The aging disc: using an ovine model to examine age-related differences in the biomechanical properties of the intralamellar matrix of single lamellae

The effect of intervertebral disc damage on the mechanical strength of the annulus fibrosus in the adjacent segment

BACKGROUND CONTEXT

A herniated intervertebral disc (IVD) is a common injury in the human population. Despite the injury being isolated to a singular IVD in the spine, it is important to look at the biomechanical effects that a damaged IVD has on the entire spine, specifically the IVD adjacent to the injury.

PURPOSE

This study examined the effects of a damaged IVD on the mechanical properties of the annulus fibrosus (AF) in the adjacent cranial IVD.

STUDY DESIGN

Basic science study using an in-vitro porcine model.

METHODS

Sixteen porcine cervical spines were used; specifically spinal levels C3/4/5 were assigned to one of two experimental groups: 1) a control group that was not subjected any injuries (n=8); 2) an experimental group that experienced an injury to the anterolateral part of the disc, reaching the nucleus pulposus but without affecting the posterior portion of the AF in the C4/5 functional spine unit (FSU) (n=8). Each specimen underwent a previously published precondition compression protocol of 300 N of compression for 15 minutes followed by a cyclical compression protocol of compression protocol of 0.5 Hz sinusoidal waveform at 300 to 1200 N for 2 hours (3600 cycles). Post compression, the C3/4 AF was dissected to obtain two multilayer samples (one anterior and one posterior) as well as a peel sample (from the posterolateral region). A tensile strength test was conducted to examine the strength of the interlamellar matrix (peel sample) and the overall strength of the AF (multilayer samples).

RESULTS

Significant results were found in the peel test samples. Specifically, experimental specimens were less stiff compared than control specimens (p<.01). In addition, experimental specimens also had a lower average strength then control specimens (p<.01). This reduction in both interlamellar strength and stiffness increases the risk of delamination in the experimental samples. In contrast, there were no differences found between the two groups when examining the AF as a whole through the multilayer tests (p>.05).

CONCLUSIONS

It appears that a damaged IVD impacts the biomechanics of the spine and specifically the mechanical properties of the adjacent IVD. Specifically, the observed weakening of the interlamellar matrix in these adjacent IVDs may predispose it to delamination and subsequently degeneration or herniation.

CLINICAL SIGNIFICANCE

These findings may help clinicians when treating patients who have experienced a disc herniation or severe degeneration, as they may potentially experience accelerated adjacent disc degeneration.

Biomechanical effect of age-related structural changes on cervical intervertebral disc: A finite element study

Previous literature has investigated the biomechanical response of healthy and degenerative discs, but the biomechanical response of suboptimal healthy intervertebral discs received less attention. The purpose was to compare the biomechanical responses and risk of herniation of young healthy, suboptimal healthy, and degenerative intervertebral discs. A cervical spine model was established and validated using the finite element method. Suboptimal healthy, mildly, moderately, and severely degenerative disc models were developed. Disc height deformation, range of motion, intradiscal pressure, and von Mises stress in annulus fibrosus were analyzed by applying a moment of 4 Nm in flexion, extension, lateral bending, and axial rotation with 100 N compressive loads. Disc height deformation in young healthy, suboptimal healthy, mildly, moderately, and severely degenerative discs was 40%, 37%, 21%, 12%, and 8%, respectively. The decreasing order of the range of motion was young healthy spine > suboptimal healthy spine > mildly degenerative spine > moderately degenerative spine > severely degenerative spine. The mean stress of annulus ground substance in the suboptimal healthy disc was higher than in the young healthy disc. The mean stress of inter-lamellar matrix and annulus ground substance in moderately and severely degenerative discs was higher than in other discs. Age-related structural changes and degenerative changes increased the stiffness and reduced the elastic deformation of intervertebral discs. Decreased range of motion due to the effects of aging or degeneration on the intervertebral disc, may cause compensation of adjacent segments and lead to progressive degeneration of multiple segments. The effect of aging on the intervertebral disc increased the risk of annulus fibrosus damage from the biomechanical point of view. Moderately and severely degenerative discs may have a higher risk of herniation due to the higher risk of damage and layers separation of annulus fibrosus caused by increased stress in the annulus ground substance and inter-lamellar matrix.

