A study of vertebra and disc geometric relations of the human cervical and lumbar spine

Finite Element Analysis Comparing the Biomechanical Parameters in Multilevel Posterior Cervical Instrumentation Model Involving Lateral Mass Screw versus Transpedicular Screw Fixation at the C7 Vertebra

STUDY DESIGN

Basic research.

PURPOSE

This finite element (FE) analysis (FEA) aimed to compare the biomechanical parameters in multilevel posterior cervical fixation with the C7 vertebra instrumented by two techniques: lateral mass screw (LMS) vs. transpedicular screw (TPS).

OVERVIEW OF LITERATURE

Very few studies have compared the biomechanics of different multilevel posterior cervical fixation constructs.

METHODS

Four FE models of multilevel posterior cervical fixation were created and tested by FEA in various permutations and combinations. Generic differences in fixation were determined, and the following parameters were assessed: (1) maximum moment at failure, (2) maximum angulation at failure, (3) maximum stress at failure, (4) point of failure, (5) intervertebral disc stress, and (6) influence of adding a C2 pars screw to the multilevel construct.

RESULTS

The maximum moment at failure was higher in the LMS fixation group than in the TPS group. The maximum angulation in flexion allowed by LMS was higher than that by TPS. The maximum strain at failure was higher in the LMS group than in the TPS group. The maximum stress endured before failure was higher in the TPS group than in the LMS group. Intervertebral stress levels at C6-C7 and C7-T1 intervertebral discs were higher in the LMS group than in the TPS group. For both models where C2 fixation was performed, lower von Mises stress was recorded at the C2-C3 intervertebral disc level.

CONCLUSIONS

Ending a multilevel posterior cervical fixation construct with TPS fixation rather than LMS fixation at the C7 vertebra provides a stiff and more constrained construct system, with higher stress endurance to compressive force. The constraint and durability of the construct can be further enhanced by adding a C2 pars screw in the fixation system.

Biomechanical analysis of cervical spine (C2-C7) at different flexed postures

Musculoskeletal diseases are often related with postural changes in the neck region that can be caused by prolonged cervical flexion. This is one of the contributing factors. When determining the prevalence, causes, and related risks of neck discomfort, having a solid understanding of the biomechanics of the cervical spine (C1-C7) is absolutely necessary. The objective of this study is to make predictions regarding the intervertebral disc (IVD) stress values across C2-C7 IVD, the ligament stress, and the variation at 0°, 15°, 30°, 45°, and 60° of cervical neck angle using finite element analysis (FEA). In order to evaluate the mechanical properties of the cervical spine (particularly, C2-C7), this investigation makes use of computed tomography (CT) scans to develop a three-dimensional FEA model of the cervical spine. A preload of 50 N compression force was applied at the apex of the C2 vertebra, and all degrees of freedom below the C7 level were constrained. The primary objective of this investigation is to assess the distribution of von Mises stress within the IVDs and ligaments spanning C2-C7 at various flexion angles: 0°, 15°, 30°, 45°, and 60°, utilizing FEA. The outcomes derived from this analysis were subsequently compared to previously published experimental and FEA data to validate the model's ability to replicate the physiological motion of the cervical spine across different flexion angles.

Tension-activated nanofiber patches delivering an anti-inflammatory drug improve repair in a goat intervertebral disc herniation model

Conventional microdiscectomy treatment for intervertebral disc herniation alleviates pain but does not repair the annulus fibrosus, resulting in a high incidence of recurrent herniation and persistent dysfunction. The lack of repair and the acute inflammation that arise after injury can further compromise the disc and result in disc-wide degeneration in the long term. To address this clinical need, we developed tension-activated repair patches (TARPs) for annulus fibrosus repair and local delivery of the anti-inflammatory factor anakinra (a recombinant interleukin-1 receptor antagonist). TARPs transmit physiologic strain to mechanically activated microcapsules embedded within the patch, which release encapsulated bioactive molecules in direct response to spinal loading. Mechanically activated microcapsules carrying anakinra were loaded into TARPs, and the effects of TARP-mediated annular repair and anakinra delivery were evaluated in a goat model of annular injury in the cervical spine. TARPs integrated with native tissue and provided structural reinforcement at the injury site that prevented aberrant disc-wide remodeling resulting from detensioning of the annular fibrosus. The delivery of anakinra by TARP implantation increased matrix deposition and retention at the injury site and improved maintenance of disc extracellular matrix. Anakinra delivery additionally attenuated the inflammatory response associated with TARP implantation, decreasing osteolysis in adjacent vertebrae and preserving disc cellularity and matrix organization throughout the annulus fibrosus. These results demonstrate the therapeutic potential of TARPs for the treatment of intervertebral disc herniation.

Finite Element Analysis of Head-Neck Kinematics in Rear-End Impact Conditions with Headrest

A detailed three-dimensional (3D) head-neck (C0-C7) finite element (FE) model was developed and used to dictate the motions of each cervical spinal segment under static physiological loadings of flexion and extension with a magnitude of 1.0 Nm and rear-end impacts. In this dynamic study, a rear-end impact pulse was applied to C7 to create accelerations of 4.5 G and 8.5 G. The predicted segmental motions and displacements of the head were in agreement with published results under physiological loads of 1.0 Nm. Under rear-end impact conditions, the effects of peak pulse acceleration and headrest angles on the kinematic responses of the head-neck complex showed rates of increase/decrease in the rotational motion of various cervical spinal segments that were different in the first 200 ms. The peak flexion rotation of all segments was lower than the combined ROM of flexion and extension. The peak extension rotation of all segments showed variation compared to the combined ROM of flexion and extension depending on G and the headrest angle. A higher acceleration of C7 increased the peak extension angle of lower levels, but the absolute increase was restricted by the distance between the head and the headrest. A change in the headrest angle from 45° to 30° resulted in a change in extension rotation at the lower C5-C6 segments to flexion rotation, which further justified the effectiveness of having distance between the head and the headrest. This study shows that the existing C0-C7 FE model is efficient at defining the gross reactions of the human cervical spine under both physiological static and simulated whiplash circumstances. The fast rate of changes in flexion and extension rotation of various segments may result in associated soft tissues and bony structures experiencing tolerances beyond their material characteristic limits. It is suggested that a proper location and angle of the headrest could effectively prevent the cervical spine from injury in traumatic vehicular accidents.

Development, calibration and validation of a comprehensive customizable lumbar spine FE model for simulating fusion constructs

Instrumentation alters the biomechanics of the spine, and therefore prediction of all output quantities that have critical influence post-surgically is significant for engineering models to aid in clinical predictions. Geometrical morphological finite element models can bring down the development time and cost of custom intact and instrumented models and thus aids in the better inference of biomechanics of surgical instrumentation on patient-specific diseased spine segments. A comprehensive hexahedral morphological lumbosacral finite element model is developed in this work to predict the range of motions, disc pressures, and facet contact forces of the intact and instrumented spine. Facet contact forces are needed to predict the impact of fusion surgeries on adjacent facet contacts in bending, axial rotation, and extension motions. Extensive validation in major physiological loading regimes of the pure moment, pure compression, and combined loading is undertaken. In vitro, experimental corridor results from six different studies reported in the literature are compared and the generated model had statistically significant comparable values with these studies. Flexion, extension and bending moment rotation curves of all segments of the developed model were favourable and within two separately established experimental corridor windows as well as recent simulation results. Axial torque moment rotation curves were comparable to in vitro results for four out of five lumbar functional units. The facet contact force results also agreed with in vitro experimental results. The current model is also computationally efficient with respect to contemporary models since it uses significantly smaller number of elements without losing the accuracy in terms of response prediction. This model can further be used for predicting the impact of different instrumentation techniques on the lumbar vertebral column.

