Intradiscal Pressure Measurements Research Articles

Comparative FEM study on intervertebral disc modeling: Holzapfel-Gasser-Ogden vs. structural rebars.

Introduction: Numerical modeling of the intervertebral disc (IVD) is challenging due to its complex and heterogeneous structure, requiring careful selection of constitutive models and material properties. A critical aspect of such modeling is the representation of annulus fibers, which significantly impact IVD biomechanics. This study presents a comparative analysis of different methods for fiber reinforcement in the annulus fibrosus of a finite element (FE) model of the human IVD. Methods: We utilized a reconstructed L4-L5 IVD geometry to compare three fiber modeling approaches: the anisotropic Holzapfel-Gasser-Ogden (HGO) model (HGO fiber model) and two sets of structural rebar elements with linear-elastic (linear rebar model) and hyperelastic (nonlinear rebar model) material definitions, respectively. Prior to calibration, we conducted a sensitivity analysis to identify the most important model parameters to be calibrated and improve the efficiency of the calibration. Calibration was performed using a genetic algorithm and in vitro range of motion (RoM) data from a published study with eight specimens tested under four loading scenarios. For validation, intradiscal pressure (IDP) measurements from the same study were used, along with additional RoM data from a separate publication involving five specimens subjected to four different loading conditions. Results: The sensitivity analysis revealed that most parameters, except for the Poisson ratio of the annulus fibers and C01 from the nucleus, significantly affected the RoM and IDP outcomes. Upon calibration, the HGO fiber model demonstrated the highest accuracy (R2 = 0.95), followed by the linear (R2 = 0.89) and nonlinear rebar models (R2 = 0.87). During the validation phase, the HGO fiber model maintained its high accuracy (RoM R2 = 0.85; IDP R2 = 0.87), while the linear and nonlinear rebar models had lower validation scores (RoM R2 = 0.71 and 0.69; IDP R2 = 0.86 and 0.8, respectively). Discussion: The results of the study demonstrate a successful calibration process that established good agreement with experimental data. Based on our findings, the HGO fiber model appears to be a more suitable option for accurate IVD FE modeling considering its higher fidelity in simulation results and computational efficiency.

Does low-magnitude high-frequency vibration reach the nucleus pulposus? – Intradiscal pressure measurements in a degenerative disc organ culture model

Does low-magnitude high-frequency vibration reach the nucleus pulposus? – Intradiscal pressure measurements in a degenerative disc organ culture model

The effect of multiplanar loading on the intradiscal pressure of the whole human spine: systematic review and meta-analysis.

For spinal load and muscle force estimation as well as for numerical model and experimental setup validation, data on human intradiscal pressure are essential. Therefore, the aim of the present meta-analysis was to summarise all in vitro measurements of human intradiscal pressure performed under defined boundary conditions, i.e. without external loading (intrinsic pressure), under axial loading (compression, traction, shear) and under single-planar bending loading (flexion, extension, lateral bending, axial rotation). Data were evaluated based on segmental level and normalised to force and moment. Regression analysis was performed to investigate coefficients of determination and statistical significance of relationships between intradiscal pressure and segmental level for the single loading conditions. 35 studies fulfilled the inclusion criteria, from which a total of 451 data points were collected for the meta-analysis. High coefficients of determination were found in axial compression (r2 = 0.875) and flexion (r2 = 0.781), while being low for intrinsic pressure (r2 = 0.266) and lateral bending (r2 = 0.385), all showing significant regression fitting (p < 0.01). Intradiscal pressure decreases from the upper cervical spine to the sacrum in all loading conditions, considering the same amount of loading for all segmental levels, while the intrinsic pressure exhibits a minimum of the regression curve in the mid-thoracic spine. Apart from its potential for numerical and experimental model validation, this dataset may help to understand the load distribution along the human spine.

Effects of motion segment simulation and joint positioning on spinal loads in trunk musculoskeletal models

