The surgical technique of penetrating keratoplasty (PK) is, in essence, very simple, perhaps deceptively simple: trepanning the corneal window and graft, placing it and suturing it. But every detail counts for an optimal final result: that's where the difficulty and excellence lie. On the other hand, PK is often combined with various techniques involving manipulation of intraocular tissues, which can greatly increase the complexity of the procedure.

Anesthesia can be done peribulbar or retrobulbar if the patient cooperates – assuring then a good hypotonia of the eye – but in young and healthy patients general anesthesia is often preferable, with good ventilation and control of blood pressure.

EXPOSURE AND PREPARATION

First of all, you have to properly expose the cornea. The pressure of the orbital eyelids and tissues can deform the eye globe so that the corneal window becomes oval or elliptical, which will cause astigmatism due to incongruence with the graft (Figure 1). To avoid this, we must check that the eye speculum adequately compensates for such compressions, or that the separator itself or a muscular traction suture does not cause them. When there is shortening of the cul-de-sac, it must be avoided to force the opening since this will produce tractions on the cornea.

If corneal neovessels exist, it is preferable to coagulate them at the level of the limbus – with a coaxial fine 23-mm bipolar diathermy, or a monopolar and a suture needle as proposed by Dua – before trepanning, since much more retraction later occurs. In the presence of tissues or foreign materials that cover the cornea (a fibrovascular pannus, calcium plaques, etc.), it will also be convenient to remove these.

In aphakic eyes or if we anticipate that they will be during the surgery, it is advisable to fix a scleral ring (Flieringa) of the appropriate size. Typically, a diameter of 18-20 mm will leave a margin of about 3-4 mm around the limbus, sufficient in case scleral hatches are required to fix an intraocular lens (IOL), a filtering surgery or a pars plana vitrectomy combined. We usually put 6 sutures (7-0 vicryl or silk), with spatulate needles so as not to perforate the thin sclera at the level of the muscular insertions. Some of these sutures can be left long to use as traction or fixed to the surgical field for safety. The younger the patient, the thinner and collapsible the sclera, and in children the ring is recommended even when they are phakic.

In patients with glaucoma, myopia, hypertension or other risk factors for expulsive hemorrhage, it is essential to achieve normal intraocular pressure (IOP) at the start of surgery to minimize the sudden decompression caused by trepanation. Apart from the orbital balloon, this may require the use of an intravenous hyperosmolar agent (mannitol, etc.). It is also useful to place the patient with a somewhat elevated head (anti-Trendelenburg) to reduce orbital venous pressure.

THE THREAT

Diameter selection

The sizes of the graft and the receiving window are the main parameters to be decided in a PK. The diameter of the window will be chosen according to the total of the cornea and the pathology present. Apart from the size of the visible lesion – the opacity – we must reach an area of sufficient thickness around the perimeter of the cut (Figure 2). A larger diameter has certain advantages: it provides a greater endothelial population; the scar is farthest from the center and will tend to induce less astigmatism and irregularity; the involvement of the visual axis would also be delayed in the event of a recurrence of stromal dystrophy. All this is limited by the greater risk of rejection when approaching the limbus, which requires a commitment and to leave a margin of at least 1-1.5 mm.

As for the donor button, if it is obtained by the epithelial side – being in a whole eye globe or a corneoscleral segment mounted on an artificial anterior chamber (AC) –, theoretically the diameters of the graft and the window should be identical when using the same trephine diameter. But if the donor is obtained by the endothelial side, its diameter will be somewhat smaller than that of the window (0.25 to 0.50 mm) for the same trephine diameter. This is because the cut from the front tends to be somewhat truncated and therefore larger than the nominal diameter of the trepan in the deep layers. On the other hand, the endothelial cut is more cylindrical, but the pressure required for the stamping makes the stromal profile somewhat concave and smaller than the nominal diameter (Figure 3). The exact difference depends on several factors and each surgeon must estimate it based on its results – variation of curvature values ​​(K) before and after –. This difference should be added to the diameter of the endothelial trephine relative to the epithelial one if it is desired that the graft and window are equal, or to the real difference desired between donor and recipient sizes.

A graft somewhat larger than the window helps to obtain a better coaptation, which requires less tension in the sutures. It also reduces the initial narrowing of the AC and the angle – problematic in phakic eyes, especially if the AC was already somewhat narrow –. By removing the sutures, the depth of AC and the corneal curvature will increase. This can be used in cases with malformation of the anterior segment or to correct hyperopia. However, above a certain value (above real 0.75 mm), grafts larger than the window tend to present folds in the deep stroma and may cause steps in the area of the joint.

A graft somewhat smaller than the window will result in a flatter cornea, which in theory may be useful for correcting myopia. However, coaptation will be more difficult, it will need more tension in the sutures and there will be more tendency to induce irregularity and astigmatism. AC will also narrow more from the initial postoperative period. In keratoconus, although there are cases with associated axial myopia, others are actually axial hyperopic. Inducing in these a flattened cornea can result in a refractive surprise.

Types of trephines and their action

The trephine is the most specific instrument of the PK, so its choice is of paramount importance. The result will depend on the quality of trepanation. And unlike the sutures that can be manipulated, removed or changed, it is not possible to correct the trepanation once it has been made. For that reason, we dedicate the following chapters to this besides this one.

The simplest type of corneal trephine is an open metal cylinder, sharpened at one end. Although effective, an open trephine always leads to truncated conical cuts, since some pressure is unavoidable, and this causes the cornea to protrude and tend to "get inside" the cylinder while trepanning (Figure 4A). Also, as soon as the eye globe is penetrated, this decompresses, and the cornea deforms: if we continue, the cut will be irregular. To minimize these problems, a shutter or plunger was added inside, such as in Franceschetti's classic trephine, which also allows to regulate the depth of the cut (Figure 5).