UBC Neck C4-C5: An Anatomically and Biomechanically Accurate Surrogate C4-C5 Functional Spinal Unit

Millions of people worldwide suffer from spinal cord injuries (SCIs) and traumatic brain injuries (TBIs) annually. Safety devices meant to protect against SCIs and TBIs, such as helmets, airbags, seat belts, and compliant floors are often evaluated with the use of anthropometric test devices (ATD s); however, there are currently no neck surrogates appropriate for the multiplane loading that often occurs in real-world scenarios leading to injury. As such, our objective in this study was to design and create an anatomically correct functional spinal unit (FSU) that produces a repeatable and biofidelic response to lateral bending, axial rotation, and quasistatic flexion-extension motion. This is a critical step in developing a biofidelic omnidirectional surrogate that can be used in future evaluations of safety devices in transportation, occupational, and sports settings. To create a biofidelic C4-C5 FSU, anatomically accurate C4 and C5 vertebrae were designed and manufactured using a 3D printer using geometry derived from the CT scans of a healthy 31-year-old male. Potential intervertebral disc and ligament surrogate materials were tested in compression and tension, respectively, to select representative materials for the surrogate intervertebral disc and cervical ligaments. The C4-C5 FSU was assembled and tested repeatedly in quasistatic flexion-extension, axial rotation, and lateral bending. Kinematic results were captured and compared to previously published cadaver data. The surrogate disc showed excellent Biofidelity (ISO/TR 9790) in compression, and the surrogate ligaments were within 25 N/mm of linear cadaveric stiffness ranges. The assembled FSU named UBC Neck C4-C5 showed good biofidelity under quasistatic axial rotation, lateral bending, flexion-extension, and coupled motion (ISO/TR 9790). However, the instantaneous centre of rotation was not similar to ex vivo or in vivo published studies. The UBC Neck C4-C5 FSU resulted in good biofidelity ratings and will inform future construction of a full surrogate neck to be used in the testing of head and neck safety equipment.

Improvement and validation of a female finite element model of the cervical spine

Although the cervical spine supports and controls the kinematics of the head, it is vulnerable to injuries during mechanical loading. Severe injuries often result in damage to the spinal cord, leading to significant ramifications. The role of gender in determining the outcome of such injuries has been established as significant. In order to better understand the essential mechanics and develop treatments or preventative measures, various forms of research have been conducted. Computational modelling is one of the most useful and extensively utilised methods, as it provides information that would otherwise be difficult to obtain. As such, the primary goal of this research is to create a new finite element of the female cervical spine that will more accurately represent the group most affected by such injuries. This work is a continuation of a previous study where a model was created from the computer tomography scans of a 46-year-old female. A functioning spinal unit consisting of the C6-C7 segment was simulated as a validation procedure. The experimental data obtained from cadaveric specimens, that assessed the range of motion of different cervical segments in flexion-extension, axial rotation, and lateral bending, was used to validate the reduced model.

Impact of extracellular matrix and collagen network properties on the cervical intervertebral disc response to physiological loads: A parametric study

Current intervertebral disc finite element models are hard to validate since they describe multi-physical phenomena and contain a huge number of material properties. This work aims to simplify numerical validation/identification studies by prioritizing the sensitivity of intervertebral disc behavior to mechanical properties. A 3D fiber-reinforced hyperelastic model of a C6-C7 intervertebral disc is used to carry out the parametric study. 10 parameters describing the extracellular matrix and the collagen network behaviors are included in the parametric study. The influence of varying these parameters on the disc response is estimated during physiological movements of the head, including compression, lateral bending, flexion, and axial rotation. The obtained results highlight the high sensitivity of the disc behavior to the stiffness of the annulus fibrosus extracellular matrix for all the studied loads with a relative increase in the disc apparent stiffness by 67% for compression and by 57% for axial rotation when the annulus stiffness increases from 0.4 to 2 MPa. It is also shown that varying collagen network orientation, stiffness, and stiffening in the studied configuration range have a noticeable effect on rotational motions with a relative apparent stiffness difference reaching 6.8%, 10%, and 22%, respectively, in lateral bending. However, the collagen orientation does not affect disc response to axial load.

The influence of the viscoelastic property of polycarbonate urethane as an artificial disc core material under various physiological motions at the L4-L5 level

Intervertebral disc (IVD) degeneration is one of the musculoskeletal disorders due to the Degenerative Disc Disease (DDD), that cause low back pain (LBP) and leads to a reduced range of motion. Spinal fusion and arthroplasty are the other surgical procedures that could replace the disc affected by DDD against artificial disc replacement (ADR). This study aims to analyse the biomechanical behaviour of proposed core material as Polycarbonate Urethane (PCU) in the L4-L5 lumbar segment for ADR with Ti-6Al-4V and Co-28Cr-6M as endplate materials and compare it to the performance of an ultra-high molecular weight polyethylene (UHMWPE) core. Finite element methods have been approached to measure the overall stress distribution along with other physiological motions like Flexion (FLEX), Extension (EXT), Axial rotation (AR) and Lateral bending (LB), respectively. Preload of 450 N compressive load, 8 N-m for Flex, 6 N-m for EXT, 6 N-m for AR and 4 N-m for LB are applied. It could be concluded that Ti-6Al-4V - PCU and Co-28Cr-6M - PCU is the best composition for the ADR for the L4-L5 level.

Biomechanical Evaluation of a Novel S-Type, Dynamic Zero-Profile Cage Design for Anterior Cervical Discectomy and Fusion with Variations in Bone Graft Shape: A Finite Element Analysis

BACKGROUND

Variations in cage design, material, and graft shape can affect osteointegration and adjacent segment range of motion (ROM) and stress after anterior cervical discectomy and fusion (ACDF) surgery. This study aimed to evaluate the biomechanical properties of a novel dynamic cervical cage design in both titanium (Ti) and polyether ether ketone (PEEK) with variations in bone graft shape using a single level ACDF (FE) model.

METHODS

A 3-dimensional C3-C6 FE model was developed using computed tomography scan data from a healthy male subject. The novel S-shaped dynamic interbody fusion cage with a zero-profile fixation was inserted at the C4-C5 level with 4 different bone graft shapes (square, circular, rectangular, and elliptical). Changes in segmental ROM and maximum von Mises stresses at the fusion and adjacent segments were analyzed.

RESULTS

Both Ti and PEEK cages showed decreased ROM at the fusion and adjacent levels for all shapes of bone graft when compared with the intact spine model. The elliptical graft, for both Ti and PEEK cages, showed a lower percentage of reduction in segmental ROM at the fusion and adjacent levels (0%-5.6%) when compared with other graft shapes (0%-12%). Maximum stresses at the fusion level were lowest in Ti cage with elliptical graft (229.8-347.6 MPa) when compared with other shapes (241.2-476.2 MPa) in flexion, extension, and lateral bending. For the bone graft, maximum stresses were highest on the elliptical-shaped bone graft in flexion and extension in the Ti cage, and in flexion and lateral bending in the PEEK cage.

