Over the past few decades, ophthalmic surgery techniques and instruments have undergone major changes and developments. From the development of small-gauge vitrectomy to advances in surgical microscopy and illumination, operative time and precision have improved dramatically. In the clinical setting, the noncontact method of cross-sectional bioimaging with OCT became an integral part of evaluation, management, and monitoring of wide range of retinal pathology.1 Development of spectral-domain OCT (SD-OCT) provided improvement in resolution and speed of acquisition, which in turn allowed for more detailed visualization of vitreoretinal pathology.2 In addition to its role in clinical management, OCT imaging plays an important role in preoperative surgical planning and postoperative evaluation, especially with epiretinal membranes (ERM), macular holes, and rhegmatogenous and tractional retinal detachments.3-8 Furthermore, new developments in digitally assisted surgical viewing are changing the ergonomics as well as enhancing the viewing and teaching capabilities in the operating rooms.
ADVANCES LEADING TO INTRAOPERATIVE OCT
Requirement of upright patient positioning and patient cooperation with the conventional tabletop OCT unit precluded its use in supine patients in the operative suite. In 2007, Bioptigen, Inc., developed a portable, handheld SD-OCT scanner, which allowed imaging of supine patients. It is mainly used for exams under anesthesia for pediatric patients with various conditions, such as retinopathy of prematurity, albinism, and shaken baby syndrome.9-12 Another commonly used handheld system is the Optovue iVue.13-16
Studies have reported feasibility of handheld intraoperative OCT (iOCT) to guide surgical decision making, for example, in cases where additional membrane peeling was required to achieve macular hole closure based on immediate evaluation of iOCT images.17,18 However, with this device configuration, surgical procedures would have to be paused to acquire images, increasing intraoperative time, and there was a higher risk of contaminating the surgical field.
Development of microscope-mounted iOCT devices led to a decrease in image capture time and improvement of reproducibility.19-21 Although this allowed for easier alignment of the system, real-time visualization of the tissue and tissue–instrument interactions were not possible until the development of microscope-integrated intraoperative OCT (MiOCT) devices.21-23 iIOCT systems incorporate the OCT optical path into the common optical pathway of the surgical microscope, allowing improved targeting and tracking of the scan beam and achieving parfocal and coaxial OCT imaging with the surgical view.
The first publication of MiOCT use in vitreoretinal surgery was in 2010, describing a custom prototype system with a research OCT integrated with a commercially available operating microscope by Cynthia A. Toth, MD, at Duke University.24 Other early prototypes included the Cirrus SD-OCT (Carl Zeiss Meditec) using the Zeiss OPMI VISU 200 surgical microscope,20 and Bioptigen EnFocus (Leica Microsystems/Bioptigen).25 Currently, 3 systems are approved by the US Food and Drug Administration. One of them is EnFocus used with the Leica surgical microscope.15,25 Another system is the Haag-Streit iOCT, which is integrated through a microscope side port,26 and the third is the RESCAN 700 (Zeiss), which is built on the Lumera 700 microscope platform.27-29
Better understanding of the vitreoretinal interface disease and intraoperative changes incurred with different surgical techniques and tissue manipulation can influence surgical decision making and possibly lead to improved surgical outcomes. Significant advances in software and hardware of MiOCT systems led to examination of their use for different conditions, such as vitreomacular traction, ERM, macular hole, and retinal detachment.15,16,30-33
In vitreomacular traction repair procedures, MiOCT provides real-time assessment of the strength of vitreomacular adhesions and allows visualization of unroofed cysts, subclinical full-thickness macular hole development, and incomplete peeling of membranes. Intraoperative identification of these subclinical changes may alter the immediate surgical approach, such as prompting the use of gas tamponade, and potentially preventing the need for reoperation.31 MiOCT use allows visualization of temporary intraoperative changes in the distance between the RPE and the ellipsoid zone and focal areas of retinal elevation at the ERM peel initiation sites (Figure 1).33 However, whether these intraoperative changes lead to significant functional changes remains to be determined. In macular hole surgery, certain configurations of macular hole and changes in macular hole geometry seen with MiOCT were associated with anatomical success.32 In retinal detachment surgery, MiOCT aids in detection of residual subretinal fluid (Figure 2A), small retinal breaks, and proliferative vitreoretinopathy membranes and can assist in completion of fluid–air exchange. In tractional retinal detachment surgeries, real-time visualization of the planes may also help achieve more precise delamination and segmentation. It may also aid in evaluation during vitrectomy for intraocular foreign body or subluxed lens removal (Figure 2B).
