Robot-assisted screw fixation in a cadaver utilizing magnetic resonance imaging-based synthetic computed tomography: toward radiation-free spine surgery. Illustrative case.

BACKGROUND: Synthetic computed tomography (sCT) can be created from magnetic resonance imaging (MRI) utilizing newer software. sCT is yet to be explored as a possible alternative to routine CT (rCT). In this study, rCT scans and MRI-derived sCT scans were obtained on a cadaver. Morphometric analysis was performed comparing the 2 scans. The ExcelsiusGPS robot was used to place lumbosacral screws with both rCT and sCT images. OBSERVATIONS: In total, 14 screws were placed. All screws were grade A on the Gertzbein-Robbins scale. The mean surface distance difference between rCT and sCT on a reconstructed software model was -0.02 ± 0.05 mm, the mean absolute surface distance was 0.24 ± 0.05 mm, and the mean absolute error of radiodensity was 92.88 ± 10.53 HU. The overall mean tip distance for the sCT versus rCT was 1.74 ± 1.1 versus 2.36 ± 1.6 mm (p = 0.24); mean tail distance for the sCT versus rCT was 1.93 ± 0.88 versus 2.81 ± 1.03 mm (p = 0.07); and mean angular deviation for the sCT versus rCT was 3.2° ± 2.05° versus 4.04°± 2.71° (p = 0.53). LESSONS: MRI-based sCT yielded results comparable to those of rCT in both morphometric analysis and robot-assisted lumbosacral screw placement in a cadaver study.

In Vitro Biomechanics of Human Cadaveric Cervical Spines With Mature Fusion.

BACKGROUND: This study sought to compare index and adjacent-level biomechanics of cadaveric specimens with mature fusion versus normal spines in intact and acutely fused conditions. METHODS: Eight human cadaveric cervical spines with mature fusion across 1 to 3 levels were studied. Intervertebral angular range of motion (ROM) was determined at fused and adjacent levels during pure moments inducing flexion-extension (FE), lateral bending (LB), and axial rotation (AR). Mature fusion data were compared to data from normal spine specimens tested intact and then with a 1-level anterior plate/graft (fresh fixation). Bone qualities were compared using dual-energy x-ray absorptiometry. RESULTS: Mean bone mineral density was significantly greater in mature fusion spines (0.632 ± 0.239 g/cm2) than in normal spines (0.489 ± 0.195 g/cm2) (P < .001). Mean ROM for levels with mature fusion was 42% (FE), 42% (LB), and 29% (AR) of the mean same-level ROM in freshly fixated specimens (P ≤ .045). The mean adjacent-level ROM in spines with mature fusion was less than in normal spines (matched levels) in all directions, with the greatest difference 1 level below fusion (FE: -38%, P < .001; LB: -42%, P < .001; AR: -49%, P = .001), followed by 1 level above fusion (FE: -23%, P = .04; LB: -22%, P = .07; AR: -28%, P = .02) and 2 levels above fusion (FE: -20%, P = .08; LB: -18%, P = .11; AR: -31%, P = .009). Mature fusion reduced the magnitude of coupled LB during AR at C6-7 and C7-T1 (P ≤ .03). CONCLUSION: Cervical spine segments with mature fusion have higher bone mass, are less flexible than freshly fixed spines, and have reduced mobility at adjacent levels.

Assessment of Surgical Procedural Time, Pedicle Screw Accuracy, and Clinician Radiation Exposure of a Novel Robotic Navigation System Compared With Conventional Open and Percutaneous Freehand Techniques: A Cadaveric Investigation.

