Dysfunctional paraspinal muscles in adult spinal deformity patients lead to increased spinal loading.

PURPOSE: Decreased spinal extensor muscle strength in adult spinal deformity (ASD) patients is well-known but poorly understood; thus, this study aimed to investigate the biomechanical and histopathological properties of paraspinal muscles from ASD patients and predict the effect of altered biomechanical properties on spine loading. METHODS: 68 muscle biopsies were collected from nine ASD patients at L4-L5 (bilateral multifidus and longissimus sampled). The biopsies were tested for muscle fiber and fiber bundle biomechanical properties and histopathology. The small sample size (due to COVID-19) precluded formal statistical analysis, but the properties were compared to literature data. Changes in spinal loading due to the measured properties were predicted by a lumbar spine musculoskeletal model. RESULTS: Single fiber passive elastic moduli were similar to literature values, but in contrast, the fiber bundle moduli exhibited a wide range beyond literature values, with 22% of 171 fiber bundles exhibiting very high elastic moduli, up to 20 times greater. Active contractile specific force was consistently less than literature, with notably 24% of samples exhibiting no contractile ability. Histological analysis of 28 biopsies revealed frequent fibro-fatty replacement with a range of muscle fiber abnormalities. Biomechanical modelling predicted that high muscle stiffness could increase the compressive loads in the spine by over 500%, particularly in flexed postures. DISCUSSION: The histopathological observations suggest diverse mechanisms of potential functional impairment. The large variations observed in muscle biomechanical properties can have a dramatic influence on spinal forces. These early findings highlight the potential key role of the paraspinal muscle in ASD.

Biomechanical Properties of Paraspinal Muscles Influence Spinal Loading-A Musculoskeletal Simulation Study.

Paraspinal muscles are vital to the functioning of the spine. Changes in muscle physiological cross-sectional area significantly affect spinal loading, but the importance of other muscle biomechanical properties remains unclear. This study explored the changes in spinal loading due to variation in five muscle biomechanical properties: passive stiffness, slack sarcomere length (SSL), in situ sarcomere length, specific tension, and pennation angle. An enhanced version of a musculoskeletal simulation model of the thoracolumbar spine with 210 muscle fascicles was used for this study and its predictions were validated for several tasks and multiple postures. Ranges of physiologically realistic values were selected for all five muscle parameters and their influence on L4-L5 intradiscal pressure (IDP) was investigated in standing and 36° flexion. We observed large changes in IDP due to changes in passive stiffness, SSL, in situ sarcomere length, and specific tension, often with interesting interplays between the parameters. For example, for upright standing, a change in stiffness value from one tenth to 10 times the baseline value increased the IDP only by 91% for the baseline model but by 945% when SSL was 0.4 µm shorter. Shorter SSL values and higher stiffnesses led to the largest increases in IDP. More changes were evident in flexion, as sarcomere lengths were longer in that posture and thus the passive curve is more influential. Our results highlight the importance of the muscle force-length curve and the parameters associated with it and motivate further experimental studies on in vivo measurement of those properties.

Investigating the active contractile function of the rat paraspinal muscles reveals unique cross-bridge kinetics in the multifidus.

PURPOSE: Various aspects of paraspinal muscle anatomy, biology, and histology have been studied; however, information on paraspinal muscle contractile function is almost nonexistent, thus hindering functional interpretation of these muscles in healthy individuals and those with low back disorders. The aim of this study was to measure and compare the contractile function and force-sarcomere length properties of muscle fibers from the multifidus (MULT) and erector spinae (ES) as well as a commonly studied lower limb muscle (Extensor digitorum longus (EDL)) in the rat. METHODS: Single muscle fibers (n = 77 total from 6 animals) were isolated from each of the muscles and tested to determine their active contractile function; all fibers used in the analyses were type IIB. RESULTS: There were no significant differences between muscles for specific force (sFo) (p = 0.11), active modulus (p = 0.63), average optimal sarcomere length (p = 0.27) or unloaded shortening velocity (Vo) (p = 0.69). However, there was a significant difference in the rate of force redevelopment (ktr) between muscles (p = < 0.0001), with MULT being significantly faster than both the EDL (p = < 0.0001) and ES (p = 0.0001) and no difference between the EDL and ES (p = 0.41). CONCLUSIONS: This finding suggests that multifidus has faster cross-bridge turnover kinetics when compared to other muscles (ES and EDL) when matched for fiber type. Whether the faster cross-bridge kinetics translate to a functionally significant difference in whole muscle performance needs to be studied further.

