Lateral Bending Motion Research Articles

Development of a spinopelvic complex finite element model for quantitative analysis of the biomechanical response of patients with degenerative spondylolisthesis.

Research on degenerative spondylolisthesis (DS) has focused primarily on the biomechanical responses of pathological segments, with few studies involving muscle modelling in simulated analysis, leading to an emphasis on the back muscles in physical therapy, neglecting the ventral muscles. The purpose of this study was to quantitatively analyse the biomechanical response of the spinopelvic complex and surrounding muscle groups in DS patients using integrative modelling. The findings may aid in the development of more comprehensive rehabilitation strategies for DS patients. Two new finite element spinopelvic complex models with detailed muscles for normal spine and DS spine (L4 forwards slippage) modelling were established and validated at multiple levels. Then, the spinopelvic position parameters including peak stress of the lumbar isthmic-cortical bone, intervertebral discs, and facet joints; peak strain of the ligaments; peak force of the muscles; and percentage difference in the range of motion were analysed and compared under flexion-extension (F-E), lateral bending (LB), and axial rotation (AR) loading conditions between the two models. Compared with the normal spine model, the DS spine model exhibited greater stress and strain in adjacent biological tissues. Stress at the L4/5 disc and facet joints under AR and LB conditions was approximately 6.6 times greater in the DS spine model than in the normal model, the posterior longitudinal ligament peak strain in the normal model was 1/10 of that in the DS model, and more high-stress areas were found in the DS model, with stress notably transferring forwards. Additionally, compared with the normal spine model, the DS model exhibited greater muscle tensile forces in the lumbosacral muscle groups during F-E and LB motions. The psoas muscle in the DS model was subjected to 23.2% greater tensile force than that in the normal model. These findings indicated that L4 anterior slippage and changes in lumbosacral-pelvic alignment affect the biomechanical response of muscles. In summary, the present work demonstrated a certain level of accuracy and validity of our models as well as the differences between the models. Alterations in spondylolisthesis and the accompanying overall imbalance in the spinopelvic complex result in increased loading response levels of the functional spinal units in DS patients, creating a vicious cycle that exacerbates the imbalance in the lumbosacral region. Therefore, clinicians are encouraged to propose specific exercises for the ventral muscles, such as the psoas group, to address spinopelvic imbalance and halt the progression of DS.

Implant Design and Cervical Spinal Biomechanics and Neurorehabilitation: A Finite Element Investigation.

The cervical spine, pivotal for mobility and overall body function, can be affected by cervical spondylosis, a major contributor to neural disorders. Prevalent in both general and military populations, especially among pilots, cervical spondylosis induces pain and limits spinal capabilities. Anterior Cervical Discectomy and Fusion (ACDF) surgery, proposed by Cloward in the 1950s, is a promising solution for restoring natural cervical curvature. The study objective was to investigate the impacts of ACDF implant design on postsurgical cervical biomechanics and neurorehabilitation outcomes by utilizing a biofield head-neck finite element (FE) platform that can facilitate scenario-specific perturbations of neck muscle activations. This study addresses the critical need to enhance computational models, specifically FE modeling, for ACDF implant design. We utilized a validated head-neck FE model to investigate spine-implant biomechanical interactions. An S-shaped dynamic cage incorporating titanium (Ti) and polyetheretherketone (PEEK) materials was modeled at the C4/C5 level. The loading conditions were carefully designed to mimic helmet-to-helmet impact in American football, providing a realistic and challenging scenario. The analysis included intervertebral joint motion, disk pressure, and implant von Mises stress. The PEEK implant demonstrated an increased motion in flexion and lateral bending at the contiguous spinal (C4/C5) level. In flexion, the Ti implant showed a modest 5% difference under 0% activation conditions, while PEEK exhibited a more substantial 14% difference. In bending, PEEK showed a 24% difference under 0% activation conditions, contrasting with Ti's 17%. The inclusion of the head resulted in an average increase of 18% in neck angle and 14% in C4/C5 angle. Disk pressure was influenced by implant material, muscle activation level, and the presence of the head. Polyetheretherketone exhibited lower stress values at all intervertebral disc levels, with a significant effect at the C6/C7 levels. Muscle activation level significantly influenced disk stress at all levels, with higher activation yielding higher stress. Titanium implant consistently showed higher disk stress values than PEEK, with an orders-of-magnitude difference in von Mises stress. Excluding the head significantly affected disk and implant stress, emphasizing its importance in accurate implant performance simulation. This study emphasized the use of a biofidelic head-neck model to assess ACDF implant designs. Our results indicated that including neck muscles and head structures improves biomechanical outcome measures. Furthermore, unlike Ti implants, our findings showed that PEEK implants maintain neck motion at the affected level and reduce disk stresses. Practitioners can use this information to enhance postsurgery outcomes and reduce the likelihood of secondary surgeries. Therefore, this study makes an important contribution to computational biomechanics and implant design domains by advancing computational modeling and theoretical knowledge on ACDF-spine interaction dynamics.

