Augmented reality (AR) is a novel technology that adds elements via computer-generated sensory data such as graphics, video, and sound in real time.1 The technology functions by enhancing people's current perception of reality,2 replacing the real world. Advanced AR technology is becoming more popular and has been applied to car navigation systems, smart phones, and tablet devices. When this technology, especially location-based AR, is applied to the field of medicine, it facilitates complicated surgeries.
Minimally invasive surgery has also been developed and used in spine surgery, including keyhole surgeries, such as transvertebral anterior cervical foraminotomy (TVACF) and posterior cervical laminoforaminotomy (PCLF). These surgical methods are more complicated than the standard, more evasive techniques, however. Anterior cervical diskectomy and fusion or posterior cervical laminoplasty are the benchmarks for cervical spondylotic myelopathy and to a certain extent contribute to positive clinical outcomes in patients with cervical radiculopathy. However, most patients with unilateral radiculopathy can be treated solely with decompression of the pathologic nerve root around the foramen.
Transvertebral anterior cervical foraminotomy can be beneficial in patients with unilateral cervical radiculopathy because nonfusion and nondiskectomy can preserve intervertebral motion. In addition, because most radiculopathies are caused by focal lesions of the intervertebral foramen, they do not require total diskectomy and vertebral fusion.3 Especially in patients who have retained physiologic alignment, nonfusion and nondiskectomy can preserve intervertebral motion. Theoretically, TVACF can reduce adjacent intervertebral degeneration compared with intervertebral fusion.
Posterior cervical laminoforaminotomy is a good option for patients who undergo posterior laminoplasty with focal severe radiculopathy. It is useful because posterior foraminotomy can be performed simultaneously in the same surgical field. However, these methods are complex and require skillful techniques, especially for controlling the appropriate drilling direction of the keyhole. The lateral and caudal side trajectory of controlling the tunnel is decided based on the findings from the cortical bone and terminal lamina of the drilled tunnel wall.3 This is a difficult decision that can cause some surgeons to think twice before performing keyhole surgeries.
O-arm imaging and navigation systems, however, have progressed. The navigation system helps surgeons use the correct direction of the keyhole. Augmented reality imaging is enabled by combined navigation systems, intraoperative imaging devices, and multifunctional operating microscopes. For example, AR imaging in a microscopic view has been used in brain surgery to create appropriate boundaries of brain tumor resection or vascular components. Augmented reality facilitates such complicated surgeries in many fields. To our knowledge however, no data are available on AR imaging in patients undergoing spine surgery, especially in complicated procedures such as TVACF or PCLF, although AR could facilitate them. The purpose of this article was to introduce the utility of AR for spine surgery. We think that AR facilitates complicated keyhole surgery even for inexperienced surgeons.
We approached the surface of the vertebra or lamina lesion using the respective surgical method. We then determined the appropriate retractor positions and kept them in the same position throughout the surgery. Doing so is important because this navigation technique requires accuracy within <1 mm. O-arm intraoperative imaging (Medtronic) was taken to make an augmented imaging model in the navigation system (StealthStation S7 Surgical Navigation System; Medtronic). At first, each starting and ending point of the expected keyhole tunnel was marked as a dot on the axial views (Figure 1, A and B). While referring to these dots, we made a start window on the ventral surface of the vertebra and an end window behind the vertebra using the software's push cursor on the coronal views (Figure 1, C and D). The remaining dots were then carefully erased, and we confirmed the start and the end windows on axial views; the start and the end windows were interpolated using the software's interpolated selection cursor (Figure 1, E and F). If the dots remained, the interpolated step would have failed. Navigation data were then transferred and linked to the microscope (Opmi Pentero 900; Zeiss Meditec) to merge the live view and AR microscopic view. We used this novel method in patients who underwent TVACF and posterior foraminotomy.
Transvertebral Anterior Cervical Foraminotomy
A Representative Case
A 63-year-old man presented with pain in the right scapula to the ulnar forearm. Conservative therapy had no effect.
Neurologic examination on admission showed right wrist extensor weakness, dysesthesia in the right ulnar forearm, no pathologic reflex, and general hyporeflexia in deep tendon reflex. The Spurling sign was positive on the right side. MRI revealed a bilateral C7 root sleeve defect and a right C8 root sleeve defect. We scheduled a C6-7 anterior cervical diskectomy and fusion and right C7-T1 TVACF.
