Motion Segment Stiffness Research Articles

Biological performance of a bioabsorbable magnesium-magnesium phosphate cement interbody fusion cage in a porcine lumbar interbody fusion model: a feasibility study.

The aim of the study was to evaluate the feasibility of a bioabsorbable cage consisting of magnesium and magnesium phosphate cement (MPC) in a porcine lumbar interbody fusion model. Twelve male Ba-Ma mini pigs underwent lumbar discectomy and fusion with an Mg-MPC cage or a PEEK cage at the L3/L4 and L4/L5 level. Computed tomography (CT) scans were made to evaluate the distractive property by comparing average disc space height (DSH) before and at 6, 12, and 24 weeks after the operation. After the lumbar spines were harvested at 6 or 24 weeks after the operation, micro-CT examination was conducted to analyze the fusion rate, and stiffness of motion segments was investigated through mechanical tests. A histological study was performed to evaluate the tissue type, inflammation, and osteolysis in the intervertebral space. CT scans showed no significant difference between the two groups in average DSH at each time point. Micro-CT scans revealed an equal fusion rate in both groups (0% at 6 weeks, 83.3% at 24 weeks). Both groups showed time-dependent increases in stability, the Mg-MPC cages achieved an inferior stiffness at 6 weeks and a comparable stiffness at 24 weeks. Histologic evaluation showed the presence of newly formed bone in both groups. However, empty spaces were observed at the interface or around the Mg-MPC cages. Compared with the PEEK cages, the Mg-MPC cages achieved comparable distraction, fusion rate, and spinal stability at 24 weeks after the operation. However, due to inferior stiffness at the early stage and fast degradation, further modification of material composition and design are necessary.

The effect of compressive loading rate on annulus fibrosus strength following endplate fracture

Intervertebral disc degeneration poses a considerable healthcare challenge, although the process is not well understood. Endplate fracture marks severe biomechanical compromise in a segment and may be correlated with degeneration of the disc. The purpose of this experiment was to investigate the relationship between endplate fracture velocity and damage to the annulus fibrosus. Following overnight-thawing, 27 frozen porcine cervical spines were dissected into motion segments (vertebra-disc-vertebra) and compressed until fracture at one of three loading rates (fast=15 mm/s, medium=1.5 mm/s, and slow=0.15 mm/s), or remained unfractured (control). Two annular samples were extracted and mechanically tested from each segment: 1) Bilayer samples underwent uniaxial tension to a stretch-ratio of 1.5; 2) Multilayer samples were delaminated with a 180° peel test configuration. All three rates of compression resulted in specimen fracture observed in the endplate and/or vertebra with varying degree of severity. Significant differences were detected in compressive strength and stiffness of motion segments when loaded at different rates of compression; interestingly these differences were not observed in the mechanical properties of the annulus fibrosus suggesting that at slow rates of loading, fracture of the endplate precedes destruction of the annulus fibrosus. In corroboration of these findings, gross and histological analysis reported no signs of annular disruption, strengthening assertions that annular damage did not occur.

The effect of posterior tethers on the biomechanics of proximal junctional kyphosis: The whole human finite element model analysis

Little is known about the effects of posterior tethers on the development of proximal junctional kyphosis (PJK). We evaluated the ability of posterior tethers to the proximal motion segment stiffness in long instrumented spinal instrumentation and fusion using a whole body human FE model. A series of finite element (FE) analysis of long segmental spinal fusion (SF) from the upper thoracic vertebra (T1) or lower thoracic vertebra (T9) to the sacrum with pedicle screws and rods were performed using an entire human body FE model (includes 234,910 elements), and compressive stresses (CS) on the anterior column, and tensile stresses (TS) on the posterior ligamentous complex (PLC) in the upper-instrumented vertebra (UIV) and the vertebra adjacent to the UIV (UIV + 1) were evaluated with posterior tethers or without posterior tethers. The models were tested at three T1 tilts (0, 20, 40 deg.), with 20% muscle contraction. Deformable material models were assigned to all body parts. Muscle-tendon complexes were modeled by truss elements with a Hill-type muscle material model. The CS of anterior column decreased with increasing T1 slope with tethers in both models, while the CS remained relatively large in T9 model compared with T1 model (T1 UIV; 0.96 to 1.56 MPa, T9 UIV; 4.79 to 5.61 MPa). The TS of the supraspinous ligament was markedly reduced in both T1 and T9 models with posterior tethers (11–35%). High vertebral CS on UIV and UIV + 1 were seen in the T9 UIV model, and the TS on the PLC were increased in both UIV models. Posterior tethers may decrease PJK development after SF with a proximal thoracic UIV, while both posterior tethers and vertebral augmentation may be necessary to reduce PJK development with a lower thoracic UIV.

