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SBK shows better short-term visual acuities over PRK, study finds
Surgeon calls sub-Bowman’s keratomileusis a ‘superior’ option for patients.
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SAN DIEGO — In a comparative contralateral study, femtosecond-enabled sub-Bowman’s keratomileusis and PRK achieved statistically similar results at 6 months’ follow-up. However, the eyes treated with sub-Bowman’s keratomileusis had significantly better visual acuities in the early postoperative period, the study authors said.
OSN Refractive Surgery Section Editor Daniel S. Durrie, MD, and Section Member Stephen G. Slade, MD, collaborated on the research effort, each operating on 50 eyes of 25 patients. Dr. Slade presented their findings at the American Society of Cataract and Refractive Surgery meeting.
Patients were randomly assigned into the PRK treatment group or sub-Bowman’s keratomileusis (SBK) treatment group according to their dominant eye. Eyes in the SBK group received an 8.5-mm, 100-µm flap created with an IntraLase laser (Advanced Medical Optics); eyes in the PRK group underwent ethanol- assisted PRK. All laser ablations were performed with a LADAR vision Custom Cornea excimer laser (Alcon).
Results through 6 months
Visual acuity fared significantly better in the SBK eyes during the immediate postoperative period, Dr. Slade told Ocular Surgery News in an interview. At 1 day postop, about half of the SBK eyes were 20/20 vs. 4% of the PRK eyes. All of the SBK eyes were 20/40 or better at day 1, as compared with about 40% of the PRK eyes, he said.
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“If you look at percent of eyes that are 20/40 or better over time, which is driving vision, 100% of the SBK were 20/40 or better at day 1 and at every visit through 6 months. So from day 1 on, 100% of our patients that had SBK were 20/40 or better. But about a third of PRK patients did not have good driving vision if they had PRK done in both eyes.
“What surprised us was that at 3 days, the results spread even further, because with PRK the epithelium is typically starting to meet in the middle, so visual acuity actually drops,” he said. He noted that, at day 3, the number of PRK eyes with 20/20 vision dropped to almost zero, whereas the SBK eyes were still improving.
Until 6 months, the SBK eyes achieved betters results in a variety of tests, including tear studies, aberrations and contrast sensitivity, Dr. Slade said. At 6 months, however, the eyes began to equilibrate, with uncorrected visual acuity of 20/20 or better in 92% of the SBK eyes and 94% of the PRK eyes, he said.
Advantages of SBK
Because SBK produces such good visual results in the early period, patients will be happier and more impressed with the treatment than if they had undergone PRK, Dr. Slade said.
“Patients would come in the next day after having LASIK, and they’d say ‘Wow!’ And that means that people get their results early, and they’re excited about them. And because of that, they will refer other patients,” he said.
Based on the results of the study, he said he now considers SBK a “superior” option for patients considering laser eye surgery.
“SBK is superior because people get their vision faster. They have 6 months where they see better,” he said. “So if two results are the same, but the SBK is faster and more comfortable, then you would say to your patients: ‘Here’s two surgeries, you’re going to get the same results at 6 months, but you’re going to get there quicker with SBK, and it’s going to hurt less.’ Which one would you pick?”
Dr. Slade added that a traditional argument in favor of PRK is that it is a safer procedure. However, he noted that the “guru of corneal biomechanics,” John Marshall, PhD, of St. Thomas’ Hospital in London, now states that the biomechanical strength of SBK is indistinguishable from PRK. Furthermore, at the 2006 European Society of Cataract and Refractive Surgeons meeting in London, he said thin-flap LASIK may be preferable to PRK for postoperative wound healing.
“With a 90-µm flap, Bowman’s is intact and the epithelium is beautiful. When we lift the membrane, we don’t disturb cells, we don’t induce a wound healing reaction, and thus we preserve the biomechanics of the cornea,” Dr. Marshall said at the conference.
Dr. Slade suggested that SBK blends the fast visual recovery of LASIK with the safety of PRK, “kind of like a hybrid of the two surgeries,” noting that he encountered no complications during the study and that his and Dr. Durrie’s results were the best they had ever achieved.
He and Dr. Durrie will continue following their cohort for at least a year, he said.
“The final thing to add is that more work is needed. So if someone says I do PRK differently and it’s much better than LASIK or SBK eyes, then I would encourage them to do the study and show us,” Dr. Slade said.
A common misconception
SBK is widely believed to mean thin-flap LASIK, but according to Dr. Slade, who coined the term, it actually refers to a more general idea of customizing the flap to the patient and the laser ablation.
“You customize the flap, just like you customize the ablation, according to the patient’s pupil size and according to the actual ablation pattern. In other words, you pick the size of the diameter and the thinness of the flap,” he said.
In the current study, he and Dr. Durrie chose to adjust diameter and depth to 8.5 mm and 100 µm, respectively, he said.
“In the future, you could do an oval for astigmatism. You could adjust the edge, making it more stepped or beveled to increase the strength. You might even come up with a smart-flap where the stromal depth varies according to the depth of the epithelium. So that’s the true concept behind SBK,” Dr. Slade said.
He noted that femtosecond lasers have enabled unprecedented customizability during flap creation.
“You might be able to make a thinner flap with the right metal blade keratome, but I do not think you can really do true SBK with a metal keratome,” Dr. Slade. “Because you really can’t adjust the diameter that well, you can’t get the perfect centration, you can’t get the planar quality of the flap, and you certainly can’t make an oval flap or adjust the shape of the edge.”
For more information:
- Stephen G. Slade, MD, can be reached at The Laser Center, 3900 Essex, Suite 101, Houston, TX 77027; 713-626-5544; fax: 713-626-7744; e-mail: sgs@visiontexas.com. Dr. Slade is a paid consultant for Advanced Medical Optics and Alcon.
- Andy Moskowitz is an OSN Staff Writer who covers all aspects of ophthalmology.