STUDY DESIGN: Cadaveric study. OBJECTIVE: To evaluate accuracy, radiation exposure, and surgical time of a new robotic-assisted navigation (RAN) platform compared with freehand techniques in conventional open and percutaneous procedures. METHODS: Ten board-certified surgeons inserted 16 pedicle screws at T10-L5 (n = 40 per technique) in 10 human cadaveric torsos. Pedicle screws were inserted with (1) conventional MIS technique (L2-L5, patient left pedicles), (2) MIS RAN (L2-L5, patient right pedicles), (3) conventional open technique (T10-L1, patient left pedicles), and (4) open RAN (T10-L1, patient right pedicles). Output included (1) operative time, (2) number of fluoroscopic images, and (3) screw accuracy. RESULTS: In the MIS group, compared with the freehand technique, RAN allowed for use of larger screws (diameter: 6.6 ± 0.6 mm vs 6.3 ± 0.5 mm; length: 50.3 ± 4.1 mm vs 46.9 ± 3.5 mm), decreased the number of breaches >2 mm (0 vs 7), fewer fluoroscopic images (0 ± 0 vs 108.3 ± 30.9), and surgical procedure time per screw (3.6 ± 0.4 minutes vs 7.6 ± 2.0 minutes) (all P < .05). Similarly, in the open group, RAN allowed for use of longer screws (46.1 ± 4.1 mm vs 44.0 ± 3.8 mm), decreased the number of breaches >2 mm (0 vs 13), fewer fluoroscopic images (0 ± 0 vs 24.1 ± 25.8) (all P < .05), but increased total surgical procedure time (41.4 ± 8.8 minutes vs 24.7 ± 7.0 minutes, P = .000) while maintaining screw insertion time (3.31.4 minutes vs 3.1 ± 1.0 minutes, P = .650). CONCLUSION: RAN significantly improved accuracy and decreased radiation exposure in comparison to freehand techniques in both conventional open and percutaneous surgical procedures in cadavers. RAN significantly increased setup time compared with both conventional procedures.

Robotic Spine Surgery: Current State in Minimally Invasive Surgery.

STUDY DESIGN: Narrative review. OBJECTIVES: Robotic systems in spinal surgery may offer potential benefits for both patients and surgeons. In this article, the authors explore the future prospects and current limitations of robotic systems in minimally invasive spine surgery. METHODS: We describe recent developments in robotic spine surgery and minimally invasive spine surgery. Institutional review board approval was not needed. RESULTS: Although robotic application in spine surgery has been gradual, the past decade has seen the arrival of several novel robotic systems for spinal procedures, suggesting the evolution of technology capable of augmenting surgical ability. CONCLUSION: Spine surgery is well positioned to benefit from robotic assistance and automation. Paired with enhanced navigation technologies, robotic systems have tremendous potential to supplement the skills of spine surgeons, improving patient safety and outcomes while limiting complications and costs.

Variations Among Human Lumbar Spine Segments and Their Relationships to In Vitro Biomechanics: A Retrospective Analysis of 281 Motion Segments From 85 Cadaveric Spines.

BACKGROUND: Biomechanical properties of intact spinal motion segments are used to establish baseline values during in vitro studies evaluating spinal surgical techniques and implants. These properties are also used to validate computational models (ie, patient-specific finite element models) of human lumbar spine segments. Our laboratory has performed a large number of in vitro mechanical studies of lumbar spinal segments, using a consistent methodology. This provides extensive biomechanical data for a large number of intact motion segments, along with donor demographic variables, bone mineral density (BMD) measurements, and geometric properties. The objective of this study was to analyze how donor demographics, BMD, and geometric properties of cadaveric lumbar spine segments affect motion segment flexibility, including the range of motion (ROM), lax zone (LZ), and stiff zone (SZ), to help improve our understanding of spinal biomechanics. METHODS: A retrospective study examined the relationships between the biomechanical properties of 281 lumbar motion segments from 85 human cadaveric spines, donor demographic variables (age, sex, weight, height, and body mass index), and specimen measurements (vertebral body height, intervertebral disc height, and BMD). RESULTS: Statistical correlation and regression analyses showed that the flexibility of a lumbar motion segment is affected by lumbar level, donor age, sex, and weight as well as the intervertebral disc height, vertebral body height, and bone quality. Increased disc height was associated with decreased ROM (axial rotation), decreased LZ (flexion-extension and axial rotation), and increased SZ (flexion-extension and lateral bending) in the male group, but increased ROM (lateral bending) in the female group. Increased vertebral body height correlated with increased LZ (lateral bending) in the female group. Increased BMD correlated with decreased ROM overall. CONCLUSIONS: Biomechanical measurements from flexibility testing of cadaveric lumbar spine segments are significantly correlated with donor demographics and specimen measurements. Many of these correlations are sex-dependent.