The trepanation must be well centered and perpendicular to the frontal plane (of the iris). Any inclination will result in an asymmetrically beveled cut. It is not easy to keep the axis always perpendicular during manual trepanation. To release the surgeon from the torsion movements and allow him to concentrate on maintaining perpendicularity, motorized trephines such as Barraquer-Mateus (Grieshaber) – variant of Franceschetti’s –, H. Geuder (open) or Asmotom ATS were developed. In our experience, the first one is very effective to obtain a very regular cylindrical section with little bevel, and to penetrate in a controlled and simultaneous way in a wide sector, almost always 360° in donors. By applying an alternating rotation, it is possible to perform complete penetrations without damaging the iris also in a part of the receptors.

Also trephines with pneumatic fixation have been devised, as the one by Hessburg-Barron (Katena), the one by Hanna (Moria), the Krumeich GTS or the aforementioned Asmotom. This fixation allows to maintain the position of the trephine and to ensure the perpendicularity throughout the cut, even though they are manual. However, if they are of the open type (like the first of this group), it is essential to stop the trepanation as soon as the globe is penetrated or before, since the fixation generates an increase in the IOP and the decompression when penetrating can be violent. The models with internal suction and shutter (Hanna, Krumeich, Asmotom) avoid this problem, although they are more complex. As the trephine becomes bulkier, visibility decreases, which can hinder correct centering.

The introduction of lasers capable of accurately cutting and minimizing collateral damage to the corneal tissue opened the possibility of creating a "perfect" and consistent trepanation between the graft and the window, since the pressures inherent to the mechanical cutting would be totally avoided (see chapter 3.2.4). However, there are still issues such as the need for good central positioning and the consequences of the method of coupling the laser system to the cornea. The applanation usually required gives rise, in advanced keratoconus, to a deformation such that a perfectly circular cut generates an oval window.

When trepanation occurs by the endothelial side, the action is usually like that of a die, by pressure. Although it is easier to avoid inclination and bevels by means of guides, die-cutting produces some compression of the tissue and this causes the concave section profile (Figure 3). The edge is here as important or more than in the carving on the anterior side. A badly sharpened punch will cause more compression of the tissue before it is cut: the cornea becomes taut and tends to separate from the base, which can generate an oval shape or irregularity of the cut. To avoid this, some models have pneumatic fixation in the base, although this can cause, in turn, deformations of another type. The trephined graft is left protected by viscoelastic on both sides.

The conclusion of the cut

Since in most cases the trephine does not penetrate 360°, complete separation of the corneal button requires another instrument. This final cut should be as clean as possible and continue the same trepanation plan without creating bevels or leaving tabs. To this end, various scissors have been designed (Castroviejo, Katzin-Troutman, etc.) that combine an adequate curvature with a vertical cut. We can also use a scalpel like Eduardo Alfonso's diamond (Rumex), with the flat tip and a short edge in a lateral notch (Figure 6). Both types of instruments work best when the residual tissue to be cut is very thin, although in case of a thick bed, it is easier to follow the plane with the scissors than with the scalpel. If some deep stromal tab remains, we will cut it with fine scissors (Vannas, etc.).

ADAPTATION OF THE GRAFT TO THE WINDOW

After the trephination, first of the graft and then of the window (as well as the possible manipulation of intraocular tissues) we will bring that to this one. The AC can be filled with viscoelastic – without excess – to protect the endothelium, in which case it is necessary to remove it at the end. If there are certain bevels, the graft can be rotated until they match the window as well as possible. The keratoscope can be helpful here, although with the graft unfixed it is difficult to assess the reflexes. It can also be rotated to leave a sector of donor gerontoxon in the upper area, so that it will be cover by the eyelid.

THE SUTURE

Many surgeons consider the suture a somewhat tedious phase of PK. It is, however, of crucial importance to achieve an optimal result, especially with regard to astigmatism and to avoid certain complications. We can consider several relevant aspects of the sutures: the materials, their general configuration and their position, length, depth and tension.

Suture materials

Since its introduction more than half a century ago, monofilament polyamide (nylon) of size 10-0 has become the standard for the suture of keratoplasty. It is a thin but resistant and elastic material, which causes little tissue reaction and has a prolonged duration. However, polyamide, due to its chemical similarity with proteins, is attacked by tissue proteases (especially in the epithelial plane), degrades and ends up breaking from the 2nd or 3rd year. This has special relevance when trying to control the astigmatism by means of the selective withdrawal of the sutures. To be effective, it is important that we can leave some sutures permanently if necessary. If they end up breaking, the entire strategy of correction – often the result of a long and laborious process – is disrupted.

Among the alternative materials to the nylon stand out the polypropylene monofilament (Prolene) and the one of polyester (Mersilene). The first is not degraded by enzymes but by UVA rays. In addition, in the available 10-0 gauge it is stiffer than nylon, which makes it difficult to control its tension. The polyester cannot really be resorbed and is therefore suitable for selective removal. The 10-0 is, like polypropylene, a bit too stiff, but the 11-0 has strength and rigidity similar to the 10-0 nylon. The thickness of 11-0 is about half that of 10-0, and therefore the knots are easier to extract, having a volume 8 times (2³) less. It produces as little or less tissue reaction than nylon, although it is somewhat less extensible and not at all hydrophilic. The latter prevents the annoying soapy films in the presence of viscoelastic during suturing. The main drawback of polyester 11-0 is its fineness, which hinders its visualization and makes it more brittle if the instruments have aggressive edges. This explains why a broken suture may appear in the first days of the postoperative period, but beyond this phase this material remains unchanged for many years.

General configuration of the suture

There are two fundamental types of suture for a keratoplasty: continuous and independent points, which can also be combined. The first has the advantage of a certain redistribution of tensions, although this only acts locally. Its simplicity and needing a single knot make it attractive. The second represent a more prolonged task since they have to be knotted one by one (that is why we avoid calling them "single points"), but they can be removed selectively and, in case one of them loosens, the effect will be only local. In a continuous suture, on the other hand, it will suffice for one handle to break or give way so that all of it loosens. It must therefore be avoided in all situations with greater risk of this happening: inflamed eyes, vascularization, children, etc.

Before placing the final suture, it is necessary to fix the graft a minimum. The first points (3 to 8, typically 4 cardinal) are the most difficult and are usually not very correct, so it is preferable that they are provisional. They can be of the material that is most comfortable for us to handle and, if it is different from the definitive one (personally we continue using 9-0 silk) it will be more difficult to forget their withdrawal at the end of the intervention.