CONCLUSIONS

Both Ti and PEEK cages showed decreased ROM at the fusion and adjacent levels for all shapes of bone graft when compared with the intact spine model. In the Ti and PEEK dynamic cages, the elliptical shape bone graft showed decreased stress on the cage and increased stress on the bone graft. Further experimental and clinical studies are needed to confirm these encouraging biomechanical results of this novel dynamic, zero-profile fusion device with elliptical bone graft in ACDF surgery.

Embodiment of intra-abdominal pressure in a flexible multibody model of the trunk and the spinal unloading effects during static lifting tasks

The role of intra-abdominal pressure (IAP) in spinal load reduction has remained controversial, partly because previous musculoskeletal models did not introduce the pressure generating mechanism. In this study, an integrated computational methodology is proposed to combine the IAP change with core muscle activations. An ideal gas relationship was introduced to calculate pressure distribution within the abdominal cavity. Additionally, based on flexible multibody dynamics, a muscle membrane element was developed by incorporating the muscular fiber deformation, inter-fiber stiffness, and volume constancy. This element was then utilized in discretizing the diaphragm and transversus abdominis, forming an IAP-muscle coupling system of the abdominal cavity. Based on this methodology, a forward dynamic simulation of spinal flexion was presented to examine the unloading effect of abdominal breathing. The results confirm that core muscle contraction during the abdominal breathing cycle can substantially reduce the forces of spinal compression together with trunk extensor muscles, and this effect is more pronounced when the IAP increase is produced by contraction of the transversus abdominis. This unloading effect still holds even with the co-activation of other abdominal muscles, providing a potential choice when designing trunk movements during weight-lifting tasks.

Development of a finite element neck model for head-first compressive impacts: Toward the assessment of motorcycle neck protective equipment

Head-first compressive impacts occur in motorcycle crashes and may result in serious to fatal neck injuries to riders. Equipment to protect the riders' necks from these injuries are available in the market; however, their effectiveness in reducing injury risk is not clear, either due to the lack of scientific evidences or assessment with any prevalently accepted standard. This paper presents a finite element ligamentous neck model, developed as a computationally efficient tool, for future use in the computational phase of assessment process of neck protective equipment. The 3D cervical spine was generated using the mean statistical dimensions of vertebrae and proposed constitutive models, provided in the scientific literature. Ligaments, for the vertebra-vertebra and Hybrid III head-vertebra ligamentous joints, were introduced with the aid of published anatomical descriptions. For validation, the response of the head-neck system under compressive loadings and the flexion-extension bending stiffness of the neck at the segment level were compared against experimental data. The advanced CORrelation and Analysis (CORA) algorithm was applied on the validation responses to assess biofidelity of the model. The results indicate that the model is functional and meets ISO/TR9790 standard as a "good" biofidelic model.

The Diagnostic Value of an X-ray-based Scoring System for Degeneration of the Cervical Spine: A Reproducibility and Validation Study

Background

In interventional pain medicine, cervical facet joint (CFJ) pain is commonly treated with CFJ denervation techniques, almost automatically assuming degeneration of the CFJs as an important cause of CFJ pain. A standard cervical X‐ray is still commonly used in the clinical evaluation of patients suspected for CFJ degeneration. Although degenerative features can be visualized by different radiological imaging techniques, the relation between radiological degenerative features of the cervical spine and pain remains controversial. Paramount in order to estimate the clinical usefulness of a radiological imaging is to establish the reproducibility of the radiological scoring system. A reproducible and clinically feasible diagnostic scoring system was developed to estimate cervical degeneration on standard cervical X‐rays.

Materials and Methods

A reproducibility study for the interpretation of degenerative abnormalities on standard cervical X‐rays was performed, using a dichotomous outcome (degenerative abnormalities present Yes/No). The estimation of intervertebral disc height loss on standard cervical X‐rays was validated with computed tomography (CT) scan measurements.

Results

Five radiological degenerative features on standard cervical X‐rays (disc height loss, anterior vertebral osteophytes, posterior vertebral osteophytes, vertebral end plate sclerosis, and uncovertebral osteoarthritis) showed a substantial to excellent reproducibility (kappa value ≥ 0.60). The qualitative definition of disc height loss used in the reproducibility study showed a substantial agreement with the actual measurements of disc height loss on CT scan (kappa value = 0.69).

Conclusion

Subjective judgment of a cervical standard X‐ray is a reproducible method to demonstrate degenerative abnormalities of the cervical spine.

Morphology and growth of the pediatric lumbar vertebrae

BACKGROUND CONTEXT

The majority of existing literature describing pediatric lumbar vertebral morphology are limited to characterization of the vertebral bodies, pedicles, and spinal canal and no study has described the rates of growth for any lumbar vertebral structure. While it is known that growth of the lumbar vertebrae results in changes in vertebral shape, the dimension ratios used to quantify these shape changes do not represent the 3D morphology of the vertebral structures. Additionally, many of the previous evaluations of growth and shape are purely descriptive and do not investigate sexual dimorphism or variations across vertebral levels.

PURPOSE

This study aims to establish a database of pediatric lumbar vertebra dimension, growth, and shape data for subjects between and ages of 1 and 19 years.

STUDY DESIGN

A retrospective study of computed tomography (CT) data.

METHODS

Retrospective, abdominal, CT scans of 102 skeletally normal pediatric subjects (54 males, 48 females) between the ages of 1 and 19 years were digitally reconstructed and manually segmented. Thirty surface landmark points (LMPs), 30 vertebral measurements, the centroid size, centroid location, and the local orientation were collected for each lumbar vertebra along with the centroid size of the LMPs comprising each subject's full lumbar spine and their intervertebral disc (IVD) heights. Nonparametric statistics were used to compare dimension values across vertebral levels and between sexes. Linear models with age as the independent variable were used to characterize dimension growth for each sex and vertebral level. Age-dependent quadratic equations were fit to LMP distributions resulting from a generalized Procrustes analysis (GPA) of the vertebrae and fixed effects models were used to investigate differences in model coefficients across levels and between sexes.

RESULTS

Intervertebral level dimension differences were observed across all vertebral structures in both sexes while pedicle widths and IVDs heights were the only measurements found to be sexually dimorphic. Dimension growth rates generally varied across vertebral levels and the growth rates of males were typically larger than those of females. Differences between male and female vertebral shapes were also found for all lumbar vertebral structures.

CONCLUSIONS

To the authors' knowledge, this is the first study to report growth rates for the majority of pediatric lumbar vertebral structures and the first to describe the 3D age-dependent shapes of the pediatric lumbar spine and vertebrae. In addition to providing a quantitative database, the dimension, growth, and shape data reported here would have applications in medical device design, surgical planning, surgical training, and biomechanical modeling.

The biomechanical study of cervical spine: A Finite Element Analysis

The biomechanical study helps us to understand the mechanics of the human cervical spine. A three dimensional Finite Element (FE) model for C3 to C6 level was developed using computed tomography (CT) scan data to study the mechanical behaviour of the cervical spine. A moment of 1 Nm was applied at the top of C3 vertebral end plate and all degrees of freedom of bottom end plate of C6 were constrained. The physiological motion of the cervical spine was validated using published experimental and FE analysis results. The von Mises stress distribution across the intervertebral disc was calculated along with range of motion. It was observed that the predicted results of functional spine units using FE analysis replicate the real behaviour of the cervical spine.

