In 1965, at the First World Cornea Congress, we presented our "peeling off" technique to perform deep anterior lamellar keratoplasty (DALK) (video 5.4.1.1)¹. In the next 50 years we developed this surgery for large diameter grafts (9.5 to 11.0 mm). In 2001 we introduced the "air-assisted peeling off" variant, which we will describe here (Figure 1, video 5.4.1.2)².

AIR-ASSISTED PEELING SURGICAL TECHNIQUE FOR DALK

The donor is obtained by trepanation on a cutting block for the desired diameter, usually between 9.5 to 11.0 mm. The endothelium is removed with a cellulose sponge. We mark the receiving cornea superficially with two trephines at 0.2 mm depth, one of the total diameters chosen, another of 7.75 mm, concentric to the first.

We introduce a 27 or 30 G needle connected to a syringe loaded with air, with the bevel facing up, into the superficial corneal stroma through the lip of the 7.75 mm mark and towards the center of the cornea. As the air is injected slowly, the stroma is made opaque by the confluence of tiny white air bubbles. It is common to see some larger bubbles, both in the anterior chamber and subconjunctival. This is probably due to a retrograde flow of air through the trabecular meshwork into the anterior chamber and outside of the globe. We perform a paracentesis to decompress the anterior chamber. Occasionally, the application of a second air injection may be necessary in order to achieve extended corneal emphysema (video 5.4.1.3).

The resection in the recipient cornea begins inside the 7.75 mm mark, with a scalpel blade. We deepen little by little along a sector of the circumference and a little towards the center until reaching the predescemetic space. This becomes evident thanks to the dark color of the stroma, due to the progressive decrease or absence of emphysema with depth (video 5.4.1.4). Once this plane is reached, we begin peeling under sustained traction on the corneal flap. We use the non-cutting edge of the blade, which produces a soft dissection, which is offered almost spontaneously and without the need to make cuts in the residual stroma (Figure 2, video 5.4.1.5).

It is vital to avoid traction-release-traction maneuvers that go against the goal of achieving a uniform donor-recipient interface. The lower cohesion of the stromal lamellae³ facilitates the dissection by peeling, which is the key to this technique and is decisive for obtaining excellent visual results. No attempt is made to reach Descemet's membrane (DM), as this increases the risk of perforation⁴. The unaltered bubbles in the anterior chamber indicate the integrity of the residual layer (Figure 3).

Once the central and deep stromal resection is completed without signs of perforation, we extend the lamellar dissection under the peripheral ring up to 1 mm outside the larger partial trepanation mark (videos 5.4.1.6 a and 5.4.1.6 b). In case of a perforation that forces us to convert to penetrating keratoplasty (PK) – something very rare with this technique –; we can use a trepanation of 7.75 for a graft with endothelium of 8 mm (video 5.4.1.7). If all went well, we trim the ring of peripheral tissue and place the large graft without endothelium that we prepared at the beginning (Figure 4).

Our usual suture pattern is a combination of 8 simple stitches with a 10-0 nylon anti-torque continuous suture. The needle is passed through the entire thickness through the cornea of ​​the donor, and deep into the receiving cornea through the plane we have dissected up to 1 mm beyond the larger diameter. The continuous suture adjusts smoothly and regularly throughout the circumference: a surgical keratoscope such as Maloney's is useful for this purpose or observing whether a central bubble in the anterior chamber is round. In cases of great ectasia, we use a donor disc 0.5 mm larger than the receiver to avoid that the large flattening causes folds in the posterior receptor layer. At the end we inject a subconjunctival combination of antibiotic and corticoid and place a therapeutic contact lens.

ADVANTAGES OF THE PEELING TECHNIQUES

The advantages of these techniques include all those that are common to DALK surgery (extraocular surgery, respect of the endothelium, lower requirements of the donor tissue, etc.). Specifically, the peeling techniques allow a predescemetic dissection very close to the DM, but without reaching it, with minimal risk of perforation. We have had to convert to PK in only one case out of more than 400 – and perhaps due to inadequate indication of the DALK, plus another one of a small pore that did not require reconversion.

