How does thoracic scoliosis surgery affect thoracolumbar spinal flexibility and lumbar intradiscal pressure? An in vitro study confirming the importance of the rib cage.

PURPOSE: To evaluate effects of spinal and rib osteotomies on the resulting spinal flexibility for surgical correction of thoracic scoliosis and to explore effects of posterior fixation on thoracolumbar segmental range of motion and lumbar intervertebral disc loading. METHODS: Six fresh frozen human thoracolumbar spine and rib cage specimens (26-45 years, two female / four male) without clinically relevant deformity were loaded with pure moments of 5 Nm in flexion/extension, lateral bending, and axial rotation. Optical motion tracking of all segmental levels (C7-S) and intradiscal pressure measurements of the lumbar spine (L1-L5) were performed (1) in intact condition, (2) after Schwab grade 1, (3) Schwab grade 2, and (4) left rib osteotomies at T6-T10 levels, as well as (5) after posterior spinal fixation with pedicle screw-rod instrumentation at T4-L1 levels. RESULTS: Schwab grade 1 and 2 osteotomies did not significantly (p > 0.05) affect spinal flexibility, whereas left rib osteotomies significantly (p < 0.05) increased segmental ranges of motion at upper and lower levels in flexion/extension and at treated levels in lateral bending. Posterior fixation caused significantly (p < 0.05) increased range of motion at upper adjacent thoracic and mid-lumbar levels, as well as significantly (p < 0.05) increased intradiscal pressure at the lower adjacent level. CONCLUSION: Low effects of Schwab grade 1 and 2 osteotomies question the impact of isolated posterior spinal releases for surgical correction maneuvers in adolescent idiopathic scoliosis, in contrast to additional concave rib osteotomies. High effects of posterior fixation potentially explain frequently reported complications such as adjacent segment disease or proximal junctional kyphosis.

In Vitro Biomechanics of the Cervical Spine: A Systematic Review.

In vitro testing has been conducted to provide a comprehensive understanding of the biomechanics of the cervical spine. This has allowed a characterization of the stability of the spine as influenced by the intrinsic properties of its tissue constituents and the severity of degeneration or injury. This also enables the preclinical estimation of spinal implant functionality and the success of operative procedures. The purpose of this review paper was to compile methodologies and results from various studies addressing spinal kinematics in pre- and postoperative conditions so that they could be compared. The reviewed literature was evaluated to provide suggestions for a better approach for future studies, to reduce the uncertainties and facilitate comparisons among various results. The overview is presented in a way to inform various disciplines, such as experimental testing, design development, and clinical treatment. The biomechanical characteristics of the cervical spine, mainly the segmental range of motion (ROM), intradiscal pressure (IDP), and facet joint load (FJL), have been assessed by testing functional spinal units (FSUs). The relative effects of pathologies including disc degeneration, muscle dysfunction, and ligamentous transection have been studied by imposing on the specimen complex load scenarios imitating physiological conditions. The biomechanical response is strongly influenced by specimen type, test condition, and the different types of implants utilized in the different experimental groups.

Biomechanics following skip-level cervical disc arthroplasty versus skip-level cervical discectomy and fusion: a finite element-based study.

BACKGROUND: Moderately increased motion at the intermediate segment (IS) after skip-level fusion may accelerate disc degeneration. However, limited biomechanical data are available that examine the effects on the IS following cervical disc arthroplasty (CDA). The purpose of this study is to investigate the biomechanical changes in the IS of the cervical spine after skip-level fusion or skip-level arthroplasty. METHODS: A finite element model of a healthy cervical spine (C2-C7) was constructed. Two surgical models were developed: (1) skip-level fusion at C3/4 and C5/6 and (2) skip-level arthroplasty at C3/4 and C5/6. A 75-N follower load and 1.0-N·m moments were applied to the top of the C2 vertebra to produce flexion, extension, lateral bending and axial rotation in the intact model. The end-points in each direction corresponding to the intact model were applied to the surgical models under displacement-control protocols. RESULTS: The ranges of motion (ROMs) of the fusion model were markedly decreased at the operated levels, while the corresponding ROMs of the arthroplasty model were similar to those of the intact spine in all directions. In the fusion model, the ROMs of the IS (C4/5) were markedly increased in all directions. The ROMs in the arthroplasty model were similar to those in the intact spine, and the ROMs of untreated segments were evenly increased. In the fusion model, the intradiscal pressure and facet contact force at were C4/5 remarkably increased and unevenly distributed among the unfused segments. In the arthroplasty model, the IS did not experience additive stress. CONCLUSION: The IS does not experience additive ROM or stress in the intervertebral disc or facet joints after skip-level arthroplasty, which has fewer biomechanical effects on the IS than does skip-level fusion. This study provides a biomechanical rationale for arthroplasty in treating patients with skip-level cervical degenerative disc disease.

