With AR systems, a digital 3D model and its structure of interest could be viewed and analyzed for operations planning, which was valuable for individualized craniotomy, positioning of structures of interest, and planning surgical approaches[9, 10]. In our study, AR was used to display the inner edge of the TSSJ. As a result, although MARNS positioning accuracy was inferior to NNS, it was easier to provide accurate TSSJ location, and it was helpful enough to design effective incisions and preserve bone flap integrity. There were several innovations we proposed for the MARNS that helped it be applied more effectively.
3.1 Tattooed QR code
In most reported studies, registration of images was most commonly carried out using fiducial markers or skin surface identification. Both techniques were found to be faster and more accurate, as well as less invasive and laborious than manual registration[11]. However, The fiducial marker-based registration method is inconvenient and time-consuming to perform in sterile environments. In addition, artificial markers increase the need for additional tracking equipments, as well as excessive preparations[12]. Skin surface identification techniques do not allow for intraoperative matching due to sterile surgical drape occlusion. In our study, tattooed QR codes were innovatively used as markers on the body. This tattooed QR code was convenient, can be reused, only needed to be designed and placed on the scalp at a specific position. Most importantly, it is capable of being used in sterile environments. We placed the QR code in front of the tragus to minimize QR code flexion and distortion, taking into account the influence of factors on the robustness of registration such as the site of the surgical incision and scalp curvature. In our opinion, this tattooed QR code serves as a useful landmark for MARNS registration.
3.2 Digital surgical design for correction of depth perception deficiency
There are several problems with AR techniques available for surgery. Depth perception has been one of the most significant[13]. For the surgeon, it is crucial to know the exact distance between surgical instruments and target organs. Based on current AR technology, the AR visualization paradigms fail to provide users with an effective way of distinguishing between accurate and inaccurate spatial alignment of virtual content to the environment[14]. Optical see-through AR has a fundamental problem, Swan et al.[15] claim. They found that egocentric depth of AR objects has been underestimated. Despite the well-matched and fused virtual model with the AR entity, it cannot provide operators with a clear understanding of the underlying structural hierarchy, which the operators desperately need. As a result, it is easy to cause a great deal of error in the projection of deep tissues or lesions on the scalp and skull due to the different angles from which they are photographed. According to our study, the digital surgical design approach we used on 3D models effectively eliminate the bias in positioning associated with poor structural hierarchy.
Digital surgical design[16] is an emerging medical science and technology tool integrated with medical surgical planning, which consists of three parts: 3D reconstruction, surgical design, and product design. It can help physicians to perform data measurement, preoperative analysis, surgical simulation, and to develop a set of surgical designs. In accordance with concepts of digital surgical design, we tried to create landmarks on both the scalp and skull to represent projections of TSSJ when reconstructing a virtual model. Finally, the design of the surgical incision and the location of the skull " keyhole " were accurately determined during retrosigmoid craniotomies.
3.3 Surgical skills
The sinus venosus can easily be injured by a bone window directly adjacent to it. The earliest 2 patients had TSS injuries when they were exposed to the venosus sinus, resulting from an inappropriate approach when grinding the bone flap. In this regard, a correct approach is also essential in preventing sinus injuries. Following is how we improved our procedure for exposing bone flaps and prevented TSS injuries from recurring. Firstly, after the projection point of TSSJ on skull is determined, a bone hole is gradually created with a grinding bur just below it to prevent damage to the venous sinus. Secondly, bite off part of the cranium along the superior border of the bony foramen, until the inner edge of the TSSJ is exposed. Then, continue grinding the skull with a small cutting bur until thin-layer bone slices are visible near the sigmoid sinus. At this time, if bleeding occurs from the guiding vein, a rapid hemostasis could be easily achieved with bone wax. Finally, the bone flap was milled according to the surgical requirements and exfoliated.