Some static mechanical properties of the lumbar intervertebral joint, intact and injured

A new method to design energy-conserving surrogate models for the coupled, nonlinear responses of intervertebral discs

The aim of this study was to design physics-preserving and precise surrogate models of the nonlinear elastic behaviour of an intervertebral disc (IVD). Based on artificial force-displacement data sets from detailed finite element (FE) disc models, we used greedy kernel and polynomial approximations of second, third and fourth order to train surrogate models for the scalar force-torque -potential. Doing so, the resulting models of the elastic IVD responses ensured the conservation of mechanical energy through their structure. At the same time, they were capable of predicting disc forces in a physiological range of motion and for the coupling of all six degrees of freedom of an intervertebral joint. The performance of all surrogate models for a subject-specific L4 | 5 disc geometry was evaluated both on training and test data obtained from uncoupled (one-dimensional), weakly coupled (two-dimensional), and random movement trajectories in the entire six-dimensional (6d) physiological displacement range, as well as on synthetic kinematic data. We observed highest precisions for the kernel surrogate followed by the fourth-order polynomial model. Both clearly outperformed the second-order polynomial model which is equivalent to the commonly used stiffness matrix in neuro-musculoskeletal simulations. Hence, the proposed model architectures have the potential to improve the accuracy and, therewith, validity of load predictions in neuro-musculoskeletal spine models.

Biomechanical consequences of cement discoplasty: An in vitro study on thoraco-lumbar human spines

With the ageing of the population, there is an increasing need for minimally invasive spine surgeries to relieve pain and improve quality of life. Percutaneous Cement Discoplasty is a minimally invasive technique to treat advanced disc degeneration, including vacuum phenomenon. The present study aimed to develop an in vitro model of percutaneous cement discoplasty to investigate its consequences on the spine biomechanics in comparison with the degenerated condition. Human spinal segments (n = 27) were tested at 50% body weight in flexion and extension. Posterior disc height, range of motion, segment stiffness, and strains were measured using Digital Image Correlation. The cement distribution was also studied on CT scans. As main result, percutaneous cement discoplasty restored the posterior disc height by 41% for flexion and 35% for extension. Range of motion was significantly reduced only in flexion by 27%, and stiffness increased accordingly. The injected cement volume was 4.56 ± 1.78 ml (mean ± SD). Some specimens (n = 7) exhibited cement perforation of one endplate. The thickness of the cement mass moderately correlated with the posterior disc height and range of motion with different trends for flexions vs. extension. Finally, extreme strains on the discs were reduced by percutaneous cement discoplasty, with modified patterns of the distribution. To conclude, this study supported clinical observations in term of recovered disc height close to the foramen, while percutaneous cement discoplasty helped stabilize the spine in flexion and did not increase the risk of tissue damage in the annulus.

Biomechanical Contributions of Spinal Structures with Different Degrees of Disc Degeneration

STUDY DESIGN

Biomechanical cadaveric study.

OBJECTIVE

The aim of this study was to evaluate the effect of degeneration on biomechanical properties of the passive structures of the lumbar spine.

SUMMARY OF BACKGROUND DATA

Although the load apportionment among the passive structures in healthy spines follows well-defined contribution patterns, it remains unknown how this load distribution and sagittal preload changes by degenerative processes of the intervertebral disc (IVD).

METHODS

Fifty lumbar spinal segments were tested in a displacement-controlled stepwise reduction study in flexion, extension, axial rotation, lateral bending, anterior, posterior and lateral shear. The intertransverse ligaments (ITLs), supraspinous and interspinous ligaments (ISL&SSL), facet joint capsules (FJC), facet joints (FJ), ligamentum flavum (LF), posterior longitudinal ligament (PLL), anterior longitudinal ligament (ALL), and spondylophytes were subsequently reduced. The results were set in relation to IVD-degeneration, quantified with Pfirrmann classification.

RESULTS

In flexion, a load redistribution from LF (-28% n.s.) and PLL (-13% n.s.) towards the IVD (+9%, n.s.) is observed comparing grade 2 to 5 IVD degeneration, whereas in all other loading directions, a reduction of IVD-contribution from -12% to -53% is recorded. In axial rotation, anterior and lateral shear, more load is shared by the FJ (+4% n.s., +23% ∗, +13% n.s.). The preload of the ALL, LF, PLL, and IVD is reduced ranging from -0.06 Nm to -0.37 Nm.

CONCLUSION

IVD degeneration is related to notable load-redistributions between the passive spinal structures. With further degeneration, reduced contribution of the LF and PLL and higher loads on the IVD are observed in flexion. In the other tested loading directions, the relative load on the IVD is reduced, whereas higher FJ-exposure in axial rotation, anterior and lateral shear is observed. Furthermore, the preload of the spinal structures is reduced. These observations can further the understanding of the degenerative cascade in the spine.Level of Evidence: N/A.

Modeling of human intervertebral disc annulus fibrosus with complex multi-fiber networks

Collagen fibers within the annulus fibrosus (AF) lamellae are unidirectionally aligned with alternating orientations between adjacent layers. AF constitutive models often combine two adjacent lamellae into a single equivalent layer containing two fiber networks with a crisscross pattern. Additionally, AF models overlook the inter-lamellar matrix (ILM) as well as elastic fiber networks in between lamellae. We developed a nonhomogenous micromechanical model as well as two coarser homogenous hyperelastic and microplane models of the human AF, and compared their performances against measurements (tissue level uniaxial and biaxial tests as well as whole disc experiments) and seven published hyperelastic models. The micromechanical model had a realistic non-homogenous distribution of collagen fiber networks within each lamella and elastic fiber network in the ILM. For small matrix linear moduli (<0.2 MPa), the ILM showed substantial anisotropy (>10%) due to the elastic fiber network. However, at moduli >0.2 MPa, the effects of the elastic fiber network on differences in stress-strain responses at different directions disappeared (<10%). Variations in sample geometry and boundary conditions (due to uncertainty) markedly affected stress-strain responses of the tissue in uniaxial and biaxial tests (up to 16 times). In tissue level tests, therefore, simulations should represent testing conditions (e.g., boundary conditions, specimen geometry, preloads) as closely as possible. Stress/strain fields estimated from the single equivalent layer approach (conventional method) yielded different results from those predicted by the anatomically more accurate apparoach (i.e., layerwise). In addition, in a disc under a compressive force (symmetric loading), asymmetric stress-strain distributions were computed when using a layerwise simulation. Although all developed and selected published AF models predicted gross compression-displacement responses of the whole disc within the range of measured data, some showed excessively stiff or compliant responses under tissue-level uniaxial/biaxial tests. This study emphasizes, when constructing and validating constitutive models of AF, the importance of the proper simulation of individual lamellae as distinct layers, and testing parameters (sample geometric dimensions/loading/boundary conditions).