Experimental evaluation of precision and accuracy of RSA in the lumbar spine

PURPOSE

Roentgen stereophotogrammetric analysis is a technique to make accurate assessments of the relative position and orientation of bone structures and implants in vivo. While the precision and accuracy of stereophotogrammetry for hip and knee arthroplasty is well documented, there is insufficient knowledge of the technique's precision and, especially accuracy when applied to rotational movements in the spinal region.

METHODS

The motion of one cadaver lumbar spine segment (L3/L4) was analyzed in flexion-extension, lateral bending and internal rotation. The specific aim of this study was to examine the precision and accuracy of stereophotogrammetry in a controlled in vitro setting, taking the surrounding soft tissue into account. The second objective of this study was to investigate the effect of different focal spot values of X-ray tubes.

RESULTS

Overall, the precision of flexion-extension measurements was found to be better when using a 0.6 mm focal spot value rather than 1.2 mm (± 0.056° and ± 0.153°; respectively), and accuracy was also slightly better for the 0.6 mm focal spot value compared to 1.2 mm (- 0.137° and - 0.170°; respectively). The best values for precision and accuracy were obtained in lateral bending for both 0.6 mm and 1.2 mm focal spot values (precision: ± 0.019° and ± 0.015°, respectively; accuracy: - 0.041° and - 0.035°).

CONCLUSION

In summary, the results suggest stereophotogrammetry to be a highly precise method to analyze motion of the lumbar spine. Since precision and accuracy are better than 0.2° for both focal spot values, the choice between these is of minor clinical relevance.

Multilevel Validation of a Male Neck Finite Element Model With Active Musculature

Computational models of the human neck have been developed to assess human response in impact scenarios; however, the assessment and validation of such models is often limited to a small number of experimental data sets despite being used to evaluate the efficacy of safety systems and potential for injury risk in motor vehicle collisions. In this study, a full neck model (NM) with active musculature was developed from previously validated motion segment models of the cervical spine. Tissue mechanical properties were implemented from experimental studies, and were not calibrated. The neck model was assessed with experimental studies at three levels of increasing complexity: ligamentous cervical spine in axial rotation, axial tension, frontal impact, and rear impact; postmortem human subject (PMHS) rear sled impact; and human volunteer frontal and lateral sled tests using an open-loop muscle control strategy. The neck model demonstrated good correlation with the experiments ranging from quasi-static to dynamic, assessed using kinematics, kinetics, and tissue-level response. The contributions of soft tissues, neck curvature, and muscle activation were associated with higher stiffness neck response, particularly for low severity frontal impact. Experiments presenting single-value data limited assessment of the model, while complete load history data and cross-correlation enabled improved evaluation of the model over the full loading history. Tissue-level metrics demonstrated higher variability and therefore lower correlation relative to gross kinematics, and also demonstrated a dependence on the local tissue geometry. Thus, it is critical to assess models at the gross kinematic and the tissue levels.

Development and validation of osteoligamentous lumbar spine under complex loading conditions: A step towards patient-specific modeling

Finite-element models are used to investigate the biomechanics of normal, diseased and surgically fused spines. Generally, nominal spine geometries are used to understand the biomechanics, which has created a need for a technique that develops patient-specific lumbar spine geometries. In the current study, a lumbar spine (T12-Sacrum) was developed using a technique that facilitates geometrical morphing, which assists in incorporating patient-specific morphologies into the model. The model evaluations can be used to propose a biomechanically suitable lumbar spine fusion procedure for patients. This study focuses on the validation of the base model under pure-moment, pure-compression and combined-compression-and-moment loadings. Experimental data from the literature were used to validate the response of the model. The L1-L2, L2-L3, L3-L4, L4-L5 and L5-sacrum segments demonstrated a range of motion of 4.5, 4.0, 5.4, 5.0 and 8.9° in flexion; 3.0, 2.5, 3.6, 3.1 and 5.2° in extension; 6.2, 5.8, 6.4, 5.0 and 6.1° in right and left lateral bending; and 2.9, 3.0, 2.9, 1.9 and 2.5° in right and left axial rotation, all under 10 Nm pure-moment loading. The L1-L2, L2-L3, L3-L4, L4-L5 and L5-sacrum discs demonstrated compressions of 1.1, 1.4, 1.6, 1.4 and 0.9 mm under 1200 N follower- or pure-compression loading. With the combined loading of 280 N follower and 7.5 Nm moment, the L1-L5 model demonstrated 11.7, 7.2, 18.3 and 10.4 degrees of range of motion in flexion, extension, bending and rotation, respectively. The model results were in good agreement with corridors from six different experimental studies and can be used for future clinical studies.

Effect of Percutaneous Endoscopic Lumbar Foraminoplasty of Different Facet Joint Portions on Lumbar Biomechanics: A Finite Element Analysis

Objective

To evaluate the influence of percutaneous endoscopic lumbar foraminoplasty of different facet joint portions on segmental range of motion (ROM) and intradiscal pressure (IDP) of L₃/L₄ and L₄/L₅ motion segments by establishing three dimensional finite element (FE) model.

Method

Computed tomography images of a male adult volunteer of appropriate age and in good condition both mentally and physically. Obtained data was used in this study from July 2020 to December 2020, and an intact L_3–5 three dimensional finite element model was successfully constructed using ANSYS and MIMICS software (model M1). The M1 was modified to simulate the foraminoplasty of different facet joint portions, with unilateral cylindrical excision (diameter = 0.75 cm) performed on the tip (model M2) and the base (model M3) of right L₅ superior facet elements along with surrounding capsular ligaments, respectively. Under the same loading conditions, the ROM and IDP of L₃/₄ and L₄/L₅ segments in states of forward flexion, backward extension, left lateral bending, right lateral bending, left axial rotation and right axial rotation were all compared.

Result

Compared with the intact model in backward extension, M2 increased the ROM of L_4/5 segment by 9.4% and IDP by 11.7%, while the ROM and IDP of M3 changed only slightly. In right axial rotation, M2 and M3 increased the ROM of L_4/5 segment by 17.9% and by 3.6%, respectively. In left axial rotation, M2 and M3 increased the ROM of L₄/L₅ segment by 7.14% and 3.6%, respectively. As for other states including forward flexion, left lateral bending, right lateral bending, the ROM and IDP were not significantly distinct between these two models. While focusing on L₃/L₄ segment, obviously changes in the ROM and IDP have not been presented and neither M2 nor M3 changed in any loading condition.

Conclusion

This study provides evidence that the base‐facet foraminoplasty of L₅ superior facet provided a higher segmental stability compared with the tip‐facet foraminoplasty in flexion and axial rotation. Meanwhile, it also shows the two types of foraminoplasty make few differences to the L_4/5 segmental biomechanics. Besides, it does not appear to impact the stability of L₃/L₄ in six states of forward flexion, backward extension, left lateral bending, right lateral bending, left axial rotation and right axial rotation when superior facet of L₅ was partially removed. These findings might be useful in understanding biomechanics of the lumbar spine after foraminoplasty performed on different portions of the facet, thus providing endoscopic surgeons a better reference for operational approach to maintain the function and mobility of the spine.

