Rehabilitating Vision
Induced refractive errors during penetrating keratoplasty (PK) may invalidate the good result of corneal transplantation, precluding the patient's full visual rehabilitation.1,2 In fact, even with a clear-corneal graft, spectacle correction of an anisometropic error may be difficult and may lead to poor visual results.
Several reports2-5 suggest the role of the disparity between graft and host diameters in the creation of post-PK myopia (ie, a 0.25-mm–difference increased the mean keratometric value up to 45.50 D compared with 43.80 D of the same size donor and recipient diameters). In one retrospective study, 30.3% of eyes achieved a refractive error within ±1.50 D of emmetropia.4 In corneal transplantation, the primary cause of postoperative corneal astigmatism is the disparity between the two wound margins.5
To correct refractive errors after PK, we distinguish between two periods (ie, before and after suture removal) that depend on the presence of the corneal sutures.
BEFORE SUTURE REMOVAL
Suture adjustment during the plastic period of surgical wound. This period is, at maximum, 6 months after surgery. In a randomized clinical trial, 77% of patients required at least one postoperative suture adjustment for high astigmatism (ie, >3.50 D) after PK. The investigators declared that intraoperative suture adjustment made the astigmatism lower and more regular after PK.6 In the presence of transplantation sutures, attempts to correct high astigmatism are achieved by applying compressive sutures along the flatter meridians. Effectiveness of these stitches tends to decrease during the postoperative follow-up, so they should only be used to modify the regularization of the wound healing.7
AFTER SUTURE REMOVAL
Refractive surgery. Even 6 months after suture removal, corneal astigmatism could change unpredictably due to irregular wound healing;8 as a result, rehab methods must accommodate for possible refractive changes during this period. The first consideration is contact lens wearing. Soft contact lenses may be recommended to prevent corneal graft damages. Wherever high postoperative astigmatism occurs, however, toric semirigid lenses must be applied. Therefore, the long-term correction of postoperative ametropia with contact lenses is not always tolerated after PK. Contact lenses may alter the donor cornea by peripheral touch that may, after a while, lead to neovascularizations with a higher risk of graft rejection.9 The importance of refractive surgery for the correction of defects induced by PK emerges:9-14 almost 20% of patients needed refractive surgery for the residual astigmatism after successful PK.
Radial keratotomy (RK). This was the first refractive surgery technique that corrected post-PK myopic defects.15,16 Corneal peripheral incisions weakened the cornea, caused protrusions and flattened the central corneal area. With transplanted corneas, graft ring scar rigidity may limit flattening and undercorrect the refractive defect. After RK, postsurgical refraction may deviate from the emmetropia to residual myopia, secondary to hypocorrection or regression of the effect, or toward hyperopia (ie, hyperopic shift).15,16 Damage may be done to the corneal transplanted graft or the corneal scar. Other postoperative complications include under- or overcorrection, glare, halos, diurnal fluctuations of refraction, subepithelial cysts and loss of contrast sensitivity.16
PRK. In the last few years, the well-known excimer laser refractive technique, PRK, has successfully replaced RK. PRK does not accurately correct high myopia17 and may also increase the risk of corneal graft damage or loss of corneal sensitivity; cause postoperative stromal reaction, halos and glare;9,14 and slower functional recovery times. Sometimes, it may force to a regraft.18
PRK destroys Bowman's layer, and the anatomic-functional damage may — in transplanted corneas — cause severe problems:9,18 The corneal graft shows little tendency to regenerate nervous fibers that may cause haze18 and severe corneal scarring.14 Removing the corneal epithelium may damage the graft or the original keratoplasty wound so much as to cause elevated or irregular astigmatisms.17
CURRENT TECHNOLOGIES
New refractive surgery techniques including phakic IOLs (eg, ICLs, iris-fixed or anterior chamber-fixed lenses) are in development, however, the follow-up is only short-term.19,20 These implants may cause graft failure for direct endothelial damage21 or chronic inflammation.
In other cases, transparent lens removal (ie, phacoemulsification of a clear lens) is a useful refractive technique with or without PCIOL implantation. In these cases, perform the clear cornea tunnel on the steepest diameter to flatten it. This valid technique may still cause endophthalmitis, graft failure for endothelial damage, graft wound rupture and high postoperative IOP.
