE RADIAL KERATOTOMY.

FIG. 5: FOLLOWING RADIAL KERATOTOMY, THE CENTER OF THE CORNEA IS FLAT AND THE MID-PERIPHERY IS IRREGULAR AND STEEP.

FIG. 6: AN RGP LENS OVER RADIAL KERATOTOMY. NOTE THE SUPERIOR POSITIONING AND THE MILD SUPRA-LATERAL DECENTRATION.

Figure 6 shows an RGP over a patient's left eye. Note the superior positioning and the mild supra-lateral decentration. Lenses will seldom center properly following RK due to this elbow effect. The degree of decentration in Figure 6, however, is acceptable.

There are three approaches to rigid contact lens fitting after radial keratotomy: superior alignment with the contact lens tucked under the upper lid so that it moves with the lid; intrapalpebral, using smaller and steeper lenses that fit within the aperture; and reverse curve lenses, which are flatter centrally and tighter peripherally. A superior alignment fit is often most successful. The diameter of intrapalpebral lenses is usually not sufficient for adequate centration. Reverse curve lenses tend to center better, but they may be too snug in the periphery for good tear exchange.

Contact lens fitting may begin three to six weeks following the last surgical procedure, depending on the surgeon's recommendation. If the incisions have not healed completely, raised areas similar to those over the wound in a transplant may be present. The wound must flatten before a contact lens can be fitted. Some fluorescein pooling around the incision sites is acceptable, but the corneal epithelium must be intact.

Initial base curve selection should be near or on the flat K of the preoperative keratometric measurements. For example, if preoperative K measurements were 45.00/47.00, fit a 45.00 base curve initially. This may be three to six diopters steeper than the postoperative keratometry, with the lens aligned over the mid-periphery of the cornea, vaulting centrally. If a bubble is present, flatten the base curve in half-diopter steps until it is gone. A trapped bubble under the lens is not acceptable.

To keep the lenses centered, use larger diameters -- 9.5mm is standard, but some eyes may require 10mm, 10.5mm and even 11.0mm. The optical zone diameter should be approximately 1.5mm less than the overall lens diameter when using 9.5mm lenses. For 10.0mm or 10.5mm lenses, maintain an 8.0mm optical zone and use 2.0mm for transition curves. The secondary curve radius should be approximately one millimeter flatter than the central base curve radius (Table 1).

Use a standard 12.25mm radius at 0.4mm wide peripheral curve or a high edge lift aspheric to provide plenty of edge lift to facilitate the flow of tears under the lens. Determine power by refracting over the diagnostic lens. When dealing with irregular corneal topographies, mathematical power computation tends to be less than accurate.

Use high oxygen flux fluorosilicone acrylate materials to ensure maximum oxygen transmission. Remember that we are fitting over corneal scars and don't want to increase the inflammatory response by creating a hypoxic situation.

Rigid lenses center over the steepest meridian of the cornea. In eyes that have undergone radial keratotomy, the steep elbow is eccentric and the lens will not center or move as it would on an unoperated eye. Therefore, they don't center well, they don't move well, and they just don't look 'normal.' When assessing the fit, remember your objectives: a fit that the patient can tolerate, a fit that the cornea can tolerate and a fit that corrects the patient's vision.

Adjust the base curve and overall lens diameter relative to fluorescein evaluation, lens movement and lens centration. Figure 7 shows a typical fluorescein pattern under a lens post RK. Note the fluorescein pooling centrally and thin at the mid-periphery; on a normal eye, this would be too tight. Figure 8 shows a lens that is too tight on an eye after radial keratotomy. Central pooling is expected, but there is too much bearing in the secondary curve area.

FIG. 7: A TYPICAL FLUORESCEIN PATTERN UNDER A LENS POST RK. NOTE THE FLUORESCEIN POOLING CENTRALLY AND THIN AT THE MID-PERIPHERY; ON A NORMAL EYE, THIS WOULD BE TOO TIGHT.

FIG. 8: A POST-RK LENS THAT IS TOO TIGHT. CENTRAL POOLING IS EXPECTED, BUT THERE IS TOO MUCH BEARING IN THE SECONDARY CURVE AREA.

FIG. 9: SOFT LENSES ARE NOT APPROPRIATE AFTER RADIAL KERATOTOMY AS VASCULARIZATION OCCURS FREQUENTLY.