Intervertebral disc needle puncture injury can be repaired using a gelatin-poly (γ-glutamic acid) hydrogel: an in vitro bovine biomechanical validation.

PURPOSE: The subtle impairments of the disc due to anular punctures may have an immediate effect on the functional integrity due to the altered intradiscal pressure, hence the subsequent catabolic degradation. This study evaluates functional restoration of needle puncture injured intervertebral discs with a newly developed injectable hydrogel using the quantitative discomanometry (QD) test. The proposed hydrogel is composed of gelatin and poly (γ-glutamic acid) (γ-PGA) and crosslinked with 1-(3-dimethylaminopropyl)-3-ethyl-carbodiimide hydrochloride (EDC). METHODS: Thirty-six bovine motion segments were distributed into six groups. Needle puncture injured discs were created in all discs except for those in the first group (intact). The second group included injured discs that received no treatment (injury). The remaining four groups included injured discs repaired with injected hydrogels fabricated with different polymer solutions and EDC concentrations including: gelatin/γ-PGA solution crosslinked with the EDC solution at a 10:1 and 40:1 ratio to form the GP/E(10:1) and GP/E(40:1) groups, respectively, and gelatin and γ-PGA solution crosslinked with the EDC solution at a 10:1 ratio to form the G/E(10:1) or P/E(10:1) groups. The QD tests were performed to evaluate disc integrity of all six groups. RESULTS: Among all hydrogel repair groups, the GP/E(10:1) group was found to have the highest leakage and saturate pressure and was the only group comparable to the intact one. CONCLUSIONS: Restoration of disc integrity secondary to needle puncture injury can be achieved via the repair with the newly developed gelatin hydrogel incorporated with γ-PGA and EDC. These slides can be retrieved under Electronic Supplementary Material.

Reduction of intradiscal pressure by the use of polycarbonate-urethane rods as compared to titanium rods in posterior thoracolumbar spinal fixation.

Loss of sagittal alignment and balance in adult spinal deformity can cause severe pain, disability and progressive neurological deficit. When conservative treatment has failed, spinal fusion using rigid instrumentation is currently the salvage treatment to stop further curve progression. However, fusion surgery is associated with high revision rates due to instrumentation failure and proximal junctional failure, especially if patients also suffer from osteoporosis. To address these drawbacks, a less rigid rod construct is proposed, which is hypothesized to provide a more gradual transition of force and load distribution over spinal segments in comparison to stiff titanium rods. In this study, the effect of variation in rod stiffness on the intradiscal pressure (IDP) of fixed spinal segments during flexion-compression loading was assessed. An ex vivo multisegment (porcine) flexion-compression spine test comparing rigid titanium rods with more flexible polycarbonate-urethane (PCU) rods was used. An increase in peak IDP was found for both the titanium and PCU instrumentation groups as compared to the uninstrumented controls. The peak IDP for the spines instrumented with the PCU rods was significantly lower in comparison to the titanium instrumentation group. These results demonstrated the differences in mechanical load transfer characteristics between PCU and titanium rod constructs when subjected to flexion-compression loading. The concept of stabilization with a less rigid rod may be an alternative to fusion with rigid instrumentation, with the aim of decreasing mechanical stress on the instrumented segments and the possible benefit of a decrease in the incidence of screw pullout.

Experimental Study of Failures of the Rigid Spinal Posterior Fixation System Under Compressive Load Conditions: A Cadaver Study.

