Effects of aging and degeneration on the human intervertebral disc during the diurnal cycle: a finite element study

Recent Advances in Coupled MBS and FEM Models of the Spine—A Review

How back pain is related to intervertebral disc degeneration, spinal loading or sports-related overuse remains an unanswered question of biomechanics. Coupled MBS and FEM simulations can provide a holistic view of the spine by considering both the overall kinematics and kinetics of the spine and the inner stress distribution of flexible components. We reviewed studies that included MBS and FEM co-simulations of the spine. Thereby, we classified the studies into unidirectional and bidirectional co-simulation, according to their data exchange methods. Several studies have demonstrated that using unidirectional co-simulation models provides useful insights into spinal biomechanics, although synchronizing the two distinct models remains a key challenge, often requiring extensive manual intervention. The use of a bidirectional co-simulation features an iterative, automated process with a constant data exchange between integrated subsystems. It reduces manual corrections of vertebra positions or reaction forces and enables detailed modeling of dynamic load cases. Bidirectional co-simulations are thus a promising new research approach for improved spine modeling, as a main challenge in spinal biomechanics is the nonlinear deformation of the intervertebral discs. Future studies will likely include the automated implementation of patient-specific bidirectional co-simulation models using hyper- or poroelastic intervertebral disc FEM models and muscle forces examined by an optimization algorithm in MBS. Applications range from clinical diagnosis to biomechanical analysis of overload situations in sports and injury prediction.

Diurnal Variation in Hydration of the Cervical Intervertebral Disc Assessed Using T2 Mapping of Magnetic Resonance Imaging

Objective

The study aimed to investigate the diurnal variation in cervical disc hydration and its relationship with cervical degeneration.

Materials and Methods

C3–C7 discs of 86 prospectively enrolled participants (37 males, 49 females; mean age ± standard deviation, 23.5 ± 2.5 years) were assessed using T2 mapping in the morning and evening. All discs were stratified by Miyazaki grade or C2–C7 Cobb angle and T2 values (T2). The degree of diurnal T2 variation (T2-DDV), defined as (morning T2 – evening T2)/morning T2 × 100%, was measured for the entire disc, annulus fibrosus (AF), nucleus pulposus (NP), and endplate zones.

Results

T2 of the entire disc decreased significantly after the daytime load (p < 0.001), with a T2-DDV of 13.3% for all discs and 16.0%, 12.2%, and 13.0% for healthy (grade I), mild degenerative (grade II), and advanced degenerative (grade III/IV) discs, respectively. T2 of regional NPs and AFs decreased significantly from morning to evening (p ≤ 0.049) except in the healthy anterior inner AF (p = 0.092). Compared with healthy discs, mild degenerative discs displayed lower T2 and T2-DDV in regional NPs (p < 0.001). Advanced degenerative discs showed higher T2-DDV in the anterior inner AF compared with healthy discs (p = 0.050). Significant diurnal T2 changes in the endplate zones were observed only in healthy discs (p = 0.013). Cervical discs in the low Cobb angle group showed higher T2-DDV in the anterior AFs and anterior NP and lower T2-DDV in the posterior AF than those in the high Cobb angle group (p ≤ 0.041).

Conclusion

This study characterized the diurnal variation in hydration of the cervical discs as assessed using T2 mapping and revealed early chemo-mechanical coupling dysfunction in degenerating discs. Cervical sagittal alignment on MRI can affect the diurnal stress patterns of the cervical discs. T2 mapping is sensitive to disc biomechanical dysfunction and offers translational potential from biomechanical research to clinical application.

Excessive mechanical strain accelerates intervertebral disc degeneration by disrupting intrinsic circadian rhythm

Night shift workers with disordered rhythmic mechanical loading are more prone to intervertebral disc degeneration (IDD). Our results showed that circadian rhythm (CR) was dampened in degenerated and aged NP cells. Long-term environmental CR disruption promoted IDD in rats. Excessive mechanical strain disrupted the CR and inhibited the expression of core clock proteins. The inhibitory effect of mechanical loading on the expression of extracellular matrix genes could be reversed by BMAL1 overexpression in NP cells. The Rho/ROCK pathway was demonstrated to mediate the effect of mechanical stimulation on CR. Prolonged mechanical loading for 12 months affected intrinsic CR genes and induced IDD in a model of upright posture in a normal environment. Unexpectedly, mechanical loading further accelerated the IDD in an Light-Dark (LD) cycle-disrupted environment. These results indicated that intrinsic CR disruption might be a mechanism involved in overloading-induced IDD and a potential drug target for night shift workers.