Spine biomechanical testing methodologies: The controversy of consensus vs scientific evidence

Biomechanical testing methodologies for the spine have developed over the past 50 years. During that time, there have been several paradigm shifts with respect to techniques. These techniques evolved by incorporating state-of-the-art engineering principles, in vivo measurements, anatomical structure-function relationships, and the scientific method. Multiple parametric studies have focused on the effects that the experimental technique has on outcomes. As a result, testing methodologies have evolved, but there are no standard testing protocols, which makes the comparison of findings between experiments difficult and conclusions about in vivo performance challenging. In 2019, the international spine research community was surveyed to determine the consensus on spine biomechanical testing and if the consensus opinion was consistent with the scientific evidence. More than 80 responses to the survey were received. The findings of this survey confirmed that while some methods have been commonly adopted, not all are consistent with the scientific evidence. This review summarizes the scientific literature, the current consensus, and the authors' recommendations on best practices based on the compendium of available evidence.

Sensitivity analysis of biomechanical effect in vertebral body of two different augmenters

BACKGROUND

Transvertebral Bone Graft and Augmentation (TBGA) has achieved good clinical effects in the treatment of osteoporotic vertebral compression fractures (OVCFs). This study aimed to investigate the postoperatively biomechanical effects of TBGA and compare the biomechanical sensitivity of two different augmenters: a cylindrical enhancement device (CED) and bone cement.

METHODS

Finite element models of the spine segment T11-L3 were created, including one model based on normal segment and the other three with L1 augmentation for pathological conditions. Three treatments were simulated including CED implant treatment A, CED implant treatment B, and bone cement treatment. The stress distribution and maximum displacement of the four models under different treatments were analyzed. A method of linear fitting of dummy variables was used to analyze the sensitivity of biomechanical parameters to the degree of osteoporosis (DO) and load.

FINDINGS

The reduction of stress with increasing DO in augmented and adjacent vertebral bodies under bone cement augmentation was less than that under CED augmentation. The stress of augmented vertebral body and the adjacent vertebral body was most sensitive to extension and rotation loading conditions. As DO increasing, the bone cement augmentation significantly increased the stress level on the upper and lower endplates.

INTERPRETATION

When the degree of osteoporosis increased, CED outperforms bone cement in terms of the stress reduction in augmented vertebral and adjacent vertebral, which could be beneficial for avoiding re-fracture. Using TBGA to treat OVCFs, especially with Plan B method, the condition of the pathological spine is closer to the original status in terms of the sensitivity to stress and the spinal range of motion. The TBGA treatment is sensitive to lateral bending and torsion, therefore patients should be advised to avoid high-risk motions like lateral bending and rotation.

Development of a multiscale model of the human lumbar spine for investigation of tissue loads in people with and without a transtibial amputation during sit-to-stand

Quantification of lumbar spine load transfer is important for understanding low back pain, especially among persons with a lower limb amputation. Computational modeling provides a helpful solution for obtaining estimates of in vivo loads. A multiscale model was constructed by combining musculoskeletal and finite element (FE) models of the lumbar spine to determine tissue loading during daily activities. Three-dimensional kinematic and ground reaction force data were collected from participants with ([Formula: see text]) and without ([Formula: see text]) a unilateral transtibial amputation (TTA) during 5 sit-to-stand trials. We estimated tissue-level load transfer from the multiscale model by controlling the FE model with intervertebral kinematics and muscle forces predicted by the musculoskeletal model. Annulus fibrosis stress, intradiscal pressure (IDP), and facet contact forces were calculated using the FE model. Differences in whole-body kinematics, muscle forces, and tissue-level loads were found between participant groups. Notably, participants with TTA had greater axial rotation toward their intact limb ([Formula: see text]), greater abdominal muscle activity ([Formula: see text]), and greater overall tissue loading throughout sit-to-stand ([Formula: see text]) compared to able-bodied participants. Both normalized (to upright standing) and absolute estimates of L4-L5 IDP were close to in vivo values reported in the literature. The multiscale model can be used to estimate the distribution of loads within different lumbar spine tissue structures and can be adapted for use with different activities, populations, and spinal geometries.

Biomechanical contribution of spinal structures to stability of the lumbar spine-novel biomechanical insights

BACKGROUND CONTEXT

The contribution of anatomical structures to the stability of the spine is of great relevance for diagnostic, prognostic and therapeutic evaluation of spinal pathologies. Although a plethora of literature is available, the contribution of anatomical structures is still not well understood.

PURPOSE

We aimed to quantify the biomechanical relevance of each of the passive spinal structure trough deliberate biomechanical test series using a stepwise reduction approach on cadavers.

STUDY DESIGN

Biomechanical cadaveric study.

METHODS

Fifty lumbar spinal segments originating from 22 human lumbar cadavers were biomechanically tested in a displacement-controlled stepwise reduction study: the intertransverse ligaments, the supraspinous and interspinous ligaments, the facet joint capsules (FJC), the facet joints (FJ), the ligamentum flavum (LF), the posterior longitudinal ligament (PLL), and the anterior longitudinal ligament were subsequently reduced. In the intact state and after each transection step, the segments were physiologically loaded in flexion, extension, axial rotation (AR), lateral bending (LB) and with anterior (AS), posterior (PS) and lateral shear (LS). Thirty-two specimens with only minor degeneration, representing a reasonably healthy subpopulation, were selected for the here presented evaluation. Quantitative values for load and spinal level dependent contribution patterns for the anatomical structures were derived.

RESULTS

Small variability between of the contribution patterns are observed. The intervertebral disc (IVD) is exposed to about 67% of the applied load in LB and during shear loading, but less by load in flexion, extension and AR (less than 35%). The FJ&FJC are the main stabilizers in AR with 49%, but provide only 10% of the stability in extension. Beside the IVD, the LF and the PLL contribute mainly in flexion (22% and 16%, respectively), while the ALL plays a major role during extension (40%) and also contributes during LB (15%). The contribution of the intertransverse ligaments and the supraspinous and interspinous ligaments are very small in all loading directions (<2% and <6%, respectively).

CONCLUSION

The IVD takes the main load in LB and absorbs shear loading, while the FJ&FJC stabilize AR. The ALL resists extension while LF and PLL stabilize flexion. With the small variability of contribution patterns, suggesting distinct adaptation of the structures to one another, the biomechanical characteristics of one structure have to be put in context of the whole spinal segment.

CLINICAL SIGNIFICANCE

The novel information on load distribution helps predict the biomechanical consequences of surgical procedures in more detail.

Relationship between the location of ligamentum flavum hypertrophy and its stress in finite element analysis

Objective

To quantitatively describe the stress of the ligamentum flavum (LF) using the finite element method and to compare the stress at different parts of the healthy LF.

Methods

Based on the high resolution computed tomography imaging data of a healthy 22‐year‐old man, three‐dimensional nonlinear L_4–5 lumbar finite element model (FEM) representing intact condition was developed. The LF, as the object of the present research, was incorporated into the spinal model in the form of solid three‐dimensional structure. The model’s validity is verified by comparing its biomechanical indices, such as range of motion and axial compression pressure displacement, with published results under specific loading conditions. To authenticate the accuracy of the solid LF, the lamina attachments, the central cross‐section, and other anatomy indicators were compared with figures in the published literature. After the average and maximum von Mises stress on the surface of LF under various working conditions were measured using ANSYS and AutoCAD software, the surface stress difference in the LF between the ventral and dorsal sides as well as the lateral and lamina parts were determined.

