Human Lumbar Motion Segments Research Articles

The influence of geometry on intervertebral disc stiffness

Geometry plays an important role in intervertebral disc (IVD) mechanics. Previous computational studies have found a link between IVD geometry and stiffness. However, few experimental studies have investigated this link, possibly due to difficulties in non-destructively quantifying internal geometric features. Recent advances in ultra-high resolution MRI provides the opportunity to visualise IVD features in unprecedented detail. This study aimed to quantify 3D human IVD geometries using 9.4 T MRIs and to investigate correlations between geometric variations and IVD stiffness. Thirty human lumbar motion segments (fourteen non-degenerate and sixteen degenerate) were scanned using a 9.4 T MRI and geometric parameters were measured. A 1kN compressive load was applied to each motion segment and stiffness was calculated. Degeneration caused a reduction (p < 0.05) in IVD height, a decreased nucleus-annulus area ratio, and a 1.6 ± 3.0 mm inward collapse of the inner annulus. The IVD height, anteroposterior (AP) width, lateral width, cross-sectional area, nucleus-annulus boundary curvature, and nucleus-annulus area ratio had a significant (p < 0.05) influence on IVD stiffness. Linear relationships (p < 0.05, r > 0.47) were observed between these geometric features and IVD compressive stiffness and a multivariate regression model was generated to enable stiffness to be predicted from features observable on clinical imaging (stiffness, N/mm = 6062 – (61.2 × AP width, mm) – (169.2 × IVD height, mm)). This study advances our understanding of disc structure–function relationships and how these change with degeneration, which can be used to both generate and validate more realistic computational models.

Intradiscal treatment of the cartilage endplate for improving solute transport and disc nutrition.

Poor nutrient transport through the cartilage endplate (CEP) is a key factor in the etiology of intervertebral disc degeneration and may hinder the efficacy of biologic strategies for disc regeneration. Yet, there are currently no treatments for improving nutrient transport through the CEP. In this study we tested whether intradiscal delivery of a matrix-modifying enzyme to the CEP improves solute transport into whole human and bovine discs. Ten human lumbar motion segments harvested from five fresh cadaveric spines (38-66 years old) and nine bovine coccygeal motion segments harvested from three adult steers were treated intradiscally either with collagenase enzyme or control buffer that was loaded in alginate carrier. Motion segments were then incubated for 18h at 37 °C, the bony endplates removed, and the isolated discs were compressed under static (0.2MPa) and cyclic (0.4-0.8 MPa, 0.2Hz) loads while submerged in fluorescein tracer solution (376Da; 0.1mg/ml). Fluorescein concentrations from site-matched nucleus pulposus (NP) samples were compared between discs. CEP samples from each disc were digested and assayed for sulfated glycosaminoglycan (sGAG) and collagen contents. Results showed that enzymatic treatment of the CEP dramatically enhanced small solute transport into the disc. Discs with enzyme-treated CEPs had up to 10.8-fold (human) and 14.0-fold (bovine) higher fluorescein concentration in the NP compared to site-matched locations in discs with buffer-treated CEPs (p < 0.0001). Increases in solute transport were consistent with the effects of enzymatic treatment on CEP composition, which included reductions in sGAG content of 33.5% (human) and 40% (bovine). Whole disc biomechanical behavior-namely, creep strain and disc modulus-was similar between discs with enzyme- and buffer-treated CEPs. Taken together, these findings demonstrate the potential for matrix modification of the CEP to improve the transport of small solutes into whole intact discs.

In Vitro Model for Lumbar Disc Herniation to Investigate Regenerative Tissue Repair Approaches

Lumbar disc herniation (LDH) is the most common reason for low back pain in the working society. New regenerative approaches and novel implants are directed towards the restoration of the disc or its biomechanical properties. Aiming to investigate these new therapies under physiological conditions, in this study, a model for LDH was established by developing a new physiological in vitro test method. In 14 human lumbar motion segments, different daily-life and worst-case activities were simulated successfully by applying a physiological range of motion and axial loading in order to create physiological intradiscal pressure. An LDH could be provoked in 11 of the 14 specimens through vertical and round annular defects of different sizes. Interestingly, the defect and the LDH hardly influenced the biomechanical properties of the disc. For the investigation of regenerative approaches in further experiments, the recommendation based on the results of this study is to create an LDH in non-degenerated motion segments by the application of the new physiological in vitro test method after setting the round annular defects to a size of 4 mm in diameter.