The effect of vertebral body stapling on spine biomechanics and structure using a bovine model

BackgroundAdolescent idiopathic scoliosis is a common condition affecting 2.5% of the general population. Vertebral body stapling was introduced as a method of fusionless growth modulation for the correction of moderate idiopathic scoliosis (Cobb angles of 20–40°), and was claimed to be more effective than bracing and less invasive than fusion.The aim of this study was to assess the effect of vertebral body stapling on the stiffness of a thoracic motion segment unit under moment controlled load, and to assess the vertebral structural damage caused by the staples. MethodsThoracic spine motion segments from 6 to 8 week old calves (n=14) were tested in flexion/extension, lateral bending, and axial rotation. The segments were tested un-instrumented, then a left anterolateral intervertebral Shape Memory Alloy (SMA) staple was inserted and the test was repeated. Data were collected from the tenth load cycle of each sequence and stiffness was calculated. The staples were carefully removed and the segments were studied with micro-computed tomography to assess physical damage to the bony structure. Visual assessment of the vertebral bone structure on micro-CT was performed. FindingsThere was no change in motion segment stiffness in flexion/extension nor in axial rotation. There was a reduction in stiffness in lateral bending with 30% reduction bending away from the staple and 12% reduction bending towards the staple. Micro-CT showed physeal damage in all the specimens. InterpretationIntervertebral stapling using SMA staples cause a reduction in spine stiffness in lateral bending. They also cause damage to the endplate epiphyses.

Resorbable plating system stabilizes tissue-engineered intervertebral discs implanted ex vivo in canine cervical spines.

Total disc replacement using tissue‐engineered intervertebral discs (TE‐IVDs) may offer a biological alternative to treat radiculopathy caused by disc degeneration. A composite TE‐IVD was previously developed and evaluated in rat tail and beagle cervical spine models in vivo. Although cell viability and tissue integration into host tissue were promising, significant implant displacement occurred at multiple spinal levels. The goal of the present study was to assess the effects of a resorbable plating system on the stiffness of motion segments and stability of tissue‐engineered implants subjected to axial compression. Canine motion segments from levels C2/C3 to C5/C6 were assessed as intact (CTRL), after discectomy (Dx), with an implanted TE‐IVD only (PLATE−), and with a TE‐IVD combined with an attached resorbable plate (PLATE+). Segments under PLATE+ conditions fully restored separation between endplates and showed significantly higher compressive stiffness than segments under PLATE− conditions. Plated segments partially restored more than 25% of the CTRL motion segment stiffness. Plate attachment also prevented implant extrusion from the disc space at 50% compressive strain, and this effect was more significant in segments from levels C3/C4 when compared to segments from level C5/C6. These results suggest that stabilization of motion segments via resorbable plating assists TE‐IVD retention in the disc space while allowing the opportunity for implants to fully integrate into the host tissue and achieve optimal restoration of spine biomechanics.

Does Resection of the Posterior Longitudinal Ligament Affect the Stability of Cervical Disc Arthroplasty?

The need for posterior longitudinal ligament (PLL) resection during cervical total disc arthroplasty (TDA) has been debated. The purpose of this laboratory study was to investigate the effect of PLL resection on cervical kinematics after TDA. Eight cadaveric cervical spine specimens were tested in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) to moments of ±1.5 Nm. After testing the intact condition, anterior C5-C6 cervical discectomy was performed followed by PLL resection and implantation of a compressible, 6-degrees-of-freedom disc prosthesis (M6-C, Spinal Kinetics Inc, Sunnyvale, California). Next, a second prosthesis was implanted at C6-C7 with PLL intact. Finally, the C6-C7 PLL was resected while the disc prosthesis remained in place. Segmental range of motion (ROM) and stiffness in the high flexibility zone around the neutral posture were analyzed using repeated measures ANOVA. At C5-C6, following TDA and PLL resection, FE, LB, and AR ROMs decreased significantly. Anterior and posterior disc height, segmental lordosis, and flexion stiffness increased significantly. At C6-C7, TDA with the PLL intact resulted in a significant increase in anterior disc height and segmental lordosis with no change in posterior disc height. FE, LB, and AR ROMs all decreased significantly, while flexion stiffness increased significantly compared to intact. PLL resection at C6-C7 did not result in a notable change compared to TDA with PLL intact. At the same level, flexion stiffness decreased following PLL resection compared to TDA with a value closer to intact. Two-level TDA (C5-C7) with PLL resection did not result in a loss of segmental stability. PLL resection did not significantly affect motion segment kinematics following cervical TDA using a prosthesis with inherent stiffness. Motion segment stiffness loss after PLL resection can be compensated for by a TDA design that can provide resistance to angular motion.

