Lisa Ferrara

Structural features and thickness of the vertebral cortex in the thoracolumbar spine.

STUDY DESIGN The thickness and structure of the vertebral body cortex were examined from sections of human cadaveric vertebrae. OBJECTIVES The objectives were to identify the principal structural features of the cortex, to directly measure the minimum and maximum thicknesses of the cortex in the thoracolumbar spine, and to compare regional variations in the structure of the cortex. SUMMARY OF BACKGROUND DATA The thickness of the vertebral cortical shell contributes to the compressive strength of the vertebral body. There is little consensus concerning the thickness and morphology of vertebral shell and endplate along the spine in existing data. METHODS Human T1, T5, T9, L1, and L5 vertebral bodies (mean age 70.4 years) from 20 cadaveric spines were sectioned and photographed. The minimum and maximum cortical thickness of the shells and endplates in the midsagittal plane were measured from magnified images. RESULTS The anterior shell thickness was significantly greater than the posterior shell and both endplates. Endplate thickness was greatest in the lower lumbar vertebrae. There was a significant decrease in cortex thickness over the central portion of endplates and shells, with a mean minimum thickness of 0.40 mm and a mean maximum thickness of 0.86 mm, with an overall mean of 0.64 +/- 0.41 mm. Increased porosity was also observed along the central regions of the cortical shells. In the lower thoracic and lumbar spine, a double-layered endplate structure was observed. CONCLUSIONS Invasive techniques provide the only means to directly resolve the thickness and distribution of bone in the vertebral cortex. The cortex thickness and structure varies along the endplates and the anterior and posterior surfaces of the vertebral body. The implications of the so called double-layered endplate structure are unknown, but indicate the need for further study.

A biomechanical comparison of facet screw fixation and pedicle screw fixation: effects of short-term and long-term repetitive cycling.

Study Design. A biomechanical study was conducted to assess the stabilization performance of transfacet pedicle screw fixation. Objective. To compare the biomechanical effects of short-term and long-term cyclic loading on lumbar motion segments instrumented with either a pedicle screw or a transfacet pedicle screw construct. Summary of Background Data. Facet screw fixation is an alternative to pedicle screw fixation that permits the use of a minimally invasive strategy. It is not known whether facet screw fixation can provide stability equivalent to pedicle screw fixation during cyclical loading. Therefore, transfacet pedicle screw fixation and standard pedicle screw fixation techniques were compared biomechanically. Methods. Lumbar motion segments were tested under short-term and long-term cyclic loading conditions. For the short-term phase, specimens were tested intact for six cycles (to 400 N or 4 Nm) in compression, flexion, extension, lateral bending, and torsion. The specimens then were instrumented with bilateral semicircular interbody spacers and pedicle screw instrumentation or transfacet pedicle screws, and the testing sequence was repeated. For the long-term phase, 12 specimens were instrumented in a similar manner and loaded to 6 Nm of flexion bending for 180,000 cycles. Results. For the short-term phase, both fixation systems had significantly greater stiffness and reduced range of motion, as compared with the intact state. No differences were observed between the fixation systems except in flexion, wherein transfacet pedicle screw specimens were significantly stiffer than traditional pedicle screw specimens. For the long-term phase, the stiffness and range of motion did not significantly increase or decrease over repetitive cycling of the instrumented specimens. Furthermore, no significant difference between the fixation systems was observed. Conclusions. The stability provided by both transfacet pedicle screw fixation and traditional pedicle screw fixation was not compromised after repetitive cycling. In this model, transfacet pedicle screw fixation appears equivalent biomechanically to traditional pedicle screw fixation.

Biomechanics of the Spine

Abstract:The field of modern biomechanics has deep historical roots from the ancient Egyptians, who documented the earliest accounts of spinal injury (2600-2200 BC), as well as from the ancient Hindus, who were noted for their treatment of spinal deformities (3500-1800 BC). Building on this foundation, the modern explosion of spinal instrumentation introduced the concept of internal fixation for spinal stabilization, further advancing the understanding of the mechanics of musculoskeletal motion. The spine is a complex mechanical structure complete with levers (vertebrae), pivots (facets and disks), passive restraints (ligaments), and actuators (muscles). Each of these elements merits special consideration and thus is addressed individually. Understanding actions and reactions, force vectors, related component vectors, and the movements and/or deformation that they cause allows the spine surgeon to apply fundamental physical principles to clinical practice. Kinematics is the application of these physical principles toward the study of the motion of rigid bodies. Knowledge of the principles and laws that are clinically relevant regarding spinal instrumentation is crucial to success. Clinical biomechanics requires the assessment of 3 key questions: 1) how do the components of the implant connect together, 2) how does the implant connect to the spine, and 3) how does the construct function biomechanically? The ability to apply basic biomechanical principles in the clinical arena provides a framework that the surgeon can use in the clinical decision-making process.