Musculoskeletal models represent spinal motion segments by spherical joints/beams with linear/nonlinear properties placed at various locations. We investigated the fidelity of these simplified models (i.e., spherical joints with/without rotational springs and beams considering nonlinear/linear properties) in predicting kinematics of the ligamentous spine in comparison with a detailed finite element (FE) model while considering various anterior-posterior joint placements. Using the simplified models with different joint offsets in a subject-specific musculoskeletal model, we computed local spinal forces during forward flexion and compared results with intradiscal pressure measurements. In comparison to the detailed FE model, linearized beam and spherical joint models failed to reproduce kinematics whereas the nonlinear beam model with joint offsets at −2 to +4mm range (+: posterior) showed satisfactory performance. In the musculoskeletal models without a hand-load, removing rotational springs, linearizing passive properties and offsetting the joints posteriorly (by 4mm) increased compression (∼32%, 17% and 11%) and shear (∼63%, 26% and 15%) forces. Posterior shift in beam and spherical joints increased extensor muscle active forces but dropped their passive force components resulting in delayed flexion relaxation and lower antagonistic activity in abdominal muscles. Overall and in sagittally symmetric tasks, shear deformable beams with nonlinear properties performed best followed by the spherical joints with nonlinear rotational springs. Using linear rotational springs or beams is valid only in small flexion angles (<30°) and under small external loads. Joints should be placed at the mid-disc height within −2 to +4mm anterior-posterior range of the disc geometric center and passive properties (joint stiffnesses) should not be overlooked.

三次元筋骨格モデルを用いた動的胸腰椎モデルの開発

Musculoskeletal modeling of the thoracolumbar spine and trunk muscle are considered clinically useful for quantifying spinal disease processes. However, current models of thoracic spine are not evaluated for dynamical load and posture, or muscle path are constructed Via-Point (straight line muscle) type. In this study, we developed novel trunk musculoskeletal models of the predicted thoracolumbar load which are constructed muscle path in more detail, and validated developed models in dynamic loading and various postures. Developed spine models segmented 17 vertebrae, a sacrum, 10 pairs of ribs, a sternum and defined 17 intervertebral. Muscle path of the trunk muscle models constructed 2 types which are Via-Point type and Wrapping type (curved line muscle along ellipsoid). We estimated intradiscal pressure with constructed models and compared it with measured values. As a result, intradiscal pressure estimated by the model using Wrapping method had an extremely strong correlation with intradiscal pressure measurements made in the thoracic spine. Our results demonstrated significance to faithfully reproduce muscle path in developed musculoskeletal models.

Intradiscal pressure measurements: A challenge or a routine?

Intradiscal pressure (IDP) is an essential biomechanical parameter and has been the subject of numerous in vivo and in vitro investigations. Although currently available sensors differ in size and measurement principles, no data exist regarding inter-sensor reliability in measuring IDP. Moreover, although discs of various species vary significantly in size and mechanics, the possible effects of sensor insertion on the IDP have never been investigated. The present in vitro study aimed to address these issues.The synchronized signals of two differently sized pressure transducers (Ø1.33 and Ø0.36mm) obtained during the measurements in two species (bovine and caprine) and their influence on the measured pressure were compared. First, the discs were subjected to three loading periods, and the pressure was measured simultaneously to assess the inter-sensor reliability. In the second test, the effect of the sensor size was evaluated by alternatingly inserting one transducer into the disc while recording the resulting pressure change with the second transducer.Although both sensors yielded similar pressure values (ICC: consistency: 0.964–0.999; absolute agreement: 0.845-0.996) when used simultaneously, the sensor size was determined to influence the measured pressure during the insertion tests. The magnitude of the effect differed between species; it was insignificant in the bovine specimens but significant in the caprine specimens, with a pressure increase of 0.31–0.64MPa (median: 0.43MPa) obtained when the larger sensor was inserted.The results suggest that sensor selection for IDP measurements requires special attention and can be crucial for species with smaller disc sizes.

Thoracolumbar spine model with articulated ribcage for the prediction of dynamic spinal loading

Musculoskeletal modeling offers an invaluable insight into the spine biomechanics. A better understanding of thoracic spine kinetics is essential for understanding disease processes and developing new prevention and treatment methods. Current models of the thoracic region are not designed for segmental load estimation, or do not include the complex construct of the ribcage, despite its potentially important role in load transmission. In this paper, we describe a numerical musculoskeletal model of the thoracolumbar spine with articulated ribcage, modeled as a system of individual vertebral segments, elastic elements and thoracic muscles, based on a previously established lumbar spine model and data from the literature. The inverse dynamics simulations of the model allow the prediction of spinal loading as well as costal joints kinetics and kinematics. The intradiscal pressure predicted by the model correlated well (R2=0.89) with reported intradiscal pressure measurements, providing a first validation of the model. The inclusion of the ribcage did not affect segmental force predictions when the thoracic spine did not perform motion. During thoracic motion tasks, the ribcage had an important influence on the predicted compressive forces and muscle activation patterns. The compressive forces were reduced by up to 32%, or distributed more evenly between thoracic vertebrae, when compared to the predictions of the model without ribcage, for mild thoracic flexion and hyperextension tasks, respectively. The presented musculoskeletal model provides a tool for investigating thoracic spine loading and load sharing between vertebral column and ribcage during dynamic activities. Further validation for specific applications is still necessary.

