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Anterior segment diagnostics aid preop assessment, boost outcomes
Ultrasound, wavefront aberrometry, Scheimpflug imaging, OCT and other technologies are expanding the frontiers of anterior segment imaging.
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Ongoing developments in anterior segment diagnostic and test instrumentation enhance preoperative assessment and optimize postoperative outcomes.
Scheimpflug photography, very high-frequency (VHF) digital ultrasound, optical coherence tomography and other technologies enable critical functions such as topography, wavefront aberrometry, pachymetry and biometry. Increasingly accurate measurements enhance cataract and refractive surgery, as well as corneal transplantation procedures.
The most recent innovations include a VHF digital ultrasound arc scanner that promises to enhance refractive outcomes for accommodating IOL and toric IOL implantation, according to one expert.
 Dan Z. Reinstein |
The Artemis 3 (ArcScan), currently in development, promises to surpass its predecessors, particularly in measuring the entire circumference of the crystalline lens capsule, which will facilitate accommodating IOL sizing and IOL power calculation, Dan Z. Reinstein, MD, MA (Cantab), FRCSC, DABO, FRCOphth, told Ocular Surgery News.
“The new thing is that behind the iris, the Artemis 3 can now scan and delineate the entire circumference of the human capsule around the lens,” Dr. Reinstein said. “By measuring the capsular bag, one may be able to better determine the effective lens position of an IOL in cataract surgery, which should increase the accuracy of the refractive result and decrease the necessity to use excimer laser corneal procedures on top of a cataract surgery.”
VHF digital ultrasound scanning offers optimal measurement precision and patient safety and good visual results despite the need to immerse the eye in saline for scanning. When physicians overcome this perceived inconvenience, Dr. Reinstein predicted they will appreciate the quality and accuracy of the information provided by VHF digital ultrasound.
The Artemis 3 is currently under review by the U.S. Food and Drug Administration.
New device combines technologies
Eric D. Donnenfeld, MD, OSN Cornea/External Disease Board Member, and Perry S. Binder, MD, said the iDesign aberrometer (Abbott Medical Optics), which combines Placido disc topography and wavefront aberrometry, is another device on the horizon.
“The iDesign will give you important information for treating complex eyes and for diagnosing irregular corneas,” Dr. Donnenfeld said. “In the future, we’ll be using a lot of wavefront treatments and some topographic-based treatments. This will give us the ability to measure eyes and use the right technology for the right patient.”
 Eric D. Donnenfeld |
The iDesign will help surgeons avoid data acquisition errors, a major downside of capturing multiple images with various devices, Dr. Binder said.
“The advantage is that you’re making one capture and you’re capturing one image directly on top of the other,” he said. “That minimizes acquisition data errors such as technician errors, patient fixation errors and translation of wavefronts from one image, where the observer may have taken an incorrect picture with Placido disc, and they come back and do the wavefront, which is commonly done today for excimer laser surgery.”
Theoretically, the addition of OCT and pachymetry may make the iDesign one of the most versatile anterior segment diagnostic tools available.
“One machine that’s going to be able to replace four can give you better data,” Dr. Binder said.
Wavefront aberrometry and Scheimpflug imaging
The ORange intraoperative wavefront aberrometer (WaveTec Vision Systems), which made its commercial debut at the American Society of Cataract and Refractive Surgery meeting in San Francisco, enables surgeons to obtain real-time refractive measurements during cataract surgery, Dr. Donnenfeld said.
“We’ve used it efficiently in toric IOLs and limbal relaxing incisions to improve outcomes, and outcomes have significantly improved with this new technology,” he said. “We have dramatically improved our rate of 20/20 patients and reduced our enhancement rates significantly through the use of intraoperative wavefront ORange.”
Dr. Donnenfeld and colleagues are now assessing the efficacy of the ORange for aphakic refraction in patients with refractive outliers and following its use in RK, PRK and LASIK.
“In these cases, IOL calculations can be difficult, and this should allow us a more accurate reading,” Dr. Donnenfeld said. “One of the caveats for cataract surgery is that every patient responds differently to a limbal relaxing incision and to a cataract incision. ORange intraoperative aberrometry allows us to assess the effect of the incision to help predict better outcomes following cataract surgery.”
The Pentacam Scheimpflug camera (Oculus) enables qualitative assessment of corneal shape, which helps surgeons better identify patients with misshapen and diseased corneas, corneal ectasia, keratoconus and other contraindications for refractive surgery, Dr. Binder said.