The central residual stroma is commonly less than 10% of the corneal total (Figures 5 and 6), which allows visual results similar to the techniques that reach the DM or PK⁵. In the technique assisted by air, it does not suppose an agent of dissection as in the one of Anwar, but a means to calibrate the depth of the same one – to recognize when we are in the pre-Descemet plane. The peeling maneuver is in fact the same as in the original technique.

The use of large diameter grafts (9.5 - 11 mm) – routine with our technique – is associated with a great refractive stability and lower astigmatism, final and from the early postoperative period. In the treatment of keratoconus and other corneal ectasias, this surgery allows very thin corneas to be approached (video 5.4.1.8), with the presence of annular implants (video 5.4.1.9) and other complicated cases. The recovery of a normal thickness allows possible refractive treatments (LASIK / PRK) (Figure 7).

We have not observed with this technique the phenomenon of late progression of astigmatism and peripheral ectasia that has been described with PKs due to old keratoconus⁶ – and that we also observe in an increasing way –. We have verified this different behavior in a patient in which one eye was operated on by PK and the other one by DALK by peeling, followed for more than 4 decades (Figure 8). We believe that this difference is due to the large diameter of our lamellar grafts. Possibly the DALKs of smaller diameters do not avoid this problem. To correct this, we performed a peripheral full-thickness annular reconstructive keratoplasty on a deep lamellar bed (see chapter 5.8.7).

The works published by Gerrit Melles since 1999 gave rise to a certain popularization of deep anterior lamellar keratoplasty (DALK). Unlike all the techniques described until then, which used an anterior approach until reaching the pre-Descemet plane, Melles uses a posterior route from the limbus. But the most originality of his technique consists in the way of visualizing the depth of the lamellar dissection¹. Because the refractive indices of the cornea and the aqueous humor are similar, it is usually difficult to distinguish the posterior corneal surface during surgery. However, by filling the anterior chamber (AC) with air, an optical interface is created whose alteration by the instruments allows us to know how close they are to it. We will describe the Melles’ technique with some contributions from our personal experience (video 5.4.2.1).

MANOEUVRES AND SIGNS TO VISUALIZE THE DEPTH OF DISSECTION

To get the proper depth to start the dissection, the first maneuver we perform is an exchange of aqueous humor by air with a blunt 30 G cannula through a paracentesis. The air bubble in the AC acts as a convex mirror, so if we approach the anterior surface of the cornea with a knife, we will see two specular images of it (Figure 1): one is the one reflected on the anterior corneal surface and the other on the posterior corneal surface. In this way we can estimate the corneal thickness, which is half the distance between the tip of the knife and the image reflected from the posterior corneal surface. It is the so-called “mirror sign”.

When we begin to introduce the knife into the thickness of the corneal stroma, we can observe a luminous semi-circular reflection near its tip. This reflex is due to the indentation of tissue during the incision and is therefore called the “indentation effect”. The space between the tip of the knife or spatula and the light reflection corresponds to the thickness of the corneal tissue not yet penetrated. It can be seen as a dark band just around the tip, and its thickness decreases as we move into the deeper layers of the stroma (Figure 2). When we approach the posterior surface (Figure 3), small folds or “fold signs“ appear (folding effect), whose number, width and mobility increase as we go deeper, which indicates the proximity of the Descemet’s membrane (DM).

SURGICAL TECHNIQUE OF MELLES FOR DALK

In his original article² Melles describes a superior approach for this surgery. We prefer to do it on the temporary or super-temporal side since, as in cataract surgery, it is important to feel comfortable when handling all the instruments and that way is easier than the superior one to maneuver with the different spatulas.