Factors affecting intradiscal pressure measurement during in vitro biomechanical tests.

ObjectivesTo assess the reliability of intradiscal pressure measurement during in vitro biomechanical testing. In particular, the variability of measurements will be assessed for repeated measures by considering the effect of specimens and of freezing/thawing cycles.MethodsThirty-six functional units from 8 porcine spines (S1: T7-T8, S2: T9-T10, S3: T12-T11, S4: T14-T13, S5: L1-L2 and S6: L3-L4) have been used. The intervertebral discs were measured to obtain the frontal and sagittal dimensions. These measurements helped locate the center of the disc where a modified catheter was positioned. A fiber optic pressure sensor (measuring range: -0.1 to 17 bar) (360HP, SAMBA Sensors, Sweden) was then inserted into the catheter. The specimens were divided into 3 groups: 1) fresh (F), 2) after one freeze/thaw cycle (C1) and 3) after 2 freeze/thaw cycles (C2). These groups were divided in two, depending on whether specimens were subjected to 400 N axial loading or not. Ten measurements (insertion of the sensor for a period of one minute, then removal) were taken for each case. Statistical analyses evaluated the influence of porcine specimen and the vertebral level using a MANOVA. The effect of repeated measurements was evaluated with ANOVA. The difference between freeze/thaw cycles were analysed with U Mann-Whitney test (P≤0.05).ResultsWithout axial loading, the F group showed 365 mbar intradiscal pressure, 473 mbar for the C1 group, and 391 mbar for the C2 group. With 400N axial load, the F group showed intradiscal pressure of 10610 mbar, the C1 group 10132 mbar, the C2 group 12074 mbar. The statistical analysis shows a significant influence of the porcine specimen (p<0.001), with or without axial loading and of the vertebral level with (p=0.048) and without load (p<0.001). The results were also significantly different between the freeze/thaw cycles, with (p<0.001) and without load (p=0.033). Repeated measurement (without load p = 0.82 and with p = 0.56) did not show significant influence.ConclusionsThe results tend to support that freezing/thawing cycles can affect intradiscal pressure measurement with significant inter-specimen variability. The use of the same specimen as its own control during in vitro biomechanical testing could be recommended.

Factors affecting intradiscal pressure measurement during in vitro biomechanical tests

Factors affecting intradiscal pressure measurement during in vitro biomechanical tests

Intradiscal pressure depends on recent loading and correlates with disc height and compressive stiffness

Intervertebral discs exhibit time-dependent deformation (creep), which could influence the relation between applied stress and intradiscal pressure. This study investigates the effect of prolonged dynamic loading on intradiscal pressure, disc height and compressive stiffness, and examines their mutual relationships. Fifteen caprine lumbar discs with 5 mm of vertebral bone on either side were compressed by 1 Hz sinusoidal load for 4.5 h. After preload, 'High' (130 ± 20 N) or 'Low' (50 ± 10 N) loads were alternated every half hour. Continuous intradiscal pressure measurement was performed with a pressure transducer needle. Each disc showed a linear relationship between axial compression and intradiscal pressure (R (2) > 0.91). The intercept of linear regression analysis declined over time, but the gradient remained constant. Disc height changes were correlated to intradiscal pressure changes (R (2) > 0.98): both decreased during High loading, and increased during Low loading. In contrast, compressive stiffness increased during High loading, and was inversely related to intradiscal pressure and disc height. Intradiscal pressure is influenced by recent loading due to fluid flow. The correlations found in this study suggest that intradiscal pressure is important for disc height and axial compliance. These findings are relevant for mechanobiology studies, nucleus replacements, finite element models, and ex vivo organ culture systems.

