Flexion-extension Loading Research Articles

Cyclic testing of standalone ALIF versus TLIF in lumbosacral spines of low bone mineral density: an ex vivo biomechanical study.

Screwed anterior lumbar interbody fusion (SALIF) alleviates the need for supplemental posterior fixation leading to reduction of perioperative morbidity. Specifically, elderly and multimorbid patients would benefit from shorter operative time and faster recovery but tend to have low bone mineral density (BMD). The current study aimed to compare loosening, defined as increase of ROM and NZ, of SALIF versus transforaminal lumbar interbody fusion (TLIF) under cyclic loading in cadaveric spines with reduced BMD. Twelve human spines (L4-S2; 6 male 6 female donors; age 70.6 ± 19.6; trabecular BMD of L5 84.2 ± 24.4mgHA/cm3, range 51-119mgHA/cm3) were assigned to two groups. SALIF or TLIF were instrumented at L5/S1. Range of motion (ROM) and neutral zone (NZ) were assessed before and after axial cyclic loading (0-1150N, 2000 cycles, 0.5Hz) in flexion-extension (Flex-Ext), lateral bending, (LB), axial rotation (AR). ROM of the SALIF specimens increased significantly in all loading directions (p ≤ 0.041), except for left AR (p = 0.053), whereas for TLIF it increased significantly in left LB (p = 0.033) and Flex (p = 0.015). NZ of SALIF showed increase in Flex-Ext and LB, whereas NZ of TLIF did not increase significantly in any motion direction. Axial compression loading caused loosening of SALIF in Flex-Ext and LB, but not TLIF at L5/S1 in low BMD specimens. Nevertheless, Post-cyclic ROM and NZ of SALIF is comparable to TLIF. This suggests that, neither construct is optimal for the use in patients with reduced BMD.

Biomechanical Analysis of Trapezoidal Thread Screw-Rod Fixation in Pedicle Section of Cervical Spine: A Finite-Element Analysis.

Cervical pedicle screw-rod fixation presents a complex approach in spinal surgery, offering enhanced spine stabilization in variable conditions considering traumatic injuries, degenerative changes, as well as orthopaedic and oncological ailments. This technique employs small diameter screw implants strategically placed to bolster the mechanical integrity of the spine. Notably, it involves minimally invasive procedures, resulting in smaller incisions and reduced patient discomfort. This study aims to assess the effects of trapezoidal thread screws in pedicle sections of the cervical spine during flexion-extension loadings, focusing on factors such as range of motion (ROM), implant stress, and stress on adjacent bone. Utilizing CT scan data, a finite element model of the cervical spine (C2-C7 vertebrae) was prepared. Trapezoidal thread screws were integrated into a single-level pedicle screw-rod fixation at the C5-C6 vertebrae. The C2 vertebra were given a compressive load of 50 N along with a moment of 1 Nm, resulting in the immobilization of the C7. The results indicate a reduction in ROM at the C5-C6 level by 69% to 77% compared to the intact spine during flexion-extension loading, with a slight increase in ROM observed at adjacent cervical spine levels. Stress analysis revealed that the trapezoidal thread screws induced stresses ranging from 24 MPa to 29 MPa in PEEK trapezoidal screw-rod implants, which fall below the material's yield stress. This suggests that the trapezoidal thread profile may be advantageous in minimizing stress concentration, attributed to its larger contact area with the vertebrae bone between the threads.

Posterior intra-articular fixation stabilizes both primary and secondary sacroiliac joints: a cadaveric study and comparison to lateral trans-articular fixation literature

