Cataract surgery has advanced over the years with phacoemulsification and small incisions to speed up recovery and avoid the creation of astigmatism. Limbal relaxing incisions (LRIs) and toric intraocular lenses (IOLs) have been added to correct preexisting astigmatism.1,2 Although the technology for enhancing and perfecting refractive outcomes has progressively improved, the detailed measurement of refractive error had not progressed beyond refraction and topography until the advent of wavefront aberrometry.3–5 Preoperative and postoperative measurement of refraction, topography, and aberrometry now help in perfecting the refractive outcome of cataract surgery. However, even these diagnostic devices are insufficient to further improve outcomes and/or confirm the efficacy of the premium IOLs to a degree of accuracy, reproducibility, and speed that modern refractive cataract surgery demands. Cataract surgery outcomes have fallen behind those of laser vision correction, so that approximately 20% of patients require follow-up and approximately 14% are still prescribed glasses.6
Having an intraoperative diagnostic device, such as an aberrometer, could enable even greater diagnostic refractive precision. Aberrometers have been applied to ophthalmology over the past two decades, being used statically for refining the outcomes of laser vision correction using traditional Shack-Hartmann aberrometry4,5 and more recently even during cataract surgery with the development of Talbot-Moiré wavefront sensors (WaveTec, Aliso Viejo, CA).7,8 The known limitations of these devices, which typically use charge-coupled device or complimentary metal-oxide semiconductor (CMOS) image sensors requiring transfer of many hundreds of thousands of pixels per frame and extensive processing overhead, results in static data acquisition and data display with a lateral resolution of several hundred microns, through a deployment of hundreds of lens-lets, and limitations due to ambient light sensitivities. Although there have been previous attempts at real-time intraoperative aberrometry during refractive surgery,9 we wish to introduce a new, dynamic, real-time, intraoperative, sequential wavefront sensor with next generation enhanced performance that does not cause or require pauses of surgery.10
Materials and Methods
A real-time, sequential wavefront aberrometer has been designed for ophthalmic applications with initial deployment to enhance outcomes in refractive cataract surgery. This core technology, referred to as HOLOS (Clarity Medical Systems, Inc., Pleasanton, CA) is a novel technology for dynamically achieving accurate wavefront images or refractive profiles for real-time, intraoperative refinement during surgery.
The simplest description for how this sequential aberrometer works is depicted in Figure 1. The wavefront emanating from the patient’s eye is optically relayed onto a variable aperture and the segmented wavefront is focused onto a single quad detector to localize its position at a moment in time. This optical relaying is accomplished by focusing the incident wavefront (from the patient) onto a mirror with a known angle of tilt. As the motor rotates the mirror (Figure 2), the returned and sequentially shifted wavefront is redirected by the polarization beam splitter through another lens onto an aperture. This aperture “selects” a portion of the relayed wavefront to pass through a focusing element and onto the single quad detector. Not shown in Figure 1 is the 830-nm super-luminescent diode collimated light source that is launched into the eye and focused on the retina to create the returned wavefront.
Figure 1. Components of the real-time, sequential wavefront aberrometer for intraoperative use (HOLOS; Clarity Medical Systems, Inc., Pleasanton, CA).
Figure 2. (A) The sequentially shifted wavefront image is optically relayed by (B) rotating an angle mirror through a variably shaped aperture onto the quad detector.
If there is a refractive error (wavefront gradient) associated with the “selected” portion of the shifted wavefront, it is detected by the quad detector as an offset from a nominal (emmetropic) location, as shown in Figure 3. The collected data on the magnitude and location of the offset error are correlated to the eye’s refractive error.
Figure 3. The shifted wavefront is detected on the quad detector by the magnitude and location of the offset error. CCD = charge-coupled device
The sequential wavefront aberrometer achieves real-time, high-resolution sampling in large part by the speed at which the mirror rotates and the number and position of samples synchronized to pulses of the super-luminescent diode per revolution. For example, if the motor speed was set for 200 revolutions per second and the samples per revolution were set for 10 samples, this would result in 2,000 refractive samples per second over the area of interest from which to determine the refractive profile of the eye. Beyond speed and sampling, the spatial resolution of wavefront sampling can also be increased by controlling the firing time of the pulsed super-luminescent diode relative to the motor rotation and by controlling the tilt angle of the rotating mirror along with reducing the size of the sampling aperture. By modifying and controlling these variables, HOLOS can achieve a localized, lateral spatial resolution in terms of where the wavefront from the eye is sampled down to 25 μm, although a lesser resolution is more typical.