Position of sutures

In order to generate a regular distribution of tension, classically it is recommended that the sutures be evenly spaced and radial. This is evident in case of independent sutures. But in a continuous suture, if the passage through the stroma is radial, the superficial loops will not be. A torsion vector is thus generated that can deform the tissue. To avoid this, we can place two radial continuous sutures of opposite direction, so that their torsional and anti-torsional effects cancel each other out. Another option is a neutral suture (atorsional), which obviates this effect by means of a non-radial passage through the stroma, with the same angle (opposite) as that of the superficial portion, so that the loops remain isosceles (Figure 7). However, it is not easy to choose the exact step angle at each position of the perimeter to obtain this configuration without the help of markers. In any case, the effect of torsion with a radial continuous suture decreases as the number of handles increases, so its relevance with the usual patterns of 16 or more turns is questionable.

Some surgeons prefer to combine the continuous suture with independent ones, in an attempt to "take the best of each world." This allows, for example, a selective removal of the radial points to control the astigmatism relatively early and leave the continuous one as safety until the scar consolidates. But nothing ensures that, when finally removing it, there will not be a new astigmatic change.

Length of the sutures

The distance of the sutures to the incision must be sufficient. If it is too short, there will be little tissue inside the handle and it may loosen more easily. If it is too long, it will create more initial compression and will loosen more when the tissue is adapted. It is not easy to define precisely what "sufficient" is; it will depend on multiple factors such as the age of the patient and the pathology, the type and material of the suture, etc., apart from the preferences of the surgeon. Typically, they tend to be about 1 mm from the cut on the inside and some more (1-2 mm) from the outside, often reaching the limbus. In any case, the length of all the sutures or handles should be as homogeneous as possible.

Depth of the sutures

To ensure a good union of the donor and recipient tissues, it is crucial that the sutures are sufficiently deep. This is a difficult aspect to judge from the overhead point of view of the surgical microscope. Having a slit lamp is very useful in this regard, almost essential. It also helps to use double prey clamps like those of Polack (video 3.2.1.1) that stabilize the tissue and allow a better visualization of the point of passage of the needle.

A superficial suture will tend to create an incomplete union of the posterior layers, so that at the end there is a scar ajar on the back, with a lambda profile (λ). It is enough that this happens in one or a few sutures or contiguous handles to generate a high astigmatism. It is therefore important to take care of the passage of each one of them. When the suture remains superficial in a generalized manner, the graft may at first be flattened by the tension of the suture, but when it is removed it will end up with very high K values (global ectasia), apart from astigmatism.

To avoid subsequent backward opening, Troutman proposed penetrating sutures (through & through) and designed "hooked" compound curvature needles. These allow deep and short passes, since if the intraocular portion of a penetrating suture is long, it can also cause a later deformation in a tent shape. The worst risk of a penetrating suture is that it creates a fistula, with epithelial invasion in the form of cysts or flat epithelization of the AC – a complication that is currently very rare but potentially fatal to the eye (see chapters 3.4.2 and 3.7.2). It was more known at the time of thicker sutures and necrotizing materials such as silk. For that reason, the classic manuals recommended passing the sutures through the middle third of the stroma and thus leaving a posterior margin of security. With monofilaments this is not necessary, and it is preferable to go through the deep third, ideally in a predescemetic plane (Figure 8).

Tension of the sutures and astigmatism control

The sutures of a PK must have the minimum tension necessary to ensure the tightness of the eye globe from the immediate postoperative period. Excessive tension produces a local deformation of the tissue and a more marked flattening of the graft and AC. Although transient, this can cause complications, from areas of desiccation or dellen – which if left unattended can leave a leucoma or cause perforation –, to anterior synechiae or even angle closure. On the other hand, it is convenient to leave a certain margin of tension, since this will yield as the encompassed tissue adapts in the postoperative period.

The initial astigmatism – and possibly also a good part of the end – will depend on how the distribution of tensions is. The one of each independent sutures can be adjusted before locking it by means of a sliding knot; that of the continuous suture can be redistributed at the end (or in the postoperative period) by running the handles with a "carousel" maneuver. In general, a freshly placed continuous suture will remain a little loose: once pre-knotted (with a single triple loop) it is necessary to tighten it, handle the loop around the perimeter, until the excess suture returns to the knot area, adjusts definitively and locks with two more simple loops. Strampelli devised an elegant tensing maneuver using two dilators of lacrimal punctum (Figure 9). This avoids traumatizing the suture as it can occur with the aggressive edges of tweezers. Once the knot of the continuous one is closed, the maneuver is repeated in the sector near the knot to bury it and, in the process, adjust the astigmatism.

To be able to control these adjustments it is essential to have a means of intraoperative astigmatism evaluation. Attempts to develop a surgical keratometer have encountered the difficulty of obtaining, in a freshly sutured graft, an image of sufficient quality for the quantitative measurement. However, the simple reflection of one or more concentric circles (keratoscope), even when viewed momentarily after washing the surface of the graft (often dry and irregular), allows the trained eye of the surgeon to estimate the astigmatism approximately. Although there are several hand models, the surgeon will have both of them occupied and it is more efficient to mount a fixed and luminous keratoscope on the microscope objective (Figure 10) – something simple to manufacture with LEDs for any moderately skilled mechanic. The patient must be placed in such a way that the frontal (iris) and the objective planes are parallel, since any inclination generates oval reflexes that distort the assessment. By adjusting the sutures until the reflection looks circular, we will have reached a minimum astigmatism. With experience, the detection limit is about 1-2 D.

Finally, it is necessary to bury the knots of the sutures. We think it preferable to do it on the graft side, since the further away from the limbus, the less inflammatory and cicatricial reaction, and the easier the withdrawal will be. The latter makes it convenient to leave them relatively superficial (in a sub-Bowman plane). The tightness of the suture is checked at the end by giving tone to the eye with balanced saline solution (BSS) or antibiotic, after having removed the viscoelastic if it was put on. The IOP can be checked with a Maklakov-Barraquer applanation tonometer calibrated for 20-30 mmHg (Ocular Instruments), leaving the IOP somewhat below (outside) the 20-mmHg mark.

