4.2.1 Microkeratome-Assisted Lenticule Preparation

Different strategies have been suggested to consistently obtain thinner cuts with a single microkeratome pass, such as stromal dehydration, ²⁰ development of a nomogram for microkeratome head setting based on central corneal thickness prior to the cut (▶Fig. 4.2), ²¹ and adjustment of the translational speed of the microkeratome in thicker donors, as slower translational speeds result in deeper cuts and thinner lenticules (▶Table. 4.1). ²²

With a single microkeratome cut, there is higher variability in thickness throughout the lenticule compared with two microkeratome passes (double pass). ^{23 , 24} This variability results in lenticules that are thinner in the center than in the periphery, ²² which has been related to the postoperative hyperopic shift observed after DSAEK. ²⁵ The predictability of the final lenticule thickness decreases as the target thickness decreases, dropping from 78% of cuts within 10 µm when the target is between 90 and 120 µm to 48% when the target falls below 90 µm. ²⁶ Nevertheless, with the proper microkeratome head selection based on central corneal thickness, the majority of lenticules fall below 131 µm. ²⁶

In 2009, Busin et al ⁸ introduced the concept of UT-DSAEK at the European Society of Cataract and Refractive Surgery meeting and proposed the use of a preoperative lenticule thinner than 100 µm by using the “microkeratome double-pass technique” for donor preparation, which would standardize the preparation of grafts resulting in consistently thinner and more planar lenticules. ^{22 , 23}

In addition to a thinner lenticule, the double-pass microkeratome technique seems to provide better tissue architecture than a single-pass preparation, with comparable postoperative outcomes to DMEK without the steep learning curve, ^{27 , 28} or the specific anterior chamber fluid dynamics required for DMEK.

The “double-pass microkeratome technique” consists an initial “de-bulking” pass of the microkeratome usually with a head between 300 and 350 µm and a second microkeratome pass that is adjusted to obtain a lenticule thickness below 100 to 130 µm. The second pass is started 180° away from the first pass and it is done in the opposite direction to obtain a more planar surface.

The head used for this step can be selected according to a nomogram developed by Busin (▶Table 4.2), ⁸ targeting a residual bed with a central thickness below 100 µm. Other algorithms have also been suggested to adjust both the first pass in relationship to the initial corneal thickness and the second pass related to stromal bed thickness prior to the cut (▶Fig. 4.3). ²³ Lenticules created with the double-pass technique tend to be more planar, that is, with a more even distribution of thickness from the center to the periphery after the second pass of the microkeratome. ²³

Pressure in the system must be standardized by raising the infusion bottle to a height of 120 cm above the level of the artificial anterior chamber and then clamping the tubing 50 cm from the entrance. Attention must be given to maintain a uniform, slow movement of the hand-driven microkeratome, requiring 4 to 6 seconds for each dissection, which will produce a planar surface that minimizes interface irregularities and hyperopic shift. ▶Fig. 4.4 shows the optical coherence tomography (OCT) images of donor tissue before (▶Fig. 4.4a, c) and after (▶Fig. 4.4b, d) microkeratome double-pass tissue preparation for UT-DSAEK, and before (▶Fig. 4.4e) and after (▶Fig. 4.4f) regular single-pass microkeratome for DSAEK preparation.

With the double-pass microkeratome technique, Busin reported 100% of the tissues with a postcut thickness below 151 µm, 95.6% below 131 µm, and 78.3% below 101 µm; and only 2.1% of the tissues were lost due to perforation. ²⁹ Woodward et al. ³⁰ obtained 65% of cuts below 100 µm and 92% below 131 µm with the double-pass technique and found no significant difference in rate of perforation when the second pass was done 180° from the first cut (23%) or at the thickest peripheral measurement in the residual bed (29%). Their higher perforation rate compared to the 7% reported by Busin ²⁹ can be related to the larger chosen head size for the second cut.

