Effect of Shear Force on Intervertebral Disc (IVD) Degeneration: An In Vivo Rat Study

Investigation of the Role of Osteoporotic Vertebra Degeneration on the Stability of the Lumbar Spine: In Silico Modelling under Compressive Loading

An evaluation of the impact of osteoporosis on loss of spinal stability, with or without intervertebral disc degeneration, using computational analysis is presented. The research also investigates the correlation between osteoporosis and intervertebral disc degeneration. Three-dimensional finite element models of human lumbar spine segments were used to assess the influence of osteoporosis on spinal stability. Five different models of age-related degeneration were created using various material properties for trabecular bone and intervertebral discs. Calculation results indicate that in a spine with osteoporosis, the deformation of the intervertebral discs can increase by more than 30% when compared to a healthy spine. Thus, intervertebral disc deformation depends not only on the degree of degeneration of the discs themselves, but their deformation is also influenced by the degree of osteoporosis of the vertebrae. Additionally, the load-bearing capacity of the spine can decrease by up to 30% with osteoporosis, regardless of the degree of intervertebral disc deformation. In conclusion, osteoporosis can contribute to intervertebral disc degeneration.

Efficacy of an Omaha system-based remote ergonomic intervention program on self-reported work-related musculoskeletal disorders (WMSDs) - A randomized controlled study

Purpose

Heavy biomechanical loadings at workplaces may lead to high risks of work-related musculoskeletal disorders. This study aimed to explore the efficacy of an Omaha System-based remote ergonomic intervention program on self-reported work-related musculoskeletal disorders among frontline nurses.

Materials and methods

From July to October 2020, 94 nurses with self-reported pain in one of the three body parts, i.e., neck, shoulder, and low back, were selected and were randomly divided into two groups. The intervention group received a newly developed remote program, where the control group received general information and guidance on health and life. Program outcome was evaluated by a quick exposure check approach.

Results

After 6 weeks, the intervention group exhibited significantly less stress in the low back, neck, and shoulder/forearms, compared to the control group (p < 0.05). In addition, the occurrence of awkward postures, such as extreme trunk flexion or twisting, was also significantly reduced (p < 0.05).

Conclusions

The newly developed Omaha System-based remote intervention program may be a valid alternative to traditional programs for frontline nurses during the COVID-19 pandemic, reducing biomechanical loadings and awkward postures during daily nursing operations.

Mechanical characterization and design of biomaterials for nucleus pulposus replacement and regeneration

Biomaterials for nucleus pulposus (NP) replacement and regeneration have great potential to restore normal biomechanics in degenerated intervertebral discs following nucleotomy. Mechanical characterizations are essential for assessing the efficacy of biomaterial implants for clinical applications. While traditional compression tests are crucial to quantify various modulus values, relaxation behaviors and fatigue resistance, rheological measurements should also be conducted to investigate the viscoelastic properties, injectability, and overall stability upon deformation. To recapitulate the physiological in vivo environment, the use of spinal models is necessary to evaluate the risk of implant extrusion and the restoration of biomechanics under different loading conditions. When designing devices for NP replacement, injectable materials are ideal to fully fill the nucleus cavity and prevent implant migration. In addition to achieving biocompatibility and desirable mechanical characteristics, biomaterial implants should be optimized to avoid implant extrusion or re-herniation post-operatively. This review discusses the most commonly used testing protocols for assessing mechanical properties of biomaterial implants and serves as reference material for enabling researchers to characterize NP implants through a unified approach whereby newly developed biomaterials may be compared and contrasted to existing devices.

The role of biomechanical factors in models of intervertebral disc degeneration across multiple length scales

Low back pain is the leading cause of disability, producing a substantial socio-economic burden on healthcare systems worldwide. Intervertebral disc (IVD) degeneration is a primary cause of lower back pain, and while regenerative therapies aimed at full functional recovery of the disc have been developed in recent years, no commercially available, approved devices or therapies for the regeneration of the IVD currently exist. In the development of these new approaches, numerous models for mechanical stimulation and preclinical assessment, including in vitro cell studies using microfluidics, ex vivo organ studies coupled with bioreactors and mechanical testing rigs, and in vivo testing in a variety of large and small animals, have emerged. These approaches have provided different capabilities, certainly improving the preclinical evaluation of these regenerative therapies, but challenges within the research environment, and compromises relating to non-representative mechanical stimulation and unrealistic test conditions, remain to be resolved. In this review, insights into the ideal characteristics of a disc model for the testing of IVD regenerative approaches are first assessed. Key learnings from in vivo, ex vivo, and in vitro IVD models under mechanical loading stimulation to date are presented alongside the merits and limitations of each model based on the physiological resemblance to the human IVD environment (biological and mechanical) as well as the possible feedback and output measurements for each approach. When moving from simplified in vitro models to ex vivo and in vivo approaches, the complexity increases resulting in less controllable models but providing a better representation of the physiological environment. Although cost, time, and ethical constraints are dependent on each approach, they escalate with the model complexity. These constraints are discussed and weighted as part of the characteristics of each model.

