In 2000, wavefront technology was introduced into clinical refractive surgery. To fulfill its potential as a key player in this market, excimer lasers needed to reproduce treatment ablation detail with greater fidelity. We believe that Advanced Medical Optics/Visx lasers ([AMO]; Visx, Santa Clara, Calif) are among the forefront in refractive laser technology. Currently, the Star (Visx) is our perfect laser.

The Star Model B laser superseded the original Taunton Technology laser (Monroe, Conn) in the early 1990s. The Star Model C laser then replaced the Model B in 1995, with its first clinical use by Bruce Jackson, MD, in Ottawa. We have used the Star laser since its introduction in 1995 and have seen a dramatic improvement in outcome of refractive surgery with wavefront-guided treatment. It is our belief that the Star is not only in vogue, but is avant-garde.

Our results with the Star laser are excellent; the modern refractive surgery is comparable with that of optometry (the standard deviation is approximately 0.30 D SE). These results are achieved because the laser itself is a stable and reliable platform. Furthermore, it is supported by custom wavefront, Fourier reconstruction, iris registration and x-y offset targeting. The laser is a practical and efficient surgical tool that is comfortable to use for extended periods.

Space For Future Development
The Star laser was engineered as a new platform; the original designers left space in the system for future developments. This system has been extensively upgraded beginning in 1998, with the Star S2 (Smoothscan), and each upgrade and advance made this laser better and easier to use than its predecessor. The AMO-Visx research and development laser group continues to generate a plethora of developments, which we anticipate shall also reach clinical practice.

One of the Star laser's most important components is a beam integrator module (Figure 1). Optical elements within the integrator break up the laser beam into seven beamlets that pass through the optical delivery system of the laser. The beamlets merge twice, first at an iris diaphragm plane, where the ablative spot size is determined, and finally at the eye. The Star S2 laser reorganized the optical design so that the second beam focus is above the cornea; this location causes smoother ablation surfaces, an important requisite of surface ablation, and increases predictability during laser ablation.

The Star S2 upgrade to Star S3 was also a significant hardware change and paved the way for the Star S4 and current Star S4-IR (iris registration). This ongoing upgrade process has included the addition of tracker cameras, infrared light-emitting diode (LED) illuminators — for x-y tracking and z focus — and cyclotorsional angular registration (Figure 2). All of these elements are necessary in a perfect laser model.

The iris diaphragm should shape the beam pulses on the cornea (known as wide-area ablation), which is a method employed by the early Star laser myopic treatment algorithm. Introducing variable spot size scanning (VSS), with spot sizes of 1.0 mm to 6.5 mm, more efficiently delivers ablation to the cornea. AMO/Visx released software and hardware upgrades for the Star laser as uniform platform upgrades. Upgrades allow surgeons and service engineers to recognize the technology level of the laser. A modern laser system should be an integrated series of components incorporating the pathway from capture at the wavefront sensor to delivery from the laser itself (Table 1). Other treatment components currently under investigation include healing and compensation of biomechanical effects resulting from a LASIK flap or surface ablation. The design philosophy was to build a smart laser providing the best possible treatment quality.

The laser pulse can be fixed (small spot scanning [SSS]) or VSS. VSS involves combining different sized spots, whereas SSS uses only a single small spot size. The Star laser uses VSS; the laser spot size diameter is between 0.65 mm to 6.5 mm. The pulse diameter varies by a factor of 10, and the area varies by a factor of 100. Because SSS systems remove smaller volumes of tissue per pulse, they are required to fire at a higher repetition rate. Thermal loading on the cornea is minimized with the Visx system because it sorts pulses. Another advantage of VSS is that the larger mean spot size reduces the effect of tolerance stack-up within the delivery system, thus producing more accurate and smoother ablations. VSS is flexible, can create complex shapes accurately, is time efficient and avoids thermal loading of the cornea.

Fourier reconstruction, Zernike polynomials and zonal reconstruction are used to measure the wavefront error. A Fourier algorithm leaves little residual wavefront error compared with Zernike reconstruction. Fourier reconstruction/application of a more complex shape requires more rigour in the treatment to the cornea. Fourier reconstruction and wavefront-guided ablation has come of age with this high-accuracy ablation targeting.

The Star laser system is designed to be accessible by the patient, especially those who are claustrophobic and/or exceptionally overweight. The reclining chair allows the patient to sit and slowly move to a supine position before being positioned under the laser. This is particularly useful if the patient has mobility issues or is elderly, which is an important consideration as this population undergoes more laser procedures.