HANDLING IRIS AND OTHER INTRAOCULAR TISSUES

Although trepanation and suture form the core of the PK technique, it will often include some manipulation of the intraocular tissues. Depending on the case, the surgery may be combined with cataract, explant or IOL exchange, glaucoma, other reconstructive techniques or that affect the vitreous-retina, which may even require the participation of several teams of surgeons. Although these issues are dealt with in specific chapters, there are in any case certain common considerations, especially concerning the iris and the pupil.

The application of a miotic in the preoperative period (pilocarpine eye drops 2%, 1 hour before) is routine to protect the lens in phakic patients. When it is necessary to induce miosis during the surgical procedure, we use 1% acetylcholine without preservatives. This happens, for example, if the pupil has dilated excessively with peri/retrobulbar anesthesia, to tighten the iris well before performing an iridotomy, or before trepanning if a combined cataract surgery has been performed previously.

The practice of one or more peripheral iridotomies is a classic step in PK. Its purpose is to avoid pupillary block and its consequence: Urrets-Zavalía syndrome – a paralytic mydriasis due to atrophy of the iris secondary to ischemia due to acute ocular hypertension in the early postoperative period. In that period, a certain eye pain may not wake up the alarm and therefore neglecting the IOP measurement. The blockage may be due to viscoelastic remnants or air bubbles, fibrin formation, narrowing of the anterior chamber by a tight suture, even a special laxity of the iris has been invoked in patients with keratoconus, etc. Given that this risk is still present despite the latest advances and due to the simplicity and harmlessness of iridotomies, its practice is still recommended in all phakic PK cases, as well as aphakic cases with the possibility of vitreous access to the pupillary area (Figure 11). In the posterior chamber pseudophakic with integrity of the zonula-capsular diaphragm they would not be essential, although they can provide greater security. In situations where there is a risk of peripheral anterior synechiae – penetrating injuries that reach the angle or after explant of angular support IOL – it is useful to perform multiple iridotomies to "isolate" the problem areas.

To perform open-air iridotomies, different types of scissors can be used (De Wecker-Barraquer, Vannas, etc.). The size should be the minimum necessary to ensure its permeability and it is preferable to place them in the upper quadrant, so that the eyelid covers them to avoid dysphotopic phenomena. Usually we do two, towards 10:30-11 h. and 1-1:30 h. To be well peripheral, the iris must have adequate tension (in miosis) and it is useful to press the ciliary groove with the ajar branches of the scissors before cutting, which also moves them away from the lens. A small wash with BSS, with a 27 G cannula, allows to check their permeability.

Although corneal transplant techniques have undergone a profound transformation in recent decades, many still use a trephine. Achieving an excellent visual rehabilitation through a cornea transplant requires using the proper cutting instruments both in donor and recipient and know the principles that guide its operation. The choice of the most suitable trephine can depend on what has been learned, practiced and available in our environment; but the objective will always be to achieve a well centered circular opening, with minimal tissue distortion and endothelial damage.

The use of trephines for therapeutic purposes dates back at least to more than 600 years¹. Trepanations have been observed in prehistoric skulls – with subsequent growth of the edge, indicative of survival – and are collected by paintings such as "The Surgeon" by Jan Sanders van Hemessen (1500-1575). The history of keratoplasty, linked to the development and improvement of corneal trephines, has been the subject of another chapter (see 1.3).

CHARACTERISTICS AND PRINCIPLES OF ACTION OF THE TREPHINES

By trephine is meant in surgery a cylindrical instrument with a sharp edge on one of its edges, capable of practicing a circular and uniform cut by rotation on the tissue. Of its characteristics such as the type of cut, material, fixing and drive, etc. (table 1) its effectiveness, reproducibility and safety will be derived before a maneuver that requires high precision such as trepanation for keratoplasty.

The ability to section a cornea depends on the strength of the tissue, the edge of the instrument and its angle of attack, and the applied and intraocular pressures. The mechanical properties of the cornea vary according to the individuals, the different pathologies and the cicatricial processes. An excellent cutting edge in pressurized globes may be insufficient in cases of hypotonia. The precision of the cut is also lost in oblique approaches. All this generates a remarkable variability in the quality of the cut – although in any case superior to what can be obtained with scalpels or scissors – and is a challenge for the design of the ideal trephine.

The desirable characteristics in a trephine include, among others: 1) Possibility of precise and reproducible centering; 2) Adequate visualization of the trepanation zone, the limbus and the central pupillary area; 3) Perpendicular execution of the cut; and 4) Perfect fixation of the eye globe during the entire procedure. Among the aspects of the trephine that can condition the characteristics of the cut we find²:

Type of cut or blade. It can be continuous, with a cylindrical blade, or with a single cutting point (Figure 1). The first one is usually made of steel and cuts evenly on the 360° edge. Its manual application generally requires an alternating action. The blade in the second one can be of steel or diamond – like Lieberman's – and moves along a conical support element. Some design has two opposing cut points, such as the Arcitome of Hanna (Moria) for arcuate incisions. With the cylindrical blades, the lateral deviation is unlikely once the cut has started, because it covers 360° and is relatively uniform. With the punctual ones, lateral deviations can occur during cutting due to changes in the pressure exerted by the surgeon and differences in tissue strength. With both systems, the angle of attack must be perpendicular to the corneal surface.

Seal and handle. The trephines may have the hollow center (bore) or be sealed by a plunger. The first allow observing the cornea for centering and as the trephine advances. The second do not allow this display but the position of the plunger can be adjusted to a certain depth of cut. In both cases, a handle is needed for each trephine diameter. Although universal handles have been designed to exchange the blades, with them the visualization is lost, and they do not protect from a possible precipitate penetration in the anterior chamber.

Type of fixation. The presence of a pneumatic fixation system facilitates the quality of the cut and its perpendicular orientation – it reduces the risk of oblique cutting. In general, these systems consist of an external suction ring and an internal ring for cutting. The vacuum applied must be uniform, which may be conditioned by irregularities in the cornea or conjunctiva in the contact area. It is important to ensure correct centering before setting the fixation.

