Stress distributions inside intervertebral discs: the validity of experimental "stress profilometry'

A comparison of traditional and net structured intersomatic cages in the lombosacral region: A biomechanical analysis for enhancing discopathy treatment

The vertebral column represents an essential element for support, mobility, and the protection of the central nervous system. Various pathologies can compromise these vital functions, leading to pain and a decrease in the quality of life. Within the scope of this study, a novel redesign of the Intersomatic Cage, traditionally used in the presence of discopathy, was proposed. The adoption of additive manufacturing technology allowed for the creation of highly complex geometries, focusing on the lumbosacral tract, particularly on the L4-L5 and L5-S1 intervertebral discs. In addition to the tensile analysis carried out using Finite Element Analysis (FEA) in static simulations, a parallel study on the range of motion (ROM) of the aforementioned vertebral pairs was conducted. The ROM represents the relative movement range between various vertebral pairs. The introduction of the intersomatic cage between the vertebrae, replacing the pulpy nucleus of the intervertebral disc, could influence the ROM, thus having significant clinical implications. For the analysis, the ligaments were modelled using a 1D approach. Their constraint reaction and deformability upon load application were analysed to better understand the potential biomechanical implications arising from the adoption of the cages. During the FEA simulations, two types of cages were analysed: LLIF for L4-L5 and ALIF for L5-S1, subjecting them to four different loading conditions. The results indicate that the stresses exhibited by cages with a NET structure are generally lower compared to those of traditional cages. This stress reduction in cages with NET structure suggests a more optimal load distribution, but it is essential to assess potential repercussions on the surrounding bone structure.

Computed tomography osteoabsorptiometry for imaging of degenerative disc disease

Background

Lower back pain is a common condition with significant morbidity and economic impact. The pathophysiology is poorly understood but is in part attributable to degenerative disc disease (DDD). The healthy intervertebral disc ensures spine functionality by transferring the perceived load to the caudally adjacent vertebrae. The exposure to recurring mechanical load is mirrored in the mineralization pattern of the subchondral bone plate (SBP), where increased bone density is a sign of repetitive localized high stress. Computed tomography -osteoabsorptiometry (CT-OAM) is a technique based on conventional CT scans that displays the mineral density distribution in the SBP as a surface-color map. The objective of this study was to measure and analyze the SBP mineral density patterns of healthy lumbar intervertebral disc (IVDs) and those suffering DDD using CT-OAM densitograms. These findings should provide in vitro insight into the long-term morphological properties of the IVD and how these differ in the state of disc degeneration.

Methods

The CT-data sets of spines from 17 healthy individuals and 18 patients displaying DDD in the lumbar spine were acquired. Individual vertebrae of both cohorts were 3D reconstructed, processed using image analysis software, and compared to one another. Maximum intensity projection of the subchondral mineralization provided surface densitograms of the SBP. The relative calcium concentration, the local maxima of mineralization, and a mean surface projection of level-defined SBPs were calculated from the densitogram and statistically compared.

Results

The inferior SBP, adjacent to degenerating disc, display an 18-41 % higher relative calcium concentration than their healthy counterparts. In the opposing superior SBPs the relative calcium content is significantly increased. Whereas it is reasonably consistent for L1-L3 (L1: 132 %, L2: 127 %, L3: 120 %), the increase grows in caudal direction (L4: 131 %, L5: 148 %, S1: 152 %). Furthermore, a change in the areal distribution of excessive mineralization can be differentiated between healthy and diseased motion segments.

Conclusions

The acquired data provide in vitro proof of the mechanical and anatomical properties of the SBP in relation to the state of disc degeneration. In conjunction with the diagnostic use of CT-osteoabsorptiometry, our data provide a basis for a non-invasive and sensitive technique that correlates with disc functionality. This could be promising in various cases, from early identification of early stages of DDD, tracking disease progression, and assessing the repercussions of surgical procedures or experimental therapies.

Joint fatigue-failure: A demonstration of viscoelastic responses to rate and frequency loading parameters using the porcine cervical spine

Fatigue-failure in low back tissues is influenced by parameters of cyclic loading. Therefore, this study quantified the effect of loading rate and frequency on the number of tolerated compression cycles. Energy storage and vertical deformation were secondarily examined. Thirty-two porcine spinal units were randomly assigned to experimental groups that differed by loading rate (4.2 kN/s, 8.3 kN/s) and loading frequency (0.5 Hz, 1 Hz). Following preload and range-of-motion tests, specimens were cyclically loaded in a neutral posture until fatigue-failure occurred or 10800 cycles were tolerated. Macroscopic dissection was performed to identify the fracture morphology, and measurements of energy storage and vertical displacement were calculated throughout the specimen lifespan (1%, 10%, 50%, 90%, 99%). Given the differences in compression dose-force-time integral-between experimental conditions, the number of sustained cycles were assessed following linear and nonlinear dose-normalization via correction factors calculated from existing risk-exposure approximations. Without dose-normalization, an 8.3 kN/s loading rate and 0.5 Hz loading frequency reduced the fatigue lifetime by 3541 and 5977 cycles, respectively (p < 0.001). Linear and nonlinear dose-normalization resulted in a significant rate × frequency interaction (p < 0.001). For a 1 Hz loading frequency, the number of sustained loading cycles did not differ between loading rates (p_adj ≥ 0.988), but at 0.5 Hz, spinal units compressed at 8.3 kN/s sustained 99% (linear) and 97% (nonlinear) fewer cycles (p_adj < 0.001). These findings demonstrate that the interacting effects of loading frequency and loading rate on spinal fatigue-failure depend on the normalization of dose discrepancies between experimental groups.

A novel in vivo mouse intervertebral disc degeneration model induced by compressive suture

Intervertebral disc degeneration (IDD) is the root cause of many musculoskeletal disorders of the spine. However, the etiology of IDD is complex and still not well understood. Animal models of IDD would be useful in deciphering the underlying mechanisms. But the existing animal models have their limitations. Therefore, to establish a novel mouse model that can simulate the human IDD process in vivo, we proposed to carefully circumcise the 2 mm-wide tail skin and then compressively sutured the defect with a simple end-to-end suture to exert excessive pressure on the disc. After 1-week, 2-week, and 4-week compression, the mice were sacrificed and the intervertebral discs were harvested for tissue analysis. The radiological, morphological, and molecular modifications of intervertebral discs were measured to characterize this model. Radiologically, the water content of the intervertebral disc decreased significantly after 2-week compression. Morphologically, the nucleus pulposus showed a decrease in volume and the number of notochordal cells. The compressive suture also broke the balance between anabolic and catabolic enzymes in nucleus pulposus, which led to the remodeling of the extracellular matrix in nucleus pulposus as the content of aggrecan and collagen II decreased. The compressive suture could induce intervertebral discs degeneration in a more reasonable way, which was solely influenced by mechanical loading, as the mice caudal vertebrae still moved freely after the operation. This kind of animal model could be adapted as a reliable in vivo mouse IDD model for the research regarding the etiology and treatments of IDD.

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.

Mechanisms of Failure Following Simulated Repetitive Lifting: A Clinically Relevant Biomechanical Cadaveric Study

STUDY DESIGN

A biomechanical analysis correlating internal disc strains and tissue damage during simulated repetitive lifting.

OBJECTIVE

To understand the failure modes during simulated safe and unsafe repetitive lifting.

SUMMARY OF BACKGROUND DATA

Repetitive lifting has been shown to lead to lumbar disc herniation (LDH). In vitro studies have developed a qualitative understanding of the effect of repetitive loading on LDH. However, no studies have measured internal disc strains and subsequently correlated these with disc damage.

METHODS

Thirty human cadaver lumbar functional spinal units were subjected to an equivalent of 1 year of simulated repetitive lifting under safe and unsafe levels of compression, in combination with flexion (13-15°), and right axial rotation (2°) for 20,000 cycles or until failure. Safe or unsafe lifting were applied as a compressive load to mimic holding a 20 kg weight either close to, or at arm's length, from the body, respectively. Maximum shear strains (MSS) were measured, and disc damage scores were determined in nine regions from axial post-test magnetic resonance imaging (MRI) and macroscopic images.

