Preoperative evaluation is fundamentally important in any medical procedure[21]. We believe that using AS-OCT has a vital role in adjusting the surgical steps for each case individually in DMEK for PK graft failure.
Common conclusions have emerged from multiple previous studies regarding the results of DMEK after PKP compared to primary DMEK[16, 18, 19, 22, 23], including higher rebubbling rates, higher endothelial cell density loss, higher late graft failure rates and more demanding surgical technique. Furthermore, glaucoma filtering surgeries, which are probably more prevalent in post-PKP eyes, are considered a significant risk factor for late graft failure, probably as a result of altering the microenvironment in the aqueous humor which accelerates endothelial cell loss[24]. Therefore, based on these eminent conclusions and our experience, additional comprehensive pre-operative AS-OCT evaluation was invested to carefully plan DMEK surgery for each case individually, as well as applying multiple adjustments to our surgical technique, in order to decrease failure rates.
Pre-operative assessment by AS-OCT aimed to evaluate both the PKP graft size and the presence of undesired posterior morphological features, including irregular bulging scars in the graft-host interphase or anterior synechiae (Figs. 1 and 2). Lavy et al.[19] showed that higher detachment rates occurred in oversized grafts. Posterior bulging of graft-host interphase scarring prevents proper graft attachment while anterior synechiae mainly impede intraoperative unfolding DMEK graft and its proper positioning. Therefore, in cases with posterior bulging and no intraoperative plan to dissect the extra tissue, it is important to plan for undersized DMEK graft to reduce the chances of positioning it underneath the PK-host interphase and enhance DMEK graft attachment post-operatively (Fig. 2, C and D). Moreover, evaluating the presence of posterior morphological features and considering their removal during surgery, may decrease intraoperative graft manipulations and facilitate its positioning (Fig. 2, E and F). In a smooth, non-bulging posterior surface of the corneal graft host interphase, it is worth considering oversizing the DMEK graft, and deliver more endothelial cells to the decompensated cornea (Fig. 2, A and B).
Intra-operative adjustments included injecting sub-tenon triamcinolone prophylactically (Fig. 3A). We believe that it significantly decreases post-operative intraocular inflammation affecting the early postoperative graft failure rate and the incidence of CME. CME is a well-known complication after intraocular surgery and has been reported to occur in 2.0–12.5% of cases after endothelial keratoplasty[25–27]. Heinzelmann et al.[28] observed a considerably elevated incidence of CME (13%) which influence visual rehabilitation. In our study, SD-OCT for macula was performed 1 month after surgery and CME was observed only in one eye (6.25%), which might be explained by the complicated clinical course of this case. Postoperative intraocular inflammation may also affect graft failure and rejection rates which are not negligible after secondary endothelial keratoplasties. Administrating a depo of corticosteroids during the surgery may also have a beneficial effect on graft survival and endothelial cells function.
In addition, paracentesis and main incision were performed in the host peripheral corneal rim without penetrating the PK graft to prevent potential graft-host wound dehiscence (Fig. 3B). No circumferential scoring of DM was performed and descemetorhexis was started from the center of the PK graft and completed in a curvilinear pattern along the PK wound in a manner resembling capsulorhexis (Fig. 3C). We believe, performing these adjustments without removing graft-host interphase sutures, may ensue in a smoother back surface, reduce the amount of Descemet remnants and may even facilitate posterior scar tissue removal. Moreover, scars in the graft-host interphase or anterior synechia, were selectively removed during surgery, if possible, based upon pre-operative AS-OCT (Fig. 3C).
After injecting the DMEK graft, the correct orientation was confirmed by intraoperative AS-OCT (Rescan), when available, or the “Montsouris” sign. Afterward, the graft was unfolded using careful indirect manipulations by tapping on the cornea surface. Then, the graft was elevated with a small air bubble beneath and was centered by the “wave maneuver”, an indirect L shaped tapping on the corneal surface (Fig. 3, D and E). We believe that these surgical steps lessen unnecessary extra manipulations on the graft and facilitate positioning it in the correct orientation and suitable position. This way of manipulating the graft on the posterior corneal surface with Descemet-stromal touch, helps to avoid endothelial-iris/IOL touch, which may also protect the graft endothelial cells.
Finally, the anterior chamber was 80–100% filled with 20% SF6 gas to pressurize the eye and support graft adherence for a longer period than air (Fig. 3F). After Two to three hours, intraocular pressure was checked and if necessary, an appropriate intervention was applied. Lavy et al.[19] attributed the tendency of delayed and extensive graft detachment, to insufficient pressurization during surgery or postoperative hypotonia. They recommended to pay attention for pressurizing the eye at the end of surgery or to extend the air-bubble time if enough pressurization cannot be achieved. Compared to other studies, we had relatively low rebubbling rate (18.75%), this may be attributed to SF6 usage and the extended volume in the anterior chamber at the end of the surgery, but also may be due to a more conservative rebubbling policy of the surgeon.
In our study, we encountered relatively low complications rates. Graft failure rate was 6.25% (late), graft detachment rate and rebubbling rate were 18.75% respectively. Interestingly, glaucoma filtering surgeries were not a significant risk factor for graft failure. However, this conclusion may be applicable at least regarding early decompensation events and not for late decompensation events due to short-term study period. We believe, based on our experience, that these successful results are attributed to our careful pre-operative AS-OCT evaluation and intra-operative adjustments to the surgical technique in DMEK after PKP graft failure. In comparison to our results, higher rebubbling and failure rates were reported by other studies. Lifshitz et al.[18] had 43% rebubbling and 43% failure rates. Lavy et al.[19] found 34% and 36% rebubbling and failure rates respectively. Heinzelmann et al.[23] reported 37% and 21% rebubbling and failure rates respectively.
Visual recovery and central pachymetry improvement were relatively fast during the study period (Table 2). Those favorable clinical outcomes are consistent with other studies as well[16, 18, 22]. The visual recovery with DMEK under failed PK contrasts sharply with the delayed and unpredictable visual rehabilitation after re-PKP[29]. Moreover, the rapid visual recovery seen after DMEK under failed PK is consistent with the visual recovery that DMEK provides in virgin eyes as compared with PK[30].
There are some limitations to our study, including its retrospective nature, small sample size and short-term follow up. Moreover, visual acuity tests were done by technicians and not experienced optometrists. Therefore, uncorrected or partially corrected, rather than best corrected, visual acuity was assessed. In addition, ECD was done only for 7 cases after surgery and comparison between donors ECD and post-operative ECD may be inconclusive due to group size difference. This was mainly because specular microscopy test was not covered by the health insurance and not all study participants could afford it. However, we encountered a predictable decrease in ECD after surgery which may be attributed to cell migration and postoperative inflammatory response[19].