Spinal Biomechanical Modelling in the Process of Lumbar Intervertebral Disc Herniation in Middle-Aged and Elderly

Lumbar disc herniation is one of the common clinical diseases of the lower lumbar spine in orthopedics. The purpose is to remove the herniated disc nucleus pulposus tissue, remove the compressed part of the disease, and relieve symptoms, such as nerve pain. In the past, biomechanics research mostly relied on in vitro measurements, but the complicated internal environment of the human body prevented us from further measurement and research. However, with the development of computer technology, the use of computer CT scanning, software three-dimensional reconstruction, and displacement study three-dimensional spine biomechanics method makes the research of biomechanics into in vitro simulation stage and has gradually become the focus of current research. The postoperative biomechanics was simulated and the comparison model was established at the same time. At the same time, we combined the clinical follow-up data and studied the clinical data for the treatment of postoperative recurrence of lumbar disc herniation. We compared and analyzed the initial operation method and the experimental results and obtained the prevention of recurrence. The results showed that when one inferior articular process was removed, the lumbar spine appeared unstable to rotate to the opposite side; when one inferior articular process was completely removed, the movement of the lumbar spine in all directions was unstable. Better research on the biomechanical properties of the spine will help the diagnosis and treatment of clinical lumbar disc herniation. Therefore, when performing posterior lumbar spine surgery, not only should the exposure of the surgical field and thorough decompression be considered, but also the biomechanical properties of the lumbar spine should be comprehensively evaluated.

A microstructure-based modeling approach to assess aging-sensitive mechanics of human intervertebral disc

BACKGROUND AND OBJECTIVE

The human body soft tissues are hierarchic structures interacting in a complex manner with the surrounding biochemical environment. The loss of soft tissues functionality with age leads to more vulnerability regarding to the external mechanical loadings and increases the risk of injuries. As a main example of the human body soft tissues, the intervertebral disc mechanical response evolution with age is explored. Although the age-dependence of the intervertebral disc microstructure is a well-known feature, no noticeable age effect on the disc stiffness is evidenced in the in-vitro experimental studies of the literature. So, if the disc intrinsic mechanics remains constant, how to explain the correlation of disc degeneration and disc functionality loss with age.

METHODS

A microstructure-based modeling approach was developed to assess in-silico the aging-sensitive mechanics of human intervertebral disc. The model considers the relationship between stress/volumetric macro-response and microstructure along with effective age effects acting at the lamellar and multi-lamellar scales. The stress-stretch and transversal responses of the different disc regions were computed for various age groups (13-18, 36, 58, 69 and 82 years old) and their evolution with age was studied.

RESULTS

While matching with in-vitro experimental data, the predicted stiffness was found to increase while passing from adolescent young discs to mature older discs and then to remain almost constant for the rest of life. Important age-related changes in the disc transversal behavior were also predicted affecting the flexibility of the disc, changing its volumetric behavior, and modifying its dimensions.

CONCLUSION

The developed approach was found able to bring new conclusions about age-dependent mechanical properties including regional dependency. The disc mechanics in terms of rigidity, radial and axial transversal responses were found to alter going from adolescent to middle age where the disc reaches a certain maturity. After reaching maturity, the mechanical properties undergo very slight changes until becoming almost constant with age.