4.2.2 Femtosecond Laser Ultrathin Lenticule Preparation

So far, despite the theoretical advantage of more predictable depth of cut and a faster recovery of visual acuity during the immediate postoperative period, femtosecond laser preparation of DSAEK donors has yielded worse long-term postoperative visual acuities and higher rate of re-grafting compared to microkeratome grafts. ³¹ This is due to the scattered laser emission and the high intraocular pressure (IOP) required during femtosecond applanation, which distorts the lenticule surface and creates ridges, ³² giving the femtosecond cuts a wavelike or concentric ring configuration in the stromal interface. ^{33 , 34} Additionally, the large cavitation bubbles increase the rate of endothelial cell loss to 50 to 65% by 12 months ^{32 , 35 , 36 , 37} and the graft detachments rates as high as 40%, ^{31 , 36 , 38} yielding overall inferior refractive outcomes compared to manual trephination. ^{31 , 32 , 33 , 36 , 38 , 39 , 40}

Docking the endothelial side of donor cornea on the applanation surface to decrease the laser emission path and scatter could result in a more predictable and regular cut; however, this comes at the cost of 30% less viable endothelial cells compared to epithelial side docking ³⁵ even when ocular viscoelastic devices (OVDs) are used on the applanation surface. ⁴¹

4.3.1 Main Corneal Incision

Several DSAEK graft insertion techniques and the use of DMEK tissue insertion devices have been developed in attempts to minimize tissue manipulation during the procedure and therefore minimize the loss of endothelial cells. Five-millimeter main incisions have been shown to have less endothelial cell loss compared to the 3.0 mm; ^{42 , 43} however, this difference does not appear to be clinically significant postoperatively. ⁴²

4.3.2 Descemetorrhexis

A deep anterior chamber facilitates complete descemetorrhexis. Filling of the anterior chamber with air, OVDs, or balanced salt solution is crucial to maintain an adequate depth that will not only facilitate removing Descemet’s membrane but also prevent complications related to sudden anterior chamber collapse such as crystalline lens damage when apposed to the cornea, iris prolapse, or less frequently, suprachoroidal hemorrhage.

Descemetorrhexis is accomplished in most instances manually with the use of a reverse Sinsky hook under retroillumination of the cornea. After scoring Descemet’s membrane, intraocular forceps are used to peel off Descemet’s membrane from the host.

A retained fragment of Descemet’s membrane can result in significant interface haze and irregularity, as well as postoperative lenticule detachment. Therefore, it is essential to assure complete descemetorrhexis, which can be confirmed by examination of the recipient’s cornea with retroillumination or direct examination of the removed Descemet’s membrane onto the recipient’s cornea.

Femtosecond laser technology is currently being considered for host descemetorrhexis for DMEK by producing an 8- to 8.25-mm diameter cut 100 µm anterior to the thinnest measured point from the epithelial side and 100 µm posterior to the thinnest cut from the anterior chamber. Results have been promising, with lower detachment rates treated successfully with re-bubbling ^{44 , 45 , 46} and endothelial cell losses of 24% with femtosecond laser descemetorrhexis comparable to 29% with manual descemetorrhexis. ^{44 , 45 , 47}

The use of DSAEK Terry scrapers to scrape the periphery of the recipient stroma after descemetorrhexis has been shown to decrease the rate of postoperative graft dislocation from 50 to 4% in case series ⁴⁸ and appears to be of help in the reattachment of partially detached lenticules. ⁴⁹

4.3.3 Lenticule Insertion

Suture pulling techniques do not seem to offer an advantage over the use of forceps in terms of endothelial cell loss, ⁵⁰ and the Busin guide has shown less endothelial cell loss compared to forceps (25 vs. 34%, respectively), but no advantages in terms of final visual acuity outcomes or rate of graft failure; ⁵¹ therefore, the choice of insertion technique depends merely on surgeon’s preference as long as the wound remains the appropriate size of 5 mm. Recently, tissue insertion devices have been designed to minimize tissue manipulation during insertion while using a smaller incision size, and endothelial cell loss appears to be less when these devices are used (average of 16% loss reported in 1 year) ⁵² compared to forceps folding techniques (35%) and sheets glide (35%).

The Busin glide and injection devices preserve the endothelium better compared to manual insertion with forceps; ^{51 , 53} however, this difference does not appear to be clinically relevant postoperatively neither in standard not in UT-DSAEK. ⁴² Therefore, insertion technique should be made by each individual surgeon to maximize the surgeon’s performance and minimize iatrogenic tissue trauma through excessive manipulation.