Working long shifts at times when the body should be at rest can have lasting effects on the intervertebral discs in the back, leading to chronic pain. Night shift workers are susceptible to developing certain health conditions because of chronic disruption to their circadian rhythms. Now, Li-Bo Jiang at Zhongshan Hospitial, Fudan University in Shanghai and co-workers across China have uncovered a link between circadian rhythm disruption and intervertebral disc degeneration. In experiments on human tissue samples and rat models, the team found that oscillation of the expression of clock-related genes and proteins was reduced in severely degenerated disc cells. Cellular clock mechanisms were disrupted in disc cells that had been repeatedly placed under mechanical strain at night. This disruption appears to influence degradation of the extracellular matrix, which the team believe may in turn accelerate disc degeneration.

Prediction of the natural frequencies of different degrees of degenerated human lumbar segments L2-L3 using dynamic finite element analysis

BACKGROUND AND OBJECTIVE

Chronic exposure to resonant environment may cause more serious injuries to human lumbar spine than other vibrations. On the condition that the natural frequency of human lumbar spine is known, excitation frequency from an external vibration source can be optimized to keep away from the natural frequency and thus avoid lumbar resonance. Therefore, this study aimed to present an approach to predict the natural frequency of the human lumbar spine.

METHODS

Four poroelastic finite element models of human L2-L3 spinal motion segments with different degrees of degeneration were established. Dynamic finite element analyses of these models during 1 h of vibration were then conducted. The mechanical parameters of these models under vibrations at different excitation frequencies were predicted. The excitation frequencies that resulted in the greatest changes in the lumbar mechanical parameters were identified as the natural frequencies of the established L2-L3 spinal motion segments.

RESULTS

Simulation results showed that the natural frequencies of the healthy and mildly degenerated L2-L3 spinal motion segments, moderately degenerated L2-L3 spinal motion segments, and seriously degenerated L2-L3 spinal motion segments were in the range of 5-7, 3-5, and 1-3 Hz, respectively.

CONCLUSIONS

The predicted results indicated that the natural frequencies of the human L2-L3 spinal motion segments gradually decreased with the severity of degeneration. These phenomena may be related to changes in the lumbar structures and materials because of degeneration. This study provided a feasible method to predict the lumbar natural frequencies for different populations, which may be helpful in optimizing external vibration sources to avoid lumbar resonance.

Diagnostic Role of Magnetic Resonance Imaging in Low Back Pain Caused by Vertebral Endplate Degeneration

Low back pain (LBP) is a common health issue worldwide with a huge economic burden on healthcare systems. In the United States alone, the cost is estimated to be $100 billion each year. Intervertebral disc degeneration is considered one of the primary causes of LBP. Moreover, the critical role of the vertebral endplates in disc degeneration and LBP is becoming apparent. Endplate abnormalities are closely correlated with disc degeneration and pain in the lumbar spine. Imaging modalities such as plain film radiography, computed tomography, and fluoroscopy are helpful but not very effective in detecting the causes behind LBP. Magnetic resonance imaging (MRI) can be used to acquire high-quality three-dimensional images of the lumbar spine without using ionizing radiation. Therefore, it is increasingly being used to diagnose spinal disorders. However, according to the American College of Radiology, current referral and justification guidelines for MRI are not sufficiently clear to guide clinical practice. This review aimed to evaluate the role of MRI in diagnosing LBP by considering the correlative contributions of vertebral endplates. The findings of the review indicate that MRI allows for fine evaluations of endplate morphology, endplate defects, diffusion and perfusion properties of the endplate, and Modic changes. Changes in these characteristics of the endplate were found to be closely correlated with disc degeneration and LBP. The collective evidence from the literature suggests that MRI may be the imaging modality of choice for patients suffering from LBP. LEVEL OF EVIDENCE: 1 TECHNICAL EFFICACY STAGE: 3.

Penguin Suit and Fetal Position Finite Element Model to Prevent Low Back Pain in Spaceflight