The effect of posterior polyester tethers on the biomechanics of proximal junctional kyphosis: a finite element analysis.

OBJECTIVE Proximal junctional kyphosis (PJK) remains problematic following multilevel instrumented spine surgery. Previous biomechanical studies indicate that providing less rigid fixation at the cranial aspect of a long posterior instrumented construct, via transition rods or hooks at the upper instrumented vertebra (UIV), may provide a gradual transition to normal motion and prevent PJK. The purpose of this study was to evaluate the ability of posterior anchored polyethylene tethers to distribute proximal motion segment stiffness in long instrumented spine constructs. METHODS A finite element model of a T7-L5 spine segment was created to evaluate range of motion (ROM), intradiscal pressure, pedicle screw loads, and forces in the posterior ligament complex within and adjacent to the proximal terminus of an instrumented spine construct. Six models were tested: 1) intact spine; 2) bilateral, segmental pedicle screws (PS) at all levels from T-11 through L-5; 3) bilateral pedicle screws from T-12 to L-5 and transverse process hooks (TPH) at T-11 (the UIV); 4) pedicle screws from T-11 to L5 and 1-level tethers from T-10 to T-11 (TE-UIV+1); 5) pedicle screws from T-11 to L-5 and 2-level tethers from T-9 to T-11 (TE-UIV+2); and 6) pedicle screws and 3-level tethers from T-8 to T-11 (TE-UIV+3). RESULTS Proximal-segment range of motion (ROM) for the PS construct increased from 16% at UIV-1 to 91% at UIV. Proximal-segment ROM for the TPH construct increased from 27% at UIV-1 to 92% at UIV. Posterior tether constructs distributed ROM at the UIV and cranial adjacent segments most effectively; ROM for TE-UIV+1 was 14% of the intact model at UIV-1, 76% at UIV, and 98% at UIV+1. ROM for TE-UIV+2 was 10% at UIV-1, 51% at UIV, 69% at UIV+1, and 97% at UIV+2. ROM for TE-UIV+3 was 7% at UIV-1, 33% at UIV, 45% at UIV+1, and 64% at UIV+2. Proximal segment intradiscal pressures, pedicle screw loads, and ligament forces in the posterior ligament complex were progressively reduced with increasing number of posterior tethers used. CONCLUSIONS Finite element analysis of long instrumented spine constructs demonstrated that posterior tethers created a more gradual transition in ROM and adjacent-segment stress from the instrumented to the noninstrumented spine compared with all PS and TPH constructs. Posterior tethers may limit the biomechanical risk factor for PJK; however, further clinical research is needed to evaluate clinical efficacy.

Intervertebral reaction force prediction using an enhanced assembly of OpenSim models

OpenSim offers a valuable approach to investigating otherwise difficult to assess yet important biomechanical parameters such as joint reaction forces. Although the range of available models in the public repository is continually increasing, there currently exists no OpenSim model for the computation of intervertebral joint reactions during flexion and lifting tasks. The current work combines and improves elements of existing models to develop an enhanced model of the upper body and lumbar spine. Models of the upper body with extremities, neck and head were combined with an improved version of a lumbar spine from the model repository. Translational motion was enabled for each lumbar vertebrae with six controllable degrees of freedom. Motion segment stiffness was implemented at lumbar levels and mass properties were assigned throughout the model. Moreover, body coordinate frames of the spine were modified to allow straightforward variation of sagittal alignment and to simplify interpretation of results. Evaluation of model predictions for level L1–L2, L3–L4 and L4–L5 in various postures of forward flexion and moderate lifting (8 kg) revealed an agreement within 10% to experimental studies and model-based computational analyses. However, in an extended posture or during lifting of heavier loads (20 kg), computed joint reactions differed substantially from reported in vivo measures using instrumented implants. We conclude that agreement between the model and available experimental data was good in view of limitations of both the model and the validation datasets. The presented model is useful in that it permits computation of realistic lumbar spine joint reaction forces during flexion and moderate lifting tasks. The model and corresponding documentation are now available in the online OpenSim repository.

Is kyphoplasty better than vertebroplasty at restoring form and function after severe vertebral wedge fractures?