Dynamic in vivo 3D atlantooccipital kinematics during multiplanar physiologic motions

Normal biomechanics of the upper cervical spine, particularly at the atlantooccipital joint, remain poorly characterized. The purpose of this study was to determine the intervertebral kinematics of the atlantooccipital joint (occiput-C1) during three-dimensional in vivo physiologic movements. Twenty healthy young adults performed dynamic flexion/extension, axial rotation, and lateral bending while biplane radiographs were collected at 30 images per second. Motion at occiput-C1 was tracked using a validated volumetric model-based tracking process that matched subject-specific CT-based bone models to the radiographs. The occiput-C1 total range of motion (ROM) and helical axis of motion (HAM) was calculated for each movement. During flexion/extension, the occiput-C1 moved almost exclusively in-plane (ROM: 17.9 ± 6.9°) with high variability in kinematic waveforms (6.3°) compared to the in-plane variability during axial rotation (1.4°) and lateral bending (0.9°) movements. During axial rotation, there was small in-plane motion (ROM: 4.2 ± 2.5°) compared to out-of-plane flexion/extension (ROM: 12.7 ± 5.4°). During lateral bending, motion occurred in-plane (ROM: 9.0 ± 3.1°) and in the plane of flexion/extension (ROM: 7.3 ± 2.7°). The average occiput-C1 axis of rotation intersected the sagittal and coronal planes 7 mm to 18 mm superior to the occipital condyles. The occiput-C1 axis of rotation pointed 60° from the sagittal plane during axial rotation but only 10° from the sagittal plane during head lateral bending. These novel results are foundational for future work on upper cervical spine kinematics.

Posterior atlantoaxial fixation of osteoporotic odontoid fracture: biomechanical analysis of the Magerl versus harms techniques in a cadaver model

BACKGROUND CONTEXTOdontoid fractures are among the most common cervical spine fractures in the elderly and are associated with increased morbidity and mortality. Clinical evidence suggests improved survival and quality of life after operative intervention compared to nonoperative treatment. PURPOSEThis study seeks to examine the stability of an osteoporotic Type II odontoid fracture following posterior atlantoaxial fixation with either the Magerl transarticular fixation technique or the Harms C1 lateral mass screws C2 pedicle screw rod fixation. STUDY DESIGNBiomechanical cadaveric study. METHODSEighteen cadaveric specimens extending from the cephalus to C7 were used in this study. Reflective marker arrays were attached to C1 and C2 and a single marker on the dens to measure movement of each during loading with C2–C3 and occiput-C1 being allowed to move freely. A biomechanical testing protocol imparted moments in flexion-extension, axial rotation, and lateral bending while a motion capture system recorded the motions of C1, C2, and the dens. The spines were instrumented with either the Harms fixation (n=9) or Magerl fixation (n=9) techniques, and a simulated Type II odontoid fracture was created. Motions of each instrumented spine were recorded for all moments, and then again after the instrumentation was removed to model the injured, noninstrumented state. RESULTSBoth Harms and Magerl posterior C1–C2 fixation allowed for C1, C2, and the dens to move as a relative unit. Without fixation the dens motion was coupled with C1. No significant differences were found in X, Y, Z translation motion of the dens, C1 or C2 during neutral zone motions between the Magerl and Harms fixation techniques. There were no significant differences found in Euler angle motion between the two techniques in either flexion-extension, axial rotation, or lateral bending motion. CONCLUSIONSOur findings suggest that both Harms and Magerl fixation can significantly reduce dens motion in Type II odontoid fractures in an osteoporotic cadaveric bone model. CLINICAL SIGNIFICANCEBoth Harms and Magerl posterior atlantoaxial fixation techniques allowed for C1, C2, and the dens to move as a relative unit following odontoid fracture, establishing more anatomic stability to the upper cervical spine.

