Arthur J. Michalek

Role of biomechanics in intervertebral disc degeneration and regenerative therapies: what needs repairing in the disc and what are promising biomaterials for its repair?

BACKGROUND CONTEXT Degeneration and injuries of the intervertebral disc (IVD) result in large alterations in biomechanical behaviors. Repair strategies using biomaterials can be optimized based on the biomechanical and biological requirements of the IVD. PURPOSE To review the present literature on the effects of degeneration, simulated degeneration, and injury on biomechanics of the IVD, with special attention paid to needle puncture injuries, which are a pathway for diagnostics and regenerative therapies and the promising biomaterials for disc repair with a focus on how those biomaterials may promote biomechanical repair. STUDY DESIGN A narrative review to evaluate the role of biomechanics on disc degeneration and regenerative therapies with a focus on what biomechanical properties need to be repaired and how to evaluate and accomplish such repairs using biomaterials. Model systems for the screening of such repair strategies are also briefly described. METHODS Articles were selected from two main PubMed searches using keywords: intervertebral AND biomechanics (1,823 articles) and intervertebral AND biomaterials (361 articles). Additional keywords (injury, needle puncture, nucleus pressurization, biomaterials, hydrogel, sealant, tissue engineering) were used to narrow the articles down to the topics most relevant to this review. RESULTS Degeneration and acute disc injuries have the capacity to influence nucleus pulposus (NP) pressurization and annulus fibrosus (AF) integrity, which are necessary for an effective disc function and, therefore, require repair. Needle injection injuries are of particular clinical relevance with the potential to influence disc biomechanics, cellularity, and metabolism, yet these effects are localized or small and more research is required to evaluate and reduce the potential clinical morbidity using such techniques. NP replacement strategies, such as hydrogels, are required to restore the NP pressurization or the lost volume. AF repair strategies including cross-linked hydrogels, fibrous composites, and sealants offer promise for regenerative therapies to restore AF integrity. Tissue engineered IVD structures, as a single implantable construct, may promote greater tissue integration due to the improved repair capacity of the vertebral bone. CONCLUSIONS IVD height, neutral zone characteristics, and torsional biomechanics are sensitive to specific alterations in the NP pressurization and AF integrity and must be addressed for an effective functional repair. Synthetic and natural biomaterials offer promise for NP replacement, AF repair, as an AF sealant, or whole disc replacement. Meeting mechanical and biological compatibilities are necessary for the efficacy and longevity of the repair.

Complex loading affects intervertebral disc mechanics and biology

BACKGROUND Complex loading develops in multiple spinal motions and in the case of hyperflexion is known to cause intervertebral disc (IVD) injury. Few studies have examined the interacting biologic and structural alterations associated with potentially injurious complex loading, which may be an important contributor to chronic progressive degeneration. OBJECTIVE This study tested the hypothesis that low magnitudes of axial compression loading applied asymmetrically can induce IVD injury affecting cellular and structural responses in a large animal IVD ex-vivo model. METHODS Bovine caudal IVDs were assigned to either a control or wedge group (15°) and placed in organ culture for 7 days under static 0.2MPa load. IVD tissue and cellular responses were assessed through confined compression, qRT-PCR, histology and structural and compositional measurements, including Western blot for aggrecan degradation products. RESULTS Complex loading via asymmetric compression induced cell death, an increase in caspase-3 staining (apoptosis), a loss of aggrecan and an increase in aggregate modulus in the concave annulus fibrosis. While an up-regulation of MMP-1, ADAMTS4, IL-1β, and IL-6 mRNA, and a reduced aggregate modulus were induced in the convex annulus. CONCLUSION Asymmetric compression had direct deleterious effects on both tissue and cells, suggesting an injurious loading regime that could lead to a degenerative cascade, including cell death, the production of inflammatory mediators, and a shift towards catabolism. This explant model is useful to assess how injurious mechanical loading affects the cellular response which may contribute to the progression of degenerative changes in large animal IVDs, and results suggest that interventions should address inflammation, apoptosis, and lamellar integrity.

Measurement of local strains in intervertebral disc anulus fibrosus tissue under dynamic shear: Contributions of matrix fiber orientation and elastin content

Shear strain has been implicated as an initiator of intervertebral disc anulus failure, however a clear, multi-scale picture of how shear strain affects the tissue microstructure has been lacking. The purposes of this study were to measure microscale deformations in anulus tissue under dynamic shear in two orientations, and to determine the role of elastin in regulating these deformations. Bovine AF tissue was simultaneously shear loaded and imaged using confocal microscopy following either a buffer or elastase treatment. Digital image analysis was used to track through time local shear strains in specimens sheared transversely, and stretch and rotation of collagen fiber bundles in specimens sheared circumferentially. The results of this study suggest that sliding does not occur between AF plies under shear, and that interlamellar connections are governed by collagen and fibrilin rather than elastin. The transverse shear modulus was found to be approximately 1.6 times as high in plies the direction of the collagen fibers as in plies across them. Under physiological levels of in-plane shear, fiber bundles stretched and re-oriented linearly. Elastin was found to primarily stiffen plies transversely. We conclude that alterations in the elastic fiber network, as found with IVD herniation and degeneration, can therefore be expected to significantly influence the AF response to shear making it more susceptible to micro failure under bending or torsion loading.