BACKGROUND: This study aimed to investigate the biomechanical effects of different interventions on astronauts lumbar intervertebral discs in a microgravity environment during spaceflight and in a gravity environment when the astronaut returns.METHODS: A finite element model of the L4L5 lumbar segment was developed with eight loading schemes representing different interventions. The loading schemes included no intervention, wearing a penguin suit, sleeping in a fetal position, wearing a penguin suit combined with sleeping in the fetal position, reclining for 4 or 16 h/d, and maintaining upright posture for 4 or 16 h/d.RESULTS: Without intervention, the microgravity environment led to increased central pore pressure, radial displacement, and water content in the lumbar intervertebral disc. Wearing a penguin suit combined with sleeping in the fetal position can reduce disc pore pressure, axial stress, radial displacement, and water content to 0.156 MPa, 11.50 kPa, 0.538 mm, and 1.390%, respectively. When astronauts return to the gravity environment, staying upright for 4 h can reduce the pore pressure, axial stress, radial displacement, and water content of the intervertebral disc to 0.222 MPa, 10.72 kPa, 0.373 mm, and 0.219%, respectively.CONCLUSION: This study showed that wearing a penguin suit and sleeping in the fetal position both have the potential to protect the lumbar intervertebral disc from the negative effects caused by microgravity. Remaining in the upright posture for 4 h per day may help squeeze out the water in the intervertebral disc safely when astronauts return to the gravity environment.Zhang S, Wang K, Zhu R, Jiang C, Niu W. Penguin suit and fetal position finite element model to prevent low back pain in spaceflight. Aerosp Med Hum Perform. 2021; 92(5):312318.

Computational Challenges in Tissue Engineering for the Spine

This paper deals with a brief review of the recent developments in computational modelling applied to innovative treatments of spine diseases. Additionally, it provides a perspective on the research directions expected for the forthcoming years. The spine is composed of distinct and complex tissues that require specific modelling approaches. With the advent of additive manufacturing and increasing computational power, patient-specific treatments have moved from being a research trend to a reality in clinical practice, but there are many issues to be addressed before such approaches become universal. Here, it is identified that the major setback resides in validation of these computational techniques prior to approval by regulatory agencies. Nevertheless, there are very promising indicators in terms of optimised scaffold modelling for both disc arthroplasty and vertebroplasty, powered by a decisive contribution from imaging methods.

Finite element investigation on the dynamic mechanical properties of low-frequency vibrations on human L2-L3 spinal motion segments with different degrees of degeneration

Exposure to low-frequency vibration is harmful to human lumbar health. However, the dynamic mechanical properties of lumbar spines with varying degrees of degeneration during time-dependent vibration remain incompletely understood. In this study, four poroelastic finite element models of human L2-L3 spinal motion segments, including the non-degeneration and the mild, moderate, and serious degeneration, were established. One-hour low-frequency vibrations with different frequencies were applied. Then, the dynamic mechanical properties of different degenerated lumbar models under the same vibration and the same lumbar model under vibrations at different frequencies were investigated. The results indicated and implied that the negative influences of 1-h vibration on the dynamic mechanical properties of the non-degenerated and mildly degenerated models were similar, but became obvious for the moderately and seriously degenerated models with time. Therefore, the damage caused by low-frequency vibration on the degenerated spinal motion segments was more serious compared with that on the healthy one. Meanwhile, the dynamic mechanical properties of the same lumbar model under vibrations at different frequencies expressed the negligible differences when the vibration frequency was not close to the lumbar natural frequency. Thus, the effects of the 1-h vibrations at different frequencies on one spinal motion segment were similar. Vibration frequency sensitivity analysis on the dynamic characteristics of human L2-L3 spinal motion segments with different degrees of degeneration.

Growing a backbone - functional biomaterials and structures for intervertebral disc (IVD) repair and regeneration: challenges, innovations, and future directions

Back pain and associated maladies can account for an immense amount of healthcare cost and loss of productivity in the workplace. In particular, spine related injuries in the US affect upwards of 5.7 million people each year. The degenerative disc disease treatment almost always arises due to a clinical presentation of pain and/or discomfort. Preferred conservative treatment modalities include the use of non-steroidal anti-inflammatory medications, physical therapy, massage, acupuncture, chiropractic work, and dietary supplements like glucosamine and chondroitin. Artificial disc replacement, also known as total disc replacement, is a treatment alternative to spinal fusion. The goal of artificial disc prostheses is to replicate the normal biomechanics of the spine segment, thereby preventing further damage to neighboring sections. Artificial functional disc replacement through permanent metal and polymer-based components continues to evolve, but is far from recapitulating native disc structure and function, and suffers from the risk of unsuccessful tissue integration and device failure. Tissue engineering and regenerative medicine strategies combine novel material structures, bioactive factors and stem cells alone or in combination to repair and regenerate the IVD. These efforts are at very early stages and a more in-depth understanding of IVD metabolism and cellular environment will also lead to a clearer understanding of the native environment which the tissue engineering scaffold should mimic. The current review focusses on the strategies for a successful regenerative scaffold for IVD regeneration and the need for defining new materials, environments, and factors that are so finely tuned in the healthy human intervertebral disc in hopes of treating such a prevalent degenerative process.