First, we open the conjunctiva at the chosen site, or, directly in the sclero-corneal limbus, we make an incision of about 5 mm in length and 0.25 mm in depth. We dissect a scleral tunnel with a semilunar knife until we have minimal access to the corneal stroma. Then a paracentesis is performed with a 15° knife to about 3-hour sectors in a clockwise direction. In our experience, this paracentesis must have the right size to allow the entrance of a 30 G cannula since, if it is greater, the air will tend to escape from the AC. Through it and with the aforementioned cannula, we aspirate the aqueous humor and exchange it for a bubble of air until the AC is completely full and the eye is firm to the touch. We usually hydrate the paracentesis so that the air does not escape during the dissection.

Through the previously made tunnel we introduce the first of the spatulas designed by Melles (No. 2110, DORC International BV, Zuidland, The Netherlands) which is really a blunt knife (Figure 4B). This instrument should advance towards the DM until the three signs (mirror, indentation, folds) described in the previous section appear (Figures 1, 2 and 3). This is the most important step of the entire surgery, since it is when we must reach the pre-Descemet plane.

Once the desired depth of dissection is achieved, we take the second Melles spatula (Figure 4C). With it we begin the dissection of the DM with lateral fan-like movements. These will be, at first, of very limited amplitude that will increase as we create some space, until we reach the limbus on both sides of the incision. It is difficult to establish precise rules on how to perform this dissection but in any case, the movements should be smooth, without tilting the spatula down, and we should always be attentive to the folds in DM. With experience, some ease is achieved in these maneuvers, which should never be done brusquely in order to avoid perforation. With the second spatula, we will dissect about 180° of corneal tissue and then move on to the third of the Melles spatulas (Figure 4D) to complete the dissection of the entire cornea to the limbus at 360°.

We will have thus created a virtual space between the deep stroma and the posterior layer (DM and predescemetic lamellae). Next, we extract air from the AC through the paracentesis, to lower the intraocular pressure and be able to perform the next step comfortably. Through the scleral or sclero-corneal pocket created initially, we inject viscoelastic softly and progressively: we will see how the posterior lamina separates little by little from the deep stroma and creates an anterior cavity or pseudo chamber (Figure 5). Before beginning this injection, we believe advisable to place a suture of nylon in the incision in order to prevent the viscoelastic from escaping.

With the well-filled pseudo-chamber, we move to trepanation. To obtain a good fixation of the pneumatic trephines (like Hessburg-Barron) we must clean off the viscoelastic that has usually escaped through the tunnel from the corneal surface with a hemostat. Once fixed, we activate the trephine until viscoelastic emerges, which indicates that we have reached the dissected stromal pocket. We release the suction and remove the trephine. Resection of the anterior corneal disc is completed with scissors for keratoplasty. As in penetrating keratoplasty (PK), the scissor blades should be kept as perpendicularly as possible to the corneal surface and with special care not to perforate the posterior lamina. For this purpose, it is useful to inject more viscoelastic between the predescemetic space and the deep dissected stroma.

After removing the anterior corneal button from the receiver, we wash the predescemetic space to eliminate any remaining viscoelastic and proceed to place and suture the donor button, steps that do not differ from the other DALK techniques. Figure 6 summarizes the steps taken with this technique in 9 images.

COMPLICATIONS

The most frequent intraoperative complication is perforation of the posterior layer during dissection, especially in keratoconus. When this occurs, we will see how the air of the AC passes to the plane of dissection. If the AC remains formed and we believe that the buttonhole will not prevent a good apposition of the DM to the donor cornea, we can carry on with the procedure. Otherwise, it will be preferable to convert to a PK.

The appearance of a paralytic middle mydriasis, possibly caused by hypertensive ischemia of the iris (Urrets-Zavalía syndrome) is a complication that has been associated with this technique, due to the prolonged pressurization with intracameral air^4,5. To avoid this, the dissection time with the AC filled with high-pressure air should be minimized and leaving the minimum possible amount of air in the AC at the end of the operation. If a DM detachment occurs in the postoperative period (anterior pseudo chamber), air should not be injected without first performing an iridotomy, preferably at 6 o'clock and/or inducing pharmacological mydriasis.