Degeneration alters structure-function relationships at multiple length-scales and across interfaces in human intervertebral discs.

Intervertebral disc (IVD) degeneration and associated back pain place a significant burden on the population. IVD degeneration is a progressive cascade of cellular, compositional, and structural changes, which results in a loss of disc height, disorganization of extracellular matrix architecture, tears in the annulus fibrosus which may involve herniation of the nucleus pulposus, and remodeling of the bony and cartilaginous endplates (CEP). These changes to the IVD often occur concomitantly, across the entire motion segment from the disc subcomponents to the CEP and vertebral bone, making it difficult to determine the causal initiating factor of degeneration. Furthermore, assessments of the subcomponents of the IVD have been largely qualitative, with most studies focusing on a single attribute, rather than multiple adjacent IVD substructures. The objective of this study was to perform a multiscale and multimodal analysis of human lumbar motion segments across various length scales and degrees of degeneration. We performed multiple assays on every sample and identified several correlations between structural and functional measurements of disc subcomponents. Our results demonstrate that with increasing Pfirrmann grade there is a reduction in disc height and nucleus pulposus T2 relaxation time, in addition to alterations in motion segment macromechanical function, disc matrix composition and cellular morphology. At the cartilage endplate-vertebral bone interface, substantial remodeling was observed coinciding with alterations in micromechanical properties. Finally, we report significant relationships between vertebral bone and nucleus pulposus metrics, as well as between micromechanical properties of the endplate and whole motion segment biomechanical parameters, indicating the importance of studying IVD degeneration as a whole organ.

Internal load-sharing in the human passive lumbar spine: Review of in vitro and finite element model studies

Human lumbar motion segment is composed of various components with distinct contributions to its gross mechanical response. By employing experimental and computational approaches, many studies have investigated the relative role of each component as well as effects of various factors such as boundary-initial conditions, load magnitude-combination-direction, load temporal regime, preload, posture, degeneration, failures and surgical interventions on load-sharing. This paper reviews and critically discusses the relevant findings of in vitro and finite element model studies on load-sharing in healthy, aged, degenerate and damaged human lumbar motion segments. Two systematic searches were performed in PubMed (October 2018 - March 2019) using three sets of concepts (“lumbar spine”, “load-sharing” and “motion segment components”) followed by a complementary generic search. The segment overall response as well as the relative role of its constituents are markedly influenced by alterations in resection sequence, boundary conditions, geometry, loading characteristics (rate, magnitude, combinations and preloads), disc hydration, bone quality, posture and time (creep and cyclic). Structural transection order affects both findings and conclusions not only in force-control protocols but also in displacement-control loading regimes. Disc degeneration, endplate fracture and surgical resections significantly alter load transmission in the lumbar spine. In summary, in vitro and finite element model studies have together substantially improved our understanding of functional biomechanics (load-sharing) of human lumbar spine in normal and perturbed conditions acting as invaluable complementary tools in clinical applications.

Biomechanical function of a balloon nucleus pulposus replacement system: A human cadaveric spine study.