Spinal motion and intradiscal pressure measurements before and after lumbar spine instrumentation with titanium or PEEK rods

Spinal instrumentation and fusion have been incriminated as contributing to adjacent segment degeneration (ASD). It has been suggested that ASD results from increased range of motion and intradiscal pressure (IDP) adjacent to instrumentation. Posterior dynamic stabilization with polyetheretherketone (PEEK) rods has been proposed as potentially advantageous compared to rigid instrumentation with titanium (Ti) rods in reducing the incidence of ASD. We evaluated segmental motions in the cadaveric spine instrumented with PEEK or Ti rods from L3 to S1, as well as the adjacent segment motions and IDP at L1–2 and L2–3. Human cadaveric spines were potted at T12–L1 and S1–2. Spinal instrumentation from L3–S1 was accomplished using pedicle screws with either PEEK or Ti rods. Specimens were subjected to displacement controlled testing: 15° flexion, 15° extension, 10° lateral bending, and 5° right axial rotation using the MTS machine (MTS, Minneapolis, MN, USA). Intradiscal pressure was measured by placing pressure transducers into the intervertebral disc at L1–2 and L2–3. Spinal motion of L2 relative to L3, and L3 relative to S1 was tracked using a three dimensional motion analysis system. Instrumentation with PEEK and Ti rods was associated with a decrease in L3–S1 motion compared to the intact state that was significant in flexion (p=0.002), and extension (p=0.0075). Instrumentation with PEEK and Ti rods was associated with an increase in IDP at L1–2 that was significant in flexion (p=0.0028). Instrumentation with either PEEK or Ti rods resulted in decreased motion at the instrumented levels while increasing IDP at the adjacent level.

Development and validation of a discretised multi-body spine model in LifeMOD for biodynamic behaviour simulation

This paper presents a discretised musculoskeletal multi-body spine model using the LifeMOD Biomechanics Modeller. This was obtained by refining spine segments in cervical, thoracic and lumbar regions into individual vertebra segments, using rotational joints representing the intervertebral discs, building various ligaments between vertebrae and implementing necessary lumbar muscles. To validate the model, two comparison studies were made with in vivo intradiscal pressure measurements of the L4–L5 disc as well as extension moments, axial force and shear force around L5–S1 obtained from spine models available in the literature. The results indicated that the present model is in good correlation with both cases and matches well with experimental data which found that the axial forces are in the range of 3929–4688 N and shear forces up to 650 N. This study provides a preliminary overview of our ongoing work towards building bio-fidelity discretised multi-body spine models for investigating various medical applications. These models can be useful for incorporation into design tools for wheelchairs or other seating systems which may require attention to ergonomics as well as assessing biomechanical behaviour between natural spines and spinal arthroplasty or spinal arthrodesis. Furthermore, these models can be combined with haptic-integrated graphic environments to help surgeons to examine kinematic behaviours of scoliotic spines and to propose possible surgical plans before spine correction operations.

Is it possible to estimate the compressive force in the lumbar spine from intradiscal pressure measurements? A finite element evaluation

Knowledge about in vivo spinal compressive forces is a basic requirement for spinal biomechanics. Their direct measurement is not yet possible. Therefore, compressive forces are estimated from in vivo measured intradiscal pressure values. However, it is still not evident how precise these estimations are.A finite element model of the spine was employed to simulate elementary body positions and the compressive force at level L4-5 was calculated. This value was compared with different estimations calculated by multiplying the intradiscal pressure with the disc's cross-sectional area and with a correction factor. A model specific and different previously employed correction factors were used. Separately, in vivo forces were estimated from previously measured pressure values.A model specific correction factor leads for all body positions to a good estimation (error <4%) of the force except for extension (error >27%). Non-model specific correction factors lead to estimation errors of up to 44%. When accounting for these limitations, in vivo forces were estimated e.g. for standing between 430N and 600N.Compressive forces can be estimated for non-extended body positions when the individual correction factor is known. In vivo forces can be estimated from intradiscal pressure values within a certain range.

New non-invasive and patient-specific method allowing intradiscal pressure change measurement induced by lumbar conservative or surgical treatments

commonly prescribed as conservative treatment of low back pain. These devices could affect the distribution of loads among passive and active lumbar subsystems. However, with only one study in this field proposed by Nachemson et al (1983), their biomechanical action such as the reduction of the intradiscal pressure (IDP) remains unknown and controversial. The goal of this study was to investigate the IDP change induced by a rigid lumbar orthosis from a new non-invasive measurement method. Method.– From the comparison of two radiographs EOS (Biospace, Paris) with and without a lumbar orthosis, the displacements and rotations of each vertebra were calculated and implemented in a patient-specific finite element modelling. The IDP measurements along each lumbar disc were extracted with a precision of 2.5 Pa. Twenty patients were tested in standing or sitting posture with a semi-rigid lumbar orthosis Lordactiv (Ormihl-Danet, Villeurbanne) in two conditions: with or without a curved rigid lumbar part. Result.– A curved rigid lumbar part was necessary to obtain significant IDP distribution change along the anteroposterior direction. Interestingly, an IDP decrease appeared in somepatients until–50 Pa.However, the IDP change appeared very posture, disc level and patient-dependent ranging from –50 to +8 Pa. Discussion.– An IDP distribution change reveals a different load sharing between posterior elements and intervertebral discs whereas decompression only could be explained by a decrease muscle activity. Especially, the intradiscal elasticity (age-dependent) and the lumbar back adipose tissue thickness are the main variables correlated with the interindividual differences. Conclusion.– These results demonstrated that wearing a rigid lumbar orthosis can significantly influence the stress-strain values in diseased discs, provided that the patient is kept an sufficient elasticity.

