Is SBK a better refractive surgery than LASIK?

October 10, 2007

Approximately 90% of vision correction surgeries performed in the United States are LASIK procedures. But you may soon start hearing about a new refractive surgery procedure called SBK (or "sub-Bowman's keratomileusis"). SBK is similar to LASIK, and may be less likely to cause dry eyes and other complications.

The limitations of LASIK

Despite its popularity and high success rates, LASIK has been associated with the development of persistent dry eyes in some patients. In rare instances after LASIK, a serious post-operative complication called corneal ectasia can develop. Ectasia is a bulging distortion of the shape of the cornea due to loss of its structural integrity. Though there can be several causes of ectasia, reducing the thickness of the cornea during LASIK surgery appears to be a risk factor.

The limitations of PRK

There is less risk of dry eyes and ectasia with PRK (also called "surface ablation"). But two factors have limited the popularity of PRK: there is significant discomfort after PRK surgery and it can take a week or more before the patient's vision is acceptable for driving.

SBK: A little like LASIK, a little like PRK

Ophthalmologists and refractive surgeons Stephen Slade, M.D. and Daniel Durrie, M.D. have pioneered a new laser vision correction procedure that may deliver the best features of LASIK and PRK, while reducing the downsides of these procedures.

The new procedure, called "sub-Bowman's keratomileusis" or "SBK," uses a femtosecond laser to create a thinner corneal flap than a typical LASIK flap. SBK flaps are approximately 110 microns thick, whereas LASIK flaps are usually 120 to 160 microns thick.

The term "sub-Bowman's" is used to describe the procedure because the depth of the flap is just thick enough to include Bowman's membrane &ndash the thin, non-cellular layer of the cornea that separates the outer epithelium from the underlying corneal tissue that is reshaped by an excimer laser in laser vision correction procedures like LASIK, PRK and (now) SBK.

By making the corneal flap thinner, SBK surgeons should be able to provide a comfortable procedure with fast visual recovery (like LASIK) and also decrease the risk of dry eye and ectasia (like PRK).

How it works

In addition to being thinner, the SBK flap is also smaller in diameter. The combination of a thinner ands smaller flap damages fewer corneal nerve endings during the procedure, compared to LASIK. Corneal nerve damage has been implicated as the cause of dry eye after LASIK surgery.

The smaller SBK flap also damages fewer of the strong corneal fibers that help the cornea maintain its proper shape. With fewer of these strong fibers affected, SBK may lessen the risk of corneal ectasia, compared to eyes that have had LASIK surgery.

Study results: SBK vs. PRK

To test the visual results and patient satisfaction of SBK vs. PRK, Drs. Slade and Durrie performed SBK on one eye and PRK on the other eye of 50 patients. Among the results of their study:

• The day after surgery 100% of the SBK eyes had visual acuity of 20/40 or better(the legal limit for driving). Even at 1 week after surgery, only 52% of PRK eyes were 20/40 or better.
• At 1 month after surgery, 90% of the SBK eyes had 20/20 visual acuity, whereas only 50% of the PRK eyes were 20/20.
• Patients reported that the SBK eye was more comfortable after surgery, and that the PRK eye had more pain, even at 1 month after surgery.
• At 3 months after surgery, twice as many patients reported better vision in the SBK eye compared to the PRK eye.

A matter of perception?

The femtosecond laser, which is also used to create corneal flaps in "all-laser" LASIK, is the technology breakthrough that has made SBK possible. The femtosecond laser allows refractive surgeons to make thinner, more predictable flaps than a bladed microkeratome, with a lower risk of flap complications. The most popular femtosecond laser is made by IntraLase Corp. (Irvine, CA).

Though SBK is very similar to all-laser LASIK, Dr. Durrie feels it's time to introduce the new name and procedure to the market. He feels the introduction of SBK may overcome some of the fears consumers have about refractive surgery. He also says it may be time to tell people who've been putting off refractive surgery because they're waiting for "the next procedure" that SBK is that new procedure.

SOURCE: Durrie: Corneal refractive surgery headed to the sub-Bowman layer. Ocular Surgery News; Vol 25, No 25. October 15, 2007. pp 10-12.

Last updated: February, 2010