Comparing Endothelial Cell Density after Sub-Bowman

1
Comparing Endothelial Cell Density after
Sub-Bowmans Keratomileusis and Photorefractive
Keratectomy for the Treatment of Myopia

• Ryan T. Smith, MD
• Daniel S. Durrie, MD
• George O. Waring IV, MD
• Priscilla Thomas, MS
• Financial Disclosure Dr. Smith, Dr. Waring and
  Ms. Thomas have no financial interest in the
  material presented. Dr. Durrie is a consultant
  for AMO and Alcon Labs.

2
Purpose

• To compare the effect of Sub-Bowmans
  Keratomileusis (SBK) and Photorefractive
  Keratectomy (PRK) on postoperative endothelial
  cell density (ECD).

3
Background

• 193-nm excimer laser is totally absorbed at the
  site of exposure and penetrate less than a
  diameter of a cell.
• Theories for endothelial cell damage
• Mechanical trauma from shock waves, thermal
  effects, actinic effects, and metabolic changes
• Most human studies show that PRK and LASIK does
  not effect corneal endothelial cell densities
• SBK uses a femtosecond laser to create a thin
  (90-110µm) corneal flap
• Effects of SBK on the corneal endothelium has yet
  to be reported

1. Collins M, Carr J. Effects of Laser In Situ
   Keratomileusis (LASIK) on the Corneal Endothelium
   3 Years Postoperatively. Am J Ophthalmol. 2001
   Jan131(1)1-6
2. Simaroj P, Kosalprapai K. Effect of Laser in
   situ Keratomileusis on the Corneal Endothelium.
   J Refract Surg. 2003 Mar-Apr19(2 Suppl)S237-40
3. Perez-Santonja J, Sakla H. Evaluation of
   Endothelial Cell Changes 1 Year After Excimer
   Laser In Situ Keratomileusis. Arch Ophthalmol.
   1997 Jul115(7)841-6.
4. Nassiri N, Farahangiz S. Corneal endothelial cell
   injury induced by mitomycin-C in photorefractive
   keratectomy Nonrandomized controlled trial. J
   Cataratc Refract Surg. 2008 Jun34(6)902-8
5. Diakonis V, Pallikaris A. Alterations in
   Endothelial Cell Density After Photorefractive
   Keratectomy With Adjuvant Mitomycin. Am J
   Ophthalmol. 2007 Jul144(1)99-103. Epub 2007 May
   23.
6. Durrie D, Slade S. Wavefront-guided excimer laser
   ablation using photorefractive keratectimy and
   sub-bowmans keratomileusis a contralateral eye
   study. J Refract Surg. 200824S77-S84.

4
Methods

• Fifty eyes of twenty-five patients were compared
  in a prospective, randomized contralateral,
  single center study to compare the effects of SBK
  and PRK on ECD at one and three months
  postoperatively
• SBK
• IntraLase (AMO, Santa Ana CA) 8.5mm diameter,
  raster pattern, superior hinge
• Flap thickness 100µm
• PRK
• 8.5mm Trephine
• 20 Ethanol for 25 sec
• Chilled with balanced salt solution
• All eyes treated with Custom LADARVision 4000
  (Alcon Labs, Ft. Worth TX) excimer laser
• PRK eyes were not treated with an antimetabolite
  such as mytomycin-C
• Endothelial cell densities was measured using
  Nidek Confocal 4 at preop, one, and three months

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Results

• SBK The average was 2995325 which decreased to
  2977358 at one month (p0.5756) and to 2931369
  at three months (p0.4106)
• PRK The average ECD was 3011329 which decreased
  to 2951327 at one month (p0.5736), and to
  2982365 at three months (p0.6513)

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Discussion

• Both PRK and LASIK have been shown to be
  predictable and effective for the correction of
  refractive errors
• This is the first contralateral study evaluating
  and comparing the effect PRK and SBK have on the
  corneal ECD.
• SBK was designed to be a hybrid procedure between
  PRK and LASIK by lessening the biomechanical
  impact of LASIK while preserving the quick visual
  recovery
• Experimental studies have shown endothelial cell
  damage with ablation depth within 40µm of
  Descemets membrane or ablation of 90 of the
  stroma
• The vast majority of studies agree that routine
  LASIK/PRK ablation depth do not cause endothelial
  cell damage
• With thinner flaps, ablation depth is reduced
  offering more endothelial protection when
  compared to standard LASIK.
• Our results suggest that SBK does not damage the
  corneal endothelium and has post operative ECD
  nearly identical to PRK

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Study Limitations

• Relatively short follow up period
• Lack of endothelial cell morphology analysis
  (coefficient of variation and percentage of
  hexagonal cells)

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Conclusions

• There is no statistically significant change in
  the ECD following SBK or PRK at 3 months
• Additionally, no statistically significant
  difference was found between the effect of SBK
  and PRK in the postoperative ECD
• Future studies with longer follow up and more
  detailed endothelial analysis are needed to
  confirm these early conclusions.