Navigation of Tumor and Metastatic Lesions in the Thoracolumbar Spine

Present Practice

For both primary vertebral column malignancies and spinal metastases with spinal cord compression, surgical intervention is considered the standard of care. In isolated primary malignancies, the indication for surgery is curative resection, which has been shown to improve local control and disease-specific survival. For spinal metastases, the indications for surgery are (1) neural element decompression and (2) fixation of an unstable tumor-affected segment.

Spinal metastases make up the core of the practices of most spinal oncologists and decision-making can be guided by either the N eurologic, O ncologic, M echanical, and S ystemic (NOMS) or L ocation, M echanical instability, N eurology, O ncology, and P atient fitness, prognosis and response to prior therapy (LMNOP) frameworks. In both systems, the epidural disease burden can be graded using the Epidural Spinal Cord Compression System ( Table 5.1 ) and mechanical instability can be graded using the Spinal Instability Neoplastic Score (SINS; Table 5.2 ).

For these patients, advances in radiation treatment modalities have made it such that extensive tumor resection is not necessary to achieve durable long-term local control. As a result, with the exception of patients with de novo deformity requiring anterior column reconstruction, minimally invasive approaches such as separation surgery via a mini-open approach are generally preferred due to their lower associated morbidity. Such approaches employ either transcutaneous or transfascial placement of instrumentation. In both cases, the lack of direct visualization means that navigation of some sort must be used for instrumentation placement. Conventionally this has been done under the guidance of two-dimensional fluoroscopy; however, many surgeons are increasingly using stereotactic navigation systems (e.g., BrainLab, StealthStation) and robotic assisted instrumentation. Both strategies have the potential to reduce the overall radiation exposure to the surgeon and surgical staff. Additionally, nearly a dozen published series have suggested that operative and clinical outcomes are superior for patients with spinal metastases treated using minimally invasive, navigation-assisted versus open techniques. As indicated in two recent systematic reviews, minimally invasive techniques appear to reduce intraoperative blood loss, hospital length of stay, and complication rates relative to open surgery. Equally as important, the pain-related and neurological outcomes appear to be equivalent.

Advances in intraoperative navigation have also allowed the application of nonsurgical interventions to the spinal metastasis population, such as radiofrequency ablation, and have enhanced the ability to plan and execute osteotomies for primary malignancies. In both primary and metastatic lesions of the vertebral column, intraoperative navigation systems may be useful for assessing the degree of tumor extraction, an especially important benefit for primary lesions given the association of greater tumor excision with superior local control.

Guidelines for Spinal Navigation

The common platforms used for cranial and spinal navigation mean that many of the techniques developed by trainees during cranial cases can be applied when learning how to apply navigation to spine oncology cases. Recently, the International Society for Computer Assisted Orthopaedic Surgery published a set of guidelines for how navigation can best be applied for spine surgery. The core principles highlighted in the guidelines include (1) the importance of verifying navigation registration pre-instrumentation using known anatomic landmarks ( Fig. 5.1A–D ), (2) maintaining a clear line of sight between the infrared camera of the navigation system and the patient-fixed reference frame, (3) continuous verification of registration accuracy by comparing the image location with the known landmarks, and (4) minimizing changes in the locoregional alignment prior to the completion of navigation-assisted maneuvers, as alignment changes can result in significant registration inaccuracies. Along these lines, the group similarly indicated that the contraindications to navigation-assisted surgery include excessive spinal mobility, the inability to place a stable and rigid tracker, and poor preoperative image quality, the latter of which limits the accuracy of the navigation system. Within the realm of tumors, significant disruption of the locoregional landmarks by oncologic disease may preclude the ability to employ spinal navigation effectively.

The group similarly acknowledged the existence of a learning curve, a finding corroborated by others. As it pertains to the adoption of this program for spine oncology, our opinion is that navigation should be integrated into the surgical workflow in a staged approach. It should begin with application to more straightforward procedures, such as the placement of pedicle screws in normal, non-dysplastic vertebrae, and gradually move toward more complex maneuvers, culminating in its application to the guidance of osteotomy cuts. In all cases, it is our opinion that the surgeon should be acquainted with the execution of the procedure without navigation prior to attempting it under navigation guidance.

Applications of Spinal Navigation to Tumors and Present Outcomes

In spine tumor surgery, the main applications of navigation technology are instrumentation placement and tumor resection. The evidence for the former is relatively limited in the spine tumor population, as the fundamentals of pedicle screw instrumentation are highly similar for patients with oncologic and degenerative disease. Within the degenerative literature, however, several series suggest that navigation-assisted instrumentation provides superior instrumentation accuracy. The recent series include those of Jing et al., Budu et al., and Yang et al. In their series of 60 patients undergoing thoracolumbar instrumentation using either O-arm (n = 191) or fluoroscopy-assisted placement (n = 150), Jing et al. found that screws placed under navigation were significantly more likely to have clinically acceptable placement. Budu et al. similarly showed in their series of 831 screws (296 navigated using BrainLab and Orbic 3D, Siemens) that screws placed under navigation were half as likely to be misplaced. Additionally, they found that the use of navigation reduced the variability in misplacement rates across surgeons. Navigation was associated with improved accuracy in those who had greater inaccuracy using fluoroscopic assistance, suggesting that it may elevate the performance of surgeons who have subpar results using fluoroscopic assistance. These results reflect the findings of an earlier systematic review by Gelalis et al. In a review of 26 prospective trials of 6617 screws, the authors found that navigation assistance was associated with superior instrumentation accuracy. They also found that, whereas free-hand placed screws tended to deviate medially (into the canal), those placed under CT-guided navigation more commonly perforated the lateral pedicle wall. Consequently, ceteris paribus, navigation-assisted placement may decrease the neurological risk profile associated with thoracic screw placement by biasing the screws laterally, away from the spinal canal and cord.

