The retinal image of a Snellen letter could be considered the negative of PSF. This patient, in effect, is qualifying his higher-order aberrations in terms of PSF. Wavefront Technology The next step in the quest to identify, measure and eventually correct the higher-order aberrations in human vision is to quantify them in Zernike, PSF and RMS terms. This, in fact, is the new vision science of wavefront analysis and the new role of wavefront aberrometry.
Thanks to such optical researchers as David Williams, Ph. We now have numerous sensing and measuring methods for wavefront analysis also called aberrometry. Dynamic spatial skiascopy, such as 3-DWave Marco Ophthalmic. Holographic imaging, such as Z-View Aberrometer Ophthonix. Each commercial wavefront aberrometer has strengths and weaknesses. So, too, does the general science of wavefront. These strengths and weaknesses will have a profound impact on the scope and precision of diagnostic refractive care, on current and evolving forms of refractive treatments surgical and non-surgical to correct lower- and higher-order aberrations, and on everyday clinical practice.
Diagnostic Applications While photoablation and conventional LASIK can adequately correct the lower-order aberrations sphere and cylinder , they aggravate and induce higher-order aberrations in the eye. The original goal of using wavefront-guided custom ablation was to correct higher-order aberrations. Now, the more realistic goal is for the surgeon to induce fewer aberrations, which are associated with the corneal biological and biomechanical insult resulting from refractive surgery.
Also, wavefront may be equal or more valuable in diagnostic applications. Actual or potential diagnostic applications include: Refraction. Through an aberrometers vector analysis software, a blended astigmatic correction of lower and higher order cylinders without the need for which is better one or two? Christopher Freeman, O.
Topography-Guided LASIK Enhancements
Dell, M. Refractive surgery has come a long way since the s, when Russian ophthalmologist Svyatoslav Fyodorov had lines of people waiting outside his clinic to undergo surgery that would rid them of glasses. Since Dr. Fyodorov first performed radial keratotomy, the field of refractive surgery has seen a transformation to lasers with small optical zones and then to lasers with large optical zones, blend zones and now custom, wavefront-guided ablations.
This is coupled with the LadarWave wavefront analyzer to make up Alcons wavefront-guided ablation platform, known as CustomCornea. This system became the first FDA-approved laser system for wavefront-guided excimer laser ablation in October The Alcon laser is also a flying spot laser with spot size of 0. It has an optical zone size up to 6. In June , the FDA expanded the treatment range for the CustomCornea wavefront platform; surgeons can now use it to treat up to 8.
The Ladarwave system is another Hartmann-Shack aberrometer with Zernicke polynomial-based conversion to the laser. It can measure up to a 7.
Laser in situ Keratomileusis (LASIK) - American Academy of Ophthalmology
This system has a treatment range of up to 6. The VISX system uses what is called variable spot scanning with a spot size that varies from 1mm to 6mm. The optical zone size is FDA approved for up to 6mm and can treat up to 6. The S4 uses the ActiveTrak, 60Hz, infrared video-based eye tracker, which measures eye movement in the x, y and z axes.
Its a Shack-Hartmann aberrometer that uses Zernike polynomials to represent each aberration. This systems software will be converted from Zernike to a higher-resolution, Fourier Transfor analy sis of the aberrometry. This, in turn, should lead to more precise laser ablations and improved visual outcomes.
Bausch and Lombs Zyoptix. This was approved by the FDA in October The Zywave aberrometer can measure aberrations beyond a 7. The z laser system features varying spot sizes of 1 to 2mm with an FDA-approved fully corrected optical zone size as large as 7. Zyoptixs FDA clinical trial data showed No patient expressed dissatisfaction with the procedure.
Meanwhile, Carl Zeiss Meditec plans to begin U. Our laser center was selected to be one of the study sites. The Zeiss laser, also a flying spot system, is an extremely fast laser that operates at Hz and uses a Hz video-based tracker. Ring dimensions used in the second pass determined the thickness, and thus the dioptric power, of the lenticle removed.
- Lasik and Beyond Lasik English Edition: Wavefront Analysis and Customized Ablation / Edition 1;
- Lasik and Beyond Lasik English Edition : Wavefront Analysis and Customized Ablation!