Aggrecan governs intervertebral discs development by providing critical mechanical cues of the extracellular matrix

Aggrecan (ACAN) is localized in the intervertebral disc (IVD) in unique compartment-specific patterns where it contributes to the tissue structure and mechanical function together with collagens. The extracellular matrix (ECM) of the IVD undergoes degenerative changes during aging, misuse or trauma, which inevitably alter the biochemical and biomechanical properties of the tissue. A deeper understanding of these processes can be achieved in genetically engineered mouse models, taking into account the multifaceted aspects of IVD development. In this study, we generated aggrecan insertion mutant mice (Acan ^iE5/iE5 ) by interrupting exon 5 coding for the G1 domain of ACAN, and analyzed the morphological and mechanical properties of the different IVD compartments during embryonic development. Western blotting using an antibody against the total core protein failed to detect ACAN in cartilage extracts, whereas immunohistochemistry by a G1-specific antibody showed weak signals in vertebral tissues of Acan ^iE5/iE5 mice. Homozygous mutant mice are perinatally lethal and characterized by short snout, cleft palate and disproportionate dwarfism. Whole-mount skeletal staining and µ-CT analysis of Acan ^iE5/iE5 mice at embryonic day 18.5 revealed compressed vertebral bodies with accelerated mineralization compared to wild type controls. In Acan ^iE5/iE5 mice, histochemical staining revealed collapsed extracellular matrix with negligible sulfated glycosaminoglycan content accompanied by a high cellular density. Collagen type II deposition was not impaired in the IVD of Acan ^iE5/iE5 mice, as shown by immunohistochemistry. Mutant mice developed a severe IVD phenotype with deformed nucleus pulposus and thinned cartilaginous endplates accompanied by a disrupted growth plate structure in the vertebral body. Atomic force microscopy (AFM) imaging demonstrated a denser collagen network with thinner fibrils in the mutant IVD zones compared to wild type. Nanoscale AFM indentation revealed bimodal stiffness distribution attributable to the softer proteoglycan moiety and harder collagenous fibrils of the wild type IVD ECM. In Acan ^iE5/iE5 mice, loss of aggrecan resulted in a marked shift of the Young's modulus to higher values in all IVD zones. In conclusion, we demonstrated that aggrecan is pivotal for the determination and maintenance of the proper stiffness of IVD and vertebral tissues, which in turn could play an essential role in providing developmental biomechanical cues.

Bony Stress and Its Association With Intervertebral Disc Degeneration in the Lumbar Spine: A Systematic Review of Clinical and Basic Science Studies

STUDY DESIGN

Translational review encompassing basic science and clinical evidence.

OBJECTIVES

Multiple components of the lumbar spine interact during its normal and pathological function. Bony stress in the lumbar spine is recognized as a factor in the development of pars interarticularis defect and stress fractures, but its relationship with intervertebral disc (IVD) degeneration is not well understood. Therefore, we conducted a systematic review to examine the relationship between bony stress and IVD degeneration.

METHODS

Online databases Scopus, PubMed and MEDLINE via OVID were searched for relevant studies published between January 1980-February 2020, using PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-analyses) guidelines. Two authors independently analyzed the data, noting characteristics and biases in various studies.

RESULTS

Thirty-two articles were included in the review: 8 clinical studies, 9 finite element modeling studies, 3 in-vivo biomechanical testing studies, and 12 in-vitro biomechanical testing studies. Of the 32 articles, 19 supported, 4 rejected and 9 made no conclusion on the hypothesis that there is a positive associative relationship between IVD degeneration and bony stress. However, sufficient evidence was not available to confirm or reject a causal relationship.

CONCLUSIONS

Most studies suggest that the prevalence of IVD degeneration increases in the presence of bony stress; whether a causal relationship exists is unclear. The literature recommends early diagnosis and clinical suspicion of IVD degeneration and bony stress. Longitudinal studies are required to explore causal relationships between IVD degeneration and bony stress.

Adjacent segments biomechanics following lumbar fusion surgery: a musculoskeletal finite element model study

PURPOSE

This study exploits a novel musculoskeletal finite element (MS-FE) spine model to evaluate the post-fusion (L4-L5) alterations in adjacent segment kinetics.

METHODS

Unlike the existing MS models with idealized representation of spinal joints, this model predicts stress/strain distributions in all passive tissues while organically coupled to a MS model. This generic (in terms of musculature and material properties) model uses population-based in vivo vertebral sagittal rotations, gravity loads, and an optimization algorithm to calculate muscle forces. Simulations represent individuals with an intact L4-L5, a preoperative severely degenerated L4-L5 (by reducing the disc height by ~ 60% and removing the nucleus incompressibility), and a postoperative fused L4-L5 segment with either a fixed or an altered lumbopelvic rhythm with respect to the intact condition (based on clinical observations). Changes in spine kinematics and back muscle cross-sectional areas (due to intraoperative injuries) are considered based on in vivo data while simulating three activities in upright/flexed postures.