Assessment of lumbar spinal disc injury in frontal crashes

Frontal vehicle crashes have been a leading cause of spinal injuries in recent years. Reconstruction of frontal crashes using computational models and spinal load analysis helps us understand the patterns of injury and load propagation during frontal crashes. By reconstructing a real crash test and using a viscoelastic crash dummy model, spinal injury patterns were analyzed. The results indicated that a moderate crash with an impact speed of 56 km/h leads to injuries in L1-L2 and L5-S1 levels (L for lumbar and S for sacral vertebrae). The largest spinal loads and injuries were mainly observed immediately after the airbag deployment when the peak of the crash acceleration transpires. Also, the effects of impulse magnitude on the spinal loads and head injury criterion (HIC) showed that HIC is more sensitive than compressive forces to the magnitude of impulse. Moreover, the effects of disc preconditioning as a major factor in the risk of injury was evaluated. The results demonstrate that as the lumbar spine is subjected to a longer preloading, it will be more vulnerable to injury; preconditioning of the discs more adversely affected the risk of injury than a 10% increase in the crash impulse. Overall the results highlight the importance of spinal injury prevention in frontal crashes.

Biomechanical Effect of L4 -L5 Intervertebral Disc Degeneration on the Lower Lumbar Spine: A Finite Element Study

OBJECTIVE

To ascertain the biomechanical effects of a degenerated L₄ -L₅ segment on the lower lumbar spine through a comprehensive simulation of disc degeneration.

METHODS

A three-dimensional nonlinear finite element model of a normal L₃ -S₁ lumbar spine was constructed and validated. This normal model was then modified such that three degenerated models with different degrees of degeneration (mild, moderate, or severe) at the L₄ -L₅ level were constructed. While experiencing a follower compressive load (500 N), hybrid moment loads were applied to all models to determine range of motion (ROM), intradiscal pressure (IDP), maximum von Mises stress in the annulus, maximum shear stress in the annulus, and facet joint force.

RESULTS

As the degree of disc degeneration increased, the ROM of the L₄ -L₅ degenerated segment declined dramatically in all postures (flexion: 5.79°-1.91°; extension: 5.53°-2.62°; right lateral bending: 4.47°-1.46°; left lateral bending: 4.86°-1.61°; right axial rotation: 2.69°-0.74°; left axial rotation: 2.69°-0.74°), while the ROM in adjacent segments increased (1.88°-8.19°). The largest percent decrease in motion of the L₄ -L₅ segment due to disc degeneration was in right axial rotation (75%), left axial rotation (69%), flexion (67%), right lateral bending (67%), left lateral bending right (67%), and extension (53%). The change in the trend of the IDP was the same as that of the ROM. Specifically, the IDP decreased (flexion: 0.592-0.09 MPa; extension: 0.678-0.334 MPa; right lateral bending: 0.498-0.205 MPa; left lateral bending: 0.523-0.272 MPa; right axial rotation: 0.535-0.246 MPa; left axial rotation: 0.53-0.266 MPa) in the L₄ -L₅ segment, while the IDP in adjacent segments increased (0.511-0.789 MPa). The maximum von Mises stress and maximum shear stress of the annulus in whole lumbar spine segments increased (L₄ -L₅ segment: 0.413-2.626 MPa and 0.412-2.783 MPa, respectively; adjacent segment of L₄ -L₅ : 0.356-1.493 MPa and 0.359-1.718 MPa, respectively) as degeneration of the disc progressively increased. There was no apparent regularity in facet joint force in the degenerated segment as the degree of disc degeneration increased. Nevertheless, facet joint forces in adjacent healthy segments increased as the degree of disc degeneration increased (extension: 49.7-295.3 N; lateral bending: 3.5-171.2 N; axial rotation: 140.2-258.8 N).

CONCLUSION

Degenerated discs caused changes in the motion and loading pattern of the degenerated segments and adjacent normal segments. The abnormal load and motion in the degenerated models risked accelerating degeneration in the adjacent normal segments. In addition, accurate simulation of degenerated facet joints is essential for predicting changes in facet joint loads following disc degeneration.

The circadian rhythm in intervertebral disc degeneration: an autophagy connection

There is one circadian clock in the central nervous system and another in the peripheral organs, and the latter is driven by an autoregulatory molecular clock composed of several core clock genes. The height, water content, osmotic pressure and mechanical characteristics of intervertebral discs (IVDs) have been demonstrated to exhibit a circadian rhythm (CR). Recently, a molecular clock has been shown to exist in IVDs, abolition of which can lead to stress in nucleus pulposus cells (NPCs), contributing to intervertebral disc degeneration (IDD). Autophagy is a fundamental cellular process in eukaryotes and is essential for individual cells or organs to respond and adapt to changing environments; it has also been demonstrated to occur in human NPCs. Increasing evidence supports the hypothesis that autophagy is associated with CR. Thus, we review the connection between CR and autophagy and the roles of these mechanisms in IDD.