Background Many studies have been conducted on the biomechanics of the spine to elucidate the fixation properties of spinal fusion surgery and the causes of instrumentation failure. Among these studies, there are some studies on load sharing in the spine and measurement using strain gauges and pressure gauges, but there is a lack of research on axial compressive loads. Methods Axial compressive load tests were performed on human cadaveric injured lumbar vertebrae fixed with pedicle screws (PS). Both the strain generated in the PS rod and the intradiscal pressure were measured. Subsequently, the stress generated in the PS rod and the load sharing of the spine and instrumentation were calculated. Results Even when only compressive load is applied, bending stress of more than 10 times the compression stress was generated in the rod, and the stress tended to concentrate on one rod. Rod deformation becomes kyphotic, in contrast to the lordotic deformation behavior of the lumbar spine. The stress shielding rate was approximately 40%, less than half. Conclusions This study obtained basic data useful for constructing and verifying numerical simulations that are effective for predicting and elucidating the causes of dislodgement and failure of spinal implants.

Ligamentous tethering and intradiscal pressure affecting the mechanical environment of scoliotic spines.

Despite several theories have been proposed to explain the progression of Adolescent Idiopathic Scoliosis (AIS), there is no consensus on the mechanical factors that control the spinal deformities. Prominent biomechanical notions focus on the geometrical asymmetry and differential growth, however, the correlation between these phenomena remains unclear. We postulate that intradiscal pressure and its connection with the supporting ligamentous structures are the reasons behind the asymmetric growth in AIS. To investigate this hypothesis, a numerical 3D patient-specific model of a scoliotic spine is constructed to carry upper body weight. Four analyses are performed: control simulation with no ligaments followed by 3 simulations, in each, a different and stiffer set of ligaments is employed. The analyses showed that intradiscal pressure is relatively high in the spine's higher-deformity region. Moreover, the stiffness effect of the ligamentous tethering correlated directly to intradiscal pressure; the stiffer the ligaments, the higher the intradiscal pressure. Due to geometrical asymmetry, the pressure is eccentric toward the concave region of deformed vertebral units. As a result, the deformed annulus fibrosus generated uplifts in the convex side of deformed vertebral units. The eccentric pressure and the uplift are opposite in location and direction creating an imbalanced mechanical environment for the spine during growth.

In vivo measurement of intradiscal pressure changes related to thrust and non-thrust spinal manipulation in an animal model: a pilot study.

BACKGROUND: The intervertebral disc is a known back pain generator and is frequently the focus of spinal manipulative therapy evaluation and treatment. The majority of our current knowledge regarding intradiscal pressure (IDP) changes related to spinal manual therapy involves cadaveric studies with their inherent limitations. Additional in vivo animal models are needed to investigate intervertebral disc physiological and molecular mechanisms related to spinal manipulation and spinal mobilization treatment for low back disorders. METHODS: Miniature pressure catheters (Millar SPR-1000) were inserted into either the L4-L5 or L5-L6 intervertebral disc of 3 deeply anesthetized adult cats (Oct 2012-May 2013). Changes in IDP were recorded during delivery of instrument-assisted spinal manipulation (Activator V® and Pulstar®) and motorized spinal flexion with/without manual spinous process contact. RESULTS: Motorized flexion of 30° without spinous contact decreased IDP of the L4-L5 disc by ~ 2.9 kPa, while physical contact of the L4 spinous process decreased IDP an additional ~ 1.4 kPa. Motorized flexion of 25° with L5 physical contact in a separate animal decreased IDP of the L5-L6 disc by ~ 1.0 kPa. Pulstar® impulses (setting 1-3) increased IDP of L4-L5 and L5-L6 intervertebral discs by ~ 2.5 to 3.0 kPa. Activator V® (setting 1-4) impulses increased L4-L5 IDP to a similar degree. Net changes in IDP amplitudes remained fairly consistent across settings on both devices regardless of device setting suggesting that viscoelastic properties of in vivo spinal tissues greatly dampen superficially applied manipulative forces prior to reaching deep back structures such as the intervertebral disc. CONCLUSIONS: This study marks the first time that feline in vivo changes in IDP have been reported using clinically available instrument-assisted spinal manipulation devices and/or spinal mobilization procedures. The results of this pilot study indicate that a feline model can be used to investigate IDP changes related to spinal manual therapy mechanisms as well as the diminution of these spinal manipulative forces due to viscoelastic properties of the surrounding spinal tissues. Additional investigation of IDP changes is warranted in this and/or other in vivo animal models to provide better insights into the physiological effects and mechanisms of spinal manual therapy at the intervertebral disc level.