In 2002, Mohammad Anwar described a method for deep anterior lamellar keratoplasty (DALK) that uses deep stromal dissection by air (pneumo-dissection). He called it the "big bubble" technique (BB) because it typically creates a macroscopic cavity filled with air between the deep stroma and Descemet's membrane (DM)¹.

The original technique is to first perform a partial trepanation of 60-80% of the corneal thickness. For this purpose, a calibrated trephine is recommended, such as Krumeich, Hanna or Hessburg-Barron. A 27 or 30 G needle is then inserted into the stroma, bent 60° at 5 mm from the tip, with the bevel facing down and attached to a syringe with 1 to 3 cc of filtered air (Figure 1). We carefully penetrate from the edge of the trepanation until the bevel is completely buried. We advance 3 to 4 mm with an orientation between radial and tangential and deepen smoothly until we approach DM as much as possible. Such semi-radial direction avoids the finest corneal apex, especially in keratoconus.

We inject the air with a moderate and constant pressure. Two things can happen mainly: A) At first the air infiltrates the central stroma and creates a whitish emphysema formed by microbubbles. Then, abruptly, we notice a sudden decrease in resistance in the syringe plunger and a large cavity or bubble appears, more or less circular, that grows rapidly until a whitish ring forms towards the limit of the trepanation (if it is about 8 mm), which seems to limit its more peripheral progression. The desired BB effect has been achieved; we stop the injection of air and remove the needle (Figure 2, video 5.4.3.1).

We now know that this type of separation (BB type 1) does not occur exactly between the posterior limit of the stroma and the DM, but ahead of this is the so-called predescemetic layer of Dua (PDL). More rarely, a transparent border bubble is formed, without the white ring and advancing more peripherally. Here the DM has been separated with cleanliness, which means a greater risk of rupture, and we talk of a BB type 2 (see chapter 5.2).

B) The air infiltrates the stroma without forming a macroscopic bubble, only an irregular emphysema that lacks a circular contour. It tends to extend towards the periphery and gets to penetrate in the anterior chamber (AC) through the trabecular meshwork – if we do not stop the injection. The latter is recommended so as not to whiten the entire stroma, in order to have another place where to try the injection again.

When the entire stroma is bleached, and we do not know if BB has been formed, we are facing a critical moment. If we puncture the central part of the bed without the posterior layers having separated, we will penetrate the AC and we will have to convert to penetrating keratoplasty. To assess whether BB has been formed or not, it is useful to introduce a small bubble in the AC². If this remains in the periphery with a kidney shape – or moves there if we injected it before trying the BB –, it indicates that the posterior surface of the cornea protrudes backwards due to the correct formation of the BB (Figure 3, video 5.4.3.2).

If, on the contrary, the small bubble is invisible under the central opaque area, the BB will not have been achieved. In this case, we can continue with manual dissection or hydrate the stroma with balanced saline solution (BSS); after waiting a moment, the cornea becomes semi-transparent and a new attempt with air is possible.

Once the BB is achieved, it is recommended to perform a paracentesis outside the trepanation area, taking care not to perforate DM. We resect the anterior layer of the corneal disc to a medium depth and only then do we allow a small amount of aqueous humor to escape through the paracentesis. We will have a fine stromal bed over the BB, whose central area we will tear with a 30° knife that we must keep parallel to the corneal surface (Figure 4). As soon as the air comes out, we remove it to avoid perforating the DM/PDL.

The collapse of the BB is noted by a sudden darkening of the whitish opaque disk or ring. We decompress more the AC and introduce through the anterior incision of the BB an iris spatula with blunt tip that we advance up to the limit of the trepanation in the 360°. We check that we have dissected the entire posterior surface of the stroma. With the help of the same spatula, we lift the stromal bed in a tent fashion and cut it with a knife to expose the DM/PDL (Figure 5, video 5.4.3.3). The sectors of said bed are cut along the edge of the trepanation with blunt-tipped scissors (Figure 6). Finally, we take the donor cornea, from which the DM-endothelium is removed, place it on the DM/PDL and proceed to suture it.