This improved accuracy offered by navigation may be even greater in deformity operations, where patients may have hypoplastic or obliquely oriented pedicles that are difficult to cannulate without navigation assistance. As deformity is not uncommon in tumor patients, similar benefits may be observed within the oncology population. Additionally, as metastatic lesions frequently disrupt the local bony anatomy, intraoperative navigation may enable surgeons to identify more easily compromised bone and select more optimal screw trajectories. De la Garza Ramos et al. recently provided evidence to support the superiority of navigation-assisted pedicle screw instrumentation in patients with spinal metastases. In a series of 62 patients instrumented with 547 total screws (249 navigated), they found that the utilization of navigation led to a significantly lower rate of pedicle breach compared to free-hand instrumentation.

By comparison, the support for navigation in facilitating tumor resection is unique to the oncology population. For the resection of primary tumors, several groups have described good results. Most of the early descriptions were for benign pathologies. One of the first descriptions was reported by Moore and McLain, who described their experience using the Viewpoint Stereotactic Navigation System (Z-kat) for the resection of an osteoid osteoma and osteoblastoma of the cervicothoracic junction. Rajasekaran et al. also used navigation to treat benign lesions. They described the application of Iso-C 3D (Seimens) fluoroscopy-guided resection to four osteoid osteomas of the mobile spine. All the lesions arose from the posterior elements and were successfully treated with piecemeal resection, leading to durable recurrence-free survival in all patients at an average of 2 years. Nagashima et al. subsequently described the application of navigation to an osteoid osteoma of the C2 pedicle. Using the StealthStation (Medtronic) navigation system and a preoperative CT, they achieved piecemeal resection with minimal morbidity; the postoperative outcomes were not reported.

A contemporaneous description of the application of intraoperative navigation to primary vertebral column malignancies was reported by Dasenbrock et al. The authors employed intraoperative navigation for partial sacrectomy and en bloc resection of three sacral chordomas. The group utilized a preoperative CT scan and the BrainLab (BrainLab) system to guide the osteotomy cuts for the partial sacrectomy. The imaging reference frame was affixed to the L5 spinous process and the superior articulating processes of S1 were used as locoregional landmarks during point-to-point mapping for registration of the system. To guide the cuts and monitor the trajectory of the high-speed drill, a tracking probe was attached to the drill. This facilitated tracking in the sagittal and axial planes in real time. The authors reported en bloc excision with negative margins and the patient remained disease free at the 44-month follow-up. The same group also applied intraoperative, image-guided navigation to resect a recurrent cervical chordoma via an endoscopic, transcervical approach with good results.

Yang et al. subsequently employed a similar technique in their series of 26 patients undergoing computer navigation-aided resection of sacral chordoma. They reported achieving marginal (R1) or negative (R0) margins in 22 cases (85%); the overall recurrence rate was only 8% at a mean of 39 months. Jeys et al. also reported good results in 23 patients treated for primary tumors of the pelvis or sacrum. Ninety-one percent of patients had clean bone and soft tissue margins and local recurrence was seen in only 13% of cases. Perhaps more importantly, the authors found the navigation system to reflect the real-time position of their surgical instruments with respect to the patient’s anatomy. The average registration error was less than 1 mm; although no comparisons were made to surgeons navigating by anatomy alone, it is likely that the navigation significantly improved surgeon accuracy. Examinations by the same group have found navigated procedures to be associated with reduced operative time, intraoperative blood loss, and iatrogenic neurological injury, suggesting that the use of navigation may also reduce the morbidity of surgical intervention.

Subsequently, Nasser et al. described their use of O-arm (Medtronic) assisted navigation for the biopsy or resection of 41 spinal column tumors of the mobile spine and sacrum. Seven patients with metastatic lesions and 13 with primary tumors underwent extensive bony resection with the assistance of navigation. Although no non-navigated group was included for comparison, the authors concluded that navigation assistance was highly useful and could improve the precision of resection margins and instrumentation placement.

More recently, Bosma et al. published results suggesting that intraoperative navigation may improve a surgeon’s ability to achieve negative margins during the resection of primary tumors of the sacrum and pelvis. Patients who underwent navigation-assisted surgery were four times more likely to have negative margins than were historic controls who underwent resection without navigation assistance. Similar comparisons have not been conducted for lesions of the mobile spine; however, Ando et al. reported their experience of 18 patients with primary spine tumors (two sarcomas) and saw no incidence of recurrence at a median of 24.5 months. Additional experience comprised solely of patients with malignant neoplasms is required though.

Clinical Case 1

Preoperative Imaging

Imaging conducted at an outside hospital revealed a large sacral mass that was hypointense on T1- and hyperintense on T2-weighted MRI sequences ( Fig. 5.2A–F ). The lesion had a large 5 × 6 × 7 cm presacral component encasing the bilateral S1–S3 roots. Repeat imaging with contrast-enhanced sequences revealed enhancing lesions of the bilateral posterior superior iliac spine consistent with skip metastases.