21

Customized Ablation and LASEK

Erin D.Stahl, MD and Daniel S.Durrie, MD

Durrie Vision Research

Overland Park, KS

WAVEFRONT MEASUREMENT AND CUSTOM ABLATION

The Progression of Technology

In looking back on the history of refractive surgery, one of the most notable elements of this surgical field is the rapid and profound advance of technology. What began as a very basic science has progressed into a field dependent on sophisticated systems for both diagnostics and treatment of errors in the human optical system.

Focusing on the evolution of technology since the advent of excimer laser refractive treatment, we have seen extensive change and improvement. Laser engineers have moved from stationary, broad-beam lasers to a level of technology capable of creating small, flying spot lasers used in conjunction with extremely fast eye tracking systems. The current laser technology allows for a high level of precision in creating ablation patterns and ensuring correct alignment during treatment (Fig.1). The software that controls these lasers has progressed from treating simple myopia to treating astigmatism, and from there into the treatment of hyperopia, astigmatic hyperopia, and mixed astigmatism. With the combination of extremely sophisticated hardware and software, we now have the ability to treat nearly any spherocylindrical correction with accurate and predictable results.

At this point in time, clinicians feel very confident in their ability to perform refractive surgery and bring patients a level of vision comparable with their preoperative vision in glasses or contact lenses. The question that arises is whether we can take patients to a level of vision quality that surpasses that of traditional spherocylindrical correction. To address this question, it is important to look at the current tools and technology used to determine the desired correction. Although we have sophisticated laser technology, the ability to make complex maps of the corneal surface with topography, and software that can be programmed to determine precise ablation patterns, we still determine treatment by sitting the patient in front of the phoroptor. We will never move beyond the visual capability of spherocylindrical correction if we do not begin to take a more sophisticated approach to measurement.

Wavefront Diagnostics

With the advent of the Star Wars anti-missile defense system, astronomers began trying to determine methods to detect and correct defects in long-range telescopes. They developed

LASEK, PRK, and excimer laser stromal surface ablation 260

Figure 1 LADARVision®. Flying, small-spot laser beam. Courtesy of Alcon Laboratories.

a system of measurement that traced light ray s through an optical sy stem and recorded defects or aberrations. As vision scientists, laser companies and clinicians began to search for a new system of diagnostics for the human eye; they turned to this concept of wavefront-sensing technology as an ideal method of measuring aberration in the human eye.

Wavefront measurement of the human eye is achieved by passing light rays in the form of a small, low-intensity laser through the eye and onto the retina. As these rays are reflected off of the retina and travel out of the eye, their path is distorted by aberrations in the optical system. As they exit from the eye, the light rays are detected using a chargecoupled device (CCD) camera. Complex software is then used to determine where each light ray has landed and, from there, which aberrations are present in the eye (Fig. 2) (Fig. 3). Because wavefront measurement of the human eye is still a nascent technology, there are numerous unique methods in development with various detection and calculation techniques (Fig. 4).

As wavefront engineers became confident in their abilities to accurately measure the human optical system, they once again returned to the realms of astronomy, physics, and optics to determine a method of analysis and interpretation for wavefront measurements. Past history with wavefront sensing suggested that engineers use the Zernike polynomial, a mathematical description of a two-dimensional surface with uniform radius, to describe optical aberrations. The Zernike equation assesses a wavefront image and breaks the complex image into an infinite number of surfaces for simple description. The optical surfaces used in a Zernike description are represented in a Zernike pyramid (Fig. 4). Each

Cust•¥€ ed ablation and LASEK 261

level of the Zernike pyramid represents an “order” for those aberrations. The secondorder aberrations as expressed on the top level of the pyramid represent the “lower-order” aberrations of defocus (sphere) and astigmatism (cylinder). These are the optical aberrations that are currently measured with a phoroptor. As we move down the pyramid

Figure 2 To collect wavefront measurements of the human optical system, a beam of light is projected into the eye, bounced off of the retina, and measured as it escapes through the cornea. A detection system then analyzes the returning beams and calculates wavefront aberrations.