With recent advances in motion-sparing techniques in spine surgery, disc nucleus replacement (DNR) has been introduced as a viable method to restore the biomechanical functions of the spine. Several methods of DNR have been proposed in the literature. However, the risk of device migration or extrusion is a major issue that should be addressed for a successful DNR. DNR using a balloon nucleus (BN) filled with pressurized fluid may be capable of reducing such risks while preserving the advantages of DNR. The objective of this study was to investigate the biomechanical functionalities of the human cadaveric lumbar motion segments with a custom made BN filled with saline at internal fluid pressure of 0.3 or 0.6 MPa in terms of axial and rotational flexibilities of the L4-L5 motion segment. Axial flexibility was quantified by the axial displacement resulting from an axial compressive force of 400 N while the rotational flexibility by the range of motions determined as the rotational angles in response to a pure moment of 6.0 Nm in flexion, extension, and right- and left-lateral bending directions. These tests were performed successively on the motion segment in the following conditions: intact, post nucleotomy, implanting BN with 0.3 MPa, and BN with 0.6 MPa. The nucleotomy was found to significantly increase both the axial and rotational flexibilities while the implantation of the BN reduced the axial and rotational flexibilities to those of the intact segment. The axial and rotational flexibilities of the segment with the BN with 0.3 MPa were greater than those of the segment with the BN with 0.6 MPa. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:167-173, 2018.

Sagittal Plane Correction Using Lateral Transpsoas Approach: Effect of Cage Angulation and Surgical Technique on Segmental Lordosis

Introduction Lordotic cage insertion through lateral transpsoas approach is being used increasingly for restoration of sagittal alignment. However, the degree of correction achieved by varying cage angle as well as ALL release and posterior element resection is not well defined. The objective of this study is to determine the degree of segmental correction which could be achieved through lateral transpsoas approach by varying cage angle as well as adding anterior longitudinal ligament (ALL) release and posterior element resection. Material and Methods Thirteen human cadaveric lumbar motion segments between L1 and L5 were dissected into single motion segments. Segmental angles and disk heights were measured under both 50N and 500N loads under the following conditions: Intact specimen, discectomy (collapsed disk simulation), insertion of parallel cage, 10° cage, 30° cage with ALL release, 30° cage with ALL release and spinous process (SP) resection, 30° cage with ALL release, SP resection, facetectomy and compression with pedicle screws. Results Segmental lordosis was not increased by with either parallel or 10° cages as compared with intact disks, and contributed small amounts of lordosis when compared with the collapsed disk condition. Placement of 30° cages with ALL release increased segmental lordosis by 10.5°. Adding SP resection increased lordosis to 12.39°. Facetectomy and compression with pedicle screws further increased gains in lordosis to 26.4° in the 50N group and 25.8° in the 500N group. Neither group experienced a decrease in either anterior or posterior disk height. Conclusion Insertion of parallel or 10° cage has little effect on lordosis. 30° cage insertion and ALL release resulted in an increase of lordosis by ~10.5°. The addition of SP resection and facetectomy allowed increases in correction of up to 26.4°. No interventions resulted in decrease in either anterior or posterior disk height suggesting gains were achieved without causing foraminal stenosis.

The effect of simulated microgravity on lumbar spine biomechanics: an in vitro study.

Disc herniation risk is quadrupled following spaceflight. This study tested the hypothesis that swelling-induced disc height increases (comparable to those reported in spaceflight) stiffen the spine and elevate annular strain and nuclear pressure during forward bending. Eight human lumbar motion segments were secured to custom-designed testing jigs and subjected to baseline flexion and compression and pure moment flexibility tests. Discs were then free-swelled in saline to varying supraphysiologic heights consistent with prolonged weightlessness and re-tested to assess biomechanical changes. Swelling-induced disc height changes correlated positively with intradiscal pressure (p<0.01) and stiffening in flexion (p<0.01), and negatively with flexion range of motion (p<0.05). Swelling-induced increases in disc height also led to increased annular surface strain under combined flexion with compression. Disc wedge angle decreased with swelling (p<0.05); this loss of wedge angle correlated with decreased flexion range of motion (R (2)=0.94, p<0.0001) and decreased stiffness fold change in extension (p<0.05). Swelling-induced increases in disc height decrease flexibility and increase annular strain and nuclear pressure during forward bending. These changes, in combination with the measured loss of lordotic curvature with disc swelling, may contribute toward increased herniation risk. This is consistent with clinical observations of increased disc herniation rates after microgravity exposure and may provide the basis for future countermeasure development.