Is there any advantage of using stand-alone cages? A numerical approach

Background

Segment fusion using interbody cages supplemented with pedicle screw fixation is the most common surgery for the treatment of low back pain. However, there is still much controversy regarding the use of cages in a stand-alone fashion. The goal of this work is to numerically compare the influence that each surgery has on lumbar biomechanics.

Methods

A non-linear FE model of the whole lumbar spine was developed to compare between two types of cages (OLYS and NEOLIF) with and without supplementary fixation. The motion of the whole spine was analysed and the biomechanical environment of the adjacent segments to the operated one was studied. Moreover, the risk of subsidence of the cages was qualitatively evaluated.

Results

A great ROM reduction occurred when supplementary fixation was used. This stiffening increased the stresses at the adjacent levels. It might be hypothesised that the overloading of these segments could be related with the clinically observed adjacent disc degeneration. Meanwhile, the stand-alone cages allowed for a wider movement, and therefore, the influence of the surgery on adjacent discs was much lower. Regarding the risk of subsidence, the contact pressure magnitude was similar for both intervertebral cage designs and near the value of the maximum tolerable pressure of the endplates.

Conclusions

A minimally invasive posterior insertion of an intervertebral cage (OLYS or NEOLIF) was compared using a stand-alone design or adding supplementary fixation. The outcomes of these two techniques were compared, and although stand-alone cage may diminish the risk of disease progression to the adjacent discs, the spinal movement in this case could compromise the vertebral fusion and might present a higher risk of cage subsidence.

Electronic supplementary material

The online version of this article (10.1186/s12938-019-0684-8) contains supplementary material, which is available to authorized users.

Application of Finite Element Analysis for Investigation of Intervertebral Disc Degeneration: from Laboratory to Clinic

Due to the ethical concern and inability to detect inner stress distributions of intervertebral disc (IVD), traditional methods for investigation of intervertebral disc degeneration (IVDD) have significant limitations. Many researchers have demonstrated that finite element analysis (FEA) is an effective tool for the research of IVDD. However, the specific application of FEA for investigation of IVDD has not been systematically elucidated before. In the present review, we summarize the current finite element models (FEM) used for the investigation of IVDD, including the poroelastic nonlinear FEM, diffusive-reactive theory model and cell-activity coupled mechano-electrochemical theory model. We further elaborate the use of FEA for the research of IVDD pathogenesis especially for nutrition and biomechanics associated etiology, and the biological, biomechanical and clinical influences of IVDD. In addition, the application of FEA for evaluation and exploration of various treatments for IVDD is also elucidated. We conclude that FEA is an excellent technique for research of IVDD, which could be used to explore the etiology, biology and biomechanics of IVDD. In the future, FEA may help us to achieve the goal of individualized precision therapy.

Finite Element Investigation of the Effects of the Low-Frequency Vibration Generated by Vehicle Driving on the Human Lumbar Mechanical Properties

Long-term exposure to low-frequency vibration generated by vehicle driving impairs human lumbar spine health. However, few studies have investigated how low-frequency vibration affects human lumbar mechanical properties. This study established a poroelastic finite element model of human lumbar spinal segments L2–L3 to perform time-dependent vibrational simulation analysis and investigated the effects of different vibrational frequencies generated by normal vehicle driving on the lumbar mechanical properties in one hour. Analysis results showed that vibrational load caused more injury to lumbar health than static load, and vibration at the resonant frequency generated the most serious injury. The axial effective stress and the radial displacement in the intervertebral disc, as well as the fluid loss in the nucleus pulposus, increased, whereas the pore pressure in the nucleus pulposus decreased with increased vibrational frequency under the same vibrational time, which may aggravate the injury degree of human lumbar spine. Therefore, long-term driving on a well-paved road also induces negative effects on human lumbar spine health. When driving on a nonpaved road or operating engineering machinery under poor navigating condition, the auto seat transmits relatively high vibrational frequency, which is highly detrimental to the lumbar spine health of a driver.