 Perry S. Binder |
“The Scheimpflug technologies available in, for instance, Pentacam, Orbscan (Bausch & Lomb) and Galilei (Zeimer) give a higher resolution but low magnification view of the cornea. It gives a qualitative assessment of corneal shape,” he said. “Using computer algorithms, one can calculate the curvature of the cornea and calculate wavefront aberrations induced by some of those shape changes in the cornea. This helps doctors in the diagnosis of warped corneas and ectatic corneas to eliminate them from refractive surgical procedures.”
The Pentacam also provides pachymetry mapping, which helps surgeons assess central corneal and peripheral thickness and screen candidates for laser refractive surgery, Dr. Donnenfeld said.
“We believe that pachymetry maps are important in assessing the symmetry of the cornea and assessing patients for excimer laser photoablation,” he said.
The Orbscan corneal topographer measures the shape of the anterior and posterior surface of the cornea, according to Bausch & Lomb’s Web site.
Axial length measurement, topography and pachymetry
The IOLMaster (Carl Zeiss Meditec) provides accurate axial length measurement, noncontact optical biometry and yields reproducible measurements between operators, according to the company’s Web site.
Dr. Donnenfeld said the IOLMaster has dramatically improved diagnostics in his practice.
“We have changed our technology dramatically in the last couple of years,” he said. “All you have to do is look at how A-scan in our practice has been completely replaced by the IOLMaster as a technology that achieves better results with a more consistent ability to achieve emmetropia following cataract surgery.”
Visante OCT (Zeiss) combines Placido disc topography and pachymetry. Overall, anterior segment OCT offers myriad benefits.
“There’s some good work that shows that OCT may be helpful in evaluating epithelial thickness,” Dr. Donnenfeld said. “That may be helpful in diagnosing keratoconus.”
Anterior segment OCT aids the measurement of LASIK flap thickness, cataract density, IOL placement and phakic IOL size.
“OCT will allow us to evaluate anterior chamber depth and angle diameter to help predict better phakic IOL measurements and give us a way of predicting lens size more reliably,” he said. “OCT should give us the ability to evaluate cataracts and have an objective measurement of cataract density. … It allows you to place [implantable Collamer lenses] and predict the right-size ICL that should be used based on OCT measurements of sulcus diameter.”
Dr. Reinstein cautioned that, “We must be aware that the ability of OCT to pick up the LASIK interface is quite variable, and when detected, the repeatability of flap thickness measurements is in the region of 5 µm, compared to 1.68 µm for Artemis VHF digital ultrasound.
OCT may be the most convenient technology for anterior segment imaging, but the evidence shows that it is not the best technology, Dr. Reinstein said.
“Our in-house studies testing epithelial thickness reproducibility with the most advanced type of corneal OCT using spectral domain techniques (Optovue, Inc), showed a repeatability of 3.4 µm, compared to 0.58 µm for the Artemis 1 VHF digital ultrasound. The inferior precision, in addition to the fact that spectral domain OCT is currently unable to obtain epithelial thickness beyond a 2-mm radius, render OCT less useful for keratoconus screening than the whole-cornea epithelial thickness maps obtained by VHF digital ultrasound.”
 Right and left eyes of the same patient demonstrating that the sulcus-to-sulcus diameter can be unpredictably different despite similar angle-to-angle measurements. |
 Artemis B-scan of a cornea 1 year after LASIK demonstrating the ability to detect the LASIK interface with clarity throughout the length of the incision with the digitally processed interface plot beneath. Images: Reinstein D |
Because OCT cannot image behind the iris, “this renders it unable to help with posterior chamber dimensions or whole lens capsule anatomy,” he said.
“OCT, in my view, is the best imaging and biometric modality for the retina surgeon, but for the anterior segment and refractive surgeons, the relative difference in the quality of information derived from current optical devices compared with what is now possible by VHF digital ultrasound will dictate that the future is in ultrasound,” Dr. Reinstein said.
He pointed out that there are physical reasons why current OCT technology does not have the sensitivity equivalent to VHF digital ultrasound for picking up LASIK interfaces and mapping epithelial thickness across the whole cornea, determining the sulcus-to-sulcus and posterior chamber dimensions, or the whole lens capsule dimensions. Within the stroma, corneal interfaces produce very weak optical discontinuities and hence a very low signal-to-noise ratio, even with current sophisticated spectral domain OCT imaging technology. In contrast, digital ultrasound easily picks up internal stromal interfaces because ultrasound scatters where physical tissue discontinuities exist, producing relatively large changes in acoustic impedance and high signal-to-noise ratio at such interfaces.
“Perhaps commercially, OCT now serves the purpose of enabling corneal and anterior segment biometry that was hitherto unavailable to the ophthalmic community at large,” he said. “But in my opinion, and I readily admit to having a financial interest in this statement, once devices enabling high-resolution ultrasound imaging and biometry become widely available, surgeons will easily forgive the inconvenience of water-cup immersion based scanning in favor of using the highest quality biometric data for cornea and all anterior segment surgery.”