Design of a novel lateral mass screw–plate system for the treatment of unstable atlas fractures: a finite element analysis

BackgroundOsteosynthesis of unstable atlas fractures preserves joint motion and therefore has a distinct advantage over a range of treatment procedures. To prevent the potential disadvantages associated with osteosynthesis, a new atlas lateral mass screw–plate (LMSP) system has been designed. However, the biomechanical role of using the LMSP system in atlas internal fixation is not known. The aim of this study was to compare the biomechanical stability of a new LMSP with traditional posterior screw and rod (PSR) fixation techniques on the occipitocervical junction (C0–C2) through finite element analysis.MethodsA nonlinear C0–C2 finite element model of the intact upper cervical spine was developed and validated. The unstable model using the PSR system was then compared with the model using the LMSP system for fixation. A vertical load of 40 N was applied to the C0 to simulate head weight, while a torque of 1.5 Nm was applied to the C0 to simulate flexion, extension, lateral bending, and axial rotation.ResultsThe range of motion of both systems was close to the intact model. Compared with the LMSP system model, the PSR system model increased flexion, extension, lateral bending, and axial rotation by 4.9%, 3.0%, 5.0%, and 29.5% in the C0–C1 segments, and 4.9%, 2.7%, 2.4%, and 22.6% in the C1–C2, respectively. In flexion, extension, and lateral bending motion, the LMSP system model exhibited similar stress to the PSR system model, while in axial rotation, the PSR system model exhibited higher stress.ConclusionsThe findings of our study indicate that the two tested system models provide comparable stability. However, better stability was achieved during axial rotation with the LMSP system, and in this system, the maximum von Mises stress was less than that of the PSR one. As the atlantoaxial joint functions primarily as a rotational joint, the use of the LMSP system may provide a more stable environment for the joint that has become unstable due to fracture.

Effect of different loads on facet joint motion during lumbar lateral bending in sitting position

ObjectiveTo study the effect of weight-bearing on lumbar facet joint during lateral bending in sitting position.MethodsTen normal healthy people (5 males and 5 females) aged 25–39 years (mean 32 ± 4.29 years) were recruited. CT scanning was used to reconstruct the lumbar spine model, and then dual fluoroscopic image system (DFIS) was used to restore the lumbar facet joint movement in sitting position. Finally, the lumbar facet joint translation distance and rotation angle were measured.ResultsIn L3-4 level, the displacement of right facet joint in Y-axis was the smallest at 0.05 ± 0.40 mm, the displacement of 0 kg left facet joint in X-axis was the largest at 1.68 ± 0.85 mm, and the rotation angle was − 0.57 ± 1.43° to 5.66 ± 2.70° at 10 kg; in L4-5 level, the displacement of right facet joint in Y-axis was the smallest at 10 kg, − 0.13 ± 0.91 mm, and the displacement of left facet joint in Z-axis was the largest at − 2.11 ± 0.88 mm, and the rotation angle was 0.21 ± 2.14° to 7.89 ± 2.59° at 10 kg; in L5-S1 level, the displacement of right facet joint in Y-axis was the smallest at 10 kg, − 0.17 ± 1.10 mm, and the displacement of 0 kg left facet joint in X-axis was the largest at 2.19 ± 2.28 mm, and the rotation angle was 0.03 ± 2.02° to 3.98 ± 0.37°.ConclusionIn sitting position, weight-bearing has certain influence on the displacement of facet joints during lumbar lateral bending movement, and this influence occurs simultaneously in translation and rotation; the left and right facet joints are not symmetrical during lumbar lateral bending movement.

Tether pre-tension within vertebral body tethering reduces motion of the spine and influences coupled motion: a finite element analysis