The diagnostic precision of computed tomography for traumatic cervical spine injury: An in vitro biomechanical investigation.

BACKGROUND: CT is considered the best method for vertebral fracture detection clinically, but its efficacy in laboratory studies is unknown. Therefore, our objective was to determine the sensitivity, precision, and specificity of high-resolution CT imaging compared to detailed anatomic dissection in an axial compression and lateral bending cervical spine biomechanical injury model. METHODS: 35 three-vertebra human cadaver cervical spine specimens were impacted in dynamic axial compression (0.5 m/s) at one of three lateral eccentricities (low 5% of the spine transverse diameter, middle 50%, high 150%) and two end conditions (19 constrained lateral translation and 16 unconstrained). All specimens were imaged using high resolution CT imaging (246 µm). Two clinicians (spine surgeon and neuroradiologist) diagnosed the vertebral fractures based on 34 discrete anatomical structures using both the CT images and anatomical dissection. FINDINGS: The sensitivity of CT was highest for fractures of the facet joint (59%) and vertebral endplate (57%), and was lowest for pedicle (13%) and lateral mass fractures (23%). The precision of CT was highest for spinous process fractures (83%) and lowest for pedicle (21%), uncinate process and lateral mass (both 23%) fractures. The specificity of CT exceeded 90% for all fractures. The Kappa value between the two reviewers was 0.52, indicating moderate agreement. INTERPRETATION: In this in vitro cervical spine injury model, high resolution CT scanning missed many fractures, notably those of the lateral mass and pedicle. This finding is potentially important clinically, as the integrity of these structures is important to clinical stability and surgical fixation planning.

Anteroposterior shear stiffness of the upper thoracic spine at quasi-static and dynamic loading rates-An in vitro biomechanical study.

To evaluate the biomechanical properties of the upper thoracic spine in anterior-posterior shear loading at various displacement rates. These data broaden our understanding of thoracic spine biomechanics and inform efforts to model the spine and spinal cord injuries. Seven T1-T2 thoracic functional spinal units were loaded non-destructively by a pure shear force up to 200 N, starting from a neutral posture. Tests were run in both posterior and anterior directions, at displacement rates of 1, 10, and 100 mm/s. The three-dimensional motion of the specimen was recorded at 1000 Hz. Individual and averaged load-displacement curves were generated and specimen stiffnesses were calculated. Due to a nonlinear response of the specimens, stiffness was defined separately for both the lower half and the upper half of the specimen range of motion. Specimens were significantly stiffer in the anterior direction than in the posterior direction, across all rates. At low displacements, the anterior stiffness averaged 230 N/mm, 76% higher than the low displacement posterior stiffness of 131 N/mm. At high displacements, anterior stiffness averaged 258 N/mm, 51% stiffer than the high displacement posterior stiffness of 171 N/mm. Shear displacement rate had a small effect on the load response, with the 100 mm/s rate causing a mildly stiffer response at low displacements in the anterior direction. Overall, the load-displacement response exhibited pseudo-quadratic behavior at 1 and 10 mm/s but became more linear at 100 mm/s. The shear stiffness in the upper thoracic spine is greatest in the anterior loading direction, being 51%-76% greater than posterior, most likely due to facet interactions. The effect of the shear displacement rate is low.

Automated Segmentation of Spinal Muscles From Upright Open MRI Using a Multiscale Pyramid 2D Convolutional Neural Network.

