Several lamellar keratoplasty dissection techniques have been described. One flaw of these techniques is that the depth of the stromal dissection can not be visualized during surgery, and that the procedures therefore bear the risk of perforation. Another flaw is that the recipient bed is created by a layer for layer removal of corneal tissue. Once started, the procedure must be completed as a lamellar or penetrating keratoplasty, although donor tissue requirements differ for each of these procedures. In lamellar keratoplasty, a donor corneal lamella is generally obtained from a fresh globe with unknown endothelial quality, or lyophilized corneal tissue. When the dissection of the recipient stromal bed cannot be completed due to inadvertent perforation, donor tissue with good quality endothelium may not be available to convert to a penetrating keratoplasty.

In this on-line course, a three-step surgical technique is described in chapter 4, to perform a deep stromal, anterior lamellar keratoplasty procedure, in which the depth of the dissection relative to the corneal thickness can be visualized during surgery. The procedure can be completed in the event of a micro-perforation, or can be aborted to perform a planned penetrating keratoplasty. In chapter 5, a refinement of the surgical technique is described. Instead of making a spatula dissection at a deep stromal level through a scleral incision, a dissection with viscoelastic was performed just above Descemets membrane, using a similar method to achieve the intended level of dissection depth.

As a first step, a deep stromal, lamellar dissection is made to a visually controlled depth. Injection of air into the anterior chamber may facilitate deep stromal dissection for four reasons. First, because the air-to-endothelium interface reflects the posterior corneal surface, its specular light-reflex may be used as a reference plane for desired dissection depth. Second, small folds in Descemets membrane can be seen during the performance of deep stromal dissections. When the anterior chamber is filled with air, these folds are accentuated, and the number, width and motility of the folds seem to indicate how close to Descemets membrane the dissection is made. Third, micro-perforations are easily noted during surgery, since a small air bubble is seen to escape from the anterior chamber into the stromal pocket, and the break in Descemets membrane is sharply outlined over the underlying air bubble. Fourth, in the event of a micro-perforation, the break in Descemets membrane is self-sealing by the air in the anterior chamber, and the dissection may be continued without loss of the intraocular pressure.

As a second step of the procedure, visco-elastic is injected into the stromal pocket to displace the entire posterior corneal surface toward the iris, thereby creating a pseudo-anterior chamber. Because the stromal pocket is made through a self sealing scleral tunnel incision, the visco-elastic remains within the pocket when pressure is applied onto the anterior corneal surface. Thus, a normal intraocular pressure can be restored after the injection of visco-elastic into the stromal pocket, and the anterior, diseased recipient corneal tissue may be excised with routine trephination techniques, without damage to the posterior corneal surface.

In one of our patients, residual visco-elastic remained in the stromal interface after surgery. After removal, the best corrected visual increased from finger counting to 0.4 in the first postoperative week. It seems therefore important to completely remove all visco-elastic at the recipient stromal bed prior to suturing the donor corneal button in place.

As a third step, a donor button is transplanted into the recipient bed using standard keratoplasty surgical instruments and techniques. Since (almost) all stroma is removed, a full-thickness donor button can be sutured into the recipient opening. When the donor tissue thickness exceeds the depth of the recipient bed, the donor button still fits because the peripheral recipient cornea is split while the dissection is made, and the excess thickness of the button only causes little separation of the recipient, posterior stromal layers.

A best corrected visual acuity of 0.7 at six months and 0.8 at 12 months compares favourably with reported series of lamellar keratoplasty, or penetrating keratoplasty. A postoperative astigmatism of about 2.5 D also compares favourably with reported series of lamellar keratoplasty, deep lamellar keratoplasty, or penetrating keratoplasty. One patient showed high astigmatism after excentric trephination. A flaw of our study is that it is a prospective, but not a comparative study, i.e. we did not study the results of matched control penetrating keratoplasties.

Compared to deep, lamellar keratoplasty techniques using manual dissection (chapter 4), the visco-dissection technique (chapter 5) may offer several advantages. First, less surgical time is required for making the dissection, since the pathological stroma over its full-thickness can be quickly separated from Descemets membrane in the recipient. Second, the procedure can be performed using instrumentation commonly available for routine penetrating keratoplasty, without the need for special surgical instruments. Third, a smooth donor-to-recipient interface is created, which may reduce the risk of interface scarring and improve the visual outcome. Fourth, an identical level of dissection depth is obtained in both donor and recipient tissues, so that theoretically a perfect anatomical restoration is achieved.

A disadvantage of the visco-dissection technique is that there is substantial risk of perforation of Descemets membrane during injection of viscoelastic into the cornea, removal of the recipient anterior lamella, or suturing of the donor button. Although Descemets membrane may easily withstand the intraocular pressure in the presence of a Descemetocèle, we found that a large area of exposed, denuded Descemets membrane is fragile, and that a microperforation has a tendency to enlarge by itself due to the elastic properties of Descemets membrane.

In the current study, a perforation occurred in 7 out of 68 eyes (10%), using spatula dissection through a scleral tunnel. A 39.2% microperforation rate has been described for conventional deep lamellar keratoplasty techniques. An advantage of the presently described technique may be that íf perforation occurs, it is likely to occur early in the surgery, whereas inadvertent perforation usually occurs after preparation of the recipient bed has almost been completed in conventional lamellar keratoplaty techniques. The visco-dissection technique may be used for indications commonly managed with a penetrating keratoplasty, for example keratoconus and corneal dystrophies. If dissection of Descemets membrane is successful, these cases may benefit from the advantages of a lamellar keratoplasty, whereas the procedure is easily converted to a penetrating keratoplasty in the event of perforation.

Another disadvantage of our techniques may be that the anterior chamber has to be opened for an aqueous to air exchange prior to performing a corneal dissection. It therefore bears a risk of intraocular infection and damage to the anterior chamber structures. The recipient endothelium may be damaged by inflating the anterior chamber with air, and/or performing a deep stromal dissection. In our ongoing clinical study, preoperative and long-term postoperative endothelial cell counts are performed to determine how endothelial cell loss with our technique compares to that after existing deep lamellar keratoplasty techniques, for which a 13% cell loss at one year has been reported.

Since the 1960s, lamellar keratoplasty may have lost its popularity due the imperfections of the existing surgical techniques rather than poor visual outcomes. Although better microkeratomes have become available with the development of laser-assisted in-situ keratomileusis (LASIK), microkeratome lamellar resections cannot be used for disorders with deep stromal opacities, variable corneal thickness, and surface irregularities. Improvement of the manual technique for lamellar keratoplasty could therefore potentially broaden the interest for the procedure again, to manage anterior corneal disorders.