RESULTS

Postoperative changes in some adjacent segment kinetics were found considerable (i.e., larger than 25%) that depended on the postoperative lumbopelvic kinematics and preoperative L4-L5 disc condition. Postoperative alterations in adjacent disc shear, facet/ligament forces, and annulus stresses/strains were greater (> 25%) than those found in intradiscal pressure and compression (< 25%). Kinetics of the lower (L5-S1) and upper (L3-L4) adjacent segments were altered to different degrees.

CONCLUSION

Alterations in segmental rotations mainly affected adjacent disc shear forces, facet/ligament forces, and annulus/collagen fibers stresses/strains. An altered lumbopelvic rhythm (increased pelvis rotation) tends to mitigate some of these surgically induced changes.

Musculoskeletal biomechanics of patients with or without adjacent segment degeneration after spinal fusion

STUDY DESIGN

A retrospective, single center, case-control study was performed.

OBJECTIVE

The present study employed patient-specific biomechanical modeling to find potential biomechanical differences after spinal fusion at L4/5 in patients with and without subsequent development of adjacent segment disease (ASD).

METHODS

The study population comprised patients who underwent primary spinal fusion at L4/5 and were either asymptomatic during > 4 years of follow-up (CTRL; n = 18) or underwent revision surgery for ASD at L3/4 (n = 20). Landmarks were annotated on preoperative and follow-up lateral radiographs, and specific musculoskeletal models were created using a custom-built modeling pipeline. Simulated spinal muscle activation and lumbar intervertebral shear loads in unfused segments were analyzed in upright standing and forward flexion. Differences between the pre- and postoperative conditions were computed for each patient.

RESULTS

The average postoperative muscle activity in the upright standing posture was 88.4% of the preoperative activity in the CTRL group (p <  0.0001), but did not significantly change from pre- to postoperatively in the ASD group (98.0%). The average shear load magnitude at the epifusional joint L3/4 during upright standing increased from pre- to postoperatively in the ASD group (+ 3.9 N, +/- 17.4 (n = 18)), but decreased in the CTRL group (- 4.6 N, +/- 23.3 (n = 20); p <  0.001).

CONCLUSION

Patient-specific biomechanical simulation revealed that spinal fusion surgery resulted in greater shear load magnitude and muscle activation and therefore greater forces at the epifusional segment in those with ASD compared with those without ASD. This is a first report of patient-specific disc load and muscle force calculation with predictive merits for ASD.

Biomechanical effects of lumbar fusion surgery on adjacent segments using musculoskeletal models of the intact, degenerated and fused spine

Adjacent segment disorders are prevalent in patients following a spinal fusion surgery. Postoperative alterations in the adjacent segment biomechanics play a role in the etiology of these conditions. While experimental approaches fail to directly quantify spinal loads, previous modeling studies have numerous shortcomings when simulating the complex structures of the spine and the pre/postoperative mechanobiology of the patient. The biomechanical effects of the L4–L5 fusion surgery on muscle forces and adjacent segment kinetics (compression, shear, and moment) were investigated using a validated musculoskeletal model. The model was driven by in vivo kinematics for both preoperative (intact or severely degenerated L4–L5) and postoperative conditions while accounting for muscle atrophies. Results indicated marked changes in the kinetics of adjacent L3–L4 and L5–S1 segments (e.g., by up to 115% and 73% in shear loads and passive moments, respectively) that depended on the preoperative L4–L5 disc condition, postoperative lumbopelvic kinematics and, to a lesser extent, postoperative changes in the L4–L5 segmental lordosis and muscle injuries. Upper adjacent segment was more affected post-fusion than the lower one. While these findings identify risk factors for adjacent segment disorders, they indicate that surgical and postoperative rehabilitation interventions should focus on the preservation/restoration of patient’s normal segmental kinematics.

Comparative efficacy of 12 non-drug interventions on non-specific chronic low back pain in nurses: A systematic review and network meta-analysis

BACKGROUND

Many non-drug interventions for decreasing non-specific chronic low back pain (NSCLBP) in nurses have been extensively studied, but the most effective approach is still unclear.

OBJECTIVE

This systematic review and network meta-analysis evaluated the efficacies of 12 non-drug interventions in reducing NSCLBP in nurses.

METHODS

PubMed, ScienceDirect, Web of Science, Cochrane, EMBASE, CINAHL, Medline, WANFANG, VIP, China Knowledge Integrated, and SinoMed were searched from their establishment to July 2019. Randomized controlled trials (RCTs) comparing non-drug interventions for NSCLBP in nurses were included and analyzed using Stata v15 statistical software.

RESULTS

A total of 31 RCTs (n= 7116) and 12 non-drug interventions were included. The first three results with the highest surface areas under the curve ranking area (SUCRAs) were low back exercise plus healthy education, single low back exercise, and yoga (SUCRAs: 79.4%, 76.2%, and 75.1%, respectively). In addition, single yoga was inferior to protective equipment (standardized mean difference [SMD] = 3.88, 95% confidence interval [CI]: 0.92 to 6.84) and multidisciplinary intervention (SMD =-4.06, 95% CI: -7.33 to -0.78).