Results

The FEM predicted a similar tendency for biomechanical indices as shown in previous studies. The lamina attachments, the central cross‐section, and the height as well as the width of the LF in the healthy FEM were in accordance with published results. In the healthy model, the average and maximum von Mises stress in the shallow layer of the LF were, respectively, 1.40, 2.28, 1.76, 1.48, 1.38 and 1.79, 2.41, 1.46, 1.42, 1.71 times that in the deep layer under a compressive preload of 500 N incorporated with flexion, extension, and lateral and rotational moments (10 Nm). The most conspicuous difference in surface stress was observed with the flexion motion, with a nearly 241% difference in the maximum stress and a 228% difference in the average stress compared to those in other states. As far as the whole dorsal side of the LF was concerned, the maximum surface stress was almost all concentrated in the dorsal neighboring facet joint portion. In addition, the maximum and average stress were, respectively, 77%, 72%, 15%, 11%, 71% and 153%, 39%, 54%, 200%, 212% higher in the lateral part than in the lamina part.

Conclusion

Based on the predisposition of LF hypertrophy in the human spine and the stress distribution of this study, the positive correlation between LF hypertrophy and its stress was confirmed.

A COMPARATIVE ANALYSIS OF THE EFFECTS OF TWO AUGMENTERS ON THE SENSITIVITY OF VERTEBRAL BIOMECHANICAL BEHAVIOR IN VERTEBROPLASTY

The treatment of osteoporotic vertebral compression fractures (OVCFS) by transvertebral bone graft and augmentation (TBGA) has achieved satisfactory clinical results, but its biomechanical effects are not clear. The purpose of this study was to investigate the biomechanical effects of TBGA and compare the biomechanical sensitivity of the augmenter used in TBGA — a cylindrical enhancement device (CED) with bone cement. The finite element (FE) model of healthy segments T11-L3 (M1) was built, and two other models with L1 augmentation (M2, M3) were established to simulate CED and bone cement treatment, respectively. The stress and displacement distribution of the three models under five physiological loads were calculated and analyzed by the FE method. Based on the results, the sensitivities of biomechanical parameters to the degree of osteoporosis (DO) and loads were analyzed by linear fitting method using dummy variables. With the increase of DO, the CED is superior to bone cement in preventing the fractures of the augmented vertebral and the adjacent vertebral under the set loading conditions. Simulating TBGA method, the model 2 with L1 reconstructed was closer to the normal T11-L3 model in terms of sensitivity of stress and displacement under different loading conditions.

Topping-off surgery vs posterior lumbar interbody fusion for degenerative lumbar disease: a finite element analysis

BACKGROUND

Adjacent segment disease (ASD) is a common complication after posterior lumbar interbody fusion (PLIF). Recently, a topping-off surgery (non-fusion with Coflex) has been developed to reduce the risk of ASD, yet whether and how the topping-off surgery can relieve ASD remains unclear. The purpose of this study was to explore the biomechanical effect of PLIF and Coflex on the adjacent segments via finite element (FE) analysis and discuss the efficacy of Coflex in preventing ASD.

METHODS

A FE model of L3-L5 segments was generated based on the CT of a healthy volunteer via three commercially available software. Coflex and PLIF devices were modeled and implanted together with the segment model in the FE software. In the FE model, a pre-compressive load of 500 N, equal to two-thirds of the human body mass, was applied on the top surface of the L3. In addition, four types of moments (anteflexion, rear protraction, bending, and axial rotation) set as 10 Nm were successively applied to the FE model combined with this pre-compressive load. Then, the range of motion (ROM), the torsional rigidity, and the maximum von Mises equivalent stress on the L3-L4 intervertebral disc and the implant were analyzed.

RESULTS

Both Coflex and PLIF reduced ROM. However, no significant difference was found in the maximum von Mises equivalent stress of adjacent segment disc between the two devices. Interestingly enough, both systems increased the torsional rigidity at the adjacent lumbar segment, and PLIF had a more significant increase. The Coflex implant had a larger maximum von Mises equivalent stress.

CONCLUSIONS

Both Coflex and PLIF reduced ROM at L3-L4, and thus improved the lumbar stability. Under the same load, both devices had almost the same maximum von Mises equivalent stress as the normal model on the adjacent intervertebral disc. But it is worthy to notice the torsional rigidity of PLIF was higher than that of Coflex, indicating that the lumbar treated with PLIF undertook a larger load to reach ROM of Coflex. Therefore, we presumed that ADS was related to a higher torsional rigidity.

Shear stiffness in the lower cervical spine: Effect of sequential posterior element injury

The aim of this study was to determine the effect of the posterior ligaments and facet joints on the shear stiffness of lower cervical functional spinal units in anterior, posterior, and lateral shear. Five functional spinal units were loaded in anterior, posterior, and right lateral shear up to 100 N using a custom-designed apparatus in a materials testing machine. Specimens were tested in three conditions: intact, with the posterior ligaments severed, and with the facet joints removed. There was a significant decrease in anterior stiffness in the 20-100 N load range from 186 (range: 98-327) N/mm in the intact condition to 105 (range: 78-142) N/mm in the disc-only condition (p = 0.03). Posterior stiffness between these condition decreased significantly from 134 (range: 92-182) N/mm to 119 (range: 83-181) N/mm (p = 0.03). There was no significant effect of posterior ligament removal on shear stiffness. No significant differences were found in the lateral direction or in the 0-20 N range for any direction. Under a 100-N shear load, the facet joints played a significant role in the stiffness of the cervical spine in the anterior-posterior direction, but not in the lateral direction.

An improved stiffness matrix model of the functional spinal unit for application to an improved understanding of pathological changes

The stiffness matrix is a useful way to describe the mechanical behaviour of the functional spinal unit, which is defined as the superior and inferior vertebrae, capsules and ligaments. This usefulness is extended by means of the concept of the "balance point". The balance point is the load application point where the coupling coefficients of the stiffness matrix are minimized. Theoretical considerations are used to demonstrate that the stiffness matrix varies with load point location and thus a single stiffness matrix does not fully characterize the motion segment as well as to derive the stiffness matrix at any one specified point from the stiffness matrix at some other specified point. Special characteristics of the stiffness matrix obtained by loading through the "balance point" were shown. Some possible advantages derived from mechanical testing using the "balance point" concept are discussed. This study validates an improved stiffness matrix model that enhances the understanding of pathological changes by setting the gold standard of the behaviour of a normal functional spinal unit.