BackgroundPosterior and lateral techniques have been described as approaches to sacroiliac joint arthrodesis. The purpose of this study was to compare the stabilizing effects of a novel posterior stabilization implant and technique to a previously published lateral approach in a cadaveric multidirectional bending model. We hypothesized that both approaches would have an equivalent stabilizing effect in flexion–extension and that the posterior approach would exhibit better performance in lateral bending and axial rotation. We further hypothesized that unilateral and bilateral posterior fixation would stabilize both the primary and secondary joints.MethodsRanges of motion (RoMs) of six cadaveric sacroiliac joints were evaluated by an optical tracking system, in a multidirectional flexibility pure moment model, between ± 7.5 N-m applied moment in flexion–extension, lateral bending, and axial rotation under intact, unilateral fixation, and bilateral fixation conditions.ResultsIntact RoMs were equivalent between both samples. For the posterior intra-articular technique, unilateral fixation reduced the RoMs of both primary and secondary joints in all loading planes (flexion–extension RoM by 45%, lateral bending RoM by 47%, and axial RoM by 33%), and bilateral fixation maintained this stabilizing effect in both joints (flexion–extension at 48%, lateral bending at 53%, and axial rotation at 42%). For the lateral trans-articular technique, only bilateral fixation reduced mean RoM of both primary and secondary sacroiliac joints, and only under flexion–extension loads (60%).ConclusionDuring flexion–extension, the posterior approach is equivalent to the lateral approach, while producing superior stabilization during lateral bend and axial rotation.

Analytical and numerical study of shape memory polymers as intervertebral discs under pure bending

Rupture of the elastic nucleus is one of the most critical challenges in total disc replacement (TDR). Smart polymers (SMPs) could be good candidates for TDR due to their unique properties such as self-healing, shape memory, adhesion control, adhesive capability and self-assembly. Herein, we investigate the behavior of SMPs as intervertebral discs (IVDs) under pure bending loading using analytical and numerical methods. For this purpose, the lumbar disc was considered as a cantilever beam and constitutive equation for pure bending mode using Euler-Bernoulli beam theory were developed in a full cycle of stress-free strain recovery. Numerical evaluation was performed to understand the effect of frozen volume fraction to determine the appropriate SMP for clinical evaluations. We conduct all analyses for the L4-L5 vertebral section because it is the most susceptible lumbar disc to damage. The numerical results show that SMPs with rescued frozen volume fraction are better representative for natural lumbar disc. Our results also indicate that SMPs that replace IVDs should be able to withstand at least 7.8° and 8.93° angle changes for pure bending and flexion-extension loads without any damage.

Stepwise reduction of bone mineral density increases the risk of cage subsidence in oblique lumbar interbody fusion patients biomechanically: an in-silico study

BackgroundCage subsidence causes poor prognoses in patients treated by oblique lumbar interbody fusion (OLIF). Deterioration of the biomechanical environment initially triggers cage subsidence, and patients with low bone mineral density (BMD) suffer a higher risk of cage subsidence. However, whether low BMD increases the risk of cage subsidence by deteriorating the local biomechanical environment has not been clearly identified.MethodsOLIF without additional fixation (stand-alone, S-A) and with different additional fixation devices (AFDs), including anterolateral single rod screws (ALSRs) and bilateral pedicle screws (BPSs) fixation, was simulated in the L4-L5 segment of a well-validated finite element model. The biomechanical effects of different BMDs were investigated by adjusting the material properties of bony structures. Biomechanical indicators related to cage subsidence were computed and recorded under different directional moments.ResultsOverall, low BMD triggers stress concentration in surgical segment, the highest equivalent stress can be observed in osteoporosis models under most loading conditions. Compared with the flexion-extension loading condition, this variation tendency was more pronounced under bending and rotation loading conditions. In addition, AFDs obviously reduced the stress concentration on both bony endplates and the OLIF cage, and the maximum stress on ALSRs was evidently higher than that on BPSs under almost all loading conditions.ConclusionsStepwise reduction of BMD increases the risk of a poor local biomechanical environment in OLIF patients, and regular anti-osteoporosis therapy should be considered an effective method to biomechanically optimize the prognosis of OLIF patients.

Biomechanical comparison of a novel tensioned cable construct versus tension band wiring for transverse patella fracture fixation.