Comparison of In Vivo Intradiscal Pressure between Sitting and Standing in Human Lumbar Spine: A Systematic Review and Meta-Analysis.

BACKGROUND: Non-specific low back pain (LBP) is highly prevalent today. Disc degeneration could be one of the causes of non-specific LBP, and increased intradiscal pressure (IDP) can potentially induce disc degeneration. The differences in vivo IDP in sitting and standing postures have been studied, but inconsistent results have been reported. The primary objective of this systematic review is to compare the differences in vivo IDP between sitting and standing postures. The secondary objective of this review is to compare effect size estimates between (1) dated and more recent studies and (2) healthy and degenerated intervertebral discs. METHODS: An exhaustive search of six electronic databases for studies published before November 2021 was conducted. Articles measuring in vivo IDP in sitting and standing postures were included. Two independent researchers conducted the screening and data extraction. RESULTS: Ten studies that met the inclusion criteria were included in the systematic review, and seven studies with nine independent groups were included in meta-analyses. The sitting posture induces a significantly higher IDP on the lumbar spine (SMD: 0.87; 95% CI = [0.33, 1.41]) than the standing posture. In studies published after 1990 and subjects with degenerated discs, there are no differences in vivo IDP between both postures. CONCLUSIONS: Sitting causes higher loads on the lumbar spine than standing in the normal discs, but recent studies do not support this conclusion. Furthermore, the degenerated discs showed no difference in IDP in both postures.

Sensitivity of the Cervical Disc Loads, Translations, Intradiscal Pressure, and Muscle Activity Due to Segmental Mass, Disc Stiffness, and Muscle Strength in an Upright Neutral Posture.

Musculoskeletal disorders of the cervical spine have increased considerably in recent times. To understand the effects of various biomechanical factors, quantifying the differences in disc loads, motion, and muscle force/activity is necessary. The kinematic, kinetic, or muscle response may vary in a neutral posture due to interindividual differences in segmental mass, cervical disc stiffness, and muscle strength. Therefore, our study aimed to develop an inverse dynamic model of the cervical spine, estimate the differences in disc loads, translations, intradiscal pressure, and muscle force/activity in a neutral posture and compare these results with data available in the literature. A head-neck complex with nine segments (head, C1-T1) was developed with joints having three rotational and three translational degrees of freedom, 517 nonlinear ligament fibers, and 258 muscle fascicles. A sensitivity analysis was performed to calculate the effect of segmental mass (5th to 95th percentile), translational disc stiffness (0.5-1.5), and muscle strength (0.5-1.5) on the cervical disc loads (C2-C3 to C7-T1), disc translations, intradiscal pressure, and muscle force/activity in a neutral posture. In addition, two axial external load conditions (0 and 40 N) were also considered on the head. The estimated intradiscal pressures (0.2-0.56 MPa) at 0 N axial load were comparable to in vivo measurements found in the literature, whereas at 40 N, the values were 0.39-0.93 MPa. With increased segmental mass (5th to 95th), the disc loads, translations, and muscle forces/activities increased to 69% at 0 N and 34% at 40 N axial load. With increased disc stiffness (0.5-1.5), the maximum differences in axial (<1%) and shear loads (4%) were trivial; however, the translations were reduced by 67%, whereas the differences in individual muscle group forces/activities varied largely. With increased muscle strength (0.5-1.5), the muscle activity decreased by 200%. For 40 vs. 0 N, the differences in disc loads, translations, and muscle forces/activities were in the range of 52-129%. Significant differences were estimated in disc loads, translations, and muscle force/activity in the normal population, which could help distinguish between normal and pathological cervical spine conditions.

Biomechanical Effect of Hybrid Dynamic Stabilization Implant on the Segmental Motion and Intradiscal Pressure in Human Lumbar Spine.

