EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.
Issue link: https://digital.eyeworld.org/i/78788
16 EW NEWS & OPINION August 2012 Tools & techniques Latest trends and techniques in DALK by Clara C. Chan, M.D. D A nterior and posterior lamellar keratoplasty have truly revolution- ized our approach for dealing with corneal pathology. While the vast majority of corneal procedures in my practice are for endothelial dysfunction, deep anterior lamellar procedures remain an important part of the corneal surgeon's toolbox for dealing with scars, dystrophies, and keratoconus. Continual incremental im- provements in anterior lamellar techniques have now allowed us to approach anterior stromal pathology with a safer procedure that avoids the potential complications of full-thickness penetrating keratoplasty such as endothelial rejection, graft dehiscence, and intraoperative choroidal hemorrhage. As corneal surgeons begin to master both anterior and posterior lamellar keratoplasty, penetrating keratoplasty will ultimately only be used for cases of com- bined endothelial dysfunction and stromal pathology. The beauty of deep anterior lamellar keratoplasty lies in the ability to convert to penetrating keratoplasty during the learning curve if a macroperforation develops and the lamellar approach becomes unsalvageable. Deep anterior lamellar keratoplasty is perhaps one of the most difficult corneal procedures to master. In this month's column, Clara C. Chan, M.D., gives her step-by-step approach for performing deep anterior lamellar keratoplasty. For those of you out there surfing the learning curve, I hope you find the tips and pearls useful. Richard Hoffman, M.D., Tools & techniques editor eep anterior lamellar ker- atoplasty (DALK), while technically more chal- lenging than traditional penetrating keratoplasty (PK), is an excellent technique for the visual rehabilitation of corneal disease in patients who have normal endothelium. For example, young patients with keratoconus, stromal dystrophies, post-refractive surgery ectasia, and corneal scars are ideal candidates for DALK. Endothelial immune graft rejection cannot occur after DALK, and the procedure al- lows for preservation of endothelial cell density when compared to PK. As an extraocular procedure, DALK also has important theoretical safety advantages such as less intraopera- tive choroidal hemorrhage risk. A report published by the American Academy of Ophthalmology1 con- cluded that DALK is equivalent to PK for the outcome measure of best Snellen corrected visual acuity, par- ticularly if the surgical technique yields minimal residual host stromal thickness. There is no advantage to DALK for refractive error outcomes. Various techniques in DALK have been reported with Anwar's big bubble technique being the most popular.2 also assist with the procedure in the creation of stepped corneal wounds3 or in the creation of a lamellar pocket and intrastromal channel for air injection.4 Although instrumen- tation has improved over the years to assist with the procedure, creation of the big bubble to separate Descemet's membrane (DM) and endothelial layers from the anterior stromal and epithelial layers remains the greatest challenge. In the follow- ing section, key steps of performing DALK along with some surgical tips are reviewed. 1. Marking the cone and sizing the graft: In my practice, the most common indication for per- forming DALK is for patients with advanced keratoconus. In these pa- tients, if the iron line created by the cone is visible, I like to mark it with a marking pen to ensure that the trephine size selected encompasses the entire cone (Figure 1). Fre- quently, these cases require a larger diameter trephination. I prefer to oversize the donor by 0.5 mm if the host trephination is less than or equal to 8.5 mm. If the host cornea The femtosecond laser may trephination is 8.75 mm or larger, no oversizing is required. The donor tissue (PK quality, in case unsalvage- able perforation of DM occurs) is prepared on a side table and left in storage media for use later on. 2. Trephination, tunnel forma- tion, big bubble formation: Trephi- nation of the host cornea may be performed freehand using a manual Weck trephine, using the Hanna trephine system (Moria Surgical, Doylestown, Pa.) with a set depth (usually 400 microns), or any other trephine of the surgeon's choice. If using the Hanna trephine, I have found it to be helpful to have a Pentacam scan (Oculus, Lynnwood, Wash.) to determine the thinnest point of the cornea.5 I then set the Hanna trephine to 100 microns less than the thinnest pachymetry read- ing. A set of tires may be used to check that you are around 80% corneal depth. Being deep enough in the cornea is a key aspect of achiev- ing a successful big bubble. To cut down deeper and to initiate the plane in which the air injection would occur, an angled 15 blade against the trephine edge can be used to cut a half clock hour slit into the posterior stroma (Figure 2). The surgeon will notice that the posterior stromal fibers are much smoother. At this point, the surgeon begins to create a tunnel into the posterior corneal stroma using the DORC red spatula (Dutch Oph- thalmic Research Center, Zuidland, the Netherlands). The deep posterior stroma should feel smooth and the spatula, when wiggled back and forth, should progress without too much resistance. If small bubbles are noted to form adjacent to the spat- ula edge, this indicates the correct deep stromal plane (Figure 3). This tunnel is then lengthened by inser- tion of a cannula or bent 30-gauge needle attached to a 3 cc syringe with air. My preference is to use the Tan DALK 27-gauge cannula (Asico, Westmont, Ill.), which is curved to follow the shape of the cornea and has a bevel-down opening and a tip that is just sharp enough to help with passage through the corneal stroma, but blunt enough so that it will not puncture. The cannula should progress through the deep stroma smoothly using a gentle wig- gle between the fingers, with only a minimal sensation of resistance. The apex of the cone in a case of kerato- conus should be avoided as this is usually the thinnest point, which can easily perforate from the pres- sure built up during the injection of air (Figure 4). Just prior to the air in- jection to create the big bubble, the cannula tip should be angled down- ward. Pushing on the syringe with a constant pressure until a sudden feeling of resistance giving away creates the big bubble. 3. Anterior chamber decom- pression and "double bubble": When a successful big bubble is achieved, the stroma will whiten and the eye becomes very firm (Fig- ure 5). Do not continue to inject air once you see that the white ring has reached the extent of the trephined edge. You do not want the DM to be completely dissected apart out to the limbus. Using a 0.12 forceps to help with counter traction, pull the can- nula out and take care not to allow air to escape, which would collapse the big bubble. A posterior paracen- tesis is performed at this time using an MVR blade to release some aque- ous fluid and to decompress the eye pressure. Take care not to perforate the DM, which has been displaced into the anterior chamber. Injecting some small air bubbles into the anterior chamber through the para- centesis at this time allows for con- firmation that the DM is pushed posteriorly if the bubbles remain in the periphery (Figure 6). Leaving some air in the anterior chamber throughout the remainder of the case helps the surgeon to see that the DM is intact. 4. Stroma removal and the "quick nick": Removal of stroma is performed in two steps. First, the an- terior stroma using a crescent blade is dissected off (Figure 6). A uniform layer of thin posterior stroma is left, and it is important to keep the field dry so that any fluid egress observed will alert the surgeon to a microper- foration. A 15 blade is then used to create a "quick nick" through the posterior layer of remaining stroma, which collapses the big bubble. I have found that by using a marking pen to mark the site for the "quick nick" and a dollop of viscoelastic to seal the incision site, the "nick" is more easily visible and can be made in a slower, more controlled fashion.6 The surgeon will notice