For the current clinical application as a real-time refractometer, a fixed mirror angle is used, allowing rotation of the prismatic mirror along a single diameter annulus for the rapid capture of sequential data points to characterize the refractive error (ie, sphere and cylinder). Dynamically changing the mirror angle (or even using different mirror technology, such as a microelectrical mechanical system) allows for the measurement of higher-order aberrations and interrogation of varying areas of interest. For example, with a certain optical design, changing the mirror angle from 0.5° to 3° allows for HOLOS to sample refractive annuli, ranging from approximately 1 to 6 mm diameter and beyond, collecting spatially resolved refractive data (aberrometry) throughout the pupillary area. Overall, the real-time, intraoperative, surgical information (ie, an intraoperative wavefront movie) is gathered with rapid sampling frequency, sufficient sampling density, variable aperture size, and a fixed mirror angle for streaming refractometry or dynamic mirror adjustment for aberrometry. For the purpose of this initial study, we selected a fixed mirror angle with a fixed aperture to primarily capture lower-order refraction over a ±4 diopter range, using a nominal 5-mm diameter annulus, specifically for the residual and corrected sphere and astigmatism with meridian during cataract surgery.
The prototype wavefront device has been compacted into a narrow profile attachment that can be fixed to an operating microscope without interfering with the surgeon’s line of sight through the scope. The portion of HOLOS, as attached beneath the microscope (Figure 4), is tapered with a width from approximately 6 cm in the front to 14 cm after the working area, and with a height of approximately 5 cm, being ergonomically friendly to leave a sufficient working distance for surgical maneuvers.
Figure 4. The narrow profile of the wavefront aberrometer can be fixed onto multiple operating microscopes (shown is the Zeiss Lumera model) (arrow) without interfering with the surgeon’s line of sight or working distance through the scope.
An intraoperative, clinical feasibility trial using HOLOS was designed in the setting of cataract surgery to confirm the viability of this technology in providing real-time refractometry data for clinical decision-making during refractive cataract surgery. The goals of this study were to demonstrate both the operational utility and clinical efficacy of the device, as follows:
Confirm the added size and dimensions do not change or limit the surgeon’s working distance or staff’s ergonomics and work flow.
Obtain high-quality refractive data regardless of the microscope or room illumination.
Not lengthen the operative time during surgery.
Capture the refraction real-time continuously.
Present the real-time qualitative and quantitative refractive data integrated within the surgical video of the eye.
Provide valuable intraoperative information for real-time diagnostic guidance of surgical maneuvers, thereby enhancing the surgical outcome.
To achieve these goals, the HOLOS intraoperative prototype device was installed onto the surgical microscopes of three surgeons: Dr. Robert Osher at the Cincinnati Eye Institute in Cincinnati, Ohio; Dr. Stephen Slade at the Slade & Baker Vision Center in Houston, Texas; and Dr. David F. Chang at the Peninsula Eye Surgery Center in Los Altos, California. Multiple cataract surgery and IOL testing situations were pursued to demonstrate the use of the device. Specifically, the evaluation of refractive data was represented qualitatively and displayed graphically within the center of the entrance pupil as an ellipse, circle, or dot, and quantitatively as streaming video display of sphere, cylinder, and axis at the bottom of the screen. This was done as a display while viewing the real-time image of the eye before, during, and after various surgical maneuvers to primarily assess the magnitude and axis of astigmatism. All subjects participated voluntarily after providing informed consent, and all measurements were performed according to the ethical principles set forth in the Association for Research in Vision and Ophthalmology Declaration of Helsinki.
The clinical HOLOS prototype device was mounted onto two different model Zeiss microscopes (Figure 4) without difficulty. The surgeons reported the device did not impact their visualization, overall ergonomics, or surgical working distances (range: 150 to 175 mm). Furthermore, the device’s performance was not altered by the strong illumination of the microscope or background room lights. Each surgery was successfully completed with the HOLOS attached throughout the length of the case.