Tension band wiring (TBW) is the most widely accepted method for patella fracture fixation. The purpose of our study was to compare the biomechanical efficacy of a novel cable construct to TBW for the fixation of transverse patella fractures. The tensioned cable construct was hypothesized to have less fracture gapping after cyclic flexion-extension loading and greater ultimate load to failure as compared to TBW. Transverse patellar osteotomies (AO/OTA 34C1.1) were performed on nine pairs of fresh-frozen human cadaveric whole legs (mean age 82.2years, range 71-101). Treatment with TBW or tensioned cable construct was randomized within each specimen pair. Fracture site displacement was measured after 5000 flexion-extension cycles from 0° to 90° at 0.5Hz. In load to failure testing, the knee was fixed at 45° of flexion and the quadriceps tendon was pulled proximally at 0.5mm/sec until patella fixation failure. Comparisons were made using paired t-tests with alpha values of 0.05. Eight paired specimens completed the cyclic loading. The tensioned cable construct had significantly less fracture gapping than TBW (2.9 vs 10.9mm; p = 0.020). Seven paired limbs underwent load to failure testing, which revealed no significant difference between the tensioned cable construct and TBW (1551.6N vs 1664.0N; p = 0.26). In this study of transverse patella fracture fixation, a tensioned cable construct demonstrated significantly less fracture gapping compared to TBW in response to cyclic loading with no significant difference in load at failure.

Annulus Calibration Increases the Computational Accuracy of the Lumbar Finite Element Model.

Mechanical simulations. Inadequate calibration of annuli negatively affects the computational accuracy of finite element (FE) models. Specifically, the definition of annulus average radius (AR) does not have uniformity standards. Differences between the elastic moduli in the different layers and parts of the annulus were not fully calibrated when a linear elastic material is used to define its material properties. This study aims to optimize the computational accuracy of the FE model by calibrating the annulus. We calibrated the annulus AR and elastic modulus in our anterior-constructed lumbar model by eliminating the difference between the computed range of motion and that measured by in vitro studies under a flexion-extension loading condition. Multi-indicator validation was performed by comparing the computed indicators with those measured in in vitro studies. The computation time required for the different models has also been recorded to evaluate the computational efficiency. The difference between computed and measured ROMs was less than 1% when the annulus AR and elastic modulus were calibrated. In the model validation process, all the indicators computed by the calibrated FE model were within ±1 standard deviation of the average values obtained from in vitro studies. The maximum difference between the computed and measured values was less than 10% under nearly all loading conditions. There is no apparent variation tendency for the computational time associated with different models. The FE model with calibrated annulus AR and regional elastic modulus has higher computational accuracy and can be used in subsequent mechanical studies.

A Cadaver-Based Biomechanical Evaluation of a Novel Posterior Approach to Sacroiliac Joint Fusion: Analysis of the Fixation and Center of the Instantaneous Axis of Rotation.

PurposeThe purpose of this study was to assess the stabilizing effect of a posterior joint fixation technique using a novel cortical allograft implant in unilateral and bilateral fixation constructs. We hypothesize that fixation would reduce the joint’s range of motion during flexion-extension, axial rotation, and lateral bending loads. We also hypothesize that fixation would shift the center of the instantaneous axis of rotation during the predominant flexion-extension motions towards the implant’s location, and that this shift would be correlated with the reduction in flexion-extension range of motion.Materials and MethodsSix cadaveric sacroiliac joint specimens were tested under intact, unilateral fixation, and bilateral fixation conditions. The total range of motion (ROM) of the sacroiliac joint in flexion–extension, lateral bending, and axial rotation were evaluated by an optical tracking system, in a multidirectional flexibility pure moment model, between ± 7.5 Nm applied moment loads. The centers of the instantaneous axis of rotation (cIAR) of the sacroiliac joint were evaluated during flexion-extension loading. A correlation analysis was performed between the ROM reduction in flexion-extension upon implantation and shift of the cIAR to the graft implantation site.ResultsUnilateral and bilateral fixations generated sacroiliac joint ROM reductions in flexion-extension, lateral bending, and axial rotation motions. Fixation shifted the cIAR to the graft implantation site. Reduction in the total range of motion had a moderate correlation with the shift of the cIAR.ConclusionOur novel posterior approach presents a multifaceted mechanism for stabilizing the joint: first, by the reduction of the total range of motion in all planes of motion; second, by shifting the centers of the instantaneous axis of rotation towards the implant’s location in the predominant plane of motion, ensuring little to no motion at the implantation site, thus promoting fusion in this region.