The hybrid dynamic stabilization system, Dynesys-Transition-Optima, represents a novel pedicle-based construct for the treatment of lumbar degenerative disease. The theoretical advantage of this system is to stabilize the treated segment and preserve the range of motion within the adjacent segment while potentially decreasing the risk of adjacent segment disease following lumbar arthrodesis. Satisfactory short-term outcomes were previously demonstrated in the Dynesys-Transition-Optima system. However, long-term follow-up reported accelerated degeneration of adjacent segments and segmental instability above the fusion level. This study investigated the biomechanical effects of the Dynesys-Transition-Optima system on segment motion and intradiscal pressure at adjacent and implanted levels. Segmental range of motion and intradiscal pressure were evaluated under the conditions of the intact spine, with a static fixator at L4-5, and implanted with DTO at L3-4 (Dynesys fixator) and L4-5 (static fixator) by applying the loading conditions of flexion/extension (±7.5 Nm) and lateral bending (±7.5 Nm), with/without a follower preload of 500 N. Our results showed that the hybrid Dynesys-Transition-Optima system can significantly reduce the ROM at the fusion level (L4-L5), whereas the range of motion at the adjacent level (L3-4) significantly increased. The increase in physiological loading could be an important factor in the increment of IDP at the intervertebral discs at the lumbar spine. The Dynesys-Transition-Optima system can preserve the mobility of the stabilized segments with a lesser range of motion on the transition segment; it may help to prevent the occurrence of adjacent segment degeneration. However, the current study cannot cover all the issues of adjacent segmental diseases. Future investigations of large-scale and long-term follow-ups are needed.

Biomechanical Properties of Paraspinal Muscles Influence Spinal Loading-A Musculoskeletal Simulation Study.

Paraspinal muscles are vital to the functioning of the spine. Changes in muscle physiological cross-sectional area significantly affect spinal loading, but the importance of other muscle biomechanical properties remains unclear. This study explored the changes in spinal loading due to variation in five muscle biomechanical properties: passive stiffness, slack sarcomere length (SSL), in situ sarcomere length, specific tension, and pennation angle. An enhanced version of a musculoskeletal simulation model of the thoracolumbar spine with 210 muscle fascicles was used for this study and its predictions were validated for several tasks and multiple postures. Ranges of physiologically realistic values were selected for all five muscle parameters and their influence on L4-L5 intradiscal pressure (IDP) was investigated in standing and 36° flexion. We observed large changes in IDP due to changes in passive stiffness, SSL, in situ sarcomere length, and specific tension, often with interesting interplays between the parameters. For example, for upright standing, a change in stiffness value from one tenth to 10 times the baseline value increased the IDP only by 91% for the baseline model but by 945% when SSL was 0.4 µm shorter. Shorter SSL values and higher stiffnesses led to the largest increases in IDP. More changes were evident in flexion, as sarcomere lengths were longer in that posture and thus the passive curve is more influential. Our results highlight the importance of the muscle force-length curve and the parameters associated with it and motivate further experimental studies on in vivo measurement of those properties.

Biomechanical Analysis of the Reasonable Cervical Range of Motion to Prevent Non-Fusion Segmental Degeneration After Single-Level ACDF.

The compensatory increase in intervertebral range of motion (ROM) after cervical fusion can increase facet joint force (FJF) and intradiscal pressure (IDP) in non-fusion segments. Guiding the post-ACDF patient cervical exercise within a specific ROM (defined as reasonable ROM) to offset the increase in FJF and IDP may help prevent segmental degeneration. This study aimed to determine the reasonable total C0-C7 ROM without an increase in FJF and IDP in non-fusion segments after anterior cervical discectomy and fusion (ACDF). A three-dimensional intact finite element model of C0-C7 generated healthy cervical conditions. This was modified to the ACDF model by simulating the actual surgery at C5-C6. A 1.0 Nm moment and 73.6 N follower load were applied to the intact model to determine the ROMs. A displacement load was applied to the ACDF model under the same follower load, resulting in a total C0-C7 ROM similar to that of the intact model. The reasonable ROMs in the ACDF model were calculated using the fitting function. The results indicated that the intervertebral ROM of all non-fusion levels was increased in the ACDF model in all motion directions. The compensatory increase in ROM in adjacent segments (C4/5 and C6/7) was more significant than that in non-adjacent segments, except for C3/4 during lateral bending. The intervertebral FJF and IDP of C0-C7 increased with increasing ROM. The reasonable ROMs in the ACDF model were 42.4°, 52.6°, 28.4°, and 42.25° in flexion, extension, lateral bending, and axial rotation, respectively, with a decreased ROM of 4.4-7.2%. The postoperative increase in FJF and IDP in non-fusion segments can be canceled out by reducing the intervertebral ROM within reasonable ROMs. This study provided a new method to estimate the reasonable ROMs after ACDF from a biomechanical perspective, and further in vitro and clinical studies are needed to confirm this.