RESULTS

Twenty percent of specimens in the safe lifting group failed before 20,000 cycles due to endplate failure, compared with 67% in the unsafe group. Over half of the specimens in the safe lifting group failed via either disc protrusion or LDH, compared with only 20% via protrusion in the unsafe group. Significant positive correlations were found between MRI and macroscopic damage scores in all regions (rs > 0.385, P < 0.049). A significant positive correlation was observed in the left lateral region for MSS versus macroscopic damage score (rs = 0.486, P < 0.037) and MSS versus failure mode (rs = 0.724, P = 0.018, only specimens with disc failure). Pfirrmann Grade 3 discs were strongly associated with subsequent LDH (P = 0.003).

CONCLUSION

Increased shear strains were observed in the contralateral side to the applied rotation as disc injury progressed from protrusion to LDH. Larger compressive loads applied to simulate unsafe lifting led to frequent early failure of the endplate, however, smaller compressive loads at similar flexion angles applied under safe lifting led to more loading cycles before failure, where the site of failure was more likely to be the disc. Our study demonstrated that unsafe lifting leads to greater risk of injury compared with safe lifting, and LDH and disc protrusion were more common in the posterior/posterolateral regions.

LEVEL OF EVIDENCE

N/A.

Analysis of the disc pressure of the upper thoracic spine using pressure-sensitive film: an experimental study in porcine model-implications for scoliosis progression

There has been few studies focusing on the disc pressure of the upper thoracic spine and it still lacks the quantitative pressure measurement of each spinal disc segment. The aim of this study was to study the pressure changes of intervertebral disc in porcine upper thoracic spine using pressure-sensitive film. Twelve porcine thoracic motion segments were harvested and successively loaded with vertical loads of 100 N, 150 N, and 200 N during 5° of anterior flexion, 5° of posterior extension and 5° of lateral bending. The resulting pressure values were measured. During anterior flexion, the anterior annulus of all segments at all loads showed higher mean pressure values than those during vertical compression, whereas the posterior annulus did not show higher mean values. During posterior extension, the anterior annulus of all segments showed lower mean pressure values than those during vertical compression, whereas the posterior annulus did not show lower mean pressure values. During lateral bending, the annulus of all segments showed higher mean pressure values than those during vertical compression. The posterior thoracic vertebra plays an important role in the motion of the upper thoracic vertebral segment and pressure distribution. During lateral bending, the concave side pressure of the annulus increases obviously, suggesting that asymmetrical force is a contributory factor for scoliosis progression.

Biomechanics of Back Pain

This paper offers a mechanistic account of back pain which attempts to incorporate all of the most important recent advances in spinal research. Anatomical and pain-provocation studies show that severe and chronic back pain most often originates in the lumbar intervertebral discs, the apophyseal joints, and the sacroiliac joints. Psychosocial factors influence many aspects of back pain behaviour but they are not important determinants of who will experience back pain in the first place. Back pain is closely (but not invariably) associated with structural pathology such as intervertebral disc prolapse and endplate fractures, although age-related biochemical changes such as those revealed by a ‘dark disc’ on MRI have little clinical relevance. All features of structural pathology (including disc prolapse) can be re-created in cadaveric specimens by severe or repetitive mechanical loading, with a combination of bending and compression being particularly harmful to the spine. Structural disruption alters the mechanical environment of disc cells in a manner that leads to cell-mediated degenerative changes, and animal experiments confirm that surgical disruption of a disc is followed by widespread disc degeneration. Some people are more vulnerable to spinal degeneration than others, largely because of their genetic inheritance. Age-related biochemical changes and loading history can also affect tissue vulnerability. Finally the concept of ‘functional pathology’ is introduced, according to which, back pain can arise because postural habits generate painful stress concentrations within innervated tissues, even though the stresses are not high enough to cause physical disruption.

Pressure distributions inside intervertebral discs under unilateral pedicle screw fixation in a porcine spine model

Background

Little data are available regarding the effects of pedicle screws on the intervertebral disc stress for different spinal segments. The aim of this study was to analyze the intervertebral disc stress in response to the placement of pedicle screws.

Methods

T3–4, T11–12, T15–L1, L3–4, and L4–5 intervertebral disc segments from six porcine spine specimens were harvested. A compressive load of 200 N was applied both before and after the pedicle screw was implanted on the left side of each target segment; the resulting pressure was measured during vertical, 5° anterior flexion, 5° posterior extension, and 5° lateral bending.

Results

The posterior intradiscal pressures of the intervertebral disc were significantly lower in the fixed group than in the unfixed group for all segments during vertical, 5° anterior flexion, and 5° posterior extension. The left pressures of the intervertebral disc were significantly lower in the fixation group for all segments. During 5° lateral bending, the left intervertebral disc pressures were significantly lower in the fixation group. Lower mean pressures were observed in the fixed group.

Conclusions

Unilateral pedicle screws can effectively reduce the pressure of the fixed lateral intervertebral disc. Moreover, it can change the pressure distribution of the intervertebral disc and reduce the pressure of the entire intervertebral disc, especially the posterior side of the intervertebral disc.

How are adjacent spinal levels affected by vertebral fracture and by vertebroplasty? A biomechanical study on cadaveric spines

BACKGROUND CONTEXT

Spinal injuries and surgery may have important effects on neighboring spinal levels, but previous investigations of adjacent-level biomechanics have produced conflicting results. We use "stress profilometry" and noncontact strain measurements to investigate thoroughly this long-standing problem.

PURPOSE

This study aimed to determine how vertebral fracture and vertebroplasty affect compressive load-sharing and vertebral deformations at adjacent spinal levels.

STUDY DESIGN

We conducted mechanical experiments on cadaver spines.

METHODS

Twenty-eight cadaveric spine specimens, comprising three thoracolumbar vertebrae and the intervening discs and ligaments, were dissected from fourteen cadavers aged 67-92 years. A needle-mounted pressure transducer was used to measure the distribution of compressive stress across the anteroposterior diameter of both intervertebral discs. "Stress profiles" were analyzed to quantify intradiscal pressure (IDP) and concentrations of compressive stress in the anterior and posterior annulus. Summation of stresses over discrete areas yielded the compressive force acting on the anterior and posterior halves of each vertebral body, and the compressive force resisted by the neural arch. Creep deformations of vertebral bodies under load were measured using an optical MacReflex system. All measurements were repeated following compressive injury to one of the three vertebrae, and again after the injury had been treated by vertebroplasty. The study was funded by a grant from Action Medical Research, UK ($143,230). Authors of this study have no conflicts of interest to disclose.

RESULTS

Injury usually involved endplate fracture, often combined with deformation of the anterior cortex, so that the affected vertebral body developed slight anterior wedging. Injury reduced IDP at the affected level, to an average 47% of pre-fracture values (p<.001), and transferred compressive load-bearing from nucleus to annulus, and also from disc to neural arch. Similar but reduced effects were seen at adjacent (non-fractured) levels, where mean IDP was reduced to 73% of baseline values (p<.001). Vertebroplasty partially reversed these changes, increasing mean IDP to 76% and 81% of baseline values at fractured and adjacent levels, respectively. Injury also increased creep deformation of the vertebral body under load, especially in the anterior region where a 14-fold increase was observed at the fractured level and a threefold increase was observed at the adjacent level. Vertebroplasty also reversed these changes, reducing deformation of the anterior vertebral body (compared with post-fracture values) by 62% at the fractured level, and by 52% at the adjacent level.

CONCLUSIONS

Vertebral fracture adversely affects compressive load-sharing and increases vertebral deformations at both fractured and adjacent levels. All effects can be partially reversed by vertebroplasty.

Porosity and Thickness of the Vertebral Endplate Depend on Local Mechanical Loading

STUDY DESIGN

Mechanical and microcomputed tomography (micro-CT) study of cadaver spines.

OBJECTIVE

To compare porosity and thickness of vertebral endplates with (1) compressive stresses measured in adjacent intervertebral discs and (2) grade of disc degeneration.

SUMMARY OF BACKGROUND DATA

Endplate porosity is important for disc metabolite transport, and yet porosity increases with age and disc degeneration. We hypothesize that porosity is largely determined by mechanical loading from adjacent discs.

