Outcomes of DALK After 'Big Bubble' Surgery
There is a growing trend among corneal surgeons to perform lamellar keratoplasty instead of PK for corneal diseases with healthy endothelium. The aim is to replace only the damaged layer(s) of the cornea while preserving the normal endothelium. For various corneal diseases that do not involve the endothelial layer (eg, keratoconus, stromal scars and lattice dystrophy), DALK is currently considered to be the preferred surgical procedure. Additionally, compared with PK the preservation of an intact DM is also believed to confer more structural integrity to the cornea. As mentioned before, DALK has two main problems: achieving a reliable baring of the DM and its poor reproducibility. Among the various techniques discussed, the BB technique is the one that offers a solution to both these problems and is therefore most widely practiced.
To our knowledge, no study has compared the outcome of DALK when the 'big bubble' was achieved with eyes with those when it was not. In this study, we found the mean corneal thickness to be higher in the FB group suggesting that despite 'very deep' manual lamellar dissection, often in two or three passes, a layer of deep stroma of variable thickness is often left behind. As the donor button is identically prepared in both techniques, the higher thickness in the FB group is entirely due to the residual deep stroma left behind.
Due to stromal irregularities that are inherently associated with manual dissection techniques, we expected contrast sensitivity and densitometry results to be different in the two groups. However, to our surprise, all visual and refractive parameters including contrast sensitivity were quite similar in the two groups. In addition, interface densitometry, as a measure of interface 'haze' was also comparable in the two groups. The mean contrast sensitivity as recorded with Pelli-Robson chart triplets read with best corrected vision was better at 9.3 in the BB group compared with 8.7 in the FB group, although this difference was not statistically significant. Similarly, the average densitometry reading in the BB group was 16.3 (range 11.2–22.7) compared with 19.6 (range, 12.6–42.7) in the FB group, again with no statistically significant difference. The absence of any dissimilarity between the two groups seems to suggest that in DALK once the depth of dissection has been extended beyond a certain point in the deep stroma, interface haze can be avoided or becomes clinically irrelevant. This is supported by other another study showing that following DALK, eyes with a residual stromal bed of <20 μm enabled patients to achieve visual quality comparable with that after PK. In this study we found the difference in mean thickness between the two groups to be approximately 65 μm, suggesting the thickness of residual bed left behind in our patients. Therefore, although we support a thinner residual bed to get better visual outcomes, our data suggest that a residual bed of <65 μm in the FB/manual dissection group would give comparable results. Direct measurement of the residual stromal thickness with optical coherence tomography (OCT) would have strengthened this argument but unfortunately OCT was not available at our centre. The differential response of the dissection at different depths of stroma can explained by differential stromal structure: anteriorly the lamellae are significantly interwoven and arranged obliquely to the corneal surface but in the posterior third of the stroma they are stacked parallel to the corneal surface. Moreover, in the deeper stroma there is lesser density and reflectivity of activated keratocytes. This could cause a different wound healing response to injury in the deeper stromal layers as compared with the superficial layers. Differences in composition of posterior and anterior stroma are well known. This is probably why we often get scarring and interface haze in cases of superficial corneal injuries and occasionally in flap-related complications of laser-assisted in situ keratomileusis, but do not find similar issues after DALK. In this regard, even in FB cases, injection of air may offer some advantage as it allows visualisation of the deep stroma and dissection can be continued till very few of the tiny bubbles are visible.
In this series, approximately half of the patients in either group managed to get reasonably good vision with spectacles and the rest who did not were mainly due to high/irregular astigmatism. Although most patients had their sutures removed at around 12 months post-surgery, some had selective removal of sutures to correct astigmatism and therefore were left with sutures in situ at the time of final examination. Despite these complications, many of these patients benefited from contact lens fitting to achieve BCVA. In this study, 72% in the BB group and 67% in the FB group achieved BCVA of logMAR 0.28 (Snellen equivalent 6/12) or better. This compares well with other reported studies even though the case mix was different. Most of the other reported studies on DALK are on patients with keratoconus while our study included other conditions such as stromal dystrophies and corneal scars, in addition to keratoconus (Table 1).
DALK produces levels of myopia similar to those of PK but over a narrower range. In our series, the average spherical equivalent and astigmatism were −4.22 (range +2.25 to −10.50) and 4.12 D (range 0.5–10.5), respectively. It was slightly more in the BB group than in the FB group with no statistically significant difference. The average postoperative spherical equivalent and astigmatism compared well with other studies.
The main limitation of this study is the small number of patients in each group. This has an implication on the statistical measurements, as some of the differences that are not so obvious due to the small sample size might become significant in a large study. This applies to the contrast sensitivity and densitometry measurements in this study, which show a slight apparent difference but could not be correlated statistically. Another limitation is that although we measured the thickness of the residual stroma in the FB/manual dissection group with Scheimpflug images, we feel that OCT might have shown the anatomy in much more detail and precision.
This study provides encouraging evidence to surgeons who are faced with an unsuccessful big bubble attempt. Manual dissection can be embarked upon, rather than converting to PK, in the knowledge that an equivalent outcome can be expected.