How Does the Rib Cage Affect the Biomechanical Properties of the Thoracic Spine? A Systematic Literature Review.

The vast majority of previous experimental studies on the thoracic spine were performed without the entire rib cage, while significant contributive aspects regarding stability and motion behavior were shown in several other studies. The aim of this literature review was to pool and increase evidence on the effect of the rib cage on human thoracic spinal biomechanical characteristics by collating and interrelating previous experimental findings in order to support interpretations of in vitro and in silico studies disregarding the rib cage to create comparability and reproducibility for all studies including the rib cage and provide combined comparative data for future biomechanical studies on the thoracic spine. After a systematic literature search corresponding to PRISMA guidelines, eleven studies were included and quantitatively evaluated in this review. The combined data exhibited that the rib cage increases the thoracic spinal stability in all motion planes, primarily in axial rotation and predominantly in the upper thorax half, reducing thoracic spinal range of motion, neutral zone, and intradiscal pressure, while increasing thoracic spinal neutral and elastic zone stiffness, compression resistance, and, in a neutral position, the intradiscal pressure. In particular, the costosternal connection was found to be the primary stabilizer and an essential determinant for the kinematics of the overall thoracic spine, while the costotransverse and costovertebral joints predominantly reinforce the stability of the single thoracic spinal segments but do not alter thoracic spinal kinematics. Neutral zone and neutral zone stiffness were more affected by rib cage removal than the range of motion and elastic zone stiffness, thus also representing the essential parameters for destabilization of the thoracic spine. As a result, the rib cage and thoracic spine form a biomechanical entity that should not be separated. Therefore, usage of entire human non-degenerated thoracic spine and rib cage specimens together with pure moment application and sagittal curvature determination is recommended for future in vitro testing in order to ensure comparability, reproducibility, and quasi-physiological validity.

Giraffes and hominins: reductionist model predictions of compressive loads at the spine base for erect exponents of the animal kingdom.

In humans, compressive stress on intervertebral discs is commonly deployed as a measurand for assessing the loads that act within the spine. Examining this physical quantity is crucially beneficial: the intradiscal pressure can be directly measured in vivo in humans, and is immediately related to compressive stress. Hence, measured intradiscal pressure data are very useful for validating such biomechanical animal models that have the spine incorporated, and can, thus, compute compressive stress values. Here, we use human intradiscal pressure data to verify the predictions of a reductionist spine model, which has in fact only one joint degree of freedom. We calculate the pulling force of one lumped anatomical structure that acts past this (intervertebral) joint at the base of the spine, lumbar in hominins, cervical in giraffes, to compensate the torque that is induced by the weight of all masses located cranially to the base. Given morphometric estimates of the human and australopith trunks, respectively, and the giraffe's neck, as well as the respective structures' lever arms and disc areas, we predict, for all three species, the compressive stress on the intervertebral disc at the spine base, while systematically varying the angular orientation of the species' spinal columns with respect to gravity. The comparison between these species demonstrates that hominin everyday compressive disc stresses are lower than those in big quadrupedal animals. Within each species, erecting the spine from being bent forward by, for example, thirty degrees to fully upright posture reduces the compressive disc stress roughly to a third. We conclude that erecting the spine immediately allows the carrying of extra loads of the order of body weight, and yet the compressive disc stress is lower than in a moderately forward-bent posture with no extra load.

A novel coupled musculoskeletal finite element model of the spine - Critical evaluation of trunk models in some tasks.