Anwar argues that with this technique the BB can be formed in 80-90% of cases if the indication is correct. Those with a history of hydrops or other weaknesses in the plane of DM should be avoided, since the air will pass to the AC without forming the BB.

The rapid popularity of this technique has led to several variants and the development of special instruments (see chapter 5.1). Currently, we have tools to confirm the formation of BB, such as endoscopic visualization³ and intraoperative OCT⁴.

To perform deep anterior lamellar keratoplasty (DALK), one of the ways to achieve separation between the deep stroma and the Descemet’s membrane (DM) is to use a viscoelastic solution. Manche first proposed this in 1999, when he observed the detachment of DM after the accidental injection of sodium hyaluronate into the corneal stroma¹. Later, DALK techniques were described in detail by means of viscodissection^2,3. This modality supposes an alternative to the one of the "big bubble" (BB) to achieve the complete separation between the deep stroma and the posterior lamina that includes the DM and the pre-Descemet layer of Dua (PDL), which allows optimal anatomical and visual results (Figure 1).

BASIC VISCODISECTION SURGICAL TECHNIQUE

For this procedure we prefer an anesthesia with good akinesia such as retrobulbar. A chemosis that can interfere with the fixation of the trepan should be avoided. We use the Hessburg-Barron suction trephines (Katena, New Jersey, USA) at a depth of 75% of the corneal thickness according to the preoperative ultrasonic pachymetry. Next, we dissect a layer of the anterior stroma with a semilunar knife. This helps to determine the depth to which the needle must penetrate for the injection of the viscoelastic. In the stromal bed created after removing the first layer, we insert a 30 G needle attached to a syringe with medium-type cohesive viscoelastic material such as Healon^® (AMO, Illinois, USA). High-density dispersive or cohesive materials should not be used. The bevel of the needle is oriented downwards and the viscoelastic is injected slowly, with a relatively high pressure of the plunger, until producing a separation or visco-bubble between the stroma and the posterior lamina (video 5.4.4.1). The needle can be inserted radially towards the center of the cornea or with a small angle to avoid the apex, and approximately parallel to the posterior surface. It must penetrate into the deep stroma just enough to prevent the reflux of the viscoelastic. The separation is slowly progressive, unlike the BB with air, and the edge of the visco-bubble extends beyond the trepanation.

Once the adequate separation of the stroma and DM/PDL is created, we proceed as in a BB air technique. The eye is decompressed through an anterior chamber paracentesis and we access the cavity filled with viscoelastic with a scalpel, with much greater security than in the BB since the viscoelastic prevents the sudden collapse of the same^1,4,5. We check with a blunt spatula in 360° that no adhesions remain and eventually we release them. We resect the deep stromal layer in front of the cavity, dividing into sectors and trimming with scissors along the trepanation. For these steps there are special scissors in which the lower branch has a blunt tip and is of different length than the upper one, or the usual Katzin type for keratoplasty can be used. These have the advantage of facilitating that the peripheral cut be vertical. To ensure hypotonia during these maneuvers it may be necessary to empty the anterior chamber repeatedly.

After exhaustive washing of the bed to eliminate any remaining viscoelastic, we place the full-thickness graft without endothelium-DM on the bed and suture it. Our usual technique employs 16 independent radial sutures of 10-0 nylon, at the highest possible non-penetrating depth, in order to obtain the best coaptation between the donor tissue and the recipient. To ensure the coaptation of DM/PDL to the graft, we can leave an air bubble in the anterior chamber – always inducing pharmacological mydriasis.

ALTERNATIVE TECHNIQUES

Both for trepanation and the initial lamellar cut of medium depth, we have also used the VisuMax^® femtosecond laser (Carl Zeiss Meditec, Germany) with good results.

Sometimes it is necessary to inject viscoelastic several times until finding the desired plane. If the infiltrated layer is above the predescemetic plane, its whitish color will prevent seeing the deeper planes. After removing it together with the viscoelastic, by means of lamellar dissection with knife, the injection is repeated until reaching the correct plane (Figure 2, video 5.4.4.2).