Microelectromechanical systems and neurosurgery: a new era in a new millennium.

MICROMACHINES AND MICROELECTROMECHANICAL SYSTEMS (MEMS) are terms that are new to neurosurgeons but certain to become “household terms” in neurosurgery in the near future. These new terms serve as an introduction to a new world of sensors, actuators, and “smart systems” that will change the ways in which neurosurgeons interact with their environment. Through the use of microelectronics and micromachining technologies, MEMS will allow neurosurgeons to perform familiar tasks with greater precision, perform tasks that previously were not done at all, and monitor physiological and biochemical parameters more accurately and with greater safety. This review provides the information necessary to understand the fundamental concepts of MEMS and their application to the neurosurgical arena. It defines the relevant terms and describes the history behind the “micromachine revolution,” the capabilities and limitations of MEMS technology, and how this revolution is germane to neurosurgery and to neurosurgeons.

The biomechanics of 1, 2, and 3 levels of vertebral augmentation with polymethylmethacrylate in multilevel spinal segments.

Study Design. Experimental biomechanics of multilevel segments with 0, 1, 2, and 3 vertebral levels of polymethylmethacrylate augmentation. Objective. To compare multilevel spinal segments with different numbers (0, 1, 2, and 3) of vertebral levels augmented with polymethylmethacrylate. Summary of Background Data. The stiffness and strength of single-level polymethylmethacrylate augmentations in individual and multilevel vertebrae treated by kyphoplasty and vertebroplasty have been studied, but the biomechanics of multilevel segments with more than 1 vertebral level augmented with polymethylmethacrylate are lacking, yet this is clinically relevant in multilevel compression fracture treatment. Materials And Methods. A total of 48 multilevel segments (T3–T5, T6–T8, T9–T11, T12–L2, and L3–L5) from 12 spines with known bone mineral density (BMD) were allocated into 6 groups based on the number of vertebral levels augmented: 0 levels (n = 13), control group; 1 level (n = 7), group 2; 2 levels, groups 3, 4, and 5 (n = 7 in each); and 3 levels (n = 7), group 6. They were compressed to failure, disarticulated into individual vertebrae, and retested. Stiffness and strength were statistically analyzed using a univariate analysis of variance comparing the main effects, using least significant difference comparisons with 0.05 probability level. Results. Strength was dependent on BMD (P < 0.001 multilevel segments, P < 0.001 individual vertebrae), with no differences among the 6 different augmentation groups, and no significant differences between augmented and nonaugmented individual vertebrae. Stiffness was dependent on BMD (P = 0.009 multilevel segments, P < 0.004 individual vertebrae), with no significant differences among the 6 different augmentation groups, and no significant differences between augmented and nonaugmented individual vertebrae. Conclusions. Multilevel segment biomechanics are dependent on BMD and not the pattern of augmentation, so the augmentation of fractured vertebrae can be extended to adjacent levels at risk for fracture to maintain stiffness and strength, thus preventing further fractures.

An in vivo Biocompatibility Assessment of MEMS Materials for Spinal Fusion Monitoring

The site-specific biocompatibility of silicon chips and commercially available silicon pressure sensor die were evaluated after implantation in caprine (goat) spine. Surgical procedures were developed to insert silicon chips into the nucleus pulposus regions of the lumbar discs and pressure sensors into autologous bone grafts for cervical spine fusion. After a six-month implantation period, the animal was sacrificed and the spinal segments were meticulously harvested and analyzed for local tissue response via gross examination and histological techniques. Gross examination of cervical and lumbar spinal segments after harvest and dissection did not reveal any visible signs of adverse reactions to the MEMS materials. Furthermore, the surrounding tissues and musculature for both spinal regions were devoid of necrosis. Histological analysis of compromised spinal segments did not reveal evidence of any adverse foreign body response by the caprine spinal tissue to the implanted MEMS materials. These preliminary results support the further development of a spinal fusion monitoring system based on implantable MEMS sensors.

Biomechanical comparison of transarticular facet screws to lateral mass plates in two-level instrumentations of the cervical spine.

