Corneal ectasia comprises of a group of conditions including keratoconus, keratoglobus pellucid marginal degeneration, surgically induced thinning and protrusion of corneal tissue, for example, after keratorefractive procedures,1 and corneal graft ectasia.2 Keratoconus usually begins at puberty and progresses till the age of 40 years.3 Keratoglobus can be congenital and can be seen in children.4 Pellucid marginal degeneration usually presents between the second to the fifth decade of life.5 Early diagnosis and treatment helps to preserve and improve vision and prevent loss of functional vision. The greater the delay in the diagnosis, the more chances that the patient will require corneal transplantation surgery.6
Conservative management includes spectacles, contact lenses (soft, rigid gas permeable, hybrid, scleral).7 Various surgical options include collagen cross-linking, intrastromal corneal ring segments (ICRS), photorefractive keratectomy, phakic intraocular lenses, and partial or full thickness corneal transplantation procedures.8–11 Patients with advanced ectasia would require keratoplasty depending upon the depth of corneal scarring. This article reviews the surgical techniques of lamellar keratoplasty with its outcomes in advanced keratoconus.
Penetrating keratoplasty (PK) has been the surgery of choice for keratoconus for over seven decades.12,13 However, anterior lamellar keratoplasty (ALK) which involves the selective removal of corneal stromal tissue is gaining popularity owing to its advantages. In 1886, Von Hippel performed the first lamellar transplant, where a full-thickness rabbit cornea was grafted on the lamellar bed of a human cornea.14 However, the visual acuity remained poor, but with advancements in techniques and innovation in instruments, deep anterior lamellar keratoplasty (DALK) has become an alternative to PK. Its popularity decreased over the years due to interface issues, at the donor-recipient interface resulting in poor visual outcomes.15 In 1965, Brown et al16 found that it was possible to leave only Descemet membrane (DM) after performing a deep lamellar dissection. In 1972, Anwar first described the dissection of host tissue close to DM.17 To avoid an inflammatory reaction and obtain a regular interface, Anwar also removed the DM-endothelium complex from the donor to be transplanted.
The advantages of ALK include lower risk of graft rejection,18 less endothelial cell loss,19,20 better graft survival, and less intraoperative and postoperative complications compared with PK, namely traumatic wound dehiscence, expulsive hemorrhage, and endophthalmitis.21 Early removal of sutures and less chances of secondary cataract formation and glaucoma, due to the early weaning of steroid eye drops are the other advantages of ALK.22 In keratoconus, the thin and steep cornea is replaced with a thicker normal donor tissue to add tectonic strength to the cornea. It also helps to reduce the irregular astigmatism and concomitant high myopia. The stromal tissue could be excised either using automated methods like microkeratome, excimer laser, and femtosecond laser for moderate cases or dissection can be deep with or without baring the DM.23–25
Indications for lamellar keratoplasty in keratoconus have been listed in Table 1. Factors influencing the type of technique for lamellar dissection in keratoconus are the keratometry readings, the location of the cone, previous history of hydrops with corneal scarring, presence of intracorneal ring segments, and the presence of concomitant cataract.
TABLE 1 -
Indications for Lamellar Keratoplasty in Keratoconus
||Any case of keratoconus with a prior indication for a full thickness penetrating keratoplasty with healthy endothelium can be considered for lamellar keratoplasty
||Advanced cases of keratoconus which are intolerant to contact lenses or are unable to undergo procedures such as collagen crosslinking, intrastromal corneal ring segments or phakic intraocular lenses
||Cases with previous hydrops who are unable to achieve good vision with contact lenses due to residual scars
||Moderate keratoconus with high risk for hydrops especially in young males with higher keratometry readings on presentation.
