Since the first introduction of the operating microscope (OM) for neurilemoma surgery in 1957, binocular microscopes have been considered essential equipment in the neurosurgical field [5]. The microscopes have also contributed to great advances in neurosurgical technique. Recently, OMs have been integrated with neuronavigation systems and offer intraoperative fluorescence imaging of vessels and tumors as well as good magnification, sufficient illumination, and highdefinition (HD) videos [15]. However, OMs are inherently limited by the ocular lens system and size. Surgeons often have to assume an uncomfortable posture for a long period of time while looking at the surgical field through a small lens eyepiece, causing eye fatigue and excessive tension in the body. In addition, the weight and size of OMs have gradually increased, limiting its mobility and increasing the space occupied in the operating theater. The surgical assistant’s view is not the same as that of the primary operating surgeon, and the observation tube or eyepiece lens for the assistant requires readjustment when the microscope is moved [7]. In addition, the current cost of an OM is more than 500 million KRW depending on the options, calling into questions the cost effectiveness.

Exoscope is a piece of equipment that has a camera mounted outside the surgical field. In the early model, the image was acquired using a low-quality endoscopic system [2,4]. However, with improvements in imaging and optical technology, a highresolution, three-dimensional (3D) video can be acquired and displayed. Therefore, the exoscope has emerged in the neurosurgical field as a new hybrid optical system between the OM and the endoscope [1,10,12].

However, many neurosurgeons are not yet familiar with the 3D exoscope and question whether it can be used as a substitute for OM. We used two types of 3D exoscopes, VOMS-100 (SOMETECH, Seoul, Korea) and VITOM 3D (Karl Storz GmbH, Tuttlingen, Germany) in brain tumor surgeries. Herein, we present the advantages and limitations of 3D exoscopes compared with the OM.

This study was approved by the Ethical Institutional Review Board (IRB) of the Catholic Medical Center in the Republic of Korea (IRB number, OC20RAS1010) and the informed consent was waived by the regulation of our IRB.

Exoscopic surgery evolved from endoscopic surgery. Exoscopy is similar to endoscopy in that both present an image produced by a scope onto a video monitor. The difference is that the camera is positioned outside the surgical field in exoscopy, allowing the surgeons to observe the surgical field of view with the naked eye [4]. Gidenberg and Labuz [2,3] first reported use of an exoscope using the endoscopic system. The authors devised a video camera mounted on a stereotactic frame and that integrated the video image with a computergenerated 3D view to allow visualization of the virtual mass at the selected depth on the monitor in real time.

Mamelak et al. [8] reported preliminary research regarding the HD exoscopic system for neurosurgery in an animal model. The authors used an 8-mm-diameter rigid lens telescope with 3-chip HD digital camera and HD monitor. The authors reported that image quality with the exoscopic system was similar to that with the OM even at high magnification, and the system was easy to manipulate and comfortable during surgery. The lack of stereopsis was a limitation but was compensated for with repeated procedures. The authors reported 16 clinical cases in the literature in which a 10-mm-diameter rigid-lens telescope with a focal distance of 20 cm was used, held by a pneumatic holder over the operative field. The authors reported the same results using an exoscopic system in which the image quality was nearly the same as with the OM, but lacking stereopsis. They also reported that positioning and focusing of exoscope system were relatively low compared to OM [9].

The main drawback of stereopsis in exoscopy was overcome by development of 3D exoscopes. Rossini et al. [13] reported preliminary experience in petrosal meningioma using VITOM 3D, a 3D technology combined with 4K resolution. The VITOM 3D showed advantages including ergonomics, versatility, and depth of field compared with the OM. However, the authors suggested that the holder arm and repositioning system, refocusing, magnification need to be ameliorated. Oertel and Burkhardt [11] reported the initial experience of using VITOM 3D in five cranial and 11 spinal surgeries and assessed instrument handling, intraoperative repositioning and handling, and comfort level, giving an excellent rating of 100%, with image quality equal to that of the OM. Beez et al. [1] reported their experience using VITOM 3D in pediatric neurosurgery. The authors answered questionnaires regarding ease of preparation, image definition, magnification, illumination, field of view, ergonomics, accessibility of the surgical field, and general user friendliness in three operations. Although the microscope was superior in magnification, field of view, illumination, and user friendliness, the authors reported the exoscope to be advantageous in ergonomics and accessibility to the surgical field. Ricciardi et al. [12] systemically reviewed 29 reports in the literature of exoscope use in neurosurgery on 574 patients. The authors concluded that the exoscope was largely considered superior or equivalent to the OM in ergonomic comfort, educational purposes, image quality, magnification, lighting, and cost, although some surgeons preferred the OM for better stereoscopic vision.

This study was the first to analyze the experience of two 3D exoscopes compared to OM using statistical methods in brain tumor surgeries. Two exoscopes with different image resolutions, VOMS-100 with full HD and VITOM 3D with 4K, were used and compared with the OM, Kinevo 900. VOMS-100 was generally inferior to the OM in overall image quality including magnification and brightness. However, the VITOM 3D was equivalent to the OM in overall image quality and brightness, except for magnification. The different results obtained between exoscopes were potentially caused by different specifications such as the image resolution of monitor and camera. All 3D exoscopes showed advantages in accessibility of instruments in the surgical field due to the relatively long working distance and direct observation of the surgical field with the naked eye. A recently developed OM, Kinevo 900, also has a long working distance, but the bulky head of the OM can hinder direct view of the surgical field. The smaller amount of space occupied is also an advantage in the modern neurosurgical operating theater because more equipment such as that for neuronavigation, devices for intraoperative neurophysiologic monitoring, or ultrasonic aspirator are used simultaneously and require space. Ergonomics and educational usefulness were not different between exoscopes and the OM in the present study, which may be due to the use of the highly advanced OM, Kinevo 900. However, the exoscopes studied were limited in integration with neuronavigation and detection of fluorescence, which may be overcome in future models.

Although not yet commonly used in brain tumor surgery, the 3D exoscope has emerged as a new optical system alternative to OMs. The 3D exoscopes can provide comparable image quality and brightness if the appropriate resolution is offered. In addition, surgeons can view the surgical field with the naked eye and easily access instruments in the surgical field. Furthermore, the size of exoscopes is generally smaller and the cost is less than those of OMs. More advanced exoscopes most likely will be used as substitutes for OMs in the future.