Rear-Impact Neck Whiplash: Role of Head Inertial Properties and Spine Morphological Variations on Segmental Rotations

Whiplash injuries continue to be a concern in low-speed rear impact. This study was designed to investigate the role of variations in spine morphology and head inertia properties on cervical spine segmental rotation in rear-impact whiplash loading. Vertebral morphology is rarely considered as an input parameter in spine finite element (FE) models. A methodology toward considering morphological variations as input parameters and identifying the influential variations is presented in this paper. A cervical spine FE model, with its morphology parametrized using mesh morphing, was used to study the influence of disk height, anteroposterior vertebral depth, and segmental size, as well as variations in head mass, moment of inertia, and center of mass locations. The influence of these variations on the characteristic S-curve formation in whiplash response was evaluated using the peak C2–C3 flexion marking the maximum S-curve formation and time taken for the formation of maximum S-curve. The peak C2–C3 flexion in the S-curve formation was most influenced by disk height and vertebral depth, followed by anteroposterior head center of mass location. The time to maximum S-curve was most influenced by the anteroposterior location of head center of mass. The influence of gender-dependent variations, such as the vertebral depth, suggests that they contribute to the greater segmental rotations observed in females resulting in different S-curve formation from men. These results suggest that both spine morphology and head inertia properties should be considered to describe rear-impact responses.

Cervical spine morphology and ligament property variations: A finite element study of their influence on sagittal bending characteristics

Cervical spine finite element models reported in biomechanical literature usually represent a static morphology. Not considering morphology as a model parameter limits the predictive capabilities for applications in personalized medicine, a growing trend in modern clinical practice. The objective of the study was to investigate the influence of variations in spinal morphology on the flexion-extension responses, utilizing mesh-morphing-based parametrization and metamodel-based sensitivity analysis. A C5-C6 segment was used as the baseline model. Variations of intervertebral disc height, facet joint slope, facet joint articular processes height, vertebral body anterior-posterior depth, and segment size were parametrized. In addition, material property variations of ligaments were considered for sensitivity analysis. The influence of these variations on vertebral rotation and forces in the ligaments were analyzed. The disc height, segmental size, and body depth were found to be the most influential (in the cited order) morphology variations; while among the ligament material property variations, capsular ligament and ligamentum flavum influenced vertebral rotation the most. Changes in disc height influenced forces in the posterior ligaments, indicating that changes in the anterior load-bearing column of the spine could have consequences on the posterior column. A method to identify influential morphology variations is presented in this work, which will help automation efforts in modeling to focus on variations that matter. This study underscores the importance of incorporating influential morphology parameters, easily obtained through computed tomography/magnetic resonance images, to better predict subject-specific biomechanical responses for applications in personalized medicine.

Requirements for an artificial intervertebral disc

Intervertebral disc degeneration is an important social and economic problem. Presently available artificial intervertebral discs (AIDs) are insufficient and the main surgical intervention is still spinal fusion. The objective of the present study is to present a list of requirements for the development of an AID which could replace the human lumbar intervertebral disc and restore its function. The list addresses geometry, stiffness, range of motion, strength, facet joint function, center of rotation, fixation, failsafety and implantation technique. Date are obtained from the literature, quantified where possible and checked for consistency. Existing AIDs are evaluated according to the presented list of requirements. Endplate size is a weak point in existing AIDs. These should be large and fit vertebral bodies to prevent migration. Disc height and wedge angle should be restored, unless this would overstretch ligaments. Finally, stiffness and range of motion in all directions should equal those of the healthy disc, except for the axial rotation to relieve the facet joints.

Adaptation of a clinical fixation device for biomechanical testing of the lumbar spine

In-vitro biomechanical testing is widely performed for characterizing the load-displacement characteristics of intact, injured, degenerated, and surgically repaired osteoligamentous spine specimens. Traditional specimen fixture devices offer an unspecified rigidity of fixation, while varying in the associated amounts and reversibility of damage to and "coverage" of a specimen - factors that can limit surgical access to structures of interest during testing as well as preclude the possibility of testing certain segments of a specimen. Therefore, the objective of this study was to develop a specimen fixture system for spine biomechanical testing that uses components of clinically available spinal fixation hardware and determine whether the new system provides sufficient rigidity for spine biomechanical testing. Custom testing blocks were mounted into a robotic testing system and the angular deflection of the upper fixture was measured indirectly using linear variable differential transformers. The fixture system had an overall stiffness 37.0, 16.7 and 13.3 times greater than a typical human functional spine unit for the flexion/extension, axial rotation and lateral bending directions respectively - sufficient rigidity for biomechanical testing. Fixture motion when mounted to a lumbar spine specimen revealed average motion of 0.6, 0.6, and 1.5° in each direction. This specimen fixture method causes only minimal damage to a specimen, permits testing of all levels of a specimen, and provides for surgical access during testing.

Influence of morphological variations on cervical spine segmental responses from inertial loading

OBJECTIVES

The objective of this study was to investigate the influence of morphological variations in osteoligamentous lower cervical spinal segment responses under postero-anterior inertial loading.

METHODS

A parametric finite element model of the C5-C6 spinal segment was used to generate models. Variations in the vertebral body and facet depth (anteroposterior), posterior process length, intervertebral disc height, facet articular process height and slope, segment orientation ranging from lordotic to straight, and segment size were parameterized. These variations included male-female differences. A Latin hypercube sampling method was used to select parameter values for model generation. Forces and moments associated with the inertial loading were applied to the generated model segments. The 7 parameters were grouped as local or global depending on the number of spinal components involved in the shape variation. Four output responses representing overall segmental and soft tissue responses were analyzed for each model variation: response angle of the segment, anterior longitudinal ligament stretch, anterior capsular ligament stretch, and facet joint compression in the posterior region. Pearson's correlation coefficient was used to compute the correlations of these output responses with morphological variations.

RESULTS

Fifty models were generated from the parameterized model using a Latin hypercube sampling technique. Variation in response angle among the models was 4° and was most influenced by change in the combined dimension of vertebral body and facet depth, followed by size of the segment. The maximum anterior longitudinal ligament stretch varied between 0.1 and 0.3 and was strongly influenced by the change in the segment orientation. The anterior facet joint region sustained tension, whereas the posterior region sustained compression. For the anterior capsular ligament stretch, the most influential global variation was segment orientation, whereas the most influential local variations were the facet height and facet angle parameters. In the case of posterior facet joint compression, segment orientation was again most influential, whereas among the local variations, the facet angle had the most influence.

CONCLUSION

Shape variations in the intervertebral disc influenced segmental rotation and ligament responses; however, the influence of shape variations in the facet joint was confined to capsular ligament responses. Response angle was most influenced by the vertebral body depth variations, explaining greater segmental rotations in female spines. Straighter spine segments sustained greater posterior facet joint compression, which may offer an explanation for the higher incidence of whiplash-associated disorders among females, who exhibit a straighter cervical spine. The anterior longitudinal ligament stretch was also greater in straighter segments. These findings indicate that the morphological features specific to the anatomy of the female cervical spine may predispose it to injury under inertial loading.

On the importance of retaining stresses and strains in repositioning computational biomechanical models of the cervical spine

Human body models are created in a specific posture and often repositioned and analyzed without retaining stresses that result from repositioning. For example, repositioning a human neck model within the physiological range of motion to a head-turned posture prior to an impact results in initial stresses within the tissues distracted from their neutral position. The aim of this study was to investigate the effect of repositioning on the subsequent kinetics, kinematics, and failure modes, of a lower cervical spine motion segment, to support future research at the full neck level. Repositioning was investigated for 3 modes (tension, flexion, and extension) and 3 load cases. The model was repositioned and loaded to failure in one continuous load history (case 1), or repositioned then restarted with retained stresses and loaded to failure (case 2). In case 3, the model was repositioned and then restarted in a stress-free state, representing current repositioning methods. Not retaining the repositioning stresses and strains resulted in different kinetics, kinematics, or failure modes, depending on the mode of loading. For the motion segment model, the differences were associated with the intervertebral disc fiber reorientation and load distribution, because the disc underwent the largest deformation during repositioning. This study demonstrated that repositioning led to altered response and tissue failure, which is critical for computational models intended to predict injury at the tissue level. It is recommended that stresses and strains be included and retained for subsequent analysis when repositioning a human computational neck model.

