Half-Top-Hat PKP: Combining the Advantages of Lamellar and Full-Thickness Keratoplasty
In 2003, Busin described a novel wound configuration for full-thickness penetrating keratoplasty (PKP)1 involving the creation of a top-hat–shaped corneal button. In this technique, the anterior surface of the donor button is smaller than the posterior surface (7 and 9 mm, respectively); the recipient bed accommodates this modified donor corneal button.
The top-hat configuration combines the advantages of PKP, including its superior refractive and visual outcome, with the wound-healing advantage of lamellar keratoplasty. Despite its presumed advantages, this technique has not been widely adopted, partly because of the difficulty of preparing of both the donor and recipient. We recently described a simpler method of achieving the advantages of a full top-hat wound configuration.2 We named this technique half-top-hat PKP.
SURGICAL TECHNIQUE
After an ophthalmic viscosurgical device (OVD) is placed on the corneal endothelium, the donor button is mounted on an artificial anterior chamber. The geometric center of the cornea is marked, and we use a 7- to 8-mm trephine to make a circular, 0.4-mm–deep incision. A lamellar stromal dissection is carried out. We use a bevel-up crescent knife, starting from the base of the incision and working toward the limbus (Figure 1). A full-thickness trephination is then carried out from the epithelial side, using a trephine blade that is 1 mm larger than the previous trephination. It is important to complete the second trephination without leaving uncut areas. We find that rotating the trephine two or three full turns after noting perforation into the anterior chamber achieves the full cut. To prevent collapse of the corneal button once perforation has occurred, an assistant should continuously inject fluid into the artificial anterior chamber.
A peripheral lamellar wing of deep stroma and endothelium (width, 0.5 mm) surrounds the central, full-thickness part of the donor button (diameter, 7–8 mm).
In contrast to the full top-hat configuration, the recipient bed in the half-top-hat PKP is prepared by a straight, full-thickness trephination. It is the same diameter previously used during the anterior trephination of the donor tissue. The donor button is positioned by sliding the peripheral wing under the recipient cornea (Figure 2). I usually use eight interrupted 10-0 nylon sutures and a single continuous 16-bite, 10-0 nylon suture. The interrupted sutures are passed through the wing; this motion assures good apposition of the wing to the inner corneal surface of the recipient.
DISCUSSION
A straight-cut PKP requires relatively tight sutures to hold the edges together until healing can sufficiently withstand the effect of intraocular pressure (IOP), which usually takes approximately 1 year. The sutures used with top-hat and half-top-hat PKPs merely prevent the donor button from sliding out of position. They need not be as tight because the IOP tends to push the healing surfaces together. In our experience with full and half-top-hat configurations, the full-thickness graft can be freed of interrupted sutures as early as 3 to 5 months after surgery, thus significantly reducing visual rehabilitation time.
We have shown the tectonic advantage of the top-hat configuration by comparing it with the wound-bursting pressure of other wound configurations (ie, traditional, mushroom, zig-zag, and Christmas tree).3 We noted that top-hat wound configuration was the most mechanically stable. Additionally, the top-hat and half-top-hat configurations, which have a larger contact surface between the donor and the recipient compared with regular PKP, may reduce the risk of graft dehiscence. At 1 year postoperative, patients who received either the top-hat or half-top-hat configuration had a significantly higher endothelial cell count compared with patients who underwent a regular PKP (unpublished data).
Although the top-hat configuration is easily created using a femtosecond laser,4,5 in our experience it is quite hard to achieve an accurate top-hat cut manually. An accurate cut on the recipient is hard to attain because the scissors used to cut the inner circle (ie, the niche for the lamellar wing) create an irregular, imperfectly round cut. In top-hat PKPs, this irregularity may cause more intra- and postoperative leakage from the graft-host interface. Additionally, creating a manual full top-hat configuration significantly lengthens the surgical procedure.
Our configuration, the half-top-hat,2 simplifies the procedure and allows better apposition between the donor and recipient corneas. The largest advantage of the half-top-hat configuration is its larger area of vertical apposition between the donor and recipient corneas as compared with a full top-hat configuration. Thus, we expect half-top-hat PKP to be more watertight.
In the half-top-hat configuration, a full-thickness recipient cornea opposes the tendency of the donor's wing to slip out, which means better tectonic support and less risk for wound dehiscence. Additionally, surgery is easier and faster compared with a full top-hat configuration.
In my experience, the learning curve for performing half-top-hat PKP is relatively short. The learning curve includes using the artificial anterior chamber as well as performing partial-thickness trephination and lamellar dissection to create the wing of the graft, all of which are not difficult procedures to master.
CONCLUSION
I believe that the half-top-hat PKP has several advantages compared with full top-hat PKP. It is easier to execute and may also be performed by surgeons who do not have access to a femtosecond laser; however, the top-hat configuration seems to be more anatomically correct if a femtosecond laser is applied.
Igor Kaiserman MD, MSc, MHA, is in the Department of Ophthalmology, Barzilai Medical Center, Ashkelon, Israel, and a Senior Lecturer of Ophthalmology, Faculty of Health Sciences, Ben-Gurion University. Dr. Kaiserman states that he has no financial interest in the products or companies mentioned. He may be reached at tel: +972 577678941; fax: +972 26416242; Igor@Dr-Kaiserman.com.
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