Use of Photorefractive Keratectomy Treated Donor Corneas for Endothelial Keratoplasty.

Corneal transplantation has been widely performed since the first successful surgery was described by Zirm. Recent data from the Eye Bank Association of America (EBAA), the accredited organization that tracks donor quality and volume of surgeries performed anually in the United States, has shown that 46,253 corneal tissues were used for transplantation in 2014 domestically while 76,431 tissues were used worldwide.[1] However, penetrating keratoplasty (PKP) procedures have significantly decreased over the last decade, whereas the number of endothelial keratoplasties (EK) have increased. In fact, the number of EKs has superseded PKPs since 2012 in the United States. EK is now the preferred surgical option for patients with endothelial pathologies, such as Fuchs’ endothelial corneal dystrophy (FECD) and pseudophakic bullous keratopathy (PBK), due to reduced complication rates and accelerated time for visual rehabilitation.[12]

According to the EBAA, the primary indication for EK is FECD.[1] The increasing world population and longer life expectancy has increased the incidence of endothelial pathologies such as FECD in the elderly population. Several EK procedures are currently being performed. Descemet's Stripping Endothelial Keratoplasty (DSEK)[3] was initially developed for replacement of the endothelium and Descemet's Membrane (DM) in a controlled fashion, without damaging the posterior corneal stroma. Further advancement led to Descemet’ Stripping Automated Endothelial Keratoplasty (DSAEK),[4] a variation of DSEK in which, the donor complex of posterior stroma, DM and endothelium are prepared with the assistance of a microkeratome. More recently Descemet Membrane Endothelial Keratoplasty (DMEK)[5] has been added to the armamentarium, which has the feature of replacing the DM and the dysfunctional endothelium without any donor stroma. Although visual outcomes, visual rehabilitation and rejection rates with DMEK have been shown to be superior to other EKs, DSAEK remains the most frequently performed EK technique.[12]

While full thickness PKP requires full thickness donor corneal tissues without any anterior or posterior pathology, for EK the presence of a healthy endothelium is sufficient. Therefore discarding donor corneas due to anterior scarring, pterygium, opacities, history of previous corneal refractive surgery, etc., theoretically limits the availability of tissues for EK cases but may not be necessary. Thus, theoretically, cornea of donors that have undergone refractive surgery could be used for EK.

Radial keratotomy (RK), the first incisional refractive procedure, was widely popular in the 1980's.[6] Later, with the development of excimer lasers, new surgical techniques such as photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK) were introduced into ophthalmic practice.[7] Thus, PRK rapidly replaced RK for treatment of myopia and is currently widely performed wordwide. The most frequent complication of PRK is subepithelial corneal haze. This complication occurs due to keratocyte mediated regrowth of the photoablated stroma. Therefore, in recent years LASIK has become the refractive surgery of choice. However, due to flap complications and higher frequency of post-surgical neuralgia, PRK is undergoing a resurgence. The application of mitomycin – C (MMC) 0.02% in PRK cases for highly myopic eyes has been shown to reduce the risk of post-surgical corneal haze by preventing the activity of corneal fibroblasts.[89] However, controversies exist between studies in this regard, with some demonstrating that adjuvant use of mitomycin-C in PRK can decrease endothelial cell density.[101112]

Currently, the EBAA does not allow utilization of corneas with previous refractive surgeries for PKP, ALK and tectonic procedures. However, donor corneas with noninfectious anterior stromal opacities and pathology are permitted for endothelial keratoplasties.[13] The screening of donor corneas include the use of slit lamp examination, specular microscopy, corneal topography and tomography. Previous RKs can be identified by the presence of radial incisions and the edge of the LASIK flap can be easily recognized, while disciform corneal haze within the central 8 millimeters can be detected in some donor corneas with history of PRK.[14] Corneal refractive surgery including PRK and LASIK, are the most popular procedures for vision correction in normal corneas. Considering the higher rates of PRK-treated patients worldwide since the 1990's and the improved protocols of eye banks for screening and preparation of donor corneas, together with the need for visual rehabilitation due to the crescent rates of EK in recent years, it would beneficial to utilize these tissues.

In their most recent study, Kanavi et al[15] evaluated the applicability and surgical outcomes of using post-PRK corneal donors for DSAEK. They demonstrate a significant asymmetrical increase in corneal thickness towards the periphery in post-PRK donors as opposed to non-PRK donors. Moreover, the mean endothelial cell count, as well as the percentage in polymegatism and hexagonality showed excellent resutls in post-PRK donor corneas (3164.6 ± 311/mm2, 39.4 ± 6.9%, 50.5 ± 11.3%, and 321.1 ± 32.8 μm2, respectively) which suggests that prior introperative use of MMC during PRK may not have caused an adverse effect on endothelial cells. The authors further evaluated the operative reports of patients who received post-PRK donors. The asymmetrical lenticules did not affect corneal clarity or the success of graft attachment during surgery. However, it should be pointed out that the impact of asymmetrical difference in donor graft thickness on possible long-term refractive changes is unclear and deserves further investigation.

In conclusion, the authors successfully demonstrated that post-PRK corneas are a valuable source that can provide a satisfactory posterior lenticule thickness with excellent endothelial cell counts. The Central Eye Bank of Iran (CEBI) allows the use of donor corneas with non-infectious anterior dystrophies for the preparation precut lenticules. Considering the increasing rates of endothelial keratopalsties performed and thus the increasing need for donors, it is important to include donor corneas with a history of refractive surgeries, such as PRK, for screening, as they can be utilized for endothelial keratoplasties. One may conclude that post-PRK donors with sufficient endothelial cell count and endothelial morphology, can be an important source of corneal tissue for endothelial keratoplasty grafts.