WITH PHAKIC INTRAOCULAR LENSES becoming increasingly popular, the importance of being able to quickly and accurately measure the anterior chamber has grown. Now, a new optical coherence tomography anterior segment scanner from Carl Zeiss Meditec (Dublin, Calif.), currently in clinical trials, shows promise as an easy and accurate way to do just that.
The new instrument is called the Visante OCT. (Visante is an amalgam of "vision" and "anterior.") It uses 1310-nm infrared light to visualize a "slice" through the anterior chamber. Unlike the 820-nm light used by the Stratus OCT, the longer wavelength readily scans through the sclera and iris, making it particularly useful for the assessment of narrow angles and measuring anterior chamber width.
Advantages and Limitations
The Visante OCT is designed to image the shape, size and position of anterior components and make precise measurements of the distances between them, including angle-to-angle, angle size in degrees, pupil diameter, anterior chamber depth, and thickness and radii of curvature of the crystalline lens. The Visante's software eliminates any measurement distortion induced by optical transmission factors. It captures the entire anterior segment in one step, with a resolution of about 10 µm.
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GEORGES BAIKOFF, MD |
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In this Visante OCT scan, iris tissue (marked by arrows) is compressed between an implanted Artisan lens and the patient's crystalline lens, resulting in pigment dispersion. Note the "crystalline lens rise" of 849 µm ( a useful
measurement defined by Georges Baikoff, MD). |
The instrument's primary limitation is that pigmentation on the posterior side of the iris blocks the penetration of infrared light. That means that in most eyes, the Visante can't measure sulcus to sulcus.
Noteworthy features include:
- The Visante OCT is non-contact. This minimizes patient discomfort, prevents chamber distortion, reduces the time required to perform a scan, and eliminates cleaning and disinfecting.
Technicians can do the scanning. According to Zeiss, using the Visante takes little time and requires minimal training.
Imaging flexibility. The Visante can create a transverse line scan along any orientation. It provides a video image of the region being examined (eight frames per second) and stores the last seven images. The examiner can then select and save the best images.
Faster imaging reduces error. The Visante can safely use 20 times more power than the Stratus OCT because 90 percent of the longer wavelength light is absorbed by the aqueous and vitreous media; absorption by retinal melanin is also weaker. This allows the Visante to scan 20 times faster with the same signal-to-noise ratio, minimizing motion error.
The Visante OCT can image through an opaque cornea. This is especially useful before a corneal graft.
It's easy to image accommodative changes. The Visante uses plus and minus spherical lenses to compensate for the patient's refractive error. Choosing the proper lens can stimulate accommodation, allowing scans with the crystalline lens in different accommodative positions.
Scans can be taken immediately after surgery. Because no contact is involved, there's no risk to the patient. Patients can be examined in the dilated or undilated state.
Use With Phakic IOLs
In addition to providing valuable preop and postop information about the anterior segment, the Visante OCT may help address several specific concerns:
- Iris pigment dispersion. Implantation of the Verisyse phakic lens, which is clipped to the front of the iris, has triggered iris pigment dispersion in some eyes. Work by Georges Baikoff, MD, using the Visante OCT, has shown that this pigment dispersion is a function of how much the iris tends to bow forward toward the cornea, rather than the anterior chamber depth.1 In some patients, scans have found the iris compressed between the crystalline lens and the phakic IOL (see page 33). The Visante can help weed out patients whose irises are likely to be a problem.
Measuring posterior phakic lens vault. This is a key to avoiding crystalline lens contact with the IOL, and making sure the IOL doesn't push the iris too far forward, causing aqueous outflow problems.
Estimating sulcus-to-sulcus. The Visante can't scan through most irises. However, a spokesman for Zeiss says the company is investigating the possibility that the sulcus-to-sulcus measurement correlates better with angle-to-angle measurement than with the white-to-white measurement. Zeiss hopes to have some clinical data on this by early 2005.
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The Visante OCT's non-contact scan is easy enough for a technician to perform. |
The Visante OCT in Practice
Georges Baikoff, MD, who has an anterior segment practice in Marseilles, France, has screened thousands of patients with a prototype of the Visante OCT during the past two years. (He has no financial interest in the device.) He implants phakic lenses in about 25 percent of his refractive surgery patients.
Dr. Baikoff believes safety is the most important consideration when implanting a phakic IOL. "I'm talking about lens width and vault, the clearance between the lens and the endothelium, and what I call the crystalline lens rise--the distance between the anterior pole of the crystalline lens and a hypothetical line connecting the 3 and 9 o'clock angle recesses at the iris root," he says.
"Knowing the crystalline lens rise is extremely important. For example, measuring this with the Visante OCT has allowed us to reliably determine the risk of pigment dispersion following implantation of an Artisan/Verisyse lens. We've also demonstrated that the crystalline lens touches many phakic IOLs (posterior and anterior) during accommodation.2
"With the Visante OCT, we can see what conditions exist in the eye with and without accommodation, helping us to determine which type of phakic lens to implant. In addition, we can predict about how many years the phakic IOL will remain safe, using our knowledge of how the crystalline lens rise will increase with age." Dr. Baikoff foresees a day when an instrument of this kind will have detailed information about physical characteristics of existing phakic IOLs programmed into it, so the surgeon can see onscreen how each IOL will fit in the patient's anatomy, both now and in coming years.
"An instrument of this kind should become standard equipment for surgeons implanting these lenses," he says.
Coming Soon
Eventually, Zeiss plans to add a pachymetry scan to the Visante. In the meantime, the company says that clinical trials for the U.S. Food and Drug Administration are under way. The company hopes the instrument will be on the market by fall 2005.
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Baikoff G, Bourgeon G, Jitsuo-Jodai H, Fontaine A, Viera Lellis F, Trinquet L. Pigment dispersion and Artisan Implants. Crystalline lens as a safety criterion. J Cataract Refract Surg. Ms 200-04 In press.
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Georges Baikoff, MD, Eric Lutun, Jay Wei, Caroline Ferraz, MD. Contact between 3 phakic intraocular lens models and the crystalline lens: An anterior chamber optical coherence tomography study. J Cataract Refract Surg 2004; 30:20072012.
Suggested Reading:
Baikoff G, Lutun E, Ferraz C. Anterior chamber optical coherence tomography study of human natural accommodation in a 19-year-old albino. J Cataract Refract Surg 2004;30:696-701
Baikoff G, Lutun E, Wei Jay, Ferraz C. Contact between three phakic intraocular lens models and the crystalline lens: An anterior chamber optical coherence tomography study. J Cataract Refract Surg 2004;30:2007-2012
Baikoff G, Lutun E, Ferraz C, Wei Jay. Static and dynamic analysis of the anterior segment with optical coherence tomography. J Cataract Refract Surg 2004;30:1843-1850
Baikoff G, Rozot P, Lutun E, Wei J. Assessment of capsular block syndrome with anterior segment optical coherence tomography. J Cataract Refract Surg. 2004; 30:2448-2450. |