Some surgeons perform maneuvers taken from other techniques, such as placing a bubble in the anterior chamber. This can be small, to detect the formation of the cavity full of viscoelastic, which as in the BB, will reject said bubble towards the periphery⁶. With a large bubble you can see the sign of the striae of Melles, which indicates we have reached the predescemetic plane. By means of intraoperative OCT, it is possible to visualize the dissection plane, although the shadow of the tip of the needle may prevent it⁷.

It has also been proposed to first perform a stromal dissection with air and, in a second time, from the same entrance of the air or from a second way, to inject the viscoelastic, either directly or preceded by an injection of balanced saline solution (BSS) to separate the posterior DM/PDL sheet. However, the injection of air produces an opacity that will make it difficult to see the formation of the visco-bubble, which is avoided by directly injecting the viscoelastic.

In some cases, it is not possible to get the visco-bubble to grow beyond a few mm, without approaching the edge of the trepanation. In this situation, it can be continued by blunt dissection of the predescemetic space with a spatula until it reaches the required size (Figure 3, video 5.4.4.3). In some pathologies, such as mucopolysaccharidoses, difficulties have been reported in the practice of DALK and both BB and visco-dissection are discouraged – although one successful case of 5 is cited –, due to the excess of glycosaminoglycans in the corneal stroma of these patients⁸.

The success in deep anterior lamellar keratoplasty (DALK) depends to a large extent on the correct selection of the patient and an adequate surgical technique. This can be descemetic or predescemetic (video 5.4.5.1). The first involves the removal of the entire corneal stroma, leaving only the Descemet’s membrane (DM) and the endothelium in the receptor, together with the predescemetic layer of Dua (PDL). We talk about predescemetic technique when there is also a thin layer of the deep residual stroma in this bed. In both cases we implant a full-thickness donor cornea to which we have extracted the endothelium and DM. The descemetic technique allows the absence of a visible interface and visual acuities comparable to those of a penetrating keratoplasty (PK) (Figure 1), whereas in the predescemetic an interface is seen by the residual stroma, which somewhat limits the visual results.

I indicate the descemetic technique in the most common cases such as keratoconus, superficial vascularized leukomas after keratitis (bacterial or herpetic), deep stromal dystrophies (macular), etc. It allows to obtain excellent visions, minor astigmatisms and minimal endothelial loss, without the possibility of rejection at this level.

The main advantage of leaving a layer of stroma in the bed is to minimize the risk of perforating it during surgery and having to reconvert to PK. Therefore, I choose the predescemetic technique in situations of higher risk: very vascularized corneas, with high risk of endothelial rejection (herpes), severe alterations of the ocular surface (pemphigoid, facial palsy, neurotrophic ulcer, limbal failure, aniridia, etc.), recurrent entities such as reticular dystrophy and in those patients in whom we try to avoid a PK at all costs, especially if they do not need excellent visual acuity. In addition, the predescemetic technique allows replacing the graft with great ease in the case that for any reason it is opacified.

DESCEMETIC TECHNIQUE (BIG BUBBLE)

In order to detach the DM/PDL from the stroma, multiple techniques have been described – described in other chapters. I personally use the one that is perhaps most widespread, that of the "big bubble" of Anwar (BB)¹. I always use retrobulbar anesthesia, which produces a complete akinesia. I fix the upper and lower rectus muscles with sutures, which result in an excellent palpebral opening, avoids torsion of the eye during trepanation and allows to control the position of the eyeball. I trephine 2/3 to 3/4 of the depth of the stroma – measured with OCT in the preoperative period – using a pre-calibrated punch or a Hessburg-Barron trephine. Then I access the paracentral stroma from the edge of the incision with an insulin needle as a guide. Through the conduit thus created I introduce a blunt cannula of John of 27 G with three holes (ASICO, Westmont, IL., USA), with which I inject about 2 ml of air. This "travels" through the corneal fibers to reach the plane of the DM/PDL, it detaches this from the stroma and creates a cavity or BB that extends peripherally to the edge of the trepanation, where usually the exit of some air bubbles can be observed.