The effects of needle puncture injury on microscale shear strain in the intervertebral disc annulus fibrosus.

BACKGROUND CONTEXT Needle puncture of the intervertebral disc (IVD) is required for delivery of therapeutic agents to the nucleus pulposus and for some diagnostic procedures. Needle puncture has also been implicated as an initiator of disc degeneration. It is hypothesized that needle puncture may initiate IVD degeneration by altering microscale mechanical behavior in the annulus fibrosus (AF). PURPOSE Quantify the changes in AF microscale strain behavior resulting from puncture with a hypodermic needle. STUDY DESIGN Cadaveric IVD tissue explant study. METHODS Annulus fibrosus explants from bovine caudal IVDs that had been punctured radially with hypodermic needles were loaded in dynamic sinusoidal shear while being imaged with a confocal microscope. Digital image analysis was used to quantify local tissue strain and damage propagation with repeated shearing. RESULTS Needle puncture changed the distribution of microscale shear strains in the AF under load from homogenous (equal to far field) to a distinct pattern of high (4× far field) and low (0.25× far field) strain areas. Repeated loading did not cause further growth of the disruption beyond the second cycle. CONCLUSIONS Needle puncture results in a drastic alteration of microscale strain behavior in the AF under load. This alteration may directly initiate disc degeneration by being detrimental to tissue-cell mechanotransduction.

Effects of torsion on intervertebral disc gene expression and biomechanics, using a rat tail model.

Study Design. In vitro and in vivo rat tail model to assess effects of torsion on intervertebral disc biomechanics and gene expression. Objective. Investigate effects of torsion on promoting biosynthesis and producing injury in rat caudal intervertebral discs. Summary of Background Data. Torsion is an important loading mode in the disc and increased torsional range of motion is associated with clinical symptoms from disc disruption. Altered elastin content is implicated in disc degeneration, but its effects on torsional loading are unknown. Although effects of compression have been studied, the effect of torsion on intervertebral disc gene expression is unknown. Methods In vitro biomechanical tests were performed in torsion on rat tail motion segments subjected to 4 treatments: elastase, collagenase, genipin, control. In vivo tests were performed on rats with Ilizarov-type fixators implanted to caudal motion segments with five 90 minute loading groups: 1 Hz cyclic torsion to ±5 ±15° and ±30°, static torsion to +30°, and sham. Anulus and nucleus tissues were separately analyzed using qRT-PCR for gene expression of anabolic, catabolic, and proinflammatory cytokine markers. Results In vitro tests showed decreased torsional stiffness following elastase treatment and no changes in stiffness with frequency. In vivo tests showed no significant changes in dynamic stiffness with time. Cyclic torsion upregulated elastin expression in the anulus fibrosus. Up regulation of TNF-&agr;and IL-1&bgr; was measured at ±30°. Conclusion. We conclude that strong differences in the disc response to cyclic torsion and compression are apparent with torsion increasing elastin expression and compression resulting in a more substantial increase in disc metabolism in the nucleus pulposus. Results highlight the importance of elastin in torsional loading and suggest that elastin remodels in response to shearing. Torsional loading can cause injury to the disc at excessive amplitudes that are detectable biologically before they are biomechanically.

Different effects of static versus cyclic compressive loading on rat intervertebral disc height and water loss in vitro.

Study Design. In vitro biomechanical study on rat caudal motion segments to evaluate association between compressive loading and water content under static and cyclic conditions. Objective. To test hypotheses: 1) there is no difference in height loss and fluid (volume) loss of discs loaded in compression under cyclic (0.15–1.0 MPa) and static conditions with the same root-mean-square (RMS) magnitudes (0.575 MPa); and 2) after initial disc bulge, tissue water loss is directly proportional to height loss under static loading. Summary of Background Data. Disc degeneration affects water content, elastic and viscoelastic behaviors. There is limited understanding of the association between transient water loss and viscoelastic creep in a controlled in vitro environment where inferences may be made regarding mechanisms of viscoelasticity. Methods. A total of 126 caudal motion segments from 21 Wistar rats were tested in compression using 1 of 6 protocols: Static loading at 1.0 MPa for 9, 90, and 900 minutes, Cyclic loading at 0.15 to 1.0 MPa/1 Hz for 90 minutes, Mid-Static loading at 0.575 MPa for 90 minutes, and control. Water content was then measured in anulus and nucleus regions. Results. Percent water loss was significantly greater in nucleus than anulus regions, suggesting some water redistribution, with average values under 1 MPa static loading of 23.0% and 14.9% after 90 minutes and 26.9% and 17.6% after 900 minutes, respectively. Cyclic loading resulted in significantly greater height loss (0.506 ± 0.108 mm) than static loading with the same RMS value (0.402 ± 0.096 mm), but not significantly less than static loading at peak value (0.539 ± 0.122 mm). Significant and strong correlations were found between percent water loss and disc height loss, suggesting water was lost through volume decrease. Conclusion. Peak magnitude of cyclic compression and not RMS value was most important in determining height change and water loss, likely due to differences between disc creep and recovery rates. Water redistribution from nucleus to anulus occurred under loading consistent with an initial elastic compression (and associated disc bulge) followed by a reduction in disc volume over time.