Ranges of Cervical Intervertebral Disc Deformation During an In Vivo Dynamic Flexion-Extension of the Neck

While abnormal loading is widely believed to cause cervical spine disc diseases, in vivo cervical disc deformation during dynamic neck motion has not been well delineated. This study investigated the range of cervical disc deformation during an in vivo functional flexion-extension of the neck. Ten asymptomatic human subjects were tested using a combined dual fluoroscopic imaging system (DFIS) and magnetic resonance imaging (MRI)-based three-dimensional (3D) modeling technique. Overall disc deformation was determined using the changes of the space geometry between upper and lower endplates of each intervertebral segment (C3/4, C4/5, C5/6, and C6/7). Five points (anterior, center, posterior, left, and right) of each disc were analyzed to examine the disc deformation distributions. The data indicated that between the functional maximum flexion and extension of the neck, the anterior points of the discs experienced large changes of distraction/compression deformation and shear deformation. The higher level discs experienced higher ranges of disc deformation. No significant difference was found in deformation ranges at posterior points of all the discs. The data indicated that the range of disc deformation is disc level dependent and the anterior region experienced larger changes of deformation than the center and posterior regions, except for the C6/7 disc. The data obtained from this study could serve as baseline knowledge for the understanding of the cervical spine disc biomechanics and for investigation of the biomechanical etiology of disc diseases. These data could also provide insights for development of motion preservation surgeries for cervical spine.

Lower Cervical Spine Motion Segment Computational Model Validation: Kinematic and Kinetic Response for Quasi-Static and Dynamic Loading

Advanced computational human body models (HBM) enabling enhanced safety require verification and validation at different levels or scales. Specifically, the motion segments, which are the building blocks of a detailed neck model, must be validated with representative experimental data to have confidence in segment and, ultimately, full neck model response. In this study, we introduce detailed finite element motion segment models and assess the models for quasi-static and dynamic loading scenarios. Finite element segment models at all levels in the lower human cervical spine were developed from scans of a 26-yr old male subject. Material properties were derived from the in vitro experimental data. The segment models were simulated in quasi-static loading in flexion, extension, lateral bending and axial rotation, and at dynamic rates in flexion and extension in comparison to previous experimental studies and new dynamic experimental data introduced in this study. Single-valued experimental data did not provide adequate information to assess the model biofidelity, while application of traditional corridor methods highlighted that data sets with higher variability could lead to an incorrect conclusion of improved model biofidelity. Data sets with continuous or multiple moment–rotation measurements enabled the use of cross-correlation for an objective assessment of the model and highlighted the importance of assessing all motion segments of the lower cervical spine to evaluate the model biofidelity. The presented new segment models of the lower cervical spine, assessed for range of motion and dynamic/traumatic loading scenarios, provide a foundation to construct a biofidelic model of the spine and neck, which can be used to understand and mitigate injury for improved human safety in the future.

SLIDE: automatic spine level identification system using a deep convolutional neural network

PURPOSE

Percutaneous spinal needle insertion procedures often require proper identification of the vertebral level to effectively and safely deliver analgesic agents. The current clinical method involves "blind" identification of the vertebral level through manual palpation of the spine, which has only 30% reported accuracy. Therefore, there is a need for better anatomical identification prior to needle insertion.

METHODS

A real-time system was developed to identify the vertebral level from a sequence of ultrasound images, following a clinical imaging protocol. The system uses a deep convolutional neural network (CNN) to classify transverse images of the lower spine. Several existing CNN architectures were implemented, utilizing transfer learning, and compared for adequacy in a real-time system. In the system, the CNN output is processed, using a novel state machine, to automatically identify vertebral levels as the transducer moves up the spine. Additionally, a graphical display was developed and integrated within 3D Slicer. Finally, an augmented reality display, projecting the level onto the patient's back, was also designed. A small feasibility study [Formula: see text] evaluated performance.

RESULTS

The proposed CNN successfully discriminates ultrasound images of the sacrum, intervertebral gaps, and vertebral bones, achieving 88% 20-fold cross-validation accuracy. Seventeen of 20 test ultrasound scans had successful identification of all vertebral levels, processed at real-time speed (40 frames/s).

CONCLUSION

A machine learning system is presented that successfully identifies lumbar vertebral levels. The small study on human subjects demonstrated real-time performance. A projection-based augmented reality display was used to show the vertebral level directly on the subject adjacent to the puncture site.

A novel distractive and mobility-enabling lumbar spinal orthosis

Purpose

Lumbar spinal orthoses are often used as non-surgical treatment and serve to support the spine and alleviate low back pain. More recently, dynamic orthoses claiming to decompress the spine have been introduced. A previously developed prototype of dynamic mobility orthosis (DMO1) was designed that provided a distractive load across the lumbar spine but required higher sagittal bending moments and was unable to maintain spinal off-loading throughout extended ranges of movement. The objective was to design a new orthosis (DMO2) that reduced bending moment buildup and sustained spinal off-loading throughout daily living ranges of flexion and extension movement.

Methods

A mechanical analog upper torso model and programmable robotic testing platform were used to design features of DMO2: a mobility-enabling component and a distractive force component. Test conditions for DMO2 were 300 N of applied vertical torso load over a range of 25° flexion to 10° extension. Loads carried by the brace were determined throughout flexion and extension ranges. Applied moments to the upper torso model and transferred moments to the spine were measured. The difference in applied and transferred moments represented brace moment effects.

Results

The DMO2 prototype improved spinal off-loading capacity from 172 N to 290 N at end-range flexion and from 247 N to 293 N at end range extension compared to the original DMO1 prototype. End-range applied moments (flexion-DMO1: 32.4 Nm/DMO2: 21.7 Nm; extension-DMO1: 15.0 Nm/DMO2: 10.9 Nm) and brace moments (flexion-DMO1: 18.6 Nm/DMO2: 6.6 Nm; extension-DMO1: 15.0 Nm/DMO2: 4.4 Nm) were also reduced.

Conclusions

A novel dynamic spinal orthosis was designed that maintained spinal off-loading throughout extended ranges of flexion and extension movement without buildup of adverse bending moments.

A finite element head and neck model as a supportive tool for deformable image registration

PURPOSE

A finite element (FE) head and neck model was developed as a tool to aid investigations and development of deformable image registration and patient modeling in radiation oncology. Useful aspects of a FE model for these purposes include ability to produce realistic deformations (similar to those seen in patients over the course of treatment) and a rational means of generating new configurations, e.g., via the application of force and/or displacement boundary conditions.

METHODS

The model was constructed based on a cone-beam computed tomography image of a head and neck cancer patient. The three-node triangular surface meshes created for the bony elements (skull, mandible, and cervical spine) and joint elements were integrated into a skeletal system and combined with the exterior surface. Nodes were additionally created inside the surface structures which were composed of the three-node triangular surface meshes, so that four-node tetrahedral FE elements were created over the whole region of the model. The bony elements were modeled as a homogeneous linear elastic material connected by intervertebral disks. The surrounding tissues were modeled as a homogeneous linear elastic material. Under force or displacement boundary conditions, FE analysis on the model calculates approximate solutions of the displacement vector field.

