Robotic-Assisted Navigation for Stereotactic Neurosurgery: A Cadaveric Investigation of Accuracy, Time, and Radiation.

BACKGROUND AND OBJECTIVES: Despite frequent use, stereotactic head frames require manual coordinate calculations and manual frame settings that are associated with human error. This study examines freestanding robot-assisted navigation (RAN) as a means to reduce the drawbacks of traditional cranial stereotaxy and improve targeting accuracy. METHODS: Seven cadaveric human torsos with heads were tested with 8 anatomic coordinates selected for lead placement mirrored in each hemisphere. Right and left hemispheres of the brain were randomly assigned to either the traditional stereotactic arc-based (ARC) group or the RAN group. Both target accuracy and trajectory accuracy were measured. Procedural time and the radiation required for registration were also measured. RESULTS: The accuracy of the RAN group was significantly greater than that of the ARC group in both target (1.2 ± 0.5 mm vs 1.7 ± 1.2 mm, P = .005) and trajectory (0.9 ± 0.6 mm vs 1.3 ± 0.9 mm, P = .004) measurements. Total procedural time was also significantly faster for the RAN group than for the ARC group (44.6 ± 7.7 minutes vs 86.0 ± 12.5 minutes, P < .001). The RAN group had significantly reduced time per electrode placement (2.9 ± 0.9 minutes vs 5.8 ± 2.0 minutes, P < .001) and significantly reduced radiation during registration (1.9 ± 1.1 mGy vs 76.2 ± 5.0 mGy, P < .001) compared with the ARC group. CONCLUSION: In this cadaveric study, cranial leads were placed faster and with greater accuracy using RAN than those placed with conventional stereotactic arc-based technique. RAN also required significantly less radiation to register the specimen's coordinate system to the planned trajectories. Clinical testing should be performed to further investigate RAN for stereotactic cranial surgery.

An investigation of range of motion preservation in fusionless anterior double screw and cord constructs for scoliosis correction.

PURPOSE: To evaluate the motion-preserving properties of vertebral body tethering with varying cord/screw constructs and cord thicknesses in cadaveric thoracolumbar spines. METHODS: In vitro flexibility tests were performed on six fresh-frozen human cadaveric spines (T1-L5) (2 M, 4F) with a median age of 63 (59-to-80). An ± 8 Nm load was applied to determine range of motion (ROM) in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) in the thoracic and lumbar spine. Specimens were tested with screws (T5-L4) and without cords. Single (4.0 mm and 5.0 mm) and double (4.0 mm) cord constructs were sequentially tensioned to 100 N and tested: (1) Single 4.0 mm and (2) 5.0 mm cords (T5-T12); (3) Double 4.0 mm cords (T5-12); (4) Single 4.0 mm and (5) 5.0 mm cord (T12-L4); (6) Double 4.0 mm cords (T12-L4). RESULTS: In the thoracic spine (T5-T12), 4.0-5.0 mm single-cord constructs showed slight reductions in FE and 27-33% reductions in LB compared to intact, while double-cord constructs showed reductions of 24% and 40%, respectively. In the lumbar spine (T12-L4), double-cord constructs had greater reductions in FE (24%), LB (74%), and AR (25%) compared to intact, while single-cord constructs exhibited reductions of 2-4%, 68-69%, and 19-20%, respectively. CONCLUSIONS: The present biomechanical study found similar motion for 4.0-5.0 mm single-cord constructs and the least motion for double-cord constructs in the thoracic and lumbar spine suggesting that larger diameter 5.0 mm cords may be a more promising motion-preserving option, due to their increased durability compared to smaller cords. Future clinical studies are necessary to determine the impact of these findings on patient outcomes.

Fixation Strength of Modified Iliac Screw Trajectory Compared to Traditional Iliac and S2 Alar-Iliac Trajectories: A Cadaveric Study.