Microendoscopic Lateral Decompression for Lumbar Foraminal Stenosis

A biomechanical study. How much of the facet joint and the pars interarticularis (PI) can be removed in microendoscopic lateral decompression (MELD) for lumbar foraminal stenosis (LFS)? MELD is a surgical modality for patients with LFS. In severe degenerative cases, unilateral facet joint resection or unilateral removal of the lateral part of the PI are sometimes needed to decompress the nerve root adequately. Twelve human lumbar motion segments were tested according to the following loading protocol: axial compression, flexion, extension, lateral bending to the right and left, and axial rotation to the right and left. This loading protocol was applied to each motion segment after MELD in 2 experiments: (1) unilateral graded facetectomy was performed in stages of 25%, 50%, 75%, and 100% using 3 segments of L3/L4 and 3 segments of L5/S1; (2) unilateral removal of the lateral part of the PI was performed in stages of 25%, 50%, 75%, and 100% using 3 segments of L3/L4 and 3 segments of L5/S1. The relative stiffness of each motion segments was determined each time. (1) Unilateral facet joint resection of >75% can lead to a significant reduction in stiffness in axial rotation at both L3/L4 and L5/S1. (2) Unilateral removal of 75% of the lateral part of the PI can lead to significant reduction in stiffness in right and left rotation at L3/L4 and in left rotation at L5/S1. (3) Unilateral removal of 100% of the lateral part of the PI can lead to a significant reduction in stiffness in right axial rotation at L5/S1. It would seem judicious to remove no >50% of the facet joint or the lateral part of the PI in order to prevent postoperative instability when using MELD for LFS.

Effects of Lumbar Disc Degeneration at the Cartilaginous End Plate and the Annulus Fibrosus on Off-Resonant Saturation Ratios Assessed by a 3D Ultrashort Echo Time Sequence at 3 T

Introduction Biological disc replacement demands a distinct understanding of referring pathologies and states of degeneration. Clinical imaging modalities are needed to apply such measurements to potential patients. The introduction of UTE sequences to whole body magnetic resonance imaging (MRI) allows for quantitative characterization of tissue consisting of components with fast signal decay. We hypothesize that UTE will enable us to differentiate states of degeneration within the EP and the AF. The aim of this study was to investigate effects of LDD on ORS ratios in the cartilaginous EP and the AF. Materials and Methods A total of 20 human lumbar motion segments of seven fresh-frozen cadaveric specimens (median age 66, range 49-66 years) were assessed using a whole-body MRI scanner. A sagittal 3D UTE sequence with an isotropic submillimeter resolution of 0.7 mm (TE 0.07 ms, TR 10.3 ms, FoV 128 mm, FA 10 degrees) with and without off-resonant saturation preparation pulse at 3 T was used. The off-resonance frequency was varied between 1 and 5 kHz. OSRs of images with and without preparation pulse were calculated for the upper and the lower EP as well as for the AF. Intervertebral discs were classified healthy (CO, n = 9), mildly degenerated (MIL, n = 6) and severely degenerated (SEV, n = 8) concerning morphology and signal alteration in conventional MR sequences. A nonparametric U-test was performed to report the level of significance. The relationship between OSR and grade of degeneration was estimated using multiple regression analysis in respect to age and body mass index (BMI). Results Upper and lower EP as well as AF in controls revealed no significant differences concerning OSR from 1 to 5 kHz. Regarding all regions (EPs and AF), the SEV group showed significantly increased OSRs ranging from 0.37 ± 0.01 (1 kHz) to 0.22 ± 0.01 (5 kHz), compared with the CO group, ranging from 0.33 ± 0.01 (1 kHz) to 0.2 ± 0.01 (5 kHz), and the MIL group, ranging from 0.32 ± 0.01 (1 kHz) to 0.2 ± 0.01 (5 kHz) ( p &lt; 0.05). The MIL group showed a tendency toward lower OSR compared with the CO group. This was underlined by the results of multiple regression analysis after adjusting for age (Table A). [Table: see text] Conclusion UTE sequences reveal clear images of human lumbar EP and AF and allow for quantitative assessment of free water. EP and AF of lumbar discs with major degeneration showed a loss of free water content compared with healthy and slightly degenerated discs, indicating a change toward a more condensed structure. EP and AF of slightly degenerated discs, however, showed a tendency to higher OSR, indicating an increase of free water. This might have occurred due to edema or focal destruction of EP and AF in early stages. Focused repair strategies and potential outcome predictors may result from this imaging modality. Disclosure of Interest None declared