RESULTS

A FE head and neck model was constructed that skull, mandible, and cervical vertebrae were mechanically connected by disks. The developed FE model is capable of generating realistic deformations that are strain-free for the bony elements and of creating new configurations of the skeletal system with the surrounding tissues reasonably deformed.

CONCLUSIONS

The FE model can generate realistic deformations for skeletal elements. In addition, the model provides a way of evaluating the accuracy of image alignment methods by producing a ground truth deformation and correspondingly simulated images. The ability to combine force and displacement conditions provides flexibility for simulating realistic anatomic configurations.

Cervical Spine Disc Deformation During In Vivo Three-Dimensional Head Movements

Although substantial research demonstrates that intervertebral disc cells respond to mechanical signals, little research has been done to characterize the in vivo mechanical environment in the disc tissue. The objective of this study was to estimate cervical disc strain during three-dimensional head movements. Twenty-nine young healthy adults performed full range of motion flexion/extension, lateral bending, and axial rotation of the head within a biplane radiography system. Three-dimensional vertebral kinematics were determined using a validated model-based tracking technique. A computational model used these kinematics to estimate subject-specific intervertebral disc deformation (C3-4 to C6-7). Peak compression, distraction and shear strains were calculated for each movement, disc level, and disc region. Peak compression strain and peak shear strain were highest during flexion/extension (mean ± 95% confidence interval) (32 ± 3 and 86 ± 8%, respectively), while peak distraction strain was highest during lateral bending (57 ± 5%). Peak compression strain occurred at C4-5 (33 ± 4%), while peak distraction and shear strain occurred at C3-4 (54 ± 8 and 83 ± 11%, respectively). Peak compression, distraction, and shear strains all occurred in the posterior-lateral annulus (48 ± 4, 80 ± 8, and 109 ± 12%, respectively). These peak strain values may serve as boundary conditions for in vitro loading paradigms that aim to assess the biologic response to physiologic disc deformations.

BIOMECHANICS OF CERVICAL SPINE FOLLOWING IMPLANTATION OF A SEMI-CONSTRAINED ARTIFICIAL DISC WITH UPWARD CENTER OF ROTATION: A FINITE ELEMENT INVESTIGATION

The objective of this study was to evaluate the effects of a semi-constrained artificial disc with upward instantaneous center of rotation (ICR) on the biomechanics of the cervical spine. A three-dimensional nonlinear finite element model of the lower cervical spine (C4–C7) was developed using computed tomography (CT) data. The FE model was validated by comparing it to previously published experimental results for flexion-extension, lateral bending and axial rotation movements. The validated model was then altered to include prosthesis at the C5–C6 level. A hybrid test protocol was used to investigate the effects of total disc replacement. The results of this study showed that this artificial disc can help maintain the same range of motion (ROM) and intradiscal pressure as the intact model for most loading conditions. We also found that loads on the facet joints increased dramatically at index level. The capsular ligaments were also found to transmit more tension during flexion at implanted level. Although the artificial disc with upward ICR was found to restore normal kinematics, and prevented increases in intradiscal pressure, it was also associated with an overloading of the facet joints and capsular ligaments leading to potentially undesirable outcomes in the long term.

Evaluation of biomechanical strength, stability, bioactivity, and in vivo biocompatibility of a novel calcium deficient hydroxyapatite/poly(amino acid) composite cervical vertebra cage

A new type of cervical vertebra cage was prepared using a novel composite, calcium deficient hydroxyapatite/poly(amino acid) (HA/PAA), and its mechanical properties, in vitro stability and bioactivity, and in vivo biocompatibility were characterized. The results showed that the axial compressive loads of the HA/PAA cage were in the range of 10058-10612 N and the lateral compressive loads were in the range of 1180-2363 N, and varied with the height of the cervical vertebra cages. After immersion in simulated body fluid (SBF) for 16 weeks, the axial compressive loads of the cage decreased from 10058 to 7131 N and the lateral compressive loads decreased from 1180 to 479 N. In addition, the weight loss decreased 6.01%, showing that HA/PAA composites had good stability during the incubation period. The pH value of SBF was also monitored during the whole soaking period; it fluctuated in the range of 6.9-7.4. Scanning electron microscope and energy dispersive spectrometer results showed the cage was bioactive with a new apatite layer attached on the surface. The histological evaluation revealed that new bone tissue bonded tightly with the surfaces of the implants, showing excellent biocompatibility. In conclusion, the HA/PAA cage showed sufficient strength, good stability, bioactivity, and biocompatibility, and has potential applications for clinical cervical vertebrae repair.

A morphometric study of the lumbar interspinous space in 100 stanford university medical center patients

Objective

With the increased use of interspinous spacers in the treatment of lumbar stenosis, knowledge of the geometry of the interspinous space is important. To prevent dislodgment of an interspinous spacer, the accurate depth and width of the interspinous space needs to be established to facilitate the best intraoperative selection of correct spacer size.

Methods

To determine the depth and width of the interspinous space, two methods are available which utilize plain film and magnetic resonance imaging (MRI). Data analysis of the interspinous depth and width was undertaken in 100 patients.

Results

The standard deviations were variable, since skin thickness (zone 1) was altered by sex and age. The difference in the zone 1 distance between adjacent interspinous processes varied according to gender (p<0.05), but was not influenced by age [p=0.32 by analysis of variance between groups (ANOVA)]. Zone 2, the supraspinous, and zone 3, the interspinous ligament depths, comprise the operative working area during insertion of an interspinous spacer. There were no differences with regard to gender or age (p>0.05). For zones 6 and 7, the interspinous distances at the narrowest and widest points, respectively, were found to decrease with the aging process, but the decrease was not statistically significant. There were no differences with regard to gender (p>0.05).

Conclusion

This study provides additional information on the interspinous space. This statistical data are valuable for use in the design of interspinous spacers.

Mechanical behaviour of electrospun fibre-reinforced hydrogels

Mechanically robust and biomimicking scaffolds are needed for structural engineering of tissues such as the intervertebral disc, which are prone to failure and incapable of natural healing. Here, the formation of thick, randomly aligned polycaprolactone electrospun fibre structures infiltrated with alginate is reported. The composites are characterised using both indentation and tensile testing and demonstrate substantially different tensile and compressive moduli. The composites are mechanically robust and exhibit large strains-to-failure, exhibiting toughening mechanisms observed in other composite material systems. The method presented here provides a way to create large-scale biomimetic scaffolds that more closely mimic the composite structure of natural tissue, with tuneable tensile and compressive properties via the fibre and matrix phases, respectively.

PREDICTING THE VERTEBRAL BODY POSITION BASED ON PALPATED SPINOUS PROCESS POSITION

Palpation is an essential skill of manual therapy. Clinical techniques of physical therapy usually assume that the movement direction of palpating spinous process (SP) is the direction of the vertebral body center (VBC). This study investigated the distance [SP–projected VBC (PVBC)] between the surface palpation of the five SPs and the radiographically projected vertebral center locations (PVBC) on the skin of the lumbar spine in 37 patients with low back pain (LBP). The measurement of SP–PVBC was intended to describe if palpation on SPs could explain the positions of the VBC. The SP–PVBC distance was the greatest at L1 (35.9 mm) and the smallest at L4 (15.1 mm). The predictive analysis investigated the relationships between SP–PVBC and the geometric measurements of the lumbar anatomical structures. The geometric characteristics of the lumbar spine affected the SP–PVBC distance in different levels, with the R2 values from 0.66–0.89, except 0.38 in the L4 level. Increases in the SP inclination as well as vertebral inclinations, and increases in the SP height (SPH) were factors that were found to be significantly related to the SP–PVBC distance (p < 0.05). The results indicate that the orientation of the VBC and the SP may not be the same, and tilting and rotation of the vertebrae may occur when applying manual techniques through SPs. Physical therapists need to be aware that the tilting or rolling effect of vertebrae may not be avoidable once the treatment is done via palpation on SPs.