Anterior Vertebral Body Tethering (VBT) is a novel fusionless treatment option for selected adolescent idiopathic scoliosis patients which is gaining widespread interest. The primary objective of this study is to investigate the effects of tether pre-tension within VBT on the biomechanics of the spine including sagittal and transverse parameters as well as primary motion, coupled motion, and stresses acting on the L2 superior endplate. For that purpose, we used a calibrated and validated Finite Element model of the L1-L2 spine. The VBT instrumentation was inserted on the left side of the L1-L2 segment with different cord pre-tensions and submitted to an external pure moment of 6 Nm in different directions. The range of motion (ROM) for the instrumented spine was measured from the initial post-VBT position. The magnitudes of the ROM of the native spine and VBT-instrumented with pre-tensions of 100 N, 200 N, and 300 N were, respectively, 3.29°, 2.35°, 1.90° and 1.61° in extension, 3.30°, 3.46°, 2.79°, and 2.17° in flexion, 2.11°, 1.67°, 1.33° and 1.06° in right axial rotation, and 2.10°, 1.88°, 1.48° and 1.16° in left axial rotation. During flexion-extension, an insignificant coupled lateral bending motion was observed in the native spine. However, VBT instrumentation with pre-tensions of 100 N, 200 N, and 300 N generated coupled right lateral bending of 0.85°, 0.81°, and 0.71° during extension and coupled left lateral bending of 0.32°, 0.24°, and 0.19° during flexion, respectively. During lateral bending, a coupled extension motion of 0.33–0.40° is observed in the native spine, but VBT instrumentation with pre-tensions of 100 N, 200 N, and 300 N generates coupled flexion of 0.67°, 0.58°, and 0.42° during left (side of the implant) lateral bending and coupled extension of 1.28°, 1.07°, and 0.87° during right lateral bending, respectively. Therefore, vertebral body tethering generates coupled motion. Tether pre-tension within vertebral body tethering reduces the motion of the spine.

Comparative finite element analysis of posterior short segment fixation constructs with or without intermediate screws in the fractured vertebrae for the treatment of type a thoracolumbar fracture

Six-screw short-segment posterior fixation for thoracolumbar fractures, which involves intermediate screws at the fractured vertebrae has been proposed to reduce the rates of kyphosis recurrence and implant failure. Yet, little is known about the mechanisms and biomechanical responses by which intermediate screws at the fracture vertebrae enhance fixation strength. The objective of this study was to investigate the biomechanical properties that are associated with the augmentation of intermediate screws in relation to the severity of type A thoracolumbar fracture using finite element analysis. Short-segment stabilization models with or without augmentation screws at fractured vertebrae were established based on finite element model of moderate compressive fractures, severe compressive fractures and burst fractures. The spinal stiffness, stresses at the implanted hardware, and axial displacement of the bony defect were measured and compared under mechanical loading conditions. All six-screw stabilization showed a decreased range of motion in extension, lateral bending, and axial rotation compared to the traditional four-screw fixation models. Burst thoracolumbar fracture benefited more from augmentation of intermediate screws at the fracture vertebrae. The stress of the rod in six-screw models increased while decreased that of pedicle screws. Our results suggested that patients with more unstable fractures might achieve greater benefits from augmentation of intermediate screws at the fracture vertebrae. Augmentation of intermediate screws at the fracture vertebrae is recommended for patients with higher wedge-shaped or burst fractures to reduce the risk of hardware failure and postoperative re-collapse of injured vertebrae.

An analytical model of seated human body exposed to combined fore-aft, lateral, and vertical vibration verified with experimental modal analysis

While most of the previous studies on biodynamics of the seated human body focused on vertical vibration in the sagittal plane, this study was designed with combined tri-axial translational vibration excitation to investigate the biodynamics in both sagittal and coronal planes. In this study, an analytical model was developed to represent the biodynamic responses of the seated human body to tri-axial translational vibration. The model consisted of thighs, pelvis, lumbar spine (L3 and L4), middle torso (thoracic spine T5-T12 and lumbar spine L1-L2), upper torso (thoracic spine T1-T6), and head (including cervical spine C1-C7). The transmissibilities from the seat to various body locations of subjects sitting with an upright backrest under single-axis and tri-axial vibration were measured. The parameters of the seated human model were determined by fitting the modelled in-line transmissibilities to the chest, L3 and pelvis along the fore-aft, lateral and vertical directions to the measured values. Five vibration modes were identified from the measured transmissibilities below 20 Hz. The first mode at 1.3 Hz featured the lateral sway of the upper body in the coronal plane. The second mode at 2.5 Hz characterised the lateral motion of the whole body accompanied by the lateral bending motion of the torso. The third mode at 3.6 Hz contained the fore-aft and pitch movements of the upper body with pitch of the pelvis. The fourth mode at 6.4 Hz included the vertical motion of the entire body with pitch of the pelvis, and the fifth mode at 10.3 Hz involved the vertical motion of the thighs with pitch and vertical motion of the pelvis. The model was verified with the measured vibration modes. This study provided a method for the identification of model parameters using the body transmissibilities and modal properties.