Is the sheep a suitable model to study the mechanical alterations of disc degeneration in humans? A probabilistic finite element model study

Intervertebral disc degeneration is one major source of low back pain, which because of its complex multifactorial nature renders the treatment challenging and thus necessitates extensive research. Experimental animal models have proven valuable in improving our understanding of degenerative processes and potentially promising therapies. Currently, the sheep is the most frequently used large animal in vivo model in intervertebral disc research. However, despite its undoubted value for investigations of the complex biological and cellular aspects, to date, it is unclear whether the sheep is also suited to study the mechanical aspects of disc degeneration in humans. A parametric finite element (FE) model of the L4-5 spinal motion segment was developed. Using this model, the geometry and the material properties of both the human and the ovine spinal segment as well as different appearances of disc degeneration can be depicted. Under pure and combined loads, it was investigated whether degenerative changes to both the human and the ovine model equivalent caused the same mechanical response. Different patterns of degeneration resulted in large variations in the ranges of motion, intradiscal pressure, ligament and facet loads. In the human, but not in the ovine model, all these results differed significantly between different degrees of degeneration. This FE model study highlighted possible differences in the mechanical response to disc degeneration between human and ovine intervertebral discs and indicates the necessity of further, more detailed, investigations.

Intrasubject repeatability of in vivo intervertebral motion parameters using quantitative fluoroscopy

PURPOSE

In vivo quantification of intervertebral motion through imaging has progressed to a point where biomarkers for low back pain are emerging. This makes possible deeper study of the condition's biometrics. However, the measurement of change over time involves error. The purpose of this prospective investigation is to determine the intrasubject repeatability of six in vivo intervertebral motion parameters using quantitative fluoroscopy.

METHODS

Intrasubject reliability (ICC) and minimal detectable change (MDC) of baseline to 6-week follow-up measurements were calculated for six lumbar spine intervertebral motion parameters in 109 healthy volunteers. A standardised quantitative fluoroscopy (QF) protocol was used to provide measurements in the coronal and sagittal planes using both passive recumbent and active weight-bearing motion. Parameters were: intervertebral range of motion (IV-RoM), laxity, motion sharing inequality (MSI), motion sharing variability (MSV), flexion translation and anterior disc height change during flexion.

RESULTS

The best overall intrasubject reliability (ICC) and agreement (MDC) were for disc height (ICC 0.89, MDC 43%) and IV-RoM (ICC 0.96, MDC 60%), and the worst for MSV (ICC 0.04, MDC 408%). Laxity, MSI and translation had acceptable reliability (most ICCs > 0.60), but not agreement (MDC > 85%).

CONCLUSION

Disc height and IV-RoM measurement using QF could be considered for randomised trials, while laxity, MSI and translation could be considered for moderators, correlates or mediators of patient-reported outcomes. MSV had both poor reliability and agreement over 6 weeks. These slides can be retrieved under Electronic Supplementary Material.

Effect of disc degeneration on the mechanical behavior of the human lumbar spine: a probabilistic finite element study

BACKGROUND CONTEXT

Intervertebral disc degeneration has been subject to numerous in vivo and in vitro investigations and numerical studies during recent decades, reporting partially contradictory findings. However, most of the previous studies were limited in the number of specimens investigated and, therefore, could not consider the vast variety of the specimen geometries, which are likely to strongly influence the mechanical behavior of the spine.

PURPOSE

To complement the understanding of the mechanical consequences of disc degeneration, whereas considering natural variations in the major spinal geometrical parameters.

DESIGN/SETTING

A probabilistic finite element study.

METHODS

A parametric finite element model of a human L4-L5 motion segment considering 40 geometrical parameters was developed. One thousand individual geometries comprising four degeneration grades were generated in a probabilistic manner, and the influence of the severity of disc degeneration on the mechanical response of the motion segment to different loading conditions was statistically evaluated.

RESULTS

Variations in the individual structural parameters resulted in marked variations in all evaluated parameters within each degeneration grade. Nevertheless, the effect of degeneration in almost all evaluated response values was statistically significant. With degeneration, the intradiscal pressure progressively decreased. At the same time, the facet loads increased and the ligament tension was reduced. The initially nonlinear load-deformation relationships became linear whereas the segment stiffness increased.

CONCLUSIONS

Results indicate significant stiffening of the motion segment with progressing degeneration and gradually increasing loading of the facets from nondegenerated to moderately degenerated conditions along with a significant reduction of the ligament tension in flexion.

Improving the Process of Adjusting the Parameters of Finite Element Models of Healthy Human Intervertebral Discs by the Multi-Response Surface Method

The kinematic behavior of models that are based on the finite element method (FEM) for modeling the human body depends greatly on an accurate estimate of the parameters that define such models. This task is complex, and any small difference between the actual biomaterial model and the simulation model based on FEM can be amplified enormously in the presence of nonlinearities. The current paper attempts to demonstrate how a combination of the FEM and the MRS methods with desirability functions can be used to obtain the material parameters that are most appropriate for use in defining the behavior of Finite Element (FE) models of the healthy human lumbar intervertebral disc (IVD). The FE model parameters were adjusted on the basis of experimental data from selected standard tests (compression, flexion, extension, shear, lateral bending, and torsion) and were developed as follows: First, three-dimensional parameterized FE models were generated on the basis of the mentioned standard tests. Then, 11 parameters were selected to define the proposed parameterized FE models. For each of the standard tests, regression models were generated using MRS to model the six stiffness and nine bulges of the healthy IVD models that were created by changing the parameters of the FE models. The optimal combination of the 11 parameters was based on three different adjustment criteria. The latter, in turn, were based on the combination of stiffness and bulges that were obtained from the standard test FE simulations. The first adjustment criteria considered stiffness and bulges to be equally important in the adjustment of FE model parameters. The second adjustment criteria considered stiffness as most important, whereas the third considered the bulges to be most important. The proposed adjustment methods were applied to a medium-sized human IVD that corresponded to the L3–L4 lumbar level with standard dimensions of width = 50 mm, depth = 35 mm, and height = 10 mm. Agreement between the kinematic behavior that was obtained with the optimized parameters and that obtained from the literature demonstrated that the proposed method is a powerful tool with which to adjust healthy IVD FE models when there are many parameters, stiffnesses, and bulges to which the models must adjust.

Uneven intervertebral motion sharing is related to disc degeneration and is greater in patients with chronic, non-specific low back pain: an in vivo, cross-sectional cohort comparison of intervertebral dynamics using quantitative fluoroscopy

PURPOSE

Evidence of intervertebral mechanical markers in chronic, non-specific low back pain (CNSLBP) is lacking. This research used dynamic fluoroscopic studies to compare intervertebral angular motion sharing inequality and variability (MSI and MSV) during continuous lumbar motion in CNSLBP patients and controls. Passive recumbent and active standing protocols were used and the relationships of these variables to age and disc degeneration were assessed.

