Navigated Spinal Deformity Correction

Background of Adult Spinal Deformity

Adult spinal deformity (ASD) is defined as an abnormal curvature or abnormal alignment of the vertebral column. ASD has been demonstrated to limit a patient’s quality of life significantly, and patients with both cervical and thoracolumbar deformity have predictable declines in health-related quality of life (HRQOL). In the most severe cases, patients with lumbar scoliosis and sagittal malalignment have quality of life measures equal to patients who have lost the ability to use their upper or lower extremities. The definition of scoliosis is a Cobb measurement greater than 10 degrees; however, in de novo adult degenerative scoliosis, coronal deformity is considered when Cobb measurements exceed 20 to 30 degrees. The Scoliosis Research Society (SRS) Schwab classification for spinal deformity classifies major coronal deformity as thoracic, thoracolumbar, or lumbar scoliosis greater than 30 degrees.

A globally aligned spine allows for the least amount of energy expenditure necessary to maintain upright posture, bipedal gait, and horizontal gaze. In the globally aligned spine, a C7 plumb line should fall between the posterior superior corner of the sacrum and the center of femoral heads, and the central sacral vertical line (CSVL) should transect the C7 spinous process. While coronal imbalance greater than 4 cm has been associated with increased pain and decreased function, sagittal imbalance correlates more closely with HRQOL and is the primary focus of adult deformity correction.

The importance of sagittal alignment was introduced with the conception of pelvic parameters and became the cornerstone for spinal deformity correction, as it was shown to correlate with the HRQOL measures. The body attempts to compensate for sagittal imbalance through a series of mechanisms including reduction of thoracic kyphosis, retrolisthesis of lumbar vertebra, pelvic retroversion, and knee flexion. It is important to identify these clinical and radiographic compensatory mechanisms as they can mask underlying spinal deformity. The most common spinopelvic parameters used to describe global sagittal alignment are pelvic incidence (PI), sacral slope (SS), pelvic tilt (PT), lumbar lordosis (LL), and sagittal vertical axis (SVA). Similarly, the C2–C7 SVA and the chin-brow vertical angle (CBVA) are used to assess the degree of cervical spinal deformity. In order to attain the greatest improvement in HRQOL, the following postoperative parameters should be achieved: PT < 25 degrees, PI-LL mismatch + 11 degrees, and SVA < 5 cm. Historically, the treatment of spinal deformity had been supportive and aimed at delaying the progressive decline in a patient’s overall health. However, currently, the treatment is corrective and aimed at improving the patient’s quality of life. Significant evidence has been published demonstrating a clinically significant improvement in the HRQOL measure following the surgical correction of ASD.

The potential to improve the quality of life of patients with spinal deformity significantly through surgery are well appreciated. However, it is biased to discuss the potential benefits of surgical intervention without discussing the high complication rate associated with performing these surgeries. In a two-year prospective multicenter study, the complication rate following ASD correction was approximately 70% and the revision surgery rate was approximately 28%. The most frequent complications following ASD surgery include rod breakage, proximal junctional kyphosis (PJK), postoperative anemia, surgical site infection, and neurologic injury. Currently, predictive analytic models are being developed to identify patients with spinal deformity that would benefit from surgical correction and are at low risk for complication. The use of modern technology with machine-based learning to predict success rates following ASD surgery is a perfect example of how modern technology is improving the field of spine surgery.

Spinal Navigation and Robotics in Spinal Deformity

Since 1911, when the first spinal fusion was performed, technology has been the fuel propelling advances in spine surgery. Engineers and like-minded inventors are perpetually advancing the field, and with developments in imaging modalities, interbody instrumentation, minimally invasive surgical (MIS) techniques, biologics, navigation software, and robotics, the face of spine surgery is constantly in flux. Despite the many avenues of technologic growth in spine, technologic advances in the last 30 years have been dominated by improving imaging modalities, redefining stereotactic image guidance, and introducing robotic assisted surgery. The Oxford dictionary defines “navigation” as the process or activity of ascertaining one’s position and planning or following a defined route. The concept of navigation is as much at the foundation of spine surgery as it is the basis for nautical or aerospace travel. At its core, spine surgery relies on visible anatomic landmarks and the mind’s eye to recreate a three-dimensional (3D) mental representation of the spine to navigate surgery safely. Intraoperative imaging technologies have expanded the ability of the spinal surgeon to visualize the 3D anatomy of the spine, relying less on presumptions based on visible landmarks. Spinal navigation expands on intraoperative imaging, utilizing stereotactic technology to allow the surgeon to visualize their instrumentation in 3D radiographic images of the patient’s spine. Robotic shared control devices represent the current pinnacle in navigation technology. With robotic technology, the surgeon plans the pedicle screw trajectories on pre- or intraoperative imaging; these are subsequently produced by an automated robotic arm.