METHODS

Forty motion segments (T8-9 to L4-5) were dissected from 23 cadavers aged 48 to 98 years. Each was subjected to 1 kN compression during which time intradiscal stresses were measured by pulling a pressure transducer along the disc's midsagittal diameter. "Stress profiles" revealed the average pressure in the nucleus, and the maximum stress in the anterior and posterior annulus. Specimens were further dissected to obtain discs with endplates (and 5 mm of bone) on either side. Microcomputed tomography scans (resolution 35 μm) were analyzed to calculate thickness and porosity in the midsagittal regions of all 80 endplates. Average values for the anterior, central, and posterior regions of each endplate were obtained. Disc degeneration was assessed macroscopically and microscopically.

RESULTS

Endplate porosity was inversely related to its thickness, being greatest in the central region opposite the nucleus, and least near the periphery. Superior endplates (relative to the disc) were 14% thicker (P < 0.001) and 4% less porous (P = 0.008) than inferior. In each of the 3 endplate regions (anterior, central, and posterior), porosity was inversely and significantly related to mechanical loading (pressure or maximum stress) in the adjacent disc region (P < 0.01 in all cases). Disc degeneration was best predicted by (reduced) nucleus pressure (R = 0.46, P < 0.001) and (reduced) maximum stress in the anterior annulus (R = 0.31, P < 0.001).

CONCLUSION

Mechanical loading is a major determinant of endplate thickness and porosity. Disc degeneration is more closely related to reduced disc stresses than to endplate thickness or porosity.

LEVEL OF EVIDENCE

N/A.

Comparison of eight published static finite element models of the intact lumbar spine: predictive power of models improves when combined together

Finite element (FE) model studies have made important contributions to our understanding of functional biomechanics of the lumbar spine. However, if a model is used to answer clinical and biomechanical questions over a certain population, their inherently large inter-subject variability has to be considered. Current FE model studies, however, generally account only for a single distinct spinal geometry with one set of material properties. This raises questions concerning their predictive power, their range of results and on their agreement with in vitro and in vivo values. Eight well-established FE models of the lumbar spine (L1-5) of different research centers around the globe were subjected to pure and combined loading modes and compared to in vitro and in vivo measurements for intervertebral rotations, disc pressures and facet joint forces. Under pure moment loading, the predicted L1-5 rotations of almost all models fell within the reported in vitro ranges, and their median values differed on average by only 2° for flexion-extension, 1° for lateral bending and 5° for axial rotation. Predicted median facet joint forces and disc pressures were also in good agreement with published median in vitro values. However, the ranges of predictions were larger and exceeded those reported in vitro, especially for the facet joint forces. For all combined loading modes, except for flexion, predicted median segmental intervertebral rotations and disc pressures were in good agreement with measured in vivo values. In light of high inter-subject variability, the generalization of results of a single model to a population remains a concern. This study demonstrated that the pooled median of individual model results, similar to a probabilistic approach, can be used as an improved predictive tool in order to estimate the response of the lumbar spine.

Biomechanical effects of semi-constrained integrated artificial discs on zygapophysial joints of implanted lumbar segments

This study aimed to optimize the design and application of semi-constrained integrated artificial discs (SIADs) using a finite element (FE) analysis following implantation, wherein the zygapophysial joints of the segment were biomechanically reconstructed. An FE model of the L4-L5 segment was constructed. Variations in the stresses on the discs and zygapophysial joints were observed during 5° anteflexion, 5° extension and 5° rotation under the 400-N applied axial load. Stresses and load translation analyses of the discs and zygapophysial joints were conducted during anteflexion, extension and rotation under the 400-N applied axial load. Following implantation of the lumbar segments, the stresses on the SIAD zygapophysial joints were not significantly different from those of physiological discs during anteflexion, and these were both marginally greater compared with those of non-constrained artificial discs (NADs). During extension, the increase in the stress on the SIAD zygapophysial joints was less than that on NAD zygapophysial joints. Stresses on the NAD zygapophysial joints were higher than those on SIAD and physiological discs during rotation. The stress on the SIAD zygapophysial joints was not significantly different from that on physiological discs during rotation. For SIADs and NADs, the stresses on the zygapophysial joints and the displacements of the discs were greater compared with those of the physiological discs during extension. The SIADs affected the variations in the stresses on the implanted segment more than the NADs, and the SIADs protected the zygapophysial joints of the implanted segment to a higher degree than the NADs.

Intervertebral disc decompression following endplate damage: implications for disc degeneration depend on spinal level and age

STUDY DESIGN

Mechanical and morphological studies on cadaveric spines.

OBJECTIVE

To explain how spinal level and age influence disc degeneration arising from endplate fracture.

SUMMARY OF BACKGROUND DATA

Disc degeneration can be initiated by damage to a vertebral body endplate, but it is unclear why endplate lesions, and patterns of disc degeneration, vary so much with spinal level and age.

METHODS

One hundred seventy-four cadaveric motion segments, from T7-T8 to L5-S1 and aged 19 to 96 years, were subjected to controlled compressive overload to damage a vertebral body. Stress profilometry was performed before and after damage to quantify changes in intradiscal pressure, and compressive stresses in the annulus. Eighty-six of the undamaged vertebral bodies were then sectioned in the midsagittal plane, and the thickness of the central bony endplate was measured from microradiographs. Regression analysis was used to compare the relative influences of spinal level, age, disc degeneration, and sex on results obtained.

RESULTS

Compressive overload caused endplate fracture at an average force of 3.4 kN, and reduced motion segment height by an average 1.88 mm. Pressure loss in the adjacent nucleus pulposus decreased from 93% at T8-T9 to 38% at L4-L5 (R = 22%, P < 0.001), and increased with age (R = 19%, P < 0.001), especially in male specimens. Stress concentrations in the posterior annulus increased after endplate fracture, with the effect being greatest at upper spinal levels (R = 7%, P < 0.001). Endplate thickness increased by approximately 50% between T11 and L5 (R = 21%, P < 0.001).

CONCLUSION

Endplate fracture creates abnormal stress distributions in the adjacent intervertebral disc, increasing the risk of internal disruption and degeneration. Effects are greatly reduced in the lower lumbar spine, and in young specimens, primarily because of differences in nucleus volume, and materials properties, respectively. Disc degeneration between L4 and S1 may often be unrelated to endplate fracture.

LEVEL OF EVIDENCE

N/A.

Effects of cord pretension and stiffness of the Dynesys system spacer on the biomechanics of spinal decompression- a finite element study

BACKGROUND

The Dynesys system provides stability for destabilized spines while preserving segmental motion. However, clinical studies have demonstrated that the Dynesys system does not prevent adjacent segment disease. Moreover, biomechanical studies have revealed that the stiffness of the Dynesys system is comparable to rigid fixation. Our previous studies showed that adjusting the cord pretension of the Dynesys system alleviates stress on the adjacent level during flexion. We also demonstrated that altering the stiffness of Dynesys system spacers can alleviate stress on the adjacent level during extension of the intact spine. In the present study, we hypothesized that omitting the cord preload and changing the stiffness of the Dynesys system spacers would abate stress shielding on adjacent spinal segments.

METHODS

Finite element models were developed for - intact spine (INT), facetectomy and laminectomy at L3-4 (DEC), intact spine with Dynesys system (IntDyWL), decompressed spine with Dynesys system (DecDyWL), decompressed spine with Dynesys system without cord preload (DecDyNL), and decompressed spine with Dynesys system assembled using spacers that were 0.8 times the standard diameter without cord pretension (DecDyNL0.8). These models were subjected to hybrid control for flexion, extension, axial rotation; and lateral bending.

RESULTS

The greatest decreases in range of motion (ROM) at the L3-4 level occurred for axial rotation and lateral bending in the IntDyWL model and for flexion and extension in the DecDyWL model. The greatest decreases in disc stress occurred for extension and lateral bending in the IntDyWL model and for flexion in the DecDyWL model. The greatest decreases in facet contact force occurred for extension and lateral bending in the DecDyNL model and for axial rotation in the DecDyWL model. The greatest increases in ROMs at L2-3 level occurred for flexion, axial rotation and lateral bending in IntDyWL model and for extension in the DecDyNL model. The greatest increases in disc stress occurred for flexion, axial rotation and lateral bending in the IntDyWL model and for extension in the DecDyNL model. The greatest increases in facet contact force occurred for extension and lateral bending in the DecDyNL model and for axial rotation in the IntDyWL model.

