Too Soon to SMILE
Despite the emergence of alternatives, femtosecond LASIK remains at the cutting edge of vision correction.
Julian Stevens |
We now have almost 30 years of expertise with the excimer laser. We started with PRK treatment on the corneal surface and initially saw regression – but that is now conquered. We had issues with early LASIK procedures, particularly with flap creation, but continued improvements to microkeratomes and in surgical technique resolved them. But even with a microkeratome, LASIK was a huge improvement over the original surface ablations, and with the introduction of the femtosecond laser for flap creation, microkeratome use became less popular, and femtosecond LASIK came to be the preferred procedure.
Wavefront-guided LASIK was introduced in 2001, which offered more predictable and improved excimer laser beam delivery. Given that the predicted and postoperative outcomes are so close, some or much of the statistical differences between them are statistical noise, so it’s clear that the procedure is becoming extremely precise. Iris registration for astigmatic improvement arrived next, improving not only astigmatic alignment, but also registration of higher order aberrations. Further, improvements in iris registration, cyclotorsion, laser calibration and stability, and very high resolution wavefront sensing have all taken place. Simply put, we are now approaching the limits of accuracy with excimer laser based corneal ablation.
However, another option has emerged. There has, for decades, been a desire to perform intrastromal corneal refractive surgery to effect corneal curvature change – a single femtosecond laser can be used for flap creation and refractive lenticule extraction (RELEX). Small lenticule extraction (SMILE) evolved from RELEX, and has become a clinically effective and widely performed procedure.
Some surgeons have started to see great benefits with this technique, but we have to judge this against the state-of-the-art LASIK benchmark. SMILE, in its current implementation, is a very interesting procedure and the subject of intense investigation. But there is a significant learning curve for new surgeons and those unfamiliar with the current VisuMax femtosecond laser. I have very experienced, highly capable colleagues at Moorfields who first used the VisuMax to perform SMILE without prior experience of creating LASIK flaps with the system. They found good outcomes difficult to master.
SMILE advocates argue that patients can return to their daily activities quickly due to the small incision nature of the procedure. But LASIK patients can return to work (or even visit the gym) the next day and have better early recovery of vision compared to SMILE patients.
When considering procedure options it’s important to consider risk versus benefit. When performing LASIK or SMILE, the potential complications include ectasia, diffuse lamellar keratitis, epithelial ingrowth, and decentration – but SMILE also brings the risk of a unique, novel complication of incomplete lenticule removal.
One potential benefit of SMILE is that it might have a biomechanical advantage over LASIK. Right now, we simply don’t know, as we don’t have true biomechanical measurement of corneal strength. There is an instrument in development for clinical use which will soon provide the answer, Intelon’s Brillouin optical scanner system (BOSS), but until then, any possible biomechanical advantage, disadvantage, or equivalence between the different procedures is unproven.
SMILE will get better as the VisuMax system improves. Better centration, cyclotorsional tracking, improved laser delivery with a cleaner lenticule interface, improvements in the surgical technique for lenticule extraction, and even custom lenticule shaping should be on the horizon. But today, SMILE does not appear to provide better long-term visual and refractive outcomes compared to wavefront-guided LASIK and does not provide faster visual recovery. Moreover, LASIK offers better control of higher-order aberrations than SMILE in its current iteration. Wavefront-guided LASIK remains at the forefront, and that’s why it has been and remains at the present time – my procedure of choice.