VISX Star S4 Platform

Colman Kraff

The AMO VISX Star S4 (Software Version. 5.22) laser platform with ActiveTrak can be used to perform conventional LASIK as well as custom surgery. (See Videos 1 & 2) Custom-Vue LASIK with this laser requires the use of the WaveScan, a separate device used to acquire and analyze wavefront images, and allows the surgeon to design an appropriate custom treatment profile. These data are then transferred to the laser. Conventional and custom treatment will each be discussed. The main focus of the discussion will be the software and hardware that is available within the United States. The AMO VISX system that is available outside the United States has software and hardware that allow the surgeon greater flexibility and treatment ranges currently restricted in the United States by FDA guidelines.

▪ Conventional LASIK

The purpose of this section is to discuss conventional LASIK with the VISX Star S4 laser. Generic comments pertaining to the use of the excimer laser to perform LASIK surgery are found in Chapter 1.

The VISX laser is one of the most widely used excimer lasers in the United States. A number of hardware and software technologies that have become available since its FDA approval in 1998 have made the system safer and easier to operate. These include Active-Trak 3-D eye tracking, variable spot scanning, autocentering, CustomVue, and iris registration. The ActiveTrak 3-D eye tracking system available on the Star S4 laser platform tracks eye movements with an undilated pupil within a zone of 2.0 mm in the z-axis and 1.5 mm in the x-y-axis. Eye movement outside this tracking zone will be sensed within 30 ms and the surgeon will be unable to fire the laser. When proper fixation is reestablished, the tracking system will resume tracking with the same reference point, and laser treatment can continue from the point of interruption.

Variable spot scanning (VSS), available on the VISX Star S4 system, represents a change from the older broad-beam laser delivery system to variable laser spot size delivery with the spot diameter varying from 2 to 6 mm. The advantage of this innovation is a smoother treatment, possibly reducing the incidence of haze after surface ablation. It also allows the laser to smoothly integrate a blend zone extending the ablation zone out to 8.0 mm. This can be used to reduce the risk of postoperative glare and haloes. The use of the blend zone feature requires at least 1 diopter (D) of treatment at the corneal plane, a well-centered flap large enough to accommodate the treatment, and a cornea that is thick enough to accommodate the added ablation depth. The added ablation depth is 8 µm for treatments up to 10 D and 11 µm for treatments beyond 10 D. There are additional advantages of VSS. The ablation with VSS is not as deep as a typical flying spot laser, and the treatment time is not as lengthy as the flying spot laser with a single-sized small spot. Finally, the VSS keeps the corneal temperature constant. This is important because the laser will ablate more tissue as the tissue temperature increases. The VSS treatment is currently only available for CustomVue treatments in the United States. Outside the United States, the VSS platform is available for use with conventional treatments.

The VISX Star S4 laser pulse repetition rate can be adjusted from 1.5 to 10 Hz for conventional treatment and from 6 to 20 Hz for custom treatment. The repetition rate for the custom treatment is variable and is set by the laser. Currently, the S4 platform is required for custom treatment.

The autocentering feature on the Star S4 laser automatically finds the geometric center of the undilated pupil, so it is unnecessary for the surgeon to find the pupil center prior to engaging the tracking device. The pupil center can vary under different lighting conditions. It is therefore important to keep the lighting as low as possible at the time of autocentering and engaging the active tracking system so that the center of the pupil during treatment will be the same as the pupil center during refraction or WaveScan.

Iris registration is available on the Star S4 platform. This supplants the need for mechanically marking the patient prior to a custom laser treatment, but is not available for conventional LASIK. This will be discussed in greater detail in the next section on LASIK with the VISX CustomVue.

One of the major advantages of the VISX Star S4 laser is the wide range of refractive errors approved by the FDA. Table 1.1 shows a comparison of the FDA-approved treatments for various lasers. Unlike many available laser systems, the VISX system has been approved for conventional treatment of myopia and hyperopia with and without astigmatism as well as mixed astigmatism. Approved patient treatment for myopia extends to -14.00 D with up to +5.00 D of astigmatism. Hyperopic treatment is up to +5.00 D with up to +3.00 D of astigmatism provided the spherical equivalent (SE) is < +6.00 D. The mixed astigmatism treatment is approved for up to +6.00 D of astigmatism, as long as the myopic sphere is less than the cylinder with the treatment in plus cylinder form. The laser is also approved for custom treatment for all three of these situations. The range of available treatment is somewhat narrower for custom than for conventional treatment. Conventional treatment at the upper limits of the approved range is less reliable than at lower levels of refractive error. Most surgeons will explore alternatives to LASIK at the extremes of refractive error (see Chapter 15 for a discussion on alternatives to LASIK).