Does the accuracy of pedicle screw placement differ between the attending surgeon and resident in navigated robotic-assisted minimally invasive spine surgery?

Robotic assistance with integrated navigation is an area of high interest for improving the accuracy of minimally invasive pedicle screw placement. This study analyzes the accuracy of pedicle screw placement between an attending spine surgeon and a resident by comparing the left and right sides of the first 101 consecutive cases using navigated robotic assistance in a private practice clinical setting. A retrospective, Institutional Review Board-exempt review of the first 106 navigated robot-assisted spine surgery cases was performed. One attending spine surgeon and one resident performed pedicle screw placement consistently on either the left or right side (researchers were blinded). A CT-based Gertzbein and Robbins system (GRS) was used to classify pedicle screw accuracy, with grade A or B considered accurate. There were 630 consecutive lumbosacral pedicle screws placed. Thirty screws (5 patients) were placed without the robot due to surgeon discretion. Of the 600 pedicle screws inserted by navigated robotic guidance (101 patients), only 1.5% (9/600) were repositioned intraoperatively. Based on the GRS CT-based grading of pedicle breach, 98.67% (296/300) of left-side screws were graded A or B, 1.3% (4/300) were graded C, and 0% (0/300) were graded D. For the right-side screws, 97.67% (293/300) were graded A or B, 1.67% (5/300) were graded C, and 0.66% (2/300) were graded D. This study demonstrated a high level of accuracy (based on GRS) with no significant differences between the left- and right-side pedicle screw placements (98.67% vs. 97.67%, respectively) in the clinical use of navigated, robot-assisted surgery.

Evaluation of abnormal styloid anatomy as a cause of internal jugular vein compression using a 3D-printed model: a laboratory investigation.

This study used a 3-dimensional (3D) craniocervical junction model of styloidogenic jugular venous compression (SJVC) syndrome to simulate and evaluate intracranial pressure (ICP) after internal jugular vein (IJV) compression by an elongated styloid process during axial rotation. The 3D-printed model created using data from an SJVC-syndrome patient included an articulating occipital-cervical junction, simplified arteriovenous system, gauge to measure simulated ICP, fixed obstruction simulating left-sided venous occlusion, and right-sided vascular tubing to simulate IJV compression. The model was rotated axially to its extreme right and left; maximum degree of motion and pressure were recorded for 3 cycles. Measurements were repeated after styloid resection in 25% increments. The extreme right rotation (11°) of the intact styloid condition yielded a mean pressure of 15.34 ±â€¯2.85 mmHg. After 25% styloid resection, extreme rotation (11°) yielded 13.96 ±â€¯2.88 mmHg. After 50%, extreme rotation increased to 16° yielding 17.41 ±â€¯3.52 mmHg; 11° rotation was 2.76 ±â€¯1.96 mmHg. After 75%, extreme rotation increased to 19° yielding -0.86 ±â€¯1.08 mmHg; 16° and 11° rotation yielded -0.69 ±â€¯1.19 and -0.86 ±â€¯1.08 mmHg, respectively. After 100%, extreme rotation to 19° yielded -1.21 ±â€¯0.60 mmHg; 16° and 11° rotation yielded -0.34 ±â€¯0.30 and 0.00 ±â€¯0.00 mmHg, respectively. Extreme left rotations (11°) yielded mean pressures of -0.17 ±â€¯0.00 (intact), -0.17 ±â€¯0.30 (25%), 2.24 ±â€¯0.79 (50%), 0.34 ±â€¯0.30 (75%), and 0.17 ±â€¯0.30 mmHg (100%). Simulated ICP increased proportionally to maximum ipsilateral axial rotation, and was highest after 50% styloid resection. Contralateral axial rotation did not increase pressure. IJV compression was relieved at 75% resection, suggesting that partial (75%) or complete styloidectomy is a potentially efficacious treatment for SJVC syndrome.