Biomechanical comparison of semirigid junctional fixation techniques to prevent proximal junctional failure after thoracolumbar adult spinal deformity correction

BACKGROUND CONTEXTAdult spinal deformity patients treated operatively by long-segment instrumented spinal fusion are prone to develop proximal junctional kyphosis (PJK) and failure (PJF). A gradual transition in range of motion (ROM) at the proximal end of spinal instrumentation may reduce the incidence of PJK and PJF, however, previously evaluated techniques have not directly been compared. PURPOSETo determine the biomechanical characteristics of five different posterior spinal instrumentation techniques to achieve semirigid junctional fixation, or “topping-off,” between the rigid pedicle screw fixation (PSF) and the proximal uninstrumented spine. STUDY DESIGNBiomechanical cadaveric study. METHODSSeven fresh-frozen human cadaveric spine segments (T8–L3) were subjected to ex vivo pure moment loading in flexion-extension, lateral bending and axial rotation up to 5 Nm. The native condition, three-level PSF (T11–L2), PSF with supplemental transverse process hooks at T10 (TPH), and two sublaminar taping techniques (knotted and clamped) as one- (T10) or two-level (T9, T10) semirigid junctional fixation techniques were compared. The ROM and neutral zone (NZ) of the segments were normalized to the native condition. The linearity of the transition zones over three or four segments was determined through linear regression analysis. RESULTSAll techniques achieved a significantly reduced ROM at T10-T11 in flexion-extension and axial rotation relative to the PSF condition. Additionally, both two-level sublaminar taping techniques (CT2, KT2) had a significantly reduced ROM at T9-T10. One-level clamped sublaminar tape (CT1) had a significantly lower ROM and NZ compared with one-level knotted sublaminar tape (KT1) at T10-T11. Linear regression analysis showed the highest linear correlation between ROM and vertebral level for TPH and the lowest linear correlation for CT2. CONCLUSIONSAll studied semirigid junctional fixation techniques significantly reduced the ROM at the junctional levels and thus provide a more gradual transition than pedicle screws. TPH achieves the most linear transition over three vertebrae, whereas KT2 achieves that over four vertebrae. In contrast, CT2 effectively is a one-level semirigid junctional fixation technique with a shift in the upper rigid fixation level. Clamped sublaminar tape reduces the NZ greatly, whereas knotted sublaminar tape and TPH maintain a more physiologic NZ. Clinical validation is ultimately required to translate the biomechanics of various semirigid junctional fixation techniques into the clinical goal of reducing the incidence of proximal junctional kyphosis and failure. CLINICAL SIGNIFICANCEThe direct biomechanical comparison of multiple instrumentation techniques that aim to reduce the incidence of PJK after thoracolumbar spinal fusion surgery provides a basis upon which clinical studies could be designed. Furthermore, the data provided in this study can be used to further analyze the biomechanical effects of the studied techniques using finite element models to better predict their post-operative effectiveness.

In-vitro 3D Analysis of Sacroiliac Joint Kinematics: Primary and Coupled Motions.

An in-vitro biomechanical study of human cadaver sacroiliac joints. Our study aimed to develop a more comprehensive understanding of the native motion of the SIJ within the context of spinal kinematics and spinal implant evaluation. Increasing attention has been given to the sacroiliac joint (SIJ) as a source of low back pain, despite its limited range of motion. We sought to characterize the rotational and translational motion in each axis utilizing standard pure moment flexion-extension (FE), lateral bending (LB), and axial rotation (AR) testing. Sixteen sacroiliac joints were evaluated from eight lumbosacral cadaver specimens (six females, two males) from subjects aged 28 to 57 years (mean age 46.8) with body mass index (BMI) 22 to 36 (mean BMI 30). Single leg stance was modeled by clamping the blocks on one ischium in a vise and letting the contralateral ischium hang freely. Pure moment loading was applied in FE, right/left AR, and right/left LB. Relative motions were collected with infrared markers. The on-axis ratio was significantly lower in LB than in FE (P = 0.012) and in AR (P = 0.017). The rotation deviation angle measured 13.9 ± 9.1° in FE, 17.1 ± 8.7° in AR, and 35.7 ± 25.7° in LB. In LB the rotational deviation angle is significantly higher than both FE and AR (P = 0.003 and P = 0.011, respectively). In-plane translation was significantly higher (P = 0.005) in FE loading than in LB loading. A nontrivial amount of rotation and translation occurred out of the expected axis of motion. The largest amount of off-axis rotation was observed in lateral bending. Relative to resultant translation, in-plane translation was lowest in lateral bending. Our results indicate that rotation of the SIJ is not fully described with the in-plane metrics which are normally reported in evaluation of fusion devices. Future studies of the SIJ may need to consider including off-axis rotation measurements when describing SIJ kinematics.Level of Evidence: 5.