Cust•¥€ ed ablation and LASEK 263

into the “higher-order” aberrations, we begin to see terms such as coma, trifoil, and spherical aberration (Fig.5).

The goal of wavefront measurement and analysis with the Zernike equation is to provide the clinician with superior tools to assess the optical system of the human eye. Instead of describing the eye with three terms (second order), as with the phoroptor, we now have the capabilities of accurately measuring and describing the eye with up to 18 terms (second through fifth order) (Fig. 6).

Figure 5 The Zernike pyramid visually describes aberrations.

LASEK, PRK, and excimer laser stromal surface ablation 264

Figure 6 The phoroptor is capable of measuring second-order aberrations, whereas wavefront is capable of measuring second-order through fifthorder aberrations.

Topographic Diagnostics

Wavefront measurement is not the only way of describing and classifying aberrations in the human eye. Corneal topography has been a staple technology in refractive surgery for many years. Corneal topography enables clinicians to acquire detailed images of the anterior and posterior aspects of the cornea. Topographic systems are also capable of determining steepness and thickness measurements essential to accurate visualization of the cornea. Methods of topographic analysis include optical slit scanning, placido disk imaging, and linear computation.

Topography will be important role in the future of diagnostic technology because of its ability to map aspects of the corneal surface. It is important to keep in mind that although wavefront diagnostics can accurately map the optical system, the corneal surface is the location of the laser treatment. It will be essential to correlate wavefront data to the corneal topographic data for treatment algorithm evaluation.

Customized Ablation

The marriage of diagnostic measurements and laser technology results in treatment algorithms individualized to each patient. These customized algorithms use the output from the wavefront sensor, topography device, or a combination of the two and designate a laser shot pattern that treats the surface of the cornea in a “pixel-by-pixel” approach to minimize aberrations at all points.

In initial clinical studies, it has been demonstrated that laser treatments based on wavefront-guided custom patterns have achieved a reduction in aberrations and increased best corrected visual acuity (BCVA) in comparison with conventional treatment. In phase

I clinical trials, Alcon Summit Autonomous showed that after wavefront-guided treatment with their CustomCornea system, 92% of custom eyes were 20/16 or better BCVA Fig. 7). It was also demonstrated that higher-order aberrations decreased after custom treatment (1). It is evident from these preliminary results that wavefront-guided ablation techniques can bring refractive techniques to the next level.

Cust•¥€ ed ablation and LASEK 265

Figure 7 Six months postoperatively, 92% of CustomCornea wavefrontguided PRK eyes achieved 20/16 or better BCVA.

Although the first steps are being made into the realm of customized ablations, there are many questions that remain unanswered. One is, what is the optimal wavefront pattern for the human optical system? Many vision scientists have suggested that a flat wavefront (all terms equal to zero) may bring maximized vision. Others have suggested that it is possible that certain aberrations, namely spherical aberration (2) and vertical coma, may optimize the visual system at values other that zero. Further research into diagnostic technology will continue to shed light on this exciting and promising field.

LASEK AND CUSTOM ABLATION

Flap Biomechanics

The biomechanical response to corneal manipulation associated with refractive surgery is a complicated and elusive phenomenon currently being addressed by many researchers. It is evident from initial studies that these biomechanical effects significantly impact refractive error and topographic appearance of the cornea. Although all refractive treatments will create some biomechanical changes, it has been shown that the creation of the LASIK stromal flap elicits significant changes in corneal topography even without laser treatment (3). Inherently, it is understood that the introduction of biomechanical corneal response with the creation of the LASIK flap will increase variability in

Cust•¥€ ed ablation and LASEK 267

Figure 9 (A) More higher-order aberrations are induced in a conventional LASIK treatment than in a conventional PRK treatment. (B) A greater percentage of eyes experienced a decrease in higher-order aberrations with PRK treatment in comparison with LASIK.