CONCLUSIONS

Low back exercise plus health education may be the best approach to reduce NSCLBP in nurses. Considering the heterogeneity, our findings need to be confirmed in future multicenter large sample RCTs in different countries.

The factors of non-specific chronic low back pain in nurses: A meta-analysis

BACKGROUND

Numerous studies have investigated factors for non-specific chronic low back pain (NSCLBP) in nurses, and have reached different conclusions. Evidence-based recommendations are required for the prevention and treatment of NSCLBP in nurses.

OBJECTIVE

This meta-analysis aimed to systematically review and quantify the factors of NSCLBP in nurses.

METHODS

Eleven databases were searched. The odds ratios (OR) with 95% confidence intervals (CIs) were pooled using meta-analysis, and either a fixed-effect or random-effect model was used based on heterogeneity across included studies.

RESULTS

Eighteen publications including 11,752 nursing staff were included. Being female (pooled estimate [95% CI], 1.56 [1.24-1.96]; P< 0.001), married (1.89 [1.38-2.60]; P< 0.001), overweight (0.63 [0.43-0.93]; P= 0.02), working at least 10 years (0.65 [0.48-0.89]; P= 0.007), and working night shifts (2.19 [1.16-4.21]; P= 0.02) were positively related to NSCLBP. Junior college education (0.60 [0.47-0.77]; P< 0.001) and job satisfaction (0.58 [0.47-0.73]; P< 0.001) were negatively related. Age (0.80 [0.50-1.27]; P= 0.34) and physical exercise (0.99 [0.39-2.49]; P= 0.98) were not related.

CONCLUSIONS

This is the first meta-analysis to quantify the risk factors for NSCLBP in nurses. Being female, married, working night shifts, overweight, working at least 10 years and dissatisfied with work are risk factors. High-quality prospective studies are required to validate the findings of this study.

Advanced Glycation End Product Inhibitor Pyridoxamine Attenuates IVD Degeneration in Type 2 Diabetic Rats

Type 2 diabetes mellitus (T2DM) is associated with advanced glycation end product (AGE) enrichment and considered a risk factor for intervertebral disc (IVD) degeneration. We hypothesized that systemic AGE inhibition, achieved using pyridoxamine (PM), attenuates IVD degeneration in T2DM rats. To induce IVD degeneration, lumbar disc injury or sham surgery was performed on Zucker Diabetic Sprague Dawley (ZDSD) or control Sprague Dawley (SD) rats. Post-surgery, IVD-injured ZDSD rats received daily PM dissolved in drinking water or water only. The resulting groups were SD uninjured, SD injured, ZDSD uninjured, ZDSD injured, and ZDSD injured + PM. Levels of blood glycation and disc degeneration were investigated. At week 8 post-surgery, glycated serum protein (GSP) levels were increased in ZDSDs compared to SDs. PM treatment attenuated this increase. Micro-MRI analysis demonstrated IVD dehydration in injured versus uninjured SDs and ZDSDs. In the ZDSD injured + PM group, IVD dehydration was diminished compared to ZDSD injured. AGE levels were decreased and aggrecan levels increased in ZDSD injured + PM versus ZDSD injured rats. Histological and immunohistochemical analyses further supported the beneficial effect of PM. In summary, PM attenuated GSP levels and IVD degeneration processes in ZDSD rats, demonstrating its potential to attenuate IVD degeneration in addition to managing glycemia in T2DM.

Greater Lumbar Extension During Dolphin Kick and Psoas Major Tightness in Swimmers With Low Back Pain

CONTEXT

In competitive swimming, many swimmers experience low back pain (LBP). Lumbar hyperextension may cause LBP, and tight hip-flexor muscle may cause lumbar extension during swimming.

OBJECTIVE

The purpose of this study was to clarify the features of the elastic moduli of the muscles and the lumbar extension when swimmers with LBP perform a dolphin kick (DK).

DESIGN

Cross-sectional study.

SETTING

Single center.

OTHER PARTICIPANTS

Eleven male college swimmers were enrolled as the LBP group (who have LBP when swimming and during a lumbar extension), and 21 male college swimmers were recruited as the control group (no LBP).

INTERVENTIONS

The elastic moduli of the psoas major, iliacus, teres major, latissimus dorsi, pectoralis major, and pectoralis minor were measured through ultrasonic shear wave elastography. The lumbar and hip extension angles during a DK were measured using a video camera. The passive hip extension and shoulder-flexion range of motion (ROM) were measured using a goniometer.

MAIN OUTCOME MEASURES

Muscle elastic moduli and lumbar extension angles during DK.

RESULTS

The characteristics, muscle elastic moduli, DK motion, and ROM were compared between the 2 groups. LBP group demonstrated significantly higher elastic modulus of the psoas major and lower modulus of pectoralis minor compared with the control group. Also, LBP group showed greater lumbar extension during a DK and less hip extension ROM than the control group.

CONCLUSIONS

The higher elastic modulus of the psoas major and greater lumbar extension during a DK may be related to the LBP in swimmers.