||Downs syndrome. In these cases anterior lamellar keratoplasty can provide better tectonic stability than penetrating keratoplasty and minimize severe ocular damage secondary to self-trauma or eye rubbing. It is also difficult to rule out rejection in these cases
||Atopy—high risk of rejection and failures
SURGICAL TECHNIQUES FOR DEEP ANTERIOR LAMELLAR KERATOPLASTY IN ECTATIC DISEASE
Manual Open Dissection
Anwar described a layer by layer manual stromal dissection after performing partial trephination of 70% to 80% corneal thickness. An anterior chamber paracentesis was done to decompress the anterior chamber. Stromal bed dissection was done with a knife with large sweeping movements since smaller movements can produce microridges. The operative field was kept as dry as possible to differentiate landmarks more clearly than if fluid was present on the cornea. Dissection was done down to DM without injecting air or fluid.26
In 1985 Archila et al27 described the technique of air injection into corneal stromal layers for dissection. An injection of 1 mL of air into the corneal stroma, facilitated the manual dissection of lamellae as long as the microbubbles were visible close to DM preventing penetration into the anterior chamber. A similar technique was described by Price.28
Anwar and Teichmann29 first described a technique of injecting a big bubble for corneal lamellar separation. A calibrated guided trephine was used to trephine the recipient cornea up to 60% to 80% of the depth. A 27- or 30-gauge needle, bent 5 mm from the tip and attached to a 1 or 3 mL air-filled syringe, was carefully advanced into the corneal stroma. Bevel down the needle was placed into the partially trephined central button, aiming in a direction halfway between a tangential and a radial one until the bevel was completely buried. This direction avoided the central part of the cornea, which is usually the thinnest area, particularly in keratoconus. The risk of puncturing DM was reduced with the bevel of the needle facing posteriorly and air entry into the pre-Descemet's plane was encouraged. A whitish circular semiopaque disk was achieved with the desired result of air injection and sudden easing of resistance of the plunger of the syringe. This technique can be repeated if the surgeon is unable to obtain the big bubble in the first attempt. An anterior chamber paracentesis was then performed at a site peripheral to the edge of the large air bubble. A partial-thickness anterior keratectomy was then performed by dissecting with a #69 Beaver blade (Becton Dickinson & Co.), leaving a layer of corneal stroma in place anterior to the air bubble. The remaining layers of corneal stroma were penetrated near the center of the cornea with the sharp tip of a pointed blade. A blunt spatula was inserted through the opening to dissect till the DM. Dua et al30 differentiated the big bubble into 2 types. Type 1 was the one described by Anwar where the air separates Dua's layer (DL) from the deep stroma, creating a large central bubble of around 8 to 9 mm in diameter. This is the preferred type of bubble in DALK. Type 2 big bubble wherein the DM is separated from the posterior surface of DL by the air bubble. This bubble is larger with a thinner wall and more susceptible to rupture. In a mixed variety, both type 1 and type 2 can coexist.
In 1999, Melles et al31 described a technique of visualizing the posterior corneal surface, using an “air to endothelium” interface, by injecting air in the anterior chamber. Using this interface as a reference plane for dissection, a 360 degrees deep stromal pocket was created with injection of viscoelastic to create a separation plane between anterior stroma and posterior corneal lamella. Trephination was performed till the stromal pocket with escape of viscoelastic. The remaining, unexcised stromal attachments of the anterior lamella were cut with curved micro scissors.
Dissection of corneal lamellae with ocular viscoelastics has been described by Melles et al in 2000.32 After exchanging the aqueous with air, viscoelastic material was injected just anterior to DM with a 30-gauge needle. Trephination of the recipient cornea was done later followed by excision of anterior corneal lamellae. Manche et al33 described another technique of viscodissection. A 25-gauge blunt cannula was used to inject viscoelastic into a pocket prepared in the deep stromal fibers after deep trephination which aided in further dissection.