CONCLUSIONS

The results reveals that removing the Dynesys system cord pretension attenuates the ROMs, disc stress, and facet joint contact forces at adjacent levels during flexion and axial rotation. Removing cord pretension together with softening spacers abates stress shielding for adjacent segment during four different moments, and it provides enough security while not jeopardizes the stability of spine during axial rotation.

Can prevention of a reherniation be investigated? Establishment of a herniation model and experiments with an anular closure device

STUDY DESIGN

Biomechanical in vitro study.

OBJECTIVE

To establish a reliable in vitro herniation model with human cadaver spines that enables evaluation of anular closure devices.

SUMMARY OF BACKGROUND DATA

Biomechanically, it is desirable to close anulus defects after disc herniation to preserve as much nucleus as possible. Multiple anular closure options exist to prevent reherniation. A reliable test procedure is needed to evaluate the efficacy and reliability of these implants.

METHODS

Two groups of human lumbar segments (n = 6 per group) were tested under cyclic loading until herniation occurred or 100,000 load cycles were applied. One group contained moderate/severe degenerated discs. A second group had mild degenerated discs. Intradiscal pressure was measured in the intact state to confirm disc quality.If herniation occurred, the extruded material was reinserted into the disc and the anulus defect was treated with the Barricaid anular closure device (Intrinsic Therapeutics, Inc., Woburn, MA). Disc height and 3-dimensional flexibility of the specimens in the intact, defect, and implanted states were measured under pure moments in each principal motion plane. Afterwards, provocation of reherniation was attempted with additional 100,000 load cycles.

RESULTS

Likelihood of herniation was strongly linked to disc degeneration and supported by the magnitude of intradiscal pressure. In moderate/severe degenerated discs, only 1 herniation was created. In mild degenerated discs, herniations were reliably created in all specimens. Using this worst-case model, herniation caused a significant reduction of disc height, which was nearly restored with the implant. In no case was reherniation or implant migration visible after 100,000 load cycles after Barricaid implantation.

CONCLUSION

We established a human herniation model that reliably produced nucleus extrusion during cyclic loading by selecting specimens with low disc degeneration. The Barricaid seems to prevent nucleus from reherniating. The reliability of this method suggests the opportunity to investigate other anulus closure devices and nucleus replacement techniques critically.

Annulus fissures are mechanically and chemically conducive to the ingrowth of nerves and blood vessels

STUDY DESIGN

Mechanical and biochemical analyses of cadaveric and surgically removed discs.

OBJECTIVE

To test the hypothesis that fissures in the annulus of degenerated human discs are mechanically and chemically conducive to the ingrowth of nerves and blood vessels.

SUMMARY OF BACKGROUND DATA

Discogenic back pain is closely associated with fissures in the annulus fibrosus, and with the ingrowth of nerves and blood vessels.

METHODS

Three complementary studies were performed. First, 15 cadaveric discs that contained a major annulus fissure were subjected to 1 kN compression, while a miniature pressure transducer was pulled through the disc to obtain distributions of matrix compressive stress perpendicular to the fissure axis. Second, Safranin O staining was used to evaluate focal loss of proteoglycans from within annulus fissures in 25 surgically removed disc samples. Third, in 21 cadaveric discs, proteoglycans (sulfated glycosaminoglycans [sGAGs]) and water concentration were measured biochemically in disrupted regions of annulus containing 1 or more fissures, and in adjacent intact regions.

RESULTS

Reductions in compressive stress within annulus fissures averaged 36% to 46%, and could have been greater at the fissure axis. Stress reductions were greater in degenerated discs, and were inversely related to nucleus pressure (R(2) = 47%; P = 0.005). Safranin O stain intensity indicated that proteoglycan concentration was typically reduced by 40% at a distance of 600 μm from the fissure axis, and the width of the proteoglycan-depleted zone increased with age (P < 0.006; R(2) = 0.29) and with general proteoglycan loss (P < 0.001; R(2) = 0.32). Disrupted regions of annulus contained 36% to 54% less proteoglycans than adjacent intact regions from the same discs, although water content was reduced only slightly.

CONCLUSION

Annulus fissures provide a low-pressure microenvironment that allows focal proteoglycan loss, leaving a matrix that is conducive to nerve and blood vessel ingrowth.

Degenerative joint disease of the spine

Degenerative changes are the manifestations of metabolic disturbances in the matrix of intervertebral disks and zygapophysial joints. Genetic factors and physical loading contribute, but the strongest correlation is with age. Degenerative changes lack any significant correlation with spinal pain. Therefore, they do not constitute a diagnosis. Internal disk disruption is a distinctive condition that is independent of degeneration and age. Its biophysical and morphologic features correlate with back pain, as do its manifestations on magnetic resonance imaging.

Retaining intradiscal pressure after annulotomy by different annular suture techniques, and their biomechanical evaluations

BACKGROUND

The adverse effects of annulotomy during lumbar discectomy have been increasingly recognized, and methods are developing to repair the annular defect. Biomechanically, the repair should retain the intra-nuclear pressure, which is doubtful using the current suture techniques. Therefore, a new suture technique was designed and tested to close a simpler type of annular incision.

METHODS

A new suture technique, the modified purse-string suture, was introduced into a re-validated nonlinear finite element human disk model after creating a standard transverse slit incision, as well as two other suture techniques: either two simple sutures, or a horizontal crossed suture, and compared their contact pressure on the cleft contact surface. Then, porcine lumbar endplate-disk-endplate complexes with transverse slit incisions were repaired using the three techniques. Quantitative discomanometry was then applied to compare their leakage pressure, as a parameter of disk integrity.

FINDINGS

In finite element model, the new technique created the greatest contact pressure along the suture range (the outer annulus), and generated a minimum contact pressure at the critical point, which was 68% and 55% higher than the other two suture techniques. In quantitative discomanometry, the new suture technique also had an average leakage pressure of 85% and 49% higher than the other two suture techniques.

INTERPRETATION

The modified purse-string suture can generate higher contact pressure than the other two techniques at finite element analysis and in realistic animal models, which aids in retaining intra-discal pressure, and should be encouraged in clinical practice.

Biomechanics of vertebral compression fractures and clinical application

Local biomechanical factors in the etiology of vertebral compression fractures are reviewed. The vertebral body is particularly vulnerable to compression fracture when its bone mineral density (BMD) falls with age. However, the risk of fracture, and the type of fracture produced, does not depend simply on BMD. Equally important is the state of degeneration of the adjacent intervertebral discs, which largely determines how compressive forces are distributed over the vertebral body. Disc height also influences load-sharing between the vertebral body and neural arch, and hence by Wolff's Law can influence regional variations in trabecular density within the vertebral body. Vertebral deformity is not entirely attributable to trauma: it can result from the gradual accumulation of fatigue damage, and can progress by a quasi-continuous process of "creep". Cement injection techniques such as vertebroplasty and kyphoplasty are valuable in the treatment of these fractures. Both techniques can stiffen a fractured vertebral body, and kyphoplasty may contribute towards restoring its height. The presence of cement can limit endplate deformation, and thereby partially reverse the adverse changes in load-sharing which follow vertebral fracture. Cement also reduces time-dependent "creep" deformation of damaged vertebrae.

The effects of a new shape-memory alloy interspinous process device on the distribution of intervertebral disc pressures in vitro

This study was designed to measure the pressure distribution of the intervertebral disc under different degrees of distraction of the interspinous process, because of a suspicion that the degree of distraction of the spinous process may have a close relationship with the disc load share. Six human cadaver lumbar spine L2-L5 segments were loaded in flexion, neutral position, and extension. The L3-L4 disc load was measured at each position using pressure measuring films. Shape-memory interspinous process implants (SMID) with different spacer heights, ranging in size from 10 to 20 mm at 2 mm increments, were used. It was found that a SMID with a spacer height equal to the distance of the interspinous process in the neutral position can share the biomechanical disc load without a significant change of load in the anterior annulus. An interspinous process stabilizing device (IPD) would not be appropriate to use in those cases with serious spinal stenosis because the over-distraction of the interspinous process by the SMID would lead to overloading the anterior annulus which is a recognized cause of disc degeneration.

Effects of a new shape-memory alloy interspinous process device on pressure distribution of the intervertebral disc and zygapophyseal joints in vitro

OBJECTIVE

To quantify the pressure distribution of lumbar intervertebral discs and zygapophyseal joints with different degrees of distraction of the interspinous processes by using a new shape-memory interspinous process stabilization device, and to research the relationship between changing disc and zygapophyseal joint loads and the degree of distraction of interspinous processes, and thus optimize usage of the implant.

