Impact of Graft Thickness on Visual Acuity After DSEK
Impact of Graft Thickness on Visual Acuity After DSEK
We saw no significant correlation between graft thickness and VA at any time point except 6 months. The statistically significant finding at 6 months may represent a type I error due to repeated statistical testing. Furthermore, only 20 pairs of data were available at 6 months, compared with 39 at last follow-up. The correlation between graft, or total corneal, thickness and VA at last follow-up was very weak. To our knowledge, this is the first study to investigate the correlation between graft thickness and VA at multiple time points. Furthermore, ours is the largest study of manually dissected graft thickness and VA.
The influence of graft thickness on VA has been the subject of several studies, yet remains controversial. We are aware of two studies that have reported a significant relationship between graft thickness and VA after DSAEK. Neff et al retrospectively analysed 33 cases of DSAEK using precut tissue. The eyes were divided into thin and thick graft groups, either side of the median postoperative graft thickness (131 μm). The thin graft group had a significantly better postoperative best spectacle-corrected visual acuity (BSCVA) than the thick graft group (p<0.01). When we compared eyes either side of the median graft thickness at last follow-up (142 μm), no such difference in VA was found.
Pogorelov et al measured central corneal and graft thickness 6 months after DSAEK in 15 eyes. They concluded that BSCVA correlates significantly with both central corneal and graft thickness (Pearson correlations −0.745 and −0.589, respectively, p<0.05). Chen et al measured corneal thickness with ultrasonic pachymetry 6 months after DSAEK. They found a weak, but statistically significant, correlation with postoperative BSCVA (r=0.117, p=0.001).
A larger number of studies have reported no significant correlation between graft thickness and VA after endothelial keratoplasty. Terry et al measured preoperative graft thickness in 65 cases of DSAEK. Graft thickness did not correlate significantly with improvement in vision (Pearson correlation =0.114, p=0.439). Neff et al noted that the mean graft thickness in those studies was higher than that of their study. This was considered to be a potential reason why earlier studies had not demonstrated a correlation. The median graft thickness of our eyes at last follow-up (142 μm) was only marginally higher than that of Neff et al (131 μm).
Price and Price conducted a study comparing the outcomes of microkeratome versus manual dissection of the graft. In those 200 cases, no significant correlation was found between central corneal thickness and BSCVA at 6 months postoperatively (p=0.25). Corneal thickness does not influence VA following deep lamellar endothelial keratoplasty either (r=0.03).
When the donor lenticule includes no stromal tissue, as in DMEK, visual advantages have been demonstrated. Unfortunately, DMEK is technically demanding and DMEK grafts are prone to tears during harvesting. The use of ultrathin grafts, of 100 μm thickness or less, has been proposed as an easier alternative procedure. There are currently limited published data regarding grafts of this thickness. Fourteen of the 51 grafts in our study (27.5%) were thinner than 100 μm at last follow-up, but this subgroup did not achieve a better final VA.
To avoid distorting any relationship, we excluded 16 eyes whose VA was impaired by either graft failure or pre-existing retinal pathology. We report a graft failure rate of 15.7%. While this is higher than some published figures, it should be remembered that this cohort represents the surgeons' early cases. It may therefore be related to an initial surgical learning curve. A systematic review of the safety and outcomes of DSEK and DSAEK found a mean primary graft failure rate of 5% (range 0–29%), and a mean graft rejection rate of 10% (range 0–45.5%). The relationship between graft thickness and VA may be confounded by graft viability.
Our mean postoperative BCVA (0.36 logMAR or 20/46 Snellen, n=36), measured between 4 and 38 months after DSEK, compares favourably with other series using manually dissected grafts. While there is evidence that DSAEK has superior visual outcomes, it requires an expensive microkeratome. Pre-cut tissue is not currently available in the UK. In our study, the median logMAR VA improved at each successive time point. There was no statistically significant difference, however, between 6 and 9 months. This supports published data that show visual rehabilitation occurs within 6 months postoperatively. Central graft thickness stabilised by 1 month postoperatively, much earlier than VA. Factors other than falling graft thickness must contribute to the improvement in VA.
Price and Price reported that DSEK induced a significant hyperopic shift, whereas DSAEK did not. We found no significant difference between pre and postoperative values for spherical equivalent or refractive cylinder. Our results support the conclusion of Rice et al: a refractive-neutral result can be achieved with manual dissection of the graft. We recognise, however, that measurement of refraction in eyes with corneal decompensation is unreliable.
The major limitation of this study is its retrospective nature. Preoperative and serial postoperative BCVA were often not available. The use of VA instead is undeniably less reliable, but should not have affected our conclusions. For measurements of both VA and thicknesses, the number of data points falls progressively at longer postoperative time points. This is partly because patients had not yet reached these time points, and partly due to missed follow-up.
The central thickness of manually dissected donor grafts does not appear to affect VA. Similarly, no significant correlation was found between central corneal thickness and VA after DSEK. Dissection of thinner lenticules of graft tissue may be unnecessary.