Nano and micro biomechanical alterations of annulus fibrosus after in situ immobilization revealed by atomic force microscopy

Annulus fibrosus is critical to bear loads and resist fluid flow in the intervertebral disc. However, the detailed biomechanical mechanism of annulus fibrosus under abnormal loading is still ambiguous, especially at the micro and nano scales. This study aims to characterize the alterations of modulus at the nano scale of individual collagen fibrils in annulus fibrosus after in-situ immobilization, and the corresponding micro-biomechanics of annulus fibrosus. An immobilization model was used on the rat tail with an external fixation device. The elastic modulus of annulus fibrosus at both the nano- and micro-scale was examined using atomic force microscopy after fixation for 4 and 8 weeks, respectively. The fibrils in inner layer showed an alteration in elastic modulus from 91.38 ± 20.19 MPa in the intact annulus fibrosus to 110.64 ± 15.58 MPa (p < 0.001) at the nano scale after immobilization for 8 weeks, while the corresponding modulus at the micro scale also underwent a change from 0.33 ± 0.04 MPa to 0.47 ± 0.04 MPa (p < 0.001). The fibril disorder after immobilization was observed by hematoxylin/eosin staining. The gene expression of annulus fibrosus was also measured by real-time reverse transcription-polymerase chain reaction, which showed the upregulation of collagen II (p = 0.003) after immobilization. The results indicated that the immobilization not only influenced the individual fibril at the nanoscale, but also the micro-biomechanical property of annulus fibrosus which is critical to define the cell response to surrounding biomechanical environment. These alterations may also lead to the change in the mechanical property of the whole disc and the load-bearing function. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 9999:1-7, 2018.

A 3-Dimensional Finite Element Analysis of Adjacent Segment Disk Degeneration Induced by Transforaminal Lumbar Interbody Fusion After Pedicle Screw Fixation

BACKGROUND

Transforaminal lumbar interbody fusion (TLIF) is an effective treatment of upper lumbar intervertebral disk herniation. However, its clinical efficacy for adjacent segment disk degeneration (ASDD) remains undefined. Therefore, the biomechanical evaluation of ASDD caused by TLIF after pedicle screw fixation (PSF) was explored via a 3-dimensional (3D) finite element analysis.

METHODS

Computed tomography images of a healthy male adult volunteer were used in this study. A L3-4 3D finite element model (model) was successfully constructed using Pro/E software, which was also used to establish the L4-5 of the TLIF, PSF, and PSF + TLIF models. Under the same loading conditions, the protrusion and retraction of the adjacent intervertebral disk and the stress distribution of the annulus fibrosis, facet joint, and articular process in the TLIF, PSF, and PSF + TLIF models were all compared.

RESULTS

Protrusion and retraction of the adjacent intervertebral disk were more notable in the PSF + TLIF model than in the PSF model under the same loading conditions. The stress of the annulus fibrosis of the PSF + TLIF model was stronger relative to that of the PSF model under flexion, extension, or lateral bending. The stress of the articular process of the PSF + TLIF model was also stronger than that of the PSF model under extension or lateral bending.

CONCLUSIONS

This study provides evidence that TLIF may aggravate ASDD after PSF. Furthermore, the findings provided in this report represent the theoretic basis for the clinical analysis of ASDD caused by TLIF after PSF.

Stabilization of the lumbar spine by spinal muscle forces producing compressive follower loads: 3-dimensional computational study

Axial compressive loads whose direction changes along the spinal curvature (so called compressive follower loads (CFLs)) was postulated as a normal physiological load in the lumbar spine in the literature. Computational analyses were conducted in this study using finite element and optimization models of the spinal system incorporating 244 fascicles of back muscles. It was feasible to find optimum solutions for spinal muscle forces generating CFLs in the lumbar spine in 3-D postures of neutral standing, flexion 40°, extension 10°, axial rotation 10°, or lateral bending 30°. FE analyses demonstrated that the lumbar spine can be in a stable condition not under all CFL generating muscle forces but under those producing CFLs along a curve parallel to the spinal curvature located in the vicinity of the base spinal curve constructed by connecting the geometrical centers of the vertebral bodies. It was also possible to estimate the stable range of the relative location of such CFL curve to the base spinal curve. These results suggest that the lumbar spine in various 3-D postures can be stabilized by spinal muscles that generate CFLs in the spine, which at least in part supports the hypothesis of CFLs as a physiological load in the lumbar spine. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:3004-3012, 2018.