METHODS

Twenty patients with CNSLBP and 20 matched controls received quantitative fluoroscopic lumbar spine examinations using a standardised protocol for data collection and image analysis. Composite disc degeneration (CDD) scores comprising the sum of Kellgren and Lawrence grades from L2-S1 were obtained. Indices of intervertebral motion sharing inequality (MSI) and variability (MSV) were derived and expressed in units of proportion of lumbar range of motion from outward and return motion sequences during lying (passive) and standing (active) lumbar bending and compared between patients and controls. Relationships between MSI, MSV, age and CDD were assessed by linear correlation.

RESULTS

MSI was significantly greater in the patients throughout the intervertebral motion sequences of recumbent flexion (0.29 vs. 0.22, p = 0.02) and when flexion, extension, left and right motion were combined to give a composite measure (1.40 vs. 0.92, p = 0.04). MSI correlated substantially with age (R = 0.85, p = 0.004) and CDD (R = 0.70, p = 0.03) in lying passive investigations in patients and not in controls. There were also substantial correlations between MSV and age (R = 0.77, p = 0.01) and CDD (R = 0.85, p = 0.004) in standing flexion in patients and not in controls.

CONCLUSION

Greater inequality and variability of motion sharing was found in patients with CNSLBP than in controls, confirming previous studies and suggesting a biomechanical marker for the disorder at intervertebral level. The relationship between disc degeneration and MSI was augmented in patients, but not in controls during passive motion and similarly for MSV during active motion, suggesting links between in vivo disc mechanics and pain generation.

Effect of sacral slope on the biomechanical behavior of the low lumbar spine

The present study investigated the influence of sacral slope (SS) on the biomechanical responses of the lumbar spine under specific physiological conditions. Firstly, based on computed tomography scan images of a 30-year-old healthy male volunteer (SS, 55°), a three-dimensional finite element (FE) model including the L4-S1 segment was established. Flexion, extension, lateral bending and torsion motions were simulated and compared with cadaveric test data in the literature to validate the lumbar spine FE model. The model was then modified with different SS values (40 and 25°) for the same simulations to describe the process of structural compensation. Numerical results showed that with the reduction of SS, the range of motions (ROMs) reduced for flexion and lateral bending, but increased for extension and torsion. For displacement, the maximum magnitudes of L4/5 annulus fibrosus (AF) reduced by 10-25% in flexion, lateral bending and torsion, but less effect was observed for extension with only a 4% drop. Nearly the same displacement distribution appeared on the L5/S1 AF with small changes in the four motions. For the stress field of L4/5 AF, in contrast to flexion, the magnitudes for extension and lateral bending varied markedly, and under torsion the value increased by ~10%. For L5/S1 AF, the stresses changed little under flexion, extension and lateral bending, but strongly declined for torsion by ~71.8%. In conclusion, the present study indicates that the change in SS due to structural compensation affects the biomechanical behavior of the spine structure, and attention should be paid to SS when conducting surgical procedures or selecting intervertebral fusion implants.

Biomechanics of the human intervertebral disc: A review of testing techniques and results

Many experimental testing techniques have been adopted in order to provide an understanding of the biomechanics of the human intervertebral disc (IVD). The aim of this review article is to amalgamate results from these studies to provide readers with an overview of the studies conducted and their contribution to our current understanding of the biomechanics and function of the IVD. The overview is presented in a way that should prove useful to experimentalists and computational modellers. Mechanical properties of whole IVDs can be assessed conveniently by testing 'motion segments' comprising two vertebrae and the intervening IVD and ligaments. Neural arches should be removed if load-sharing between them and the disc is of no interest, and specimens containing more than two vertebrae are required to study 'adjacent level' effects. Mechanisms of injury (including endplate fracture and disc herniation) have been studied by applying complex loading at physiologically-relevant loading rates, whereas mechanical evaluations of surgical prostheses require slower application of standardised loading protocols. Results can be strongly influenced by the testing environment, preconditioning, loading rate, specimen age and degeneration, and spinal level. Component tissues of the disc (anulus fibrosus, nucleus pulposus, and cartilage endplates) have been studied to determine their material properties, but only the anulus has been thoroughly evaluated. Animal discs can be used as a model of human discs where uniform non-degenerate specimens are required, although differences in scale, age, and anatomy can lead to problems in interpretation.

Incorporating Six Degree-of-Freedom Intervertebral Joint Stiffness in a Lumbar Spine Musculoskeletal Model-Method and Performance in Flexed Postures

Intervertebral translations and rotations are likely dependent on intervertebral stiffness properties. The objective of this study was to incorporate realistic intervertebral stiffnesses in a musculoskeletal model of the lumbar spine using a novel force-dependent kinematics approach, and examine the effects on vertebral compressive loading and intervertebral motions. Predicted vertebral loading and intervertebral motions were compared to previously reported in vivo measurements. Intervertebral joint reaction forces and motions were strongly affected by flexion stiffness, as well as force-motion coupling of the intervertebral stiffness. Better understanding of intervertebral stiffness and force-motion coupling could improve musculoskeletal modeling, implant design, and surgical planning.

Lying Down Instability Undetected on Standing Dynamic Radiographs

It is well known that spinal instability should be evaluated in the standing lateral position. Standing dynamic flexion and extension radiographs are usually used to assess spinal instability. Here, we report a patient who experienced distraction instability while in the supine position rather than the standard standing position. To our knowledge, this is the first report of lying-down instability undetected on standing dynamic flexion and extension radiographs. We discuss the pathophysiological mechanism of this uncommon but possible entity and provide a review of the literature.

Fundamental biomechanics of the spine--What we have learned in the past 25 years and future directions

Since the publication of the 2nd edition of White and Panjabi׳s textbook, Clinical Biomechanics of the Spine in 1990, there has been considerable research on the biomechanics of the spine. The focus of this manuscript will be to review what we have learned in regards to the fundamentals of spine biomechanics. Topics addressed include the whole spine, the functional spinal unit, and the individual components of the spine (e.g. vertebra, intervertebral disc, spinal ligaments). In these broad categories, our understanding in 1990 is reviewed and the important knowledge or understanding gained through the subsequent 25 years of research is highlighted. Areas where our knowledge is lacking helps to identify promising topics for future research. In this manuscript, as in the White and Panjabi textbook, the emphasis is on experimental research using human material, either in vivo or in vitro. The insights gained from mathematical models and animal experimentation are included where other data are not available. This review is intended to celebrate the substantial gains that have been made in the field over these past 25 years and also to identify future research directions.

A history of spine biomechanics. Focus on 20th century progress

The application of mechanical principles to problems of the spine dates to antiquity. Significant developments related to spinal anatomy and biomechanical behaviour made by Renaissance and post-Renaissance scholars through the end of the 19th century laid a strong foundation for the developments since that time. The objective of this article is to provide a historical overview of spine biomechanics with a focus on the developments in the 20th century. The topics of spine loading, spinal posture and stability, spinal kinematics, spinal injury, and surgical strategies were reviewed.