METHODS

Six cadaver lumbar specimens (L(2)-L(5)) were loaded. The loads in disc and zygapophyseal joints were recorded at each L(3-4) disc level. Implants with different spacer heights were then placed by turn and the pressure measurements repeated.

RESULTS

An implant with 10 mm spacer height does not significantly share the load. A 12 mm implant reduces the posterior annulus load, and meanwhile decreases the zygapophyseal joints pressure, but only in extension. A 14 mm implant shares the loads of posterior annulus, nucleus, and zygapophyseal joints in extension and the neutral position, but slightly increases the anterior annulus' load. Though 16-20 mm implants do decrease the loads in the posterior annulus and zygapophyseal joints, the anterior annulus' load was apparently increased.

CONCLUSION

Different degrees of distraction of the interspinous processes lead to different load distribution on the intervertebral disc. The implant tested is not appropriate in cases of serious spinal stenosis because of the contradiction that, while over-distraction of the interspinous processes decreases the posterior annulus and the zygapophyseal joints load and distracts the intervertebral foramina, it leads to a marked increase in the load of the anterior annulus, which is recognized to accelerate disc degeneration.

Degenerative anular changes induced by puncture are associated with insufficiency of disc biomechanical function

STUDY DESIGN

In vivo experiments to examine physiologic consequences and in vitro tests to determine immediate biomechanical effects of anular injury by needle puncture.

OBJECTIVE

To determine whether a relationship exists between induction of degenerative changes in anulus fibrosus (AF) and compromised disc biomechanical function according to injury size.

SUMMARY OF BACKGROUND DATA

Various studies in intervertebral disc mechanics, degeneration, and regeneration involve the creation of a defect in the anulus fibrosus (AF). However, the impact of the puncture, itself, on biomechanical function and disc health are not understood.

METHODS

For in vivo experiments, rat caudal discs subjected to percutaneous anular punctures using different gauge size hypodermic needles (18, 22, 26 g) and nonpunctured controls were examined histologically up to 4 weeks postsurgery. For in vitro biomechanical testing, healthy motion segments were isolated and their creep compression response assessed immediately after needle puncture.

RESULTS

We found that needle size-dependence of creep compression behavior paralleled the size-dependence of degenerative changes in the AF. Specifically, 18-g punctures resulted in inward bulging of the AF, lamellar disorganization, and cellular changes. These changes were not seen in 22- and 26-g punctured discs. Biomechanical tests showed that only 18-g needle punctures led to significant changes in disc mechanics. Importantly, a statistically significant association was found between needle sizes that caused biomechanical changes and induction of degenerative changes in the AF.

CONCLUSION

Our findings suggest that injury sizes large enough to disrupt biomechanical function are needed to drive degenerative changes in rat caudal disc AF. Based on the data, we believe that small anular defects become sealed, allowing the disc to function normally and the AF to heal. Larger defects appear to require longer wound closure times, and may prolong the duration of impaired disc function.

Evaluation of load transfer characteristics of a dynamic stabilization device on disc loading under compression

In the current study, finite element analyses were conducted to examine the biomechanical capability of a newly design dynamic stabilization system, FlexPLUS, to restore the load transmission of degenerated intervertebral L4-L5 lumbar motion segment spine under compression. Detailed three-dimensional FE models of L4-L5 motion segment and the FlexPLUS were developed. Compressive loading up to 1000N was applied to the intact L4-L5 model, the L4-L5 models with slight and moderate degenerated disc, and the implanted L4-L5 model. Further more, the load transmission characteristics of Dynesys and a rigid rod was also simulated for comparison. The resultant load-displacement curves and the load transferred through annulus under various conditions were compared. The predicted axial displacement of L4 top surface against applied compressive force of the intact L4-L5 model agreed well with experimental data. The predicted results showed that degenerated disc has significant effect on the lumbar segment load bearing capacity. Not only the stiffness of the segment was greatly increased, the uniform nature of the disc stress distribution was also altered. The FlexPLUS can effectively reduce the disc loading of degenerated model. Although the non-uniform load distribution pattern through annulus was not improved, the overall stress magnitude was greatly reduced to the level of intact model for grade II degeneration.

In vitro disc pressure profiles below scoliosis fusion constructs

STUDY DESIGN

Biomechanical human cadaveric study comparing straight and scoliotic spines with healthy and degenerated L4/5 discs.

OBJECTIVE

To describe the biomechanical environment of discs under various spinal alignments by measuring the coronal intradiscal pressure profiles.

SUMMARY OF BACKGROUND DATA

Abnormal loading of the lumbar discs in the concavity of scoliotic curves may accelerate disc degeneration, which may be related to pain.

METHODS

Eight intact human cadaver spines (T1-S1; mean donor age 47 years old) underwent radiographs, DEXA, and MRI and were graded for disc degeneration. Each specimen was instrumented in a normal (straight coronal) spinal alignment from T4-L4. Intradiscal pressure profiles for the L4/5 disc and resultant moments were obtained under axial follower loads up to 1500 N. Testing was repeated for bilateral 3-cm decompensation. Posterior instrumentation was used to induce scoliosis (thoracic and lumbar curve average = 25 degrees, fractional lumbosacral curve average = 5 degrees), and testing was repeated for all load states.

RESULTS

MRI found 4 healthy (grade I and II) and 4 degenerated (grade III to V) L4/5 discs. Scoliosis and decompensation significantly increased coronal moments (P < 0.003). Disc pressures increased linearly with greater applied loads for all specimens. Healthy L4/5 discs exhibited uniform pressure profiles with normal spinal alignment and minimal effect with simulated scoliosis or decompensation. For degenerated discs, there was a relative pressure profile depression in the nucleus relative to the anulus region; with spinal malalignment, either due to scoliotic curvature, decompensation, or both, there was disc pressure profile asymmetry. The ratio of maximum intradiscal pressure at the concavity relative to the convexity was 1.1 (range, 1.0-1.2) for healthy discs and 3.6 (range, 2.2-4.4) for degenerated discs in the scoliotic specimens (P = 0.008).

CONCLUSION

Disc pressure profilometry below long spinal constructs found asymmetric loading with the greatest loads at the concave inner anulus, especially in the presence of disc degeneration, scoliosis, and decompensation. For the degenerated cases, there was substantial disc pressure profile asymmetry despite only mildly severe scoliotic curvatures. These results suggest that scoliosis surgeons should minimize end-vertebra tilt, maximize lumbar curve, and balance correction at the time of surgical intervention. These results combined with prior animal studies suggest a compounding effect of asymmetric loading and progression of disc degeneration.

Can compressive stress be measured experimentally within the annulus fibrosus of degenerated intervertebral discs?

The aims were to assess the ability of a pressure transducer to measure compressive stress within the annulus fibrosus of degenerated intervertebral discs. Measurements could help to explain the mechanisms of disc failure and low back pain.

The methods used were as follows. Thirteen full-depth cores of annulus, 7 mm in diameter, were removed from the middle and outer annuli of two severely degenerated human discs and constrained within a metal cylinder. Then static compressive forces were applied by a planeended metal indenter of diameter 6.8 mm, while a strain-gauged pressure transducer, side mounted in a needle of diameter 0.9 mm and calibrated in saline, was pulled through the issue. The transducer output was converted into stress, and the average measured stress was compared with the nominal applied stress. Measurements were repeated at up to 21 load levels, with the transducer oriented vertically and horizontally.

The results showed that the measured and applied stress were linearly related (average r2=0.98) with a mean gradient (calibration factor) of 0.98 (vertical stress) and 0.92 (horizontal stress). Gradients ranged between 1.28 and 0.73. Damaged transducers grossly under-recorded ‘stress’ even though their output remained proportional to applied load.

It was concluded that pressure transducers can measure compressive stress inside a degenerated human annulus. The tissue is sufficiently deformable to allow efficient coupling of stress between the matrix and transducer membrane. Damage to the transducer can give misleading results.

Stress in lumbar intervertebral discs during distraction: a cadaveric study

BACKGROUND CONTEXT

The intervertebral disc is a common source of low back pain (LBP). Prospective studies suggest that treatments that intermittently distract the disc might be beneficial for chronic LBP. Although the potential exists for distraction therapies to affect the disc biomechanically, their effect on intradiscal stress is debated.

