As previously underlined, the main concern regarding intraoperative navigation with O-arm or similar technologies is the amount of radiation given to the patients. For example, the use of an intraoperative CT for the instrumentation of 17 vertebrae in a slim patient gives a mean dose of 32.4 milliSievert (mSv) [16] or more than half of the US annual dose limit for surgeons (50 mSv) and almost 21 times the exposure for a spine X-ray (1.5 mSv). The total dose can raise up to 80.9 mSv if the patient is obese [16]. Such an exposure can cause complications related to radiations. Stochastic and deterministic effects of radiation exposure are well known, and the administration of X-rays to patients must be limited. This has been recently underlined in a paper that demonstrated an increased incidence of cancer in patients that were previously treated for adolescent idiopathic scoliosis [17]. Moreover, surgeons are continuously exposed to radiations during their surgeries, due to the use of these kinds of intraoperative navigation systems. Ultimately, both patients and surgeons increase their risk of stochastic and deterministic effects due to radiation exposure.

MySpine technology allows a guided implant of the screws with a minimal radiation exposure for the patient and with no exposure for surgeons. Each guide is crafted based on a low-dose CT protocol that has been developed specifically for this technique. This protocol allows the patients to be exposed to a radiation dose that varies between 0.9 and 2.5 mSv for the analysis of 15–19 vertebrae. Obviously, with this low-dose exposure, the quality of the resulting images of CT scan is poor, but the cortical bone of the different vertebrae is trustworthily represented. Adjacent soft tissues and inner vertebral trabeculae aren’t well shown, but this doesn’t affect guides planning and crafting (Fig. 5.2).

Once the fusion area is determined and a low-dose CT scan is performed for the image acquisition of the patient’s vertebral anatomy, a 3D model of the whole spine is created. Each individual vertebra is reconstructed and the ideal entry points and trajectory of the screws planned. Screw dimensions (length, diameter) are decided at this stage, based on anatomical features of each vertebra (Fig. 5.3). If the preoperative planning satisfies surgeon’s needs, the tubular guides are crafted using a 3D printer. Otherwise the surgeon can change preoperative planning using specifically designed software. Each screw parameter is alterable: entry points, orientation on the transverse and sagittal planes, screw length and diameter. Regarding the guides, these can be planned as open, semi-open, or closed, depending on the spinous process fit. The semi-open or closed guides allow a more strong fit but supraspinous ligament section is mandatory. On the other hand the open configuration allows the preservation of the supraspinous ligament but decreases the contact between the guide and the bony tissue. This however doesn’t affect guide stability and accuracy. The guides are made of medical grade polyamide with a powder particle size of 60 μm and a layer height lower than 0.1 mm, allowing a submillimetric precision. Each guide is provided with the corresponding vertebral model to verify the fit and check the entry points of the screws before applying them on the patient.

Each standard MySpine guide has a specific inferior surface that allows a perfect contact with the corresponding vertebral body. Three key contact points between guides and vertebrae are necessary: the spinous process, the laminae, and the transverse processes. These bony landmarks should be carefully dissected during spine preparation to avoid any violation of the bony contour of the posterior arch. Soft tissues must be completely detached, allowing a direct contact between the guide and the bone. Any soft tissue that is left on the spine can change the fit of the guides on the vertebra and consequently affect screw precision. Once the spine is adequately dissected, the most cranial guide is inserted. Usually this guide has a cranial semi-open design, to allow the preservation of the upper part of the supraspinous ligament. Once the tubular guide is properly placed, the entry points can be identified and flattened with a burr. Awls and probes are then inserted in the tubular guide that drives the instruments in guided directions. Once the pedicle is prepared, the pre-planned screw is inserted along the tubes. An evolution of this system uses low profile guides with less contact points and a K-wire with cannulated instruments and screws. The advantage is to reduce spine dissection and muscle detachment, even if K-wire stability in poor quality bone can affect the final position of the screw. Once screws are inserted the screwdrivers are detached and the guide removed, and the surgeon moves caudally to the subsequent vertebra. Once the planned vertebrae are instrumented, rods are inserted and deformity corrected following the surgeon’s preferred technique. Decortication of the spine and bone grafting are then mandatory to obtain spinal fusion and long lasting corrections.

MySpine accuracy has been preliminarily tested, and the results are exposed in two different papers published in 2014 and 2015. In the first manuscript, the system has been applied to four patients with severe scoliosis, with the implant of a total of 76 pedicle screws. In this series 84% of the screws were completely intrapedicular, and this value rose up to 96.1% considering the screws with <2 mm of pedicle violation. No hardware-related complications occurred, and no medial violation of the pedicle was observed.