The analysis of mechanical changes of percutaneous vertebroplasty

In this article, a numerical analysis method of mechanical changes for percutaneous vertebroplasty is presented. Firstly, a 3D geometric model of lumbar functional spinal units has been built by segmenting computed tomography images and performing 3D reconstruction. On the basis of the geometric model, two kinds of 3D finite element models (FEM) for preoperative and postoperative vertebrae are created. Then, a numerical calculation method on FEM for the analysis of mechanical changes has been developed. The simulating calculation can reveal the stress and strain distribution and deformation of the preoperative and postoperative vertebrae. The approach could provide useful mechanical changes analysis and a fresh perspective into how the procedure can be implemented more effectively toward the goal of preventing osteoporosis-related fractures.

What have we learned from finite element model studies of lumbar intervertebral discs in the past four decades?

Finite element analysis is a powerful tool routinely used to study complex biological systems. For the last four decades, the lumbar intervertebral disc has been the focus of many such investigations. To understand the disc functional biomechanics, a precise knowledge of the disc mechanical, structural and biochemical environments at the microscopic and macroscopic levels is essential. In response to this need, finite element model studies have proven themselves as reliable and robust tools when combined with in vitro and in vivo measurements. This paper aims to review and discuss some salient findings of reported finite element simulations of lumbar intervertebral discs with special focus on their relevance and implications in disc functional biomechanics. Towards this goal, the earlier investigations are presented, discussed and summarized separately in three distinct groups of elastic, multi-phasic transient and transport model studies. The disc overall response as well as the relative role of its constituents are markedly influenced by loading rate, magnitude, combinations/preloads and posture. The nucleus fluid content and pressurizing capacity affect the disc compliance, annulus strains and failure sites/modes. Biodynamics of the disc is affected by not only the excitation characteristics but also preloads, existing mass and nucleus condition. The role of fluid pressurization and collagen fiber stiffening diminish with time during diurnal loading. The endplates permeability influences the time-dependent response of the disc in both loaded and unloaded recovery phases. The transport of solutes is substantially influenced by the disc size, tissue diffusivity and endplates permeability.

Biomechanical comparison of anterior lumbar interbody fusion: stand-alone interbody cage versus interbody cage with pedicle screw fixation -- a finite element analysis

Background

Anterior lumbar interbody fusion (ALIF) followed by pedicle screw fixation (PSF) is used to restore the height of the intervertebral disc and provide stability. Recently, stand-alone interbody cage with anterior fixation has been introduced, which eliminates the need for posterior surgery. We compared the biomechanics of the stand-alone interbody cage to that of the interbody cage with additional PSF in ALIF.

Methods

A three-dimensional, non-linear finite element model (FEM) of the L2-5 segment was modified to simulate ALIF in L3-4. The models were tested under the following conditions: (1) intact spine, (2) destabilized spine, (3) with the interbody cage alone (type 1), (4) with the stand-alone cage with anterior fixation (SynFix-LR®; type 2), and (5) with type 1 in addition to PSF (type 3). Range of motion (ROM) and the stiffness of the operated level, ROM of the adjacent segments, load sharing distribution, facet load, and vertebral body stress were quantified with external loading.

Results

The implanted models had decreased ROM and increased stiffness compared to those of the destabilized spine. The type 2 had differences in ROM limitation of 8%, 10%, 4%, and 6% in flexion, extension, axial rotation, and lateral bending, respectively, compared to those of type 3. Type 2 had decreased ROM of the upper and lower adjacent segments by 3-11% and 3-6%, respectively, compared to those of type 3. The greatest reduction in facet load at the operated level was observed in type 3 (71%), followed by type 2 (31%) and type 1 (23%). An increase in facet load at the adjacent level was highest in type 3, followed by type 2 and type 1. The distribution of load sharing in type 2 (anterior:posterior, 95:5) was similar to that of the intact spine (89:11), while type 3 migrated posterior (75:25) to the normal. Type 2 reduced about 15% of the stress on the lower vertebral endplate compared to that in type 1. The stress of type 2 increased two-fold compared to the stress of type 3, especially in extension.

Conclusions

The stand-alone interbody cage can provide sufficient stability, reduce stress in adjacent levels, and share the loading distribution in a manner similar to an intact spine.

Effects of lumbar arthrodesis on adjacent segments: differences between surgical techniques

STUDY DESIGN

A finite element analysis.

OBJECTIVE

To evaluate the differences between surgical techniques in terms of the effects of arthrodesis on adjacent segments.

SUMMARY OF BACKGROUND DATA

Augmentation with posterior rigid fixation combined with transpedicular screw insertion, which is one of the most popular techniques for lumbar arthrodesis, shows benefits in immediate stabilization and a higher fusion rate but is reportedly correlated with greater stress on adjacent segments. However, the increased stress on adjacent segments needs further evaluation because the differences of the effects on adjacent segments between surgical techniques, including anterior lumbar interbody fusion, posterior lumbar interbody fusion, and semirigid fixation, have not yet been determined.

METHODS

A finite element model of the human lumbar spine was developed. Three spinal segments (L2-L5) were used to investigate. The intact spinal model was validated by comparing it with previously reported models. Then, 4 arthrodesis models were analyzed and compared: (1) anterior lumbar interbody fusion model; (2) posterior lumbar interbody fusion model; (3) semirigid fixation model combined with posterior lumbar interbody fusion; and (4) rigid fixation model combined with posterior lumbar interbody fusion.

RESULTS

Among these 4 models, the rigid fixation model showed the greatest amount of stress, with increased intervertebral disc pressure and contact force of the facet joints of both upper and lower adjacent segments. The second highest stress levels were seen in the semirigid fixation model and the lowest stress levels were seen in the anterior lumbar interbody fusion model.

CONCLUSION

Although bony fusion had been completed, the effects of lumbar arthrodesis on adjacent segments could vary according to the surgical technique used for arthrodesis. Semirigid fixation combined with arthrodesis deserves careful consideration and further detailed study because it may cause less stress on adjacent segments than rigid fixation while maintaining the benefits of the latter procedure.

Determination of the translational and rotational stiffnesses of an L4-L5 functional spinal unit using a specimen-specific finite element model

The knowledge of spinal kinematics is of paramount importance for many aspects of clinical application (i.e. diagnosis, treatment and surgical intervention) and for the development of new spinal implants. The aim of this study was to determine the translational and rotational stiffnesses of a functional spinal unit (FSU) L4-L5 using a specimen-specific finite element model. The results are needed as input data for three-dimensional (3D) multi-body musculoskeletal models in order to simulate vertebral motions and loading in the lumbar spine during daily activities. Within the modelling process, a technique to partition the constitutive members and to calibrate their mechanical properties for the complex model is presented. The material and geometrical non-linearities originating from the disc, the ligaments and the load transfer through the zygapophysial joints were considered. The FSU was subjected to pure moments and forces in the three anatomical planes. For each of the loading scenarios, with and without vertical and follower preload, the presented technique provides results in fair agreement with the literature. The novel representation of the nonlinear behaviour of the translational and rotational stiffness of the disc as a function of the displacement can be used directly as input data for multi-body models.

Biomechanical evaluation of an expandable cage in single-segment posterior lumbar interbody fusion

STUDY DESIGN

Controlled laboratory study.