Biomechanics in Annulus Fibrosus Degeneration and Regeneration

Degenerative disc degeneration (DDD) is the major cause of low back pain, which seriously affects the life of patients. Current surgical and conservative treatments only relieve the pain temporarily, yet fail to restore the normal biomechanics and functions of healthy spine. Indeed, high recurrence of disc herniation commonly happens after discectomy. Degenerative changes in biomechanical and structural properties of the intervertebral disc (IVD), including fissures in annulus fibrosus (AF) and volume loss of nucleus pulposus (NP), mainly contribute to DDD development. AF plays a critical role in the biomechanical properties of IVD as it structural integrity is essential to confine NP and maintain physiological intradiscal pressure under loading. Maintaining the homeostasis of AF and NP, and thereby IVD, requires regulation of their biomechanics, which is also involved in the onset and subsequent development of AF degeneration. Therefore, it is essential to understand the biomechanical changes of AF during degeneration, which can also provide valuable insights into the repair and regeneration of AF. In this review, we focus on the biomechanical properties of AF tissue associated with its homeostasis and degeneration, and discuss the biomechanical stimulus required for regeneration of AF. We also provide an overview of recent strategies to target and modulate cell mechanics toward AF regeneration.

Individual Collagen Fibril Thickening and Stiffening of Annulus Fibrosus in Degenerative Intervertebral Disc

STUDY DESIGN

In vitro study using rat intervertebral discs (IVDs).

OBJECTIVE

To explore the alteration of annulus fibrosus collagen fibrils after loading on IVD and to investigate the degeneration pathogenesis at the nanoscale.

SUMMARY OF BACKGROUND DATA

Abnormal loading can lead to IVD degeneration, but the precise mechanism has been hitherto elusive, especially at the nanoscale.

METHODS

A rat IVD loading model was used, which combined bending of the tail by 40° with compressive loading of 1.8, 4.5, and 7.2 N of the rat tail using an external fixation device. The structure and the elastic modulus of individual collagen fibrils within IVD Co8-Co9 was examined 2 weeks after loading at the nanoscale using atomic force microscopy.

RESULTS

Significant fibril disorder and a decrease in cell number within the annulus fibrosus after loading was observed at the microscale as judged by hematoxylin/eosin staining, suggesting initiation of rupture of the structure and degradation of the IVD. The annulus fibrosus collagen fibrils underwent a change in diameter and elastic modulus from 170 ± 18 to 310 ± 24 nm (P < 0.001) and 0.86 ± 0.12 to 1.27 ± 0.30 GPa (P = 0.003), respectively when measured on the concave side after a loading of 7.2 N. Thus the loading process resulted in a thickening and stiffening of collagen fibrils with a difference between the inner and outer layers.

CONCLUSION

The results of the present study indicated that abnormal loading was not only associated with disorder at the microscale, but also alteration of the collagen fibrils at the nanoscale, possibly leading to changes in the mechanical and physiological environment around the cells of the annulus fibrosus.

LEVEL OF EVIDENCE

N/A.

Joint analysis of IVD herniation and degeneration by rat caudal needle puncture model

Intervertebral disc (IVD) degeneration is responsible for various spine pathologies and present clinical treatments are insufficient. Concurrently, the mechanisms behind IVD degeneration are still not completely understood, so as to allow development of efficient tissue engineering approaches. A model of rat IVD degeneration directly coupled to herniation is here proposed in a pilot study. Disc injury is induced by needle puncture, using two different needles gauges: a low caliber 25-G needle and a high caliber 21-G needle. Histological, biochemical, and radiographic degeneration was evaluated at 2 and 6 weeks post-injury. We show that the larger caliber needle results in a more extended histological and radiographic degeneration within the IVD, compared to the smaller one. TUNEL quantification indicates also increased cell death in the 21-G group. Analyses of collagen type I (Picrosirius red staining), collagen type II (immunofluorescence), and GAG content (Blyscan assay) indicate that degeneration features spontaneously recover from 2 to 6 weeks, for both needle types. Moreover, we show the occurrence of hernia proportional to the needle gauge. The number of CD68+ macrophages present, as well as cell apoptosis within the herniated tissue are both proportional to hernia volume. Moreover, hernias formed after lesion tend to spontaneously diminish in volume after 6 weeks. Finally, MMP3 is increased in the hernia in the 21-G group at 2 weeks. This model, by uniquely combining IVD degeneration and IVD herniation in the same animal, may help to understand mechanisms behind IVD pathophysiology, such as hernia formation and spontaneous regression. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:258-268, 2017.

Fusion angle affects intervertebral adjacent spinal segment joint forces-Model-based analysis of patient specific alignment

This study addresses the hypothesis that adjacent segment intervertebral joint loads are sensitive to the degree of lordosis that is surgically imposed during vertebral fusion. Adjacent segment degeneration is often observed after lumbar fusion, but a causative mechanism is not yet clearly evident. Altered kinematics of the adjacent segments and potentially nonphysiological mechanical joint loads have been implicated in this process. However, little is known of how altered alignment and kinematics influence loading of the adjacent intervertebral joints under consideration of active muscle forces. This study investigated these effects by simulating L4/5 fusions using kinematics-driven musculoskeletal models of one generic and eight sagittal alignment-specific models. Models featured different spinopelvic configurations but were normalized by body height, masses, and muscle properties. Fusion of the L4/5 segment was implemented in an in situ (22°), hyperlordotic (32°), and hypolordotic (8°) fashion and kinematic input parameters were changed accordingly based on findings of an in vitro investigation. Bending motion from upright standing to 45° forward flexion and back was simulated for all models in intact and fused conditions. Joint loads at adjacent levels and moment arms of spinal muscles experienced changes after all types of fusion. Hypolordotic configuration led to an increase of adjacent segment (L3/4) shear forces of 29% on average, whereas hyperlordotic fusion reduced shear by 39%. Overall, L4/5 in situ fusion resulted in intervertebral joint forces closest to intact loading conditions. An artificial decrease in lumbar lordosis (minus 14° on average) caused by an L4/5 fusion lead to adverse loading conditions, particularly at the cranial adjacent levels, and altered muscle moment arms, in particular for muscles in the vicinity of the fusion. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:131-139, 2017.