STUDY DESIGN: Randomized trial. OBJECTIVE: To implement an algorithm enabling the automated segmentation of spinal muscles from open magnetic resonance images in healthy volunteers and patients with adult spinal deformity (ASD). SUMMARY OF BACKGROUND DATA: Understanding spinal muscle anatomy is critical to diagnosing and treating spinal deformity.Muscle boundaries can be extrapolated from medical images using segmentation, which is usually done manually by clinical experts and remains complicated and time-consuming. METHODS: Three groups were examined: two healthy volunteer groups (N = 6 for each group) and one ASD group (N = 8 patients) were imaged at the lumbar and thoracic regions of the spine in an upright open magnetic resonance imaging scanner while maintaining different postures (various seated, standing, and supine). For each group and region, a selection of regions of interest (ROIs) was manually segmented. A multiscale pyramid two-dimensional convolutional neural network was implemented to automatically segment all defined ROIs. A five-fold crossvalidation method was applied and distinct models were trained for each resulting set and group and evaluated using Dice coefficients calculated between the model output and the manually segmented target. RESULTS: Good to excellent results were found across all ROIs for the ASD (Dice coefficient >0.76) and healthy (dice coefficient > 0.86) groups. CONCLUSION: This study represents a fundamental step toward the development of an automated spinal muscle properties extraction pipeline, which will ultimately allow clinicians to have easier access to patient-specific simulations, diagnosis, and treatment.

The effect of end condition on spine segment biomechanics in compression with lateral eccentricity.

During axial impact compression of the cervical spine, injury outcome is highly dependent on initial posture of the spine and the orientation, frictional properties and stiffness of the impact surface. These properties influence the "end condition" the spine experiences in real-world impacts. The effect of end condition on compression and sagittal plane bending in laboratory experiments is well-documented. The spine is able to escape injury in an unconstrained flexion-inducing end condition (e.g. against an angled, low friction surface), but when the end condition is constrained (e.g. head pocketing into a deformable surface) the following torso can compress the aligned spine causing injury. The aim of this study was to determine whether this effect exists under combined axial compression and lateral bending. Over two experimental studies, twenty-four human three vertebra functional spinal units were subjected to controlled dynamic axial compression at two levels of laterally eccentric force and in two end conditions. One end condition allowed the superior spine to laterally rotate and translate (T-Free) and the other end condition allowed only lateral rotation (T-Fixed). Spine kinetics, kinematics, injuries and occlusion of the spinal canal were measured during impact and pre- and post-impact flexibility. In contrast to typical spine responses in flexion-compression loading, the cervical spine specimens in this study did not escape injury in lateral bending when allowed to translate laterally. The specimen group that allowed lateral translation during compression had more injuries at high laterally eccentric force, saw greater peak canal occlusions and post-impact flexibility than constrained specimens.

Larger muscle fibers and fiber bundles manifest smaller elastic modulus in paraspinal muscles of rats and humans.

The passive elastic modulus of muscle fiber appears to be size-dependent. The objectives of this study were to determine whether this size effect was evident in the mechanical testing of muscle fiber bundles and to examine whether the muscle fiber bundle cross-section is circular. Muscle fibers and fiber bundles were extracted from lumbar spine multifidus and longissimus of three cohorts: group one (G1) and two (G2) included 13 (330 ± 14 g) and 6 (452 ± 28 g) rats, while Group 3 (G3) comprised 9 degenerative spine patients. A minimum of six muscle fibers and six muscle fiber bundles from each muscle underwent cumulative stretches, each of 10% strain followed by 4 minutes relaxation. For all specimens, top and side diameters were measured. Elastic modulus was calculated as tangent at 30% strain from the stress-strain curve. Linear correlations between the sample cross sectional area (CSA) and elastic moduli in each group were performed. The correlations showed that increasing specimen CSA resulted in lower elastic modulus for both rats and humans, muscle fibers and fiber bundles. The median ratio of major to minor axis exceeded 1.0 for all groups, ranging between 1.15-1.29 for fibers and 1.27-1.44 for bundles. The lower elastic moduli with increasing size can be explained by relatively less collagenous extracellular matrix in the large fiber bundles. Future studies of passive property measurement should aim for consistent bundle sizes and measuring diameters of two orthogonal axes of the muscle specimens.

