Since Melles introduced the first successful approach to posterior lamellar keratoplasty in 1998, endothelial keratoplasty has rapidly gained surgical preference over penetrating keratoplasty. The first report of precut DSAEK tissue was published in 2008 by Chen et al.,[14] who achieved a mean graft thickness of 169 ± 36 µm. Since then, numerous techniques have been described to achieve reliable and reproducible graft lenticule thickness, targeting visual acuities comparable to DMEK. Aside from single- and double-pass nomograms, the most recent techniques to thin the cornea before the microkeratome pass include swelling the stroma for 60 seconds with a balanced salt solution[15] or drying with polyvinyl alcohol or cellulose sponges,[9, 12] increasing the AAC pressure[12] or directing sterile airflow over the exposed stroma.[7]
While DMEK has been used for ideal cases with high visual acuity potential because it offers faster recovery and a lower rejection rate,[16] DSAEK is now generally reserved for eyes with ocular comorbidities. In this study, most of the patients undergoing surgery presented associated conditions such as glaucoma, ocular perforating and penetrating injuries, or retinal detachment or corneal scarring, in which the aim of surgery was to relieve the pain of bullous keratopathy, not to achieve good visual acuity due to stromal fibrosis. The focus of this study was therefore not best corrected visual acuity.
In this study, a nomogram for DSAEK was created using a single-pass technique in which the targeted graft thickness 1-month after surgery below 120 µm was achieved in 81.25% of the cases (39 eyes). Twenty eight eyes (58.33%) had a central graft thickness less than 100 µm, and 18 eyes (37.50%) had a central graft thickness below 80 µm. Considering that the swelling process resolves in 3 to 6 months, if these grafts had been measured at the 6-month follow-up most graft thicknesses would have been included in the UT-DSAEK category. Romano et al.[9] published a case series of 10 corneas in which a decrease was seen in the graft thickness from the time it was cut until 3 months of up to 38%. These changes have also been reported in other studies in which graft thinning occurred up to 3 months,[9, 17] 6 months[18, 19] and even 1 year[20] after DSAEK. We preferred to measure donor graft thickness at the 1-month follow-up as most of the patients did not live in the proximity of the clinic and could not return for an appointment at 6 months. Only 9 of the 48 lamellae were thicker than 120 µm, indicating the reproducibility of the procedure. Most authors describe variability in graft thickness, and the standard deviation ≥ 33 µm reported here is very similar to that of other studies.[20–22]
In contrast to the assertion of Cheung et al.,[23] that deep microkeratome cuts produce more variability in cut depth than shallow cuts, we achieved thinner graft thicknesses using microkeratome heads with a thicker blade (Table 1). We believe this variability may be associated with the preservation medium used for the donor cornea. Fresh corneas and corneas stored at 4ºC do not undergo the swelling process as do cultured corneas and may vary substantially in thickness measured at the time of surgery. Among the different corneal preservation media, there is some variability depending on preservation time. In a recent study involving seventeen pairs of donor corneas, two preservation media, Optisol-GS and Cornea Cold, were compared, showing a mean thickness at day 7 of 644 mm in the Cornea Cold group and 591 mm in the Optisol GS group. This gap was reduced after 21 days of storage, when the mean thickness was 714 mm in the Cornea Cold group and 708 mm in the Optisol GS group.[24] Thus, although the same nomogram could be used, small adjustments to hydration status or tissue consistency should be applied to avoid perforation.
The Moria microkeratome has been used and studied in different articles reporting lenticule thicknesses of 63 ± 29 µm at 5 months,[4] 69.9 ± 20.8 µm at 3 months[9] and 89.4 ± 26.2 µm at 6 months.[25] However, most of these studies used the double-pass microkeratome technique,[8, 25, 26] which involves significant risk for perforation, endothelial cell loss and stromal surface irregularity. Other studies have used a single 350 µm blade head in 10 corneas,[9] a combination of femtosecond[10] or a manual rotation system.[10, 26] In our study, we assessed the variable of microkeratome cutting speed, confirming that there was no variation with respect to the fast or slow speed. This does not occur with other microkeratomes such as the automated Amadeus II, which uses a head advancement speed of 3.0, 2.0 and 1.5 mm/sec and achieves statistically significant results with mean graft thicknesses one month postoperatively of 99.33 ± 16.97 µm.[13]
The limitations of this study are the relatively small sample size and the short follow-up period. The short follow-up was unavoidable as many of the patients were not local to the clinic and could not return for an appointment at 3–6 months. For this reason, we did not extend the follow up period as this would have resulted in a significant dropout rate. The major limitation of microkeratome dissection may be its poor accuracy in determining the final thickness of the dissected tissue, especially when preparing grafts for UT-DSAEK. This nomogram succeeded in 81.25% of the cases in achieving a graft thickness below 120 µm, taking into consideration that had the measurement been performed at 3 months, the number of cases below 120 µm would have been higher. This nomogram can be further refined by including additional variables (i.e., storage medium, AAC pressure, drying or swelling process).
In conclusion, using an automated microkeratome with a customized single-pass nomogram allows ultrathin grafts with reduced variability in thickness to be obtained. Only a single pass is required, reducing the risk of donor graft damage or waste. No complications were encountered during flap preparation with this technique.