PURPOSE

To determine if distraction alone, distraction combined with flexion, or distraction combined with extension can reduce nucleus pulposus pressure and posterior annulus compressive stress in cadaveric lumbar discs compared with simulated standing or lying.

STUDY DESIGN

Laboratory study using single cadaveric motion segments.

OUTCOME MEASURES

Strain gauge measures of nucleus pulposus pressure and compressive stress in the anterior and posterior annulus fibrosus.

METHODS

Intradiscal stress profilometry was performed on 15 motion segments during 5 simulated conditions: standing, lying, and 3 distracted conditions. Disc degeneration was graded by inspection from 1 (normal) to 4 (severe degeneration).

RESULTS

All distraction conditions markedly reduced nucleus pressure compared with either simulated standing or lying. There was no difference between distraction with flexion and distraction with extension in regard to posterior annulus compressive stress. Discs with little or no degeneration appeared to distribute compressive stress differently than those with moderate or severe degeneration.

CONCLUSIONS

Distraction appears to predictably reduce nucleus pulposus pressure. The effect of distraction therapy on the distribution of compressive stress may be dependent in part on the health of the disc.

Determination of vertebral endplate deformation under load using micro-computed tomography

Endplate strength plays an important role in preventing vertebral failure of normal vertebrae and in cases where surgical intervention has replaced the disc with an implant or has altered the vertebral loading. We have developed a non-contact method based on the principles of image guided failure analysis, mechanical testing, and micro-computed tomography analysis, which allows for in vitro quantification of endplate deformation under axial load. The method allows for the implementation of a repeated measures experimental design, each specimen acting as its own control. Our methodology was validated using cadaveric functional spine units, loaded stepwise from 200 N to a maximum of 2000 N. The loading protocol was repeated over two days, allowing time for recovery of the disc mechanical properties. We found no meaningful difference in measured force, stiffness, and endplate deformation between day 1 and day 2. The mean fiducial registration error was less than 0.015 mm for all three axes. Endplate deformation could be reproducibly estimated. The root mean squared error was 0.03 mm, which is the effective precision of the method. Using this micro-CT based method, the effect of interbody implants, grafts, disc replacement strategies, and surgical procedures such as nucleotomy and vertebral cement augmentation on endplate mechanical behaviour can be ascertained.

The internal mechanical properties of cervical intervertebral discs as revealed by stress profilometry

Extensive anatomical differences suggest that cervical and lumbar discs may have functional differences also. We investigated human cervical discs using "stress profilometry". Forty-six cadaveric cervical motion segments aged 48-90 years were subjected to a compressive load of 200 N for 20 s, while compressive 'stress' was recorded along the posterior-anterior midline of the disc using a pressure transducer, side-mounted in a 0.9 mm diameter needle. Stress profiles were repeated with the transducer orientated horizontally and vertically, and with the specimen in neutral, flexed and extended postures. Profiles were repeated again following creep loading (150 N, 2 h) which simulated diurnal water loss in vivo. Stress profiles were reproducible, and measured "stress" at each location was proportional to applied load. Stress profiles usually showed a hydrostatic nucleus with regions of higher compressive stress concentrated anteriorly in flexion, and posteriorly in extension. Stress concentrations increased in degenerated discs and following creep. Some features were unique to cervical discs: many showed a stress gradient across their central regions, even though vertical and horizontal stresses were equal to each other, and stress concentrations in the posterior annulus were generally small. Central regions of many cervical discs show the characteristics of a "tethered fluid" which can equalise stress over small distances, but not large. This may be attributable to their fibrous texture. The small radial diameter of the cervical posterior annulus may facilitate buckling and thereby prevent it from sustaining high compressive stresses.

Torsion-induced pressure distribution changes in human intervertebral discs: an in vitro study

STUDY DESIGN

Biomechanical testing of human cadaveric lumbar specimens was performed to evaluate the effects of torsional torque on intradiscal pressure and disc height.

OBJECTIVE

Evaluate the effects of small torsion torques on intradiscal pressure and disc height in human lumbar specimens.

SUMMARY OF BACKGROUND DATA

Nuclear depressurization in addition to an instantaneous disc height increase were found in previous porcine research when small (<2 degrees) axial vertebral rotations were applied. If applicable to human spines, this phenomenon may support spinal manipulation for the relief of low back pain.

METHODS

Six human lumbar cadaveric functional spine units (FSU) were loaded in the neutral position with 600 N axial compression. Intranuclear pressure measurements were then obtained at 0, 0.5, 1.0, and 2.0 Nm of torsion. Posterior elements were removed and measurements were repeated for the disc body unit (DBU).

RESULTS

There was no statistically significant difference in nuclear pressure or intervertebral disc height with different torsion torques among or between the FSUs and DBUs. However, a disc height increase ranging from 0.13 mm to 0.16 mm occurred with the insertion of a 1.85-mm diameter pressure probe cannula.

CONCLUSIONS

Small torsion torques showed no significant difference in intradiscal pressures or disc heights. This is an unlikely mechanism for the perceived benefits of spinal manipulation.

Outer annulus tears have less effect than endplate fracture on stress distributions inside intervertebral discs: relevance to disc degeneration

BACKGROUND

Annulus tears and endplate fracture are common lesions in human intervertebral discs. Both cause degenerative changes in animal models, but the time course appears to be different. The purpose of the present experiment is to compare the effects of outer annulus tears and endplate fracture on intradiscal stresses. We hypothesise that endplate fracture provides a greater stimulus for disc degeneration.

METHODS

Seven cadaveric lumbar "motion segments" aged 49-70 years were compressed at 2 kN while the distribution of compressive stress was measured in each disc by pulling a 1.3 mm-diameter pressure transducer along its mid-sagittal diameter. Measurements were repeated after rim tears were simulated by 10 mm-deep scalpel cuts into the outer anterior annulus. The first cut was horizontal, 15 mm to the right of the disc midline, near the junction with the upper endplate. The second cut was vertical, 15 mm to the left of the disc midline, at mid-disc height. The third cut was horizontal, in the disc midline and at mid-disc height, so that the cut passed through the needle hole of the pressure transducer. Stress profiles were recorded in three postures and at two load levels, after each cut. Stress measurements were repeated a final time following compressive overload sufficient to fracture the endplate.

FINDINGS

Outer annulus tears had negligible effect on compressive stress distributions in the annulus fibrosus more than 15 mm from the scalpel cut, and they caused nucleus pressure to fall by only 1% (SD 1.3%, NS). In contrast, endplate fracture reduced nucleus pressure by 37% (P=0.004) and increased maximum stress within the posterior annulus by 93% (P=0.033).

INTERPRETATION

Outer annulus tears have less (immediate) effects on intradiscal compressive stresses than endplate fracture, supporting our hypothesis.

Intervertebral disc degeneration can predispose to anterior vertebral fractures in the thoracolumbar spine

Mechanical experiments on cadaveric thoracolumbar spine specimens showed that intervertebral disc degeneration was associated with reduced loading of the anterior vertebral body in upright postures. Reduced load bearing corresponded to locally reduced BMD and inferior trabecular architecture as measured by histomorphometry. Flexed postures concentrated loading on the weakened anterior vertebral body, leading to compressive failure at reduced load.

INTRODUCTION

Osteoporotic fractures are usually attributed to age-related hormonal changes and inactivity. However, why should the anterior vertebral body be affected so often? We hypothesized that degenerative changes in the adjacent intervertebral discs can alter load bearing by the anterior vertebral body in a manner that makes it vulnerable to fracture.

MATERIALS AND METHODS

Forty-one thoracolumbar spine "motion segments" (two vertebrae and the intervertebral disc) were obtained from cadavers 62-94 years of age. Specimens were loaded to simulate upright standing and flexed postures. A pressure transducer was used to measure the distribution of compressive "stress" inside the disc, and stress data were used to calculate how compressive loading was distributed between the anterior and posterior halves of the vertebral body and the neural arch. The compressive strength of each specimen was measured in flexed posture. Regional volumetric BMD and histomorphometric parameters were measured.