The ultimate goal of using computer assisted spinal navigation and robotics in ASD is to improve the precision and accuracy of the spine surgeon, which will lead to fewer complications and improved surgical outcomes. The theoretical advantages of navigation and robotics are (1) improved accuracy of pedicle screw placement, (2) improved ability to perform minimally invasive spine surgery, (3) decreased radiation exposure to the spine surgeon, and (4) avoiding surgeon fatigue and human error. The remainder of this chapter will focus on the application of navigation and robotics for use in ASD.

Accuracy of Pedicle Screw Placement

The accuracy of pedicle screw placement has been the most investigated topic for both spinal navigation and robotic assisted surgery. In deformity surgery, the placement of accurate pedicle screws becomes increasingly more difficult because of the loss of anatomic landmarks in revision settings, rotatory scoliosis requiring unique pedicle screw trajectories, and the presence of dysplastic pedicles ( Fig. 4.1 ). Spinal navigation and robotics are tools that can facilitate accurate screw insertion in the aforementioned difficult clinical scenarios, and the use of CT spinal navigation has been shown to have equivalent pedicle screw insertion accuracy in primary and revision clinical cases.

A number of studies have been performed comparing the accuracy of pedicle screw placement between free-hand techniques, fluoroscopic techniques, navigated techniques, and robotic techniques. A large meta-analysis compared pedicle screws placed using either conventional fluoroscopy, 2D fluoroscopic navigation, or 3D fluoroscopic navigation, and found accuracy rates of 68.1%, 84.3%, and 95.5%, respectively. In a randomized controlled trial (RCT) that compared 100 consecutive patients with pedicle screws placed using either conventional techniques or navigation, the pedicle breach rate was 13.4% in the conventional group and 4.6% in the navigated group. A prospective RCT of 1116 thoracolumbar pedicle screws demonstrated that robotic assisted pedicle screw placement was associated with better accuracy, a lower rate of violating the proximal facet capsule, and a lower rate of medial pedicle breach compared to the free-hand technique. Similar results of improved accuracy with spinal navigation have been reported in the deformity setting. An RCT comparing navigated and nonnavigated pedicle screw accuracy in patients with spinal deformity demonstrated a pedicle breach rate of 2% with navigation and 23% without navigation. In a review of patients with neuromuscular scoliosis and dysplastic apical pedicles, the accurate placement of apical pedicle screws was achieved in 79% of pedicles utilizing navigation and 67% of pedicles without navigation. A systematic review compared navigated and non-navigated pedicle screw insertion in scoliosis patients and found a higher pedicle screw perforation rate without the use of navigation. However, no difference in revision treatment rates for screw malposition was seen between the groups.

In addition to thoracolumbar pedicle screws, navigation has been shown to be effective for the instrumentation of cervical pedicle screws and lumbopelvic fixation. A number of technical guides and case series have demonstrated the usefulness of CT-guided navigation for the placement of S2-alar-iliac screws. Cervical pedicle screws are technically demanding with a lateral breach threatening the vertebral artery and a medial breach placing the spinal cord at risk. The free-hand technique for subaxial cervical pedicle screws hm 14.3% to 29.1%. Cervical pedicle screw placement using 3D navigation has an accuracy of 89.7%. In the upper cervical spine, CT navigation has been shown to be an effective tool for the placement of C1 and C2 instrumentation. However, the single head-to-head study comparing free-hand to navigated techniques for C2 pars screw placement found higher accuracy with the free-hand technique.