Placing the sutures prior to viscoelastic removal will provide a pressurized eye to facilitate suture placement and reduce postoperative astigmatism. It is of equal or more importance to insert the tissue and then tie the sutures prior to unfolding, as not doing so increases the chances of tissue expulsion through the open main incision, especially when inserting ultrathin lenticules.

Transient interface fluid has been associated with transient interface opacities; ⁵⁴ therefore, fluid and OVDs need to be thoroughly removed from the donor–host interface. The use of venting incision has been controversial due to the increased risk of epithelial downgrowth into the interface. The author uses a Lindstrom laser in situ keratomileusis (LASIK) flap roller, but any other blunt instrument can be used to “sweep” the corneal surface during lenticule adhesion with full air fill in the anterior chamber to remove any fluid present in the interface.

In a case series where strategies have been incorporated to assure complete removal of interface fluid, such as corneal massage with a LASIK flap roller and venting incisions in DSAEK, the rate of graft dislocation decreased significantly to 6% compared to 50% when none of these techniques were used. ⁵⁵ The use of intraoperative anterior segment OCT (AS-OCT) can also confirm full graft attachment at the end of DMEK procedures, as visualization of the Descemet’s membrane is difficult in some instances. ^{56 , 57}

4.3.4 Insertion of Air into the Anterior Chamber

To minimize the risk of postoperative pupillary block, the diameter of pupillary dilation should be noted during the preoperative examination. Pupils with large dilation diameters in dim light conditions, mainly greater than the diameter of the desired postoperative air bubble, might not need a peripheral iridotomy, and this decision is at the surgeon’s discretion. If this information is not available or if the pupil diameter in dim light is small, an inferior peripheral iridotomy performed during the surgery can prevent pupillary block from a large air bubble. In phakic patients, special care must be made to avoid violating the crystalline lens capsule. If a peripheral iridotomy is deferred, adequate dilation must be achieved during the immediate postoperative period to assure that pupil diameter is larger than the size of the air bubble in the anterior chamber.

To further minimize the risk of pupillary block, an adequate air–fluid exchange once the graft is attached is needed. ⁵⁸ The air bubble in the anterior chamber should be free-floating and not connected to air posterior to the iris. This can be verified intraoperatively by rotating the patient’s head and watching the air bubble movement within the anterior chamber or by observing the size of the bubble during the air fluid exchange, which should continue to decrease and not remain the same size, indicating that air continues to come forward from behind the iris. Once the air bubble is free-floating and absence of air behind the iris has been confirmed, air can be added to the bubble until the desired size is achieved. Air should not extend beyond the dilated pupil if there is not a patent peripheral iridotomy present.

Sulfur hexafluoride (SF₆) gas can also be used instead of air for donor lenticule support ^{59 , 60} as it remains in the anterior chamber longer and has higher surface tension and buoyancy. Ten percent SF₆ does not result in significantly different detachment and re-bubbling rates compared to 100% air; ⁶¹ however, the use of 20% SF₆ does result in significantly lower detachment rates, especially after DMEK ^{62 , 63 , 64 , 65 , 66} with no significant increase in the risk of pupillary block, ⁶⁷ or corneal toxicity in in vitro studies. ⁶⁸ It is critical to avoid 100% SF₆ as this will expand postoperatively, resulting in increased IOP that may necessitate its removal.

Intraoperative optical coherence tomography (iOCT) has been shown to be an adjuvant in both DSAEK and DMEK for close and detailed visualization of the graft–host interface to rule out the presence of fluid, remaining viscoelastic, remnants of host’s Descemet’s membrane, or air. ^{54 , 69} Due to the logistic and financial implications of iOCT, it is currently not widely used but it has the potential to reduce postoperative issues with interface fluid if it can become routinely available in the operating room setting.

4.5.1 Refractive Outcomes

Typically, vision after DSAEK improves gradually over the first 4 to 6 weeks as the edema resolves; so perhaps if manifest refraction remains essentially stable between two consecutive visits after this point, a prescription for spectacles can be provided. However, there is still a trend for improvement with 20/40 or better visual acuity in more than 80% of patients at 6 months, and this should be discussed with the patient as their prescription could change some due to corneal remodeling over time. These changes can be negligible or significant, as this trend for improvement in visual acuity can even continue for as long as 3 years. ⁷³