Pedicle screw accuracy in clinical utilization of minimally invasive navigated robot-assisted spine surgery.

In the emerging field of robot-assisted spine surgery, the radiographic evaluation of pedicle screw accuracy in clinical application is an area of high interest. This study describes the pedicle screw accuracy of the first 56 consecutive cases in which navigated robotic assistance was used in a private practice clinical setting. A retrospective, Institutional Review Board-exempt review of the first 56 navigated robot-assisted spine surgery cases was performed. Pedicle screw malposition, reposition, and return to operating room (OR) rates were collected. A CT-based Gertzbein and Robbins system (GRS) was used to classify pedicle screw accuracy. In the first 56 robotic cases, 356 total pedicle screws were placed. Eight screws were placed without the robot due to surgeon discretion. Of the 348 pedicle screws inserted by navigated robotic guidance, only 2.6% (9/348) were repositioned, resulting in a 97.4% (339/348) successful screw placement rate. The average age was 64, and 48% were female. Average body mass index was 31 kg/m2. Based on the GRS CT-based grading, 97.7% (340/348) were graded A or B, 1.7% (6/348) screws were graded C, and only 0.6% (2/348) of screws were graded D. Two complications, explantation of interbody and vacuum-assisted wound closure, were reported as requiring a return to the OR, but these were not related to robotic guidance or pedicle screws. This study demonstrated a high level of accuracy (97.7%) in the first 56 cases using navigated, robot-assisted surgery based on the GRS. There were two non-screw-related complications requiring return to the operating room.

Navigated robotic assistance improves pedicle screw accuracy in minimally invasive surgery of the lumbosacral spine: 600 pedicle screws in a single institution.

BACKGROUND: In the emerging field of robot-assisted spine surgery, radiographic evaluation of pedicle screw accuracy in the surgical setting is of high interest. Advances in medical imaging have improved the accuracy of pedicle screw placement, from fluoroscopy-guided to computer-aided navigation. METHODS: A retrospective, institutional review board-exempt review of the first 106 navigated robot-assisted spine surgery cases was performed. Radiographic evaluation of preoperative and postoperative computerized tomography (CT) scans were collected. RESULTS: In the first 106 cases, 630 lumbosacral pedicle screws were placed. Thirty screws were placed in five patients without the robot because of surgeon discretion. Of the 600 pedicle screws inserted by navigated robotic guidance, only 1.5% (9/600) were repositioned intraoperatively. CONCLUSION: This study demonstrated a high level of accuracy (98.2%) in terms of grade A or B pedicle screw breach scores in the clinical use of navigated, robot-assisted surgery in its first 101 cases.

Comparing the Biomechanical Stability of Cortical Screw Trajectory Versus Standard Pedicle Screw Trajectory for Short- and Long-Segment Posterior Fixation in 3-Column Thoracic Spinal Injury.

BACKGROUND: Information on the performance of posterior fixation with cortical screw (CS) versus pedicle screw (PS) trajectories for stabilizing thoracolumbar burst fractures is limited. Therefore, we sought to analyze stability with CS versus PS in short- and long-segment fixations using a 3-column spinal injury model. METHODS: Nondestructive flexibility tests: (1) intact, (2) intact + short fixation, (3) intact + long fixation, (4) after burst fracture, (5) short fixation + burst fracture, and (6) long fixation + burst fracture using thoracic spine segments (7 CS, 7 PS). RESULTS: With CS, the range of motion (ROM) was significantly greater with short-segment than with long-segment fixation in all directions, with and without burst fracture (P ≤ .008). With PS and burst fracture, ROM was significantly greater with short fixation during lateral bending and axial rotation (P < .006), but not during flexion-extension (P = .10). Groups with CS versus PS were not significantly different after burst fracture during flexion-extension and axial rotation, with short (P ≥ .58) or long fixation (P ≥ .17). During lateral bending, ROM was significantly greater with CS versus PS, without burst fracture (long fixation, P = .02) and with burst fracture (short and long fixation, P ≤ .001). CONCLUSIONS: CS trajectory is a valid alternative to PS trajectory for thoracic spine fixation in 3-column spinal injuries, and long-segment fixation is superior to short-segment fixation with either.