Aggrecan-like biomimetic proteoglycans (BPGs) composed of natural chondroitin sulfate bristles grafted onto a poly(acrylic acid) core for molecular engineering of the extracellular matrix

Biomimetic proteoglycans (BPGs) were designed to mimic the three-dimensional (3D) bottlebrush architecture of natural extracellular matrix (ECM) proteoglycans, such as aggrecan. BPGs were synthesized by grafting native chondroitin sulfate bristles onto a synthetic poly(acrylic acid) core to form BPGs at a molecular weight of approximately ∼1.6 MDa. The aggrecan mimics were characterized chemically, physically, and structurally, confirming the 3D bottlebrush architecture as well as a level of water uptake, which is greater than that of the natural proteoglycan, aggrecan. Aggrecan mimics were cytocompatible at physiological concentrations. Fluorescently labeled BPGs were injected into the nucleus pulposus of the intervertebral disc ex vivo and were retained in tissue before and after static loading and equilibrium conditioning. BPGs infiltrated the tissue, distributed and integrated with the ECM on a molecular scale, in the absence of a bolus, thus demonstrating a new molecular approach to tissue repair: molecular matrix engineering. Molecular matrix engineering may compliment or offer an acellular alternative to current regenerative medicine strategies.

STATEMENT OF SIGNIFICANCE

Aggrecan is a natural biomolecule that is essential for connective tissue hydration and mechanics. Aggrecan is composed of negatively charged chondroitin sulfate bristles attached to a protein core in a bottlebrush configuration. With age and degeneration, enzymatic degradation of aggrecan outpaces cellular synthesis resulting in a loss of this important molecule. We demonstrate a novel biomimetic molecule composed of natural chondroitin sulfate bristles grafted onto an enzymatically-resistant synthetic core. Our molecule mimics a 3D architecture and charge density of the natural aggrecan, can be delivered via a simple injection and is retained in tissue after equilibrium conditioning and loading. This novel material can serve as a platform for molecular repair, drug delivery and tissue engineering in regenerative medicine approaches.

Biomechanical analysis of lumbar decompression surgery in relation to degenerative changes in the lumbar spine - Validated finite element analysis

BACKGROUND

There are no studies about the biomechanical analysis of lumbar decompression surgery in relation to degenerative changes of the lumbar spine. Therefore, the purpose of this study was to compare, by using finite element (FE) analysis, the biomechanical changes of the lumbar spine in terms of annulus stress and nucleus pressure after two different kinds of lumbar decompression surgery in relation to disc degenerative changes.

METHODS

The validated intact and degenerated FE models (L2-5) were used in this study. In these two models, two different decompression surgical scenarios at L3-4, including conventional laminectomy (ConLa) and the spinous process osteotomy (SpinO), were simulated. Therefore, a total of six models were simulated. Under preloading, 7.5 Nm moments of flexion, extension, lateral bending, and torsion were imposed. In each model, the maximal von Mises stress on the annulus fibrosus and nucleus pressure at the index segment (L3-4) and adjacent segments (L2-3 and L4-5) were analyzed.

RESULTS

The ConLa model and disc degeneration model demonstrated a larger annulus stress at the decompression level (L3-4) under all four moments than were seen in the SpinO model and healthy disc model, respectively. Therefore, the ConLa model with moderate disc degeneration showed the highest annulus stress at the decompression level (L3-4). However, the percent change of annulus stress at L3-4 from the intact model to the matched decompression model was less in the moderate disc degeneration model than in the healthy disc model.

CONCLUSIONS

Although the ConLa model with moderate disc degeneration showed the highest annulus stress, the degenerative models would be less influenced by the decompression technique.

Effects of resting modes on human lumbar spines with different levels of degenerated intervertebral discs: a finite element investigation

BACKGROUND

The negative effect of long-term working load on lumbar is widely known. However, insertion of different resting modes on long-term working load, and its effects on the lumbar spine is rarely studied. The purpose of this study was to investigate the biomechanical responses of lumbar spine with different levels of degenerated intervertebral discs under different working-resting modes.

METHODS

Four poroelastic finite element models of lumbar spinal segments L2-L3 with different grades of disc degeneration were developed. Four different loading conditions represented four different resting frequencies, namely, no rest, one-time long rest, three-time moderate rests, and five-time short rests, on the condition that the total resting time was the same except in the no rest mode. Loading amplitudes of diurnal activities included 100 N, 300 N, and 500 N.

RESULTS

With increasing resting frequency, the axial effective stress and fluid loss decreased, whereas the pore pressure and radial displacement increased. Under different resting frequencies, the changing rate of each biomechanical parameter was different.

CONCLUSIONS

Under a situation of fixed total resting time, high resting frequency was advisable. If sufficient resting frequency was unavailable for healthy people as well as patients with mildly and moderately degenerated intervertebral discs, they could similarly benefit from relatively less resting frequencies. However, one-time rest will not be useful in cases where intervertebral discs were seriously degenerated. Reasonable working-resting modes for different degrees of disc degeneration, which could assist patients achieve a better restoration, were provided in this study.