Discussion
There is a growing trend among corneal surgeons to perform lamellar keratoplasty instead of PK for corneal diseases with healthy endothelium. The aim is to replace only the damaged layer(s) of the cornea while preserving the normal endothelium. For various corneal diseases that do not involve the endothelial layer (eg, keratoconus, stromal scars and lattice dystrophy), DALK is currently considered to be the preferred surgical procedure. Additionally, compared with PK the preservation of an intact DM is also believed to confer more structural integrity to the cornea. As mentioned before, DALK has two main problems: achieving a reliable baring of the DM and its poor reproducibility. Among the various techniques discussed, the BB technique is the one that offers a solution to both these problems and is therefore most widely practiced.
To our knowledge, no study has compared the outcome of DALK when the 'big bubble' was achieved with eyes with those when it was not. In this study, we found the mean corneal thickness to be higher in the FB group suggesting that despite 'very deep' manual lamellar dissection, often in two or three passes, a layer of deep stroma of variable thickness is often left behind. As the donor button is identically prepared in both techniques, the higher thickness in the FB group is entirely due to the residual deep stroma left behind.
Due to stromal irregularities that are inherently associated with manual dissection techniques, we expected contrast sensitivity and densitometry results to be different in the two groups. However, to our surprise, all visual and refractive parameters including contrast sensitivity were quite similar in the two groups. In addition, interface densitometry, as a measure of interface 'haze' was also comparable in the two groups. The mean contrast sensitivity as recorded with Pelli-Robson chart triplets read with best corrected vision was better at 9.3 in the BB group compared with 8.7 in the FB group, although this difference was not statistically significant. Similarly, the average densitometry reading in the BB group was 16.3 (range 11.2–22.7) compared with 19.6 (range, 12.6–42.7) in the FB group, again with no statistically significant difference. The absence of any dissimilarity between the two groups seems to suggest that in DALK once the depth of dissection has been extended beyond a certain point in the deep stroma, interface haze can be avoided or becomes clinically irrelevant. This is supported by other another study showing that following DALK, eyes with a residual stromal bed of <20 μm enabled patients to achieve visual quality comparable with that after PK. In this study we found the difference in mean thickness between the two groups to be approximately 65 μm, suggesting the thickness of residual bed left behind in our patients. Therefore, although we support a thinner residual bed to get better visual outcomes, our data suggest that a residual bed of <65 μm in the FB/manual dissection group would give comparable results. Direct measurement of the residual stromal thickness with optical coherence tomography (OCT) would have strengthened this argument but unfortunately OCT was not available at our centre. The differential response of the dissection at different depths of stroma can explained by differential stromal structure: anteriorly the lamellae are significantly interwoven and arranged obliquely to the corneal surface but in the posterior third of the stroma they are stacked parallel to the corneal surface. Moreover, in the deeper stroma there is lesser density and reflectivity of activated keratocytes. This could cause a different wound healing response to injury in the deeper stromal layers as compared with the superficial layers. Differences in composition of posterior and anterior stroma are well known. This is probably why we often get scarring and interface haze in cases of superficial corneal injuries and occasionally in flap-related complications of laser-assisted in situ keratomileusis, but do not find similar issues after DALK. In this regard, even in FB cases, injection of air may offer some advantage as it allows visualisation of the deep stroma and dissection can be continued till very few of the tiny bubbles are visible.
In this series, approximately half of the patients in either group managed to get reasonably good vision with spectacles and the rest who did not were mainly due to high/irregular astigmatism. Although most patients had their sutures removed at around 12 months post-surgery, some had selective removal of sutures to correct astigmatism and therefore were left with sutures in situ at the time of final examination. Despite these complications, many of these patients benefited from contact lens fitting to achieve BCVA. In this study, 72% in the BB group and 67% in the FB group achieved BCVA of logMAR 0.28 (Snellen equivalent 6/12) or better. This compares well with other reported studies even though the case mix was different. Most of the other reported studies on DALK are on patients with keratoconus while our study included other conditions such as stromal dystrophies and corneal scars, in addition to keratoconus (Table 1).
DALK produces levels of myopia similar to those of PK but over a narrower range. In our series, the average spherical equivalent and astigmatism were −4.22 (range +2.25 to −10.50) and 4.12 D (range 0.5–10.5), respectively. It was slightly more in the BB group than in the FB group with no statistically significant difference. The average postoperative spherical equivalent and astigmatism compared well with other studies.
The main limitation of this study is the small number of patients in each group. This has an implication on the statistical measurements, as some of the differences that are not so obvious due to the small sample size might become significant in a large study. This applies to the contrast sensitivity and densitometry measurements in this study, which show a slight apparent difference but could not be correlated statistically. Another limitation is that although we measured the thickness of the residual stroma in the FB/manual dissection group with Scheimpflug images, we feel that OCT might have shown the anatomy in much more detail and precision.
This study provides encouraging evidence to surgeons who are faced with an unsuccessful big bubble attempt. Manual dissection can be embarked upon, rather than converting to PK, in the knowledge that an equivalent outcome can be expected.
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