The Effect of Posterior Lumbar Spinal Surgery on Biomechanical Properties of Rat Paraspinal Muscles 13 Weeks After Surgery.

STUDY DESIGN: Preclinical study in rodents. OBJECTIVE: To investigate changes in biomechanical properties of paraspinal muscles following a posterior spinal surgery in an animal model. SUMMARY OF BACKGROUND DATA: Posterior spine surgery damages paraspinal musculature per histological and imaging studies. The biomechanical effects of these changes are unknown. METHODS: 12 Sprague-Dawley rats were divided equally into sham and surgical injury (SI) groups. For sham, the skin and lumbodorsal fascia were incised at midline. For SI, the paraspinal muscles were detached from the vertebrae, per normal procedure. Thirteen weeks postsurgery, multifidus and longissimus biopsies at L1, L3, and L5 levels were harvested on the right. From each biopsy, three fibers and three to six bundles of fibers (∼10-20 fibers ensheathed in their extracellular matrix) were tested mechanically to measure their passive elastic modulus. The collagen content and fatty infiltration of each biopsy were also examined histologically by immunofluorescence staining. Nonparametric statistical methods were used with a 1.25% level of significance. RESULTS: A total of 220 fibers and 279 bundles of fibers were tested. The elastic moduli of the multifidus and longissimus fibers and longissimus fiber bundles were not significantly different between the SI and sham groups. However, the elastic modulus of multifidus fiber bundles was significantly greater in the SI group compared to sham (SI median 82 kPa, range 23-284; sham median 38 kPa, range 23-50, P = 0.0004). The elastic modulus of multifidus fiber bundles in the SI group was not statistically different between spinal levels (P = 0.023). For histology, only collagen I deposition in multifidus was significantly greater in the SI group (median 20.8% vs. 5.8% for sham, P < 0.0001). CONCLUSION: The surgical injury increased the passive stiffness of the multifidus fiber bundles. Increased collagen content in the extracellular matrix is the likely reason and these changes may be important in the postoperative compensation of the spine.Level of Evidence: N/A.

Estimation and assessment of sagittal spinal curvature and thoracic muscle morphometry in different postures.

Spine models are typically developed from supine clinical imaging data, and hence clearly do not fully reflect postures that replicate subjects' clinical symptoms. Our objectives were to develop a method to: (i) estimate the subject-specific sagittal curvature of the whole spine in different postures from limited imaging data, (ii) obtain muscle lines-of-action in different postures and analyze the effect of posture on muscle fascicle length, and (iii) correct for cosine between the magnetic resonance imaging (MRI) scan plane and dominant fiber line-of-action for muscle parameters (cross-sectional area (CSA) and position). The thoracic spines of six healthy volunteers were scanned in four postures (supine, standing, flexion, and sitting) in an upright MRI. Geometry of the sagittal spine was approximated with a circular spline. A pipeline was developed to estimate spine geometry in different postures and was validated. The lines-of-action for two muscles, erector spinae (ES) and transversospinalis (TS) were obtained for every posture and hence muscle fascicle lengths were computed. A correction factor based on published literature was then computed and applied to the muscle parameters. The maximum registration error between the estimated spine geometry and MRI data was small (average RMSE∼1.2%). The muscle fascicle length increased (up to 20%) in flexion when compared to erect postures. The correction factor reduced muscle parameters (∼5% for ES and ∼25% for TS) when compared to raw MRI data. The proposed pipeline is a preliminary step in subject-specific modeling. Direction cosines of muscles could be used while improving the inputs of spine models.