Analysis of quantitative magnetic resonance imaging and biomechanical parameters on human discs with different grades of degeneration

To establish relationships between quantitative MRI (qMRI) and biomechanical parameters in order to help inform and interpret alterations of human intervertebral discs (IVD) with different grades of degeneration.

Evaluation of quantitative magnetic resonance imaging, biochemical and mechanical properties of trypsin‐treated intervertebral discs under physiological compression loading

To investigate the influence of targeted trypsin digestion and 16 hours compression loading on MR parameters and the mechanical and biochemical properties of bovine disc segments.

Height and torsional stiffness are most sensitive to annular injury in large animal intervertebral discs

BACKGROUND CONTEXT Acute annulus fibrosus injury has been identified as a contributing factor to intervertebral disc (IVD) degeneration. Injuries as small as those resulting from needle injection result in localized mechanical disruption via fiber breakage, but it is unknown whether these injuries initiate degeneration locally or through changes in the mechanical behavior of the entire disc. However, in vitro biomechanical studies of injury are limited to a single type of injury or measurements in only one or two degrees of freedom. PURPOSE The aim of this study is to provide a comprehensive assessment of the joint level mechanical response to IVD injuries of various sizes in a large animal model. We hypothesize that annular injuries will affect disc mechanics differently depending on size, location, and mode of loading. We further hypothesize that a large injury to one side of the disc will induce a bending moment reaction under axial compression, which may decrease spinal column stability. STUDY DESIGN A comprehensive biomechanical study investigating effects of small and large injuries on IVD pressurization and six-degree-of-freedom stiffness behaviors using bovine motion segments. METHODS Bovine caudal motion segments were subjected to a series of annular injuries ranging from 21-gauge needle puncture to 10-mm scalpel incisions and evaluated before and after injury with both mechanical testing under multiple degrees of freedom (axial compression, flexion-extension, lateral bending, and torsion) and nucleus pulposus (NP) fluid pressurization tests. RESULTS Mechanical tests showed that axial torsional stiffness and disc height under resting compressive load were the parameters most sensitive to large annular injury. Bending and compressive stiffnesses, as well as bending moments induced by axial compression, were not significantly changed by scalpel incisions. Additionally, large injuries resulted in altered relaxation behavior after NP pressurization indicative of increases in both radial bulge compliance and fluid flow rates. CONCLUSIONS These findings suggest that loss of disc height, torsional stiffness, and NP fluid pressurization are the immediate results of acute annular injury and are therefore those properties that IVD repair strategies must strive to restore or maintain. The lack of change in bending stiffness and moment under compression suggests that acute annular tears alone are not sufficient to induce off-axis motion and instability.

Effects of enzymatic digestion on compressive properties of rat intervertebral discs

Enzymatic treatments were applied to rat motion segments to establish structure-function relationships and determine mechanical parameters most sensitive to simulated remodeling and degeneration. Rat caudal and lumbar disc biomechanical behaviors were evaluated to improve knowledge of their similarities and differences due to their frequent use during in vivo models. Caudal motion segments were assigned to four groups: soaked (control), genipin treated, elastase treated, and collagenase treated. Fresh lumbar and caudal discs were also compared. The mechanical protocol involved five force-controlled loading stages: equilibration, cyclic compression-tension, quasi-static compression, frequency sweep, and creep. Crosslinking was found to have the greatest effect on IVD properties at resting stress. Elastins role was greatest in tension and at higher force conditions, where GAG content was also a contributing factor. Collagenase treatment caused tissue compaction, which impacted mechanical properties at both high and low force conditions. Equilibration creep and cyclic compression-tension tests were the mechanical tests most sensitive to alterations in specific matrix constituents. Caudal and lumbar motion segments had many similarities but biomechanical differences suggested some distinctions in collagenous structure and water transport characteristics in addition to the geometric differences. Results provide a basis for interpreting biomechanical changes observed in animal model studies of degeneration and remodeling, and underscore the need to maintain and/or repair collagen integrity in IVD health and disease.