Femtosecond laser technology has been incorporated into corneal transplantation surgery to improve visual outcomes and speed visual recovery. Previous retrospective studies comparing visual outcomes of FE-DALK and S-DALK had conflicting results, with some showing faster wound healing and visual recovery, and others demonstrating no significant differences in visual outcomes between the two techniques.7–9 To the best of our knowledge, this study is the first randomized controlled trial comparing the two. No significant differences were found between FE-DALK and S-DALK in terms of keratometric and visual outcomes, nor any other outcome measure.
Postoperative astigmatism is a significant factor affecting visual outcomes after keratoplasty. Values of postoperative astigmatism and mean arithmetic SIA did not differ significantly between FE-DALK and S-DALK at 6, 12 and 15 months after surgery, indicating similar astigmatic outcomes. However, looking at the 15-month vector analysis, it appears that the mean SIA corneal vector value was lower in FE-DALK compared with S-DALK (Fig. 4). This indicates that the direction of surgically induced astigmatism in FE-DALK was quite evenly distributed. In S-DALK, however, the induction of astigmatism tended to occur around the 114-degree axis, as evident by the larger mean corneal vector value directed along this axis at 15 months postoperatively. The reason for this difference between the two techniques is unclear. One possible explanation could be that manual recipient trephination is less symmetric than femtosecond trephination. Manual trephination is always completed using scissors. Therefore, certain areas may be more ergonomically difficult for the surgeon to cut than others, producing a consistent deformation which could lead to induced astigmatism in a more consistent axis as was evident in the S-DALK group. Another possible explanation may be related to the fact that FE-DALK was in a mushroom configuration while S-DALK was in a straight-cut configuration. Use of a mushroom configuration, especially one that is created by laser on both the donor and recipient, allows better alignment of host and donor tissues with less chance of step formation or tissue slippage at the graft-host junction. Some areas are more prone to graft-host mismatch, such as the inferior thinner host cornea in keratoconus patients, and in such cases, a consistent vector of induced astigmatism may be more prominent in S-DALK than in FE-DALK. Therefore, the differences in vectoral values found between the groups may suggest an advantage of FE-DALK. However, this did not translate into better overall outcomes in corneal astigmatism or vision.
Li et al. retrospectively compared FE-DALK and S-DALK and found improved UCVA, BSCVA, and astigmatism following straight-cut FE-DALK.8 In a previous study by our group, earlier visual recovery was identified with mushroom-shaped FE-DALK compared with S-DALK at 3 months. However, visual results were comparable at 6 months and 1 year.9 This current trial showed no superior visual outcomes of FE-DALK over S-DALK at 6, 12 and 15 months after surgery. This is consistent with findings reported by Alio et al., in a retrospective study that showed no significant difference in visual outcomes between FE-DALK (n = 25) and S-DALK (n = 25).7
One of the rationales for using femtosecond laser in DALK is the potential to improve the accuracy with which the optimal depth that is required to form a big bubble is achieved, while minimizing the risk of perforating Descemet’s membrane. This will potentially reduce the rate of conversion to PK and increase the rate of successful big bubble formation. In this trial, no strong statistical evidence was found between FE-DALK and S-DALK neither in rates of PK conversion nor in rates of successful big bubble formation. It should be noted, however, that the big-bubble success rate was 67% in FE-DALK and only 52% in S-DALK. Since the current trial was powered to investigate the primary outcomes of astigmatism and SIA, it may not have been sufficiently powered to address the differences in big-bubble success. This should be further investigated in larger prospective studies. Modified FE-DALK techniques are now emerging whereby in addition to femtosecond-assisted trephination, the femtosecond laser is also being used to create an intrastromal channel for the insertion of an air cannula up to the precisely required big-bubble depth under the guidance of either preoperative or intraoperative optical coherence tomography (OCT). Initial reports in recent literature show big-bubble success rates of 90–100% using these modifications.16–19 As these modified techniques continue to mature, future clinical trials comparing them to S-DALK may be warranted.
This randomized controlled trial had several limitations. First, to keep the postoperative scenario as close to a real-life setting as possible and to ensure optimal patient safety and benefit, surgeons removed corneal sutures at their discretion to optimize their patients’ postoperative outcomes. This fact may have been a confounder of the comparison of postoperative astigmatism between the groups. Second, participants were undergoing both routine postoperative follow-up exams their surgeon’s clinic and study visits at the study site. Therefore, some of the patients were not motivated to come in for an additional study visit and did not attend their 15-month follow-up.
In conclusion, in this randomized controlled trial, femtosecond DALK and standard DALK showed comparable functional and anatomical outcomes.