Background contextThe vertebral augmentation procedures, vertebroplasty and kyphoplasty, can relieve pain and facilitate mobilization of patients with osteoporotic vertebral fractures. Kyphoplasty also aims to restore vertebral body height before cement injection and so may be advantageous for more severe fractures. PurposeThe purpose of this study was to compare the ability of vertebroplasty and kyphoplasty to restore vertebral height, shape, and mechanical function after severe vertebral wedge fractures. Study design/settingThis is a biomechanical and radiographic study using human cadaveric spines. MethodsSeventeen pairs of thoracolumbar “motion segments” from cadavers aged 70–98 years were injured, in a two-stage process involving flexion and compression, to create severe anterior wedge fractures. One of each pair underwent vertebroplasty and the other kyphoplasty. Specimens were then compressed at 1 kN for 1 hour to allow consolidation. Radiographs were taken before and after injury, after treatment, and after consolidation. At these same time points, motion segment compressive stiffness was assessed, and intervertebral disc “stress profiles” were obtained to characterize the distribution of compressive stress on the vertebral body and neural arch. ResultsOn average, injury reduced anterior vertebral body height by 34%, increased its anterior wedge angle from 5.0° to 11.4°, reduced intradiscal (nucleus) pressure and motion segment stiffness by 96% and 44%, respectively, and increased neural arch load bearing by 57%. Kyphoplasty caused 97% of the anterior height loss to be regained immediately, although this reduced to 79% after consolidation. Equivalent gains after vertebroplasty were significantly lower: 59% and 47%, respectively (p<.001). Kyphoplasty reduced vertebral wedging more than vertebroplasty (p<.02). Intradiscal pressure, neural arch load bearing, and motion segment compressive stiffness were restored significantly toward prefracture values after both augmentation procedures, even after consolidation, but these mechanical effects were similar for kyphoplasty and vertebroplasty. ConclusionsAfter severe vertebral wedge fractures, vertebroplasty and kyphoplasty were equally effective in restoring mechanical function. However, kyphoplasty was better able to restore vertebral height and reverse wedge deformity.

The effect of repeated loading and freeze-thaw cycling on immature bovine thoracic motion segment stiffness.

There is growing interest in the biomechanics of "fusionless" implant constructs used for deformity correction in the thoracic spine; however, there are questions over the comparability of in vitro biomechanical studies from different research groups due to the various methods used for specimen preparation, testing and data collection. The aim of this study was to identify the effect of two key factors on the stiffness of immature bovine thoracic spine motion segments: (1) repeated cyclic loading and (2) multiple freeze-thaw cycles, to aid in the planning and interpretation of in vitro studies. Two groups of thoracic spine motion segments from 6- to 8-week-old calves were tested in flexion/extension, right/left lateral bending and right/left axial rotation under moment control. Group A was tested with continuous repeated cyclic loading for 500 cycles with data recorded at cycles 3, 5, 10, 25, 50, 100, 200, 300, 400 and 500. Group (B) was tested after each of five freeze-thaw sequences, with data collected from the 10th load cycle in each sequence. Results of testing showed that for Group A: flexion/extension stiffness reduced significantly over the 500 load cycles (-22%; p = 0.001), but there was no significant change between the 5th and 200th load cycles. Lateral bending stiffness decreased significantly (-18%; p = 0.009) over the 500 load cycles, but there was no significant change in axial rotation stiffness (p = 0.137). Group B: there was no significant difference between mean stiffness over the five freeze-thaw sequences in flexion/extension (p = 0.813) and a near-significant reduction in mean stiffness in axial rotation (-6%; p = 0.07). However, there was a statistically significant increase in stiffness in lateral bending (+30%; p = 0.007). Study findings indicate that comparison of in vitro testing results for immature thoracic bovine spine segments between studies can be performed with up to 200 load cycles without significant changes in stiffness. However, when testing protocols require greater than 200 cycles, or when repeated freeze-thaw cycles are involved, it is important to account for the effect of cumulative load and freeze-thaw cycles on spine segment stiffness.

Intervertebral discs from spinal nondeformity and deformity patients have different mechanical and matrix properties

Background contextIt is well-established that disc mechanical properties degrade with degeneration. However, prior studies utilized cadaveric tissues from donors with undefined back pain history. Disc degeneration may present with pain at the affected motion segment, or it may be present in the absence of back pain. The mechanical properties and matrix quantity of discs removed and diagnosed for degeneration with patient chronic pain may be distinct from those with other diagnoses, such as spinal deformity. PurposeTo test the hypothesis that discs from nondeformity segments have inferior mechanical properties than deformity discs owing to differences in matrix quality. Study design/settingIn vitro study comparing the mechanical and matrix properties of discs from surgery patients with spinal nondeformity and deformity. MethodsWe analyzed nucleus and annulus samples (8–11 specimens per group) from surgical discectomy patients as part of a fusion or disc replacement procedure. Tissues were divided into two cohorts: nondeformity and deformity. Dynamic indentation tests were used to determine energy dissipation, indentation modulus, and viscoelasticity. Tissue hydration at a physiologic pressure was assessed by equilibrium dialysis. Proteoglycan, collagen, and collagen cross-link content were quantified. Matrix structure was assessed by histology. ResultsWe observed that energy dissipation was significantly higher in the nondeformity nucleus than in the deformity nucleus. Equilibrium dialysis experiments showed that annulus swelling was significantly lower in the nondeformity group. Consistent with this, we observed that the nondeformity annulus had lower proteoglycan and higher collagen contents. ConclusionsOur data suggest that discs from nondeformity discs have subtle differences in mechanical properties compared with deformity discs. These differences were partially explained by matrix biochemical composition for the annulus, but not for the nucleus. The results of this study suggest that compromised matrix quality and diminished mechanical properties are features that potentially accompany discs of patients undergoing segmental fusion or disc replacement for disc degeneration and chronic back pain. These features have previously been implicated in pain via instability or reduced motion segment stiffness.