Increase in facet joint loading after nucleotomy in the human lumbar spine

Low-back pain has been related to degenerative changes after nucleotomy. Although several etiologies for pain after nucleotomy have been proposed, there is evidence of pain arising in the facet joints in general, which may be related to changes in load transfer. This study addresses the effect of nucleotomy on facet joint loading.Nine human lumbar motion segments (age: 40–59 years) were loaded in axial compression and extension-flexion. Reaction forces were compared with soft tissue structures sequentially removed. After nucleotomy the facets supported significantly greater load, almost doubling from a median of 8.6% of the applied external force to 15.8%. Force transmission related to the capsular ligament increased significantly from an intact median of 1.2–5.1% after nucleotomy. No correlation was observed between force increase on the facets and the proportion of disc nucleus removed.Even a small quantity of nucleus removal (range: 0.7–1.7g) increased the forces transmitted over the facet joints, both with and without capsular ligaments. This suggests that the proportion of material removed might not be important clinically with regard to facet joint degeneration and pain.

Finite element investigation of the effect of a bifid arch on loading of the vertebral isthmus

BackgroundThe biomechanical effect of a bifid arch as seen in spina bifida occulta and following a midline laminectomy is poorly understood. PurposeTo test the hypothesis that fatigue failure limits will be exceeded in the case of a bifid arch, but not in the intact case, when the segment is subjected to complex loading corresponding to normal sporting activities. Study designFinite element analysis. MethodsFinite element model of an intact L4-S1 human lumbar motion segment including ligaments was used. A section of the L5 vertebral arch and spinous process was removed to create the model with a midline defect. The models were loaded axially to 1 kN and then combined with axial rotation of 3°. Bilateral stresses, alternating stresses, and shear fatigue failure on both models were assessed and compared. ResultsUnder 1 kN axial load, the von Mises stresses observed in midline defect case and in the intact case were very similar (differences <5 MPa) having a maximum at the ventral end of the isthmus that decreases monotonically to the dorsal end. However, under 1 kN axial load and rotation, the maximum von Mises stresses observed in the ipsilateral L5 isthmus in the midline defect case (31 MPa) was much higher than the intact case (24.2 MPa), indicating a lack of load sharing across the vertebral arch in the midline defect case. When assessing the equivalent alternating shear stress amplitude, this was found to be 22.6 MPa for the midline defect case and 13.6 MPa for the intact case. From this, it is estimated that shear fatigue failure will occur in less than 70,000 cycles, under repetitive axial load and rotation conditions in the midline defect case, whereas for the intact case, fatigue failure will occur only after more than 10 million cycles. ConclusionsA bifid arch predisposes the isthmus to early fatigue fracture by generating increased stresses across the inferior isthmus of the inferior articular process, specifically in combined axial rotation and anteroposterior shear.

Robotic application of a dynamic resultant force vector using real-time load-control: Simulation of an ideal follower load on Cadaveric L4–L5 segments