Influence of cervical spine sagittal alignment on range of motion after corpectomy: a finite element study.

Sagittal alignment of the cervical spine might influence the development of radiological adjacent segment pathology (RASP) after central corpectomy (CC). Range of motion (ROM) of the adjacent segments is closely linked to the development of RASP. To investigate the ROM of the adjacent segments after CC, we developed a C2-T1 finite element (FE) model. The model with a lordotic sagittal alignment served as the baseline model. Models with straight and kyphotic alignment were generated using mesh morphing methods. Single-level corpectomy at C5 was done on these models. Segmental ROMs of intact and corpectomized spines were compared for physiologic flexion-extension loads. The flexion ROM decreased by an average of 13% with the change in sagittal alignment from lordosis to kyphosis; however, a consistent decrease was not observed in extension. After CC, the ROM increased by an average of 95% and 31% in the superior and inferior adjacent segments. With kyphotic change in the sagittal alignment, the postoperative increase in flexion ROM exhibited a decreasing trend, while this was not seen in extension. Kyphotic changes of the intact spine resulted in segmental stiffening, and after corpectomy, it resulted in inconsistent variations of segmental extension ROMs.

Effect of heterotopic ossification after bryan-cervical disc arthroplasty on adjacent level range of motion: A finite element study

BackgroundQuantitative bone re-modelling theories suggest that bones adapt to mechanical loading conditions. Follow-up studies have shown that total disc replacement (TDR) modifies stress patterns in the bones, leading to heterotopic ossification (HO). Although there are a few studies on HO using finite element models (FEM), its effect on the adjacent levels and change in range of motion (ROM) have not been adequately investigated. This study interfaces the HO using bone re-modelling algorithm with a finite element solution and investigates the subsequent changes in segmental ROM. MethodsA FEM of the human cervical spine (C3–C7) was developed for this study, with material properties obtained from literature. The motion of the segments in the sagittal, frontal and transverse planes under combined loading conditions of 1 Nm moment and 73.6 N compression were validated against experimental corridors. The natural disc between the C5–C6 segment was replaced with the Bryan artificial cervical disc, and changes in sagittal ROM were compared before and after HO. The process of HO was simulated using a bone remodelling algorithm using strain energy density (SED) as the mechanical stimuli. Results and conclusionOur study demonstrates the feasibility of using SED calculations from the flexion-extension loading conditions for prediction of HO after ADR. The current findings suggest that the nature of trabecular stresses, and the subsequent rate and location of HO formation could differ based on the geometric design and nature of constraint for different artificial discs. The Bryan disc significantly reduced ROM at the implanted level in flexion. However, in extension, ROM increased at the implanted level and decreased slightly at the adjacent levels. After HO, ROM drastically reduced at the implanted level in both extension and flexion, and showed a minor increase in the adjacent levels, indicating that biomechanical behavior of high-grade HO is similar to a fused segment, thereby reducing the intended and initial motion preservation.