Study Design. In vitro biomechanical comparison of transarticular facet screws to lateral mass plates in two level instrumentations of the cervical spine. Objective. Lateral mass plates are costly, and screw placement is difficult. Facet screws have never been tested as an alternative in the cervical spine. This biomechanical study compared cervical transarticular facet screws to lateral mass plates in two-level instrumentations of human cadaveric cervical spines. Summary of Background Data. Translaminar facet screws have been shown to have similar biomechanical performance to pedicle screw fixation in the lumbar spine, especially in flexion. They have proven to be fast, safe, and effective, with authors reporting 94% to 100% fusion rates in single-level lumbar fusions. However, a biomechanical comparison of transarticular facet screws to lateral mass plates in cervical spine instrumentations has not been reported. Methods. Thirteen human cadaveric cervical motion segments (C2–C4, C5–C7) were tested before and after instrumentation, with either transarticular facet screws or lateral mass plates, in flexion, extension, lateral bending, and torsion. Specimens were subjected to six cycles under a load of 2 Nm. Results. Both fixation systems significantly reduced range of motion (ROM) and increased stiffness compared with the intact state in flexion, extension, lateral bending, and torsion. There were also no significant differences between the facet screws and plates in any of the four directions. To compare the two systems, ROM of each was analyzed relative to the uninstrumented state. Flexion was 0.26 (or 26% of the intact state) for the transarticular facet screws versus 0.20 for the lateral mass plates (P = 0.34), extension was 0.10 versus 0.07 (P = 0.43), lateral bending was 0.17 versus 0.15 (P = 0.52), and torque was 0.25 versus 0.38 (P = 0.12). Load to failure testing failed to indicate any differences between the two methods of fixation because all the specimens failed elsewhere. Conclusion. This study proves that transarticular facet screws and lateral mass plates are equivalent in two-level instrumentations of the cervical spine. This is the first biomechanical study to test transarticular facet screws in this context.

Biomechanical analysis of sacral screw strain and range of motion in long posterior spinal fixation constructs: effects of lumbosacral fixation strategies in reducing sacral screw strains.

Study Design. A cadaveric biomechanical experiment was conducted to assess the range of motion (ROM) and screw strain at S1 in a long instrumented spinal fusion construct to compare the effects of various surgical strategies for L5–S1 stabilization. Objective. To directly quantify and compare S1 screw strains and lumbosacral ROM for 4 different L2–S1 posterior segmental instrumented fusion constructs: an L2-S1 pedicle screw (PS) construct alone and PS with each of 3 different augmentations, anterior lumbar intebody fusion (ALIF), anterior axial interbody threaded rod (AxiaLITR), or iliac screws. Summary of Background Data. Iliac screws and anterior interbody devices are commonly used as augmentation to reduce the incidence of S1 screw loosening in long fusion constructs. Alternatives, such as AxiaLITR, may provide similar biomechanical advantages without many of the same long-term limitations and morbidities. Methods. Pure moment flexibility testing was performed in 6 cadaveric lumbosacral spines. Specimens were tested with 4 instrumentation constructs: (.1) PS L2–S1, (2) PS with ALIF, (3) PS with AxiaLITR, and (4) PS with iliac screws. Bilateral S1 PS were instrumented with strain gauges, directly measuring screw loading while simultaneously measuring L5–S1 ROM with a noncontact camera system. Results. Average S1 screw strains were the greatest with the PS group and were reduced by 38% with the ALIF group, 75% with the AxiaLITR group, and 78% with the iliac screw group in flexion-extension (P < 0.05). Similar trends were observed in torsion (P < 0.05). Strains in lateral bending were smaller in magnitude and were similar among all 4 constructs. The AxiaLITR and iliac screw groups demonstrated a similar ROM and significant reduction in ROM at L5–S1 compared with both the PS and ALIF groups (P ⩽ 0.02 and P < 0.03). Conclusion. The results of this study indicated that iliac screws and AxiaLITR provide similar stability at L5–S1, while significantly reducing the strain on the S1 screws.

Effects of angle and laminectomy on triangulated pedicle screws

We aimed to demonstrate the effect of angle and laminectomy on paired pedicle screws to determine whether a 90 degrees screw angle is optimal as has been previously suggested. According to the angle between right and left screws, 28 calf vertebrae were divided into three groups and instrumented as follows: Group I: 60 degrees screw angle; Group II: 90 degrees angle; Group III: 60 degrees angle with laminectomy. The screws were connected using rods and cross-fixators and tested to peak pullout force. Triangulated pedicle screws provided 76.5% more pullout strength than single screws. Most of the specimens failed through loss of convergence angle (toggling of screws on the rods) and subsequent uni- or bilateral screw pullout. Mean+/-SD peak loads were: Group I: 2071+/-622 N; Group II: 1753+/-497 N; Group III: 2186+/-587 N. The differences were not significant (p>0.05). 90 degrees triangulation was not associated with a superior pullout performance versus conventional 60 degrees triangulation, suggesting that achieving additional triangulation angle is not necessary to obtain increased pullout strength. Laminectomy did not alter the effect of triangulation on fixation strength.

Micromachines in spine surgery.

The old cliché “We’re not in Kansas anymore,” taken from Dorothy’s comment to Toto in “The Wizard of Oz,” is a statement that is often used in the context of our rapidly changing times. We most certainly have entered another time and era regarding technical and scientific advancements in spine surgery. Much of the information that is imparted herein regards technology that is not yet fully developed nor is its potential fully realized. In fact, much of it relates to predictions (the lead author’s predictions) about the future of spine surgery. The authors, like Yogi Berra, “hate to make predictions . . . , especially about the future.” Such predictions often place hope, optimism, and bias ahead of reality.