Objectives: Our aim was to compare corneal densi­tometry values in keratoconic eyes undergoing deep anterior lamellar keratoplasty with the “big bubble” and manual lamellar dissection techniques, as well as penetrating keratoplasty.

Materials and Methods: This retrospective study evaluated the postoperative outcomes of 31 eyes treated with big bubble-deep anterior lamellar keratoplasty, 23 eyes with manual lamellar dissection-deep anterior lamellar keratoplasty, and 37 eyes with penetrating keratoplasty. Corneal densitometry, best corrected visual acuity, and other corneal tomographic parameters were recorded at 1, 6, 12, and 24 months postoperatively. The groups were compared with respect to the outcomes.

Results: There were no significant differences between the groups with respect to the anterior, central, and total corneal densitometry values at 0 to 2 mm and 2 to 6 mm. At 1 month postoperatively, the densitometric value in the posterior corneal layer of the 0- to 2-mm and 2- and 6-mm corneal zones was higher in the manual lamellar dissection-deep anterior lamellar keratoplasty group than in the big bubble-deep anterior lamellar kera­toplasty and penetrating keratoplasty groups (P = .001). At 6, 12, and 24 months, the manual lamellar dissection-deep anterior lamellar keratoplasty and penetrating keratoplasty groups had similar values in both zones in the posterior layer. The difference between manual lamellar dissection-deep anterior lamellar keratoplasty and big bubble-deep anterior lamellar keratoplasty in both zones in the posterior layer was statistically significant at 6, 12, and 24 months postoperatively (P < .05). The best corrected visual acuity of big bubble-deep anterior lamellar kerat­oplasty and penetrating keratoplasty groups was better than that of the manual lamellar dissection-deep anterior lamellar keratoplasty group (P = .047).

Conclusions: Corneal densitometry values may vary depending on the type of keratoplasty used in keratoconus patients. Higher densitometry values may affect visual quality in keratoplasty patients by decreasing or filtering the light that reaches the retina.

Key words : Corneal ectasia, Corneal transplantation, Haze, Topography, Visual quality

Introduction

Keratoconus is a bilateral and noninflammatory ectatic disorder, characterized by progressive steep­ening, thinning, and irregular astigmatism.^1,2 As the shape of the cornea changes, myopic and irregular astigmatic changes occur that compromise visual function. As progression continues, spectacle correction may not satisfy the patients, and contact lens wear may be necessary to improve vision in daily life.³ For severe cases, keratoplasty may be required for visual improvement.⁴

Deep anterior lamellar keratoplasty (DALK) has advantages over penetrating keratoplasty (PK) in the surgical treatment of keratoconus because it provides similar visual outcomes and preserves the recipient endothelium.^5,6 Thus, DALK eliminates endothelial rejection and complications associated with “open sky” surgery.⁵ However, there is a longer learning curve for successful DALK compared with PK. The most common technique for DALK is the “big bubble” (BB) method, in which air is injected between the posterior stroma and the pre-Descemet layer.^7,8 In this way, pneumatic dissection helps to expose the Descemet membrane (DM). However, the desired BB cannot be achieved in some situations, and manual lamellar dissection (MLD) may be required to finish the procedure.

The Pentacam HR (Oculus, Wetzlar, Germany) is a noncontact optical system that enables the mea­surement of anterior and posterior corneal topographic maps, 3-dimensional analysis of the anterior chamber, pachymetry, and densitometry and is used to detect light scattering and corneal transparency.^9,10 Some patients may complain of glare and photophobia despite a clear graft. It can be assumed that different types of keratoplasty techniques may affect backward light scattering and visual acuity. With the Pentacam HR, backward light scattering is measured quan­titatively as corneal densitometry.

The purpose of this study was to compare corneal densitometry values in keratoconic eyes undergoing DALK with BB and MLD techniques and PK.