Temporal Progression of Acute Spinal Cord Injury Mechanisms in a Rat Model: Contusion, Dislocation, and Distraction.

Traumatic spinal cord injuries (SCIs) occur due to different spinal column injury patterns, including burst fracture, dislocation, and flexion-distraction. Pre-clinical studies modeling different SCI mechanisms have shown distinct histological differences between these injuries both acutely (3 h and less) and chronically (8 weeks), but there remains a temporal gap. Different rates of injury progression at specific regions of the spinal cord may provide insight into the pathologies that are initiated by specific SCI mechanisms. Therefore, the objective of this study was to evaluate the temporal progression of injury at specific tracts within the white matter, for time-points of 3 h, 24 h, and 7 days, for three distinct SCI mechanisms. In this study, 96 male Sprague Dawley rats underwent one of three SCI mechanisms: contusion, dislocation, or distraction. Animals were sacrificed at one of three times post-injury: 3 h, 24 h, or 7 days. Histological analysis using eriochrome cyanide and immunostaining for MBP, SMI-312, neurofilament-H (NF-H), and ß-III tubulin were used to characterize white matter sparing and axon and myelinated axon counts. The regions analyzed were the gracile fasciculus, cuneate fasciculus, dorsal corticospinal tract, and ventrolateral white matter. Contusion, dislocation, and distraction SCIs demonstrated distinct damage patterns that progressed differently over time. Myelinated axon counts were significantly reduced after dislocation and contusion injuries in most locations and time-points analyzed (compared with sham). This indicates early myelin damage often within 3 h. Myelinated axon counts after distraction dropped early and did not demonstrate any significant progression over the next 7 days. Important differences in white matter degeneration were identified between injury types, with distraction injuries showing the least variability across time-points These findings and the observation that white matter injury occurs early, and in many cases, without much dynamic change, highlight the importance of injury type in SCI research-both clinically and pre-clinically.

The effect of vertebral level on biomechanical properties of the lumbar paraspinal muscles in a rat model.

INTRODUCTION: Passive mechanical properties of the paraspinal muscles are important to the biomechanical functioning of the spine. In most computational models, the same biomechanical properties are assumed for each paraspinal muscle group, while cross-sectional area or fatty infiltration in these muscles have been reported to differ between the vertebral levels. Two important properties for musculoskeletal modeling are the slack sarcomere length and the tangent modulus. This study aimed to investigate the effect of vertebral level on these biomechanical properties of paraspinal muscles in a rat model. METHODS: The left paraspinal muscles of 13 Sprague-Dawley rats were exposed under anesthesia. Six muscle biopsies were collected from each rat: three from multifidus (one per each of the L1, L3, and L5 levels) and similarly three from longissimus. Each biopsy was cut into two halves. From one half, two to three single muscle fibers and two to six muscle fiber bundles (14 ± 7 fibers surrounded in their connective tissue) were extracted and mechanically tested in a passive state. From the resulting stress-strain data, tangent modulus was calculated as the slope of the tangent at 30% strain and slack sarcomere length (beyond which passive force starts to develop) was recorded. The other half of each biopsy, which represented the muscle at the fascicle level, was snap frozen, sectioned, stained for Collagen I and its area fraction was measured. To evaluate the effect of spinal level on these biomechanical properties of multifidus and longissimus, one-way repeated measures ANOVA (p < 0.05) was performed for tangent modulus and slack sarcomere length, while for collagen I content linear mixed-models analysis was adopted. RESULTS: In total, 192 fibers and 262 fiber bundles were mechanically tested. For both muscle groups, no significant difference in tangent modulus of the single fibers was detected between the three spinal levels (p = 0.9 for multifidus and p = 0.08 for longissimus). Similarly, the tangent modulus values for the fiber bundles were not significantly different between the three spinal levels (p = 0.13 for multifidus and p = 0.49 for longissimus). In both muscle groups, the slack sarcomere lengths were not different among the spinal levels except for multifidus fibers (p = 0.02). Collagen I area fraction in muscle fascicles averaged 6.8% for multifidus and 5.3% for longissimus and was not different between the spinal levels. DISCUSSION: The results of this study highlighted that the tangent modulus, slack sarcomere length, and collagen I content of the lumbar paraspinal muscles are independent of spinal level. This finding provides the basis for the assumption of similar mechanical properties along a paraspinal muscle group.