Biomechanical Effects of an Oblique Lumbar PEEK Cage and Posterior Augmentation.

OBJECTIVE: Lumbar interbody spacers are widely used in lumbar spinal fusion. The goal of this study is to analyze the biomechanics of a lumbar interbody spacer (Clydesdale Spinal System, Medtronic Sofamor Danek, Memphis, Tennessee, USA) inserted via oblique lumbar interbody fusion (OLIF) or direct lateral interbody fusion (DLIF) approaches, with and without posterior cortical screw and rod (CSR) or pedicle screw and rod (PSR) instrumentation. METHODS: Lumbar human cadaveric specimens (L2-L5) underwent nondestructive flexibility testing in intact and instrumented conditions at L3-L4, including OLIF or DLIF, with and without CSR or PSR. RESULTS: OLIF alone significantly reduced range of motion (ROM) in flexion-extension (P = 0.005) but not during lateral bending or axial rotation (P ≥ 0.63). OLIF alone reduced laxity in the lax zone (LZ) during flexion-extension (P < 0.001) but did not affect the LZ during lateral bending or axial rotation (P ≥ 0.14). The stiff zone (SZ) was unaffected in all directions (P ≥ 0.88). OLIF plus posterior instrumentation (cortical, pedicle, or hybrid) reduced the mean ROM in all directions of loading but only significantly so with PSR during lateral bending (P = 0.004), without affecting the compressive stiffness (P > 0.20). The compressive stiffness with the OLIF device without any posterior instrumentation did not differ from that of the intact condition (P = 0.97). In terms of ROM, LZ, or SZ, there were no differences between OLIF and DLIF as standalone devices or OLIF and DLIF with posterior instrumentation (CSR or PSR) (P > 0.5). CONCLUSIONS: OLIF alone significantly reduced mobility during flexion-extension while maintaining axial compressive stiffness compared with the intact condition. Adding posterior instrumentation to the interbody spacer increased the construct stability significantly, regardless of cage insertion trajectory or screw type.

First spine surgery utilizing real-time image-guided robotic assistance.

Robotics in spinal surgery has significant potential benefits for both surgeons and patients, including reduced surgeon fatigue, improved screw accuracy, decreased radiation exposure, greater options for minimally invasive surgery, and less time required to train residents on techniques that can have steep learning curves. However, previous robotic systems have several drawbacks, which are addressed by the innovative ExcelsiusGPSTM robotic system. The robot is secured to the operating room floor, not the patient. It has a rigid external arm that facilitates direct transpedicular drilling and screw placement, without requiring K-wires. In addition, the ExcelsisuGPSTM has integrated neuronavigation, not present in other systems. It also has surveillance marker that immediately alerts the surgeon in the event of loss of registration, and a lateral force meter to alert the surgeon in the event of skiving. Here, we present the first spinal surgery performed with the assistance of this newly approved robot. The surgery was performed with excellent screw placement, minimal radiation exposure to the patient and surgeon, and the patient had a favorable outcome. We report the first operative case with the ExcelsisuGPSTM, and the first spine surgery utilizing real-time image-guided robotic assistance.

Impact of Connector Placement and Design on Bending Stiffness of Spinal Constructs.