Discussion
We saw no significant correlation between graft thickness and VA at any time point except 6 months. The statistically significant finding at 6 months may represent a type I error due to repeated statistical testing. Furthermore, only 20 pairs of data were available at 6 months, compared with 39 at last follow-up. The correlation between graft, or total corneal, thickness and VA at last follow-up was very weak. To our knowledge, this is the first study to investigate the correlation between graft thickness and VA at multiple time points. Furthermore, ours is the largest study of manually dissected graft thickness and VA.
The influence of graft thickness on VA has been the subject of several studies, yet remains controversial. We are aware of two studies that have reported a significant relationship between graft thickness and VA after DSAEK. Neff et al retrospectively analysed 33 cases of DSAEK using precut tissue. The eyes were divided into thin and thick graft groups, either side of the median postoperative graft thickness (131 μm). The thin graft group had a significantly better postoperative best spectacle-corrected visual acuity (BSCVA) than the thick graft group (p<0.01). When we compared eyes either side of the median graft thickness at last follow-up (142 μm), no such difference in VA was found.
Pogorelov et al measured central corneal and graft thickness 6 months after DSAEK in 15 eyes. They concluded that BSCVA correlates significantly with both central corneal and graft thickness (Pearson correlations −0.745 and −0.589, respectively, p<0.05). Chen et al measured corneal thickness with ultrasonic pachymetry 6 months after DSAEK. They found a weak, but statistically significant, correlation with postoperative BSCVA (r=0.117, p=0.001).
A larger number of studies have reported no significant correlation between graft thickness and VA after endothelial keratoplasty. Terry et al measured preoperative graft thickness in 65 cases of DSAEK. Graft thickness did not correlate significantly with improvement in vision (Pearson correlation =0.114, p=0.439). Neff et al noted that the mean graft thickness in those studies was higher than that of their study. This was considered to be a potential reason why earlier studies had not demonstrated a correlation. The median graft thickness of our eyes at last follow-up (142 μm) was only marginally higher than that of Neff et al (131 μm).
Price and Price conducted a study comparing the outcomes of microkeratome versus manual dissection of the graft. In those 200 cases, no significant correlation was found between central corneal thickness and BSCVA at 6 months postoperatively (p=0.25). Corneal thickness does not influence VA following deep lamellar endothelial keratoplasty either (r=0.03).
When the donor lenticule includes no stromal tissue, as in DMEK, visual advantages have been demonstrated. Unfortunately, DMEK is technically demanding and DMEK grafts are prone to tears during harvesting. The use of ultrathin grafts, of 100 μm thickness or less, has been proposed as an easier alternative procedure. There are currently limited published data regarding grafts of this thickness. Fourteen of the 51 grafts in our study (27.5%) were thinner than 100 μm at last follow-up, but this subgroup did not achieve a better final VA.
To avoid distorting any relationship, we excluded 16 eyes whose VA was impaired by either graft failure or pre-existing retinal pathology. We report a graft failure rate of 15.7%. While this is higher than some published figures, it should be remembered that this cohort represents the surgeons' early cases. It may therefore be related to an initial surgical learning curve. A systematic review of the safety and outcomes of DSEK and DSAEK found a mean primary graft failure rate of 5% (range 0–29%), and a mean graft rejection rate of 10% (range 0–45.5%). The relationship between graft thickness and VA may be confounded by graft viability.
Our mean postoperative BCVA (0.36 logMAR or 20/46 Snellen, n=36), measured between 4 and 38 months after DSEK, compares favourably with other series using manually dissected grafts. While there is evidence that DSAEK has superior visual outcomes, it requires an expensive microkeratome. Pre-cut tissue is not currently available in the UK. In our study, the median logMAR VA improved at each successive time point. There was no statistically significant difference, however, between 6 and 9 months. This supports published data that show visual rehabilitation occurs within 6 months postoperatively. Central graft thickness stabilised by 1 month postoperatively, much earlier than VA. Factors other than falling graft thickness must contribute to the improvement in VA.
Price and Price reported that DSEK induced a significant hyperopic shift, whereas DSAEK did not. We found no significant difference between pre and postoperative values for spherical equivalent or refractive cylinder. Our results support the conclusion of Rice et al: a refractive-neutral result can be achieved with manual dissection of the graft. We recognise, however, that measurement of refraction in eyes with corneal decompensation is unreliable.
The major limitation of this study is its retrospective nature. Preoperative and serial postoperative BCVA were often not available. The use of VA instead is undeniably less reliable, but should not have affected our conclusions. For measurements of both VA and thicknesses, the number of data points falls progressively at longer postoperative time points. This is partly because patients had not yet reached these time points, and partly due to missed follow-up.
The central thickness of manually dissected donor grafts does not appear to affect VA. Similarly, no significant correlation was found between central corneal thickness and VA after DSEK. Dissection of thinner lenticules of graft tissue may be unnecessary.