OBJECTIVE: Traditional iliac (TI) screws require extensive dissection, involve offset-connectors, and have prominent screw heads that may cause patient discomfort. S2 alar-iliac (S2AI) screws require less dissection, do not need offset connectors, and are less prominent. However, the biomechanical consequences of S2AI screws crossing the alar-iliac joint is unknown. The present study investigates the fixation strength of a modified iliac (MI) screw, which has a more medial entry point and reduced screw prominence, but does not cross the alar-iliac joint. METHODS: Eighteen sacropelvic spines were divided into 3 groups (n = 6): TI, S2AI, and MI. Each specimen was fixed unilaterally with S1 pedicle screws and pelvic fixation according to its group. Screws were loaded at ±10 Nm at 3Hz for 1000 cycles. Motion of each screw and rod strain above and below the S1 screw was measured. RESULTS: Toggle of the S1 screw was lowest for the TI group, followed by the MI and S2AI groups, but there were no significant differences (P = 0.421). Toggle of the iliac screw relative to the pelvis was also lowest for the TI group, followed by the MI group, and was greatest for the S2AI group, without significant differences (P = 0.179). Rod strain was similar across all groups. CONCLUSIONS: No statistically significant differences were found between the TI, S2AI, and MI techniques with regard to screw toggle or rod strain. Advantages of the MI screw include its lower profile and a medialized starting point eliminating the need for offset-connectors.

Comprehensive Evaluation of Accessory Rod Position, Rod Material and Diameter, Use of Cross-connectors, and Anterior Column Support in a Pedicle Subtraction Osteotomy Model: Part I: Effects on Apical Rod Strain: An In Vitro and In Silico Biomechanical Study.

STUDY DESIGN: In silico finite element study. OBJECTIVE: The aim of this study was to evaluate the effect of six construct factors on apical rod strain in an in silico pedicle subtraction osteotomy (PSO) model: traditional inline and alternative Ames-Deviren-Gupta (ADG) multi-rod techniques, number of accessory rods (three- vs. four-rod), rod material (cobalt-chrome [CoCr] or stainless steel [SS] vs. titanium [Ti]), rod diameter (5.5 vs. 6.35 mm), and use of cross-connectors (CC), or anterior column support (ACS). SUMMARY OF BACKGROUND DATA: Rod fracture following lumbar PSO is frequently reported. Clinicians may modulate reconstructs with multiple rods, rod position, rod material and diameter, and with CC or ACS to reduce mechanical demand or rod contouring. A comprehensive evaluation of these features on rod strain is lacking. METHODS: A finite element model (T12-S1) with intervertebral discs and ligaments was created and validated with cadaveric motion data. Apical rod strain of primary and accessory rods was collected for 96 constructs across all six construct factors, and normalized to the Ti two-rod control. RESULTS: Regardless of construct features, CoCr and SS material reduced strain across all rods by 49.1% and 38.1%, respectively; increasing rod diameter from 5.5 mm to 6.35 mm rods reduced strain by 32.0%. Use of CC or lumbosacral ACS minimally affected apical rod strain (<2% difference from constructs without CC or ACS). Compared to the ADG technique, traditional inline reconstruction reduced primary rod strain by 32.2%; however, ADG primary rod required 14.2° less rod contouring. The inline technique produced asymmetrical loading between left and right rods, only when three rods were used. CONCLUSION: The number of rods and position of accessory rods affected strain distribution on posterior fixation. Increasing rod diameter and using CoCr rods was most effective in reducing rod strain. Neither CC nor lumbosacral ACS affected apical rod strain. LEVEL OF EVIDENCE: N/A.

Comprehensive In Silico Evaluation of Accessory Rod Position, Rod Material and Diameter, Use of Cross-connectors, and Anterior Column Support in a Pedicle Subtraction Osteotomy Model: Part II: Effects on Lumbosacral Rod and Screw Strain.

