Cornea
Transplant techniques: PK and DALK by design

At a glance:
PK vs. DALK
• Ease of procedure and eye banking has contributed to popularity of PK
• DALK offers the following advantages: rapid healing; reduced rejection;
improved safety;
• Despite advanges, surgical challenges of DALK limit its popularity

Turning an experienced eye on these two techniques

Steps of the DALK procedure
Source: Sonia H. Yoo, M.D.

When it comes to types of corneal transplants penetrating keratoplasty (PK) still tops the list in the United States by a sizeable margin, according to Sadeer B. Hannush, M.D., assistant professor of ophthalmology, Jefferson Medical College, Philadelphia, and attending surgeon, Cornea Service, Wills Eye Institute, Philadelphia. “In 2008 of 52,000 grafts 32,000 were penetrating, 17,000 endothelial and about 1,100 were anterior lamellar keratoplasty,” Dr. Hannush said.

The PK standard

Part of the reason that PK remains so entrenched in the lexicon here is the relative simplicity of the procedure and the fact that it has been around for some time. “PK is so common because it has been actively around for about 40 years,” Dr. Hannush said. “Most surgeons trained in the US and abroad are comfortable doing the procedure.” He sees one word as summing up why so many continue to use the procedure. “Familiarity, familiarity, familiarity—that’s why PK is so common,” he said.
Sonia H. Yoo, M.D., associate professor of clinical ophthalmology, Bascom Palmer Eye Institute, Miller School of Medicine, University of Miami, Miami, sees eye banking as a key an influence in the emergence of PK. “Two things helped to catapult the success of PK,” she said. “One is eye banking where we could actually acquire tissue and store it for a while and the other is the use of drugs such as topical steroids to help prevent against rejection.”
Practitioners rely on PK to treat a variety of different diseases. “PK is used for a number of different clinical conditions including keratoconus, corneal edema from either Fuchs’ dystrophy, or a pseudophakic bullous neuropathy, or aphakic bullous keratopathy, and corneal scars from trauma or infection,” Dr. Yoo said.
The procedure itself is very straightforward and can be done in one of two ways. “The most prevalent way is to use a trephination system usually with a circle or a blade like a cookie cutter, and we remove the central 2/3rds of the patient’s cornea,” Dr. Hannush said. “Similarly we punch the central 2/3rds of a donor cornea full thickness and transplant it into the recipient and suture it with 10-0 nylon suture.”
More recently a few centers have begun performing femtosecond-assisted penetrating keratoplasty, dubbed by some as IEK (IntraLase Enabled Keratoplasty). “Basically this means that the opening—the trephinations in the recipient and in the donor, are done with a laser as opposed to a blade,” Dr. Hannush said. “The arguable advantage is that you can shape the wound in a way that makes it heal faster, may require fewer sutures, may allow for earlier suture removal and may create less postoperative astigmatism.”
Such astigmatism has been the bane of corneal transplantation. Dr. Hannush refers to a cartoon that he frequently includes in his lectures on this. “We show a patient sitting in a corneal surgeon’s chair and the surgeon says, ‘Your transplant is crystal clear Mr. Jones and Mr. Jones asks, ‘Well, how come I can’t see doc?’” Dr. Hannush said. “The reason Mr. Jones cannot see is that he has a lot of astigmatism induced by wound healing and suture placement and suture removal that precludes good vision.”
The hope is that femtosecond-assisted PK may create a smoother surface with less astigmatism and therefore better vision. However, only a limited number of these have yet been performed. “I would say that the literature has less than 100 of those grafts reported,” Dr. Hannush said.
In recent years there have been tissue targeted outgrowths of the PK procedure such as endothelial keratoplasty (EK) and deep anterior lamellar keratoplasty (DALK). With (EK), which has quickly become popular, the posterior 2% of the cornea, including Decemet’s membrane and endothelium are removed from the recipient and replaced by donor tissue.
Indications for this include Fuchs’ dystrophy and other sources for corneal endothelial dysfunction such as pseudophakic bullous keratopathy or pseudophakic corneal edema, according to Dr. Hannush. “EK has replaced PK or is replacing PK for the indication of endothelial dysfunction,” he said.

