Slade & Baker Vision Center

Articles By Dr. Stephen G. Slade, MD

Sub Bowman’s Keratomileusis: The Case for a New “K” in Refractive Surgery

By Stephen G. Slade, MD, Houston, Texas

Whether surface or lamellar excimer laser surgery is better has been a long running debate in Ophthalmology. In general, even though different studies have found LASIK and PRK very similar in longer-term follow-up, in the immediate post-operative of 1 to 6 months, LASIK delivered better results and that drove the move towards LASIK as the procedure of choice. More recently, Allan and Shortt undertook a comprehensive review of all peer-reviewed published literature, as well as the FDA database, comparing outcomes in LASIK vs. PRK.1 The conclusion reached was that LASIK was a better procedure to PRK in safety and efficacy although the majority of the published literature dates prior to 2001., More recently, surface ablation has received a renewed interest as it has been perceived as safer in terms of ectasia. What if there is a better technique, a hybrid of the two procedures, that combined the faster recovery and better patient comfort of LASIK with any safety advantages of PRK?  For several years, Drs. Slade and Durrie have worked with customizing the LASIK flap to the patient and the ablation much as we customize the ablation to the patient. We have named this technique Sub Bowman’s Keratomileusis (SBK).

Prospective Comparison

In attempt to try and help answer this long-running debate between surface and lamellar, Daniel S. Durrie, MD, Overland Park, Kan., and Stephen G. Slade MD have recently completed a clinical study to compare PRK against SBK. In this study we used a smaller diameter flap (8.0 mm) and a thinner flap (≤100 µm). Our goal was to create a prospective, randomized study with a, contralateral eye design to make it as evenly matched as possible. We also agreed to use newer technology and techniques, so all patients received a customized ablation, PRK was performed using an established technique and all flaps were created with a 60-kHz femtosecond laser (IntraLase Femtosecond Laser, Advanced Medical Optics, Irvine, CA).

In some respects, the results confirmed what many of us had suspected, and it also led us to conclude that SBK may be just what we expected, a hybrid of PRK and LASIK. SBK may give refractive surgeons the biomechanical stability of surface ablation with the quicker visual recovery and relatively pain-free experience of LASIK.

Clinical Results

The study of 50 patients (100 eyes) was conducted at two sites (Slade in Houston, Texas and Durrie in Overland Park, Kansas) last spring, with one eye undergoing a femtosecond-laser assisted LASIK procedure (with an intended flap thickness of 100 µm) and the fellow eye undergoing a PRK procedure. Because it was a contralateral study, the two groups (Slade and Durrie) were almost even matched in terms of pre-operative mean refractive error: the femtosecond laser group was -3.64 D (-2.00 to -5.75 D) (SD = 0.97) and the mean manifest cylindrical refraction was -0.63 D (0 to -3.00 D), while the PRK group was -3.68 D (-2.00 to -5.75 D) (SD = 1.06) with a mean manifest cylindrical refraction was -0.64 D (0 to -2.75 D). The mean preoperative BSCVA was 20/17 in both groups (Slade and Durrie) with a range of 20/12 to 20/20 (SD = 2.47).

Because we wanted to truly analyze the outcomes, as well as to look at the biomechanical effects these two approaches have on the cornea, we utilized both standard, and newer, diagnostic testing. This included corneal topography, wavefront aberrometry (LADARWave, Alcon Laboratories, Inc., Ft. Worth, TX); contrast visual acuity, corneal sensitivity (Cochet-Bonnet aesthesiometer); Visante OCT imaging (Carl Zeiss Meditec, Oberkochen, Germany); corneal hysteresis (Ocular Response Analyzer, Reichert, Depew, NY); and, corneal response factor and, light scatter and optical visual acuity assessment (OQAS, Visometrics, Terrassa, Spain) at Dr. Slade’s clinic. At Dr. Durrie’s clinic, patients underwent two additional tests: Confocal through microscopy and Artemis High-frequency Ultrasound Imaging (Ultralink LLC, St. Petersburg, FL).

The full results from this Slade/Durrie study are due to be published later in the year, however, we can report that the visual results confirm that the femtosecond-laser group had a quicker return to functional vision than the PRK group. In fact, 100% of the femtosecond eyes could see 20/40 uncorrected on the first post-operative day compared to 42% in the PRK group. At 1 month post-op, 88% of the femtosecond eyes were 20/20 or better, uncorrected, compared to 48% of the PRK eyes.