LASEK, PRK, and excimer laser stromal surface ablation 268

POSSIBLE DISADVANTAGES OF CUSTOM TREATMENT AND

LASEK

Stromal Biomechanics Poorly Understood

Although LASEK seems to be the natural solution to the problems with LASIK flap biomechanics and intrastromal variables, the effects of laser treatment on the surface of the cornea are still only vaguely understood. As refractive techniques with excimer lasers have progressed, our understanding of corneal response to spherocylindrical correction has been greatly increased. This understanding has led to changes in nomogram and laser patterns leading to improved and more predictable results. A possible hurdle in the movement toward customized surface treatments with LASEK is the challenge of understanding the effects on the cornea when it is subjected to often asymmetric and highly variable ablation patterns.

Healing Variables

Another challenge in moving into more specific treatment parameters is understanding the effects of healing on stromal and epithelial biomechanics. Although single laser pulses

Figure 10 Case study. One eye LASIK and one eye LASEK.

are capable of treating extremely small aberrations with tissue removal of approximately 0.25 microns, it seems logical that a single epithelial cell (5 microns) may have the capability of filling in that treatment site and negating its effect. Additionally, the epithelial surface is not perfectly uniform, because the cells in different areas will

Cust•¥€ ed ablation and LASEK 269

undergo hypertrophy and hyperplasia in a natural effort to smooth the surface. Epithelial remodeling will undoubtedly affect small treatments to some extent. Healing variables will need to be addressed in determining the treatment capabilities of custom treatments.

PRELIMINARY DATA-LASEK HIGHER-ORDER

ABERRATIONS

Although we are still very early in our experience with analyzing wavefront effects of LASEK treatment, the initial clinical data reveal positive results. We present a case study looking at the higher-order aberrations of trifoil, coma, tetrafoil (4,2), and spherical aberration induced by a single patient with the LASIK procedure in one eye and the LASEK procedure in the other eye. The graph in Figure 10 demonstrates that postoperative LASIK aberrations (blue bars) are higher than postoperative LASEK aberrations (yellow bars) in trifoil, coma, tetrafoil, and spherical aberration (4,2). Additionally, the magnitudes of trifoil are (4,2) decreased preoperatively to postoperatively in the eye treated with LASEK, whereas the LASIK eye experienced increased postoperative aberrations in all terms. These results suggest that the techniques of the LASEK procedure, most notably the abandonment of the stromal flap, help to decrease postoperative aberrations associated with refractive surgery.

CONCLUSIONS

The field of refractive surgery has made incredible strides in the past decade. We have made the leap from incisional surgery to precise excimer laser ablations. With the excimer laser, we have seen further evolution from broad-beam lasers to small flying spot lasers coupled with sophisticated eye-tracking systems. Wavefront technology is helping us transition from a point at which the current lasers are capable of performing more precise treatments than we can measure with the phoropter to a new position at which we can now measure with more accuracy than we can treat. This will help spur laser companies and surgeons to further improve lasers and refractive surgical techniques to keep up with the diagnostic equipment. The LASEK procedure may help keep aberrations created by the surgery itself to a minimum and thus is showing promise in helping us with this next step in the evolution of refractive surgery.

REFERENCES

1.McDonald MB. Presentation, Wavefront-Guided PRK with CustomCornea®. American Society of Cataract and Refractive Surgery, April 2001.

2.Holladay JT, Dudeja DR, Chang J. Functional vision and corneal changes after laser in situ keratomileusis determined by contrast sensitivity, glare testing, and corneal topography. J Cataract Refract Surg; 1999; 25(5):663–669.

3.Roberts C. The cornea is not a piece of plastic. J Refract Surg; 2000 (16(4)):407–413.

4.McDonald MB, Magruder GB. Presentation,“Wavefront-Guided Outcomes with CustomCornea®.” American Academy of Ophthalmology, November 2000.