Cervical Range of Motion Assessment through Inertial Technology: A Validity and Reliability Study

Inertial technology has spread widely for its comfortable use and adaptability to various motor tasks. The main objective of this study was to assess the validity of inertial measurements of the cervical spine range of motion (CROM) when compared to that of the optoelectronic system in a group of healthy individuals. A further aim of this study was to determine the optimal placement of the inertial sensor in terms of reliability of the measure, comparing measurements obtained from the same device placed at the second cervical vertebra (C2), the forehead (F) and the external occipital protuberance (EOP). Twenty healthy subjects were recruited and asked to perform flexion–extension, lateral bending, and axial rotation movements of the head. Outcome measurements of interest were CROM and mean angular velocities for each cervical movement. Results showed that inertial measurements have good reliability (0.75 < ICC < 0.9). Excellent reliability (ICC > 0.9) was found in both flexion and right lateral bending angles. All parameters extracted with EOP placement showed ICC > 0.62, while ICC < 0.5 was found in lateral bending mean angular velocities both for F and C2 placements. Therefore, the optimal sensor’s positioning emerged to be EOP. These results suggest that inertial technology could be useful and reliable for the evaluation of the CROM.

An Immunological and Biomechanical Comparison of PEEK-Zeolite and PEEK Interbody Fusion Devices.

A laboratory study comparing PEEK-zeolite and PEEK spinal implants in an ovine model. This study challenges a conventional spinal implant material, PEEK, to PEEK-zeolite using a non-plated cervical ovine model. Although widely used for spinal implants due to its material properties, polyether ether ketone (PEEK) is hydrophobic, resulting in poor osseointegration, and elicits a mild non-specific foreign body response. Zeolites are negatively-charged aluminosilicate materials that are hypothesized to reduce this pro-inflammatory response when used as a compounding material with PEEK. Fourteen skeletally mature sheep were each implanted with one PEEK-zeolite interbody device and one PEEK interbody device. Both devices were packed with autograft and allograft material and randomly assigned to one of two cervical disc levels. The study involved two survival timepoints (12 and 26wk) and biomechanical, radiographic, and immunological endpoints. One sheep expired from complications not related to the device or procedure. Biomechanical evaluation was based on measures of segmental flexibility, using a six-degrees-of-freedom pneumatic spine tester. Radiographic evaluation was performed using microCT scans in a blinded manner by three physicians. Levels of the pro-inflammatory cytokines interleukin-1β, interleukin-6, and tumor necrosis factor alpha at the implant were quantified using immunohistochemistry. PEEK-zeolite and PEEK exhibited equivalent range of motion in flexion extension, lateral bending and axial torsion. Motion was significant reduced for implanted devices at both time points as compared to native segments. Radiographic assessments of fusion and bone formation were similar for both devices. PEEK-zeolite exhibited lower levels of interleukin-1β (P=0.0003) and interleukin-6 (P=0.0300). PEEK-zeolite interbody fusion devices provide initial fixation substantially equivalent to PEEK implants but exhibit a reduced pro-inflammatory response. PEEK-zeolite devices may reduce the chronic inflammation and fibrosis previously observed with PEEK devices.

Bio-inspired skeletal model and kinematics of humanoid spine and ribs

Development of humanoids with human like range of motion for spine and torso is of great research interest. Present article covers the construction of the humanoid spine and ribs by considering spine as a hyper redundant mechanism with ribs attached in the thoracic region. To demonstrate the joints and motion characteristics, a thorough discussion of the biomechanical aspects of the spine is provided. A three-segment hyper redundant mechanism is utilised to imitate the human mimetic spine. The study considers flexion-extension and lateral bending motions of the spine. In the proposed formulation, a piecewise constant curvature approximation could simulate spine motion during flexion-extension movement. A simplified configuration with each section of spine controlled by two cables for flexion-extension and two cables for lateral bending is adopted. Additionally, a three dimensional CAD model of proposed system is developed and simulated using ADAMS. The constant curvature model and ADAMS simulation results closely match the human range of motion.