MIOCT PROSPECTIVE STUDY DATA
Early studies of iOCT included only small retrospective case reports, until the PIONEER study, which was the first large prospective study that evaluated feasibility and utility of the microscope-mounted Bioptigen iOCT.34 Surgeons in this study reported that their surgical approach in retinal membrane peeling procedures was changed by iOCT in 8% of cases.34
The DISCOVER study evaluated the EnFocus and RESCAN 700 MiOCT systems. It was a single-site, multisurgeon prospective investigational device study for both anterior and posterior segment surgery.25,28,29 The 1-year results of the EnFocus system in this study showed that iOCT was feasible in 92% of cases. Per surgeon reports, it was most useful for membrane peeling procedures, providing valuable information in 65% of cases and leading to a change in surgeons’ decision making in 35% of cases.25 The RESCAN 700 portion of the DISCOVER study similarly demonstrated the feasibility of real-time iOCT in ophthalmic surgery.29 In the RESCAN 700 system, the real-time OCT images are projected on a heads-up display, allowing the surgeon to manipulate scan length, location, and angle through video monitor display or the foot pedal control. This study showed that imaging was successfully obtained in 99% of cases and provided new and different information to the surgeon in 19% of cases.29
Further software and hardware changes will be necessary to address the current MiOCT systems’ limitations, such as challenges related to visualization of iOCT data on external screens vs surgical oculars, difficulties with imaging peripheral retina, and light scattering and shadowing from surgical instruments. In systems with in-ocular heads-up display systems, the size of OCT images and the visual field are limited by the size of the surgical oculars. Use of an external monitor for viewing OCT images displays a larger image, but it requires the surgeon to look away from the surgical field. Additionally, MiOCT systems show deterioration of image while evaluating peripheral retina, limiting its use in evaluating peripheral pathology. There are reports of intraocular probe-type OCTs that may be used to scan target tissue anywhere in the eye.35-38 However, further studies are necessary to evaluate this technology.
Currently, most surgical instruments lead to light scattering and shadowing, limiting to some degree the real-time visualization of retina manipulation (Figure 2C and 2D). The amount of shadowing varies depending on instrument material, configuration, thickness, and relative orientation to the optical axis of the OCT.30 Development of instruments that minimize scatter and shadowing will allow for more precise tissue manipulation. Ehlers et al reported use of semitransparent rigid plastic material instruments that allowed decreased light scatter and improved visibility of adjacent tissue as well as the tissue immediately underlying the instruments.39 Furthermore, development of new software algorithms may assist in software-based processing of the image to minimize shadowing as well as localize the beam to the area of interest.
One of the most recent advances in MiOCT technology developed at Duke University allows 4-dimensional (4D) MiOCT visual feedback through real-time volumetric imaging.40 4D MiOCT acquires, processes, and renders volumes in real time, which allows for enhanced visualization of tissue deformation and instrument motion and decreases the need for constant tracking of the moving object. 4D MiOCT is not currently commercially available.
HEADS-UP DIGITALLY ASSISTED SURGICAL VIEWING
Recent developments in 3-dimensional (3D) heads-up primary vitreoretinal surgery viewing are gaining popularity. The Ngenuity digitally assisted vitreoretinal surgery system (Alcon, Inc.) and TrueVision visualization system allow surgeons to maintain a heads-up position instead of having to look down through the microscope oculars. Ngenuity hardware is comprised of a 2-sensor, single 3D high-dynamic-range camera mounted in place of the microscope oculars. The cameras are connected to a central processing unit (CPU), which then projects the live feed onto a 55” OLED medical display. The monitor is mounted on a stand, which can be adjusted and rotated. The surgeon wears polarized “passive” glasses to see the images in 3D.
Reported advantages of this system include high magnification, improved ergonomics for the surgeon, a decrease in required endoillumination through enhanced digital signal processing, improved depth of field, ability to overlay diagnostic studies, including iOCT data, and enhanced teaching and observation capabilities.41,42
Improved ergonomics is one of the advantages of this system. Without the need to lean forward and look into microscope oculars, the surgeon can sit back in the chair and use the backrest for back support. This setup can reduce back and neck strain.43 In a few published reports, surgeons were able to switch to this technique without difficulties.41,42 Image quality depends on specific conditions, such as distance of the display from the surgeon, angle of the display relative to the surgeon, and minimization of glare. The monitor positioning must be as straight as possible to achieve optimal image quality. Because an assistant sits perpendicular to the patient’s head, there is a need for a head turn toward the screen, which may require more time for the assistant to adapt.
With the surgery displayed on the screen, anyone in the room wearing 3D glasses is able to see how the surgery is going. This provides an important educational benefit, allowing trainees to observe exactly what the surgeon is doing. Also, attendings at teaching institutions can better guide fellows and residents through difficult surgical maneuvers.
One of the important benefits of Ngenuity is being able to use lower endoillumination levels by increasing the camera’s iris aperture, potentially decreasing phototoxicity, especially for macular cases. Another benefit includes real-time image processing and color manipulation, which can allow better visualization of the vitreous and decreased glare. The increase in depth of field and wider field of view also helps in complex cases, including proliferative vitreoretinopathy/complex retinal detachment cases, intraocular foreign body, and scleral fixated intraocular lens cases.44
Further developments to allow more real-time digital signal processing could enhance this technology.
Real-time integration of OCT technology and heads-up 3D surgical viewing in the operating room have started to make an impact on surgeon ergonomics, surgical techniques, management, and decision making. As the MiOCT systems offer immediate image-guidance for the surgeon, they may improve our understanding of effects of surgical manipulation on tissues and possibly allow us to explain and predict variations in postoperative visual outcomes. Randomized control trials are needed to evaluate the effect of the reported intraoperative changes seen with iOCT on long-term patient outcomes.
Also, MiOCT may offer benefits to regenerative and gene therapy in the future, improving precision of delivery of a therapeutic agent. New advances, such as improvement to iOCT aiming and tracking, optimization of heads-up display or external monitor to maximize feedback while minimizing distractions, improvements in digital image processing, and development of OCT-compatible instruments, will promote further integration of MiOCT and heads-up surgical viewing systems. RP
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