TREATMENT ZONES

The treatment zones available for conventional LASIK using the VISX Star S4 laser for the correction of myopia are 6.0 and 6.5 mm. As stated earlier, the blend zone will extend the treatment zone to a diameter of 8.0 mm. Hyperopic treatment requires a treatment zone of 9.0 mm with the deepest part of the ablation occurring at 5.0 mm, that is, 2.5 mm from the center of the ablation. Mixed astigmatism treatment also requires an ablation zone of 9.0 mm. When planning these treatments, the flap must be of adequate size and centration to accommodate the entire treatment zone.

NOMOGRAM ADJUSTMENT

Several nomograms are available for treatment of myopia and myopia with astigmatism. Some surgeons use rules of thumb rather than a nomogram specifying the adjustment for specific corrections and/or decade of age. Because a starting place is necessary, nomograms for myopia and hyperopia are provided. When beginning to plan treatments, the reader is encouraged to seek out the advice of experienced colleagues who use the VISX laser in the specific laser suite that will be used. Advice from faculty at a VISX certification course or from the medical directors at the company is also of value.

MYOPIA

Because ablation within the corneal stroma using the VISX laser to correct myopia results in a greater refractive effect than surface ablation, an adjustment to the intended treatment is needed to prevent an overcorrection. The nomogram-adjusted treatment is what will be entered into the laser. The Bansal-Kay nomogram was designed only for treatment of myopia with the VISX STAR S2 laser system, but continues to be effective for conventional myopia treatment with the S4 laser system. In
this nomogram, the percentage reduction increases with age and with the amount of correction in the SE. The percentage reduction is applied only to the spherical component of the desired correction. The astigmatism component is left unadjusted. This nomogram is a useful starting point; however, surgeons should always carefully evaluate postoperative data from many cases and adjust their preferred nomogram accordingly.

TABLE 7.1 Bansal-Kay LASIK Myopia Nomogram for VISX STAR Laser

Diopters	Age 18-20	21-25	26-30	31-35	36-40	41-45	46-55	≥56
0-1	4	4	4	5	6	7	7	8
1-2	4	4	4	5	6	7	7	8
2-3	4	4	5	5	7	8	8	8
3-4	5	5	5	6	7	8	8	8
4-5	5	5	6	6	8	8	8	9
5-6	5	5	6	6	8	9	9	10
6-7	6	6	7	7	9	10	10	11
7-8	7	7	7	8	9	10	11	12
8-9	8	8	8	8	10	11	12	12
9-10	9	9	9	9	11	11	13	13
10-11	10	10	10	10	12	13	14	14
11-12	11	11	12	12	13	13	15	15
Notes: Multiply percentage by spherical equivalent and subtract from the sphere in the cycloplegic refraction.
Nomogram target = plano, adjust to undercorrect.
Individual results will vary with each laser, surgeon, humidity, and altitude.
Some data have been extrapolated.
Recommend starting laser ablation within 15 seconds of lifting flap.
Source: From Dr. Jay Bansal, unpublished data, with permission.

Use of the Bansal-Kay nomogram (Table 7.1) requires five steps:

Step 1: Calculate the SE from the cycloplegic refraction (CR). For corrections with >7 D of myopia, the vertex distance of the CR should be noted. The VISX Star S4 laser uses 12.5 mm as the default vertex distance and converts this to the refraction at the corneal plane. The correct vertex distance should be entered in patients with high myopia.

Step 2: Find the appropriate reduction percentage on the nomogram by locating the correct diopter range in the left-hand column and the correct patient age range in the top row.

Step 3: Multiply the SE by the selected percentage reduction to obtain the appropriate diopter reduction.

Step 4: Reduce the sphere in the desired refractive correction by the calculated diopter reduction. (Note: Some surgeons use the CR routinely. Some use the cylinder axis from the manifest refraction. Others routinely use the manifest refraction (MR) in younger patients provided it is close to the CR.)