The result is a characteristic peripheral annular image, formed by air in the stroma, while the center of the cornea remains relatively transparent where the BB is. The eyeball is at this moment at high tension, so I perform a paracentesis in the limbus through which aqueous humor will emerge. Sometimes other types of BB² can be formed but in any case, the next step is to dissect the superficial 2/3 of the stroma with a semilunar knife to expose the roof of the BB. After completely emptying the anterior chamber (AC) to achieve hypotonia, I proceed to puncture said ceiling, which causes the collapse of the BB. Through that hole I insert the branch of a scissors with a blunt tip and divide the stromal layer into 4 quadrants, then cut them vertically following the trephination. I irrigate profusely the bed of the DM/PDL to eliminate any foreign material deposited. The donor button is placed and sutured with the common technique described below.

PREDESCEMETIC TECHNIQUE (PLANE BY PLANE)

It is similar to the previous one in terms of anesthesia, preparation and initial partial trepanation. Injection of air into the cornea is not used. The goal is to remove all the stroma except a thin layer of about 40-50 μm in front of the DM/PDL. To do this, I first dissect the superficial layer with a semilunar knife, which exposes the deep stroma. I fill the AC with air to get an adequate intraocular pressure. With an insulin needle bent with the bevel up, I scratch the peripheral stromal bed until it creates a ridge that allows me to start a new dissection on the entire surface with the semilunar knife. I cut this new lamina 360° with scissors and repeat the maneuver 2 or 3 times until, when scarifying the peripheral bed with the needle, a small pore is produced through which the air comes out of the AC. This indicates that the bed is already very thin, but because there is a certain predescemetic tissue, the pore does not tend to tear as it would with bare DM/PDL. I continue as in the other technique (bed irrigation, graft suture, etc.) (Figure 2).

PREPARATION OF THE DONOR TISSUE

I prefer to trepan the donor cornea from the epithelial side, which in general requires the use of an artificial AC. I use a trepan with the same diameter as in the receiver or 0.25 mm larger. I remove the DM-endothelium from the button obtained with blunt forceps.

SUTURE

After verifying that the adaptation of the donor button to the receiving bed is correct, I proceed to place four cardinal sutures, taking care to distribute the donor tissue in the four quadrants symmetrically. For this it is useful to reform the AC with air and observe how the classic rhomboidal figure is formed. Normally I place a continuous anti-torsional suture – in the form of isosceles triangles – and superficial, which only aims to level the Bowman layers of the donor and the receiver. Once the suture is passed, I empty the AC to create hypotonia, remove the cardinal sutures, tense the continuous one and knot it so that the knot is inside the stroma. In the end I reform the AC with saline solution to give tension to the eyeball and distribute the tensile forces. In patients with altered ocular surface (chemical injuries, pemphigoid, etc.) I prefer independent sutures, which I place somewhat deeper and tightened to avoid loosening in the postoperative period and steps to form between the edges.

REPLACEMENT OF THE OPACIFIED GRAFT

The predescemetic technique is used, as previously mentioned, in patients at higher risk. In cases of limbal insufficiency, a large diameter DALK may provide transient amplifying cells (TAC), capable of maintaining a certain degree of epithelial regeneration^3,4. This would explain the prolonged transparency of some of these grafts. In any case, when opacification occurs, it can be replaced with great ease. With the back of a 15° knife or other relatively blunt instrument, it is possible to reopen the scar in 360° and then lift the old graft with a forceps, place a new one and suture it. Although the vision that is achieved is usually limited and multiple successive replacements may be necessary, in some patients this option seems less risky than choosing a PK or a keratoprosthesis.

In recent years different techniques of deep anterior lamellar keratoplasty (DALK) have been described, with the common goal of reaching a predescemetic plane with the lowest possible residual bed and without the risk of perforation of the Descemet’s membrane (DM) associated with the descemetic techniques.