RESULTS

In the upright posture, compressive load bearing by the neural arch increased with disc degeneration, averaging 63 +/- 22% (SD) of applied load in specimens with severely degenerated discs. In these specimens, the anterior half of the vertebral body resisted only 10 +/- 8%. The anterior third of the vertebral body had a 20% lower trabecular volume fraction, 16% fewer trabeculae, and 28% greater intertrabecular spacing compared with the posterior third (p < 0.001). In the flexed posture, flexion transferred 53-59% of compressive load bearing to the anterior half of the vertebral body, regardless of disc degeneration. Compressive strength measured in this posture was proportional to BMD in the anterior vertebral body (r2 = 0.51, p < 0.001) and inversely proportional to neural arch load bearing in the upright posture (r2 = 0.28, p < 0.001).

CONCLUSIONS

Disc degeneration transfers compressive load bearing from the anterior vertebral body to the neural arch in upright postures, reducing BMD and trabecular architecture anteriorly. This predisposes to anterior fracture when the spine is flexed.

Discogenic origins of spinal instability

STUDY DESIGN

Cadaveric motion segment experiment.

OBJECTIVE

To show how two physical aspects of disc degeneration (dehydration and endplate disruption) contribute to spinal instability.

SUMMARY OF BACKGROUND DATA

The origins of spinal instability and its associations with back pain are uncertain. METHODS.: Twenty-one cadaveric thoracolumbar motion segments aged 48 to 90 years were secured in cups of dental plaster and loaded simultaneously in bending and compression to simulate full flexion, extension, and lateral bending movements. Vertebral movements, recorded using a two-dimensional "MacReflex" motion analysis system, were analyzed to calculate neutral zone (NZ), range of motion (ROM), bending stiffness (BS), horizontal translational movements, and the location of the center of rotation (COR). Intradiscal "stresses" were measured by pulling a miniature pressure transducer through the disc along its midsagittal diameter. All experiments were repeated after each of two treatments, which simulated physical aspects of disc degeneration: creep loading to dehydrate the disc and compressive overload to disrupt the endplate. Results were analyzed using ANOVA and linear regression.

RESULTS

Motion segment height was reduced by 1.0 (SD 0.3) mm during creep and by a further 1.7 (0.6) mm after endplate disruption. In flexion and lateral bending, the combined treatments increased NZ and ROM by 89% to 298%, and increased the "instability index" (NZ/ROM) by 43% to 61%. Translational movements increased by 58% to 86%, whereas BS decreased by 42% to 48%. In extension, ROM and NZ were little affected, although the COR moved closer to the apophyseal joints. Measures of instability increased most in lateral bending, and following endplate disruption. Stress concentrations in the posterior anulus fibrosus increased markedly after endplate disruption.

CONCLUSIONS

Two physical aspects of disc degeneration (dehydration and endplate disruption) cause marked segmental instability. Back pain associated with instability may be attributable to stress concentrations in degenerated discs.

Dynamic stabilization in the surgical management of painful lumbar spinal disorders

STUDY DESIGN

A literature review.

OBJECTIVE

To evaluate the mechanisms of action and effectiveness of posterior dynamic stabilization devices in the management of painful spinal disorders.

SUMMARY OF BACKGROUND DATA

Dynamic stabilization may provide pain relief by altering the transmission of abnormal loads across the degenerated disc space.

METHODS

A Medline search was conducted.

RESULTS

Articles supporting abnormal load transmission across the disc space and clinical reviews of currently available posterior dynamic systems were included.

CONCLUSIONS

Posterior dynamic stabilization systems may provide benefit comparable to fusion techniques, but without the elimination of movement. Further study is required to determine optimal design and clinical indications.

Can vertebroplasty restore normal load-bearing to fractured vertebrae?

STUDY DESIGN

Cadaver motion segments were used to evaluate the effects of vertebroplasty on spinal loading following vertebral fracture.

OBJECTIVES

To determine if vertebroplasty reverses fracture-induced changes in the distribution of compressive stress in cadaver motion segments.

SUMMARY OF BACKGROUND DATA

Vertebroplasty involves reinforcement of vertebrae by injection of cement and is now being used increasingly to treat osteoporotic vertebral fractures. However, its effects on spinal load-bearing are largely unknown. We hypothesize that vertebroplasty, following vertebral fracture, helps to equalize stress acting on the intervertebral disc and adjacent vertebral bodies.

METHODS

Nineteen cadaver thoracolumbar motion segments (age 64-90 years) were induced to fracture by compressive overload. Specimens were then subjected to vertebroplasty, and subsequently creep loaded for 1 hour at 1.5 kN. The compressive stress acting on the intervertebral disc was measured before and after fracture, after vertebroplasty, and after creep, by pulling a pressure transducer mounted in a 1.3-mm needle across the disc's midsagittal diameter. This information was then used to calculate neural arch load-bearing. At each time point, measurements were also made of compressive stiffness.

RESULTS

Vertebral fracture reduced motion segment compressive stiffness, decompressed the adjacent nucleus, increased stress concentrations in the posterior anulus, and increased neural arch load-bearing, all by a significant amount. Vertebroplasty partially, but significantly, reversed all of these fracture-induced changes.

CONCLUSIONS

Vertebroplasty reduces stress concentrations in the anulus and neural arch resulting in a more even distribution of compressive stress on the intervertebral disc and adjacent vertebral bodies.

Intradiscal electrothermal therapy can alter compressive stress distributions inside degenerated intervertebral discs

STUDY DESIGN

Mechanical testing of cadaveric motion segments.

OBJECTIVES

To test the hypothesis that intradiscal electrothermal therapy (IDET) can affect the internal mechanical functioning of lumbar discs.

SUMMARY OF BACKGROUND DATA

The clinical efficacy of IDET is variable, and its mode of action uncertain.

METHODS

Eighteen lumbar motion segments (64-97 years old) were incubated at 37 degrees C. A miniature pressure transducer, side mounted in a 1.3-mm diameter needle, was used to measure the distribution of compressive "stress" along the midsagittal diameter of each disc while it was compressed at 1.5 kN. Measurements were repeated in 3 simulated postures. Standard IDET was performed using biplanar radiography to confirm the placement of the heating element and an independent thermocouple to measure temperature in the inner lateral anulus. Stress profilometry was repeated immediately after IDET.

RESULTS

Peak temperatures in the inner lateral anulus during IDET averaged 40.0 degrees C (standard deviation [STD] 2.3). Stress measurements repeated before IDET differed by less than 8%, and a sham IDET procedure produced no consistent changes. After IDET, pressure in the nucleus decreased by 6% to 13% (P < 0.05), and stress concentrations in the anulus were reduced by an average 0.28 MPa (P < 0.004). In 12 of the 18 specimens, anulus stress concentrations were reduced by more than 8%, and in these "responders," mean reduction was 78%. Stress concentrations were increased by more than 8% in 2 specimens.

CONCLUSIONS

IDET has a significant but inconsistent effect on compressive stresses within intervertebral discs. These results may partly explain the variable clinical success of IDET.

Intervertebral disc degeneration can lead to "stress-shielding" of the anterior vertebral body: a cause of osteoporotic vertebral fracture?

STUDY DESIGN

Mechanical testing of cadaveric lumbar motion segments.

OBJECTIVES

To test the hypothesis that degenerative changes in the intervertebral discs can influence loading of the anterior vertebral body in a manner that makes it vulnerable to fracture.

SUMMARY OF BACKGROUND DATA

Measurements of systemic bone loss do not fully explain the patterns of osteoporotic vertebral fractures.

METHODS

Thirty-three cadaveric lumbar motion segments (aged 19-82 years) were subjected to 2 kN of compressive loading while positioned to simulate habitual erect standing postures and forwards bending. Intradiscal stresses were measured in each posture by pulling a miniature pressure transducer along the midsagittal diameter of the disc. "Stress profiles" were then integrated over area to calculate the force acting on the anterior and posterior halves of the vertebral body. These forces were subtracted from the applied 2 kN to determine the compressive force on the neural arch.

RESULTS

In motion segments with nondegenerated discs, <5% of the compressive force was resisted by the neural arch, and forces on the vertebral body were always distributed evenly, irrespective of posture. However, with severely degenerated discs, neural arch load-bearing increased to 40% in the erect posture, and the compressive force on the vertebral body was concentrated anteriorly in forwards bending, and posteriorly in erect posture.