Residual motion of cortical versus pedicle screw constructs after decompression, interbody fusion and cross-link augmentation

PurposeTo compare the residual range of motion (ROM) of cortical screw (CS) versus pedicle screw (PS) instrumented lumbar segments and the additional effect of transforaminal interbody fusion (TLIF) and cross-link (CL) augmentation.MethodsROM of thirty-five human cadaver lumbar segments in flexion/extension (FE), lateral bending (LB), lateral shear (LS), anterior shear (AS), axial rotation (AR), and axial compression (AC) was recorded. After instrumenting the segments with PS (n = 17) and CS (n = 18), ROM in relation to the uninstrumented segments was evaluated without and with CL augmentation before and after decompression and TLIF.ResultsCS and PS instrumentations both significantly reduced ROM in all loading directions, except AC. In undecompressed segments, a significantly lower relative (and absolute) reduction of motion in LB was found with CS 61% (absolute 3.3°) as compared to PS 71% (4.0°; p = 0.048). FE, AR, AS, LS, and AC values were similar between CS and PS instrumented segments without interbody fusion. After decompression and TLIF insertion, no difference between CS and PS was found in LB and neither in any other loading direction. CL augmentation did not diminish differences in LB between CS and PS in the undecompressed state but led to an additional small AR reduction of 11% (0.15°) in CS and 7% (0.05°) in PS instrumentation.ConclusionSimilar residual motion is found with CS and PS instrumentation, except of slightly, but significantly inferior reduction of ROM in LB with CS. Differences between CS and PS in diminish with TLIF but not with CL augmentation.

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

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

Adding sacral anchors through an S1 alar screw and multirod construct as a strategy for lumbosacral junction augmentation: an in vitro comparison to S1 pedicle screws alone with sacroiliac fixation.

Achieving solid fusion of the lumbosacral junction continues to be a challenge in long-segment instrumentation to the sacrum. The purpose of this study was to test the condition of adding sacral anchors through an S1 alar screw (S1AS) and multirod construct relative to using S1 pedicle screws (S1PSs) alone with sacroiliac fixation in lumbosacral junction augmentation. Seven fresh-frozen human lumbar-pelvic spine cadaveric specimens were tested under nondestructive moments (7.5 Nm). The ranges of motion (ROMs) in extension, flexion, left and right lateral bending (LB), and axial rotation (AR) of instrumented segments (L3-S1); the lumbosacral region (L5-S1); and the adjacent segment (L2-3) were measured, and the axial construct stiffness (ACS) was recorded. The testing conditions were 1) intact; 2) bilateral pedicle screw (BPS) fixation at L3-S1 (S1PS alone); 3) BPS and unilateral S2 alar iliac screw (U-S2AIS) fixation; 4) BPS and unilateral S1AS (U-S1AS) fixation; 5) BPS and bilateral S2AIS (B-S2AIS) fixation; and 6) BPS and bilateral S1AS (B-S1AS) fixation. Accessory rods were used in testing conditions 3-6. In all directions, the ROMs of L5-S1 and L3-S1 were significantly reduced in B-S1AS and B-S2AIS conditions, compared with intact and S1PS alone. There was no significant difference in reduction of the ROMs of L5-S1 between B-S1ASs and B-S2AISs. Greater decreased ROMs of L3-S1 in extension and AR were detected with B-S2AISs than with B-S1ASs. Both B-S1ASs and B-S2AISs significantly increased the ACS compared with S1PSs alone. The ACS of B-S2AISs was significantly greater than that of B-S1ASs, but with greater increased ROMs of L2-3 in extension. Adding sacral anchors through S1ASs and a multirod construct was as effective as sacropelvic fixation in lumbosacral junction augmentation. The ACS was less than the sacropelvic fixation but with lower ROMs of the adjacent segment. The biomechanical effects of using S1ASs in the control of long-instrumented segments were moderate (better than S1PSs alone but worse than sacropelvic fixation). This strategy is appropriate for patients requiring advanced lumbosacral fixation, and the risk of sacroiliac joint violation can be avoided.

Yeditepe spine mesh: Finite element modeling and validation of a parametric CAD model of lumbar spine

Finite element analysis is a powerful tool that is often used to study the biomechanical response of the spine. The primary objective of this study was to illustrate the mechanical behavior of a previously proposed parametric CAD spine model in comparison with a segmented FSU model and the literature. In this study, two finite element models of the L4-L5 spinal level were developed from the same patient's CT scan data. The first was developed using well-known segmentation methods, whereas the second was developed from the new by using a novel parametric CAD model. Both models were subjected to the same loading and boundary conditions to perform flexion, extension, lateral bending and axial rotation motions. The segmented finite element model was observed to be in good agreement with the literature. The parametric finite element model results were also observed to be in good agreement with the segmented finite element model and with the literature except under extension.