Spine musculoskeletal (MS) models make simplifying assumptions on the intervertebral joint degrees-of-freedom (rotational and/or translational), representation (spherical or beam-like joints), and properties (linear or nonlinear). They also generally neglect the realistic structure of the joints with disc nuclei/annuli, facets, and ligaments. We aim to develop a novel MS model where trunk muscles are incorporated into a detailed finite element (FE) model of the ligamentous T12-S1 spine thus constructing a gold standard coupled MS-FE model. Model predictions are compared under some tasks with those of our earlier spherical joints, beam joints, and hybrid (uncoupled) MS-FE models. The coupled model predicted L4-L5 intradiscal pressures (R2 â‰… 0.97, RMSE â‰… 0.27 MPa) and L1-S1 centers of rotation (CoRs) in agreement to in vivo data. Differences in model predictions grew at larger trunk flexion angles; at the peak (80°) flexion the coupled model predicted, compared to the hybrid model, much smaller global/local muscle forces (~38%), segmental (~44%) and disc (~22%) compression forces but larger segmental (~9%) and disc (~17%) shear loads, ligament forces at the lower lumbar levels (by up to 57%) and facet forces at all levels. The spherical/beam joints models predicted much greater muscle forces and segmental loads under larger flexion angles. Unlike the spherical joints model with fixed CoRs, the beam joints model predicted CoRs closer (RMSE = 2.3 mm in flexion tasks) to those of the coupled model. The coupled model offers a great potential for future studies towards improvement of surgical techniques, management of musculoskeletal injuries and subject-specific simulations.

Biomechanical analysis of lumbar fusion with proximal interspinous process device implantation.

Lumbar spinal fusion may cause adjacent segment degeneration (ASD) in the long term. Recently, inserting an interspinous process device (IPD) proximal to the fusion has been proposed to prevent ASD. The aim of this study was to investigate the biomechanics of lumbar fusion with proximal IPD implantation (LFPI) under both static loads and whole body vibration (WBV). A previously validated finite element (FE) model of the L1-5 lumbar spine was modified to simulate L4-5 fusion. Three different IPDs (Coflex-F, Wallis and DIAM) were inserted at the L3-4 segment of the fusion model to construct the LFPI models. The intact and surgical FE models were analyzed under static loads and WBV, respectively. Under static loading conditions, LFPI decreased range of motion (ROM) and intradiscal pressure (IDP) at the transition segment L3-4 compared with the fusion case. At the segment (L2-3) adjacent to the transition level, LFPI induced higher motion and IDP than rigid fusion. Under WBV, vibration amplitudes of the L3-4 IDP and L4-5 facet joint force (FJF) decreased by more than 54.3% after surgery. The LFPI model with the DIAM system offered the most comparable biomechanics to the intact model under static loads, and decreased the dynamic responses of the L4-5 FJF under WBV. The LFPI model with the Wallis and Coflex-F systems could stabilize the transition segment, and decrease dynamic responses of the L3-4 IDP. The DIAM system may be more suitable in LFPI.

Biomechanical effects of uncinate process excision in cervical disc arthroplasty.

BACKGROUND: Studies on the role of uncinate process have been limited to responses of the intact spine and patient's outcomes, and procedures to perform the excision. The aim of this study was to determine the role of uncinate process on the biomechanical response at the index and adjacent levels in three artificial discs used in cervical disc arthroplasty. METHODS: A validated finite element model of cervical spine was used. Flexion, extension, and lateral moments and follower load were applied to Bryan, Mobi-C, and Prestige LP artificial discs at C5-C6 level with and without uncinate process. Ranges of motion at index level and adjacent caudal and cranial segments, intradiscal pressures at adjacent segments, and facet loads at index level and adjacent segments were obtained. Data were normalized with respect to the preservation of uncinate process. FINDINGS: Uncinate process removal increased motions up to 27% at index and decreased up to 10% at adjacent levels, decreased disc pressures up to 14% at adjacent segments, decreased facet loads at adjacent segments up to 14%, while at index level, change in loads depended on mode and arthroplasty, with Mobi-C responding with up to 51% increase and Bryan disc up to 11% decrease, while Prestige LP increased loads by 17% in extension and decreased by 9%% in lateral bending. INTERPRETATION: As surgical selection is based on morphology and surgeon's experience, the present computational findings provide quantitative information for an optimal choice of the device and procedure, while further studies (in vitro/clinical) would be required.