OBJECTIVE: To evaluate the stability of multiple rod-connector construct designs using a mechanical 4-point bending testing frame. METHODS: A mechanical study was used to evaluate the bending stiffness of 3 connectors across 12 different configurations of rod-connector-rod constructs. Stability was evaluated in flexion-extension and lateral bending. Combinations of rods having 1 of 3 diameters (4.0 mm, 5.5 mm, and 6.0 mm) connected by 1 of 3 connector types (parallel open, snap-on, and hinged) were compared. Configurations with single connectors and with double connectors with variable spacing were also compared to simulate revision surgery conditions. RESULTS: Constructs consisting of 4.0-mm rods connected to 4.0-mm rods were significantly less stiff as the total number of connectors used in a series exceeded 2. When single-connector configurations were compared, parallel open rod connectors demonstrated greater stiffness in flexion-extension than hinged open connectors, whereas hinged open connectors demonstrated greater stiffness in lateral bending. Using double connectors increased stiffness of 4.0- to 4.0-mm rod configurations in flexion-extension and lateral bending, 4.0- to 6.0-mm rod configurations in flexion-extension, and 5.5- to 6.0-mm rod configurations in lateral bending. Spacing the double connectors significantly improved lateral bending stiffness of 4.0- to 4.0-mm and 5.5- to 6.0-mm rod configurations. CONCLUSIONS: Our data indicate that the design, number, and placement of rod connectors have a significant impact on the bending stiffness of a surgical construct. Such mechanical data may influence construct design in primary and revision surgeries of the cervical spine and cervicothoracic junction.

Biomechanical Evaluation of Cervicothoracic Junction Fusion Constructs.

OBJECTIVE: We studied the effect of different cervicothoracic construct design variables on biomechanical stability in vitro. METHODS: Six fresh-frozen human cadaveric spines (C5-T4) were used. After intact analysis, each specimen was destabilized and reconstructed, with all groups having 4.0-mm pedicle screws placed at T1-T3. The 2 hook-rod constructs included interlaminar hooks at C6 and C7, with either 3.5-mm or 4.0-mm rods (C6-T3). The 2 screw-rod constructs tested included lateral mass screws at C6 and C7, with either 3.5-mm or 4.0-mm rods (C6-T3). The 2 screw-connector-rod constructs tested included lateral mass screws at C6 and C7, with either 3.5-mm or 4.0-mm rods; 1 rod spanned C6-C7 with a connector to a second rod of the same size spanning T1-T3. Global (C6-T3) and intervertebral (C6-C7, C7-T1, T1-T2, and T2-T3) ranges of motion were compared for each construct. RESULTS: In terms of global (C6-T3) stability, 3.5-mm versus 4.0-mm rod constructs were not significantly different, regardless of whether the construct was hook-rod, screw-rod, or screw-connector-rod. The hook-rod constructs provided less stability compared with the screw-rod and screw-connector-rod constructs in lateral bending (P < 0.04) and axial rotation (P < 0.001). The screw-rod constructs demonstrated a similar range of motion to that of the screw-connector-rod constructs, except for significantly less axial rotation at the C6-C7 level with 3.5-mm rods (P = 0.04). CONCLUSIONS: We found that the rod diameter of a construct does not appear to significantly influence the biomechanical stability of subaxial constructs. The screw-rod construct resulted in certain biomechanical advantages compared with the screw-connector-rod construct, and both were significantly superior to the hook-rod construct.

Biomechanical evaluation of the ProDisc-C stability following graded posterior cervical injury.

OBJECTIVEThere are limited data regarding the implications of revision posterior surgery in the setting of previous cervical arthroplasty (CA). The purpose of this study was to analyze segmental biomechanics in human cadaveric specimens with and without CA, in the context of graded posterior resection.METHODSFourteen human cadaveric cervical spines (C3-T1 or C2-7) were divided into arthroplasty (ProDisc-C, n = 7) and control (intact disc, n = 7) groups. Both groups underwent sequential posterior element resections: unilateral foraminotomy, laminoplasty, and finally laminectomy. Specimens were studied sequentially in two different loading apparatuses during the induction of flexion-extension, lateral bending, and axial rotation.RESULTSRange of motion (ROM) after artificial disc insertion was reduced relative to that in the control group during axial rotation and lateral bending (13% and 28%, respectively; p < 0.05) but was similar during flexion and extension. With sequential resections, ROM increased by a similar magnitude following foraminotomy and laminoplasty in both groups. Laminectomy had a much greater effect: mean (aggregate) ROM during flexion-extension, lateral bending, and axial rotation was increased by a magnitude of 52% following laminectomy in the setting of CA, compared to an 8% increase without arthroplasty. In particular, laminectomy in the setting of CA introduced significant instability in flexion-extension, characterized by a 90% increase in ROM from laminoplasty to laminectomy, compared to a 16% increase in ROM from laminoplasty to laminectomy without arthroplasty (p < 0.05).CONCLUSIONSForaminotomy and laminoplasty did not result in significant instability in the setting of CA, compared to controls. Laminectomy alone, however, resulted in a significant change in biomechanics, allowing for significantly increased flexion and extension. Laminectomy alone should be used with caution in the setting of previous CA.