Control depth of the cut. It can be obtained by: a) A mechanical stop in the lumen of the trephine. This can hinder visualization – if it is opaque – and limit penetration of prominent corneas as in advanced keratoconus. b) Progressive advance of the blade by thread or similar device.

CLASSIC TREPHINES

Among the classic models of manual trepanning we highlight those described by Steinberg (1843), von Hippel (1891) (see chapter 1.3), Castroviejo (1934) or Arruga (1939). This type of circular trepan could be used freehand, although in general they were mounted on a handle. To limit the penetration depth, they often incorporated a center piece or plunger. The one of Franceschetti (1950) allows to regulate it to a predetermined value, and Barraquer and Mateus developed in 1970 with Grieshaber a motorized version of the same one. However, the importance of the central visualization – which the piston prevents – explains the posterior tendency to the hollow models or with transparent shutter.

The motorized trephines facilitate a more regular and predictable cut than with a manual one, since these are more unstable and require alternating rotation actions. The motorization frees the surgeon from having to rotate the trephine and allows him to concentrate on centering and perpendicularity. However, with the introduction of the pneumatic fixation, very regular and reproducible cuts can be achieved despite a manual action, more or less mechanized.

THE ERGONOMIC MARKING TREPHINE HANDLE

One of the first designs to solve the problem of maintaining the central visibility with the handle for interchangeable trephines of different diameters was the one presented in 1998 by Duronio, O Brien and Gottsch³, which they named Ergonomic Marking Trephine Handle (EMTH) (Figure 2). It is a reusable instrument that combines marker and trephine. It consists of an external cylindrical piece, with distal surface to mark and attached to the handle, and another internal (core) that is attached to the different trephines, provided with a crosshair, rotating spokes and that is mounted to the first by a thread. The surgeon can hold it by the handle from the nasal or temporal side and position it on the cornea without obstructing the central vision area, while the other hand rotates the spokes of the piece attached to the trephine. The distal part of the external piece has eight sharp edges spaced 45°, which mark the cornea to the limbus to guide the suture and leave spaces that allow the surgeon to see the cornea during trepanation. The EMTH is designed to work with single-use trephine blades (Storz E3050), but only accepts those <8 mm. The depth of cut is controlled by the number of turns, with a design that advocates that of Hessburg-Barron.

MANUAL TREPHINES WITH PNEUMATIC FIXATION

The pneumatic fixation of a trephine, apart from obtaining its stability without depending on the constant attention of the surgeon, improves the quality of the cut by increasing the intraocular pressure. The possibility of tilt of the trephine is reduced and the cuts with inappropriate morphologies, the superficial or too deep cuts are avoided. Among its drawbacks we can mention its larger size and therefore some discomfort of use, and the need to ensure a good centering from its application. Those of Hessburg-Barron, Hanna, Krumeich and Lieberman stand out.

Hessburg-Barron’s trephine

The Hessburg-Barron trephine (Katena) is perhaps the most widely used today (Figure 3). It consists on its external part of a suction chamber attached to a 6 ml syringe with spring on the handle of the plunger to create negative pressure. The assembly is thus fixed to the peripheral cornea, which allows stable trepanation. The internal part is the trephine blade, which advances by manual thread at a rate of 250 μm depth for each rotation of 360°. This allows its use in both penetrating and anterior lamellar keratoplasty. It is available in sizes from 6 to 9 mm, at 0.5 mm intervals. The most recent models have 16 marking points spaced evenly in the suction ring, to assist in the placement of the sutures. This system can be especially useful in perforated or hypotonic eyes, in which it is very difficult to tread freehand.

Hanna's trephine

The trephine designed by Khalil Hanna (Moria) «for corneal microsurgery»⁴ has a characteristic truncated cone shape similar to a Goldmann lens (Figure 4). It integrates an external chamber of 360° for the pneumatic fixation, and in its distal part eight suction holes are opened thanks to which it is fixed to the limbus. Inside, a single-use steel blade is mounted, available in sizes from 7 to 11 mm, which rotates and advances thanks to a mechanism that is operated from a crown disposed obliquely from the upper edge. For each rotation of the crown, the blade descends 88 μm. You can preset the depth you want to reach. To ensure centering, the center of the cornea is marked and coincided with the central opening of the trephine, which bears a transparent obturator.

Krumeich’s trephine

The Guided Trephine System (GTS) designed by Jorg Krumeich⁵ allows to perform with great precision cuts for penetrating or anterior lamellar keratoplasty or circular keratotomy. It consists of a series of concentric cylindrical pieces containing the blade and independent mechanisms for rotation and advance by thread. This complex is assembled by means of a guide-rail system on a Barraquer microkeratome suction ring, which is fixed to the limbus of the recipient, or to a special artificial anterior chamber to carve the donor (Figure 5).

Centering is achieved through the suction ring. If the center of the receiving cornea is previously marked, it is possible to check the centering through a central hole which, like in Hanna's trepan, has a transparent obturator. Among its advantages we can highlight: a) It allows trepanning both the donor and the recipient cornea on the epithelial side; b) It allows to predefine the depth of cut, especially useful for lamellar techniques; and c) It allows, in expert hands, the complete trepanation in 360° without needing scissors or other instruments. Among its disadvantages: its size, which makes it difficult to use in eyes with narrow palpebral opening or marked enophthalmos; the limbal suction ring, which may not achieve fixation in the presence of filtering blebs or chemosis (see chapter 3.2.3).

Lieberman's trephine

The Lieberman⁶ single point cutter trephine is constructed with cones that rotate one above the other, with a wide central opening that allows centering control. The external cone consists of an upper wheel – which the surgeon holds with his non-dominant hand – attached to a lower wheel that will be in contact with the eye and contains the suction device (10-15 mmHg). The inner cone rotates and acts as a carrier for a sliding mechanism together with a single-point, single-use cutting point blade. By means of an upper crown, the blade rotates and lowers a few μm for each round (Figure 6). The openings allow to visualize under the microscope both sides of the incision, whose diameter can be adjusted. It includes two cutting systems: one that orients the blade vertically for penetrating keratoplasty and another that tilts it 20° for lamellar grafts.