- The Meaning of Relativity (Routledge Classics).
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This technique involved less trauma to the corneal cap, but the results remained suboptimal secondary to unpredictable refractive changes and irregular astigmatism. In the late s, Ruiz developed an automated microkeratome to improve predictability of the microkeratome cut. Automated advancement of the microkeratome head and development of an adjustable suction ring for control of the second pass, represented major advances.
The lack of predictable tissue removal in the second pass and the small optical zone of the excised refractive lenticle, however, often left the patient with unacceptable optical aberrations. In , Pallikarisperformed the first LASIK procedure, using the excimer laser instead of the second microkeratome pass to remove tissue to induce the refractive change. The excimer laser produced better optical results by. The final critical step in achieving consistently excellent results with lamellar refractive surgery was modifying the microkeratome to stop the pass before creating a full free cap.
Leaving a narrow hinge of tissue allowed the outer cornea to become a flap that is reflected out of the way during the laser exposure, then returned to its original position and allowed to adhere through natural corneal dehydration to the underlying stromal bed Figure 1. Repositioning the flap in its original position and avoiding distortion induced by sutures are critical in reducing irregular astigmatism. Ongoing improvements in microkeratomes continue to make them safer and easier to use. Advances in femtosecond scanning lasers are another important improvement in creating the LASIK flap that allow for greater accuracy and safety.
Figure 1. Schematic representation of laser in situ keratomileusis with formation of a corneal flap followed by stromal ablation. The flap is then replaced into its original position. Many variations of the mechanical microkeratome exist, but the basic principles of the microkeratome cutting head and role of the suction ring are the same.
The suction ring's two functions are adhering to the globe, providing a stable platform for the microkeratome cutting head; and raising eye pressure to a high level, firming the cornea so it cannot move away from the cutting blade. Dimensions of the suction ring determine the diameter of the flap and size of the stabilizing hinge. The suction ring is connected to a vacuum pump typically controlled by an on-off foot pedal.
The hinge initially was located nasally in the Barraquer-designed microkeratomes. Sliding microkeratome has easiest corneal access when approaching from the temporal side, leaving the nasal area last to be cut. Popular microkeratomes were introduced that allow the head to pivot on a post, resulting in an arcing path that leaves the superior zone the last to be cut. A superior hinge was considered a better location because it resisted disturbance of the flap by the wiping motion of the upper eyelid.
Nasal hinges had the drawback that many pupils are located nasal of the corneal center.
The nasal hinge, even if shifted maximally to the nasal limbus, might impinge on a large treatment zone. There is evidence that a nasal hinge location spares some flap innervation from the nasal long ciliary nerves as they enter the cornea, and there may be less tendency for postoperative dry eye in LASIK patients with nasal hinge flaps compared to superior hinge flaps. In contrast, one study found more rapid recovery of corneal sensation in patients with superior compared to nasal hinges, although all recovered sensation to the preoperative levels 6 to 12 months after surgery. An alternative methodology for creating flaps is now widely used in clinical practice.
Numerous laser platforms using femtosecond Nd-YAG laser pulse technology can create a lamellar dissection within the stroma. Each laser pulse creates a discrete area of photodisruption of the collagen.
Lasik and Beyond Lasik Wavefront Analysis and Customized Ablations
Thousands of adjacent pulses are scanned across the cornea in a controlled pattern that results in a flap where the computer is programmed for flap diameter, depth, and hinge location and size. Advocates point to potential for better depth control, avoiding complications like buttonhole perforations and epithelial defects, and precise control of flap dimensions and location. Tanna et al.
Patel et al.
High-contrast visual acuity, contrast sensitivity, and forward light scatter also did not differ between treatments at any examination. Chan et al.
- Full Image;
- Refractive surgery in Europe: not just LASIK any more.
- Diversity in Organizations.
- (PDF) Management of unsuccessful LASIK surgery | Renato Ambrósio - diavendoubtlibma.ml!
- LASIK Surgery-Minatomirai Eye Clinic (Yokohama).
As with PRK, careful attention must be given to pre-existing systemic or ocular conditions that may interfere with healing.
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