Standard in-vitro spine testing methods have focused on application of isolated and/or constant load components while the in-vivo spine is subject to multiple components that can be resolved into resultant dynamic load vectors. To advance towards more in-vivo like simulations the objective of the current study was to develop a methodology to apply robotically-controlled, non-zero, real-time dynamic resultant forces during flexion–extension on human lumbar motion segment units (MSU) with initial application towards simulation of an ideal follower load (FL) force vector.A proportional-integral-derivative (PID) controller with custom algorithms coordinated the motion of a Cartesian serial manipulator comprised of six axes each capable of position- or load-control. Six lumbar MSUs (L4–L5) were tested with continuously increasing sagittal plane bending to 8Nm while force components were dynamically programmed to deliver a resultant 400N FL that remained normal to the moving midline of the intervertebral disc. Mean absolute load-control tracking errors between commanded and experimental loads were computed. Global spinal ranges of motion and sagittal plane inter-body translations were compared to previously published values for non-robotic applications.Mean TEs for zero-commanded force and moment axes were 0.7±0.4N and 0.03±0.02Nm, respectively. For non-zero force axes mean TEs were 0.8±0.8N, 1.3±1.6Nm, and 1.3±1.6N for Fx, Fz, and the resolved ideal follower load vector FLR, respectively. Mean extension and flexion ranges of motion were 2.6°±1.2° and 5.0°±1.7°, respectively. Relative vertebral body translations and rotations were very comparable to data collected with non-robotic systems in the literature.The robotically coordinated Cartesian load controlled testing system demonstrated robust real-time load-control that permitted application of a real-time dynamic non-zero load vector during flexion–extension. For single MSU investigations the methodology has potential to overcome conventional follower load limitations, most notably via application outside the sagittal plane. This methodology holds promise for future work aimed at reducing the gap between current in-vitro testing and in-vivo circumstances.

Estimation of shear load sharing in moderately degenerated human lumbar spine

Shear load sharing between intervertebral discs and apophyseal joints was investigated experimentally in human lumbar motion segments with moderately degenerated intervertebral discs. ‘Motion-Segments’ (21–42 years, n=6) and ‘Disc-Segments’ (22–42 years, n=6) were subjected to shear in 0° flexion, using a modified materials testing machine, while immersed in a Ringer bath at 37°C. Initially, two cycles of anterior and posterior shear loading up to 200N (50N/s) were applied, to evaluate stiffnesses in both directions. Specimens were then exposed to 15mm of anterior displacement at a rate of 0.5mm/s. A physiological compressive load of 500N was applied throughout. The initial 5mm of the load–displacement curves were approximated with 6th order polynomials for evaluation of the mean behaviour in each group. ‘Disc-Segments’ were 66% (p=0.002) and 43% (p=0.026) less stiff than ‘Motion-Segments’ for anterior and posterior shear directions, respectively. ‘Disc-Segments’ exhibited 44% lower peak shear load (p=0.015) than ‘Motion-Segments’. All specimens in the ‘Disc-Segments’ group showed damage either at the interface between the endplates and the disc. The intervertebral disc contributes 38% to initial anterior shear load-bearing, increasing to 66% at 5mm displacement. Some over-estimation of disc load-bearing might have been caused by the comparison of segments from different levels. The apophyseal joints make a substantial contribution (65–55%) to anterior shear load-bearing over the initial 2mm of shear displacement but this decreases with increasing shear displacement.

Weibull Analyses of the Fatigue Life of Human Tissues

It is likely that many musculoskeletal disorders (MSDs) are the result of a gradual accumulation of damage that results from a process of fatigue failure. Studies on the mechanical responses of cadaveric materials to loading provide much of our knowledge of the properties of human tissues and should be of considerable interest to the ergonomist. The purpose of the present investigation was to perform Weibull analyses on data provided in fatigue failure studies of human extensor digitorum longus (EDL) and human lumbar motion segments to evaluate the reliability and fatigue life when these materials are loaded at various levels of their ultimate strength.

Validation of vibration testing for the assessment of the mechanical properties of human lumbar motion segments