Posteriorly Directed Shear Loads and Disc Degeneration Affect the Torsional Stiffness of Spinal Motion Segments

Finite element study. To analyze the effects of posterior shear loads, disc degeneration, and the combination of both on spinal torsion stiffness. Scoliosis is a 3-dimensional deformity of the spine that presents itself mainly in adolescent girls and elderly patients. Our concept of its etiopathogenesis is that an excess of posteriorly directed shear loads, relative to the body's intrinsic stabilizing mechanisms, induces a torsional instability of the spine, making it vulnerable to scoliosis. Our hypothesis for the elderly spine is that disc degeneration compromises the stabilizing mechanisms. In an adult lumbar motion segment model, the disc properties were varied to simulate different aspects of disc degeneration. These models were then loaded with a pure torsion moment in combination with either a shear load in posterior direction, no shear, or a shear load in anterior direction. Posteriorly directed shear loads reduced torsion stiffness, anteriorly directed shear loads increased torsion stiffness. These effects were mainly caused by a later (respectively earlier) onset of facet joint contact. Disc degeneration cases with a decreased disc height that leads to slackness of the annular fibers and ligaments caused a significantly decreased torsional stiffness. The combination of this stage with posterior shear loading reduced the torsion stiffness to less than half the stiffness of a healthy disc without shear loads. The end stage of disc degeneration increased torsion stiffness again. The combination of a decreased disc height, that leads to slack annular fibers and ligaments, and posterior shear loads very significantly affects torsional stiffness: reduced to less than half the stiffness of a healthy disc without shear loads. Disc degeneration, thus, indeed compromises the stabilizing mechanisms of the elderly spine. A combination with posteriorly directed shear loads could then make it vulnerable to scoliosis. N/A.

Which factors prognosticate rotational instability following lumbar laminectomy?

Reduced strength and stiffness of lumbar spinal motion segments following laminectomy may lead to instability. Factors that predict shear biomechanical properties of the lumbar spine were previously published. The purpose of the present study was to predict spinal torsion biomechanical properties with and without laminectomy from a total of 21 imaging parameters. Radiographs and MRI of ten human cadaveric lumbar spines (mean age 75.5, range 59-88 years) were obtained to quantify geometry and degeneration of the motion segments. Additionally, dual X-ray absorptiometry (DXA) scans were performed to measure bone mineral content and density. Facet-sparing lumbar laminectomy was performed either on L2 or L4. Spinal motion segments were dissected (L2-L3 and L4-L5) and tested in torsion, under 1,600 N axial compression. Torsion moment to failure (TMF), early torsion stiffness (ETS, at 20-40 % TMF) and late torsion stiffness (LTS, at 60-80 % TMF) were determined and bivariate correlations with all parameters were established. For dichotomized parameters, independent-sample t tests were used. Univariate analyses showed that a range of geometric characteristics and disc and bone quality parameters were associated with torsion biomechanical properties of lumbar segments. Multivariate models showed that ETS, LTS and TMF could be predicted for segments without laminectomy (r (2) values 0.693, 0.610 and 0.452, respectively) and with laminectomy (r (2) values 0.952, 0.871 and 0.932, respectively), with DXA-derived measures of bone quality and quantity as the main predictors. Vertebral bone content and geometry, i.e. intervertebral disc width, frontal area and facet joint tropism, were found to be strong predictors of ETS, LTS and TMF following laminectomy, suggesting that these variables could predict the possible development of post-operative rotational instability following lumbar laminectomy. Proposed diagnostic parameters might aid surgical decision-making when deciding upon the use of instrumentation techniques.