Characterization of chondroitinase-induced lumbar intervertebral disc degeneration in a sheep model intended for assessing biomaterials

Intervertebral disc (IVD) degeneration (IVDD) leads to structural and functional changes. Biomaterials for restoring IVD function and promoting regeneration are currently being investigated; however, such approaches require validation using animal models that recapitulate clinical, biochemical, and biomechanical hallmarks of the human pathology. Herein, we comprehensively characterized a sheep model of chondroitinase-ABC (ChABC) induced IVDD. Briefly, ChABC (1 U) was injected into the L_1/2 , L_2/3 , and L_3/4 IVDs. Degeneration was assessed via longitudinal magnetic resonance (MR) and radiographic imaging. Additionally, kinematic, biochemical, and histological analyses were performed on explanted functional spinal units (FSUs). At 17-weeks, ChABC treated IVDs demonstrated significant reductions in MR index (p = 0.030) and disc height (p = 0.009) compared with pre-operative values. Additionally, ChABC treated IVDs exhibited significantly increased creep displacement (p = 0.004) and axial range of motion (p = 0.007) concomitant with significant decreases in tensile (p = 0.034) and torsional (p = 0.021) stiffnesses and long-term viscoelastic properties (p = 0.016). ChABC treated IVDs also exhibited a significant decrease in NP glycosaminoglycan: hydroxyproline ratio (p = 0.002) and changes in microarchitecture, particularly in the NP and endplates, compared with uninjured IVDs. Taken together, this study demonstrated that intradiscal injection of ChABC induces significant degeneration in sheep lumbar IVDs and the potential for using this model in evaluating biomaterials for IVD repair, regeneration, or fusion.

Spatiotemporal Expression of 3-B-3(-) and 7-D-4 Chondroitin Sulfation, Tissue Remodeling, and Attempted Repair in an Ovine Model of Intervertebral Disc Degeneration

OBJECTIVE

Examination of intervertebral disc (IVD) regeneration in an ovine annular lesion model.

HYPOTHESIS

Sulfation motifs are important functional determinants in glycosaminoglycans (GAGs). Previous studies have correlated 3-B-3(-) and 7-D-4 chondroitin sulfate (CS) motifs in tissues undergoing morphogenetic transition in development. We hypothesize that these motifs may also be expressed in degenerate IVDs and may represent a reparative response.

DESIGN

Induction of disc degeneration by 5 mm or 6 × 20 mm lesions in the annulus fibrosus (AF) over 6 or 3 to 6 months postoperation (PO). Tissue sections were stained with toluidine blue-fast green, 3-B-3(-) and 7-D-4 CS-sulfation motifs were immunolocalized in 3-month PO 6 × 20 mm lesion IVDs. Sulfated glycosaminoglycan (GAG), 3-B-3(-), and 7-D-4 epitopes were quantitated by ELISIA (enzyme-linked immunosorbent inhibition assay) in extracts of AF (lesion site and contralateral half) and nucleus pulposus (NP) 0, 3, and 6 months PO.

RESULTS

Collagenous overgrowth of lesions occurred in the outer AF. Chondroid metaplasia in ~20% of the 6 × 20 mm affected discs resulted in integration of an outgrowth of NP tissue with the inner AF lamellae preventing propagation of the lesion. 3-B-3(-) and 7-D-4 CS sulfation motifs were immunolocalized in this chondroid tissue. ELISIA quantified CS sulfation motifs demonstrating an increase 3 to 6 months PO in the AF lesion and a reduction in sulfated GAG not evident in the contralateral AF.

CONCLUSIONS

(1) Outer annular lesions underwent spontaneous repair. (2) Chondroid metaplasia of the inner 6 × 20 mm defect prevented its propagation suggesting an apparent reparative response.

Disc cell therapy with bone-marrow-derived autologous mesenchymal stromal cells in a large porcine disc degeneration model

PURPOSE

Disc regeneration through matrix-assisted autologous mesenchymal stromal cell therapy seems promising against disc degeneration with convincing results in small animal models. Whether these positive results can be transferred to larger animal models or humans is unclear.