Experimental modal analysis is a non-destructive measurement technique, which applies low forces and small deformations to assess the integrity of a structure. It is therefore a promising method to study the mechanical properties of the spine in vivo. Previously, modal parameters successfully revealed artificially induced spinal injuries. The question remains however, whether experimental modal analysis can be applied successfully in human spinal segments with mechanical changes due to physiological processes. Since quasi-static mechanical testing is considered the “gold standard” for assessing intervertebral stiffness, the purpose of our study was to examine if the mechanical properties derived from vibration testing and quasi-static testing correlate.Six cadaver human spines (L1–L5) were loaded quasi-statically in bending and torsion, while an optical system measured the angular rotations of the individual motion segments. Subsequently, the polysegmental spines were divided into L2–L3 and L4–L5 segments and a shaker was used to vibrate the upper vertebra, while its response was obtained from accelerometers in anteroposterior and mediolateral directions. From the resulting frequency response function the eigenfrequencies (ratio between stiffness and mass) and vibration modes (pattern of motion) were determined.The vibration results showed clear eigenfrequencies for flexion–extension (mean 121.83Hz, SD 40.05Hz), lateroflexion (mean 132.17, SD 34.80Hz) and axial rotation (mean 236.17Hz, SD 81.45Hz). Furthermore, the correlation between static and dynamic tests was significant (r=0.73, p=0.01). In conclusion, the findings from this study show that experimental modal analysis is a valid method to assess the mechanical properties of human lumbar motion segments.

Shear strength of the human lumbar spine

BackgroundShear loading is recognised as a risk factor for lower back pain. Previous studies of shear loading have either not addressed the influence of age, bone mineral density, axial height loss due to creep or were performed on animal specimens. MethodsIntact human lumbar motion segments (L2–3) were tested in shear using a modified materials testing machine, while immersed in a Ringer bath at 37°C. Vertebrae were rigidly embedded in neutral posture (0° flexion) and subjected to a constant axial compression load of 500N. Shear was applied to three groups: ‘Young-No-Creep’ (20–42years), ‘Young-Creep’ (22–38years, creep 1000N for 1h) and ‘Old-No-Creep’ (44–64years). Failure was induced by up to 15mm of anterior shear displacement at a rate of 0.5mm/s. The trabecular and apophyseal joint bone mineral densities were evaluated from computed tomography images of the intact lumbar spines. FindingsPeak shear force correlated positively with trabecular bone mineral density for specimens tested without axial creep. No significant differences were observed with respect to age. During shear overload specimens increased in height in the axial direction. InterpretationTrabecular bone mineral density can be used to predict the peak force of lumbar spine in shear in neutral posture.

Relaxation Response of Lumbar Segments Undergoing Disc-Space Distraction

A biomechanical study of human cadaveric lumbar spine segments undergoing disc-space distraction for insertion of anterior lumbar interbody implants. To measure the distraction force and its relaxation during a period of up to 3 hours after disc-space distraction as a function of the distraction magnitude and disc level. Interbody implants depend on compressive preload produced by disc-space distraction (annular pretension) for initial stabilization of the implant-bone interface. However, the amount of preload produced by disc-space distraction due to insertion of the implant and its subsequent relaxation have not been quantified. Twenty-two fresh human lumbar motion segments (age: 51 ± 14.8 years) were used. An anterior lumbar discectomy was performed. The distraction test battery consisted of a tension stiffness test performed before and after each relaxation test, 2 distraction magnitudes of 2 and 4 mm, and a recovery period before each distraction input. The distraction forces and lordosis angles were measured. RESULTS.: Peak distraction force was significantly larger for the 4-mm distraction (431.8 ± 116.4 N) than for the 2-mm distraction (204.9 ± 55.5 N) (P < 0.01). The distraction force significantly decreased over time (P < 0.01), approximating steady-state values of 146.1 ± 47.3 N at 2-mm distraction and 289.8 ± 92.8 N at 4-mm distraction, respectively. The distraction force reduced in magnitude by more than 20% of peak value in the first 15 minutes and reduced by approximately 30% of the peak value at the end of the testing period. The spine segment relaxed by the same amount of force, regardless of the disc level (P > 0.05). The "tightness of fit" that the surgeon notes immediately after interbody device insertion in the disc space degrades in the very early postoperative period, which could compromise the stability of the bone-implant interface.