Effect of degeneration on the six degree of freedom mechanical properties of human lumbar spine segments

While the effects of disc degeneration on compression and rotation motions have been studied, there is no data for shear loading. Clinical research has shown that those with low back pain (a potential consequence of degeneration) experience a 75% greater lateral shear force than those without it. Therefore, the aim was to compare the effect of degeneration on spine segment stiffness and phase angle in each of six degree of freedom (6DOF) loading directions. Fourteen intact functional spinal units (FSU) were dissected from human lumbar spines (mean (SD) age 76.2 (11) years, Thompson grades 3 (N = 5, mild), 4 (N = 6, moderate), 5 (N = 3, severe)). Each FSU was tested in ±6DOFs while subjected to a physiological preload, hydration, and temperature (37°C) conditions in a hexapod robot. A one-way ANOVA between degenerated groups was performed on stiffness and phase angle for each DOF. Significant differences in stiffness were found between mild and moderate degenerative groups in lateral shear (p = 0.001), and axial rotation (p = 0.001), where moderate degeneration had decreased stiffness. For phase angle, significant differences were seen in anterior shear (p = 0.017), and axial rotation (p = 0.026), where phase angle for mild degeneration was less than moderate. Trends of stiffness and phase angle changes between degenerative groups were similar within each DOF. Clinically, the identification of the DOFs that are most affected by degeneration could be used in rehabilitation to improve supplemental stabilization of core muscle groups. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1399-1409, 2016.

Mechanics and biology in intervertebral disc degeneration: a vicious circle

Intervertebral disc degeneration is a major cause of low back pain. Despite its long history and large socio-economical impact in western societies, the initiation and progress of disc degeneration is not well understood and a generic disease model is lacking. In literature, mechanics and biology have both been implicated as the predominant inductive cause; here we argue that they are interconnected and amplify each other. This view is supported by the growing awareness that cellular physiology is strongly affected by mechanical loading. We propose a vicious circle of mechanical overloading, catabolic cell response, and degeneration of the water-binding extracellular matrix. Rather than simplifying the disease, the model illustrates the complexity of disc degeneration, because all factors are interrelated. It may however solve some of the controversy in the field, because the vicious circle can be entered at any point, eventually leading to the same pathology. The proposed disease model explains the comparable efficacy of very different animal models of disc degeneration, but also helps to consider the consequences of therapeutic interventions, either at the cellular, material or mechanical level.

The effect of disc degeneration on anterior shear translation in the lumbar spine

Many pathologies involving disc degeneration are treated with surgery and spinal implants. It is important to understand how the spine behaves mechanically as a function of disc degeneration. Shear loading is especially relevant in the natural and surgically stabilized lumbar spine. The objective of our study was to determine the effect of disc degeneration on anterior translation of the lumbar spine under shear loading. We tested 30 human cadaveric functional spinal units (L3-4 and L4-5) in anterior shear loading. First, the specimens were imaged in a 1.5 T magnetic resonance scanner. The discs were graded according to the Pfirrmann classification. The specimens were then loaded up to 250 N in anterior shear with an axial compression force of 300 N. Motion of the vertebrae was captured with an optoelectronic camera system. Inter- and intra-observer reliability for disc grading was determined (Cohen's and Fleiss' Kappa), and a non-parametric test was performed on the translation data to characterize the effect of disc degeneration on this parameter. We found fair to moderate agreement between and within observers for the disc grading. We found no significant effect of disc degeneration on anterior shear translation (Kruskal-Wallis ANOVA). Our results indicate that disc degeneration, as classified with the Pfirrmann scale, does not predict lumbar spinal motion in shear.

Effects of shear force on intervertebral disc: an in vivo rabbit study

PURPOSE

A new in vivo rabbit model was developed to investigate the effects of shear force on intervertebral disc (IVD).

METHODS

Japanese white rabbits (n = 38) were used for this study. The L4/5 discs in Group A (n = 10) were subjected to a constant shear force (50 N) using a custom-made external loading device for 1 month; in Group B (n = 10) for 2 months; whereas in Group C (n = 10), loading device was attached to the spine but the discs remained unloaded. Group D (n = 8) was a non-operated intact control group. After loading, the loading devices were taken out and the animals were given X-ray and MRI examination. After X-ray and MRI examination, the animals were euthanized for histological analysis.