The Use of a Novel Injectable Hydrogel Nucleus Pulposus Replacement in Restoring the Mechanical Properties of Cyclically Fatigued Porcine Intervertebral Discs

Repeated flexion and extension of an intervertebral disc has been shown to affect the angular stiffness of spinal motion segments and is a barometer of the mechanical integrity of the disc. A degenerated disc that loses height causes higher levels of stress on the annulus and facet joints which may increase its level of degeneration; restoring disc height may therefore help to slow this degenerative cascade. Previous research has indicated that nucleus implants have the potential to improve the mechanical characteristics of a disc and an implant that is custom-fit to the intervertebral disc yields the best results with respect to decreasing annular degeneration. Two groups of porcine spinal motion segments were exposed to repeated flexion and extension. One group was then injected with a novel hydrogel while the other group was used as a control. Both groups were then exposed to another round of cyclic flexion and extension to examine the effect that the hydrogel had on restoring the original mechanics to the motion segments. Angular stiffness was restored to the group which received the hydrogel injection in addition to a significant improvement in specimen height. No significant changes were seen in the group which did not receive an injection. It would appear that use of the novel injectable hydrogel is able to restore angular stiffness to cyclically fatigued spinal motion segments. It is also important to note that continued repetition of the event causing specimen fatigue after performing hydrogel injection will result in an eventual return to the same fatigued state.

Mesenchymal Stem Cells Mediate Disk Regeneration by Suppression of Fibrosis in Nucleus Pulposus

Introduction Confluent fibrosis, manifested as over-accumulation of fibrous matrix during tissue repair or remodeling, leads to scar formation and often organ failure. Studies suggest that 68% of protruded intervertebral disks and 44% of prolapsed IVD exhibit signs of fibrosis in the nucleus pulposus (NP) and that disk scarring indicates advanced stages of degeneration. It is not clear how fibrosis contributes to the initiation or progression of disk degeneration, and conversely if the prevention of fibrotic events plays a role in IVD repair or regeneration. Mesenchymal stem cells (MSCs) may arrest IVD degeneration and cross-talk with IVD cells via cell-cell contact or long-range signaling, implying MSCs may modulate disk microenvironment and function indirectly. Lupine is a reliable model for IVD degeneration studies as their lumbar spine bears biological relevance to humans regarding mechanics and age-related changes. Studies showed that puncturing to the annulus could transform the NP into a fibrocartilaginous phenotype, recapitulating the fibrotic features of human disk degeneration. We here tested if MSCs can rectify IVD degeneration and its function through modulation of fibrotic events, and investigated the pathways of the action. Materials and Methods Total 6-month-old NZW rabbits with puncture-induced lumbar disk degeneration were intradiscally implanted with Chinchilla bone marrow MSCs, labeled by Qtracker and encapsulated in oligopeptide-hydrogel at 5 x 10E3 cells per disk ( n = 8). Saline or hydrogel alone was injected as control. Treated disk levels were monitored by 3T MRI and radiographs up to 1 year. At 4 to 12 weeks posttreatment, the mechanical integrity of NP and joint segment were investigated by confined compression and motion stiffness tests. The collagen fibril diameter ( n = 120) and the porosity of the fibril meshwork in the NP were analyzed in situ using scanning electron microscopy and morphometric analyzer. The elastic modulus of collagen fibrils ( n = 75) in the NP was measured by nanoindentation using atomic force microscopy. Collagen I and II content was assessed by immunofluorescence. Human bone marrow MSC-conditioned media (MSC-CM) was collected and used to culture alginate-encapsulated NP cells harvested from patients with Pfirrmann grade III/IV disk degeneration. After 7 days, expression of fibrotic genes was studied using real-time PCR ( n = 3). Results T2-weighted MRI and disk height analysis validated that MSCs arrested the degeneration. Motion segment stiffness was significantly attenuated in MSC-treated disks, indicating MSCs alleviated ankylosis caused by the degeneration and restored the spinal segment function. MSCs regenerated the swelling pressure and compressive strength of the NP, implying that MSCs regenerated joint function through preserving the biphasic tissue properties. Collagen molecules in the degenerated NP aggregated into gigantic fibrils, about 2-fold larger in diameter, resulting in fewer fibers and larger interfibrillar space. However, MSCs inhibited the deregulated higher-order assembly of fibrils. MSCs significantly reduced collagen I deposition and prevented the increase of fibril elastic modulus in the NP associated with degeneration. These suggest that the abnormal collagen fibrils are not solely resulted from fibril fusion but also a heterotypic change of collagen composition, and that the positive effects of MSCs are coupled to a regulation of fibrillogenesis in the NP matrix. MSC-CM repressed the expression of fibrosis-related genes MMP12 and HSP47 in degenerative human NP cells. Conclusion MSC transplantation can alleviate fibrotic diseases including pulmonary, renal, and cardiac fibrosis through pathways independent of MSC differentiation. Our findings imply that MSCs also rescue disk degeneration and recover disk function by suppressing fibrotic events. The action of MSCs is elicited through a regulation of fibrillogenesis in part via paracrine signaling to the degenerative NP cells. We propose a model in which MSCs reinforce the mechanical integrity of NP by normalizing the interplay between collagen meshwork and proteoglycan. Disk repair by augmenting the antifibrotic function of MSCs or use of other antifibrotic agents warrants further investigation. This study is supported by the Research Grant Council of Hong Kong and HKU Foundation. I confirm having declared any potential conflict of interest for all authors listed on this abstract Yes Disclosure of Interest None declared

Which factors prognosticate spinal instability following lumbar laminectomy?