Biomechanical analysis of two insertion sites for the fixation of the sacroiliac joint via an oblique lateral approach

BackgroundThe sacroiliac joint is an important source of low back pain. In severe cases, sacroiliac joint fusion is used to reduce pain, but revision rates can reach 30%. The lack of initial mechanical stability may lead to pseudarthrosis, thus not alleviating the patient's symptoms. This could be due to the damage induced to the interosseous ligament during implant insertion. Decoupling instrumentation steps (drilling-tapping and implant insertion) would allow verifying this hypothesis. Moreover, no biomechanical studies have been published on sacroiliac joint fixation with an oblique lateral approach, while it has important clinical advantages over the direct lateral approach. MethodsEight cadaveric human pelves with both ischia embedded were tested in three sequential states: intact, drilled-tapped and instrumented with one cylindrical threaded implant with an oblique lateral trajectory. Specimens were assigned one of two insertion sites (distal point; near the posterior superior iliac spine, and proximal point; anterosuperior to the distal point) and tested in compression and flexion-extension. Vertical and angular displacements of the sacroiliac joint were measured locally using digital image correlation methods. FindingsIn compression, instrumentation significantly reduced vertical displacements (17% (SD 22%), P = 0.04) but no difference was found for angular displacements or flexion-extension loads (P > 0.05). Drilling-tapping did not change the stability of the sacroiliac joint (P > 0.05); there was no statistical difference between the insertion sites (P > 0.05). InterpretationsInsertion of one implant through either the distal or proximal insertion site with an oblique lateral approach significantly reduced vertical displacements of the sacroiliac joint in compression, a predominant load of this joint. Research ethics committeePolytechnique Montreal: CÉR-1617-30.

Effects of Lumbosacral Arthrodesis on the Biomechanics of the Sacroiliac Joint.

Background:It is unclear whether the sacroiliac joint is vulnerable to adjacent segment disease. Clinical studies have suggested that many patients who have undergone lumbar arthrodesis will develop adjacent segment disease, which may contribute to sacroiliac joint degeneration. The purpose of the present study was to examine whether arthrodesis in the lumbar spine results in altered biomechanics at the sacroiliac joint that could contribute to adjacent segment disease within the joint.Methods:With use of human cadavers in a biomechanical laboratory study, the effects of lower-lumbar arthrodesis and sacroiliac screws on the biomechanics of the sacroiliac joint were assessed. Human cadaveric pelves with lumbar spines were biomechanically tested in flexion-extension, rotation about the vertical axis, and compression along the vertical axis with single and double-leg support. Four conditions were compared: (1) intact, (2) L4-L5 arthrodesis, (3) L4-S1 arthrodesis, and (4) left sacroiliac screw. Construct vertical and horizontal motions at the anterior and posterior surfaces of the sacroiliac joint were measured.Results:Significant measurable increases in motion of the sacroiliac joint related to arthrodesis of the lumbar spine occurred with flexion-extension loading (p < 0.05). No significant changes were observed for rotation about the vertical axis or compression along the vertical axis with single and double-leg support.Conclusions:After 360°, 1 or 2-level lumbosacral spine arthrodesis, the sacroiliac joint showed a significant increase in rotational motion with flexion-extension loading. Increases in horizontal translation with axial rotation loading and vertical translation with axial compression loading were not significant.Clinical Relevance:The risk of significant alteration of normal sacroiliac kinematics should be considered in all patients undergoing 360° lumbosacral arthrodesis.

Effects of different severities of disc degeneration on the range of motion of cervical spine.

Aims and Objectives:The human spine degenerates with age. Intervertebral disc degeneration occurs in the cervical spine. The objective of this study is to determine the effects of degenerative disc diseases on the range of motion (ROM) of the human cervical spinal column using a validated finite-element model.Materials and Methods:The validated intact and healthy C2–T1 finite-element model simulated the cortical shell, cancellous core, posterior elements of the vertebrae, and spinal ligaments (longitudinal, capsular, spinous and ligamentum flava, and nucleus and annulus of the discs). Three different stages of the disc disease, that is, mild, moderate, and severe, were simulated at the C5–C6, C6–C7, and C5–C6–C7 discs, respectively, and they were termed as upper single level, lower single level, and bi-level (BL) models, respectively. The material properties and geometry of the disc(s) were altered to simulate the different stages of degeneration. The external mechanical loading was applied in the sagittal mode, via flexion–extension motions and the magnitude was 2.0 Nm for each mode. They were applied to each of the healthy and disc degeneration models, and for each of the three severities of degeneration. The ROM at adjacent and index levels was extracted and normalized with respect to the healthy (baseline) spine.Results:A nonuniform distribution in the ROM was found for different disc degeneration states, segmental levels, and flexion–extension loading modes. The specific results for each and level are reported in the results section of the paper.Conclusion:Closer follow-up times may be necessary in symptomatic patients with progressive disease, especially with BL involvements.