Different Origins of Intervertebral Disk Degeneration in the Upper and Lower Lumbar Spine

Introduction The “weak link” in the thoracolumbar spine is the vertebral body endplate, which is easily damaged in compression. Vertebral endplate fracture can decompress the adjacent intervertebral disk and allow the annulus to collapse inwards.1 This can lead to disk degeneration in animal models2 and in patients.3 However, population MRI studies indicate that endplate damage is strongly associated with adjacent disk degeneration only in the upper lumbar spine.4 We hypothesize that lower lumbar disks are less affected by endplate fracture. Materials and Methods Mechanical experiments were performed on cadaveric “motion segments” comprising two vertebrae and the intervening disk and ligaments. Each specimen was compressed to failure, until the elastic limit was just exceeded, and measurements of intradiscal (nucleus) pressure were compared before and after failure, at a reference load of 1 kN. Pressure measurements were made with a strain-gauged pressure transducer side-mounted near the tip of a 1.3-mm-diameter catheter. Multiple linear regression (SPSS v16) was used to analyze factors that influence the fall in nucleus pressure. Results A total of 143 specimens aged 19 to 96 years (mean 67 years) were tested, from all spinal levels between T7-8 and L5-S1. Radiographs confirmed the endplate as the site of compressive failure, and damage caused an average specimen height loss of 1.9 mm (STD 0.9 mm). The resulting fall in nucleus pressure averaged 59% (STD 37%), and increased with age and (pre-existing) disk degeneration ( p &lt; 0.001). Apart from specimen height loss, the greatest determinant of nucleus decompression was spinal level, with decompression averaging 70 to 90% in the lower thoracic spine, and falling linearly from T11-12 to just 6% at L5-S1. Spinal level explained 28% of the variance in nucleus decompression ( p &lt; 0.001). Conclusion The relatively small effects observed in lower lumbar disks may be due to the relatively large height and anteroposterior diameter of the nucleus at L4-5 and L5-S1: a larger nucleus volume would lead to a smaller drop in pressure for the same amount of endplate damage. Results suggest that compressive overload may well initiate disk degeneration in the upper lumbar and thoracic spine, explaining the strong relationship between disk degeneration and Schmorl's nodes at these levels.4 At L4-5 and L5-S1, however, another mechanism is likely to be important: these lower lumbar disks are particularly susceptible to radial fissure formation, leading to disk prolapse, in response to high loading in bending.1 The concept of two distinct mechanisms of disc degeneration (‘annulus-driven’ in the lower lumbar spine, and “endplate-driven” at higher spinal levels) may explain why the genetic influence in disk degeneration varies with spinal level.5 I confirm having declared any potential conflict of interest for all authors listed on this abstract Yes Disclosure of Interest None declared Adams MA, Freeman BJ, Morrison HP, Nelson IW, Dolan P. Mechanical initiation of intervertebral disc degeneration. Spine 2000;25(13):1625–1636 Holm S, Holm AK, Ekstrom L, Karladani A, Hansson T. Experimental disc degeneration due to endplate injury. Journal of Spinal Disorder and Techniques 2004;17(1):64–71 Kerttula LI, Serlo WS, Tervonen OA, Paakko EL, Vanharanta HV. Post-traumatic findings of the spine after earlier vertebral fracture in young patients: clinical and MRI study. Spine 2000;25(9):1104–1108 Mok FP, Samartzis D, Karppinen J, Luk KD, Fong DY, Cheung KM. ISSLS prize winner: Prevalence, determinants, and association of Schmorl nodes of the lumbar spine with disc degeneration: a population-based study of 2449 individuals. Spine (Phila Pa 1976) 2010;35(21):1944–1952 Battie MC, Videman T, Levalahti E, Gill K, Kaprio J. Genetic and environmental effects on disc degeneration by phenotype and spinal level: a multivariate twin study. Spine 2008;33(25):2801–2808