In patients that underwent DMEK, 98% of patients have visual acuity better than 20/40 and 79% better than 20/25 on postoperative month 6. ⁷⁴ At 1 year, 41% of patients have been reported to have distance-corrected visual acuity (DCVA) of 20/20 or better, 80% 20/25 or better, and 98% 20/30 or better. ⁷⁵ Refractive stability in DMEK appears to occur sooner than in DSAEK, on postoperative month 3. ⁷⁴

There are many reports disputing the association between preoperative lenticule thickness and postoperative outcomes. ^{76 , 77 , 78 , 79 , 80} In these reports, preoperative and/or postoperative donor thicknesses were higher than 130 µm (higher than 145 µm in preoperative and 142 µm in postoperative measurements). Additionally, when preoperative graft thicknesses below 125 µm have been analyzed separately, the percentage of patients with better visual acuities trended towards better visual outcomes. ^{24 , 76 , 79 , 81} In several publications where only thinner lenticules have been studied, the results are still conflicting. Some show a significant improvement in visual outcomes and rate of detachment and dislocation when lenticules are thinner below 130 µm. ^{82 , 83} Others have shown no significant correlation between postoperative graft lenticule thickness and visual outcomes. ^{10 , 84 , 85} The current evidence associating graft thickness with postoperative visual outcomes is inconclusive and current available studies do not yet present robust data and statistical analyses to prove or disprove this relationship, ⁸⁶ specifically with lenticules less than 130 µm.

After EK, there is a degree of corneal deturgescence, mostly during the first postoperative month. ^{11 , 80 , 85 , 87 , 88} This causes a decrease in corneal light scatter, especially in the subepithelial surface and donor–host interface during the postoperative follow-up, which may be responsible for most of the improvement in visual acuity. Graft thickness has not been directly correlated to the degree of deturgescence; however, the stabilization of postoperative graft thickness is reached earlier with thinner lenticules. ^{88 , 89} The main difference between outcomes of different types of EK seems to lie within this deturgescence period, with better visual acuities achieved faster with DMEK, UT-DSAEK, and DSAEK. By postoperative month 6, 42% of DMEK patients, 26% of UT-DSAEK patients, and 11.1% of DSAEK patients after DSAEK have corrected distance visual acuity (CDVA) of 20/20 or better (▶Fig. 4.5a). By the same period, 91% of DMEK patients have CDVA of 20/30 or better and 95% of patients after both DSAEK and UT-DSAEK have CDVA of 20/40 or better (▶Fig. 4.5b). By the first postoperative year, the proportion of DMEK patients with CDVA of 20/20 remains stable, and 80 and 98% reach 20/25 and 20/30, respectively. ⁷⁵ At this same period, 29% of UT-DSAEK and 14% of DSAEK patients reach CDVA of 20/20. ^{29 , 73} By year 2, however, this gap between DSAEK and UT-DSAEK closes, with visual acuities of 20/20 or better in 34% of DSAEK and 48.8% of UT-DSAEK patients. ^{29 , 73}

There is an average hyperopic shift after EK. ^{90 , 91 , 92} This seems to be related to tissue architecture and the negative lens effect that lenticules have, which is due to increased peripheral lenticule thickness. In correlation analyses, a thicker lenticule center, a larger lenticule diameter, and negative curvature profiles have all been correlated with a higher refractive shift, ^{81 , 91} with stronger correlation when a thicker lenticule and a negative lenticule curvature profile are combined. ^{13 , 25 , 81} This hyperopic shift occurs after all endothelial keratoplasties to a certain degree. In DSAEK cases, this shift averages at +1.1 D ⁹⁰ (range + 0.7 to +1.5 D). When UT lenticules are used, this shift decreases to + 0.85 D (range − 4.50 to + 3.25 D) when a single microkeratome pass is used for preparation ⁸⁸ and + 0.78 D (range + 0 to + 3.25 D) when the double-pass microkeratome technique is used. ²⁹ After DMEK, there is still a hyperopic shift but it is considerably lower at + 0.24 D (−1.50 to + 2.25 D). ⁷⁵

Despite negligible changes in the anterior corneal topography after all types of EK, ^{13 , 25 , 81} there is an increase in posterior corneal HOAs after both DSAEK ^{11 , 13 , 83 , 92 , 93} and, to a lower degree, DMEK. ⁸⁹ No studies are available in regards to HOAs after UT-DSAEK.