Preliminary investigation of spinal level and postural effects on thoracic muscle morphology with upright open MRI.

OBJECTIVE: Spinal-muscle morphological differences between weight-bearing and supine postures have potential diagnostic, prognostic, and therapeutic applications. While the focus to date has been on cervical and lumbar regions, recent findings have associated spinal deformity with smaller paraspinal musculature in the thoracic region. We aim to quantitatively investigate the morphology of trapezius (TZ), erector spinae (ES) and transversospinalis (TS) muscles in upright postures with open upright MRI and also determine the effect of level and posture on the morphological measures. METHODS: Six healthy volunteers (age 26 ± 6 years) were imaged (0.5 T MROpen, Paramed, Genoa, Italy) in four postures (supine, standing, standing with 30° flexion, and sitting). Two regions of the thorax, middle (T4-T5), and lower (T8-T9), were scanned separately for each posture. 2D muscle parameters such as cross-sectional area (CSA) and position (radius and angle) with respect to the vertebral body centroid were measured for the three muscles. Effect of spinal level and posture on muscle parameters was examined using 2-way repeated measures ANOVA separately for T4-T5 and T8-T9 regions. RESULTS: The TZ CSA was smaller (40%, P = .0027) at T9 than at T8. The ES CSA was larger at T5 than at T4 (12%, P = .0048) and at T9 than at T8 (10%, P = .0018). TS CSA showed opposite trends at the two spinal regions with it being smaller (16%, P = .0047) at T5 than at T4 and larger (11%, P = .0009) at T9 than at T8. At T4-T5, the TZ CSA increased (up to 23%), and the ES and TS CSA decreased (up to 10%) in upright postures compared to supine. CONCLUSION: Geometrical parameters that describe muscle morphology in the thorax change with level and posture. The increase in TZ CSA in upright postures could result from greater activation while upright. The decrease in ES CSA in flexed positions likely represents passive stretching compared to neutral posture.

Quantitative identification and segmentation repeatability of thoracic spinal muscle morphology.

OBJECTIVE: MRI derived spinal-muscle morphology measurements have potential diagnostic, prognostic, and therapeutic applications in spinal health. Muscle morphology in the thoracic spine is an important determinant of kyphosis severity in older adults. However, the literature on quantification of spinal muscles to date has been limited to cervical and lumbar regions. Hence, we aim to propose a method to quantitatively identify regions of interest of thoracic spinal muscle in axial MR images and investigate the repeatability of their measurements. METHODS: Middle (T4-T5) and lower (T8-T9) thoracic levels of six healthy volunteers (age 26 ± 6 years) were imaged in an upright open scanner (0.5T MROpen, Paramed, Genoa, Italy). A descriptive methodology for defining the regions of interest of trapezius, erector spinae, and transversospinalis in axial MR images was developed. The guidelines for segmentation are laid out based on the points of origin and insertion, probable size, shape, and the position of the muscle groups relative to other recognizable anatomical landmarks as seen from typical axial MR images. 2D parameters such as muscle cross-sectional area (CSA) and muscle position (radius and angle) with respect to the vertebral body centroid were computed and 3D muscle geometries were generated. Intra and inter-rater segmentation repeatability was assessed with intraclass correlation coefficient (ICC (3,1)) for 2D parameters and with dice coefficient (DC) for 3D parameters. RESULTS: Intra and inter-rater repeatability for 2D and 3D parameters for all muscles was generally good/excellent (average ICC (3,1) = 0.9 with ranges of 0.56-0.98; average DC = 0.92 with ranges from 0.85-0.95). CONCLUSION: The guidelines proposed are important for reliable MRI-based measurements and allow meaningful comparisons of muscle morphometry in the thoracic spine across different studies globally. Good segmentation repeatability suggests we can further investigate the effect of posture and spinal curvature on muscle morphology in the thoracic spine.