CONCLUSIONS

Severe disc degeneration causes the anterior vertebral body to be stress-shielded during the usual erect posture, and yet severely loaded whenever the spine is flexed. This could help to explain why this region is frequently the site of osteoporotic fracture, and why forward bending movements often precipitate the injury.

Neural arch load-bearing in old and degenerated spines

We validate a technique for measuring neural arch load-bearing in cadaveric spines, and use it to test the hypothesis that such load-bearing rises to high levels in old and degenerated spines. Fifty-nine cadaveric lumbar motion segments, aged 19-92 yr, were subjected to compressive creep loading to reduce intervertebral disc water content and height to in vivo levels. The distribution of compressive "stress" within the disc was then measured by pulling a miniature pressure transducer, side-mounted in a 1.3mm-diameter needle, along its mid-sagittal diameter. During these measurements, the motion segment was subjected to a compressive load of 2 kN, and positioned in 2 degrees of extension to simulate erect standing. Measurements of compressive "stress" were integrated over disc area, and this force subtracted from the applied 2 kN to give the force resisted by the neural arch. An empirical calibration factor was applied to normalise results from each disc to values obtained under conditions when all of the compressive force could be assumed to pass through the disc. Disc degeneration was graded macroscopically on a scale of 1-4. Validation tests showed that calculated values of disc loading were proportional to actual applied load (r(2)>0.96) and predicted it with errors of 2-8%. Neural arch load-bearing in non-degenerated specimens was generally less than 20%, but averaged 49% for specimens aged over 70 yr. Multiple regression showed that neural arch load bearing (%)=14.4 x disc degeneration score+0.46 x age-35. These results indicate a substantial shift in vertebral load-bearing with increasing age and degeneration.

Dynamic stabilization devices in the treatment of low back pain

Soft stabilization has an important role in the treatment of the degenerative lumbar spine. Fusion of one or two motion segments may not make a big difference in the total range of motion of the lumbar spine, but preserving flexibility of a motion segment may prevent adjacent segment disease and may permit disc replacement, even when facet joints need to be excised. If a favorable environment is created in the motion segment by unloading the disc and permitting near normal motion, the disc may be able to repair itself or may supplement the reparative potential of gene therapy. Although soft stabilization seems promising, one should take a cautious approach to any new implant system. An implant for fusion only has to serve a temporary stabilization until fusion has taken place; on the other hand, a soft stabilization system has to provide stability throughout its life. Implant loosening following fusion surgery is common in the presence of pseudarthrosis. After soft stabilization, the implant has to stay anchored to the bone despite allowing movement. This sounds like a daunting task. The flexibility of the implant system, however, should be able to protect it from loosening at the anchor point into the bone. Finally, the soft stabilization system is intended to load-share with the disc and the facet joint only partially and unloads the motion segment. Any mismatch between the kinematics of the implant system and the motion segment, in particular any discrepancy between their IAR, would result in the implant bearing unexpected load at certain ranges of motion. If that happens, it would guarantee an early implant failure or loosening. The need for strict bench testing in the laboratory, therefore, cannot be over-emphasized. The few soft stabilization systems that have had clinical applications so far have produced a clinical outcome comparable to that of fusion. No prospective randomized controlled trial has been reported yet, which is an essential requirement for practice of evidence-based medicine.

Internal Stress Distribution in Cervical Intervertebral Discs

There is concern that cervical interbody fusion can result in accelerated degenerative changes occurring at adjacent spinal levels. The cervical spine clearly evolved to be mobile. It would seem to be desirable for spinal surgeons to have an alternative to fusion, and spinal arthroplasty is an appealing concept. The Bristol Disc is a mechanical device comprising two articulating components that result in motion with 6 df. It has been shown to have favorable kinematics when compared with intact and fused cadaveric spines. The current study attempts to record changes in the distribution of stresses within cervical intervertebral discs adjacent to the artificial disc or a simulated fusion. The technique used to measure intradiscal stress distributions is based on earlier work by McNally and Adams on lumbar intervertebral discs. The study generated stress profiles through cervical intervertebral discs statically loaded in four different postures in addition to recording changes in intradiscal pressure within both the nucleus and the annulus during flexion. Similar stress profiles were recorded from intact specimens and those with the artificial joint inserted. The artificial joint resulted in reduced stresses in the annulus compared with spines with a simulated fusion. The study demonstrates how different testing conditions can result in researchers being confronted with paradoxical data, and the simulation of muscle forces is recommended.

Recent advances in lumbar spinal mechanics and their significance for modelling

Mathematical models are often used to quantify the overall forces and moments acting on the lumbar spine. However, if the purpose of the research is to explain how spinal tissues can be injured, it is necessary to distribute the overall forces and moments between (and within) different spinal structures, because it is the concentration of force which causes injury, and elicits pain. This paper reviews recent experimental evidence concerning the distribution of forces and moments acting on the lumbar spine. Lordotic postures increase loading of the posterior annulus and apophyseal joints, whereas moderately flexed postures tend to equalise compressive stress across the disc, and unload the apophyseal joints. Sustained compression reduces the volume and pressure of the nucleus pulposus, while increasing compressive stresses in the annulus and neural arch. Sustained compression also reduces disc height, giving some slack to collagen fibres in the intervertebral disc and ligaments, and causing them to resist bending less. Disc degeneration has a similar effect on disc height, and stress distributions. On the other hand, discs and ligaments can be subjected to greater bending moments following a period of sustained or repetitive bending, because sustained bending impairs the normal protective reflex from the back muscles, and repetitive bending fatigues the back muscles, reducing their ability to protect the spine. Incorporating this information into mathematical models will make them better able to identify which activities are most likely to injure the lumbar spine in life.

Stress distribution changes in bovine vertebrae just below the endplate after sustained loading

OBJECTIVE

To describe the pattern of stress distribution in the vertebral body just behind the endplate, and to document its changes due to sustained loading.

METHODS

Twelve fresh bovine coccygeal motion segments were dissected and tested. Each specimen was axially loaded with a sustained compressive force of 50% of its estimated compressive strength. Before loading, after 1.5 h and after 3 h of loading, the distribution of the axial pressure under the bottom vertebra (i.e., just below its top endplate) was recorded at three force levels (25%, 37.5% and 50% of the estimated compressive strength), using pressure-sensitive film.

RESULTS

Stress distribution over the endplate was found to be fairly uniform. At low compression forces, the stress was the highest centrally. With increased compression and after sustained compression the uniformity improved through a significant redistribution of stress to the periphery. No stress peaks were found to occur after sustained loading.

CONCLUSION

Stress peaks after sustained loading cannot explain the occurrence of endplate fractures in sustained cyclic compression in non-degenerated discs. Competing explanations, such as creep, and fatigue failure, would appear more likely candidates.

RELEVANCE

It has been hypothesised that compression induced fractures of the lumbar vertebral endplate constitute an important etiological factor for low back pain. Competing theories exist on the fracture mechanism in sustained loading and these would have different implications with respect to prevention. The present study evaluated one of these theories.

Spinal posture and prior loading history modulate compressive strength and type of failure in the spine: a biomechanical study using a porcine cervical spine model

OBJECTIVE

The purpose of this study was to investigate the effect of posture and loading history on the compressive strength and site of failure in the spine.

DESIGN

An in vitro experiment was performed using a porcine cervical model that provided a homogeneous population of young healthy spines.

BACKGROUND

The distribution of stresses amongst the many load bearing tissues of the spine is altered throughout the day by posture and the history of loading, but it is not clear how this modulates tissue damage or the risk of injury.

METHODS

48 porcine cervical spines were harvested and dissected into motion segments containing two vertebrae and the intervening disc (C3/4 and C5/6). Compressive loads and rotational torques (flexion/extension) were applied so that the effects of four loading histories (hydrated, neutral dehydration, flexed dehydration, superhydrated) and two failure postures (neutral, flexed) could be examined. Levels of dehydration were based on those reported over the course of a day. Dissection techniques and X-rays were used to document tissue damage. RESULTS. Specimens had a lower yield point (43--63%) and ultimate compressive strength (23--47%) when in a flexed posture than when in a neutral posture. When injured in a neutral posture, superhydrated specimens had a lower strength (22--29%) than dehydrated specimens. Loading history also modulated the site of failure.

CONCLUSIONS

The spine may be more prone to injury early in the morning when the discs are at their greatest level of hydration and/or when they are in a fully flexed posture.