PurposeReduced strength and shear stiffness (SS) of lumbar motion segments following laminectomy may lead to instability. The purpose of the present study was to assess a broad range of parameters as potential predictors of shear biomechanical properties of the lumbar spine.MethodsRadiographs and MRI of all lumbar spines were obtained to classify geometry and degeneration of the motion segments. Additionally, dual X-ray absorptiometry (DXA) scans were performed to measure bone mineral content and density (BMC and BMD). Facet sparing lumbar laminectomy was performed either on L2 or L4, in 10 human cadaveric lumbar spines (mean age 72.1 years, range 53–89 years). Spinal motion segments were dissected (L2–L3 and L4–L5) and tested in shear, under simultaneously loading with 1600 N axial compression. Shear stiffness, shear yield force (SYF) and shear force to failure (SFF) were determined and statistical correlations with all parameters were established.ResultsFollowing laminectomy, SS, SYF, and SFF declined (by respectively 24, 41, and 44%). For segments with laminectomy, SS was significantly correlated with intervertebral disc degeneration and facet joint degeneration (Pfirrmann: r = 0.64; Griffith: r = 0.70; Lane: r = 0.73 and Pathria: r = 0.64), SYF was correlated with intervertebral disc geometry (r = 0.66 for length; r = 0.66 for surface and r = 0.68 for volume), BMC (r = 0.65) and frontal area (r = 0.75), and SFF was correlated with disc length (r = 0.73) and BMC (r = 0.81). For untreated segments, SS was significantly correlated with facet joint tropism (r = 0.71), SYF was correlated with pedicle geometry (r = 0.83), and SFF was correlated with BMC (r = 0.85), BMD (r = 0.75) and frontal area (r = 0.75). SS, SYF and SFF could be predicted for segments with laminectomy (r 2 values respectively: 0.53, 0.81 and 0.77) and without laminectomy (r 2 value respectively: 0.50, 0.83 and 0.83).ConclusionsSignificant loss of strength and SS are predicted by BMC, BMD, intervertebral disc geometry and degenerative parameters, suggesting that low BMC or BMD, small intervertebral discs and absence of osteophytes could predict the possible development of post-operative instability following lumbar laminectomy.

Influence of Interpersonal Geometrical Variation on Spinal Motion Segment Stiffness

A validated finite element model of an L3-L4 motion segment is used to analyze the effects of interpersonal differences in geometry on spinal stiffness. The objective of this study is to determine which of the interpersonal variations of the geometry of the spine have a large effect on spinal stiffness. This will improve patient-specific modeling. The parameters that define the geometry of a motion segment are vertebral height, disc height, endplate width, endplate depth, spinous process length, transverse process width, nucleus size, lordosis angle, facet area, facet orientation, and the cross-sectional areas of the ligaments. All these parameters differ between patients. The influence of each parameter on spinal stiffness is largely unknown and such knowledge would greatly help in patient-specific modeling of the spine. The range of interpersonal variation of each of the geometric parameters was set at mean±2SD (covering 95% of the population). Subsequently, we determined the effect of each of these ranges on the bending stiffness in flexion, extension, axial rotation, and lateral bending. Disc height had the largest influence; a maximal disc height reduced the spinal stiffness to 75-86% of the mean motion segment stiffness, and a minimal disc height increased the spinal stiffness to 154-226% of the mean motion segment stiffness. Lordosis angle, transversal and longitudinal facet angle, endplate depth, and area of the capsular ligament also had a substantial influence (>5%) on the stiffness, but considerable less than the influence of the disc height. Ligament areas, nucleus size, spinous process length, and length of processes are of negligible effect (<2%) on the stiffness. The disc height should be accurately determined in patients to estimate the spinal stiffness. Ligament areas, nucleus size, spinous process length, and transverse process width do not need patient-specific modeling.