Biomechanical evaluation of interbody fixation with secondary augmentation: lateral lumbar interbody fusion versus posterior lumbar interbody fusion.

BACKGROUND: Many approaches to the lumbar spine have been developed for interbody fusion. The biomechanical profile of each interbody fusion device is determined by the anatomical approach and the type of supplemental internal fixation. Lateral lumbar interbody fusion (LLIF) was developed as a minimally invasive technique for introducing hardware with higher profiles and wider widths, compared with that for the posterior lumbar interbody fusion (PLIF) approach. However, the biomechanics of the interbody fusion construct used in the LLIF approach have not been rigorously evaluated, especially in the presence of secondary augmentation. METHODS: Spinal stability of 21 cadaveric lumbar specimens was compared using standard nondestructive flexibility studies [mean range of motion (ROM), lax zone (LZ), stiff zone (SZ) in flexion-extension, lateral bending, and axial rotation]. Non-paired comparisons were made among four conditions: (I) intact; (II) with unilateral interbody + bilateral pedicle screws (BPS) using the LLIF approach (referred to as the LLIF construct); (III) with bilateral interbody + BPS using the PLIF approach (referred to as the PLIF construct); and (IV) with no lumbar interbody fusion (LIF) + BPS (referred to as the no-LIF construct). RESULTS: With bilateral pedicle screw-rod fixation, stability was equivalent between PLIF and LLIF constructs in lateral bending and flexion-extension. PLIF and LLIF constructs had similar biomechanical profiles, with a trend toward less ROM in axial rotation for the LLIF construct. CONCLUSIONS: LLIF and PLIF constructs had similar stabilizing effects.

Technique: open lumbar decompression and fusion with the Excelsius GPS robot.

The Excelsius GPS (Globus Medical, Inc.) was approved by the FDA in 2017. This novel robot allows for real-time intraoperative imaging, registration, and direct screw insertion through a rigid external arm-without the need for interspinous clamps or K-wires. The authors present one of the first operative cases utilizing the Excelsius GPS robotic system in spinal surgery. A 75-year-old man presented with severe lower back pain and left leg radiculopathy. He had previously undergone 3 decompressive surgeries from L3 to L5, with evidence of instability and loss of sagittal balance. Robotic assistance was utilized to perform a revision decompression with instrumented fusion from L3 to S1. The usage of robotic assistance in spinal surgery may be an invaluable resource in minimally invasive cases, minimizing the need for fluoroscopy, or in those with abnormal anatomical landmarks. The video can be found here: https://youtu.be/yVI-sJWf9Iw .

Biomechanical Analysis of an Expandable Lumbar Interbody Spacer.