Mechanical properties of spinal cord grey matter and white matter in confined compression.

To better understand the link between spinal cord impact and the resulting tissue damage, computational models are often used. These models typically simulate the spinal cord as a homogeneous and isotropic material. Recent research suggests that grey and white matter tissue differences and directional differences, i.e. anisotropy, are important to predict spinal cord damage. The objective of this research was to characterize the mechanical properties of spinal cord grey and white matter tissue in confined compression. Spinal cords (n = 12) were harvested immediately following euthanasia from Yorkshire and Yucatan pigs. The spinal cords were flash frozen (60 s at -80 °C) and prepared into four types of test samples: grey matter axial, grey matter transverse, white matter axial, and white matter transverse. Each sample type was thawed, and subsequently tested in confined compression within 6 h of euthanasia. Samples were compressed to 10% strain at a quasi-static strain rate (0.001/sec) and allowed to relax for 120 s. A quasi-linear viscoelastic model combining a first-order exponential with a 1-term Prony series characterized the loading and relaxation responses respectively. The effect of tissue type (grey matter vs. white matter), direction (axial vs. transverse), and their interaction were evaluated with a two-way ANOVA (p < 0.05) with peak stress, aggregate modulus, and relaxation time as dependent variables. This study found grey matter to be 1.6-2 times stiffer than white matter and both grey and white matter were isotropic in compression. These findings should be emphasized when studying SCI biomechanics using computational models.

The Effect of Compression Applied Through Constrained Lateral Eccentricity on the Failure Mechanics and Flexibility of the Human Cervical Spine.

In contrast to sagittal plane spine biomechanics, little is known about the response of the cervical spine to axial compression with lateral eccentricity of the applied force. This study evaluated the effect of lateral eccentricity on the kinetics, kinematics, canal occlusion, injuries, and flexibility of the cervical spine in translationally constrained axial impacts. Eighteen functional spinal units were subjected to flexibility tests before and after an impact. Impact axial compression was applied at one of three lateral eccentricity levels based on percentage of vertebral body width (low = 5%, medium = 50%, high = 150%). Injuries were graded by dissection. Correlations between intrinsic specimen properties and injury scores were examined for each eccentricity group. Low lateral force eccentricity produced predominantly bone injuries, clinically recognized as compression injuries, while medium and high eccentricity produced mostly contralateral ligament and/or disc injuries, an asymmetric pattern typical of lateral loading. Mean compression force at injury decreased with increasing lateral eccentricity (low = 3098 N, medium = 2337 N, and high = 683 N). Mean ipsilateral bending moments at injury were higher at medium (28.3 N·m) and high (22.9 N·m) eccentricity compared to low eccentricity specimens (0.1 N·m), p < 0.05. Ipsilateral bony injury was related to vertebral body area (VBA) (r = -0.974, p = 0.001) and disc degeneration (r = 0.851, p = 0.032) at medium eccentricity. Facet degeneration was correlated with central bony injury at high eccentricity (r = 0.834, p = 0.036). These results deepen cervical spine biomechanics knowledge in circumstances with coronal plane loads.

The effect of posture on lumbar muscle morphometry from upright MRI.