METHODS

Fibrin matrix-assisted autologous bone-marrow-derived mesenchymal stromal cell therapy was compared to acellular fibrin matrix therapy in a porcine in vivo model. First, disc degeneration was induced by annular puncture and partial nucleotomy with a large 16G-needle, and 12 weeks later, disc therapy was performed in a second surgery with a thinner 26G needle. Seventy-two lumbar discs from 12 aged adult pigs were evaluated by histology, micro-CT, and gene expression analysis 13 and 24 weeks after nucleotomy and 1 and 12 weeks after treatment, respectively.

RESULTS

Radiologic disc height was not significantly different in both treatment groups. In the semi-quantitative histologic degeneration score, significant disc degeneration was still evident 1 week after treatment both in the mesenchymal stromal cell group and in the acellular fibrin matrix group. 12 weeks after treatment, degeneration was, however, not further increased and mesenchymal-stromal-cell-treated discs showed significantly less disc degeneration in the annulus fibrosus (p = 0.02), whereas reduction in the nucleus pulposus did not reach statistical significance. Cell treatment compared to matrix alone found less Col1 gene expression as a marker for fibrosis and more expression of the trophic factor BMP2 in the nucleus pulposus, whereas the inflammation marker IL1ß was reduced in the annulus fibrosus.

CONCLUSIONS

Disc treatment with fibrin matrix-assisted autologous mesenchymal stromal cells reduced degenerative findings compared to acellular fibrin matrix alone. Regenerative changes, however, were not significant for all parameters showing limitations in a large biomechanically demanding model with aged discs. These slides can be retrieved under Electronic Supplementary Material.

Age-related changes in shock absorption capacity of the human spinal column

Background

The spinal column possesses shock absorption properties, mainly provided by the intervertebral discs. However, with the process of senescence, all structures of the spine, including the discs, undergo degenerative changes. It may lead to alteration of the mechanical properties of the spinal motion segment and diminished capacity for vibration attenuation.

Objective

The objective of this study was to investigate the age-related changes in shock absorption properties of the spine.

Patients and methods

A total of 112 individuals divided into three groups according to age (third, fifth, and seventh decades of life) were enrolled in this study. The transmissibility of vibrations through the spine was measured in a standing position on a vibration platform by accelerometers mounted at the levels of S2 and C0. Registered signals were described using four parameters: VMS (variability), peak-to-peak amplitude (PPA), and spectral activity in two bands F2 (0.7–5 Hz) and F20 (15–25 Hz).

Results

In all age groups, signals registered at C0 were characterized by significantly lower values of VMS, PPA, and F20, when compared to level S2. Simultaneously, the parameter F20 significantly differed among all age groups when C0 vibrations were analyzed: 2.43±1.93, 5.02±3.61, and 10.84±5.12 for the third, fifth, and seventh decades of life, respectively.

Conclusion

The human spinal column provides vibration attenuation; however, this property gradually declines with the aging process.

Modelling the failure precursor mechanism of lamellar fibrous tissues, example of the annulus fibrosus

The aims of this study were to assess the damage and failure strengths of lamellar fibrous tissues, such as the anterior annulus fibrosus (AF), and to develop a mathematical model of damage propagation of the lamellae and inter-lamellar connections. This level of modelling is needed to accurately predict the effect of damage and failure induced by trauma or clinical interventions. 26 ovine anterior AF cuboid specimens from 11 lumbar intervertebral discs were tested in radial tension and mechanical parameters defining damage and failure were extracted from the in-vitro data. Equivalent 1D analytical models were developed to represent the specimen strength and the damage propagation, accounting for the specimen dimensions and number of lamellae. Model parameters were calibrated on the in-vitro data. Similar to stiffness values reported for other orientations, the outer annulus was found stronger than the inner annulus in the radial direction, with failure at higher stress values. The inner annulus failed more progressively, showing macroscopic failure at a higher strain value. The 1D analytical model of damage showed that lamellar damage is predominant in the failure mechanism of the AF. The analytical model of the connections between lamellae allowed us to represent separately damage processes in the lamellae and the inter-lamellar connections, which cannot be experimentally tested individually.