Biomechanical Performances of an Oblique Lateral Interbody Fusion Cage in Models with Different Bone Densities: A Finite Element Analysis.

Finite element models of the L3-S1 vertebrae were reconstructed using computed tomography scans. We compared the biomechanical performances of an oblique lateral interbody fusion (OLIF) cage in different bone density mode. Low bone density is an els.key factor limiting the use of stand-alone OLIF cage. Four models-intact (M0), normal bone density with OLIF (M1), bone mass loss with OLIF (M2), and osteoporotic with OLIF (M3)-were created based on 3-dimensional scans. Flexion, extension, and lateral bending movements (each lasting 10N·m) were performed on the superior surface of the L3 vertebra with a compressive preload of 500N. Range of motion (ROM), peak stresses in the L4-5 cortical endplates, cage stress, and adjacent intervertebral disk stress were evaluated. ROMs during different physiological movements were similar to those reported by previous researchers. Compared with that in M0, L4-5 ROMs of all movements decreased in M1, M2 and M3, most evidently in M3. Stress distribution in the cortical endplates rose to 7.8% in M1 and M2, even 16.2% in M3. Cage stress increased by less than 8.1% in M1 and M2, but by 25.3% in M3, especially in the movements of extension and right rotation. Compared with that in M0, L3-4 and L5-S1 intervertebral disk stress increased with bone density in all the other models, by up to 69.8% and 98.3%, respectively. As osteoporosis worsened, stress in the adjacent intervertebral disk also increased. Stand-alone OLIF in M3 is not recommended because of the risk of cage subsidence. OLIF in M1 and M2 achieved similar results in various lumbar spine movements. In M1 and M2 model (T > - 2.5), the L4-L5 showed reduced mobility in all directions, increased rigidity, limited cage displacement, lessened deformation, and better stability.

Effects of rod diameter on kinematics of posterior cervical spine instrumented constructs: an ex vivo study.

Posterior cervical spine fixation is a robust strategy for stabilizing the spine for a wide range of spinal disorders. With the evolution of spinal implant technology, posterior fixation with lateral mass screws in the subaxial spine is now common. Despite interest in variable rod diameters to meet a wide range of clinical needs such as trauma, revision, and deformity surgery, indications for use of posterior cervical spine fixation are not clear. This laboratory investigation evaluates the mechanical stability and kinematic properties of lateral mass fixation with various commercially available rod diameters. The authors conducted an ex vivo experiment using 13 fresh-frozen human cervical spine specimens, instrumented from C3 to C6 with lateral mass screws, to evaluate the effects of titanium rod diameter on kinematic stability. Each intact spine was tested using a kinematic profiling machine with an optoelectrical camera and infrared sensors applying 1.5-Nm bending moments to the cranial vertebra (C2) simulating flexion-extension, lateral bending, and axial rotation anatomical motions. A compressive follower preload of 150 N was applied in flexion-extension prior to application of a bending moment. Instrumented spines were then tested with rod diameters of 3.5, 4.0, and 4.5 mm. The kinematic data between intact and surgical cases were studied using a nonparametric Wilcoxon signed-rank test. A multivariable, multilevel linear regression model was built to identify the relationship between segmental motion and rod diameter. Instrumentation resulted in significant reduction in range of motion in all three rod constructs versus intact specimens in flexion-extension, lateral bending, and axial rotation (p < 0.05). The maximum reductions in segmental ROM versus intact spines in 3.5-, 4.0-, and 4.5-mm rod constructs were 61%, 71%, and 81% in flexion-extension; 70%, 76%, and 81% in lateral bending; and 50%, 60%, and 75% in axial rotation, respectively. Segmental motion at the adjacent segments (C2-3 and C6-7) increased significantly (p < 0.05) with increasing rod diameter. The 4.5-mm rod construct had the greatest increase in motion compared to the intact spine. With increasing rod diameters from 3.5 to 4.0 mm, flexion-extension, lateral bending, and axial rotation across C3-6 were significantly reduced (p < 0.05). Similar trends were observed with a statistically significant reduction in motion in all anatomical planes when the rod diameter was increased to 4.5 mm. Although the increase in rod diameter resulted in a more rigid construct, it also created an increase (p < 0.05) in the kinematics of the adjacent segments (C2-3 and C6-7). Whether this increase translates into adverse long-term clinical effects in vivo requires further investigation and clinical assessment.

