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A proprietary device for optical coherence tomography (Visante, Carl Zeiss Meditec) seems to be a highly useful addition to clinical practice. The instrument, according to one physician, may be the best resource thus far to evaluate placement of intracorneal segments, femtosecond laser-enabled keratoplasty, and penetrating keratoplasty.
Mexico City, Mexico-A proprietary device for optical coherence tomography ([OCT] Visante, Carl Zeiss Meditec) seems to be a highly useful addition to clinical practice, according to Ramon Naranjo-Tackman, MD, because it may be the best resource to date for evaluating placement of intracorneal segments, femtosecond laser-enabled keratoplasty (IntraLase, Advanced Medical Optics), and penetrating keratoplasty.
"There are differences in the sensitivity among OCT devices," said Dr. Naranjo-Tackman, a member of the cornea service, Hospital "Luis Sanchez Bulnes" de la Asociación para Evitar la Ceguera en Mexico, Mexico City. "[This] OCT machine uses 1,310 nm for imaging the anterior segment and 820 nm for the posterior segment. Similar to a B-scan ultrasound using light instead of sound, this OCT is a noncontact device with high resolution."
The instrument produces 80 scans per second with axial resolution up to 18 μm deep; laterally and transversally, the resolution can be as high as 60 μm.
Most patients evaluated using this OCT technology have undergone LASIK. In light of this fact, patients are required to have the flaps and residual stromal thickness evaluated by 6 months after the procedure. So far, such testing has happened in 95% of patients, he said.
In addition to refractive surgery, however, Dr. Naranjo-Tackman said that the OCT device is suitable for evaluating lamellar keratopathy as well as such new techniques as femtosecond laser-assisted keratoplasty, posterior chamber refractive lenses, glaucoma implants, and accommodative and pseudoaccommodative lenses. He also suggested that dynamic measurements of accommodation might be possible with the technology.
The OCT device provides measurements that are about 50 μm thinner than the measurements achieved with ultrasound, roughly 2.5% less. The learning curve associated with using this instrument, however, meant that when technicians in his practice first started using the machine, Dr. Naranjo-Tackman said, he noticed that the standard deviation with the first 100 eyes measured was as high as 14.3 μm. More recently, the standard deviation was 9.1 μm, he added.
In Dr. Naranjo-Tackman's department, he and his colleagues are using the OCT instrument routinely for intracorneal segments, femtosecond laser-enabled keratoplasty, and penetrating keratoplasty. To validate the results achieved with the device, he said he uses a third observer because a definite difference exists between the results obtained by two technicians.
"Depending on the number of eyes that have been evaluated, the difference can range from 2 to 22 μm," he said. "As the technicians become more familiar with the machine, their abilities improve and the difference decreased to 13 μm."
When considering which technology to compare OCT findings, the investigators first used an instrument (Confoscan 4, Nidek) that combines a confocal microscope, a non-contact endothelial microscope, and a pachymeter to measure the thickness of a flap created using a femtosecond laser and a 120-μm head.
In a case of marginal pellucid degeneration, Dr. Naranjo-Tackman said, the machine seems to be very useful. He said the measurements agreed with the corneal measurements as they became thinner in the inferior cornea. In a case of leukoma after PRK, the image showed the mushroom-shaped femtosecond laser-enabled keratoplasty graft.
"With the increasing number of lamellar keratoplasties being performed, both anterior and posterior, the OCT [device] seems to be the best resource thus far," he said. When the investigators compared the OCT instrument with the Scheimpflug camera (Pentacam, Oculus), the former also seemed to be a useful clinical device.
"The advantage of being able to use various machines is that they can provide a clinical profile of an individual patient," Dr. Naranjo-Tackman said. "The OCT is a device that is a 'must' in clinical practice. It is a noncontact device that does not require pupil dilation, [although] there is a definite learning curve. Many new technologies are emerging, and it is necessary to use them to evaluate patients preoperatively and postoperatively."