COMPARISON AMONG MANUAL TREPHINES WITH PNEUMATIC FIXATION

Although the differences between these trephines (table 2) could be reflected in the results obtained with each of them, there is little literature on the subject. For Moshirfar et al.⁷, Hanna's trephine would be more accurate and consistent than the Hessburg-Barron one on the quality of the cut in cadaver eyes, by means of optical coherence tomography and histology. However, Wifen et al.⁸, did not observe differences in the keratometric results of the keratoplasties performed with these two trephines and the same suture technique.

Although penetrating keratoplasty (PK) has a high success rate, a high final residual astigmatism may result in a limited visual result. The use of more advanced trephines such as the Guided Trephine System (GTS), manual, or the motorized Asmotom ATS, as well as other techniques that we will describe, allow us to address this complex problem and achieve a lower and regular postoperative astigmatism.

ORIGIN OF THE DISPARITY IN THE TREPANATION OF THE RECIPIENT AND THE DONOR

Usually, the donor cornea is cut from the endothelial side with a trephine-die (punch) (Figure 1b). The compression that thus suffers flattens it against the trephination block and the diameter of the endothelial cut is slightly inferior to the epithelial. When trepanation is performed from the epithelial side (Figure 1a), the opposite happens: the diameter of the endothelial cut is somewhat greater than the epithelial one. This phenomenon is common to all trephines and more accentuated if there is no obturator in its bore that maintains the shape of the cornea. To compensate, it is advisable to use for the donor a nominal diameter of the die at least 0.2 mm greater than that used in the receiver.

The trepanation systems with pneumatic fixation allow a more vertical cut and avoid the inclination of the blade. Mathematical estimates indicate that a 10-degree trephine inclination can induce 6 or more diopters (D) of astigmatism. Especially if combined with an endothelial die in the donor, the use of a manual trephine in the receiver induces an average astigmatism of 5 D and, in our opinion, its use as a routine instrument for a PK should be avoided.

THE KRUMEICH GTS GUIDED TREPHINE SYSTEM

The first prototype of the GTS¹ system was designed by Jorg Krumeich towards 1988. Unlike other designs, the GTS allows to make cuts of equal diameter in the donor and in the receiver with the same trephine, since the size is exactly perpendicular thanks to an external guide which is attached to a suction ring at -800 mbar (600 mmHg vacuum) on the patient's eye, without increasing the intraocular pressure (Figure 2). The blades are for single use and diameters between 7 and 8 mm (Figure 3a). A glass obturator inside helps maintain the shape of the cornea during the cut, allowing it to be trepanned completely from the epithelial side without the need for scissors (Figure 3d).

To cut the donor tissue, the corneoscleral segment is placed in an artificial anterior chamber pressurized by infusion at about 18-28 mmHg depending on the height of the bottle (Figure 4a). The GTS trephine is assembled on the artificial anterior chamber, which has guides similar to those of the suction ring for the receiver (Figure 4b). The trepanation of the donor, on the epithelial side, uses a blade of the same diameter as in the receiver. In this way the geometries of both will be identical.

This explains that, in our series of more than 2,500 cases of PK with the GTS – excluding those that presented extreme anomalies of corneal curvature and/or thickness –, the average astigmatism was between 2 and 4 D. The GTS system can be used also in lamellar keratoplasty and to perform circular keratotomy for PK astigmatism. Krumeich et al.² compared the refractive results in PK and deep anterior lamellar keratoplasty (DALK) with the GTS and obtained highly satisfactory results. We think it is an ideal instrument for corneal surgeons. The GTS system is also useful in surgery combined with cataract and/or vitrectomy procedures. The use of a temporary keratoprosthesis in these cases avoids the risks of open surgery³.

DEUTSCHMANN’S MOTORIZED TREPHINE (Asmotom ATS)

A motorized version of the GTS system was developed by the Deutschmann group around 1991 and is known as Asmotom ATS⁴, although it is not commercially available at present. The trephine has two suction systems: in the central concave piece that is applied to the cornea and in a peripheral ring to the blade, which is mounted on the same handpiece – unlike the GTS, there is no independent suction ring for the receiver – (Figure 5). The depth of trepanation can be adjusted with the interposition of gauge rings of different thicknesses. The Asmotom allows the complete 360° cutting – without need of scissors – of both the donor and the receiver per epithelial side, and without requiring an artificial anterior chamber or additional instrumentation (video 3.2.3.1). Although this makes identical trepanations possible, Deutschmann recommended oversizing the donor 0.2 mm, something that seems inexplicable to us. The system has blades of different diameters as required by the present pathology.

THE CONTINUOUS ATORSIONAL SUTURE

In addition to the quality of trepanation, the suture technique is very important to reduce astigmatism in the initial postoperative period. In our experience, continuous suture has been shown to be superior to independent sutures. To obtain continuous sutures that do not cause torsion of the tissue (atorsional), we have designed a special 8-point star corneal marker⁵. Thus, the surgeon can suture following exactly the mark and perform a double continuous suture with a total of 16 isosceles loops (Figure 6). The adjustment of the tension of these sutures is always carried out under the control of the operative keratoscope.

THE INTRACORNEAL METALLIC RING OF KRUMEICH

In patients with peripheral stromal scars or thin corneas, the Krumeich metallic ring can be used to stabilize the graft-receptor junction and avoid the appearance of high astigmatism⁶. Some authors have used it successfully in the prevention of neovascularization and in the reduction of the immune reaction in high-risk keratoplasty⁷. The reinforcement of the scar bed with the Krumeich ring has proved especially useful in post-traumatic cases (Figure 7).

The advantages that the femtosecond laser (LFs) can provide for corneal transplant surgery include: (1) more precise and regular cuts; (2) absence of damage of intraocular structures; (3) less damage of the donor endothelium; (4) customization of the cutting pattern; (5) perfect graft-recipient adaptation; (6) greater contact surface between donor and recipient, which promotes better and faster healing of the interface; (7) stronger and more stable surgical wound due to greater activation of the keratocytes, which allows an earlier removal of suture; (8) less induced astigmatism, and (9) reduction of visual rehabilitation time¹.

