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• Re:Re:Trephination size and success of big-bubble formation in deep anterior lamellar keratoplasty for keratoconus.
  Alireza Peyman
  Published on: 21 April 2016
• Re:Trephination size and success of big-bubble formation in deep anterior lamellar keratoplasty for keratoconus.
  Dr. Sepehr Feizi
  Published on: 21 April 2016
• Trephination size and success of big-bubble formation in deep anterior lamellar keratoplasty for keratoconus.
  Alireza Peyman
  Published on: 21 April 2016

• Published on: 21 April 2016

  Re:Re:Trephination size and success of big-bubble formation in deep anterior lamellar keratoplasty for keratoconus.

  • Alireza Peyman, Assistant Professor

  The response by Feizi et al is comprehensive and notable. But the condition could be simply explicated by a clear demonstration of actual data in a table representing DALK success-failure versus different trephination diameters. If the authors accept to publish these data, every reader would be able to deduce the practically applicable end result.

  Conflict of Interest:

  None dec...

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  The response by Feizi et al is comprehensive and notable. But the condition could be simply explicated by a clear demonstration of actual data in a table representing DALK success-failure versus different trephination diameters. If the authors accept to publish these data, every reader would be able to deduce the practically applicable end result.

  Conflict of Interest:

  None declared

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  Conflict of Interest:
  None declared.

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• Published on: 21 April 2016

  Re:Trephination size and success of big-bubble formation in deep anterior lamellar keratoplasty for keratoconus.

  • Dr. Sepehr Feizi
  • Other Contributors:
    • Mohammad Ali Javadi, Seyed-Hashem Daryabari

  We would like to acknowledge our appreciation for Dr. Peyman for paying close attention to our article and raising important points. Now, big-bubble deep anterior lamellar keratoplasty (DALK) is a corneal transplantation technique of choice for corneal stromal pathologies not involving endothelium such as keratoconus. The principle shortcoming of this technique is that it is technically challenging. Any attempts to incr...

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  We would like to acknowledge our appreciation for Dr. Peyman for paying close attention to our article and raising important points. Now, big-bubble deep anterior lamellar keratoplasty (DALK) is a corneal transplantation technique of choice for corneal stromal pathologies not involving endothelium such as keratoconus. The principle shortcoming of this technique is that it is technically challenging. Any attempts to increase the success rate of big-bubble formation should be commended as it is a critical step and difficult for many surgeons. Our article aimed to investigate the possible influence of patient- and surgery-related variables on this rate in a homogeneous group of keratoconus patients who were operated on by a single experienced surgeon.1 The exploratory analysis revealed that among various factors, recipient sex and trephination size significantly influenced the rate of achieving a bare Descemet's membrane (DM).1 Following are points in response to the critiques posed by Dr. Peyman: 1) As mentioned in the Methods section, the influence of the independent variables on the success rate of big-bubble formation was first investigated using univariate analysis. Only the variables which had a significant association at a univariate level were entered into multiple regression analysis. In our final confirmatory analysis, there was a limited number of variables including recipient sex, vertical corneal diameter, corneal thickness, anterior chamber depth, and recipient trephination size. Therefore, it is very unlikely that multiple comparisons led to a false positive correlation between the variables. Additionally, comparisons between the bare DM group and the manual dissection group using Chi-square and Mann-Whitney tests revealed significant differences between the two groups in terms of patient sex distribution and recipient trephination size confirming the results yielded by multiple regression analyses. This indicates that the significant associations found in our study were not caused by chance. 2) Providing no evidence, Dr. Peyman anecdotally reports that when the recipient trephination size is large, it is difficult to complete the procedure as big-bubble DALK. However, Huang et al.2 used two different recipient trephination sizes (7.75 mm and 8.25 mm) and reported that big- bubble DALK was successfully completed in 89.4% of the 7.75-mm group and in 84.8% of the 8.25-mm group (P=0.60). It is possible that due to some technical problems Dr. Peyman has with large diameter trephines in DALK, he has been convinced that the larger diameter of trephination could potentially cause more failure of the procedure. 3) According to Dr. Peyman's experience, cutting the recipient cornea in a case of incomplete small bubbles is difficult and could create holes or ruptures in peripheral posterior layers. However, corneal surgeons who master different techniques of DALK such as manual dissection technique and Melle's technique can safely complete dissection using viscoelastic materials or blunt spatula once the dissection plane is reached. 4) Dr. Peyman refers to a study by Dua et al.3 to explain the reason for difficult expansion of the big bubble to the border of trephination when a large trephine is used. In the initial article, Dua et al.3 revealed that pre-Descemet posterior stromal layer (PDL) ended before the termination of DM. In the subsequent study, however, they provided evidence that PDL extends beyond the edge of the big bubble to insert into the trabecular meshwork.4 Therefore, it is possible to separate posterior stroma from the PDL far to the corneal periphery. We conducted a study comparing the rate of achieving a bare DM during big-bubble DALK using central versus peripheral air injection (the manuscript has been submitted). Using surgical calipers, we precisely measured the size of achieved bubbles which ranged from 7.0 to 10.5 mm. These data indicate the big bubble can successfully be enlarged beyond that Dr. Peyman mentioned. To summarize, DALK grafts employing a larger diameter recipient bed provide several advantages including low graft astigmatisms, stable postoperative refractive outcomes, and better graft biomechanics.2,6 Our recent study adds a new advantage to the application of a large trephine size in keratoconic eyes which increases the likelihood of successful big bubble formation during Anwar's DALK technique.1

