Durrie: Corneal refractive surgery headed to the sub-Bowman layer

The latest in a series of reports from OSN’s Section Editors Summit brings to the forefront the next wave of refractive surgery — sub-Bowman’s keratomileusis.

I want to bring to your attention a topic that you are going to be hearing a lot about in refractive surgery. We could refer to this as the “Next K.”

We have seen RK, PRK, epi-K and LASIK, all stages in the evolution toward something that Stephen Slade, and I have named sub-Bowman’s keratomileusis, or SBK.

SBK refers to a procedure that is similar to LASIK, while trying to combine the advantages of LASIK and PRK in one.

It involves creating a corneal flap just below Bowman’s layer with a femtosecond laser. At this point I think it is open to the various manufacturers of femtosecond technology to see who will play in this space besides IntraLase (Advanced Medical Optics), which is what we have been using. The procedure is not branded by IntraLase – it comes from ophthalmology.

The flap is a bit smaller than traditional LASIK flaps at 8.5 mm. They are also shallower at 100 µm, so we are talking about very thin flaps.

The ‘next new thing’

Our data have so far shown SBK to have some of the advantages of both LASIK and surface ablation, such as decreased postoperative dryness, decreased loss of corneal sensitivity and decrease in higher-order aberrations.

Daniel S. Durrie

So why would we change the name? I think the time has come for a name change and to introduce to the market the “next new thing.”

It is time for the medical community to start thinking about change because we have not had great market penetration. There are still a lot of people wearing glasses out there. There are a lot of people who have not had refractive surgery, in general, that are good candidates.

If you ask them, “Why don’t you have PRK?” The answers are well known: pain, haze, slow visual recovery.

If you ask them, “Well, why don’t you have LASIK?” They answer, “Well, I’ve heard there are dry eyes. I’ve heard there’s halo. I’ve heard there’s a chance of ectasia.”

That’s basically the fear set, in addition to the cost factor.

Many people will say, “I’m waiting for the next procedure.”

So maybe we ought to give them the next procedure.

Smaller and shallower cuts

John Marshall, a renowned expert on corneal biomechanics, has been exploring these concepts for years. His general concept is that as you look at the cornea, the stronger fibers in the cornea are in the anterior and the periphery.

So if you make cuts that are smaller and shallower, you would be cutting fewer of the strong fibers. If you perform PRK, you create an insult that causes haze and pain. If you cut deeper, you create an unstable condition. But if you go into the sub-Bowman’s space, that might be an ideal situation.

Last year, Dr. Steve Slade and I did a two-site, randomized, prospective study in which we performed ethanol- assisted surface ablation in one eye and SBK in the other eye on 100 eyes of 50 patients. We both used the LADAR Vision Custom Cornea laser (Alcon) and created the flaps with an IntraLase laser.

At 1 month follow up, looking at the PRK eyes vs. the SBK eyes, 50% of PRK eyes had 20/20, vs. 90% of the SBK eyes. The advantage of a contralateral eye study is that you have the same patient so you can really evaluate the difference.

At 3 months, the PRK eyes did catch up. Yet if you look at the legal driving limit of 20/40, 100% of SBK eyes were at 20/40 just 1 day after surgery. Even at 1 week, only 52% of PRK eyes could have passed a driver’s test. That is a significant difference.

If you ask patients which eye had more pain, they reported that the PRK eye had more pain, even past 1 month.

If you look at which eye had better vision, at 3 months twice as many patients reported better vision in the SBK eye compared to the PRK eye. We also found that higher order aberrations were fewer in the SBK eye compared to the surface ablation eye, even after 6 months.

The idea that PRK has a better ablation platform for custom ablation is not true, either. In fact, we really have not found any advantage to PRK in these types of visual acuity studies, and patients are happier with the results.

How do we know it’s better? We have evaluated aberrations with a technology called Optical Quality Analysis System, or OQAS (Visiometrics), developed in Spain by Pablo Artal. This device measures lower order aberrations, higher order aberrations and scatter.

We have found that we can make the flaps thin because of the accuracy of IntraLase. We only found a 4-µm standard deviation in flap thickness when we measured individual flaps with optical coherence tomography.

We also found no difference in Schirmer’s test or lissamine green staining in SBK vs. PRK eyes. Dryness symptoms were worse up to 1 month in the PRK eyes, disproving the generally accepted notion that LASIK produces more dry eye than surface ablation. In terms of corneal sensation, we are cutting fewer nerves with the IntraLase and SBK.

A good move for refractive surgery industry

Basically, we are moving beyond LASIK and PRK. The femtosecond laser has enabled us to do this. This will ultimately be very good for the refractive surgery industry to start moving in this direction. Maybe we will even hit the front page of Time magazine again.

All of us know that LASIK is much better than patients think it is. But we have been branded with the same name and it has become commoditized also.

Looking to the future, the era we should be moving into is flap customization for the individual eye so that we can achieve better biomechanical results with our wound healing. This will drive the industry.

We know we can not go too thin with the flap because this will create its own set of problems such as flap folds or tearing. And, because epithelial thickness varies, we need better tools to measure preoperative epithelial thickness.

In our study, we achieved excellent results creating 112-µm flaps. I do not see any reason to go thinner than this until we have a better way to measure thickness.

Microkeratome blades and other excimer lasers can play a role in this new realm also. Yet, scientifically, I would rather have the physicians driving this change, rather than companies trying to figure out the sweet spot of the cornea.

Finally, in order to make this change to SBK, we will have to change our offices. We have the term LASIK all over our office, so we will have to expunge many things – and train our staff in the correct terminology – in order to get where we want to be.

For more information:

• Daniel S. Durrie, MD, can be reached at 5520 College Blvd., Suite 200, Overland Park, KS 66211; 913-491-3330; fax: 913-491-9650; e-mail: ddurrie@durrievision.com. Dr. Durrie was a clinical investigator for IntraLase, now owned by Advanced Medical Optics, and a paid consultant for Alcon.