Conversely, Dr. Binder said anterior segment OCT is useful in diagnosing intracorneal infections; LASIK interface complications such as diffuse lamellar keratitis and interface fluid syndrome; anterior segment complications in cases of cloudy corneas such as dislocated Descemet’s stripping endothelial keratoplasty and Descemet’s membrane endothelial keratoplasty transplants; unusual angle complications associated with complicated cataract surgery; and other conditions that are not as easily diagnosed using ultrasound. – by Matt Hasson
References:
- Fea AM, Annetta F, Cirillo S, et al. Magnetic resonance imaging and Orbscan assessment of the anterior chamber. J Cataract Refract Surg. 2005;31(9):1713-1718.
- Li Y, Netto MV, Shekhar R, Krueger RR, Huang D. A longitudinal study of LASIK flap and stromal thickness with high-speed optical coherence tomography. Ophthalmology. 2007;114(6):1124-1132.
- Lovisolo CF, Reinstein DZ. Phakic intraocular lenses. Surv Ophthalmol. 2005;50(6):549-587.
- Oh J, Shin HH, Kim JH, Kim HM, Song JS. Direct measurement of the ciliary sulcus diameter by 35-megahertz ultrasound biomicroscopy. Ophthalmology. 2007;114(9):1685-1688.
- Pop M, Payette Y, Mansour M. Predicting sulcus size using ocular measurements. J Cataract Refract Surg. 2001;27(7):1033-1038.
- Reinstein DZ, Archer TJ, Gobbe M. Corneal epithelial thickness profile in the diagnosis of keratoconus. J Refract Surg. 2009;25(7):604-610.
- Reinstein DZ, Archer TJ, Gobbe M. Stability of LASIK in topographically suspect keratoconus confirmed non-keratoconic by Artemis VHF digital ultrasound epithelial thickness mapping: 1-year follow-up. J Refract Surg. 2009;25(7):569-577.
- Reinstein DZ, Archer TJ, Gobbe M, Silverman RH, Coleman DJ. Epithelial thickness in the normal cornea: three-dimensional display with Artemis very high-frequency digital ultrasound. J Refract Surg. 2008;24:571-581.
- Reinstein DZ, Archer TJ, Gobbe M, Silverman RH, Coleman DJ. Epithelial, stromal and total corneal thickness in keratoconus: three-dimensional display with Artemis very high-frequency digital ultrasound [Published online ahead of print May 15, 2009]. J Refract Surg. doi: 1.9999/1081597X-200905015-06.
- Reinstein D, Archer T, Gobbe M, Silverman R, Coleman DJ. Repeatability of layered corneal pachymetry with the Artemis very high-frequency digital ultrasound arc-Scanner. J Refract Surg. In press.
- Reinstein DZ, Archer TJ, Silverman RH, Rondeau MJ, Coleman DJ. Correlation of anterior chamber angle and ciliary sulcus diameters with white-to-white corneal diameter in high myopes using Artemis VHF digital ultrasound. J Refract Surg. 2009;25(2):185-194.
- Werner L, Izak AM, Pandey SK, Apple DJ, Trivedi RH, Schmidbauer JM. Correlation between different measurements within the eye relative to phakic intraocular lens implantation. J Cataract Refract Surg. 2004;30(9):1982-1988.
- Perry S. Binder, MD, can be reached at 2500 Sixth Ave., Suite 307, San Diego, CA 92103; 858-922-8699; fax: 619-702-7963; e-mail: garrett23@aol.com. Dr. Binder has no direct financial interest in the products discussed in this article, nor is he a paid consultant for any companies mentioned.
- Eric D. Donnenfeld, MD, can be reached at OCLI, 2000 N. Village Ave., Rockville Centre, NY 11570; 516-766-2519; fax: 516-766-3714; e-mail: eddoph@aol.com. Dr. Donnenfeld is a consultant to AMO and WaveTec.
- Dan Z. Reinstein, MD, FRCSC, can be reached at London Vision Clinic, 8 Devonshire Place, London W1G 6HP, United Kingdom; 44-207-224-1005; fax: 44-207-224-1055; e-mail: dzr@londonvisionclinic.com. Dr. Reinstein is an author of patents pertaining to VHF digital ultrasound technology administered by the Weill Medical College of Cornell University and has a minor financial interest in the Artemis technology (Arcscan Inc., Morrison, Colo.) He is a consultant for excimer and femtosecond lasers to Carl Zeiss Meditec AG.