Femtosecond lasers have advanced the reproducibility in lamellar keratoplasty. They can create precise computer-guided corneal lamellar cuts at any depth, together with trephination cuts of the required diameter. The laser creates an increased surface area and interlocking surfaces, which provide more stable and faster healing graft-host interfaces.25,34 Since the current technology measures distance from the interface anteriorly, sometimes the residual bed is of different thicknesses, and baring of DM is currently not possible.35 Newer software enhancements have also enabled the creation of a tunnel, for placement of a cannula/needle as close as possible to DM, to achieve a big bubble. The use of real-time intraoperative guidance, has enabled this ability.36
INTRAOPERATIVE CONSIDERATIONS IN CASES OF ADVANCED KERATOCONUS
Modifications of Standard Big Bubble DALK Technique
Some modifications can be done in the original big bubble DALK technique described by Anwar. After partial trephination, manual dissection is started in the periphery with a crescent blade to remove the anterior stroma. The dissection is performed keeping the blade flat to the surface with sweeping movements. The angle of the blade is then adjusted according to the curvature of the cone completing the anterior keratectomy (Fig. 1A–C). It is easier to visualize the cone mentally as a 3-dimensional shape. This will allow the surgeon to accurately angle the blade appropriately. Anterior keratectomy is done for easier access to the posterior stroma since it allows the surgeon to change the plane of the needle to be placed parallel to the iris surface which can help reduce the risk of DM perforation. A 27-gauge needle is marked with a surgical marker pen at its tip and placed paracentrally in the stroma to make a tunnel. This is followed by air injection with a blunt cannula which is placed in the same tunnel. This will help to get an evenly distributed big bubble. A brave slash incision is done to release the big bubble tension followed by anterior chamber paracentesis. A blunt dissector (ASICO, IL) can be used along with Anwar scissors (Duckworth and Kent Ltd, Hertfordshire, UK) for dissection till DM or DL is reached. An air bubble can be placed in anterior chamber which can help to identify any microperforation that occurs, and also by keeping the pressure low will prevent any microperforation from enlarging. Air also provides a good contrast for visualization of any creases in the graft-host interface especially more common in advanced ectasia. To reduce the postoperative myopic shifts in DALK, same size donor–recipient trephination has been recommended by Huang et al.37 After removal of the DM and the endothelium from the donor tissue, it is sutured to the recipient bed with adjustable 10–0 nylon interrupted sutures. Any creases noted after suturing (Fig. 1D) can be displaced to outside the visual axis by placing a marginal dissector in the interface, after which the creases are ironed out to create a smooth central visual axis (Fig. 1E).
DALK in a Case With Previous Intracorneal Ring Segments
Fontana et al38 has reported DALK in keratoconus with ICRS a few months after ICRS removal. Big bubble formation was achieved in 60% of the cases, whereas layer-by-layer stromal dissection was required in the rest to expose a deep stromal plane in the central optical zone.
Figure 2A.1 and 2A.2 shows preoperative and postoperative images of a 27-year-old male, 6 weeks after ICRS removal. His best-corrected visual acuity (BCVA) before ICRS removal was 6/60. While removing the ICRS, an incision can be made at the ring edge with a diamond knife, edges can be dissected with sinskey hook and then the ring can be removed with forceps. There is a risk of corneal perforation from prior inlay insertion or due to removal, hence it is advisable to wait for the stroma to heal and then perform DALK surgery. After 6 weeks, he underwent DALK. Anterior keratectomy with crescent blade was performed followed by injection of an air bubble in the deep stromal layers to achieve a big bubble as previously described (Fig. 2A.3). His BCVA gradually improved after removal of all sutures. Three years after DALK, his BCVA was 6/6 with refractive error of +0.50/−1.50 × 95 (Fig. 2A.4).
