Different laser types and different procedure shapes are available for keratoplasty incisions. Three lasers are approved by the US Food and Drug Administration for corneal cuts: IntraLase (IntraLase Corp., division of Abbott Medical Optics); Femtec (20/10 Perfect Vision; GmbH, Heidelberg, Germany) and FemtoLDV (Ziemer Ophthalmic Systems AG, Port, Switzerland).7,8
Different shapes with different indications have been proposed, some of which include: top hat, mushroom, zig-zag, Christmas tree, zig-squared, tongue and groove.3,9
We chose the zig-zag technique () for keratoconus patients because the angled incisions from zig-zag-shaped keratoplasty had been described as being more biomechanically favourable, optical coherence tomography images showing excellent alignment of the anterior and posterior layers of the transplant, smooth transition along the anterior surface and excellent host–graft alignment.3
Diagram of femtosecond laser-enabled keratoplasty zig-zag incision.
In a previously reported study with other corneal disorders, the femtosecond laser-generated zig-zag-shaped incisions resulted in a more rapid recovery of BSCVA and induced less astigmatism compared with conventional blade trephination PKP.10
These data together with the reduced procedure time1
influenced us to make this our procedure of choice. Similar studies have reported results on a small number of keratoconus patients2,10,11
and demonstrated faster recovery of BSCVA and less induced astigmatism at 3 months, but they did not report on visual outcomes at a later time point or make comparison with the mechanical techniques.
Farid et al10
reported on 49 eyes of 43 patients who underwent zig-zag FLEK versus 17 eyes of 14 patients who underwent conventional PKP. All PKP were closed with an identical 24-bite running nylon suture technique. The postoperative follow-up ranged from 1 to 12 months. There was a significant difference in average astigmatism between the groups at postoperative months 1 (p=0.013) and 3 (p=0.018). By month 3, the average astigmatism was 3 dioptres (D) in the FLEK zig-zag group and 4.46 D in the conventional group. Of the patients with normal macular and optic nerve function, a significant difference in BSCVA was seen at month 1 (p=0.0003) and month 3 (p=0.006), with 81% of the FLEK zig-zag group versus 45% of the conventional group achieving BSCVA of 20/40 or greater by month 3 (p=0.03). They concluded that the FLEK zig-zag-shaped incision results in a more rapid recovery of BSCVA and induces less astigmatism compared with conventional PKP.
Bahar et al4
compared 23 eyes that had top-hat FLEK with 35 eyes that had conventional PKP At 12 months postoperatively, the mean cylinder was similar between the two groups: 3.6 D (SD 1.9) in the FLEK group and 4.1 D (SD 1.8) in the PKP group. The mean endothelial cell loss was significantly lower at 12 months in the FLEK group compared with the PKP group (32.4% vs 40.8%). The mean time to suture removal was 4.1 months (SD 1.2) in the FLEK group versus 9.7 months (SD 1.1) in the traditional PKP group.
Huer et al12
analysed eight eyes that underwent FLEK and concluded that long-term control of postoperative astigmatism remains an issue after FLEK, and that suture technique still plays an important role in postoperative astigmatism magnitude. Chamberlain et al13
compared postoperative outcomes in zig-zag FLEK with conventional PKP with 2years of postoperative follow-up. The authors found a significant improvement in astigmatism before but not after the 6-month postoperative follow-up period. However, they did not find significant visual improvement of BSCVA in the FLEK group compared with the conventional PKP group.
Our results on a larger number of keratoconus patients showed a significant improvement of BSCVA and astigmatism early on. Faster visual recovery to the level of being able to obtain a driver's licence and allowing patients to return to functional work earlier represents a great benefit, especially in this young group of patients who are in the most productive period of their lives.
Despite the fact that there was no statistically significant difference between the FLEK and the PKP groups in terms of topographically induced astigmatism at 6 months, the FLEK procedure has become our procedure of choice. There are several reasons for this: (1) visual acuity is better at both postoperative time intervals (3 and 6 months) in our study, which is critical in a young group of patients who are in their actively productive phase of life; (2) the procedure produces a stronger and more stable wound because of its larger surface area, making the eye less prone to corneal rupture; (3) the procedure is much faster with the eye being open for a very short period of time, when the laser is in or very near the operating room, making this a much safer procedure and significantly reducing the chance of intra-operative complications. An added benefit, as suggested by the study by Bahar et al
is that there is less endothelial cell loss in the FLEK group versus the PKP group, suggesting that the grafts may survive longer in this group of patients.
Even though PKP has been the standard treatment for advanced keratoconus, the incidence of cases with keratoconus that need a PKP has reduced in recent years due to alternative treatments, such as corneal cross-linking, and a major trend to deep anterior lamellar keratoplasty, in order to preserve the patient's own endothelium.
Future research with different pattern types geared to improving donor–host alignment might also ultimately allow us to achieve our goal of an astigmatism-free or at least significantly reduced astigmatic result in the FLEK procedure.