In this prospective, randomized, clinical, single-center study, patients with the diagnosis of KC or FUCHS undergoing PK were randomly assigned from the waiting list of the LIONS Cornea Bank Saar-Lor-Lux, Trier/Westpfalz, Germany, to two different methods of corneal trephination (EXCIMER and FEMTO). Assignments were made between January 2012 and December 2013 using a random-number table by the study nurse. All patients signed an informed consent form. The study was approved by the Ethics Committee of Saarland University, Germany (number 201/11).
Neither the patient, the surgeon, nor other team members could influence the assignment. However, patients were informed the day before surgery, which trephination technique was going to be performed and they could withdraw their informed consent during the complete time of the study. But no patient refused to participate. Only one eye of each patient was included. All surgical procedures were performed by one surgeon (BS), under general anesthesia.
For the present study, the exclusion criteria were previous ocular surgery, rejection reaction following PK, and simultaneous cataract or other surgery. Therefore, in the present study, 68 eyes of 68 patients (23 female and 45 male) undergoing PK were included. The EXCIMER group (35 eyes) consisted of 17 eyes with KC and 18 eyes with FUCHS. The FEMTO group (33 eyes) consisted of 17 eyes with KC and 16 eyes with FUCHS.
Trephination and suturing techniques
EXCIMER trephination was performed using a 193-nm excimer laser MEL70 (Carl Zeiss Meditec, Jena, Germany) along with metal masks at a 25-Hz repetition rate and 1.2-mm spot diameter. For donor trephination from the epithelial side by using the EXCIMER, a curved circular metal aperture mask (diameter, 8.1 mm in KC and 7.6 mm in FUCHS; central opening, 3.0 mm for centration; thickness, 0.5 mm; weight, 0.173 g; eight orientation teeth, 0.15 × 0.3 mm) was positioned on a corneoscleral button (16-mm diameter) fixed in an artificial anterior chamber under microscopic control. After perforation, the remaining stromal lamellae and Descemet’s membrane were cut with curved corneal microscissors. The donor oversize was 0.1 mm in all cases. For recipient trephination, a corresponding circular metal mask was used (diameter, 12.9 mm; central opening, 8.0 mm for KC and 7.5 mm for FUCHS; thickness, 0.5 mm; weight, 0.29 g; eight orientation notches, 0.15 × 0.3 mm). The laser beam was guided automatically along the edge of the mask without ablating the central cornea. After focal corneal perforation, the remaining deep stromal lamellae and Descemet’s membrane were cut with curved corneal microscissors.
FEMTO trephination was performed using a 60-KHz IntraLase™ femtosecond laser (Abbott Medical Optics, Abbott Park, IL, USA). The mushroom profile (8.5-mm upper and 7.5-mm lower diameters) was used in patients with KC, and the top-hat profile (7.5-mm upper and 8.5-mm lower diameters) was used in patients with FUCHS. In all cases, we used 0.1 μJ less energy than the maximum energy in the posterior side cut, 0.5 μJ less energy than the maximum energy in the anterior side cut, and 0.4 μJ less energy than the maximum energy in the ring lamellar cut (range 2.3–2.9 μJ). The eight alignment incisions in both the donor and recipient were created as follows: energy, 1.5 μJ; length, 1000-μm width, 50 μm; spot separation, 6 μm; line separation, 6 μm; and layer separation, 5 μm. The radial offsets were + 2 in all recipients (i.e., all the alignment incisions were outside the trephination) and − 2 in all donors (i.e., all the alignment incisions were inside the trephination). On the anterior side cuts, the spot separation and layer separation were both 3 μm; in the ring lamellar cut (spiral pattern), the tangential spot separation was 5 μm and the radial spot separation was 4 μm; and on the posterior side cut, the spot separation was 3 μm and the layer separation was 2 μm. The depth of the lamellar cut of the donor and recipient was 2/3 of the mean corneal thickness of the graft and recipient’s eye, respectively. All side cut diameters (anterior and posterior side cuts) were 0.1 mm larger than the resulting diameter, thus overlapping each other. The donor cornea was placed into an artificial anterior chamber (Polytech, Roßdorf, Germany) to achieve trephination from the epithelial side. Each laser procedure requires a disposable glass interface, which applanates the cornea during the laser procedure. For laser trephination of the recipient’s cornea, the eye was fixated by means of a vacuum suction ring. The glass cone interface was placed within the suction ring so that the cornea was applanated. After laser trephination, the corneal button was removed with forceps and a spatula under microscopic control. If necessary, microscissors were used to complete the incision .
In all patients, a peripheral iridotomy was performed at the 12-o’clock position. After temporary fixation of the donor button in the recipient bed with eight interrupted sutures, a permanent wound closure was achieved using a 16-bite double-running diagonal cross-stitch suture (10–0 nylon) described by Hoffmann . We attempted to suture as deep as 90% of the total corneal thickness in both groups in all eyes. The eight interrupted sutures were placed at the site of the orientation teeth with the EXCIMER and at the site of the alignment incisions with the FEMTO as well as possible. In cases of wound gaping or graft override, additional interrupted sutures were used to ensure proper donor-host alignment at the end of surgery.
Methods and main outcome measures
ECD (EM 3000 specular microscopy; Tomey, Erlangen, Germany), central corneal thickness (CCT; three devices: Pentacam HR Scheimpflug tomography, Wetzlar, Germany; Casia SS-1000 swept-source Fourier domain OCT, Tomey, Erlangen-Tennenlohe, Germany; EM 3000, Tomey), and corneal thickness at the thinnest point of the cornea (TCT; two devices: Pentacam HR and Casia SS-1000) were measured before removing the first suture (first follow-up, “all-sutures-in” [11.4 ± 1.9 months, range 3–19 months]) and after removing the second suture (second follow-up, “all-sutures-out” [22.6 ± 3.8 months, range 19–38 months]), but before any additional surgical intervention. Specular microscopic images acquired using EM3000 were processed using the automatic cell analysis software EM-1100 (software version 1.5, Tomey). We used the measurement results for analysis, if at least 120 cells were measured by the software.
The primary analysis is comparison of EXCIMER vs. FEMTO before removing of the first suture with respect to ECD (cells/mm2). All other comparisons are secondary analyses and have to be interpreted exploratively. With respect to primary analysis and assuming a clinically relevant group difference of 300 cells/mm2, the following sample size calculation holds: a sample size of 35 in each group will have 80% power to detect a difference in means of 300 cells/mm2, assuming that the common standard deviation is 440 cells/mm2 using a two-group t test with a 0.05 two-sided significance level (nQuery Advisor, version 7.0, Statistical Solutions, Cork, Ireland).
Statistical analysis was performed using IBM SPSS Statistics for Windows, version 19.0 (IBM Corp., Armonk, NY, USA).