DALK in Previous Hydrops
The incidence of acute hydrops in keratoconus has been reported to be 2.8%.39 PK may be required for visual rehabilitation in most cases since there is some degree of permanent scarring even after spontaneous resolution. However, these patients may be at a greater risk of graft rejection as compared to nonhydrops eyes.40 DALK can be performed in patients with previous hydrops. Pre-DM DALK is a technique in which a variable amount of residual stroma is retained in the recipient bed after stromal dissection without total exposure of the DM.41 Good clinical outcomes have been reported with pre-DM DALK after hydrops in keratoconus.42 Pre-DM dissection with a crescent blade43,44 is limited to keratoconus corneas with moderate steepness and keratometric (K) readings (≤ 60 D Kmax). Figure 2B.1–6 illustrates the preoperative, intraoperative, and postoperative images of a 24-year-old male with previous hydrops in keratoconus who underwent DALK with moderately steep cornea (Kmax 52.4D). The cornea was trephined up to two-thirds of its thickness (Hanna trephine; Moria, Antony, France), followed by free-hand dissection of anterior corneal lamellae with a crescent blade (Beaver, Waltham, MA) following the curvature of the cone and keeping the blade of the crescent blade flat to the surface of the iris. Stromal dissection was done layer by layer. A blunt marginal dissector (Asico, Westmont, IL) was used in the central scarred area with care to avoid perforation after reaching a designated residual corneal stromal thickness < /= 100 microns. The donor cornea was then sutured onto the recipient's stromal bed with two 10–0 nylon running sutures.
In cases of steep keratoconus corneas after hydrops (>60 D Kmax), it is difficult to reach a pre-DM plane at an acceptable depth manually. The steep curvature of the cornea increases the risk of perforation and can lead to uneven dissection. If DM baring is attempted it is advisable to attempt the DM break area last and use stromal hydration for dissection. Figure 2C.1–8 shows preoperative, intraoperative, and postoperative images of a 34-year-old man with a central corneal scar after hydrops in keratoconus and a Kmax of 68.7 D. The patient was treated with a manual DM baring DALK. After initial trephination of the anterior two-thirds of the corneal thickness, the anterior keratectomy was done free-hand with a crescent blade. Stromal hydration was then used to debulk the peripheral cornea, before approaching the central hydrops scar. The marginal dissector was used to manually dissect the periphery, leaving the fragile central scarred area untouched. Removal of the residual central stroma will be performed last and will result in some leakage of aqueous, through the exposed scarred DM. Intracameral air injection before final stromectomy will act as a tamponade and also deflate the aqueous volume to keep the pressure low and prevent scar extension. The donor cornea was then sutured onto the recipient's stromal bed with interrupted sutures with 10–0 nylon.
DALK in Posterior Keratoconus
In posterior keratoconus (keratoconus posticus), the posterior corneal surface has a conical protrusion secondary to decreased stromal tissue. It is usually congenital and is mostly nonfamilial.45 Due to the presence of corneal scarring and very steep posterior corneal curvature, it is advisable to perform manual dissection to avoid risk of DM perforation. Figure 3A and B demonstrates anterior segment slit lamp image and optical coherence tomography respectively of a case of posterior keratoconus. It shows central area of corneal thinning with posterior excavation associated with stromal scarring. This patient underwent manual DALK. After partial trephination with 8-mm trephine, the recipient bed was manually dissected from the periphery toward the center to remove the anterior stromal lamellae. The surrounding stroma was carefully dissected till the DM was reached. A 7.75-mm donor cornea was sutured to the recipient bed with 10–0 nylon interrupted sutures. On postoperative day 1, graft was well attached to the recipient DM (Fig. 3C). Fifteen months after surgery, his BCVA RE 6/12 with manifest refraction of −2.50DS/−1.0DC × 135 (Fig. 3D).
Cataract Management in Keratoconus
The presence of cataract in keratoconus can present with many challenges for the surgeon. It is difficult to achieve accurate keratometry and axial length readings for intraocular lens power calculation.46 Optical biometry in eyes with keratoconus typically overestimates the corneal power and underestimates the intraocular lens (IOL) target power resulting in postoperative hyperopia.47 The use of stabilizing procedures like collagen cross-linking and ICRS before optical biometry can aid in preoperative lens selection and provide more predictable surgical outcomes.48 Postoperative use of rigid gas permeable or scleral lenses can help patients achieve their best visual potential.