OBJECTIVE

To evaluate the biomechanical characteristics of a new expandable interbody cage in single-segment posterior lumbar interbody fusion (PLIF) using cadaveric lumbar spines.

SUMMARY OF BACKGROUND DATA

One of the popular methods of treating lumbar spine pathologies involves a posterior lumbar interbody fusion using bilateral interbody nonexpandable cages. However, this method can require extensive bony removal and nerve root retraction. Expandable interbody cages may decrease the risk associated with PLIFs.

METHODS

Biomechanical testing was performed on 5 fresh frozen L4/L5 mobile functional spinal units using a custom testing system that permits 6 df and a digital video digitizing system. The specimens were tested intact, postdiscectomy, after interbody cage placement, and after cage placement and pedicle screw fixation. Each specimen was tested from 0.5 to 8.0 N·m for extension, flexion, lateral bending, and rotation, and from 5 to 300 N for axial compression. The angular displacement, stiffness, disc height, and sagittal alignment were determined.

RESULTS

When the cage was supplemented with pedicle screw fixation, the mean angular displacement for rotation and lateral bending was significantly less than all other conditions (P < 0.05). The percentage range of motion (% ROM) showed a statistically significant decrease in lateral bending (P < 0.05) for cage alone vs. postdiscectomy. For the pedicle screw construct, rotation showed a significantly lower percentage ROM compared with all other constructs (P < 0.05), and lateral bending and extension-flexion showed a significantly lower percentage ROM compared with postdiscectomy (P < 0.05). For all motions, stiffness of the cage and pedicle screw construct was greater than intact, with only rotation showing a statistically significant increase (P < 0.05). Anterior disc height was restored to intact after cage alone (P < 0.05). Sagittal alignment did not show statistically significant differences.

CONCLUSION

PLIF using expandable lumbar interbody cage requires pedicle screw fixation.

DETERMINATION OF LOAD TRANSMISSION AND CONTACT FORCE AT FACET JOINTS OF L2–L3 MOTION SEGMENT USING FE METHOD

In the human spine, it is well known that facet joints play a significant role in load transmission and providing stability. It has also been hypothesized to be one of most probable sources of low back pain. Experimental determination of the load-bearing role of lumbar intervertebral joints, such as the facets joints, under axial compression has not been a straightforward task. In this study, the role of the facets in load transmission through a L2–L3 motion segment under axial compression is investigated using a L2–L3 finite element (FE) model, incorporated with an accurate three-dimensional geometry of facet joints with the inclusion of surface-to-surface continuum contact representation. The effects of osteoarthritis on facet force and biomechanical behaviors are also investigated by assuming friction at the facet joints. The study shows that the facet joints resisted 8% more in load for joints with osteoarthritics as compared with the normal joints. High percentage increase in contact facet force was also predicted for joint with osteoarthritics deformity. The use of the analytical FE model provided yet another efficient alternative for predicting the load transmission and contact force for degenerative joints, so as to provide a better understanding of the biomechanics of the spine as well as the pathophysiology of the various spinal disorders and degenerative conditions.

SIGNIFICANCE OF THE ANNULUS PROPERTIES TO FINITE ELEMENT MODELING OF INTERVERTEBRAL DISCS

Current mathematical material laws work quite accurately with conventional engineering materials because they are linear and isotropic. These laws are much less effective, however, at representing living tissues. Biomechanical engineers therefore often face the problems of modelling material non-linearities, particularly with the soft tissues, muscles and tendons of the human body. The non-linear and often anisotropic structure of these materials makes any mathematical representation very difficult. In particular, the lack of a good general mathematical model of the intervertebral disc has hampered the study of spinal mechanics, as the disc annulus is a nonlinear fibrous tissue with highly directional properties. Previous studies have concentrated on the overall behavior of discs and this has been largely explained but knowledge is still very limited on the effect of the individual disc components. This means that current mathematical models are poor when it comes to describing a prolapsed disc, where there has been failure of at least one of the components. This finite element study described here focuses on the disc annulus properties and their effect on disc behavior. The novelty of this study lies in the material formulation of the annulus fibrosus, which changes its Young's modulus according to a non-linear curve.

Design of next generation total disk replacements

To improve the treatments for low back pain, new designs of total disk replacement have been proposed. The question is how well these designs can act as a functional replacement of the intervertebral disk. Four finite element models were made, for four different design concepts, to determine how well they can mimic the physiological intervertebral disk mechanical function. The four designs were a homogenous elastomer, a multi-stiffness elastomer, an elastomer with fiber jacket, and a hydrogel with fiber jacket. The best material properties of the four models were determined by optimizing the model behavior to match the behavior of the intervertebral disk in flexion-extension, axial rotation, and lateral bending. It was shown that neither a homogeneous elastomer nor a multi-stiffness elastomer could mimic the non-linear behavior within the physiological range of motion. Including a fiber jacket around an elastomer allowed for physiological motion in all degrees of freedom. Replacing the elastomer by a hydrogel yielded similar good behavior. Mimicking the non-linear behavior of the intervertebral disk, in the physiological range of motion is essential in maintaining and restoring spinal motion and in protecting surrounding tissues like the facet joints or adjacent segments. This was accomplished with designs mimicking the function of the annulus fibrosus.

Mechanical Characterization of a Viscoelastic Disc for Lumbar Total Disc Replacement

A viscoelastic artificial disc may more closely replicate normal stiffness characteristics of the healthy human disc compared with first-generation total disc replacement (TDR) devices, which do not utilize viscoelastic materials and are based on a ball and socket design that does not allow loading compliance. Mechanical testing was performed to characterize the durability and range of motion (ROM) of an investigational viscoelastic TDR (VTDR) device for the lumbar spine, the Freedom® Lumbar Disc. ROM data were compared with data reported for the human lumbar disc in the clinical literature. Flexibility and stiffness of the VTDR in compression, rotation, and flexion/extension were within the parameters associated with the normal human lumbar disc. The device constrained motion to physiologic ranges and replicated normal stress/strain dynamics. No mechanical or functional failures occurred within the loads and ROM experienced by the human disc. Fatigue testing of the worst case VTDR device size demonstrated a fatigue life of 50 years of simulated walking and 240 years of simulated significant bends in both flexion/extension and lateral bending coupled with axial rotation, with no functional failures. These results indicate that the VTDR evaluated in this mechanical study is durable and has the ability to replicate the stiffness and mechanics of the natural, healthy human lumbar disc.

Identification of spinal tissues loaded by manual therapy: a robot-based serial dissection technique applied in porcine motion segments

STUDY DESIGN

Serial dissection of porcine motion segments during robotic control of vertebral kinematics.

OBJECTIVE

To identify which spinal tissues are loaded in response to manual therapy (manipulation and mobilization) and to what magnitude.

SUMMARY OF BACKGROUND DATA

Various theoretical constructs attempt to explain how manual therapies load specific spinal tissues. By using a parallel robot to control vertebral kinematics during serial dissection, it is possible to quantify the loads experienced by discrete spinal tissues undergoing common therapeutic procedures such as manual therapy.