STUDY DESIGN: In silico finite element study. OBJECTIVE: The aim of this study was to evaluate effects of six construct factors on rod and screw strain at the lumbosacral junction in an in silico pedicle subtraction osteotomy (PSO) model: traditional inline and alternative Ames-Deviren-Gupta (ADG) multi-rod techniques, number of accessory rods (three-rod vs. four-rod), rod material (cobalt-chrome [CoCr] or stainless steel [SS] vs. titanium [Ti]), rod diameter (5.5 vs. 6.35 mm), and use of cross-connectors (CC), or anterior column support (ACS). SUMMARY OF BACKGROUND DATA: Implant failure and pseudoarthrosis at the lumbosacral junction following PSO are frequently reported. Clinicians may modulate reconstructs with multiple rods, rod position, rod material, and diameter, and with CC or ACS to reduce mechanical demand. An evaluation of these features' effects on rod and screw strains is lacking. METHODS: A finite element model (T12-S1) with intervertebral discs and ligaments was created and validated with cadaveric motion data. Lumbosacral rod and screw strain data were collected for 96 constructs across all six construct factors and normalized to the Ti 2-Rod control. RESULTS: The inline technique resulted in 12.5% to 51.3% more rod strain and decreased screw strain (88.3% to 95%) compared to ADG at the lumbosacral junction. An asymmetrical strain distribution was observed in the three-rod inline technique in comparison to four-rod, which was more evenly distributed. Regardless of construct features, rod strain was significantly decreased by rod material (CoCr > SS > Ti), and increasing rod diameter from 5.5 mm to 6.35 mm reduced strain by 9.9% to 22.1%. ACS resulted in significant reduction of rod (37.8%-59.8%) and screw strains (23.2%-65.8%). CONCLUSION: Increasing rod diameter, using CoCr rods, and ACS were the most effective methods in reducing rod strain at the lumbosacral junction. The inline technique decreased screw strain and increased rod strain compared to ADG. LEVEL OF EVIDENCE: N/A.

Mechanisms of Failure Following Simulated Repetitive Lifting: A Clinically Relevant Biomechanical Cadaveric Study.

STUDY DESIGN: A biomechanical analysis correlating internal disc strains and tissue damage during simulated repetitive lifting. OBJECTIVE: To understand the failure modes during simulated safe and unsafe repetitive lifting. SUMMARY OF BACKGROUND DATA: Repetitive lifting has been shown to lead to lumbar disc herniation (LDH). In vitro studies have developed a qualitative understanding of the effect of repetitive loading on LDH. However, no studies have measured internal disc strains and subsequently correlated these with disc damage. METHODS: Thirty human cadaver lumbar functional spinal units were subjected to an equivalent of 1 year of simulated repetitive lifting under safe and unsafe levels of compression, in combination with flexion (13-15°), and right axial rotation (2°) for 20,000 cycles or until failure. Safe or unsafe lifting were applied as a compressive load to mimic holding a 20 kg weight either close to, or at arm's length, from the body, respectively. Maximum shear strains (MSS) were measured, and disc damage scores were determined in nine regions from axial post-test magnetic resonance imaging (MRI) and macroscopic images. RESULTS: Twenty percent of specimens in the safe lifting group failed before 20,000 cycles due to endplate failure, compared with 67% in the unsafe group. Over half of the specimens in the safe lifting group failed via either disc protrusion or LDH, compared with only 20% via protrusion in the unsafe group. Significant positive correlations were found between MRI and macroscopic damage scores in all regions (rs > 0.385, P < 0.049). A significant positive correlation was observed in the left lateral region for MSS versus macroscopic damage score (rs = 0.486, P < 0.037) and MSS versus failure mode (rs = 0.724, P = 0.018, only specimens with disc failure). Pfirrmann Grade 3 discs were strongly associated with subsequent LDH (P = 0.003). CONCLUSION: Increased shear strains were observed in the contralateral side to the applied rotation as disc injury progressed from protrusion to LDH. Larger compressive loads applied to simulate unsafe lifting led to frequent early failure of the endplate, however, smaller compressive loads at similar flexion angles applied under safe lifting led to more loading cycles before failure, where the site of failure was more likely to be the disc. Our study demonstrated that unsafe lifting leads to greater risk of injury compared with safe lifting, and LDH and disc protrusion were more common in the posterior/posterolateral regions. LEVEL OF EVIDENCE: N/A.