Currently the use of LFs not only has a place within the field of penetrating keratoplasty (PK), but has also been extensively tested in lamellar transplants, both anterior and endothelial.

TYPES OF CUTS AND PROFILES OF DISSECTION OF THE CORNEAL TISSUE

The different complex profiles of carving or dissection of the corneal tissue are generated by the combination of several types of simple incisions (Figure 1):

- Posterior side cut. Incision or plane of vertical dissection, of cylindrical morphology with a determined diameter, which extends from the posterior side of the cornea towards the surface until coinciding with a more anterior lamellar cut.

- Anterior side cut. An incision analogous to the previous one, but which starts from the anterior side of the cornea towards the deep stroma, until it coincides with a more posterior lamellar cut.

- Ring lamellar cut. Laminar incision parallel to the corneal surface (plane) with an external and internal diameter, whose edges must coincide with the anterior and posterior side cuts.

- Full lamellar cut. Equivalent to a ring lamellar incision whose internal diameter is equal to zero. It must coincide with the side cuts only in their external diameter.

The computer software of the laser – in particular that of the Intralase system (AMO) – makes possible the free combination of these incisions so that the surgeon can personalize the design of the graft and the recipient bed, and thus optimize the clinical results of each case. The designs with stepped incisions provide a greater donor-recipient contact surface, avoid postoperative leaks and accelerate healing, as was already demonstrated by José Barraquer² and later by Busin³. The most commonly used complex cut patterns or profiles are:

- Top hat profile (Figure 2). Composed of three simple incisions (posterior side of greater diameter, ring lamellar and anterior side of smaller diameter) that imitate the form of said hat. This architecture maximizes the posterior tissue to be transplanted, preserving the anterior cornea. Therefore, it is suitable for diseases that mainly affect the posterior cornea: posterior and endothelial stromal dystrophies, and other causes of advanced edematous (bullous) keratopathy when endothelial keratoplasty is contraindicated.

- Mushroom profile. It is the inverted version of the top hat (Figure 3), composed of a narrower posterior side cut than the anterior one, both connected through a ring lamellar one. Unlike the previous one, it maximizes the anterior tissue to be transplanted and preserves the posterior cornea more. Therefore, it is more convenient for diseases that mainly affect the anterior corneal stroma: keratoconus and other ectasias, corneal stromal dystrophies, central leucoma, etc. In cases where the recipient’s endothelium and Descemet’s membrane (DM) are intact, the addition to this profile of a complete lamellar cut together with a non-penetrating posterior side cut, allows performing deep anterior lamellar keratoplasty techniques, since it facilitates the visualization and direct access to the posterior stroma for performing pneumo-dissection techniques.

- Zig-zag profile. It consists of an anterior side cut and a posterior side cut of a different diameter, arranged obliquely to the corneal surface (usually 45°) (Figure 4). Both are connected by a lamellar ring cut as in the previous types. The diameter of the side cuts can be modified as desired to preserve more anterior or posterior cornea. Zig-zag morphology increases the area of donor-recipient contact and by avoiding vertical incisions it could offer better biomechanical properties, as well as stronger surgical scars⁴.

- Anvil profile. It results from combining the zig-zag and mushroom profiles⁶. It consists of an oblique anterior side cut (45°) with a larger diameter at the back, a ring lamellar incision and a vertical posterior side cut (Figure 5). Its objective is to increase the contact surface while preserving as much posterior tissue as possible.

BASIC CONCEPTS

Regardless of the favorite or optimal profile for each case, it is important to understand a series of basic concepts when designing:

- Overlapping incisions. For the formation of a complex cut profile it is necessary that the simple incisions that form it overlap or communicate, in order to obtain clear and continuous edges that allow to dissect and extract the tissue with ease. For this, the side and lamellar cuts must cross between 10 and 30 μm depending on the degree of edema or opacity of the receiving cornea and the desired treatment time.

- Depth of lamellar cutting. As a general rule, the lamellar cut is established at a depth of 50% of the corneal thickness, both in the donor and in the receiver in case of normal corneal hydration, thus creating two cylinders (anterior and posterior) of the same height.

- Oversizing the graft. Manual trepanning of the recipient cornea from the epithelial side and of the donor from the endothelial side usually leaves a donor-recipient size disparity that requires oversizing the donor cornea by 0.25 mm to compensate for it. On the other hand, the LFs creates trepanation of identical diameter in both corneas, independently of the keratometry and therefore it is recommended to use the same diameter. This improves the adaptation of the edges and the visual result^4,6. The surgeon may choose to oversize the graft to manipulate the refraction or depth of the anterior chamber (AC).

- Deeper rear point. In cases of PK, the posterior depth of the cut in the donor cornea should be set below the highest pachymetric value in the area of the incision, so as to ensure a clean cut and full thickness. For the receiving cornea, it must be considered, however, that the patient should normally be transferred from the laser room to the operating room. Therefore, it is recommended to avoid penetrating incisions, to avoid leaks and a flattening of the AC during the transfer. Therefore, the posterior depth of the cut in the receiving cornea is generally established 70 μm above the lowest pachymetric value in the area of the incision⁷.

PREPARATION OF THE FABRICS WITH THE FEMTOSECOND LASER

Trepanation of the donor cornea. It requires an artificial AC that fixes the cornea in an appropriate position and generates a pressure similar to that of an actual patient during a LASIK-like procedure. To obtain a homogeneous compression of the tissue, bubbles under the cornea should be avoided during this dissection (Figure 6).

Trepanation of the receiving cornea. In most cases it is carried out in a room different from the rest of the surgical procedure. To prevent leakage of aqueous humor during transport between both rooms, it is necessary to leave a safety band of non-dissected posterior tissue^7,8.