The femtosecond-laser group also had lower levels of higher order aberrations at 1 and 3 months (coma and spherical aberration) compared to the PRK eyes. High and low contrast acuity and retinal image quality also favored the femtosecond group.  More importantly, the patients preferred their femtosecond laser eye to the PRK eye citing less pain and better vision during the first three post-operative months.

OCT measurements showed that the femtosecond-laser created flaps had a uniform (planar) thickness with a mean of 112 ± 5 µm (range 87 to 118µm).  The average standard deviation in flap thickness was 4 µm.

Finally, the biomechanical results were evaluated using two relatively new devices: the Optical Response Analyzer (Reichert) and the Pascal Dynamic Contour Tonometer (SMT/Ziemer Ophthalmic Group). Both devices measure how the cornea responds to biomechanical changes. The results showed that the femtosecond group eyes were statistically significantly better than the PRK eyes. Interestingly, none of the tests showed that PRK had a biomechanical advantage over a flap-based approach, although the clinical significance of the ORA and the Pascal tonometer still need to be proven. 

John Marshall has led the way investigating the strength of an eye after SBK compared to PRK by studying human cadaver eyes and believes that SBK and PRK are virtually indistinguishable in terms of biomechanical stabiliy. In fact, with the effect of wound healing in a patient, Marshall believes SBK may indeed be stronger than PRK.

The New K in Refractive Surgery

Now many surgeons are already doing laser vision correction with a femtosecond laser or a mechanical microkeratome and creating a flap of ≤100 µm on a regular basis. So what’s the difference between this technique and SBK?

At Slade and Baker we have defined SBK as a customized corneal flap that creates a flap based on the requirements of the patient, as well as the type of excimer laser and ablation being used. The intended thickness is between 90 and 110 µm. In the study discussed above, the corneal flap diameter was 8.5 mm. However, in my practice, I routinely use flaps with diameters of between 7 and 8 mm depending on which of my excimer lasers and what pattern I am using.

Key to creation of an SBK flap is the ability to create a planar corneal flap with a consistent thickness across the entire area. Although there are a number of mechanical microkeratomes that can create a 100-µm flap, the reality is that the method of flap creation used by mechanical keratomes tends to create a flap that is more meniscus in shape, so it is thicker in the periphery and thinner in the center. In my opinion, the most consistent, predictable method for creation of an SBK flap is with a femtosecond laser. This is supported by a number of clinical studies that have compared flap thickness of mechanical microkeratomes with the femtosecond laser.2-5 There are many more parameters in which we can customize flaps. We are looking at shaping the edge to improve strength, controlling the depth by the variations in the epithelium and oval flaps for astigmatism.

What Do Patients Want?

In recent years, refractive surgery volumes have been relatively flat or have even declined. In many respects this is related to the drawbacks associated with the two procedures we currently rely upon. With PRK, patients must cope with pain, haze and slow visual recovery. With LASIK, there is an increased incidence of dry eye, halos and glare, as well as the risk of corneal ectasia.

In this study, we found that there was less sensitivity and fluctuating of vision through one month in the SBK group. There was also less dry eye, double vision and blurry vision, glares/halos, grittiness and difficulty with night driving through three months post-op. Overwhelmingly, patients preferred the vision in their SBK eye out to three months by a margin of 2 to 1.

We know from the longer follow-up that by six months, the two groups, both Slade and Durrie, become relatively equal. This was not a surprise. But the point is that patients will make up their minds about the success or failure of their laser vision procedure in the first few months. More importantly, it’s during this time that they will talk to their family and friends about their experiences. Certainly more study is needed but with SBK, we may now have a technique that enables us to take the best of PRK and LASIK, while avoiding the negatives – giving us the ability bring back the “wow” to refractive surgery.

References

1. Shortt AJ, Bunce C, Allan BDS. Evidence for superior efficacy and safety of LASIK over photorefractive keratectomy for correction of myopia. Ophthalmology. 2006:113;1897-1908.

2. Duffey RJ. Thin flap laser in situ keratomileusis: flap dimensions with the Moria LSK-One manual microkeratome using the 100-micron head. J Cataract Refract Surg 2005;31:1159-62.

3. Taneri S. Laser in situ keratomileusis flap thickness using the Hansatome microkeratome with zero compressions heads. J Cataract Refract Surg 2006;32:72-7.

4. Flanagan GW, Binder PS. Precision of flap measurements for laser in situ keratomileusis in 4428 eyes. J Refract Surg 2003;19:113-23.

5. Shemesh G, Dotan G, Lipshitz I. Predictability of corneal flap thickness in laser in situ keratomileusis using three different microkeratomes. 2002;18:S347-51.