If there is significant cataract resulting in poor vision and if the corneal pathology allows sufficient surgical view for a safe cataract surgery, then the option of cataract surgery with simple monofocal intraocular lens can be considered before DALK if the patient will continue to wear a contact lens. If adequate vision is obtained with contact lenses, the need for corneal surgery would be obviated. DALK should be offered before cataract surgery if the corneal pathology does not allow adequate surgical view for safe cataract surgery or if the patient needs improved refractive outcome.49Figure 4A–D shows the preoperative, postoperative slit lamp images and corneal topography images (Pentacam, Oculus, Germany) of a 48-year-old male with cataract and keratoconus. He was planned for cataract surgery with intraocular lens implantation due to the presence of significant cataract (Nuclear sclerosis grade 4, posterior subcapsular cataract grade 4 LOCS3).50 The keratometry readings were high (K1: 50.1 D, K2: 56.4 D); therefore, IOL power calculation was not accurate by ultrasound biometry. Calculated IOL power was −22.50 D with target postoperative refraction of −0.10 D. Therefore, he underwent DALK surgery first followed by cataract surgery after 15 months, with stabilization of the keratometry readings (K1: 43.9 D, K2: 47.4 D) after removal of sutures.
Toric IOL implantation after cataract surgery can be considered in patients with regular astigmatism and stable keratometric readings.51 In severe cases of keratoconus toric IOLs have shown less predictable refractive outcomes.52Figure 5A–D shows the corneal topography images (Orbscan, Bausch and Lomb, Rochester, NY) before and after DALK and slit lamp images of post DALK and post-cataract surgery in a 68-year-old man. The keratometric readings before DALK were 45.7 D and 55.7 D with −10 D astigmatism. The patient underwent DALK for the severe keratoconus. Two days after DALK surgery, the patient required anterior chamber rebubbling with air, since the donor graft was not well attached to the recipient bed and there was presence of double anterior chamber. Nine months after DALK, the corneal topography stabilized with keratometric readings of 43.2 D and 47.1 D with −3.8 D of astigmatism. He underwent phacoemulsification with toric intraocular lens implantation. After 3 years and 9 months, his postoperative BCVA was 6/7.5.
There are conflicting reports on the data comparing DALK and PK. The variation in postoperative visual acuity and contrast sensitivity after DALK has been correlated to the thickness of the host residual stromal bed after dissection.53 Many studies have found a higher percentage of patients achieving 20/20 visual acuity with PK compared with DALK.11,54 The best visual acuity is achieved in DALK cases that have a residual bed of less than 20 μm, which is ideal for obtaining visual results when similar to PK.53 No difference in BCVA, graft survival, or keratometric outcomes between patients undergoing DALK or PK was reported by the Cochrane review.55 This is different compared to DALK for other indications, for example, post-infectious scars in which long-term graft survival is significantly better in DALK compared to PK. However, most studies are limited in their duration, that is, 10 years. The global life expectancy has increased year on year, and considering patients having keratoplasty for keratoconus are in their 20s to 30s, there are few if any studies that look at survival beyond 15 to 20 years.56 Hence, it is likely a second graft would need to be performed sometime in the lifetime of the patient, for which we know the graft survival is not as good.57 Henein et al18 reported reduced rejection and refractive astigmatism with DALK but better visual outcomes with PK.
Studies have reported significant reductions in the number of PKs performed for keratoconus since the introduction of collagen cross-linking; however, there are a proportion of people that will progress and will eventually develop advanced ectasia and need a keratoplasty.58 In conclusion, although DALK is a technically challenging procedure with a steeper learning curve, its advantages over PK, that is, lower risk of graft rejection18 and less intraoperative and postoperative complications21,22 makes it the treatment of choice even in the most difficult surgical scenarios.
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