Materials and Methods

This retrospective, nonrandomized, comparative study included 91 eyes in patients who underwent DALK and PK because of keratoconus. The study was performed in adherence to the tenets of the Declaration of Helsinki. Approval for the study was granted by the Local Ethics Committee. Patients were selected from the chart notes of patients who presented at the Cornea and External Disease Service in Ankara City Hospital. Patients included in the study were > 18 years old with significant keratoconus who could not tolerate contact lens wear, had a complete ophthalmological examination, and had completed postoperative follow-up of at least 24 months. Patients were excluded from the study if they had any systemic diseases or ocular pathologies other than keratoconus, such as cataract, uveitis, glaucoma, trauma, vitreoretinal disorders, dry eye, or corneal infections, or if the graft was rejected or DM perforation was experienced during DALK surgery. Specular microscopic images of the cornea endothelium were obtained for the donor cornea preoperatively by the eye bank technicians, and eligible grafts were from donors aged 17 to 70 years with a preoperative, baseline eye bank determined endothelial cell density of 2400 to 3400 cells/mm².

New Pentacam software (Figure 1) has enabled objective corneal densitometry analysis to be performed.10,11 Corneal backscattered light is measured in 4 annular zones of the cornea (0-2 mm, 2-6 mm, 6-10 mm, and 10-12 mm). The zones are centered on the apex of the cornea. Densitometry analysis is also performed by corneal depth, which includes the anterior (the most anterior 120 μm), central, and posterior (the most posterior 60 μm) layers (Figure 1). In this study, all images were taken by a trained technician in a darkened room before any other examination or drop instillation. Good quality images were recorded, as determined by Pentacam analysis. The peripheral zones are outside the optical zone and may lead to artifacts due to corneal scarring between donor and host tissue. Hence, backscattering values were analyzed in the 0- to 2-mm and 2- to 6-mm corneal zones but not in the 6- to 10-mm and 10- to 12-mm zones. The corneal densitometry values were compared between the groups of keratoconus patients who underwent BB-DALK, MLD-DALK, and PK at 1, 6, 12, and 24 months after surgery. All the sutures were removed from the patients at 12 to 18 months. In addition to densitometry values, uncorrected visual acuity and best corrected visual acuity (BCVA) and corneal topographic values were recorded.

Surgical procedure
All surgical procedures were performed by 1 of 3 experienced cornea surgeons (E. E. Karaca, D. Özek, and Ö. E. Kemer) under general anesthesia, using the same surgical procedure. In the BB group, the Anwar technique was used.¹² A 7.5-mm diameter trephination of 60% to 80% stromal depth was performed with a suction trephine. To achieve BB, air was injected with a 30-gauge needle bent to an angle of 45° with the bevel facing down. After BB formation, a peripheral paracentesis was applied to allow the aqueous humor to escape and to lower the intraocular pressure. After a brave slash, a viscoelastic substance was injected between the stroma and DM. The stroma was divided into 4 quadrants and removed gently from the DM. In cases of failed BB, the MLD technique, as defined by Anwar and colleagues, was performed.¹³ A donor cornea, 7.75 mm in diameter and devoid of DM, was sutured with 16 single 10-0 nylon sutures into the recipient bed. Penetrating keratoplasty was applied with a standard technique using a Hessburg-Barron trephine (JEDMED Instrument Co.).14 In all cases, an 8.00- to 8.25-mm graft was used for PK.

Moxifloxacin ophthalmic drops (Vigamox 0.5%; Alcon Laboratories) and dexamethasone ophthalmic drops (Maxidex, 0.1%; Alcon Laboratories) were administered 6 times a day for 2 weeks pos­toperatively. After 2 weeks, the drops were adjusted according to graft status at postoperative visits. The patients were assessed every week in the first month and every month in the first year. Ruptured and loose sutures were removed if necessary. At 12 to18 months, all sutures were removed.

Statistical analyses
Data obtained in the study were analyzed sta­tistically using SPSS Statistics software (version 22.0; IBM). Continuous data were stated as mean ± standard deviation values, and categorical variables were stated as number of cases (n) and percentage (%). Significance of the difference between the mean values of 3 groups was tested with the analysis of variance test and post hoc test. Spearman correlation analysis was used to determine the relationship between corneal densitometry and visual acuity. A Bonferroni-corrected P value < .05 was considered statistically significant.

