1. Comparing Endothelial Cell Density after Sub-Bowman’s 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-Bowman’s 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
Collins M, Carr J. Effects of Laser In Situ Keratomileusis (LASIK) on the Corneal Endothelium 3 Years Postoperatively. Am J Ophthalmol Jan;131(1):1-6
Simaroj P, Kosalprapai K. Effect of Laser in situ Keratomileusis on the Corneal Endothelium. J Refract Surg Mar-Apr;19(2 Suppl):S237-40
Perez-Santonja J, Sakla H. Evaluation of Endothelial Cell Changes 1 Year After Excimer Laser In Situ Keratomileusis. Arch Ophthalmol Jul;115(7):841-6.
Nassiri N, Farahangiz S. Corneal endothelial cell injury induced by mitomycin-C in photorefractive keratectomy: Nonrandomized controlled trial. J Cataratc Refract Surg Jun;34(6):902-8
Diakonis V, Pallikaris A. Alterations in Endothelial Cell Density After Photorefractive Keratectomy With Adjuvant Mitomycin. Am J Ophthalmol Jul;144(1): Epub 2007 May 23.
Durrie D, Slade S. Wavefront-guided excimer laser ablation using photorefractive keratectimy and sub-bowman’s keratomileusis: a contralateral eye study. J Refract Surg. 2008;24:S77-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

5. Results
SBK: The average was 2995±325 which decreased to 2977±358 at one month (p=0.5756) and to 2931±369 at three months (p=0.4106)
PRK: The average ECD was 3011±329 which decreased to 2951±327 at one month (p=0.5736), and to 2982±365 at three months (p=0.6513)

6. 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 Descemet’s 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
Durrie D, Slade S. Wavefront-guided excimer laser ablation using photorefractive keratectimy and sub-bowman’s keratomileusis: a contralateral eye study. J Refract Surg. 2008;24:S77-S84.
Collins M, Carr J. Effects of Laser In Situ Keratomileusis (LASIK) on the Corneal Endothelium 3 Years Postoperatively. Am J Ophthalmol Jan;131(1):1-6.
Simaroj P, Kosalprapai K. Effect of Laser in situ Keratomileusis on the Corneal Endothelium. J Refract Surg Mar-Apr;19(2 Suppl):S
Perez-Santonja J, Sakla H. Evaluation of Endothelial Cell Changes 1 Year After Excimer Laser In Situ Keratomileusis. Arch Ophthalmol Jul;115(7):841-6.
Jones S, Azar R. Effects of Laser in Situ Keratomileusis (LASIK) on the Corneal Endothelium. Am J Ophthalmol Apr;125(4):

7. Study Limitations Relatively short follow up period
Lack of endothelial cell morphology analysis (coefficient of variation and percentage of hexagonal cells)

8. 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.