EyeWorld Asia-Pacific September 2018 Issue

53 EWAP CORNEA September 2018 graft survival (p<0.001) and visual outcomes (p<0.001) were signifi- cantly better for penetrating grafts, compared to DALK, for the same indications. Graft failure at 3 years was 12% after DALK compared to less than 5% for PKs, accord- ing to data from more than 3,000 registry eyes. 5 This was largely corroborated by the outcomes of a UK trial that related the reason for DALK’s higher overall failure rate compared to PK to early graft failures. The risk of graft failure for DALK was almost double that of PK (p=0.02), with 19% of DALK failures occurring in the first 30 postoperative days, compared with only 2% of PKs. 6 “We had a high early failure rate for DALK at the time this graft registry data was collected, how- ever, the technique has since then evolved,” Dr. Allan said. “We’ve come on from doing deep stromal dissection to Descemet’s baring techniques, which have become widespread. The key message here is that if you have good tech- niques, you have good results.” Over time, better techniques appear to be yielding improved graft survival rates. Big bubble DALK is a Descemet membrane baring technique that is cur- rently credited with the highest degree of success, although it has not yet entirely replaced PK. This technique involves corneal trephi- nation for 60–80% of its thick- ness. Air is injected into the deep stroma, causing a separation of the pre-Descemet membrane from the overlying stroma, which is then bared and left intact, allowing the donor transplant to be placed on top, host endothelium intact. The idea is that by limiting the proce- dure to the stroma, there is a much lower threat to graft survival long term. According to a single surgeon series in 158 eyes with a follow-up of 4 years, the failure rate (eyes re- quiring regrafting) was just under 2% using the big bubble technique in keratoconus patients. DALK pro- vided stable long-term visual and refractive outcomes, with a reduced risk of graft rejection, postopera- tive complications, and late ECD, compared to standard PK. 7 “Problems in big bubble DALK can be understood with refer- ence to Harminder Dua’s work on the anatomy of the pre-Descemet membrane, 8 which inserts into the stroma at about 6 to 8 millimeters diameter,” Dr. Allan said. “We saw that all of our perforations were oc- curring when we were dissecting in the periphery or trying to expand the bubble toward an 8-mm con- ventional DALK graft margin. With that in mind, we have moved on to another technique, mini bubble femto DALK.” Dr. Allan developed this vari- ant of the big bubble technique, in which he uses a femtosecond laser to create a mushroom graft with a 6-mm Descemet’s membrane- baring optical center and a 9-mm outer diameter. Donor preparation is facilitated by mounting the cor- nea over air for laser interface ap- planation. Air is compressible, and this helps consistent applanation at a firm pressure for clean laser trephination. The reciprocal dissec- tion in the host stroma is taken to within approximately 75 µm of the thinnest point at 6 mm. This helps deep cannula location for success- ful big bubble formation. “I like to take the endothelium off the donor, clean the interface, and put the button on, with inter- rupted sutures for the best results,” Dr. Allan said. “The mini bubble technique respects the pre-DM layer anatomy. It has a nice edge profile with no mismatches. There are reduced perforation rates versus conventional DALK, and we are hoping that the 9-mm graft will reduce the risk of late ectasia pro- gression peripheral to the graft, a common problem in conventional transplantation for keratoconus.” What if the corneal transplant practice is not colocated with a refractive surgery facility? Dr. Al- lan thinks that this will not be a problem going forward. “You’ve heard of precut donors, maybe in the future we will be seeing precut patients, too.” EWAP References 1. Reinhart WJ, et al. Deep anterior lamellar keratoplasty as an alternative to penetrat- ing keratoplasty: a report by the American Academy of Ophthalmology. Ophthalmol. 2011;118:209–18. 2. Kelly TL, et al. Corneal transplantation for keratoconus: a registry study. Arch Ophthal- mol. 2011;129:691–7. 3. Kelly TL, et al. Repeat penetrating corneal transplantation in patients with keratoconus. Ophthalmol. 2011;118:1538–42. 4. Borderie VM, et al. Long-term results of deep anterior lamellar versus penetrating keratoplasty. Ophthalmol. 2012;119:249–55. 5. Coster DJ, et al. A comparison of lamellar and penetrating keratoplasty outcomes: a registry study. Ophthalmol. 2014;121: 979–87. 6. Jones MN, et al. Penetrating and deep anterior lamellar keratoplasty for keratoco- nus: a comparison of graft outcomes in the United Kingdom. I nvest Ophthalmol Vis Sci. 2009;50:5625–9. 7. Romano V, et al. Long-term outcomes of deep anterior lamellar keratoplasty in patients with keratoconus. Am J Ophthalmol . 2015;159:505–11. 8. Dua HS, et al. Human corneal anatomy re- defined: a novel pre-Descemet's layer (Dua’s layer). Ophthalmol . 2013;120:1778–85. Editors’ note: Dr. Allan has no finan- cial interests related to his comments. Contact information Allan: bruce.allan@ucl.ac.uk