METHODS

In 9 porcine cadavers, manual therapy was provided to L3 and the kinematic response of L3-L4 recorded. The exact kinematic trajectory experienced by L3-L4 in response to manual therapy was then replayed to the isolated segment by a parallel robot equipped with a 6-axis load cell. Discrete spinal tissues were then removed and the kinematic pathway replayed. The change in forces and moments following tissue removal were considered to be those applied to that specific tissue by manual therapy.

RESULTS

In this study, both manual therapies affected spinal tissues. The intervertebral disc experienced the greatest forces and moments arising from both manipulation and mobilization.

CONCLUSION

This study is the first to identify which tissues are loaded in response to manual therapy. The observation that manual therapy loads some tissues to a much greater magnitude than others offers a possible explanation for its modest treatment effect; only conditions involving these tissues may be influenced by manual therapy. Future studies are planned to determine if manual therapy can be altered to target (or avoid) specific spinal tissues.

The viscoelastic standard nonlinear solid model: predicting the response of the lumbar intervertebral disk to low-frequency vibrations

Due to the mathematical complexity of current musculoskeletal spine models, there is a need for computationally efficient models of the intervertebral disk (IVD). The aim of this study is to develop a mathematical model that will adequately describe the motion of the IVD under axial cyclic loading as well as maintain computational efficiency for use in future musculoskeletal spine models. Several studies have successfully modeled the creep characteristics of the IVD using the three-parameter viscoelastic standard linear solid (SLS) model. However, when the SLS model is subjected to cyclic loading, it underestimates the load relaxation, the cyclic modulus, and the hysteresis of the human lumbar IVD. A viscoelastic standard nonlinear solid (SNS) model was used to predict the response of the human lumbar IVD subjected to low-frequency vibration. Nonlinear behavior of the SNS model was simulated by a strain-dependent elastic modulus on the SLS model. Parameters of the SNS model were estimated from experimental load deformation and stress-relaxation curves obtained from the literature. The SNS model was able to predict the cyclic modulus of the IVD at frequencies of 0.01 Hz, 0.1 Hz, and 1 Hz. Furthermore, the SNS model was able to quantitatively predict the load relaxation at a frequency of 0.01 Hz. However, model performance was unsatisfactory when predicting load relaxation and hysteresis at higher frequencies (0.1 Hz and 1 Hz). The SLS model of the lumbar IVD may require strain-dependent elastic and viscous behavior to represent the dynamic response to compressive strain.

Biomechanical comparison of a new stand-alone anterior lumbar interbody fusion cage with established fixation techniques - a three-dimensional finite element analysis

Background

Initial promise of a stand-alone interbody fusion cage to treat chronic back pain and restore disc height has not been realized. In some instances, a posterior spinal fixation has been used to enhance stability and increase fusion rate. In this manuscript, a new stand-alone cage is compared with conventional fixation methods based on the finite element analysis, with a focus on investigating cage-bone interface mechanics and stress distribution on the adjacent tissues.

Methods

Three trapezoid 8° interbody fusion cage models (dual paralleled cages, a single large cage, or a two-part cage consisting of a trapezoid box and threaded cylinder) were created with or without pedicle screws fixation to investigate the relative importance of the screws on the spinal segmental response. The contact stress on the facet joint, slip displacement of the cage on the endplate, and rotational angle of the upper vertebra were measured under different loading conditions.

Results

Simulation results demonstrated less facet stress and slip displacement with the maximal contact on the cage-bone interface. A stand-alone two-part cage had good slip behavior under compression, flexion, extension, lateral bending and torsion, as compared with the other two interbody cages, even with the additional posterior fixation. However, the two-part cage had the lowest rotational angles under flexion and torsion, but had no differences under extension and lateral bending.

Conclusion

The biomechanical benefit of a stand-alone two-part fusion cage can be justified. This device provided the stability required for interbody fusion, which supports clinical trials of the cage as an alternative to circumferential fixations.

Biomechanics of the lumbar spine after dynamic stabilization

Target of the study was to predict the biomechanics of the instrumented and adjacent levels due to the insertion of the DIAM spinal stabilization system (Medtronic Ltd). For this purpose, a 3-dimensional finite element model of the intact L3/S1 segment was developed and subjected to different loading conditions (flexion, extension, lateral bending, axial rotation). The model was then instrumented at the L4/L5 level and the same loading conditions were reapplied. Within the assumptions of our model, the simulation results suggested that the implant caused a reduction in range of motion of the instrumented level by 17% in flexion and by 43% in extension, whereas at the adjacent levels, no significant changes were predicted. Numerical results in terms of intradiscal pressure, relative to the intact condition, predicted that the intervertebral disc at the instrumented level was unloaded by 27% in flexion, by 51% in extension, and by 6% in axial rotation, while no variations in pressure were caused by the device in lateral bending. At the adjacent levels, a change of relative intradiscal pressure was predicted in extension, both at the L3/L4 level, which resulted unloaded by 26% and at the L5/S1 level, unloaded by 8%. Furthermore, a reduction in terms of principal compressive stress in the annulus fibrosus of the L4/L5 instrumented level was predicted, as compared with the intact condition. These numerical predictions have to be regarded as a theoretical representation of the behavior of the spine, because any finite element model represents only a simplification of the real structure.

Human internal disc strains in axial compression measured noninvasively using magnetic resonance imaging

STUDY DESIGN

Internal deformations and strains were measured within intact human motion segments.

OBJECTIVE

Quantify 2-dimensional internal deformation and strain in compression of human intervertebral discs using MRI.

SUMMARY OF BACKGROUND DATA

Experiments using radiographic or optical imaging have provided important data for internal disc deformations. However, these studies are limited by physical markers and/or disruption of the disc structural integrity.

METHODS

MR images were acquired before and during application of a 1000 N axial compression. Two-dimensional internal displacements, average strains, and the location and direction of peak strains were calculated using texture correlation, a pattern matching algorithm.

RESULTS

The average height loss was 0.4 mm, which corresponded to 4.4% compressive strain. The inner AF radial displacement was outward, even with degeneration; the average outward displacement of the inner AF (0.16 mm) was less than the outer AF (0.36 mm). High shear peak strains (2%-26%) occurred near the endplate and at the inner AF. Shear was higher in the anterior AF compared to the posterior.

CONCLUSION

This technique allows quantification of displacement and strain within the intact disc. The radial displacements of inner AF suggest NP translation under compression. Peak tensile radial strains occurred as vertical bands throughout the anulus, which may contribute to radial tears and herniations. The tensile axial and shear strains at the interface between the AF and endplate could be related to the occurrence of rim lesions. Peak strains at the endplate are likely due to the AF curvature and the oblique fibers angle at fiber insertion sites. In the future, this technique may be used to measure disc strain under a variety of loading conditions, such as bending or torsion, and could also be used to study the mechanical effects of disc degeneration and potential clinical interventions.