Lumbar Intervertebral Joint Loads as Determined by Patient Specific Musculoskeletal Modeling for Two Types of Sagittal Spino-Pelvic Alignment with Different Occurrence of Symptomatic Adjacent Segment Degeneration

Introduction Biomechanical consequences of fusion leading to adjacent segment degeneration (ASD) have been investigated in different studies, and adverse consequences of sagittal alignment on intervertebral disk (IVD) loads have been proposed as a potential cause. This study investigates IVD loading by means of patient-specific musculoskeletal modeling of potential anatomical factors that may predispose to ASD as recently identified in a retrospective clinical study.1 The current study thus attempts to quantify the reaction forces in the prospective adjacent IVDs as a consequence of spino-pelvic alignment in nonfused spines. Materials and Methods A publicly available model for the human lumbar spine2 was employed and slightly modified for this study. Most notable alterations were adjustments to muscle attachment points, scaling, and changes in the vertebral and sacral reference systems. Joint centers were maintained and length-dependent muscle properties were scaled to maintain the original muscle characteristics. For each patient from the clinical study,1 important landmark coordinates (positions of endplates T12-S1 and acetabulum) were extracted from a digital lumbar spine sagittal radiograph and models were created. Three subjects were excluded due to insufficiently clear radiographs. The models were grouped according to the anatomical relationship between lumbar lordosis (LL) and pelvic incidence (PI) (δPILL = PI-LL). Models above a threshold for ▵PILL, which clinically corresponded to patients having had revision surgery, were assigned to group ASD + ( n = 25); whereas models below the threshold and without symptomatic ASD were assigned to a control group (CTRL-, n = 34). To study only the effect of sagittal alignment, the height of the lumbar spine was normalized and segmental weights and center of mass positions were kept the same for all models. Simulations for static postures of neutral upright standing, 30° and 60° flexion and 15° extension were performed. Joint reaction loads between groups were compared in MATLAB with a Wilcoxon rank sum test. Results Computation of sagittal parameters LL and PI based on extracted landmarks matched well with measures determined in Reference1 and led to the same categorization of subjects. Validation of compression forces was performed with in vivo measurements3 (291N in our model vs. experimental values between 191 and 365N). Reported shear forces4 (L5-S1: 134N, L4-L5: 15N) were compared against those predicted by our model (L5-S1: 103N, L4-L5: 25N). Furthermore, load tendency for different postures were compared to intradiscal pressure values from the literature5,6, both normalized to neutral standing. Good agreement was achieved for flexion (239% vs. 220 and 260%, respectively) and extension (160 vs. 120% for both). Comparison of joint reaction loads for the two groups showed the most prominent differences in shear force and joint moments at the L3-L4 IVD joint level (Fig. 1). Significantly higher shear forces were observed in the ASD + group for flexed postures, while differences in neutral position were not significant. Flexion moments were significantly different among the two groups for all postures, while magnitudes were higher for the ASD + group for 60 degrees flexed postures only. (Fig. 1 shear force and flexion moments at the IVD joint L3-L4 for both groups) Conclusion The simulations showed that the IVDs at the third lumbar level of patients who needed revision surgery due to development of symptomatic ASD were exposed to significantly higher loading regimes before fusion in flexed postures. This might be an indication that not only the biomechanical effect of a fusion, but already a difference in disk loading history prior to fusion could affect the risk of ASD. Simulations of fusion for the two groups will show if the predictive capability of Reference1 can be supported by mechanical consequences on the loading of the IVD. I confirm having declared any potential conflict of interest for all authors listed on this abstract Yes Disclosure of Interest None declared Rothenfluh et al. Spino-pelvic malalignment predisposes to adjacent segment disease after lumbar spinal fusion(submitted). Christophy et al. A Musculoskeletal model for the lumbar spine. Biomech Model Mechanobiol 2011 Rohlmann et al. Loads on internal spinal fixators measured in different body positions. European Spine Journal, 1999. Arjmand et al. Comparison of trunk muscle forces and spinal loads estimated by two biomechanical models. Clinical Biomechanics 2009. Wilke et al. New in vivo measurements of pressures in the intervertebral disk in daily life Spine 1999. Sato et al. In vivo intradiscal pressure measurement in healthy individuals and in patients with ongoing back problems Spine 1999.