Biomechanical evaluation of an expandable meshed bag augmented with pedicle or facet screws for percutaneous lumbar interbody fusion

Objective To evaluate the biomechanics of lumbar motion segments instrumented with stand-alone OptiMesh system augmented with posterior fixation using facet or pedicle screws and the efficacy of discectomy and disc distraction. Background context OptiMesh bone graft containment system has been used for vertebral compression fractures and percutaneous lumbar interbody fusion. The filled mesh bag serves as the interbody device providing structural support to the motion segment being fused. No biomechanical data of this new device are available in the literature. Methods Twenty-four fresh human cadaveric lumbar motion segments were divided into two groups. In the control group, multidirectional flexibility testing was conducted after an intact condition and standard transforaminal lumbar interbody fusion (TLIF) procedure. In the OptiMesh group, testing was performed following intact, stand-alone OptiMesh procedure, OptiMesh with facet screws (placed using the transfacet approach), and OptiMesh with pedicle screws and rods. Range of motion (ROM) was calculated for each surgical treatment. The lordosis and disc height change of intact and instrumented specimens were measured in the lateral radiographs to evaluate the disc space distraction. In the OptiMesh group, cyclic loading in flexion extension (FE) was applied to measure cage subsidence or collapse (10,000 cycles at 6 Nm). After biomechanical testing, all the specimens were dissected to inspect the discectomy and end plate preparation. The area of discectomy was measured. Results The mean ROM of the intact specimens was 2.7°, 7.4°, and 7.2° in axial torsion (AT), lateral bending (LB), and FE, respectively. There was no difference between the control group and OptiMesh group. The mean ROM of the stand-alone OptiMesh system decreased to 2.4°, 5.1°, and 4.3° in AT, LB, and FE. The ROM decreased to 0.9° in AT, 2.2° in LB, and 0.9° in FE with OptiMesh system and facet screws. On average, OptiMesh system with pedicle screws and rods reduced the ROM to 1.3° in AT, 1.6° in LB, and 1.1° in FE. Compared with the intact condition and stand-alone OptiMesh system, both posterior fixation options had significant statistical difference (p<.001). In AT, ROM of facet screws was lower than that of pedicle screws (p<.05). There was no statistical difference between the facet and pedicle screws in LB and FE (p>.05). The mean volume of bone graft packed into each bag was 8.3±1.5 cc. The average increase of lordosis was 0.6°±1.0° after meshed bag was deployed. The average distraction achieved by the OptiMesh system was 1.0±0.6 mm. The average prepared area of discectomy was 42% of the total disc. The disc height change after cyclic loading was 0.2 mm. No subsidence or collapse was noticed. Conclusions The OptiMesh system offers large volume of bone graft in the disc space with small access portals. The OptiMesh system had similar construct stability to that of standard TLIF procedure when posterior fixation was applied. However, the amount of distraction was limited without additional distraction tools. With the anterior support provided by the expandable meshed bag, facet screws had comparable construct stability to that of pedicle screws. Slightly higher stability was observed in facet screws in AT.

Biomechanical Assessment of Minimally Invasive Decompression for Lumbar Spinal Canal Stenosis

A biomechanical study on the cadaveric human lumbar spine. We focused on a biomechanical comparison of the changes wrought on motion segments after a minimally invasive decompression and after a conventional medial facetectomy. Minimally invasive posterior decompression using a microscope or an endoscope is becoming popular for elderly patients with lumbar spinal canal stenosis. An advantage of the technique is that the cauda equina and nerve roots are in clear view and the facet joints, paravertebral muscles, and spinous process are well preserved. Eight human lumbar motion segments were used in this study. Each specimen was tested according to the following loading protocol: axial compression, flexion, extension, lateral bending to the right and to the left, and axial rotation to the right and to the left. This loading protocol was applied to each motion segment after the following surgical interventions: (1) left fenestration, (2) bilateral decompression via unilateral approach, (3) medial facetectomy, and (4) total facetectomy. The relative stiffness of the motion segments was determined and compared with a normalized stiffness for the specimen when intact. Bilateral decompression via unilateral approach produces less biomechanical effect in terms of stiffness changes as compared with medial facetectomy. Bilateral decompression leaves the spine more than 80% as stiff as the intact spine. These results go toward supporting a minimally invasive bilateral decompression. Minimally invasive bilateral decompression, as opposed to a conventional medial facetectomy, preserves the facet joints as much as possible. Preserving the facet joints during the decompression should produce less postoperative instability.