The C6-7 anterior cervical diskectomy and fusion was performed as usual, without any problem. How to make the AR model of TVACF (Figure 2, A; Supplemental Digital Content 1, http://links.lww.com/JG9/A4). The AR imaging model was reflected on the microscopic view (Figure 2, B; Supplemental Digital Content 1, http://links.lww.com/JG9/A4). The yellow circle is the outline of the tunnel. The surgeon drilled the vertebral bone just along this outline (Figure 2, C and D), reached a herniated disc, and removed it (Figure 2, E and F). The final view of the keyhole tunnel is compatible with the AR model (Figure 2, G).
The postoperative course was uneventful, and no neurologic deficit was observed. The patient's pain in the right arm diminished after surgery. Postoperative CT nearly corresponded to the intraoperative AR model (Figure 3). There has been no recurrence for the past 20 months.
Posterior Cervical Laminoforaminotomy
A Representative Case
A 76-year-old man presented with dysesthesia in the second and third fingers of his left hand. The patient felt elaborate motor deficit bilaterally. Conservative therapy had no effect.
Neurologic examination on admission showed weakness in the left deltoid, biceps, and wrist extensor; no pathologic reflex; and hyporeflexia in the left biceps and brachioradialis tendon reflexes. The Spurling sign was positive on the left side. MRI revealed developmental spinal canal stenosis at C3-6 and a left C6 root sleeve defect. We scheduled C3-6 laminoplasty and a left C5-6 PCLF.
How to make the AR model of PCLF is shown in Figure 4, A. (Supplemental Digital Content 2, http://links.lww.com/JG9/A5). The AR imaging model was reflected on the microscopic view (Figure 4, B; Supplemental Digital Content 2, http://links.lww.com/JG9/A5). The yellow outline is the outline of the tunnel. The surgeon drilled the vertebral bone just along this outline (Figure 4, C and D), reached a herniated disc, and removed it (Figure 4, E). The final view of the keyhole tunnel is compatible with the AR model. After that, C3-6 double door laminoplasty was performed as usual, without any problem.
The postoperative course was uneventful, and no neurologic deficit was observed. His pain in the left arm reduced after surgery. Postoperative CT nearly corresponded to the intraoperative AR model (Figure 5). There has been no recurrence for the past 20 months.
Successful surgery is possible with guided AR visualization combined with O-arm imaging and navigation systems. This technique facilitates complicated keyhole surgeries in the spine. Augmented reality navigation eliminates surgeons' hesitation associated with the orientation of the surgical field. In addition, it allows inexperienced surgeons to safely perform complicated surgeries. As a result, keyhole spine surgery may become more widely used among spine surgeons.
It enables surgeons to navigate the electric drill using a sure-track indicated on the navigation map. It is also useful when deciding the accurate direction of surgery. However, navigation without the real-time microscopic view results in deviating a little from accurate surgical planning in very fine surgical fields, such as keyhole surgeries. A 1-mm error can lead to unsuccessful keyhole surgeries because TVACF and PCLF are performed in the 6-mm tunnel. Furthermore, although standard navigation shows a point only on the navigation display, AR imaging shows the model plan in the real-world surgical field.4,5 Although some researchers have reported surgery using image injection into the microscope,6–9 no data are available for spine surgery.
Augmented reality imaging in a microscopic view has also been used for brain surgery. However, brain shift occurs because of a loss of cerebrospinal fluid following an error in navigation after dural incision. However, keyhole spine surgeries have no association with the brain shift. Therefore, keyhole spine surgeries are preferable to the AR navigation system.
Two groups have reported on the application of AR imaging in spinal surgery.10,11 However, almost all reports used special devices that are not usually used in surgeries. One of the strengths of our AR method is that we used only devices that are already used in brain surgery. As a result, our method is more economical and practical for many surgeons in the Department of Neurosurgery. Although keyhole spine surgery is difficult and complicated, AR technology overcomes these problems.
The authors thank K. Hirata for navigation system technical assistance. They also thank Y. Tsuzuki and M. Koshizuka for their radiographic technical assistance and K. Ando and S. Oyama for the set-up and arrangement of the operating room.
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