Lumbar Intervertebral Joint Loads as Determined by Patient Specific Musculoskeletal Modeling for Two Types of Sagittal Spino-Pelvic Alignment with Different Occurrence of Symptomatic Adjacent Segment Degeneration

IntroductionBiomechanical consequences of fusion leading to adjacent segment degeneration (ASD) have been investigated in different studies, and adverse consequences of sagittal alignment on intervertebral disk (IVD) loads have been proposed as a potential cause. This study investigates IVD loading by means of patient-specific musculoskeletal modeling of potential anatomical factors that may predispose to ASD as recently identified in a retrospective clinical study.1 The current study thus attempts to quantify the reaction forces in the prospective adjacent IVDs as a consequence of spino-pelvic alignment in nonfused spines. Materials and MethodsA publicly available model for the human lumbar spine2 was employed and slightly modified for this study. Most notable alterations were adjustments to muscle attachment points, scaling, and changes in the vertebral and sacral reference systems. Joint centers were maintained and length-dependent muscle properties were scaled to maintain the original muscle characteristics. For each patient from the clinical study,1 important landmark coordinates (positions of endplates T12-S1 and acetabulum) were extracted from a digital lumbar spine sagittal radiograph and models were created. Three subjects were excluded due to insufficiently clear radiographs. The models were grouped according to the anatomical relationship between lumbar lordosis (LL) and pelvic incidence (PI) (δPILL = PI-LL). Models above a threshold for ▵PILL, which clinically corresponded to patients having had revision surgery, were assigned to group ASD + (n = 25); whereas models below the threshold and without symptomatic ASD were assigned to a control group (CTRL-,n = 34). To study only the effect of sagittal alignment, the height of the lumbar spine was normalized and segmental weights and center of mass positions were kept the same for all models. Simulations for static postures of neutral upright standing, 30° and 60° flexion and 15° extension were performed. Joint reaction loads between groups were compared in MATLAB with a Wilcoxon rank sum test. ResultsComputation of sagittal parameters LL and PI based on extracted landmarks matched well with measures determined in Reference1 and led to the same categorization of subjects. Validation of compression forces was performed with in vivo measurements3 (291N in our model vs. experimental values between 191 and 365N). Reported shear forces4 (L5-S1: 134N, L4-L5: 15N) were compared against those predicted by our model (L5-S1: 103N, L4-L5: 25N). Furthermore, load tendency for different postures were compared to intradiscal pressure values from the literature5,6, both normalized to neutral standing. Good agreement was achieved for flexion (239% vs. 220 and 260%, respectively) and extension (160 vs. 120% for both). Comparison of joint reaction loads for the two groups showed the most prominent differences in shear force and joint moments at the L3-L4 IVD joint level (Fig. 1). Significantly higher shear forces were observed in the ASD + group for flexed postures, while differences in neutral position were not significant. Flexion moments were significantly different among the two groups for all postures, while magnitudes were higher for the ASD + group for 60 degrees flexed postures only. (Fig. 1 shear force and flexion moments at the IVD joint L3-L4 for both groups) ConclusionThe simulations showed that the IVDs at the third lumbar level of patients who needed revision surgery due to development of symptomatic ASD were exposed to significantly higher loading regimes before fusion in flexed postures. This might be an indication that not only the biomechanical effect of a fusion, but already a difference in disk loading history prior to fusion could affect the risk of ASD. Simulations of fusion for the two groups will show if the predictive capability of Reference1 can be supported by mechanical consequences on the loading of the IVD.I confirm having declared any potential conflict of interest for all authors listed on this abstractYesDisclosure of InterestNone declaredRothenfluh et al. Spino-pelvic malalignment predisposes to adjacent segment disease after lumbar spinal fusion(submitted).Christophy et al. A Musculoskeletal model for the lumbar spine. Biomech Model Mechanobiol 2011Rohlmann et al. Loads on internal spinal fixators measured in different body positions. European Spine Journal, 1999.Arjmand et al. Comparison of trunk muscle forces and spinal loads estimated by two biomechanical models. Clinical Biomechanics 2009.Wilke et al. New in vivo measurements of pressures in the intervertebral disk in daily life Spine 1999.Sato et al. In vivo intradiscal pressure measurement in healthy individuals and in patients with ongoing back problems Spine 1999.