Liberation of tissues. With the mushroom configuration as an example, with a Sinskey’s hook or a similar blunt instrument the three planes present in this profile are dissected. First, an entrance is made perpendicular to the corneal surface to access the anterior side cut and dissect the anterior cylinder completely. Then the tip of the hook is twisted to find the lamellar cut and dissect it 360°, before proceeding with the separation of the posterior side cut. This is done in a similar way in the donor and receiver, with the difference that in this the cut will not be full thickness and will have to be completed with a knife and curved scissors following the back circumference of the profile, being careful not to damage their edges (video 3.2.4.1).

PENETRATING KERATOPLASTY ASSISTED BY FEMTOSECOND LASER

The cutting profiles that the LFs allow to obtain make the donor and recipient corneas adapt with micrometric precision. This can improve the visual prognosis and shorten the rehabilitation time of these patients (Figure 7)⁹. The first studies that compared PK assisted with LFs (f-PK) with conventional PK showed initial results in favor of the former: earlier visual rehabilitation, better corrected visual acuity (VA) and lower astigmatism, both manifest and topographic^10-13. It was, however, a follow-up of less than one year (even <6 months). Later, in a series of cases performed with a zig-zag profile and a mean follow-up of 2 years, it was observed that the topographic astigmatism in the f-PK group was only inferior during the first 6 months, and the differences were no longer significant after this period¹⁴. Neither could differences in VA be demonstrated at any time during the follow-up, only a selective somewhat earlier removal of the suture in the f-PK group. One of the few studies that compared the results of the different cutting patterns did not find significant differences between the profile in the top hat and the zig-zag profiles¹⁵.

DEEP ANTERIOR LAMELLAR KERATOPLASYT ASSISTED BY FEMTOSECOND LASER

The crucial moment of deep anterior lamellar keratoplasty (DALK) is usually the obtaining of pneumo-dissection (big bubble; BB) (see section 5 of this work). For this, the insertion of the cannula or needle at a suitable depth prior to the injection of air is critical. With the manual trephines the depth of the cut has a high variability, and in many cases it is difficult to ensure the sufficiently deep injection of the air, which leads to the failure of the technique with the consequent extension of the surgical time or conversion to PK. The use of LFs in DALK (f-DALK) partially solves this problem, since it allows to accurately and reliably determine the depth of the cut and therefore insert the needle in the appropriate predescemetic plane that facilitates the formation of the BB^16,17.

Irregularity and variability in corneal thickness in eyes with advanced ectasia or dense stromal scars limits the ability of the laser to produce a uniform deep lamellar plane¹⁸. LFs are used for side trepanation, both in the donor and in the recipient, leaving a minimal residual stroma of security in the latter, and all the more can be used to make a lamellar dissection plane in the middle stroma as it is also done in some manual techniques to facilitate access to deep planes. The mushroom cut profile is our favorite for f-DALK, since it minimizes posterior resection, although others can be used. In the donor cornea, a full thickness trepanation is performed, and the DM-endothelium is removed. The one of the receiver will be non-penetrating, with a depth of the anterior side cut of 60% of the thinnest pachymetric value in that area, and of 80% for the posterior side cut, which are joined by a lamellar ring cut of 1 mm width (Figures 8 and 9) or by the aforementioned medial lamellar plane. The dissection of the recipient DM and endothelium is continued with conventional techniques (BB or others).

The mechanical stability of the mushroom configuration created with LFs has been shown to be superior to traditional cylindrical trepanation¹⁹. Recently it has been proposed to add a tunnel in the posterior area of the cut, to facilitate the insertion of the cannula during the BB technique and improve the success rate, although this has not been confirmed for now.

Patterns of healing of the graft-receptor interphase

In a previous study of our group we established a gradation of scarring observed at the interphase, recorded photographically by illumination with a slit lamp at a 45° angle (table 1). In f-DALK with a mushroom pattern, 52% of the cases show scarring grade 3 or 4²⁰, more intense than with the manual technique. This probably results from the combination of a larger graft-receptor contact area together with an increased activation of stromal keratocytes in relation to the amount of energy of the LFs used.

Visual and refractive results

Like f-PK, f-DALK seems to accelerate visual recovery and shows superior visual results during the immediate postoperative period, but without significant differences after the sixth month²¹. A recent study of our group did not find significant differences in the visual or refractive results obtained through DALK, whether performed with LFs or manually, after one year of follow-up. We observed a faster visual recovery in the group with LFs, where there was also a higher degree of scarring²⁰.

SUPERFICIAL ANTERIOR LAMELLAR KERATOPLASTY ASSISTED BY FEMTOSECOND LASER

Currently superficial anterior lamellar keratoplasty (SALK or ALK) is in some disuse due to the good results of the deep and the effectiveness of the PTK with excimer laser for the most superficial opacities. However, in cases where the latter is not enough but only the anterior stroma is affected, SALK may be an alternative before considering more aggressive surgeries such as DALK. The LFs would be an advance with respect to manual anterior stromal dissection or microkeratome, for its reliability and accuracy²² (see chapter 4.3.5).

ENDOTHELIAL KERATOPLASTY ASSISTED BY FEMTOSECOND LASER

The possibility of using LFs in endothelial keratoplasty is dealt with in another chapter of this work (6.4.3), although its practical use has been almost completely abandoned, because of the superiority of the microkeratome in endothelial lamellar keratoplasty (DSAEK) and because of the expansion of Descemet-endothelial keratoplasty (DMEK), where LFs has no application²³. Only in some special cases LFs has certain advantages: when it is necessary to remove part of the posterior stroma of the receiver, as was done in the old manual techniques of ELK (DLEK of Melles). The dissection with LFs can be superior here, as shown by a case of deep stromal opacity in secondary hypercupremia (Figure 10)²³.

KERATOPROSTHESIS ASSISTED BY FEMTOSECOND LASER

Keratoprosthesis surgery is indicated in cases of bilateral blindness due to severe corneal opacity and with acceptable visual potential, in which PK has a poor prognosis. These techniques are considered as the last step in the treatment of such patients. Its high rate of intra and postoperative complications is associated in part with its penetrating nature, which is avoided with interlamellar type models such as KeraKlear (see section 8). Recently, we have proposed using LFs to dissect a predescemetic bed and thus facilitate the implantation of this keratoprosthesis^24,25 (Figure 11).