Influence of different fusion techniques in lumbar spine over the adjacent segments: A 3D finite element study

The most conventional technique to treat the intervertebral disc degeneration consists on fusing the affected segment with a posterior screw fixation and sometimes with the insertion of a cage in the intersomatic space. However, this kind of surgeries had controversial results in the adjacent discs. The aim of this work was to prove the stabilization of the spine and the decompression of the disc and to analyze the influence over the adjacent segments. With this purpose, four different models were built and simulated under different loading conditions. The stabilization of the spine was ensured by the screw fixation which reduced dramatically the relative motion in the affected segment. On the other hand, the pore pressure showed a high fall in the operated models proving the decompression of the neural structures. In the adjacent segments, the ROM increased up to 50% in the upper disc and 70% in the lower one. The pore pressure and principal stresses also increased after both surgeries. The observed results suggested that the fusion procedure could trigger a cascade degeneration effect over the adjacent discs, while it is also seen that cage insertion helps to maintain disc height in a better way than screw fixation only.

Long-Term Creep Behavior of the Intervertebral Disk: Comparison between Bioreactor Data and Numerical Results

The loaded disk culture system is an intervertebral disk (IVD)-oriented bioreactor developed by the VU Medical Center (VUmc, Amsterdam, The Netherlands), which has the capacity of maintaining up to 12 IVDs in culture, for approximately 3 weeks after extraction. Using this system, eight goat IVDs were provided with the essential nutrients and submitted to compression tests without losing their biomechanical and physiological properties, for 22 days. Based on previous reports (Paul et al., 2012, 2013; Detiger et al., 2013), four of these IVDs were kept in physiological condition (control) and the other four were previously injected with chondroitinase ABC (CABC), in order to promote degenerative disk disease (DDD). The loading profile intercalated 16 h of activity loading with 8 h of loading recovery to express the standard circadian variations. The displacement behavior of these eight IVDs along the first 2 days of the experiment was numerically reproduced, using an IVD osmo-poro-hyper-viscoelastic and fiber-reinforced finite element (FE) model. The simulations were run on a custom FE solver (Castro et al., 2014). The analysis of the experimental results allowed concluding that the effect of the CABC injection was only significant in two of the four IVDs. The four control IVDs showed no signs of degeneration, as expected. In what concerns to the numerical simulations, the IVD FE model was able to reproduce the generic behavior of the two groups of goat IVDs (control and injected). However, some discrepancies were still noticed on the comparison between the injected IVDs and the numerical simulations, namely on the recovery periods. This may be justified by the complexity of the pathways for DDD, associated with the multiplicity of physiological responses to each direct or indirect stimulus. Nevertheless, one could conclude that ligaments, muscles, and IVD covering membranes could be added to the FE model, in order to improve its accuracy and properly describe the recovery periods.

The effect of creep on human lumbar intervertebral disk impact mechanics

The intervertebral disk (IVD) is a highly hydrated tissue, with interstitial fluid making up 80% of the wet weight of the nucleus pulposus (NP), and 70% of the annulus fibrosus (AF). It has often been modeled as a biphasic material, consisting of both a solid and fluid phase. The inherent porosity and osmotic potential of the disk causes an efflux of fluid while under constant load, which leads to a continuous displacement phenomenon known as creep. IVD compressive stiffness increases and NP pressure decreases as a result of creep displacement. Though the effects of creep on disk mechanics have been studied extensively, it has been limited to nonimpact loading conditions. The goal of this study is to better understand the influence of creep and fluid loss on IVD impact mechanics. Twenty-four human lumbar disk samples were divided into six groups according to the length of time they underwent creep (tcreep = 0, 3, 6, 9, 12, 15 h) under a constant compressive load of 400 N. At the end of tcreep, each disk was subjected to a sequence of impact loads of varying durations (timp = 80, 160, 320, 400, 600, 800, 1000 ms). Energy dissipation (ΔE), stiffness in the toe (ktoe) and linear (klin) regions, and neutral zone (NZ) were measured. Analyzing correlations with tcreep, there was a positive correlation with ΔE and NZ, along with a negative correlation with ktoe. There was no strong correlation between tcreep and klin. The data suggest that the IVD mechanical response to impact loading conditions is altered by fluid content and may result in a disk that exhibits less clinical stability and transfers more load to the AF. This could have implications for risk of diskogenic pain as a function of time of day or tissue hydration.