Step 5: This final refraction is given to the laser engineer to enter into the laser computer.

Example. Patient A is 26 years old and has a CR of -3.50 + 1.00 × 90 and, therefore, a SE of -3.00 D. Based on the nomogram (Table 7.1), a 5% correction is necessary to achieve the desired refractive result.

5% × -3.00 (SE) = 0.15 (the sphere in the CR is reduced by this amount)

-3.50 + 1.00 × 90 + 0.15

-3.35 + 1.00 × 90 (Enter this into the VISX system.)

An alternative rule of thumb used at Wills Eye Hospital involves three steps.

Step 1: Convert the intended correction to minus cylinder form;

Step 2: Reduce the sphere by 10% for patients under age 50 years and 15% for those older than age 50; and

Step 3: Reduce the cylinder 5-10% for refractive astigmatism <1.5 D and 10-15% for >1.5 D.

HYPEROPIA

For LASIK surgery or photorefractive keratectomy (PRK) with a conventional ablation to correct hyperopia with or without astigmatism using the VISX Star S4 laser, the MR should not differ from the CR by more than 0.75 D. With the VISX laser, regression of the ablation effect is expected to occur over a 3- to 6-month period. Therefore, enhancement should not be considered until the surgeon is certain the correction has stabilized. Because of this anticipated regression, a compensatory nomogram-adjusted boost to the spherical portion of the treatment is required for LASIK surgery. It is usually not required for surface ablation, that is, for PRK the desired correction can be directly entered into the VISX laser. The nomogram adjustment will induce myopia in the short run and patients should be warned about this possibility preoperatively. Hyperopic patients of presbyopic age or older will usually appreciate the improved unaided near vision that occurs in the short run.

Similar to the myopia nomogram, the percentage adjustment increases with increased SE and age. In contrast, however, the nomogram adjustment is added to the sphere portion of the treatment. The astigmatism correction is left unadjusted.

Example. Patient B is 56 years old and has a CR of +2.50 + 1.00 × 90 and, therefore, a SE of +3.00 D. Based on the hyperopic nomogram (Table 7.2), a 30% addition is necessary to achieve the desired corrected value needed for LASIK surgery.

30% × +3.00 D (SE) = 0.90 (the sphere in the CR is increased by this amount)

+2.50 + 1.00 × 90 + 0.90

+3.40 + 1.00 × 90 (Enter this into the VISX system.)

This hyperopic nomogram was originally designed for the VISX Star S2 laser. It still has utility for use with the Star S4 laser; however, individual surgeons need to modify the nomogram based on an analysis of postoperative results from many cases. The laser suite conditions (e.g., temperature and humidity) and the particular VISX laser can affect the surgical outcome.

MIXED ASTIGMATISM

The mixed astigmatism treatment on the VISX Star S4 laser is not a nomogram-adjusted treatment. In other words, the desired treatment can be entered directly into the laser computer. Surgeons differ with regard to whether they enter the CR or modify the MR retaining the astigmatism power and axis, but reducing the sphere to be consistent with the CR. The FDA-approved mixed astigmatism profile for conventional and custom ablation is a distinct advantage of the VISX laser, because other laser systems may not have such approval, requiring off-label multi-card treatments to achieve the desired result.

ABLATION DEPTH CALCULATION

Myopia

To calculate the ablation depth, the non-nomogram-adjusted refraction is used. This cannot be overemphasized. Ablation depth rules of thumb for the VISX laser are 12 µm/D of SE for myopia correction with a 6.0-mm treatment zone and 15-µm/D for a 6.5-mm treatment zone. The addition of an astigmatic component to a myopic treatment will make the SE less than the sphere. Therefore, the resultant ablation depth will be less when astigmatism is being treated in addition to myopia.

Since the last edition of this book, LASIK surgeons have become evermore conscious of reducing ectasia risk by reducing stromal ablation when possible. Using the blend zone can allow the surgeon to choose a 6.0-mm optical zone rather than a 6.5-mm optical zone without a noticeable increase in nighttime glare or haloes. This saves 3 µm/D of stromal ablation. The addition of an 8.0-mm blend zone adds 8 µm to the maximum ablation depth for treatments of <10 D and 11 µm for treatments >10 D. This is not a per-diopter addition, but an addition to the overall ablation depth.