If residual high astigmatisms occur after transplantation suture removal, Troutman proposed relaxing incisions (ie, nonpenetrating incisions that relieve stromal tensions and induce corneal bending).22 This technique has poor predictability of results. We prefer curved keratotomies for irregular astigmatism.7,22 On the contrary, astigmatic LASIK is our preference for regular bow-tie astigmatism. Correcting irregular astigmatisms are possible through new customized excimer laser ablations: Software links the laser to topographic or aberrometric patterns, personalizing treatments to each patient.14,23
LASIK is an effective treatment for post-PK refractive error, even for high residual astigmatism after the removal of graft sutures.10 The advantages of LASIK are well known. First, correction of high myopia has good and fast results.20 Second, LASIK preserves Bowman's membrane and the anatomical structure of the cornea; this may cause the difference in results between LASIK and PRK.9,24 In fact, removing Bowman's layer during PRK is believed to expose the corneal stroma to a foreign environment (eg, toxic substances that are dangerous for the keratocytes); the stroma overreacts through fibrosis and haze formation.9,14,18
Although LASIK presents potential benefits, there are also limitations. Creating the corneal flap is difficult, and mechanical microkeratomes may damage the cornea. Additionally, do not perform deeper laser ablations. This will avoid damage to the endothelium of the graft. Complications arising from LASIK include: an incomplete cut, caused by mechanical obstruction with lashes, speculum, drape or power failure; free cup, where the risk is higher with a flat <38.00 D cornea, exsiccation or poor alignment; an irregular cut, due to spare suction or chemosis; a thin flap or buttonhole, where the risk is higher when curved >48.00 D cornea. Corneal thickness is always higher than expected. With the Automated Corneal Shaper microkeratome (ACS; Baush & Lomb, Rochester, New York), the flap is about 43 to 46 µm thinner.25
Our first LASIK results for the correction of PK refractive errors were presented in 2000. We showed good visual results and great patient satisfaction.10 Today, we perform LASIK after PK with a 30-Hz femtosecond laser (Intralase; Intralase Corp, Irvine, California) to make the intrastromal cut of the flap (intra-LASIK). This cut is possible, even with great disparity between donor and host wound margins and with corneas steeper than 48.00 D or flatter than 38.00 D, without the risk of buttonhole or free-cup flaps. Intra-LASIK achieves more predictable flap thickness compared with mechanical microkeratomes (14/12 µm SD) and improves (1) epithelial preservation (ie, reduced epithelial ingrowth, flap complications) and (2) accuracy of mean refractive spherical equivalent and cylinder.26
OUR FIRST EXPERIENCE
Femtosecond laser-assisted corneal flaps were performed on seven patients (eight eyes) to correct mean refractive error after PK (-6.15 ±1.58 SD; range, -4.60 to -8.75). The interval between LASIK and PK ranged between 18 and 36 months and not fewer than 6 months from suture removal (Figure 1). Central corneal pachymetry ranged between 552 µm and 617 µm. A femtosecond laser made the lamellar cut and an excimer laser (Chiron Technolas 217 C-LASIK; Bausch & Lomb) was used for the stromal ablation. Corneal flap diameters ranged between 7.75 mm and 8.5 mm; flap thickness was always 120 µm. Preoperative BSCVA was 1.0 in three cases, 0.6 in three cases and 0.1 in two cases. Early postoperative mean spherical equivalent disclosed 2.25 D ±0.65 SD and mean cylinder 5.25 D ±0.20 SD of the preoperative spherical and cylinder error, respectively (Figure 2). All patients experienced improved UCVA and BCVA (Figure 3) without note of relevant intraoperative or postoperative complications. Flap lifting disclosed no difference from standard LASIK performed with Intralase.
We are waiting for a longer follow-up to demonstrate the validity of intra-LASIK, but currently, this refractive technique results to be the best rehabilitation option after PK.
Emilio Balestrazzi, MD, is from the Catholic University of Sacro Cuore – Eye Clinic, in Rome. Professor Balestrazzi states that he has no financial interest in the products or companies mentioned. He may be reached at emilio.balestrazzi@rm.unicatt.it or +39 0630156008.
Luigi Mosca, MD, is from the Catholic University of Sacro Cuore – Eye Clinic, in Rome. Dr. Mosca states that he has no financial interest in the products or companies mentioned. He may be reached at l.mosca@tim.it or +39 0630156008.
Romina Fasciani, MD, is from the Catholic University of Sacro Cuore – Eye Clinic, in Rome. Dr. Fasciani states that she has no financial interest in the products or companies mentioned. She may be reached at romina.fasciani@tiscali.it or +39 0630156008.
Laura Guccione, MD, is from the Catholic University of Sacro Cuore – Eye Clinic, in Rome. Dr. Guccione states that she has no financial interest in the products or companies mentioned. She may be reached at lauraguccione@libero.it or +39 0630156008.
Ciro Tamburrelli, MD, is from the Catholic University of Sacro Cuore – Eye Clinic, in Rome. Dr. Tamburrelli states that he has no financial interest in the products or companies mentioned. He may be reached at citam@tiscalinet.it or +39 0630156008.
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