Laser Pearls
Calibration. I like to calibrate the laser regularly. When powering up the laser, it is important to cut a calibration plastic. This assesses the accuracy of the flat, cylinder and spheres and detects artefact features, which may indicate that the optics need attention. After the first patient treatment, temperature and humidity can change locally around the laser. This condition necessitates a second laser calibration. After this double calibration, the laser is then stable with any gas boost being an indication for further calibration.

Iris Registration. Iris registration and x-y offsetting has dramatically improved the targeting of the ablation and has brought out the benefit of the Fourier wavefront reconstruction. As with any image matching and mapping, however, it is important to follow procedure and check the technology.

The original image on the Wavescan system (Visx) should be well focused. Iris details have less contrast when there is poor focus. The Wavescan version 3.62 displays the outer iris boundary (magenta); the pupil/inner iris boundary (blue); the corneal geometric centre (magenta cross); the pupil centroid (blue cross); and the corneal vertex (yellow diamond). The green box displayed in the lower right hand corner indicates that iris features have been successfully mapped.

The upper text data display is important in presenting the horizontal limbus diameter and the pupil size in the principal meridia. Commonly, the pupil is elliptical. When a pupil dilates to a large diameter, its centroid can move considerably. The Hartman-Shack spot reconstruction can then be created with a considerable offset with respect to the constricted pupil and the mid-dilated pupil (Figures 4,5). When wavefront scanning, the pupil size should ideally be slightly larger than that of the capture size. If the surgical optical zone is 6.5 mm in diameter, then a Wavescan pupil of 6.75 mm to 7.0 mm is ideal.

The outer iris boundary should be correctly mapped. Some corneas have a discrete boundary from clear cornea to sclera, whereas some corneas have arcus, which may falsely provide outer iris boundary detection. Posterior embryotoxon may also confuse the recognition, as can a Chayet sponge, which acts as a false white scleral boundary. The cornea-scleral contrast gradient is being detected, and it is important for the operator to visually check.

The system optimizes efficient iris feature recognition, and it is not designed to specifically provide a biometric patient identification. Correctly identify the patient presurgery instead of relying on technology to provide identification. Iris recognition, rather than registration, is a differing application of this technology. The x-y offset is based on fixed geometry mapping and has potentially greater benefit than cyclotorsional registration, although they are complementary.

Iris registration vertical offset key points. The key features of otational registration and x-y offsetting propelled the Star platform forward in outcome results during its introduction in 2004 to 2005. Some key points in iris registration vertical offset are:
• When wavescanning, attempt to elevate the upper eyelid as much as possible; otherwise, the upper corneal boundary is an extrapolation;
• Contact lens wearers commonly have upper limbal vascularisation and may have a gradual transition from sclera to cornea. Therefore, check that the boundary is correct; and
• When surgery is being performed and there is an eyelid speculum in situ there is commonly 360º limbal visibility. The situation is not synonymous with the point at Wavescan capture and vertical (y) offset generated onscreen should alert the surgeon to check the data.
If the decision is made that the offset is erroneous, then it is straightforward to engage iris registration and rotate the patient's head to create a zero or near-to-zero rotational error. The iris registration can then be turned off. Treatment is centered over the pupil, and there is no offset compensation, but cyclotorsional registration has been performed before beginning the laser ablation.

Custom For All and Presbyopia
Some improvements for future laser upgrades are still necessary. The future of the excimer laser lies in customised vision correction for all. This may not necessarily be the best anatomical correction — that is achieving the ultimate flat wavefront or perfect topography — but a sophisticated functional correction with large depth of focus or multifocality. This may require adaptive optics for patient subjective testing. The Star has the capability to perform presbyopic-shaped ablations, however, such treatments are in their infancy. We have yet to overcome the steep learning curve of understanding the tolerances of the human eye. The Wavescan-Star laser system is a complete excimer laser treatment platform and has proven to be reliable, accurate and stable. As with any complex system with many components, each aspect of the system is being evaluated and worked upon for continued development.

Julian Stevens, FRCS, FRCOphth, is a consultant ophthalmic surgeon, and director of refractive surgery at Moorfields Eye Hospital, in London. He is a member of the Cataract and Refractive Surgery Today Europe Editorial Board. Dr. Stevens did not provide financial disclosure information. He may be reached at JulianStevens@compuserve.com.

Anita Arora, MRCOphth, is a resident surgeon at Moorfields Eye Hospital, in London.