Biomechanical In Vitro Test of a Novel Dynamic Spinal Stabilization System Incorporating Polycarbonate Urethane Material Under Physiological Conditions.

Posterior dynamic stabilization systems (PDSS) were developed to provide stabilization to pathologic or hypermobile spinal segments while maintaining the healthy biomechanics of the spine. Numerous novel dynamic devices incorporate the temperature and moisture dependent material polycarbonate urethane (PCU) due to its mechanical properties and biocompatibility. In this study, standardized pure moment in vitro tests were carried out on human lumbar spines to evaluate the performance of a device containing PCU. An environmental chamber with controlled moisture and temperature was included in the setup to meet the requirements of testing under physiological conditions. Three test conditions were compared: (1) native spine, (2) dynamic instrumentation, and (3) dynamic instrumentation with decompression. The ranges of motion, centers of rotation, and relative pedicle screw motions were evaluated. The device displayed significant stiffening in flexion-extension, lateral bending, and axial rotation load directions. A reduction of the native range of motion diminished the stiffening effect along the spinal column and has the potential to reduce the risk of the onset of degeneration of an adjacent segment. In combination with decompression, the implant decreased the native range of motion for flexion-extension and skew bending, but not for lateral bending and axial rotation. Curve fittings using the sigmoid function were performed to parameterize all load-deflection curves in order to enhance accurate numerical model calibrations and comparisons. The device caused a shift of the center of rotation (COR) in the posterior and caudal direction during flexion-extension loading.

Repetitive trunk loading leads to faster trunk movement in response to external perturbation

The purpose of the present study was to examine trunk movement pattern responses to mechanical perturbation before and after two different repetitive trunk flexion-extension loading schemes. Spatial and temporal parameters were studied to understand the trunk recovery from an anteriorly directed perturbation. Eighteen male and female subjects (18–27 yrs) participated in active and passive trunk flexion-extension, performed seven days apart. Subjects performed 60 trunk flexion-extension repetitions in each condition. Subjects either volitionally moved their trunks (active condition) or relaxed while a dynamometer controlled the movements (passive condition). Trunk perturbations occurred before and immediately after two 30 repetition sessions. Initial responses included latency measures of trunk displacement and peak trunk velocity (VP). Temporal measures included perturbation onset to initial trunk movement (TD), movement initiation to VP (TMIPV), and perturbation onset to VP (TPPV). Recovery measures included peak recovery velocity (VPR), recovery time (TR), velocity slope (VS), and recovery slope (RS). Differences between loading sessions were present for TPPV (p < .05), TR (p < .05), and RS (p < .05). Overall, the results indicate repetitive loading leads to lower resistance to perturbation, but faster recovery from perturbation although no differences to active or passive repetitive loading was observed.

Human and bovine spinal disc mechanics subsequent to trypsin injection

ObjectiveTo investigate the biomechanical effects of injections of a protease on the characteristics of bovine coccygeal and human lumbar disc motion segments. MethodsMechanics of treated tissues were measured immediately after injection and 3 h after injection. Motion segments underwent axial rotation and flexion-extension loading. ResultsStiffness and neutral zone parameters experienced significant changes over time, with bovine tissues more strongly affected than human cadaver tissues. This was true in both axial rotation and flexion-extension. The treatment type significantly affected the neutral zone measurements in axial rotation. Hysteresis parameters were impacted by control injections. ConclusionThe extrapolation of bovine coccygeal motion testing results to human lumbar disc mechanics is not yet practical. The injected treatment may have a smaller impact on disc mechanics than time in testing. Viscoelasticity of human lumbar discs may be impacted by any damage to the annulus fibrosis induced by needlestick. The Translational Potential of this ArticlePreclinical testing of novel spinal devices is essential to the design validation and regulatory processes, but current testing techniques rely on cadaveric testing of primarily older spines with essentially random amounts of disc degeneration. The present work investigates the viability of using trypsin injections to create a more uniform preclinical model of disc degeneration from a mechanics perspective, for the purpose of testing spinal devices. Such a model would facilitate translation of new spinal technologies to clinical practice.

