Overall, 224 eyes of 196 consecutive patients (114 males, 82 females; mean age 37.6 ± 15.1 years; range 13–74 years) undergoing DALK for KC were included in the analysis. Topographic localization of the conus was within the central 8.0 mm of the cornea in 104 eyes (46.4% of the total) (group A) and beyond 8.0 mm from the corneal center in 120 eyes (53.6%) (group B). Concerning graft diameter, 148 eyes (66.1% of the total) underwent 8.0-mm DALK (group 1) while 76 eyes (33.9%) underwent 9.0-mm DALK (group 2). Of 148 eyes undergone 8.0 mm DALK, 71 eyes (31.7% of the total) had a topographic localization of the ectasia within the central 8.0 mm of the cornea before surgery (group 1A), while 77 eyes (34.4%) showed extension of the corneal ectasia beyond 8.0 mm (group 1B). Of 76 eyes undergone 9.0-mm DALK, 33 eyes (14.7% of the total) showed a corneal ectasia within 8.0 mm (group 2A), while 43 eyes (19.2%) showed a corneal ectasia beyond 8.0 mm (group 2B) (Fig. 2).
Preoperative Evaluation
Preoperative values of BCVA did not differ significantly between group 1 and group 2 (1.09 ± 0.50 LogMAR [95% CI, 1.01–1.17] vs 1.22 ± 0.51 LogMAR [95% CI, 1.11–1.34], p > 0.05), while a significant difference was found between group A and group B (0.96 ± 0.47 LogMAR [95% CI, 0.87–1.05] vs 1.29 ± 0.48 LogMAR [95% CI, 1.20–1.37], p < 0.001). There was no statistically significant difference in preoperative BCVA of eyes with the same topographic localization of conus who received grafts of different diameter, neither between group 1A and group 2A (0.93 ± 0.46 LogMAR [95% CI, 0.82–1.03] vs 1.03 ± 0.48 LogMAR [95% CI, 0.87–1.19], p = 0.15), nor between group 1B and group 2B (1.24 ± 0.48 LogMAR [95% CI, 1.13–1.34] vs 1.37 ± 0.48 LogMAR [95% CI, 1.23–1.51], p = 0.07).
Preoperative values of topographic astigmatism did not differ significantly between group 1 and group 2 (5.07 ± 3.00 D [95% CI, 4.58–5.55] vs 4.71 ± 2.74 D [95% CI, 4.09–5.32], p > 0.05), while a significant difference was found between group A and group B (4.21 ± 2.21 D [95% CI, 3.79–4.64] vs 5.58 ± 3.29 D [95% CI, 4.99–6.17], p < 0.001).
There was no statistically significant difference in preoperative topographic cylinder of eyes receiving grafts of different diameter in the presence of similar corneal ectasia localization, neither between group 1A and group 2A (4.29 ± 2.34 D [95% CI, 3.75–4.84] vs 4.04 ± 1.88 D [95% CI, 3.40–4.68], p = 0.30), nor between group 1B and group 2B (5.78 ± 3.35 D [95% CI, 5.03–6.53] vs 5.22 ± 3.16 D [95% CI, 4.27–6.16], p = 0.19).
Surgery
DALK was completed in all cases and no conversion to PK was needed. Pneumatic dissection formed a type 1 bubble in 151 out of 224 eyes (67.4%), while type 2 bubble in 12 eyes (5.4%), all belonging to group B. In case of pneumatic dissection failure, OVD-assisted dissection was performed successfully in 21 eyes (34.4% of the attempts), while the remaining 40 eyes (65.6%) underwent manual dissection.
There was a statistically significant difference in big bubble formation according to the size of the conus. In particular, it occurred more frequently in eyes belonging to group B (group 1A [n = 49, 69.0%] vs group 1B [n = 62, 80.5%], χ2 = 24.53, p < 0.01; group 2A [n = 20, 60.6%] vs group 2B [n = 32, 74.4%], χ2 = 19.20, p < 0.01). Conversely, there was no statistically significant difference in bubble type formation between group 1 and group 2 DALK (group 1 [n = 111, 75.0%] vs group 2 [n = 52, 68.4%], χ2 = 1.10, p > 0.1).
Postoperative Evaluation
Overall, BCVA and topographic cylinder improved significantly in all groups at T1 compared to T0 (T0 BCVA 1.13 ± 0.50 LogMAR [95% CI, 1.07–1.20] vs T1 BCVA 0.15 ± 0.16 LogMAR [95% CI, 0.13–0.17], p < 0.001; T0 topographic cylinder 4.94 ± 2.92 D [95% CI, 4.56–5.33] vs T1 topographic cylinder 4.19 ± 2.45 D [95% CI, 3.87–4.51], p = 0.001). Concerning the diameter of the graft, postoperative BCVA was not significantly different between group 1 and group 2 (respectively, 0.14 ± 0.17 LogMAR [95% CI, 0.12–0.17] vs 0.15 ± 0.15 LogMAR [95% CI, 0.12–0.19], p = 0.68). However, a significant difference in postoperative topographic cylinder was found between the two groups (respectively, 4.45 ± 2.60 D [95% CI, 4.04–4.87] vs 3.67 ± 2.02 D [95% CI, 3.21–4.12] p = 0.02).
Concerning the localization of the conus, postoperative BCVA and topographic cylinder were not significantly different between group A and group B regardless of the graft diameter (respectively, 0.13 ± 0.14 LogMAR [95% CI, 0.11–0.16] vs 0.16 ± 0.18 LogMAR [95% CI, 0.13–0.19], p = 0.24 and 3.99 ± 2.18 D [95% CI, 3.57–4.41] vs 4.36 ± 2.64 D [95% CI, 3.88–4.83], p = 0.26).
There was no statistically significant difference in postoperative topographic cylinder between group 1 and group 2 when considering eyes with corneal ectasia < 8.0 mm (group 1A vs group 2A, 4.15 ± 2.19 D [95% CI, 3.64–4.66] vs 3.65 ± 2.13 D [95% CI, 2.92–4.38], p = 0.14). Conversely, a statistically significant difference was found when considering eyes with corneal ectasia ≥ 8.0 mm (group 1B vs group 2B, 4.74 ± 2.90 D [95% CI, 4.09–5.38] vs3.68 ± 1.94 D [95% CI, 3.10–4.26], p = 0.02).
There was no statistically significant difference in postoperative BCVA when comparing group 1 and group 2, neither for eyes with corneal ectasia < 8.0 mm (group 1A vs group 2A, 0.12 ± 0.12 LogMAR [95% CI, 0.09–0.15] vs 0.16 ± 0.17 LogMAR [95% CI, 0.10–0.21], p = 0.11), nor for eyes with corneal ectasia ≥ 8.0 mm (group 1B vs group 2B, 0.16 ± 0.19 LogMAR [95% CI, 0.12–0.21] vs 0.15 ± 0.14 LogMAR [95% CI, 0.11–0.19], p = 0.33).
Complications
Intraoperative microperforations occurred in 5 eyes in group 1 (3.4%) and in 13 eyes in group 2 (17.1%). A statistically higher rate was reported in group 2 (χ2 = 12.80, p < 0.01). In all cases microperforations were managed conservatively and there was no need for conversion. Two cases of severe corneal neovascularization (defined as corneal neovessels going beyond the graft-host junction) were reported, both in eyes that underwent 9.0-mm DALK. Stromal immune rejection was more common in eyes that underwent 9.0-mm DALK (5 out of 76 eyes, 6.6%) compared to 8.0-mm DALK (4 out of 148 eyes, 2.7%) but the difference was not statistically significant (χ2 = 1.96, p > 0.1).