The effects of creep and recovery on the in vitro biomechanical characteristics of human multi-level thoracolumbar spinal segments

BackgroundSeveral physiological and pathological conditions in daily life cause sustained static bending or torsion loads on the spine resulting in creep of spinal segments. The objective of this study was to determine the effects of creep and recovery on the range of motion, neutral zone, and neutral zone stiffness of thoracolumbar multi-level spinal segments in flexion, extension, lateral bending and axial rotation. MethodsSix human cadaveric spines (age at time of death 55–84years) were sectioned in T1–T4, T5–T8, T9–T12, and L1–L4 segments and prepared for testing. Moments were applied of +4 to −4Nm in flexion-extension, lateral bending, and axial rotation. This was repeated after 30min of creep loading at 2Nm in the tested direction and after 30min of recovery. Displacement of individual motion segments was measured using a 3D optical movement registration system. The range of motion, neutral zone, and neutral zone stiffness of the middle motion segments were calculated from the moment-angular displacement data. FindingsThe range of motion increased significantly after creep in extension, lateral bending and axial rotation (P<0.05). The range of motion after flexion creep showed an increasing trend as well, and the neutral zone after flexion creep increased by on average 36% (P<0.01). The neutral zone stiffness was significantly lower after creep in axial rotation (P<0.05). InterpretationThe overall flexibility of the spinal segments was in general larger after 30min of creep loading. This higher flexibility of the spinal segments may be a risk factor for potential spinal instability or injury.

Combining 3D tracking and surgical instrumentation to determine the stiffness of spinal motion segments: A validation study

Abstract The spine is a complex structure that provides motion in three directions: flexion and extension, lateral bending and axial rotation. So far, the investigation of the mechanical and kinematic behavior of the basic unit of the spine, a motion segment, is predominantly a domain of in vitro experiments on spinal loading simulators. Most existing approaches to measure spinal stiffness intraoperatively in an in vivo environment use a distractor. However, these concepts usually assume a planar loading and motion. The objective of our study was to develop and validate an apparatus, that allows to perform intraoperative in vivo measurements to determine both the applied force and the resulting motion in three dimensional space. The proposed setup combines force measurement with an instrumented distractor and motion tracking with an optoelectronic system. As the orientation of the applied force and the three dimensional motion is known, not only force–displacement, but also moment-angle relations could be determined. The validation was performed using three cadaveric lumbar ovine spines. The lateral bending stiffness of two motion segments per specimen was determined with the proposed concept and compared with the stiffness acquired on a spinal loading simulator which was considered to be gold standard. The mean values of the stiffness computed with the proposed concept were within a range of ±15% compared to data obtained with the spinal loading simulator under applied loads of less than 5 Nm.

Is kyphoplasty better than vertebroplasty in restoring normal mechanical function to an injured spine?

Introduction Kyphoplasty is gaining in popularity as a treatment for painful osteoporotic vertebral body fracture. It has the potential to restore vertebral shape and reduce spinal deformity, but the actual clinical and mechanical benefits of kyphoplasty remain unclear. In a cadaveric study, we compare the ability of vertebroplasty and kyphoplasty to restore spine mechanical function, and vertebral body shape, following vertebral fracture. Methods Fifteen pairs of thoracolumbar “motion segments” (two vertebrae with the intervening disc and ligaments) were obtained from cadavers aged 42–96 years. All specimens were compressed to induce vertebral body fracture. Then one of each pair underwent vertebroplasty and the other kyphoplasty, using 7 ml of polymethylmethacrylate cement. Augmented specimens were compressed for 2 hours to allow consolidation. At each stage of the experiment, motion segment stiffness was measured in bending and compression, and the distribution of loading on the vertebrae was determined by pulling a miniature pressure transducer through the intervertebral disc. Disc pressure measurements were performed in flexed and extended postures with a compressive load of 1.0–1.5 kN. They revealed the intradiscal pressure (IDP) which acts on the central vertebral body, and they enabled compressive load-bearing by the neural arch ( F N) to be calculated. Changes in vertebral height and wedge angle were assessed from radiographs. The volume of leaked cement was determined by water displacement. Volumetric bone mineral density (BMD) of each vertebral body was calculated using DXA and water displacement. Results Vertebral fracture reduced motion segment compressive stiffness by 55%, and bending stiffness by 39%. IDP fell by 61–88%, depending on posture. F N increased from 15% to 36% in flexion and from 30% to 58% in extension ( P < 0.001). Fracture reduced vertebral height by an average 0.94 mm and increased vertebral wedging by 0.95° ( P < 0.001). Vertebroplasty and kyphoplasty were equally effective in partially restoring all aspects of mechanical function (including stiffness, IDP, and F N), but vertebral wedging was reduced only by kyphoplasty ( P < 0.05). Changes in mechanical function and vertebral wedging were largely maintained after consolidation, but height restoration was not. Cement leakage was similar for both treatments. Conclusions Vertebroplasty and kyphoplasty were equally effective at restoring mechanical function to an injured spine. Only kyphoplasty was able to reverse minor vertebral wedging.