Efficacy of interspinous process lumbar fusion with recombinant human bone morphogenetic protein-2 delivered with a synthetic polymer and β-tricalcium phosphate in a rabbit model

As a powerful bone-inducing cytokine, rhBMP-2 has been used as a bone graft substitute in combination with animal-derived collagen to achieve interbody or posterolateral spinal fusion. Successful interspinous process fusion using rhBMP-2 in combination with synthetic carrier materials would offer a safe, minimally invasive spinal fusion option for the treatment of spinal disorders. The aims of the present study were to achieve interspinous process fusion by implanting rhBMP-2-retaining degradable material instead of bone grafting and to evaluate efficacy for vertebral stabilization. A polymer gel (200 mg), β-tricalcium phosphate powder (400 mg), and rhBMP-2 (0, 30, 60 or 120 μg) were mixed to generate a plastic implant, which was then placed during surgery to bridge the L5-6 interspinous processes of 58 rabbits. Control animals received implants either without rhBMP-2 or with autogenous bone chips from the iliac crest. L5-6 vertebrae were recovered 8 weeks postoperatively. Interspinous process fusion was evaluated by radiography, biomechanical bending test, intradiscal pressure (IDP) measurement, and histology. In bending tests, strength of fusion was significantly greater in BMP60 and BMP120 groups than in sham, BMP0, BMP30 or autogenous bone groups. IDP at L5-6 was significantly reduced in BMP60 and BMP120 groups compared to sham, BMP0, BMP30, and autograft groups. Histologically, coronal sections of the fusion mass showed a bone mass bridging both spinous processes. Solid interspinous process fusion was achieved in rabbit models by 8 weeks after implanting the biodegradable bone-inducing material. These results suggest a potential new less-invasive option without bone grafting for the treatment of lumbar disorders.

Development of a More Physiological Loading Protocol for Spine In Vitro Flexibility Testing

Abstract With motion preserving systems, whose behavior is dependent on the loading applied, it is becoming more important to produce a loading environment that better simulates the situation in vivo. Several studies show that the spine experiences high compressive loads that change as a function of position. The purpose of this study was to apply a high compressive dynamic follower load and determine the moment required to produce a physiological range of motion in vitro. Six human specimens (L2-L3) were subjected to a pure moment, in combination with a high compressive dynamic follower load. Appropriate compressive loads were obtained from literature based on in vivo intradiscal pressure measurements. The moments necessary to produce pre-defined angles of rotation in flexion, extension, lateral bending, and axial rotation (in vivo literature values) were recorded. The follower load was attached laterally in flexion-extension and axial rotation and anterior-posteriorly in lateral bending. Tests were also conducted using two traditional loading protocols for comparison: ±10 Nm (no follower load); and ±10 Nm with a 600 N constant follower load, in terms of range of motion (ROM), helical axis of motion (HAM), and flexibility coefficients.The new loading protocol resulting from this study consisted of a compressive follower load of 800 N in the neutral position, a flexion moment of 35 Nm combined with a maximum compressive follower load of 2000 N, an extension moment of 10 Nm combined with 900 N, a moment of ±15 Nm in lateral bending with 1100 N, and a moment of ±20 Nm in axial rotation with 1250 N. The anterior-posterior follower load fixation in lateral bending allowed more unrestrained movement. The moments necessary to produce physiological motion under a dynamic compressive follower load are higher than what is currently used and are comparable to calculated in vivo moments.

Role of optimization criterion in static asymmetric analysis of lumbar spine load

A common method for load estimation in biomechanics is the inverse dynamics optimization, where the muscle activation pattern is found by minimizing or maximizing the optimization criterion. It has been shown that various optimization criteria predict remarkably similar muscle activation pattern and intra-articular contact forces during leg motion. The aim of this paper is to study the effect of the choice of optimization criterion on L4/L5 loading during static asymmetric loading. Upright standing with weight in one stretched arm was taken as a representative position. Musculoskeletal model of lumbar spine model was created from CT images of Visible Human Project. Several criteria were tested based on the minimization of muscle forces, muscle stresses, and spinal load. All criteria provide the same level of lumbar spine loading (difference is below 25%), except the criterion of minimum lumbar shear force which predicts unrealistically high spinal load and should not be considered further. Estimated spinal load and predicted muscle force activation pattern are in accordance with the intradiscal pressure measurements and EMG measurements. The L4/L5 spine loads 1312 N, 1674 N, and 1993 N were predicted for mass of weight in hand 2, 5, and 8 kg, respectively using criterion of mininum muscle stress cubed. As the optimization criteria do not considerably affect the spinal load, their choice is not critical in further clinical or ergonomic studies and computationally simpler criterion can be used.