Evaluation of Spinous Process Tethering at the Proximal End of Rigid Constructs: In Vitro Range of Motion and Intradiscal Pressure at Instrumented and Adjacent Levels.

BACKGROUND: Adult spinal deformity surgery requires use of long thoracolumbar instrumentation, which is associated with risk of postoperative proximal junctional kyphosis (PJK). Tethering has been used in spinal surgery but not around the spinous process (SP) in the context of preventing PJK. METHODS: Researchers applied a nondestructive hybrid loading protocol to 7 T8-L2 cadaveric specimens in flexion-extension, lateral bending, and axial rotation (AR). A rigid construct (pedicle screws and rods) and 1- and 2-level SP constructs were tested, as was a hand-tie technique. SP tethering (SPT) constructs use clamps on both sides of the SP; SPT helix constructs use 1 clamp and wrap around the SP. RESULTS: All tether constructs showed greater motion at the instrumented level and less motion at adjacent levels compared to rigid constructs. In AR, 1- and 2-level SPT constructs restricted first instrumented level motion to a greater extent when compared with other tether constructs (P ≤ .05). Passing the band through the T10 SP did not produce significant biomechanical differences compared to passing it through the T9-T10 interspinous ligament (P > .05). Hand-tied constructs demonstrated more motion compared to tensioned constructs (P > .05). Intradiscal pressure results corroborated motion data. CONCLUSIONS: SPT at the proximal end of a rigid construct produced more favorable biomechanical outcomes at instrumented and adjacent levels than were seen with a completely rigid construct. Clinical research is needed to determine whether these methods reduce the risk of PJK among patients. LEVEL OF EVIDENCE: 3. CLINICAL RELEVANCE: This work sheds light on the biomechanical stability of proximal tethering constructs in an effort to enhance the surgeon's ability to reduce rates of proximal junctional kyphosis and failure in thoracolumbar spinal fusion surgery.

In vitro Analysis of the Intradiscal Pressure of the Thoracic Spine.

The hydrostatic pressure of the nucleus pulposus represents an important parameter in the characterization of spinal biomechanics, affecting the segmental stability as well as the stress distribution across the anulus fibrosus and the endplates. For the development of experimental setups and the validation of numerical models of the spine, intradiscal pressure (IDP) values under defined boundary conditions are therefore essential. Due to the lack of data regarding the thoracic spine, the purpose of this in vitro study was to quantify the IDP of human thoracic spinal motion segments under pure moment loading. Thirty fresh-frozen functional spinal units from 19 donors, aged between 43 and 75 years, including all segmental levels from T1-T2 to T11-T12, were loaded up to 7.5 Nm in flexion/extension, lateral bending, and axial rotation. During loading, the IDP was measured using a flexible sensor tube, which was inserted into the nucleus pulposus under x-ray control. Pressure values were evaluated from third full loading cycles at 0.0, 2.5, 5.0, and 7.5 Nm in each motion direction. Highest IDP increase was found in flexion, being significantly (p < 0.05) increased compared to extension IDP. Median pressure values were lowest in lateral bending while exhibiting a large variation range. Flexion IDP was significantly increased in the upper compared to the mid- and lower thoracic spine, whereas extension IDP was significantly higher in the lower compared to the upper thoracic spine, both showing significant (p < 0.01) linear correlation with the segmental level at 7.5 Nm (flexion: r = -0.629, extension: r = 0.500). No significant effects of sex or age were detected, however trends toward higher IDP in specimens from female donors and decreasing IDP with increasing age, potentially caused by fibrotic degenerative changes in the nucleus pulposus tissue. Sagittal and transversal cuttings after testing revealed possible relationships between nucleus pulposus quality and pressure moment characteristics, overall leading to low or negative intrinsic IDP and non-linear pressure-moment behavior in case of fibrotic tissue alterations. In conclusion, this study provides insights into thoracic spinal IDP and offers a large dataset for the validation of numerical models of the thoracic spine.