MANUAL DISSECTION TECHNIQUES

In the "dry dissection" technique of Rama et al^1,2, the depth of the dissection plane is reached by making a superior initial cut with a diamond cutter 1 mm inside the diameter of the trephination ring. The depth of this cut is determined by subtracting 20 μm to the finest corneal thickness, estimated by ultrasonic pachymetry and OCT, and from it a pocket is cut from which the periphery is dissected with a sharp disk cutter (Alcon, Fort Worth , TX, USA) and the central cornea with a blunt spatula (Janach, model J2401.1a)² .

The diamond-assisted technique (Dia-DALK) of Vajpayee et al³ makes an incision from 11 to 12 h in the ring marked at 7.75 mm, with a diamond knife calibrated at 30 μm less than corneal pachymetry. Peripheral dissection is performed from this cut with thin, curved scissors (Cindy Scissors, Bausch & Lomb) and central dissection with lamellar dissectors.

The folding technique (tuck-in lamellar keratoplasty), also described by the Vajpayee group⁴, improves the apposition of the edges in corneas with peripheral thinning as in keratoglobus, which allows a more precocious epithelization. In the receiver, after the partial trephination of 8.5 mm and the anterior stromal dissection, a peripheral intrastromal pocket is created with a semilunar knife, reaching up to 0.5 mm outside the limbus. In the donor cornea a partial trepanation of 300 μm of 8.5 mm in diameter is performed, followed by the removal with Vannas' scissors of a lamina external to this section. Subsequently, the donor cornea is cut with an 11-12 mm punch, the DM is removed, placed in the bed and sutured.

In cases of pellucid marginal degeneration, the folding is partial with a pocket only in the lower 180°.

In the technique of Weiyun Shi for advanced keratoconus⁵ the two relevant steps are the respect of about 2 mm of the deep stroma on the apex of the cone and the realization of a «gradual pressure suture» that is achieved by first fixing four less tense cardinal sutures, that firmly apposite only the posterior stroma, before placing 12 independent sutures at the usual tension. This allows to move the central folds of the DM towards the periphery and prevents them from affecting the central area (Figure 1).

HYDRODISECTION AND HYDRODELAMINATION TECHNIQUES

Sugita and Kondo⁶ were the first to publish the technique of hydro-delamination, which consists of injecting balanced saline into the deep stroma: the thickening of the residual stroma created by the edema facilitates its dissection until reaching a deep plane. Amayem and Anwar⁷ inject the fluid by quadrants at the edge of the trepanned disk. Some authors have concluded that hydro-delamination is the fastest and simplest dissection technique⁸.

PNEUMODISSECTION TECHNIQUES

From the description of pneumo-dissection as the technique of the «big bubble» (BB) by Anwar⁹, various modifications have been proposed, such as the injection of a small bubble in the anterior chamber that when pushed to the periphery confirms the formation of BB¹⁰, or the resection of a stromal layer¹¹ to facilitate the introduction of the air cannula in the predescemetic depth.

The "microbubble incision" technique of Riss et al¹² allows to rescue the DALK in cases in which the BB has not been formed. More air is injected until the emphysema of the posterior stroma is formed, which is dissected until the anterior chamber is visualized. With a 15° knife guided perpendicularly the wall of some bubble of the residual stroma is cut. This is repeated if there is another bubble deeper until reaching the descemetic plane.

The technique for epithelialized descemetocele of Gabison et al allows the edges of the latter to be easily peeled off by means of a modified BB, which reduces the great risk of perforation of the DM¹³. Although the history of hydrops is often considered a contraindication for BB, Ramamurthi and Ramaesh describe in 4 cases a modification that combines a controlled injection of air and manual dissection layer by layer. After dissecting the stroma to a depth of 95% and injecting viscoelastic into the anterior chamber, they eliminate the area of ​​fibrosis, including DM. They report visions better than 0.5 at one year and a mean endothelial count of 2,044 cells/mm² in the area of ​​excised DM¹⁴.