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Sub-Bowman's keratomileusis (SBK) aims to combine the advantages of LASIK with the biomechanical stability of surface ablation. Results of a prospective study comparing SBK with advanced surface ablation in fellow eyes suggest it may be fulfilling its goals.
Dr. Durrie presented data from a two-center study he performed with Stephen Slade, MD, enrolling 50 patients who had their dominant eye randomly assigned to femtosecond laser-assisted SBK (Intralase FS, Advanced Medical Optics; 8.5-mm, 100-μm flap) or advanced surface ablation (8.5-mm ethanol-assisted PRK) and underwent the alternate procedure in the contralateral eye. All ablations were performed with an excimer laser (LadarVision 4000, Alcon Laboratories) using a wavefront-guided technique (CustomCornea, Alcon Laboratories) with a 6.5-mm optical zone and a 1.25-mm blend zone.
Evaluation of corneal biomechanics was based on measurements obtained after 3 to 6 months using a dynamic applanation system (Ocular Response Analyzer, Reichert Ophthalmic Instruments), a digital contact tonometer (Pascal Dynamic Contour Tonometer, Ziemer Ophthalmic Systems), topography (Orbscan, Bausch & Lomb), and wavefront changes. None of those endpoints showed any significant differences between the two procedures.
"Research from Cynthia Roberts, PhD, indicates a significant benefit of PRK compared with LASIK in terms of biomechanical stability. None of the tests we have done have shown any biomechanical advantage of PRK over SBK. We are hopeful that SBK may have the biomechanical features of PRK over the longer term. This is important research, and we need to move ahead and continue our follow-up in this exciting field," he said.
The idea for SBK-using a thinner, smaller corneal flap compared with traditional LASIK-was originated by John Marshall, PhD, King's College, London, based on his research on corneal biodynamics. The goal was to develop a refractive surgery procedure that would offer the biomechanical stability advantages of surface ablation while avoiding issues with haze, discomfort, and delayed visual recovery.
Dr. Durrie explained that although the initial aim was to create a thin flap just below Bowman's membrane using the femtosecond laser, early studies were performed to determine just how thin that flap could be. In a study using anterior segment optical coherence tomography to analyze flap characteristics, Dr. Durrie and colleagues determined that the 60-kHz femtosecond laser (IntraLase) could be used reliably to create thin planar flaps with minimal variation in thickness.
"We studied 25 eyes and analyzed 16 thickness measurements for each flap and found there was interindividual as well as intraindividual consistency in flap thickness. A single outlier had a flap thickness of 87 μm, but it was still easy to lift," said Dr. Durrie, president, Durrie Vision, Overland Park, KS.
"Based on those findings, we knew it was safe to try to use the femtosecond laser to try to go thinner with our flaps," he added.