Operationally, the technology is coaxially oriented with the surgeon’s view of the eye, so by default, the device was automatically aligned to the eye once it was viewed and roughly centered under the operating microscope. Once focused, the eye’s refractive information was immediately available to the surgeon, via a small external display, allowing the surgeon to assimilate both the qualitative and quantitative details using virtually no additional time for set up and/or data collection. The feedback by all three surgeons regarding the time added for refraction was reported as “imperceptible.”
The refractive outcome of the real-time sampling was visualized in an image overlaying a live eye image as viewed through the microscope and presents the refractive error both qualitatively and quantitatively. The qualitative representation of refractive error was seen as a circle for spherical error, a thin ellipse for astigmatism, and a dot for emmetropia. A practical sampling of the 180 frames/second with a single fixed aperture for localization of lower-order aberrations enables a real-time visualization of qualitative refractive data coaxially aligned with the image of the eye and quantitatively as sphere, cylinder, and axis at the bottom of the screen.
During these trials, each surgeon independently recognized and tested the potential for improved clinical efficacy of using HOLOS’ real-time refractometry in the following settings.
Identify the astigmatic axis during surgery in place of the preoperative markings.Figure 5 reveals a single image from streaming video of a pseudophakic eye with residual mixed astigmatism (against the rule). The magnitude and axis of astigmatism are shown graphically by the yellow ellipse in the center of the image’s pupil and numerically by the displayed sphere, cylinder, and axis at the bottom of the screen. The precise axis of astigmatism is verified and, with further studies, may offer a degree of reliability that exceeds current techniques.
Figure 5. A captured frame from streaming video of HOLOS (Clarity Medical Systems, Inc., Pleasanton, CA) in a pseudophakic eye with residual mixed astigmatism (against the rule), where the magnitude and axis is qualitatively shown by the central yellow ellipse and quantitatively displayed as sphere, cylinder, and axis at the bottom of the frame.
The potential to manage LRIs by guiding their placement location and using real-time feedback to titrate their length until astigmatic neutralization. For the correction of astigmatism, LRIs can be made and centered precisely over the axis indicated by the refractometry within the streaming video. As the incisions are placed, the qualitative and quantitative changes in refraction can be monitored to adjust the length of the incisions, so as to titrate the outcome for astigmatic neutralization.
Validating the potential efficacy to eliminate the use of preoperative markings with toric IOLs, because the real-time refraction can guide the toric IOL rotation until minimization or ideally neutralization of cylinder. A series of still images were pulled from the wavefront “movie” of a patient receiving a toric IOL (Alcon SN60T1; Alcon Laboratories, Inc., Fort Worth, TX). The first frame (Figure 6A) captures the placement of the toric IOL roughly 90° away from the final axial alignment. The qualitative and quantitative refractive wavefront data were continually streamed, confirming the expected increase in sphere and cylinder and the shift in axis. As the surgeon began to rotate the toric IOL (Figure 6B), the real-time refractometry data continued streaming, confirming a reduction in refractive error (sphere and cylinder) for a given rotational change. The last frame (Figure 6C) demonstrates the surgeon is close to the final alignment of the toric IOL by the intraoperative display of neutral astigmatism (+0.25 diopters) and near emmetropic target refraction. Throughout this case and others, the continuous real-time refractive feedback provided confirmation of the source and causal relationship of the patient’s improved refraction with each procedural step. In princciple, marking the axis preoperatively is not necessary, because intraoperative assessment is more reliable than any preoperative mark would provide.
Figure 6. Gradually improving refraction detected by HOLOS (Clarity Medical Systems, Inc., Pleasanton, CA) during the rotational alignment of a toric intraocular lens from (A) approximately 90° away to (B) midway improvement to (C) its final optimal alignment with astigmatic neutralization.