OBJECTIVE: Recently developed expandable interbody spacers are widely accepted in spinal surgery; however, the resulting biomechanical effects of their use have not yet been fully studied. We analyzed the biomechanical effects of an expandable polyetheretherketone interbody spacer inserted through a bilateral posterior approach with and without different modalities of posterior augmentation. METHODS: Biomechanical nondestructive flexibility testing was performed in 7 human cadaveric lumbar (L2-L5) specimens followed by axial compressive loading. Each specimen was tested under 6 conditions: 1) intact, 2) bilateral L3-L4 cortical screw/rod (CSR) alone, 3) WaveD alone, 4) WaveD + CSR, 5) WaveD + bilateral L3-L4 pedicle screw/rod (PSR), and 6) WaveD + CSR/PSR, where CSR/PSR was a hybrid construct comprising bilateral cortical-level L3 and pedicle-level L4 screws interconnected by rods. RESULTS: The range of motion (ROM) with the interbody spacer alone decreased significantly compared with the intact condition during flexion-extension (P = 0.02) but not during lateral bending or axial rotation (P ≥ 0.19). The addition of CSR or PSR to the interbody spacer alone condition significantly decreased the ROM compared with the interbody spacer alone (P ≤ 0.002); and WaveD + CSR, WaveD + PSR, and WaveD + CSR/PSR (hybrid) (P ≥ 0.29) did not differ. The axial compressive stiffness (resistance to change in foraminal height during compressive loading) with the interbody spacer alone did not differ from the intact condition (P = 0.96), whereas WaveD + posterior instrumentation significantly increased compressive stiffness compared with the intact condition and the interbody spacer alone (P ≤ 0.001). CONCLUSIONS: The WaveD alone significantly reduced ROM during flexion-extension while maintaining the axial compressive stiffness. CSR, PSR, and CSR/PSR hybrid constructs were all effective in augmenting the expandable interbody spacer system and improving its stability.

Biomechanical Stability Afforded by Unilateral Versus Bilateral Pedicle Screw Fixation with and without Interbody Support Using Lateral Lumbar Interbody Fusion.

OBJECTIVE: To determine the stability of fusion constructs with unilateral pedicle screw (UPS) or bilateral pedicle screw (BPS) fixation with and without an interbody implant using the lateral lumbar interbody (LLIF) approach. METHODS: Standard nondestructive flexibility tests were performed on 13 cadaveric lumbar specimens to assess spinal stability of intact specimens and 5 configurations of posterior and interbody instrumentation. Spinal stability was determined as mean range of motion in flexion-extension, lateral bending, and axial rotation. Nonpaired comparisons were made for these 6 conditions: 1) intact; 2) unilateral interbody via the LLIF approach (LLIF construct); 3) unilateral interbody + unilateral pedicle screws (UPS) via the LLIF approach (LLIF + UPS); 4) unilateral interbody + bilateral pedicle screws (BPS) using the LLIF approach (LLIF+BPS); 5) UPS alone; and 6) BPS alone. RESULTS: UPS and BPS, with and without interbody support, significantly reduced range of motion during the majority of directions of loading. BPS alone provided greater stability than UPS alone and LLIF alone in all directions of motion except axial rotation. With interbody support, there was no significant difference in stability between BPS and UPS across all movement directions. CONCLUSIONS: These biomechanical results suggest that fixation in the lumbar spine with an interbody support using an LLIF approach with UPS is a promising alternative to BPS. Although BPS provides greater immediate stability compared with UPS, in the presence of a lateral interbody implant, UPS and BPS provide equivalent stability. In addition, LLIF does not appear to contribute significantly to immediate stability when BPS is used.

Contribution of Neural Elements to Thoracic Stability.

AIM: Studies of spinal biomechanics typically do not focus on the contributions to range of motion (ROM) of the primary components of the spinal canal, dura, arachnoid, pia, spinal cord, nerve roots, ligaments, and vessels. We sought to determine the stability offered by these soft tissues in vitro. MATERIAL AND METHODS: Human cadaveric segments were tested intact, after osteoligamentous destabilization, and after transection of T8-9 spinal canal components. Specimens were induced into flexion, extension, axial rotation, and lateral bending using non-constraining, non-destructive pure moment while tracking motion response stereophotogrammetrically. The range of motion (ROM) was compared in each condition after adjusting for soft tissue creep. RESULTS: After spinal canal element transection, ROM increased in all directions (mean 4.7%). This increase was most pronounced during lateral bending (p=0.055). The cumulative ROM from all directions of loading showed a statistically significant mean increase of 3.3% (p=0.040). CONCLUSION: Sectioning of canal elements was found to cause a measurable increase in ROM. Although nonviable tissues were tested, living tissues are also likely to contribute to spinal stability.