The goal of refractive surgery is to correct visual defects through lenticular or corneal surgical procedures. Of all the defects, presbyopia has been the most challenging to correct, especially because its causes are complex and not fully understood. Presbyopia is also the only refractive error that is considered to be progressive in nature, although we do not really know if it truly progresses.
An effective surgical option for the correction of presbyopia is attractive because there is an enormous pool of presbyopic patients who desire good uncorrected vision. The mission of presbyopic surgery should be to eliminate the symptoms of this refractive error rather than to correct the anatomical defect itself. If we can provide patients with good functional vision at multiple distances, then it is not necessary to restore true accommodative function in the form of a dynamically adjusting optical power of the eye.1
One potential site for correction of presbyopia is the cornea, the most common target of refractive surgery and the most accessible part of the eye's optical system. Another site could be the anterior chamber, but this is as yet theoretical and would likely prove extremely difficult in reality. The human crystalline lens may be replaced with a multifocal or accommodating IOL, many styles of which are now available or under development.
For several years, LASIK has been the most popular surgical approach to correct myopia, hyperopia, and astigmatism. It is a corneal procedure familiar to surgeons and patients, and it is popular because it can be performed bilaterally on the same day, is relatively quick and painless, and has proven to be safe.
LASIK is an appropriate approach to presbyopia correction because of the absence of postoperative haze and its associated refractive complications, the absence of regression, and the presence of a regular and thin flap that can protect a multifocal profile created in the stroma.
We have developed a new procedure for the elimination of the refractive error implied in presbyopia. It is called presbyopic multifocal LASIK (PML).2 This procedure can be performed in presbyopic patients with myopia, hyperopia,3 astigmatism, or emmetropia. Multifocality is created on the cornea using a multistep treatment in which several independently calculated ablations are performed at various optical zones.
The central cornea is considered an excellent site for distance vision because it is relatively aberration-free; aberrations in the corneal periphery may actually be useful for near focus.
The curve we obtain with PML is prolate and smooth, thanks to a transition zone. Analyzing the curve in detail, it is not multifocal (more than one step) but rather aspheric. The nomograms will be available soon.
The multistep process is a highly customizable procedure with four phases. First, astigmatism is treated as if we were doing a purely classic astigmatic laser correction. Second, we treat the distance defect in a zone on the central cornea. Third, we treat the near defect in a larger optical zone. Finally, other zones may be created to ensure that the overall power of the cornea is emmetropic, even though there are varying powers in the different optical zones.
We began performing PML in 2002, with further software development in 2007. We started cautiously, treating 10 patients in only their nondominant eye. We were convinced that a bilateral approach would be more effective, so we adjusted and refined the nomograms toward this goal.
Our studies so far were performed with the Technolas 217z laser (Bausch & Lomb, Rochester, New York), employing a thin LASIK flap. However, we believe that the procedure is theoretically technology-independent and can work with any keratome and laser combination, including flap creation with a femtosecond laser. (A flap of 70–80 µm is the best to protect and not to disturb the multifocal profile of the stroma beneath it.)
In our experience, 95% of patients achieve 20/20 and J2 or better UCVA binocularly. Patient satisfaction levels have been high, and there have been no retreatments to date. Typically, patients experience a myopic shift during the first month postoperatively and then stabilize at plano. Retreatment rate is approximately 2% (mostly ±0.05 D for far).
In the PML technique, the same amount or less tissue than other laser vision correction procedures is ablated, so although pachymetry is important, it is no more a factor than in monofocal LASIK. Patients must be willing to participate in training their eyes to see at near with the multifocal correction.
In our experience so far, PML is a successful procedure that has been well-accepted by patients and is in high demand at our institute. Currently, approximately 50% of our presbyopic patients are PML candidates. The results experienced by patients have generated many referrals. Ongoing research is required to determine whether and when regression may occur.
Roberto Pinelli, MD, is the Scientific Director of the Istituto Laser Microchirurgia Oculare, Brescia, Italy. Dr. Pinelli states that he has no financial interest in the products mentioned. He may be reached at pinelli@ilmo.it.