Effectiveness of novel fabrics to resist punctures and lacerations from white shark (Carcharodon carcharias): Implications to reduce injuries from shark bites.

Increases in the number of shark bites, along with increased media attention on shark-human interactions has led to growing interest in preventing injuries from shark bites through the use of personal mitigation measures. The leading cause of fatality from shark bite victims is blood loss; thus reducing haemorrhaging may provide additional time for a shark bite victim to be attended to by emergency services. Despite previous shark-proof suits being bulky and cumbersome, new technological advances in fabric has allowed the development of lightweight alternatives that can be incorporated onto traditional wetsuits. The ability for these fabrics to withstand shark bites has not been scientifically tested. In this report, we compared two types of recently developed protective fabrics that incorporated ultra-high molecular weight polyethylene (UHMWPE) fibre onto neoprene (SharkStop and ActionTX) and compared them to standard neoprene alternatives. We tested nine different fabric variants using three different tests, laboratory-based puncture and laceration tests, along with field-based trials involving white sharks Carcharodon carcharias. Field-based trials consisted of measuring C. carcharias bite force and quantifying damages to the new fabrics following a bite from 3-4 m total length C. carcharias. We found that SharkStop and ActionTX fabric variants were more resistant to puncture, laceration, and bites from C. carcharias. More force was required to puncture the new fabrics compared to control fabrics (laboratory-based tests), and cuts made to the new fabrics were smaller and shallower than those on standard neoprene for both types of test, i.e. laboratory and field tests. Our results showed that UHMWPE fibre increased the resistance of neoprene to shark bites. Although the use of UHMWPE fibre (e.g. SharkStop and ActionTX) may therefore reduce blood loss resulting from a shark bite, research is needed to assess if the reduction in damages to the fabrics extends to human tissues and decreased injuries.

The Biomechanics of the Inter-Lamellar Matrix and the Lamellae During Progression to Lumbar Disc Herniation: Which is the Weakest Structure?

While microstructural observations have improved our understanding of possible pathways of herniation progression, no studies have measured the mechanical failure properties of the inter-lamellar matrix (ILM), nor of the adjacent lamellae during progression to herniation. The aim of this study was to employ multiscale, biomechanical and microstructural techniques to evaluate the effects of progressive induced herniation on the ILM and lamellae in control, pre-herniated and herniated discs (N = 7), using 2 year-old ovine spines. Pre-herniated and herniated (experimental) groups were subjected to macroscopic compression while held in flexion (13°), before micro-mechanical testing. Micro-tensile testing of the ILM and the lamella from anterior and posterolateral regions was performed in radial and circumferential directions to measure failure stress, modulus, and toughness in all three groups. The failure stress of the ILM was significantly lower for both experimental groups compared to control in each of radial and circumferential loading directions in the posterolateral region (p < 0.032). Within each experimental group in both loading directions, the ILM failure stress was significantly lower by 36% (pre-herniation), and 59% (herniation), compared to the lamella (p < 0.029). In pre-herniated compared to control discs, microstructural imaging revealed significant tissue stretching and change in orientation (p < 0.003), resulting in a loss of distinction between respective lamellae and ILM boundaries.

The equivalence of multi-axis spine systems: Recommended stiffness limits using a standardized testing protocol.

The complexity of multi-axis spine testing often makes it challenging to compare results from different studies. The aim of this work was to develop and implement a standardized testing protocol across three six-axis spine systems, compare them, and provide stiffness and phase angle limits against which other test systems can be compared. Standardized synthetic lumbar specimens (n=5), comprising three springs embedded in polymer at each end, were tested on each system using pure moments in flexion-extension, lateral bending, and axial rotation. Tests were performed using sine and triangle waves with an amplitude of 8Nm, a frequency of 0.1Hz, and with axial preloads of 0 and 500N. The stiffness, phase angle, and R2 value of the moment against rotation in the principal axis were calculated at the center of each specimen. The tracking error was adopted asa measure of each test system to minimize non-principal loads, defined as the root mean squared difference between actual and target loads. All three test systems demonstrated similar stiffnesses, with small (<14%) but significant differences in 4 of 12 tests. More variability was observed in the phase angle between the principal axis moment and rotation, with significant differences in 10 of 12 tests. Stiffness and phase angle limits were calculated based on the 95% confidence intervals from all three systems. These recommendations can be used with the standard specimen and testing protocol by other research institutions to ensure equivalence of different spine systems, increasing the ability to compare in vitro spine studies.