HOLOS’ real-time refractive data integrated with the real-time surgical video of the patient’s eye, with real-time measurement of sphere, cylinder, and axis throughout a refractive cataract surgery, provides potentially diagnostic feedback to surgical steps performed. The practical evaluation of residual cylinder during an LRI placement and rotational accuracy while positioning a toric IOL has the potential to make this device extremely useful in the accurate management of astigmatism.1,2 In our patients, the feedback by the surgeons regarding the time added for assessing the refractive data was reported as “imperceptible,” and is believed to actually have saved time by avoiding the necessity of marking the astigmatic axis preoperatively.
Beyond the real-time analysis of cylinder, the residual spherical error after IOL implantation could also be monitored after LASIK or in other complex eyes, so that one can avoid a refractive surprise after surgery.11 This was not specifically tested in our patients, but certainly could be implemented as a final check at the end of the refractive cataract surgery. With this kind of diagnostic flexibility, both spherical and cylindrical outcomes could be more tightly controlled, which would be of great value with premium channel IOLs.12 Although a direct comparison of time, accuracy, and convenience was not made with this technology and that of the WaveTec ORange (WaveTec Vision) or ORA intraoperative aberrometry (Optiwave Refractive Analysis),7,8 it is anticipated that based on the clinical experience in this study, HOLOS will be an improvement because it is continuously sampled and without any time delay for set-up and analysis.
Further studies of this technology in comparison with WaveTec and/or other standard diagnostic techniques are recommended for the future. We believe this technology will be a helpful adjunct during femtosecond laser refractive cataract surgery13–15 because the effect of laser intrastromal relaxing incisions and specially shaped transcorneal entry incisions can be titrated and perhaps linked with close loop feedback for enhancing the surgical outcomes.
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- Holland E, Lane S, Horn JD, Ernest P, Arleo R, Miller KM. The AcrySof Toric intraocular lens in subjects with cataracts and corneal astigmatism: a randomized, subject-masked, parallel-group, 1-year study. Ophthalmology. 2010;117:2104–2111. doi:10.1016/j.ophtha.2010.07.033 [CrossRef]
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- Cheng X, Himebaugh NL, Kollbaum PS, Thibos LN, Bradley A. Test-retest reliability of clinical Shack-Hartmann measurements. Invest Ophthalmol Vis Sci. 2004;45:351–360. doi:10.1167/iovs.03-0265 [CrossRef]
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- Shulman M. The cataracts are gone-and so is the need for glasses. With the latest implantable lenses, you can see near and far. US News World Rep. 2007;143:64,66.
- Packer M. Effect of intraoperative aberrometry on the rate of postoperative enhancement: retrospective study. J Cataract Refract Surg. 2010;36:747–755. doi:10.1016/j.jcrs.2009.11.029 [CrossRef]
- Chen M. Correlation between ORange (Gen 1, pseudophakic) intraoperative refraction and 1-week postcataract surgery autorefraction. Clin Ophthalmol. 2011;5:197–199. doi:10.2147/OPTH.S17489 [CrossRef]
- Krueger RR, Gomez P, Herekar S. Intraoperative wavefront monitoring during laser thermal keratoplasty. J Refractive Surg. 2003;19:S602–S607.
- , inventors; Clarity Medical Systems, Inc., assignee. Sequential wavefront sensor. US patent 7,445,335. January20, 2006.
- Gimbel HV, Sun R. Accuracy and predictability of intraocular lens power calculation after laser in situ keratomileusis. J Cataract Refract Surg. 2001;27:571–576. doi:10.1016/S0886-3350(00)00795-1 [CrossRef]
- Hayashi K, Manabe S, Yoshida M, Hayashi H. Effect of astigmatism on visual acuity in eyes with a diffractive multifocal intraocular lens. J Cataract Refract Surg. 2010;36:1323–1329. doi:10.1016/j.jcrs.2010.02.016 [CrossRef]
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- Palanker DV, Blumenkranz MS, Andersen D, et al. Femtosecond laser-assisted cataract surgery with integrated optical coherence tomography. Sci Transl Med. 201017;2:58ra85. doi:10.1126/scitranslmed.3001305 [CrossRef]
- Masket S, Sarayba M, Ignacio T, Fram N. Femtosecond laser-assisted cataract incisions: architectural stability and reproducibility. J Cataract Refract Surg. 2010;36:1048–1049. doi:10.1016/j.jcrs.2010.03.027 [CrossRef]