Braving DALK

Meanwhile, DALK, which is used for anything other than endothelial dysfunction has languished a bit in the United States. “This can be used for stromal dystrophies, all the ecstatic disorders as well as corneal scars after infection,” Dr. Hannush said. “DALK is now starting in the U.S. and more commonly outside of the U.S. to replace PK for these indications.”
The DALK procedure, which can be done in a couple of different ways, is a challenging one finds Dr. Yoo. “Traditionally with DALK we have trephined to nearly 90% or 95% through the corneal stroma and stopped right in front of Decemet’s membrane,” she said. “Then we manually dissected the corneal stroma off of Decemet’s membrane.”
Recently some have taken to using Amwar’s big bubble technique in conjunction to using a trephine to get through most of the corneal stroma. “The way it works is that a bubble is placed in the posterior stroma with essentially a cannula, with sort of a blunt needle which is put into the posterior stroma and then air is injected,” Dr. Yoo said. “Then essentially the air dissects Decemet’s off of the posterior stroma.”
This can be difficult on a number of fronts. “Sometimes it’s difficult to know exactly that you’re in the correct plane—that you’re deep enough and yet not all the way through Decemet’s membrane,” Dr. Yoo said. “Sometimes it’s hard even when you’re in the right plane when you inject the air to actually get Decemet’s to come off of the posterior stroma, particularly in younger corneas.”
There are a variety of advantages to the DALK procedure over PK. Two advantages that Dr. Yoo finds are more rapid healing and also the fact that there are no worries about rejection. “The advantage is there is relatively rapid rehabilitation,” she said. “However, I think that the biggest advantage is that there is no risk of endothelial rejection because you’re keeping the patients own endothelium.” This can be particularly important for younger patients. “For patients where you’re really doing a transplant because they have a scar, their own endothelium maybe better than any donor,” she said. “Because it’s a young person for example that needs to live with this cornea for the next 50 years then it is appealing that the patient is not going to have a chance of endothelial rejection.”
Another advantage is that the procedure is in principle potentially safer, Dr. Hannush finds. “The eye is at no time completely open,” he said. “If you are successful in removing the front 98% then you still have Decemet’s membrane on, the eye is not completely open and therefore the risk of posterior pressure or superchorroidal hemorrhage is lower.”
Theoretically it may also offer better results. “Because you are leaving the last layer of the eye, Decemet’s membrane, the assumption is that the whole is going to be more uniform,” Dr. Hannush said. “So, when you sew in your donor insert even though you have to use stitches, hopefully it will be done with more regularity.”
In addition, there is also the potential for earlier healing. “Because it’s not a full thickness procedure you may be able to remove sutures earlier if sutures need to be removed for astigmatic control,” Dr. Hannush said.
Finally, because the risk of rejection is lower long-term follow up may not be as important as with a full-thickness transplant. “So in many parts of the world where people are traveling from one country to another to have the work done and who can never come back to see the operating surgeon again it’s a huge advantage,” Dr. Hannush said. “You don’t have to have as trained a surgeon in your locale following you—(you can make do with ) a surgeon that may not be able to identify the signs and symptoms of rejection as well as the operating surgeons, simply because we don’t need to identify them because we don’t have them with this treatment.”
Despite such advantages few surgeons in the US perform this procedure. Dr. Hannush sees two reasons. “It is a surgically challenging procedure,” he said. “Also, it is not as big an improvement of PK for the same indication as EK is over PK for endothelial dysfunction.” Most are eager to adapt EK because it can offer a huge improvement in outcomes for patients. “With EK you have an intact globe, you have a small incision, and you have a predictable refractive outcome because you are not changing the anterior curvature of the cornea and thus there are no stitches in the transplant and much quicker visual rehabilitation,” Dr. Hannush said.
Unfortunately, the motivation to move to DALK is not nearly as clear cut, he finds. “You still need to use sutures, you still have to deal with astigmatism and the visual rehabilitation may not necessarily be quicker,” he said. “Add to that the old adage that necessity is the mother of invention.” The fact is in the United States at least there has been no great need to find ways to keep the patient’s own endothelium. “We have been very fortunate or blessed and we have plenty of very good donor tissue for penetrating keratoplasty,” Dr. Hannush said. “With 102 eye banks doing a wonderful job with eye banking we are always able to transplant the entire cornea with reasonably good results even though we only need to transplant the anterior 98%.”
Outside of the United States, however, it’s a different story. “Elsewhere in the world there is quite a paucity of good donor tissue with good endothelium,” Dr. Hannush said. “While they have enough donor tissue, they may not have healthy endothelium, which would not be appropriate for full thickness transplantation, but would definitely be appropriate for DALK.” Dr. Yoo agrees that the donor tissue situation is pivotal here. “We are very fortunate that we have a strong eye banking system in this country,” she said. “Because of that doctors haven’t been pushed to go with DALK.”
For those who do opt for this procedure Dr. Yoo stresses that it’s important to have an alternative plan in place in case Decemet’s membrane is inadvertently perforated. “If it’s a small perforation and it’s in the periphery sometimes the surgery can still be salvaged and the surgeon can still proceed with DALK,” Dr. Yoo said. However, sometimes the perforation is larger and is in the center precluding DALK. In that case it is necessary to do PK and surgeons must be prepared. “The donor tissue should essentially be prepared last,” Dr. Yoo said. “They’re probably better off having PK quality tissue and then after they’re finished preparing the recipient to then go ahead and prepare the donor tissue just in case they have to convert.”
Going forward, Dr. Yoo thinks that if DALK can be made into a more automated procedure that it will become more popular with better visual results and recovery. She sees this as akin to manual RK and ALK which with the invention of the microkeratome were transformed into LASIK. “LASIK became something that was very popular because it was something that was relatively easy to do, the results were reproducible and visual outcomes were excellent,” Dr. Yoo said. “I see that the future of DALK will hopefully move in this direction where we can take this procedure and make it more automated using lasers for example to make these very precise cuts—I see that as the next phase in the evolution of DALK.”

Editors’ note: Drs. Hannush and Yoo have no financial interests related to their comments.

Contact information:

Hannush: 215-752-8564, Sbhannush@comcast.net
Yoo: 305-326-6322, syoo@med.miami