RESULTS

After 1 and 2 months of loading, radiographic findings showed significant disc height narrowing in L4/5 discs of the animals in loading groups, and slight lumbar spondylolisthesis in some animals of Group B. MRI showed a significant decrease in nucleus pulposus (NP) area and signal intensity from T2-weighted images. Histologically, loss of normal NP cells and disorganization of the architecture of the annulus occurred, and proteoglycan stain decreased.

CONCLUSIONS

The results of this study suggest that disc degeneration can be induced by hyper-physiological shear loading in the rabbit IVD. Long-term shear loading may result in structural disc failure inducing lumbar spondylolisthesis and progressive disc degeneration, which, however, has to be proven by further studies.

A standardized representation of spinal quality of motion

The experimentally determined torque-rotation curve of the lumbar spine is mathematically described with a proposed dual-inflection point Boltzmann equation. The result is a method for describing functional spinal unit motion data. The benefit of the model is that each of the coefficients has a specific meaning in relation to the torque-rotation curve: the points A and B identify the respective minimum and maximum rotations of the functional spinal unit, m1 and m2 indicate the inflection points of the curve where the stiffness changes markedly, and α1 and α2 are associated with the rates of change of the curve at m1 and m2, respectively. The dual-inflection point Boltzmann captures the full quality of motion of the spinal segment and can also be used to derive relevant parameters such as range of motion, midrange stiffness, and hysteresis.

Pelvic incidence-lumbar lordosis mismatch results in increased segmental joint loads in the unfused and fused lumbar spine

PURPOSE

Symptomatic adjacent segment disease (ASD) has been reported to occur in up to 27 % of lumbar fusion patients. A previous study identified patients at risk according to the difference of pelvic incidence and lordosis. Patients with a difference between pelvic incidence and lumbar lordosis >15° have been found to have a 20 times higher risk for ASD. Therefore, it was the aim of the present study to investigate forces acting on the adjacent segment in relation to pelvic incidence-lumbar lordosis (PILL) mismatch as a measure of spino-pelvic alignment using rigid body modeling to decipher the underlying forces as potential contributors to degeneration of the adjacent segment.

METHODS

Sagittal configurations of 81 subjects were reconstructed in a musculoskeletal simulation environment. Lumbar spine height was normalized, and body and segmental mass properties were kept constant throughout the population to isolate the effect of sagittal alignment. A uniform forward/backward flexion movement (0°-30°-0°) was simulated for all subjects. Intervertebral joint loads at lumbar level L3-L4 and L4-L5 were determined before and after simulated fusion.

RESULTS

In the unfused state, an approximately linear relationship between sagittal alignment and intervertebral loads could be established (shear: 0° flexion r = 0.36, p < 0.001, 30° flexion r = 0.48, p < 0.001; compression: 0° flexion r = 0.29, p < 0.01, 30° flexion r = 0.40, p < 0.001). Additionally, shear changes during the transition from upright to 30° flexed posture were on average 32 % higher at level L3-L4 and 14 % higher at level L4-L5 in alignments that were clinically observed to be prone to ASD. Simulated fusion affected shear forces at the level L3-L4 by 15 % (L4-L5 fusion) and 23 % (L4-S1 fusion) more for alignments at risk for ASD.

CONCLUSION

Higher adjacent segment shear forces in alignments at risk for ASD already prior to fusion provide a mechanistic explanation for the clinically observed correlation between PILL mismatch and rate of adjacent segment degeneration.

Region specific response of intervertebral disc cells to complex dynamic loading: an organ culture study using a dynamic torsion-compression bioreactor

The spine is routinely subjected to repetitive complex loading consisting of axial compression, torsion, flexion and extension. Mechanical loading is one of the important causes of spinal diseases, including disc herniation and disc degeneration. It is known that static and dynamic compression can lead to progressive disc degeneration, but little is known about the mechanobiology of the disc subjected to combined dynamic compression and torsion. Therefore, the purpose of this study was to compare the mechanobiology of the intervertebral disc when subjected to combined dynamic compression and axial torsion or pure dynamic compression or axial torsion using organ culture. We applied four different loading modalities [1. control: no loading (NL), 2. cyclic compression (CC), 3. cyclic torsion (CT), and 4. combined cyclic compression and torsion (CCT)] on bovine caudal disc explants using our custom made dynamic loading bioreactor for disc organ culture. Loads were applied for 8 h/day and continued for 14 days, all at a physiological magnitude and frequency. Our results provided strong evidence that complex loading induced a stronger degree of disc degeneration compared to one degree of freedom loading. In the CCT group, less than 10% nucleus pulposus (NP) cells survived the 14 days of loading, while cell viabilities were maintained above 70% in the NP of all the other three groups and in the annulus fibrosus (AF) of all the groups. Gene expression analysis revealed a strong up-regulation in matrix genes and matrix remodeling genes in the AF of the CCT group. Cell apoptotic activity and glycosaminoglycan content were also quantified but there were no statistically significant differences found. Cell morphology in the NP of the CCT was changed, as shown by histological evaluation. Our results stress the importance of complex loading on the initiation and progression of disc degeneration.