Results

We evaluated 31 eyes treated with BB-DALK, 23 eyes with MLD-DALK, and 37 eyes with PK. The demographic data of the patients are shown in Table 1. The preoperative spherical equivalent, maximum keratometry, and central corneal thickness values were similar among the groups. Preoperative topographic astigmatism was determined to be statistically significantly higher in the MLD-DALK group than in the BB-DALK group (P = .027). There was a significant difference with respect to the preoperative thinnest corneal thickness (P = .004). Postoperatively, no statistically significant difference was determined between the groups with respect to these topographical parameters (Table 2). The preoperative and postoperative BCVA with spectacles values of the groups are shown in Table 3. At the final visit, the BCVA of the BB-DALK and PK groups was better than that of the MLD-DALK group (P = .047).

The densitometric values of different depths and zones are presented in Tables 4 to 7. There were no significant differences between the groups with respect to anterior, central, and total corneal densitometry at 0 to 2 mm and 2 to 6 mm. At 1 month postoperatively, higher densitometric values were determined in the 0- to 2-mm and 2- to 6-mm corneal zones in the posterior corneal layer in the MLD-DALK group than in the BB-DALK and PK groups (P = .001). At 6, 12, and 24 months, the MLD-DALK and PK groups had similar values in both zones in the posterior layer. The differences between the MLD-DALK and BB-DALK groups with respect to both zones in the posterior layer were statistically significant at 6, 12, and 24 months postoperatively (P < .05).

The results of the correlations between BCVA and corneal densitometry at the final visit are presented in Table 8. No significant correlation was found in the BB-DALK and PK groups. The anterior and total corneal densitometry values in the 0- to 2-mm zone were correlated with BCVA (r = 0.422, P = .045; r = 0.648, P = .001, respectively).

Discussion

In recent years, DALK has become more widespread instead of PK for treating advanced keratoconus. In this technique, healthy endothelium is preserved, and only the damaged layer of the cornea is removed. Thus, DALK maintains the structural integrity much better than PK and reduces the graft rejection rate.15 However, exposing the DM can be difficult without a successful BB. When the BB method fails, MLD can be used to expose the DM and continue DALK surgery. However, as the learning curve for DALK is longer than for PK, PK is still used for cases of macroperforation of the DM or large stromal scars. To the best of our knowledge, this is the first study to evaluate corneal densitometric analysis of BB-DALK, MLD-DALK, and PK using Pentacam HR densitometric software.The results of this study demonstrated that the mean postoperative corneal densitometry values of keratoconus patients treated with BB-DALK, MLD-DALK, and PK were similar except in the posterior corneal region during a 24-month follow-up period. The mean posterior corneal densitometry values in the 0- to 2-mm and 2- to 6-mm corneal zones were higher in the MLD-DALK group than in the BB-DALK group at 24 months postoperatively. There was also a transient difference between MLD-DALK and PK in the first month after surgery, but this was limited to the first month. These results suggest that interface irregularity in MLD-DALK may decrease the clarity and could affect the quality of vision.

Stromal irregularities caused by manual dis­section were assumed to affect densitometry measurements transiently. Compared with PK, the densitometry values were quite similar, although the difference in the posterior layer in the BB-DALK group persisted during the follow-up period. In addition, there was significant difference between the groups with respect to BCVA, with poorer visual acuity determined in the MLD-DALK group than in the BB-DALK and PK groups. Alzahrani and colleagues compared the corneal clarity of keratoconus patients who underwent PK and DALK.16 Although the sample was relatively small and included only DALK with BB, patients treated with DALK had significantly higher full-depth and posterior layer corneal densitometry values in the central 0- to 2-mm zone. In addition, there was a significant correlation between BCVA and corneal densitometry at full thickness in the DALK group. In the current study, there was a significant positive correlation between anterior and total corneal densitometry values in the 0- to 2-mm zone and BCVA (logMAR) in MLD-DALK group. However, there was no significant correlation in any densitometry value and BCVA in BB-DALK and PK groups. This result also suggests that interface irregularity in MLD-DALK may decrease the clarity of the cornea and cause deterioration the visual acuity.