  References 1- Feizi S, Javadi MA, Daryabari SH. Factors influencing big-bubble formation during deep anterior lamellar keratoplasty in keratoconus. Br J Ophthalmol. 2015; In press. 2- Huang T, Hu Y, Gui M, et al. Large-diameter deep anterior lamellar keratoplasty for keratoconus: visual and refractive outcomes. Br J Ophthalmol. 2015; 99:1196-1200. 3- Dua HS, Faraj LA, Said DG, et al. Human corneal anatomy redefined: a novel pre-Descemet's layer (Dua's layer). Ophthalmology. 2013;120:1778- 1785. 4- Dua HS, Faraj LA, Branch MJ, et al. The collagen matrix of the human trabecular meshwork is an extension of the novel pre-Descemet's layer (Dua's layer). Br J Ophthalmol. 2014;98:691-697. 5- Feizi S, Einollahi B, Yazdani S, et al. Graft biomechanical properties after penetrating keratoplasty in keratoconus. Cornea. 2012;31:855-858. Sepehr Feizi, MD, MSc Assistant Professor of Ophthalmology, Shahid Beheshti University of Medical Sciences, Tehran, Iran. Email: sepehrfeizi@yahoo.com Conflict of Interest: None declared

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  Conflict of Interest:
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• Published on: 21 April 2016

  Trephination size and success of big-bubble formation in deep anterior lamellar keratoplasty for keratoconus.

  • Alireza Peyman, Assistant professor

  We enthusiastically read the article entitled: "Factors influencing big-bubble formation during deep anterior lamellar keratoplasty in keratoconus" by Feizi et al (1). Authors have included many variables to find factors influencing success in the big bubble formation and defined meaningful P value for statistical significance to less than 0.05. From statistical viewpoint multiple comparisons could increase the false di...

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  We enthusiastically read the article entitled: "Factors influencing big-bubble formation during deep anterior lamellar keratoplasty in keratoconus" by Feizi et al (1). Authors have included many variables to find factors influencing success in the big bubble formation and defined meaningful P value for statistical significance to less than 0.05. From statistical viewpoint multiple comparisons could increase the false discovery rate and an increase in type I error. A method of multiple testing corrections, like Bonferroni correction, or other more complex statistical adjustment is necessary to decrease the rate of apparently meaningful findings which may be due to random chance alone (2). According to our unpublished experience with big bubble deep anterior lamellar keratoplasty (DALK) in keratoconus, the bubble is difficult to spread to peripheral cornea in most cases. In larger trephinations of the recipient cornea, the surgeon needs to create a larger diameter bubble. Trying to extend the bubble sometimes will end with rupture of Dua's and Descemet's layers. On the other hand cutting the recipient cornea in a case of small bubble diameter is difficult and could create holes or ruptures in peripheral posterior layers with subsequent failure. This problem might be explained by the anatomy of the pre-Descemet's (Dua's) layer that allow expansion of type-1 bubbles up to diameter of 8.5 millimeters (3). As many of our successful DALK big bubbles are formed anterior to Dua's layer, the larger the diameter of trephination could potentially cause more failure of the procedure. In our experience trephination of recipient more than 8.25 mm could be associated with increased failure of the DALK. Revising the statistical analysis of the study by Feizi et al (1) and adjustment for the level of significance of multiple comparisons may reveal different results to this valuable article. ? References:

  1. Feizi S, Javadi MA, Daryabari SH. Factors influencing big-bubble formation during deep anterior lamellar keratoplasty in keratoconus. The British journal of ophthalmology. 2015. 2. Streiner DL. Best (but oft-forgotten) practices: the multiple problems of multiplicity-whether and how to correct for many statistical tests. The American journal of clinical nutrition. 2015. 3. Dua HS, Faraj LA, Said DG, Gray T, Lowe J. Human corneal anatomy redefined: a novel pre-Descemet's layer (Dua's layer). Ophthalmology. 2013;120(9):1778-85.

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