Integrated Fixation Cage Loosening Under Fatigue Loading.

Screw loosening is a well-known adverse event in traditional spinal fusion instrumentation. This phenomenon may hinder segmental stability of the spine leading to bony non-union. In recent years numerous lumbar integrated fixation cages (IFC) have been introduced that offer a low profile alternative to a standard cage with an anterior plate (AP+C). The fixation approach for IFCs is different than a traditional anterior approach; therefore, it is unclear whether IFCs may loosen from the surrounding bone over time. The purpose of this study was to quantify screw loosening of IFC devices compared to AP+C implants under fatigue loading using micro-CT and image processing techniques. L2-3 and L4-5 functional spinal units (FSUs) were obtained from nine human lumbar spines. These FSUs were then reconstructed with either AP+C or IFC implants designed to attach to vertebral bodies using four screws (two top and two bottom for AP+C; two medial and two lateral for IFC). The reconstructed specimens were fatigued in flexion-extension load of ±3 Nm at 1Hz for first 5,000 cycles and it was increased to ±5 Nm until 20,000 cycles. After removing screws to prevent image artifact, micro-CT scans were performed on all FSUs post-fatigue. These images were post-processed to calculate three-dimensional volumes around screw holes created due to damage at the screw-implant interface. IFC screws had significantly greater (p=0.008) screw hole volumes compared to AP+C screws after fatigue testing. This increased screw hole volume for IFC devices was mainly due to loosening in medial screws. Medial screws had significantly greater (p<0.003) screw hole volumes compared to lateral IFC screws and all AP+C screws. There was no difference (p>0.888) between the screw hole volumes of lateral IFC, top AP+C, and bottom AP+C screws. This study elucidated screw-loosening mechanisms in integrated fixation cages under simulated physiological loading. In particular, spatial differences in fixation was observed for IFC screws across the vertebra where medial screws loosened at a greater frequency compared to lateral screws post-fatigue. This novel technique may also be used to quantitatively investigate screw fixation post-fatigue testing in a variety of spinal devices.

A Female Ligamentous Cervical Spine Finite Element Model Validated for Physiological Loads.

Mathematical cervical spine models allow for studying of impact loading that can cause whiplash associated disorders (WAD). However, existing models only cover the male anthropometry, despite the female population being at a higher risk of sustaining WAD in automotive rear-end impacts. The aim of this study is to develop and validate a ligamentous cervical spine intended for biomechanical research on the effect of automotive impacts. A female model has the potential to aid the design of better protection systems as well as improve understanding of injury mechanisms causing WAD. A finite element (FE) mesh was created from surface data of the cervical vertebrae of a 26-year old female (stature 167 cm, weight 59 kg). Soft tissues were generated from the skeletal geometry and anatomical literature descriptions. Ligaments were modeled with nonlinear elastic orthotropic membrane elements, intervertebral disks as composites of nonlinear elastic bulk elements, and orthotropic anulus fibrosus fiber layers, while cortical and trabecular bones were modeled as isotropic plastic-elastic. The model has geometrical features representative of the female cervical spine-the largest average difference compared with published anthropometric female data was the vertebral body depth being 3.4% shorter for the model. The majority the cervical segments compare well with respect to biomechanical data at physiological loads, with the best match for flexion-extension loads and less biofidelity for axial rotation. An average female FE ligamentous cervical spine model was developed and validated with respect to physiological loading. In flexion-extension simulations with the developed female model and an existing average male cervical spine model, a greater range of motion (ROM) was found in the female model.