The response of wound healing after surgery may induce corneal haze and increase corneal densitometry.¹⁶ After DALK surgery, extracellular matrix remodeling and keratocyte repopulation occur. In the first 4 hours after corneal injury, significant apoptosis develops.¹⁷ After apoptosis of keratocytes, the activation of other keratocytes and inflammatory cells begins, and thus, new keratocytes proliferate and migrate from neighboring keratocytes in 2 to 4 days.^18,19 The activation of keratocytes induces matrix metalloproteinase synthesis and contributes to tissue remodeling. Some of the keratocytes transform into myofibroblasts in 1 to 2 weeks, which is also important in reorganization of the cornea.²⁰ This complex process leads to corneal haze formation.²¹ The increased posterior corneal layer densitometry value in MLD-DALK in the current study could have been due to mechanical injury in these patients. Manual dissection probably causes more keratocyte activation compared with the BB technique. In addition, the preoperative topo­graphic parameters, including maximum keratometry, pachymetry, and astigmatism values, show that patients in the MLD-DALK group had more advanced keratoconus than those in the BB-DALK group. In this study, it was not possible to evaluate endothelial cell density of the keratoconus patients with specular microscopy preoperatively. However, patients with more advanced keratoconus might have worse endothelial cell density and function and, hence, more edematous and opaque corneas postoperatively.²² The BCVA and corneal densitometry of the patients with BB-DALK and PK were quite similar in the 24-month follow-up period. In contrast to the current study, Alzahrani and colleagues demonstrated that the patients with BB-DALK had higher densitometry values than PK.16 However, there were fewer patients in that study, and only BB-DALK and PK patients were compared.

Providing the same dissection plane in the manual dissection technique is quite difficult compared with the BB technique. Exposing the DM is much easier in BB-DALK. The duration of MLD-DALK is longer than BB-DALK and PK. The frustrating nature of the MLD-DALK surgical process may increase the densitometry in the posterior layer as a result of interface problems. Scorcia and colleagues compared BB-DALK and visco-bubble DALK in keratoconus patients.²³ It was reported that stromal interface irregularity resulted in poorer BCVA and higher corneal densitometry in the posterior layer during a 3-month follow-up period. However, Scorcia and colleagues stated that this effect on interface quality did not cause a long-term difference in BCVA and densitometry. In the current study, the residual stroma in the BB-DALK and MLD-DALK groups could not be compared. However, the postoperative central corneal thickness values in both techniques were similar. Javadi and colleagues compared the visual outcomes of BB and a manual dissection group.²⁴ The BB-DALK group was seen to have better visual outcomes than the MLD-DALK group despite similar residual stroma. Similarly, Fontana and colleagues showed better visual outcomes in a big bubble group compared with manual dissection.²⁵ Both of these studies assumed that the remaining posterior stroma in manual dissection led to poor visual outcomes compared with BB-DALK as it is not possible to achieve a bare DM with the techniques of MLD-DALK. The visual outcomes of DALK were shown to be similar to those of PK when there was a < 65-µm residual stromal bed.⁵ Similarly, the current study results showed that postoperative densitometry and visual outcomes were comparable between PK and BB-DALK.

This study had some limitations, primarily the retrospective design, small sample size, the lack of information on endothelial cell density comparison between groups, and the inability to measure the thickness of the recipient bed after manual dissection. However, to the best of our knowledge, it is the first study to compare corneal densitometry values among BB-DALK, MLD-DALK, and PK in keratoconus patients. Therefore, the results are promising. Another limitation was the lack of other stromal pathologies in the patients who underwent keratoplasty. Further studies are required for comparisons to be made with other stromal pathologies to be able to enhance our knowledge about densitometry.

In conclusion, corneal densitometry provides quantitative evaluation about different DALK techniques and PK. The outcomes of lamellar and penetrating surgery could be compared objectively with the use of Pentacam. Higher densitometry may affect visual quality in keratoplasty patients by decreasing or filtering the light that reaches the retina.

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