TABLE 7.2 VISX Laser Nomogram for Primary and Secondary Hyperopia

Preoperative Manifest SE				Secondary Hyperopia
Age (y)	0-2 D (%)	2-4 D (%)	4-6 D (%)	Post-RK (%)	Post-LASIK (%)
20-29	0	+10	+20	-4	-14
30-39	+10	+18	+25	-2	-10
40-49	+17	+23	+28	-1	-4
50-59	+30	+30	+30	0	+4
60-69	+35	+32	+31	+2	+8
Notes: Table indicates the percentage of the preoperative manifest spherical equivalent that is added to or subtracted from the spherical component of the correction. The full cylinder is entered into the laser. For example, a +2.00 + 2.50 × 90 degrees refraction in a 33-year-old with primary hyperopia would be adjusted as follows: +3.25 D (SE) × 0.18 = 0.59, which results in a treatment of +2.59 + 2.50 × 90 degrees.
The nomogram was derived from a regression analysis for trends by Drs. Lindstrom, Hardten, and associates, using a Hansatome and the VISX S2 Smoothscan at 10 Hz. Actual results were included for age groups when there were enough patients to influence the results of the regression analysis.
Source: Adapted from Lindstrom RL, Linebarger EJ, Hardten DR, et al. Early results of hyperopic and astigmatic laser in situ kertomilleusis in eyes with secondary hyperopia. Ophthalmology. 2000;107:1862, with permission.

The following example will illustrate how to calculate the ablation depth for a refraction of -3.50 + 1.25 × 120 degrees using the rules of thumb. The first step would be to decide on the appropriate treatment zone. If the zone were 6.5 mm with an 8.0-mm blend zone, the ablation depth per diopter conversion factor would be 15 µm/D. Multiply the SE by the conversion factor. The blend zone treatment would add an additional 8 µm of treatment. In this case, the SE would be 3.50 – 1.25/2 = 2.88 D. Simply multiply 2.88 D × 15 µm/D = 43.2 µm. Add an additional 8 µm for the blend zone for a total of 51.2 µm or roughly 51 µm for the ablation. Remember, this is for myopia treatment on the VISX laser only; other lasers may ablate more or less per diopter of treatment.

An alternative method for determining the ablation depth with the VISX laser is to use the appropriate ablation depth tables for the 6.0-mm zones (Table 7.3) and 6.5-mm zones (Table 7.4). These tables were generated by entering the various non-nomogram-adjusted corrections for the given zone into the laser. These tables continue to have utility despite being generated on an earlier version of the laser; however, changes in laser delivery can influence the ablation depth. Similar tables can easily be created as newer generations of lasers are developed. Most important is to avoid using the nomogram-adjusted treatment to calculate ablation depth, which would underestimate the true ablation depth.

The tables are used in the following manner. After deciding which zone to use, look for the appropriate spherical power in the left column and the amount of cylinder on the top row. If the astigmatic power in the CR is in between two values, simply extrapolate between the values.

For example, if you were calculating the ablation depth for a refraction of -3.50 + 1.25 × 120 degrees for a 6.5-mm treatment zone with a blend zone to 8.0 mm, look at the 6.5-mm table for -3.50 D. The ablation depth for -3.50 D is 54 µm. Added astigmatism correction will lessen the ablation depth. The ablation depth for a hypothetical refraction of -3.50 + 1.00 × 120 degrees is reduced to 39 µm. At a refraction of -3.50 + 2.00 × 120 degrees, the ablation depth shown on the table is decreased to 31 µm. Because 1.25 is one-quarter of the way from 1.00 to 2.00 D, reduce the ablation by one-quarter the difference of the depth for 1.00 and 2.00 D of astigmatism. The appropriate reduction in this case would be 8 × 0.25 or 2. The ablation depth would be 39 µm – 2 µm = 37 µm. An additional 8 µm must be added because of the blend zone for a total ablation depth of 45 µm. This is similar to the 51-µm depth estimated by the conversion factor method. It is always safer to choose the greater of the
two estimates to determine if the RSB will be adequate.

TABLE 7.3 Ablation Depth Myopia Treatment with VISX STAR S4 6.0-mm Zone