Inflammation induces irreversible biophysical changes in isolated nucleus pulposus cells

Intervertebral disc degeneration is accompanied by elevated levels of inflammatory cytokines that have been implicated in disease etiology and matrix degradation. While the effects of inflammatory stimulation on disc cell metabolism have been well-studied, their effects on cell biophysical properties have not been investigated. The hypothesis of this study is that inflammatory stimulation alters the biomechanical properties of isolated disc cells and volume responses to step osmotic loading. Cells from the nucleus pulposus (NP) of bovine discs were isolated and treated with either lipopolysaccharide (LPS), an inflammatory ligand, or with the recombinant cytokine TNF-α for 24 hours. We measured cellular volume regulation responses to osmotic loading either immediately after stimulation or after a 1 week recovery period from the inflammatory stimuli. Cells from each group were tested under step osmotic loading and the transient volume-response was captured via time-lapse microscopy. Volume-responses were analyzed using mixture theory framework to investigate two biomechanical properties of the cell, the intracellular water content and the hydraulic permeability. Intracellular water content did not vary between treatment groups, but hydraulic permeability increased significantly with inflammatory treatment. In the 1 week recovery group, hydraulic permeability remained elevated relative to the untreated recovery control. Cell radius was also significantly increased both after 24 hours of treatment and after 1 week recovery. A significant linear correlation was observed between hydraulic permeability and cell radius in untreated cells at 24 hours and at 1-week recovery, though not in the inflammatory stimulated groups at either time point. This loss of correlation between cell size and hydraulic permeability suggests that regulation of volume change is disrupted irreversibly due to inflammatory stimulation. Inflammatory treated cells exhibited altered F-actin cytoskeleton expression relative to untreated cells. We also found a significant decrease in the expression of aquaporin-1, the predominant water channel in disc NP cells, with inflammatory stimulation. To our knowledge, this is the first study providing evidence that inflammatory stimulation directly alters the mechanobiology of NP cells. The cellular biophysical changes observed in this study are coincident with documented changes in the extracellular matrix induced by inflammation, and may be important in disease etiology.

Aquaporin-1 and aquaporin-3 expressions in the intervertebral disc of rats with aging

OBJECTIVE

The intervertebral disc (IVD) undergoes biochemical and morphologic degenerative changes during the process of aging. Aquaporins (AQPs) are a family of water channel proteins that facilitate water and small solute movement in tissues and may have a potential role in the aging degeneration of IVDs. One of the important problems in understanding disc degeneration is to find cellular molecules which contribute to the pathogenesis of IVDs. XThe aim of this study was to demonstrate the expression of aquaporin 1 and 3 in nucleus pulposus (NP), annulus fibrosus (AF) cells of rat lumbar intervertebral discs from both young and aged animals using immunohistochemistry.

MATERIAL AND METHODS

Twenty Wistar-albino rats were included in the study. The rats were separated into two groups: 2-month-old rats (n=10) as the young group, 18-month-old rats (n=10) as the old group. The intervertebral disc tissues obtained from the lumbar spine (L1-L4, 4 discs) were used for immunohistochemical staining of AQP-1 and 3.

RESULTS

This study demonstrated that AQP-1 and AQP-3 immunoreactivity significantly decreased in NP and AF of aged rats compared to the young rats.

CONCLUSION

We suggest that AQP-1 and 3 may contribute to the age related degeneration of the intervertebral disc.

Large residual strains are present in the intervertebral disc annulus fibrosus in the unloaded state

The intervertebral disc annulus fibrosus (AF) is subjected to high circumferential tensile stresses resulting from nucleus pulposus pressurisation under axial compression. In other pressure containing tissues, such as blood vessel walls, residual compressive stresses along the inside surface of the tissues without pressurisation reduce peak tensile stresses under pressurisation. This study hypothesised that similar patterns of residual stress exist in the annulus fibrosus. Accurate characterisation of residual stresses is essential for both the incorporation of nonlinear material descriptions into models of the disc as well as the design of effective annulus repair strategies. By imaging nine bovine caudal discs before and after the release of residual stresses via incision, we measured a mean residual stretch of 0.86 ± 0.13 at the inner AF and 1.02 ± 0.08 at the outer AF. These stretch values were used to calculate a gradient of residual stress ranging from -230 ± 22 kPa of compression at the inner AF to 54 ± 0.2 kPa of tension at the outer AF. Material models of AF have assumed that the AF was in a stress free reference state when there are no external loads. However, this study documents that there are large residual stresses in the AF even without external loads. The release of residual tension in the outer AF by herniation, needle injection or incisions makes closure difficult and may accelerate degeneration of the surrounding tissue. Retention of these residual stresses may be essential to maintaining disc mechanical function and to producing viable AF repair techniques.