Soft lenses are not appropriate after radial keratotomy as vascularization occurs frequently (Fig. 9). Vessels begin to leak lipids, followed by scarring, vascular fans and cellular infiltrates. Soft lenses will not stabilize the fluctuation in vision that tends to occur after radial keratotomy. Rigid gas permeable contact lenses after radial keratotomy facilitate favorable molding of the surface and control of fluctuation.

EPIKERATOPLASTY AND KERATOMILEUSIS

Epikeratoplasty and keratomileusis, used to correct higher amounts of myopia, have been largely abandoned due to poor results. Today, surgeons prefer LASIK for large myopic corrections. After keratomileusis, a haze frequently develops and it is difficult to achieve visual acuity of better than 20/40. Figure 10 shows an epi-onlay with approximately 10 diopters of oblique cylinder. This patient subsequently required a penetrating keratoplasty for corneal opacity.

PHOTOREFRACTIVE KERATECTOMY

Contact lens fitting is straightforward after PRK. A starting point is to select the same lens the patient wore before surgery -- either rigid or soft -- and change the power. Soft lenses may be fit because there are no incision lines and neovascularization is of no greater concern than for any other soft contact lens wear.

Figure 11 shows an eye after photorefractive keratectomy. It has a fairly even pattern, but the ablation is eccentric to the visual axis. Following PRK, eyes are often left with some anterior stromal haze, which tends to decrease with time.

PENETRATING KERATOPLASTY

Indications for refractive modifications after penetrating keratoplasty are anisometropia, and regular or irregular astigmatism. There is certainly a visual and cosmetic preference for contact lens wear. A contact lens can provide a regular, smooth, spherical, anterior refractive surface which can facilitate near-normal visual acuity.

Corneal sensitivity is transiently lessened after penetrating keratoplasty, and even though it's generally easier for patients to adapt to a contact lens, epithelial breakdown may occur. Patients should not be fit with contact lenses sooner than three months after surgery, and preferably not until four months have passed, as early fitting may cause inflammation leading to rejection of the graft. The sutures must be buried, i.e., there should be no fluorescein staining and the wounds must have proper apposition. The smoothness of the wound is the primary indicator for when contact lens fitting may begin.

Postoperative astigmatism is primarily influenced by the suturing technique and the graft sizing. For example, an 8.0mm button graft in a 7.0mm bed will result in a steep graft. If the graft is eccentric, irregular astigmatism will result. Fortunately, current surgical techniques allow greater control over the amount and axis of astigmatism as the eye is healing. The axis is more important than the amount of astigmatism. Data suggest that combined, interrupted and running suture placement permits better wound control. Selective suture removal should be based on photokeratography for postoperative astigmatic control. With-the-rule astigmatism is easier to fit than against-the-rule or oblique.

COMPLICATIONS AND RECOMMENDATIONS

Complications do occur when fitting eyes after surgery. Chronic 3 and 9 o'clock staining may progress into vascularized limbic keratitis which may compromise the graft. It can be corrected by flattening the secondary and peripheral areas. The lens should have uninhibited excursion out to the limbus, without running into a bearing problem.

Sometimes a fit does not look 'right,' but it works. A less-than-optimum fit is appropriate as long as it does not compromise corneal integrity, as long as there is no staining, and as long as there is full fluorescein exchange over the entire corneal surface.

It's important to examine the eye at the dispensing visit before placing the lens to ensure that the epithelium is intact and all sutures have remained buried. Begin with three hours wear the first day, increasing one hour a day but not exceeding 12 hours per day before the next examination, with an ultimate maximum of 16 hours.

Do not give a plunger to these patients; they may mistake the graft wound for the outline of a contact lens. Be certain that patients are able to insert and remove their lenses manually and discourage the use of these appliances.

CONCLUSION

Although the postsurgical cornea presents some unique challenges to the contact lens practitioner, our goals remain the same:

• maximize vision;
• maintain comfort; and
• minimize interference with corneal function and structure.

By adhering to these principles, we will achieve success. CLS

There is ongoing research into other types of refractive surgery including intracorneal implants, which are as predictable as intraocular lenses. Theoretically, there is no dioptric limit to the amount of correction that can be achieved with this procedure. Using a microkeratome, the surgeon opens the corneal stroma, then inserts the lens and sutures the anterior cornea back in place. The implant is a hydrogel lens that has been pre-cut for the patient's prescription. As predictable as this procedure is, however, errors may occur. In the photo, the implant is out of position; as a result, the patient was undercorrected and required a contact lens. Current research also includes intracorneal ring implants for vision correction. This procedure is undergoing FDA Phase 3 investigation.