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Noninvasive technique helps to identify implanted IOL type
Laser spectroscopy can identify the material of an already-implanted IOL. This may help physicians faced with new patients whose records are unavailable.
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GALVESTON, Texas — Occasionally it becomes necessary for physicians to treat an IOL implanted in the eye of a patient whom they have never seen before and whose medical records are not available. Because different treatments for a variety of eye disorders can have a negative effect on certain IOL materials, it is important to be able to find a noninvasive way to ascertain the material of the IOL in question before beginning treatment.
In a study conducted by ayne F. March, MD, and his team of investigators, Raman spectroscopy was used to identify the type of IOL implanted after previous cataract surgery. The study concluded that silicone IOLs appear to be more vulnerable to laser posterior capsulotomy. The silicone lens tends to show dark pits on the IOL surface after Nd:YAG laser surgery, according to the study.
Laser peaks identify IOLs
Raman spectra contain specific details about the chemical composition of the material being evaluated. Dr. March had developed a technique using Raman spectroscopy in some in vivo applications. His Raman spectroscopy uses a confocal system that allows for integration depth, by which the Raman scattered light is collected. The Raman signal is collected only from the IOL, while signals from other ocular tissue are excluded.
Dr. March recently modified his technique so that lower laser power could be used to establish a noninvasive technique for registering the type of IOL implanted in a patient’s eye.
The Raman spectra of the IOLs were obtained using Raman spectroscopy, a confocal probe, a laser wavelength of 514.4 nm, a laser power of 95 µW and an exposure of 1 second. The system also consists of a working distance of 11 mm microscope entrance lens, with a light-gathering power and with a numeric aperture of 0.5 to deliver the probing light beam and to collect the Raman backscattered light.
A holographic beam splitter guides the Raman-scattered light into a larger aperture lens, focusing the light on a fiber.
The entrance lens was mounted on a micropositioner for accurate focusing on the IOL in the patient’s eye. The collecting fiber, which acts as a pinhole in the confocal system, was connected to the spectrometer, which was equipped with a liquid-nitrogen-cooled, charge-coupled-device camera. Choosing the diameter of the fiber set the integration depth within the sample. A 400-µm fiber was used, yielding a sample integration depth of 1 mm.
An argon laser, which emitted beams at 514.5 nm, was used. A probing laser power of 95 µW and an exposure time of 1 second was used throughout the study. For alignment and initial focusing on the IOL, the beam was attenuated to about 14 µW.
The signal-to-noise ratio was im proved by a factor of 2 by increasing the slit setting on the spectrometer to 400 µm from 200 µm, the value used in the previous in vitro study. Although the ratio decreased the instrument’s spectral resolution by about 33%, the increased signal-to-noise ratio enabled in vivo measurement using an incident laser power of 95 µW versus the 1 mW power used in the laser in vitro study.
Correct identification
The study included six patients ranging in age from 46 to 76 years who were legally blind as a result of age-related macular degeneration, proliferative diabetic retinopathy or glaucoma. Three patients had a fellow eye that was not legally blind; therefore, no spectrum was obtained from that eye.
Patients were asked to fixate with the fellow eye on an illuminated target in the distance. Except for one patient who had an anterior chamber IOL, the pupils of all eyes were dilated with tropicamide and phenylephrine drops 30 minutes before the measurements were taken.
During initial focusing of the Raman spectroscopy, the entrance lens was positioned about 10 mm from the cornea. Next, the optical density filter was placed in the exciting beam path and the laser light was turned on. The IOL was detected, typically after 4 to 6 Raman scans, by moving the entrance lens toward the cornea. When the IOL was located, the optical density filter was moved aside and a regular Raman scan was collected.
Three IOL materials were characterized spectroscopically: PMMA, acrylic and silicone. Most of the unique Raman spectral features of the three IOL types were located in the higher scan region of the spectra, 2,800 to 3,100 cm-1.
In the tests, PMMA exhibited a large peak at 2,946 cm-1 with a small peak at each shoulder at 2,840 cm-1 and 3,000 cm-1.
The acrylic IOL showed a broader structure, with two peaks located at 2,917 cm-1 and 2,939 cm-1 and another peak located at the 3,055 cm-1 mark.
In the silicone IOL, a large peak was located at 2,900 cm-1, accompanied by another peak with about half the amplitude of 2,900 cm-1 peak at 2,961 cm-1 and a smaller peak at 3,048 cm-1. The low intensity of the spectra around 3,390 cm-1, which is indicative of oxygen-hydrogen bonds, showed that the silicone IOL did not contain much water.
All nine IOLs in the study were correctly identified using the confocal Raman system. The spectra were available for real-time interpretation, which Dr. March said he believes provides a fast and patient-friendly diagnostic tool to obtain information on an implanted IOL’s composition.
The study concluded that the Raman spectra approach is accurate because signals from the surrounding tissues are too weak to interfere with the detection of the IOL and because the confocal approach limits the interference of undesired Raman spectral features, such as aqueous humor.
Results from Dr. March’s study suggests that when other sources of spectra influence the results, the Raman spectral features in the longer spectral region provide the information to discriminate among IOLs. The lower spectral region can be used, combined with the longer spectral region, to interpret the data when other substances, such as IOL deposits, interfere.
Future use
According to Dr. March, all the procedures in the study were uneventful. No patient reported discomfort or complications during the procedure. No side effects during or after the data collection were observed.
Dr. March reported that in terms of safety, the study parameters were below the standards set forth by the American National Standards Institute (ANSI), which states that a 95-µW laser beam operating at 514.5 nm delivered over 1 cm2 can be used for 15 seconds continuously. The study suggests that because the beam was only used for 1 second, this scanning device is 15 times less than the standard set by ANSI.
The study also suggests other possible applications for the Raman technique, including characterization of implantable contact lenses. Confocal Raman spectroscopy can be used to discriminate between the IOL and the phakic lens and to estimate the distance between the lenses, as each has unique Raman spectra, he said.
Other possible applications include using the device to set up, characterize and assess IOL coatings, which can have distinctive Raman spectra, according to Dr. March. The Raman technique could also be used to monitor in vivo changes in or deposits on IOLs, he said.
For Your Information:Reference:
- Wayne F. March, MD, can be reached at the Department of Ophthalmology, 700 University Blvd., Suite 300, University of Texas Medical Branch, Galveston, TX 77555-1141; (409) 747-5811; fax: (409) 747-5812; e-mail: WFmarch@yahoo.com.
- Erckens RJ, March WF, et al. Noninvasive Raman spectroscopic identification of intraocular lens material in the living human eye. J Cataract Refract Surg. 2000;27:1065-1070.