Disproportion of end plates and the lumbar intervertebral disc herniation

BACKGROUND CONTEXT

It is suggested that the shape of the vertebral end plates may play a role in the development of abnormalities in the intervertebral disc. On midsagittal magnetic resonance images of the spine in patients with lumbar intervertebral disc herniation, a notable disproportion frequently exists between the end plates of two vertebrae to which the disc is attached. There is apparently no study in the literature examining possible association of this disproportion with development of disc herniation.

PURPOSE

To determine whether a disproportion between two neighboring vertebral end plates is associated with the presence of disc herniation at the same level.

STUDY DESIGN

Case-control study.

PATIENT SAMPLE

Two hundred fifty patients with primary lumbar disc herniation in the case group and 250 age- and sex-matched normal individuals in the control group.

OUTCOME MEASURES

On midsagittal sections, the difference of anteroposterior diameter of upper and lower end plates neighboring a herniated (in the case group) or normal (in the control group) intervertebral disc was calculated and expressed as "difference of end plates" or "DEP."

METHODS

Subjects with previous spinal surgery, spondylolisthesis, or a significant vertebral deformity were excluded. For the main outcome variable, DEP was calculated at the level with herniated intervertebral disc in the case group, and the mean value was compared with mean DEP at the same level in the controls.

RESULTS

Mean DEP was significantly higher in the case group at both L4-L5 (2.45±0.28 vs. 2.08±0.27 mm, p=.02) and L5-S1 (3.32±0.18 vs. 2.51±0.13 mm, p<.001) levels. Similar differences were only marginally insignificant at L2-L3 (1.96±0.14 mm in the cases vs. 1.33±0.15 mm in the controls, p=.07) and L3-L4 (2.17±0.11 mm in the cases vs. 1.55±0.09 mm in the controls, p=.06) levels, with no significant difference at L1-L2 level (1.81±0.10 mm in the cases vs. 1.28±0.09 mm in the controls, p=.12). Each 1 mm increase of DEP at L4-L5 and L5-S1 levels was associated with 53% and 56% elevation in disc herniation risk at the corresponding levels, respectively.

CONCLUSIONS

Difference of end plate is a significant and probably independent risk factor for lumbar disc herniation.

INFLUENCE OF THE CONSTITUTIVE MATERIAL BEHAVIOR MODEL ASSIGNED TO THE ANNULUS FIBROSUS AND THE NUCLEUS PULPOSUS ON THE BIOMECHANICAL PERFORMANCE OF A MODEL OF THE CERVICAL SPINE: A FINITE ELEMENT ANALYSIS STUDY

One feature of the literature on finite element analysis of models of cervical spine segment(s) is that an assortment of constitutive models has been used for the elastic behavior of the annulus fibrosus (AF) and the nucleus pulposus (NF). The extent to which the model assigned to each of these tissues affects the values of the biomechanical parameters of interest of the model is lacking. This issue was the subject of the present study. We used a three-dimensional solid model of the C4–C6 motion segment units (which comprised the vertebral bodies, the bony posterior elements (transverse processes, pedicles, laminae, spinous processes, and facet joints), the intervertebral discs (IVDs), the endplates, and the five major ligaments) and eight combinations of constitutive models. It was found that (1) the influence of the constitutive material models used depended on the tissue considered, with some, such as the posterior endplate of C5 and the cancellous bone of C6, showing marked sensitivity, while others, such as the cancellous bone of C4 and the cortical bone of C5, were moderately affected; and (2) the biomechanical performance of the spine model is more sensitive to the material behavior model used for the AF than it is to that used for the NF. These results suggest that experimental and computational efforts expended in obtaining the most appropriate constitutive model for the elastic behavior of the two parts of the IVD, in particular the AF, are justified.

FINITE ELEMENT ANALYSIS OF A MODEL OF SIMULATED VERTEBRAL CEMENT AUGMENTATION: INFLUENCE OF THE REPRESENTATION OF THE SHAPE OF THE CEMENT DOMAIN ON BIOMECHANICAL PARAMETERS

Vertebral cement augmentation is rapidly becoming the modality of choice for treating patients who are experiencing severe and persistent pain because of osteoporosis-induced vertebral compression fracture(s). The resulting cement domain (the part of the vertebral body (VB) filled with the cement) has an irregular or complicated shape. In literature reports of finite element analysis (FEA) of models of simulated vertebral cement augmentation, a variety of representations of the shape of the cement domain have been used. In the literature, only very limited attention has been given to the issue of the influence of cement domain shape representation on biomechanical parameters for a given combination of model and loading. This issue is the subject of the present work, with the model being of the L1-L3 motion segments. Augmentation of an unfractured L2 (prophylactic augmentation) was simulated, three cement domain shapes were considered — namely, solid cylinder, with rounded edges; two prolate spheroids; and oblate spheroid — and the applied loading comprised a simultaneous application of a uniform compressive pressure of 0.53 MPa (equivalent to an 800-N compression load) and a counter-clockwise-acting axial rotation moment of 1 Nm to the superior surface of L1. It was found that (1) while the cement domain shape representation has a marked influence on the mean von Mises stress (σAVM), the maximum von Mises stress (σMVM), and the strain energy density (MSED) distribution in the cement domain, its influence on each of these parameters in each of the biological tissues in the model as well as on the total segmental range of motion is minimal and (2) for σAVM and σMVM, the lowest value of each of these parameters was obtained when the oblate spheroid model was used. From both clinical and computational perspectives, these findings are significant. For example, the latter finding suggests that there is scope for researching the combination of key process variables used, such as the cement chemistry, the cement delivery system, and the augmentation technique/approach, that would ensure that the final cement domain shape in cement-augmented VBs of patients be oblate spheroid on a consistent and predictable basis.

EFFECT OF MUSCLES ACTIVATION ON HEAD-NECK COMPLEX UNDER SIMULATED EJECTION

A detailed three-dimensional head-neck (C0–C7) finite element (FE) model developed based on the actual geometry of an embalmed human cadaver specimen was exercised to dictate the motions of the cervical spine under dynamic loadings. The predicted results analyzed under vertex drop impact were compared against experimental study to validate the FE model. The validated C0–C7 FE model was then further analyzed to investigate the response of the whole head-neck complex under 10G-ejection condition. From the simulation of ejection process, obvious hyper-flexion of the head-neck complex could be found. The peak flexion angles of all the lower motion segments were beyond physiological tolerance indicating a potential injury in these regions. Furthermore, the stress values in the spine were also related to the magnitudes of rotation of the motion segments. During the acceleration onset stage, the maximum stresses in the bone components were low. After that, the stress values increased sharply into the dangerous range with increased rotational angles. The effect of muscles in alleviating the potential damage in the neck is significant. It was implied that it is important for pilots to stiffen the neck before ejection to avoid severe cervical injury.