Corneal surface irregularity and/or astigmatism may decrease best spectacle corrected visual acuity after deep anterior lamellar keratoplasty. The visual acuity may then be improved by fitting a rigid gas permeable (RGP) or soft toric contact lens, a contact lens and spectacle combination, a piggyback system (a combination of soft and rigid contact lenses), or a hybrid lens. A RGP lens may be the most effective, since this type of lens corrects high degrees of regular and irregular astigmatism and has high oxygen permeability.

For unoperated corneas, most contact lens fitting methods use keratometry values in combination with the fluorescein pattern for selection of the base curve of the initial trial lens. After keratoplasty, the central keratometry values may not be representative for the entire corneal surface area, as in virgin corneas. In fact, the radii within the central 3.2 mm optical zone of a transplanted cornea often show no correlation with the radii in more peripheral areas of the same cornea. Because a good fit depends on the best possible overall support of the contact lens across the cornea, for transplanted corneas videotopography values may be used to gain a better insight into the entire donor-and-host corneal surface areas. A more detailed analysis of the videotopography map may further facilitate the selection of the base curve, and the entire fitting procedure.

Figure 9.1.The elevated wound edge (arrows) after deep anterior lamellar keratoplasty may be the area for contact lens bearing.

We hypothesized that the bearing of the contact lens may be improved if the dioptric values over the transplant wound were used for selection of the base curve, for two reasons. First, the elevated wound ridge will always be in touch with the contact lens, irrespective of the selected base curve (Figure 9.1). Hence, the best possible contact lens bearing over the ridge may be expected to give a maximum of comfort in contact lens wearing. Second, the dioptric values over the wound ridge may be the only true values displayed on the videotopography map, because the algoritm will smoothen out the dioptric values over the central cornea and the wound edge, so that the central area is displayed too flat. The peripheral cornea is almost never displayed, because the irregular ring images in this area are discarded by the software. We therefore decided to choose the base curve radius of the first trial-lens according to the flattest dioptric value displayed over the circular transplant wound on the absolute scale of the videotopography map.

In virgin corneas, the central radius is used for selection of the contact lens base curve, so that the lens power equals the spectacle correction calculated back to the corneal plane. With our technique for lens fitting after deep anterior lamellar keratoplasty, the base curve is chosen according to the largest radius over the transplant wound. Hence, the base curve radius may not have any correlation with the central radius of the transplanted cornea, i.e. the effective corneal power. As a result, a large shift in refractive error is induced due to the vault between the contact lens and the central cornea, i.e. the tear compartment that creates a positive tear lens.

Figure 9.2. The ring segments of a topographical image. The double running sutures are in situ. The only area where the peripheral ring segments over the wound ridge are displayed is at 2 –3 oclock region. This area was used to select the base curve radius of the trial lens.

Thirteen (26%) patients were referred to the contact lens unit of our hospital for contact lens fitting, 3.9 (± 1.31) months after surgery. Seven patients had astigmatism of 4 diopters or more, and six had an anisometropia of 4 D or more. Both the videotopography (Alcon Eye Map, Alcon Laboratory inc. Fort Worth, TX, Software Version 5.50.03) ring map (Figure 9.2) and the color map (Figure 9.3) were used to determine the flattest dioptric value displayed over the circular transplant wound. The radius of the dioptric value was used to select the initial trial lens with an identical base curve / radius.

Example: A 34 year-old female with bilateral keratoconus became RGP contact lens intolerant because of corneal ectasia. Her left eye had a preoperative visual acuity of 0.25 with a contact lens. Five months after deep anterior lamellar keratoplasty, she was referred to the contact lens unit because of 4D of irregular astigmatism. Best spectacle corrected visual acuity of her left eye was 0.25 with Sph +0.5 C -4.0 x 160. Slit lamp examination showed a well-centered, clear lamellar transplant with the sutures in situ.

Figure 9.3. The green colored region with a refractive power of 43.69 D is the translation of the tangential measured points of figure 2a. The Dioptric power is pointed out by the cursor. The base curve of the trial-lens is 7.60 mm.

On the videotopography ring map and color map (Figures 9.2 and 9.3), the flattest dioptric value of 43.75 D (radius = 7.60 mm) over the transplant wound was displayed at three ´o clock. Hence, an initial trial-lens was chosen with a base curve of 7.60 mm, i.e. a radius identical to the flattest area displayed over the wound, with an optical zone diameter of 8.5 mm, Sph – 7.5 and a diameter of 12.0 mm (Figure 9.4). The visual acuity with the contact lens was 0.8. Daily wearing time of the contact lens was 16 hours.

The tetra-curved trial set we used consisted of lenses with an optical zone of 8.5 millimeter. The first peripheral curve was 0.5 mm flatter than the base curve radius and had a diameter of 10.5 mm. The second peripheral curve was 1.5 mm flatter than the base curve and had an overall diameter of 12.0 mm. The edge radius was 12.5 mm. All transitions were soft blended. In all cases we used a very high Dk contact lens material such as Boston XO (Polymer Technology Corp, Wilmington MA, USA) or FluoroPerm 151 (Paragon Vision Sciences, Mesa AZ, USA) to avoid corneal edema.

Mean best corrected spectacle correction before contact lens fitting averaged Mean best spectacle corrected visual acuity (BSCVA) before contact lens fitting was 0.4 (± 0.1), and improved to 0.8 (± 0.1) after lens fitting (p < 0.001). None of the patients developed rejection periods or infiltrates. Progressive graft vascularisation did occur in one patient 10 months after lens fitting, due to a loose suture. After suture removal, contact lens wear could be continued.

Figure 9.4. The fluorescein pattern shows bearing of the contact lens over the elevated wound ridge.

The most comfortable lens fit was obtained when the lens had a slightly superior position and receives support from the upper lid, with good movement and a relatively flat fluorescein pattern. During primary gaze, upward gaze and between blinks, the upper edge of the lens was retained under the upper lid. The bottom edge of the lens should be above the lower lid but the edge of the optical zone below the inferior pupillary margin under primary gaze circumstances.

Compared to contact lens fitting in virgin corneas, the lens edge was elevated more than normal to increase the tear meniscus. This was acceptable because the upper lid does not have to pass over the lens edge. This concept of lens performance, as if the corneal lens was attached to the upper lid during blinking, may facilitate the tear-flow underneath the lens during blinking and eye movements.

In summary the principals of the fitting technique are:

• Large overall diameters with a minimum of 11.0 millimeters to avoid lens instability and decentration
• Use the circular wound edge as an alignment zone and have trial lenses with an optical zone diameter of 8.5 millimeters
• Good clearance in the periphery
• A high Dk contact lens material