Femtosecond Laser and Microkeratome-Assisted DSAEK
The influence of endothelial graft thickness on visual acuity in DSAEK is controversial. However, studies on this subject have focused either on relatively thick grafts (160–170 μm) or on grafts prepared by manual dissection. Assessment of grafts of less than 130 μm shows a positive correlation between thickness and visual acuity, suggesting that an effect may exist under a certain thickness. Even though other factors may also influence visual acuity, such as the extension and duration of preoperative corneal oedema and the irregularity of the anterior cornea surface, graft thickness seems to be an important factor.
Few techniques for creating grafts thinner than 100 μm have been presented. Therefore, to evaluate the influence of graft thickness on visual acuity we must have a technique that will consistently yield thin grafts.
The technique we describe in this paper involves performing a femtosecond laser first to prevent the variability of a double microkeratome cut and reduce the risk of perforation. Avoidance of a double femtosecond cut reduces the risk of a rough stromal bed and endothelial damage.
Despite aiming for grafts of less than 100 μm, a target graft thickness figure of 110 μm was used in the formula for safety reasons. Thus, approximately 410 μm of corneal tissue is cut with a 300 μm microkeratome head. Because the microkeratome we use usually cuts more than 300 μm, we found that having 110 μm as the target would actually yield grafts thinner than 100 μm without perforation. Final graft thickness is obtained by programming the laser to cut at a customised depth for each cornea, depending on the initial pachymetry, in order to always leave the same thickness (approximately 410 μm) for the microkeratome cut. Accordingly, there was no correlation between initial donor thickness and graft thickness at any time point because the first cut essentially makes all corneas equal in terms of thickness. This avoids using nomograms and performing pachymetry between cuts, and means only one microkeratome head is needed, here a 300 μm head, but other heads will probably give a good result as well. Since the microkeratome cut depth depends on several factors, such as intrachamber pressure, tissue thickness and manual rotation speed, these factors are kept as constant as possible to reduce variability. An important aspect of this technique is that the thinner cut is performed first, leaving a thick cornea for the microkeratome. Because the anterior stroma is more compact, this approach avoids irregular cuts and buttonholes during the microkeratome cut. A further advantage is that equipment already available in many cornea centres can be used. One limitation of this technique is that donor corneas of less than 500 μm cannot be cut because the Intralase has a minimum ablation depth of 90 μm.
This new approach shows endothelial cell loss comparable with that reported for standard DSAEK and visual results similar to published DMEK results. In general, endothelial cell loss can result from overmanipulation. However, we used the same DSAEK technique as we did previously with thick grafts; there were no complications, and the grafts unfolded successfully in the anterior chamber in all procedures. Moreover, it is also known that organ cultured corneas systematically carry non-viable endothelial cells that are implicated in cell death and go undetected when trypan blue staining is used. There were no differences in terms of visual acuity, graft thickness and endothelial cell loss between patients undergoing DSAEK alone and those having concurrent cataract surgery. Concerning visual acuity, patients without concurrent cataract surgery were already pseudophakic, which could explain why no differences were found. Concerning graft thickness and endothelial cell loss, because cataract surgery is performed before Descemet stripping and graft insertion it is not likely to influence these parameters.
In conclusion, grafts of less than 100 μm could be safely and consistently obtained. Although the initial results are very promising, further studies with larger patient cohorts and longer follow-up times are necessary. Studies should also clarify the ideal graft thickness for DSAEK, in terms of both visual recovery and endothelial cell loss.
Discussion
The influence of endothelial graft thickness on visual acuity in DSAEK is controversial. However, studies on this subject have focused either on relatively thick grafts (160–170 μm) or on grafts prepared by manual dissection. Assessment of grafts of less than 130 μm shows a positive correlation between thickness and visual acuity, suggesting that an effect may exist under a certain thickness. Even though other factors may also influence visual acuity, such as the extension and duration of preoperative corneal oedema and the irregularity of the anterior cornea surface, graft thickness seems to be an important factor.
Few techniques for creating grafts thinner than 100 μm have been presented. Therefore, to evaluate the influence of graft thickness on visual acuity we must have a technique that will consistently yield thin grafts.
The technique we describe in this paper involves performing a femtosecond laser first to prevent the variability of a double microkeratome cut and reduce the risk of perforation. Avoidance of a double femtosecond cut reduces the risk of a rough stromal bed and endothelial damage.
Despite aiming for grafts of less than 100 μm, a target graft thickness figure of 110 μm was used in the formula for safety reasons. Thus, approximately 410 μm of corneal tissue is cut with a 300 μm microkeratome head. Because the microkeratome we use usually cuts more than 300 μm, we found that having 110 μm as the target would actually yield grafts thinner than 100 μm without perforation. Final graft thickness is obtained by programming the laser to cut at a customised depth for each cornea, depending on the initial pachymetry, in order to always leave the same thickness (approximately 410 μm) for the microkeratome cut. Accordingly, there was no correlation between initial donor thickness and graft thickness at any time point because the first cut essentially makes all corneas equal in terms of thickness. This avoids using nomograms and performing pachymetry between cuts, and means only one microkeratome head is needed, here a 300 μm head, but other heads will probably give a good result as well. Since the microkeratome cut depth depends on several factors, such as intrachamber pressure, tissue thickness and manual rotation speed, these factors are kept as constant as possible to reduce variability. An important aspect of this technique is that the thinner cut is performed first, leaving a thick cornea for the microkeratome. Because the anterior stroma is more compact, this approach avoids irregular cuts and buttonholes during the microkeratome cut. A further advantage is that equipment already available in many cornea centres can be used. One limitation of this technique is that donor corneas of less than 500 μm cannot be cut because the Intralase has a minimum ablation depth of 90 μm.
This new approach shows endothelial cell loss comparable with that reported for standard DSAEK and visual results similar to published DMEK results. In general, endothelial cell loss can result from overmanipulation. However, we used the same DSAEK technique as we did previously with thick grafts; there were no complications, and the grafts unfolded successfully in the anterior chamber in all procedures. Moreover, it is also known that organ cultured corneas systematically carry non-viable endothelial cells that are implicated in cell death and go undetected when trypan blue staining is used. There were no differences in terms of visual acuity, graft thickness and endothelial cell loss between patients undergoing DSAEK alone and those having concurrent cataract surgery. Concerning visual acuity, patients without concurrent cataract surgery were already pseudophakic, which could explain why no differences were found. Concerning graft thickness and endothelial cell loss, because cataract surgery is performed before Descemet stripping and graft insertion it is not likely to influence these parameters.
In conclusion, grafts of less than 100 μm could be safely and consistently obtained. Although the initial results are very promising, further studies with larger patient cohorts and longer follow-up times are necessary. Studies should also clarify the ideal graft thickness for DSAEK, in terms of both visual recovery and endothelial cell loss.