Effect of degeneration on the six degree of freedom mechanical properties of human lumbar spine segments.

While the effects of disc degeneration on compression and rotation motions have been studied, there is no data for shear loading. Clinical research has shown that those with low back pain (a potential consequence of degeneration) experience a 75% greater lateral shear force than those without it. Therefore, the aim was to compare the effect of degeneration on spine segment stiffness and phase angle in each of six degree of freedom (6DOF) loading directions. Fourteen intact functional spinal units (FSU) were dissected from human lumbar spines (mean (SD) age 76.2 (11) years, Thompson grades 3 (N = 5, mild), 4 (N = 6, moderate), 5 (N = 3, severe)). Each FSU was tested in ±6DOFs while subjected to a physiological preload, hydration, and temperature (37°C) conditions in a hexapod robot. A one-way ANOVA between degenerated groups was performed on stiffness and phase angle for each DOF. Significant differences in stiffness were found between mild and moderate degenerative groups in lateral shear (p = 0.001), and axial rotation (p = 0.001), where moderate degeneration had decreased stiffness. For phase angle, significant differences were seen in anterior shear (p = 0.017), and axial rotation (p = 0.026), where phase angle for mild degeneration was less than moderate. Trends of stiffness and phase angle changes between degenerative groups were similar within each DOF. Clinically, the identification of the DOFs that are most affected by degeneration could be used in rehabilitation to improve supplemental stabilization of core muscle groups. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1399-1409, 2016.

Effect of potting technique on the measurement of six degree-of-freedom viscoelastic properties of human lumbar spine segments.

Polymethyl methacrylate (PMMA) and Wood's Metal are fixation media for biomechanical testing; however, the effect of each potting medium on the measured six degree-of-freedom (DOF) mechanical properties of human lumbar intervertebral discs is unknown. The first aim of this study was to compare the measured 6DOF elastic and viscoelastic properties of the disc when embedded in PMMA compared to repotting in Wood's Metal. The second aim was to compare the surface temperature of the disc when potted with PMMA and Wood's Metal. Six human lumbar functional spinal units (FSUs) were first potted in PMMA, and subjected to overnight preload in a saline bath at 37 °C followed by five haversine loading cycles at 0.1 Hz in each of 6DOF loading directions (compression, left/right lateral bending, flexion, extension, left/right axial rotation, anterior/posterior, and lateral shear). Each specimen was then repotted in Wood's Metal and subjected to a 2-h re-equilibrating preload followed by repeating the same 6DOF tests. Outcome measures of stiffness and phase angle were calculated from the final loading cycle in each DOF and were expressed as normalized percentages relative to PMMA (100%). Disc surface temperatures (anterior, left/right lateral) were measured during potting. Paired t-tests (with alpha adjusted for multiple DOF) were conducted to compare the differences in each outcome parameter between PMMA and Wood's Metal. No significant differences in stiffness or phase angle were found between PMMA and Wood's Metal. On average, the largest trending differences were found in the shear DOFs for both stiffness (approximately 35% greater for Wood's Metal compared to PMMA) and phase angle (approximately 15% greater for Wood's Metal). A significant difference in disc temperature was found at the anterior surface after potting with Wood's Metal compared to PMMA, which did not exceed 26 °C. Wood's Metal is linear elastic, stiffer than PMMA and may reduce measurement artifact of potting medium, particularly in the shear directions. Furthermore, it is easier to remove than PMMA, reuseable, and cost effective.