Thursday, March 30, 2006

Customized Ablation #1: I've Seen the Future....and it's CustomCornea!

This Technology Update column is the first of a series of eight articles chronicling the development of Customized Ablation or Wavefront Driven LASIK. This column was published in the June 15th issue of Ocular Surgery News following both the 1999 ASLMS (American Society for Lasers in Medicine and Surgery) and ASCRS (American Society of Cataract and Refractive Surgery) meetings.

I've Seen the Future.....and it's Custom Cornea!

Technology Update

Irving J. Arons
Spectrum Consulting

What if you could more accurately predict and successfully restore a patients vision to 20/20 with refractive surgery. What if, in some cases, you could actually achieve "Ted Williams" type vision (20/16) or better for your patients who would want that. Would you be interested in finding out more?

Since I first heard Marguerite McDonald's presentation on custom cornea at last fall's AAO meeting (1998), I've been intrigued with the idea, and have wanted to know more about how it was going to be accomplished. The idea of not only correcting vision to emmetropia (20/20), but enhancing it to 20/16 or better was exciting and this was only reinforced by the presentations given at a Bausch & Lomb press conference at the AAO, where George Waring declared 20/12 by 2012, now changed to 20/10 by 2010 in current B&L Surgical advertisements! (For more on both of these presentations, see my columns from the AAO in the January 1st issue of OSN.)

Now, after hearing a presentation given by Tim Turner, director of research at Orbtek, on the "Orbscan Corneal Topography Laser-Link System" (at the recent American Society of Lasers in Medicine and Surgery [ASLMS] meeting in Orlando), and especially after viewing a tape of a course given at the recent ASCRS meeting by Marguerite McDonald, Ronald Krueger, Ray Applegate and George Pettit on "Wavefront-Guided Customized Ablations -- Beyond Topography-Assisted Refractive Surgery", sponsored by Autonomous Technology -- now a subsidiary of Summit Technology, I am convinced that topographically-linked ablation, and especially wavefront-guided ablations, are the future of refractive surgery.

As reported last January, several companies have announced that they are in the process of combining corneal topography with their excimer laser systems to be able to perform "custom" ablations. Among those are LaserSight, working with Humphrey-Zeiss and Orbtek; VISX, working with EyeSys Premier and Orbtek, among others; B&L Surgical/Chiron with Technomed and Orbtek; Schwind with Technomed; and Aesculap-Meditec, also with the Orbtek system and possibly Technomed.

In Dr. Turner's presentation, he explained how the first-generation of topographically linked laser ablations, which depend on the capture of topographic information (corneal curvature and elevation) supplied by such placido reflection systems as Computed Anatomy and EyeSys Premier, will probably be the first out of the block, along with the Orbscan slit scanning system. The Orbscan is capable of providing additional information such as the thickness of the cornea and the condition of its rear surface, through what Orbtek calls CIPTA, or Corneal Interactive Programmed Topographic Ablation. However, by combining the Orbscan with ultrasound to determine the ocular axial length and lens thickness, a simulation of the retinal point spread function (PSF) can be determined. Point spread functions can be calculated for any surface or any sequence of surfaces. Thus a PSF can be calculated from anterior corneal data alone, but it may or may not approximate the true retina PSF, depending on the optical importance of the other internal ocular surfaces. However, this does not include local aberration-inducing variations. So, Orbtek has created a simulated next-generation measurement technique called WAVE, or Wavefront Ablation Vision Enhancement. WAVE treats patient vision rather than corneal geometry (elevation and surface curvature), by targeting idealistic post-operative surface topography with the optical PSF optimized. It calculates wavefront aberration by reverse ray tracing of all of the rays emanating from a theoretical point source of light at the fovea and back through a modeled crystalline lens and the measured cornea. Orbscan measurements are used by WAVE to fully characterize the cornea (both anterior and posterior surfaces), and other data (refraction, ultrasonic axial length and lens thickness) are used to model the crystalline lens. These additions aid in the simulation of the true retinal PSF and the ocular wave aberration. When wave aberrations are directly measurable, they can also be included in the WAVE strategy. The WAVE system is still experimental and has not yet been used on any human patients.

The major difference between what Orbtek and Autonomous Technology are attempting to do is in the measurement of aberrations. As noted above, Orbtek uses theoretical measurements and modeling to simulate an aberration pattern, while Autonomous actually measures the total aberration of the eye by shooting a laser pulse onto the retina, through the lens, and obtains an actual aberration pattern of the return waves, thus getting a more accurate pattern of what aberrations are present in the complete optical system.

Autonomous uses a proprietary wavefront sensor to capture the wavefront pattern returning from the laser-induced spot on the retina. It then converts this information into a 3D map of the aberrations to determine what changes are needed in the corneal surface to compensate for the aberrations present in the eye, and to overcome them. Autonomous has built a prototype device, called the CustomCornea Measurement Device, and, under an IRB, has placed it in an ophthalmologist's office in Florida in order to take measurements on patients under a real-life situation, beginning in April. In an in-house study of 103 eyes, including some people with 20/20 or better vision and some with known pathologies, the custom cornea maps provided by a prototype device, appeared to bear out the theory -- emmetropes, as well as those with sharper vision have less aberrations and can be distinguished from those with aberrated vision. In the near future, this information will be programmed into the LadarVision small spot scanning laser system to provide the potentially enhanced vision, possibly reaching the theoretical diffraction limits of the retina of 20/8!

Of course, all of this would not be possible without the extremely accurate tracking system available on the Autonomous laser. Since the accurate placement of the "compensating" ablation pattern is necessary, by linking the wavefront analysis device to the LadarVision tracking laser, the precise pattern of ablation correction can be obtained, centered on the visual axis, and "customcornea" achieved. The company hopes to begin CustomCornea treatments this summer, depending on regulatory approvals and, hopefully, within the next two years, this technology could become a commercial reality.

Editors note: This column was not put online by OSN. The Janury 1st column mentioned, is also not online, but was picked up by a sister publication, OCULAR SURGERY NEWS EUROPE/ASIA-PACIFIC EDITION, and published in February 1999. (Which is linked.)

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