Biomechanical effects of interbody cage height on adjacent segments in patients with lumbar degeneration: a 3D finite element study

ObjectiveTo investigate the biomechanical effects of interbody cage height on adjacent segments in patients with lumbar degeneration undergoing transforaminal lumbar interbody fusion (TLIF) surgery, so as to provide references for selection of interbody cage.MethodsThe finite element model of normal lower lumbar spine (L3–S1) was built and validated, then constructed three different degenerative segments in L3–L4, and the cages with different height (8, 10, 12, 14 mm) were implanted into L4–L5 disc. All the twelve models were loaded with pure moment of 7.5 N m to produce flexion, extension, lateral bending and axial rotation motions on lumbar spine, and the effects of cage height on range of motion (RoM) and intervertebral pressure in lumbar spine were investigated.ResultsThe RoM of adjacent segments and the maximum stress of intervertebral discs increased with the increase in cage height, but this trend was not obvious in mild and moderate degeneration groups. After implantation of four different height cages (8, 10, 12, 14 mm), the RoM of L3/L4 segment reached the maximum during extension. The RoM of mild degeneration group was 2.07°, 2.45°, 2.48°, 2.54°, that of moderate degeneration group was 1.79°, 1.97°, 2.05°, 2.05°, and that of severe degeneration group was 1.43°, 1.66°, 1.74°, 1.74°. The stress of L3–L4 intervertebral disc reached the maximum during flexion. The maximum stress of L3–L4 intervertebral disc was 20.16 MPa, 20.28 MPa, 20.31 MPa and 20.33 MPa in the mild group, 20.58 MPa, 20.66 MPa, 20.71 MPa and 20.75 MPa in the moderate group, and 21.27 MPa, 21.40 MPa, 21.50 MPa and 21.60 MPa in the severe group.ConclusionFor patients with mild-to-moderate lumbar degenerative disease who need to undergo TLIF surgery, it is recommended that the height of fusion cage should not exceed the original intervertebral space height by 2 mm, while for patients with severe degeneration, a fusion cage close to the original intervertebral height should be selected as far as possible, and the intervertebral space should not be overstretched.

A biomechanical study of ligament tethers strengthening for the prevention of proximal junctional kyphosis after posterior long-segment spinal fusion

BackgroundProximal junctional kyphosis is a known complication of posterior long-segment thoracolumbar fusion. Here, the biomechanical effectiveness of ligament tethers strengthening and vertebral body augmentation, in proximal junctional kyphosis prevention was explored using the finite element analysis. MethodsBased on a validated model of T1-L5 with the pedicle screw system instrumented T8-L5, strengthening models with different strategies were created to assess the range of motion in proximal vertebrae, vertebrae stress, pedicle screw stress, and pressure on intervertebral discs during extension, flexion, lateral bending, and axial rotation motions. Strengthening strategies included two- and three-level posterior ligament tethers (TE-T7-T9 and TE-T6-T9), and tethers with T7 &T8 vertebral body augmentation (TECE-T7-T9 and TECE-T6-T9). FindingsCompared to the spinal fusion model, the ligament tethers strengthening significantly reduced the flexion-extension range of motion difference among the proximal vertebrae. During the flexion-extension motion, the T8 vertebra stresses in the TE-T7-T9, TE-T6-T9, TECE-T7-T9, and TECE-T6-T9 models were distinctively reduced, the values decreased by 26.8%, 28.3%, 28.8%, and 9.6%, respectively, during flexion, and by 21.9%, 35.2%, 23%, and 18.6%, respectively, during extension. In the strengthening models, the maximum stresses on the T7/T8 intervertebral disc in the TE-T6-T9 model were reduced by 13.8% during flexion and by 14.7% during extension. InterpretationBased on our results, the ligament strengthening configuration of the three-level posterior tethers produced a more gradual transition in range of motion, vertebrae stresses, and intervertebral discs stress between the fused and non-fused segments, especially during flexion-extension, which may significantly decrease the proximal junctional kyphosis biomechanical risk.