PURPOSE: To assess the effect of upright, seated, and supine postures on lumbar muscle morphometry at multiple spinal levels and for multiple muscles. METHODS: Six asymptomatic volunteers were imaged (0.5 T upright open MRI) in 7 postures (standing, standing holding 8 kg, standing 45° flexion, seated 45° flexion, seated upright, seated 45° extension, and supine), with scans at L3/L4, L4/L5, and L5/S1. Muscle cross-sectional area (CSA) and muscle position with respect to the vertebral body centroid (radius and angle) were measured for the multifidus/erector spinae combined and psoas major muscles. RESULTS: Posture significantly affected the multifidus/erector spinae CSA with decreasing CSA from straight postures (standing and supine) to seated and flexed postures (up to 19%). Psoas major CSA significantly varied with vertebral level with opposite trends due to posture at L3/L4 (increasing CSA, up to 36%) and L5/S1 (decreasing CSA, up to 40%) with sitting/flexion. For both muscle groups, radius and angle followed similar trends with decreasing radius (up to 5%) and increasing angle (up to 12%) with seated/flexed postures. CSA and lumbar lordosis had some correlation (multifidus/erector spinae L4/L5 and L5/S1, r = 0.37-0.45; PS L3/L4 left, r = - 0.51). There was generally good repeatability (average ICC(3, 1): posture = 0.81, intra = 0.89, inter = 0.82). CONCLUSION: Changes in multifidus/erector spinae muscle CSA likely represent muscles stretching between upright and seated/flexed postures. For the psoas major, the differential level effect suggests that changing three-dimensional muscle morphometry with flexion is not uniform along the muscle length. The muscle and spinal level-dependent effects of posture and spinal curvature correlation, including muscle CSA and position, highlight considering measured muscle morphometry from different postures in spine models.

A neck compression injury criterion incorporating lateral eccentricity.

There is currently no established injury criterion for the spine in compression with lateral load components despite this load combination commonly contributing to spinal injuries in rollover vehicle crashes, falls and sports. This study aimed to determine an injury criterion and accompanying tolerance values for cervical spine segments in axial compression applied with varying coronal plane eccentricity. Thirty-three human cadaveric functional spinal units were subjected to axial compression at three magnitudes of lateral eccentricity of the applied force. Injury was identified by high-speed video and graded by spine surgeons. Linear regression was used to define neck injury tolerance values based on a criterion incorporating coronal plane loads accounting for specimen sex, age, size and bone density. Larger coronal plane eccentricity at injury was associated with smaller resultant coronal plane force. The level of coronal plane eccentricity at failure appears to distinguish between the types of injuries sustained, with hard tissue structure injuries more common at low levels of eccentricity and soft tissue structure injuries more common at high levels of eccentricity. There was no relationship between axial force and lateral bending moment at injury which has been previously proposed as an injury criterion. These results provide the foundation for designing and evaluating strategies and devices for preventing severe spinal injuries.

Effect of Velocity and Duration of Residual Compression in a Rat Dislocation Spinal Cord Injury Model.

Early decompression of the traumatically injured and persistently compressed spinal cord is intuitively beneficial for neurological outcome. Despite considerable pre-clinical evidence of a neurological benefit to early decompression, the effect of early surgical decompression in clinical spinal cord injury (SCI) remains less clear. The discrepancy between pre-clinical and clinical results may be due to differences between the biomechanical variables used in pre-clinical animal models and the biomechanical conditions occurring in clinical injuries. These pre-clinical variables include region of spinal cord, velocity of impact, and injury mechanism. In this study, the effect of velocity and duration of residual compression on injury severity were evaluated using a novel, rodent model of cervical dislocation SCI. Fifty-two male Sprague-Dawley rats were included in five groups: two timings of decompression (24 min, 240 min), two velocities (10 mm/sec, 500 mm/sec), and a sham group. All injuries involved a 1.45-mm dorsal dislocation of the C6 vertebra relative to C5 with subsequent residual compression of 0.8 mm. Animals were evaluated for motor function using the Martinez open field, grip strength, and grooming tests for 6 weeks post-injury. Immunohistochemistry and histology following sacrifice were conducted with counts for NeuN- and choline acetyltransferase (ChAT)-positive neurons, and length of cavitation. Behavioral testing and histological analysis revealed that injuries induced by the high velocity were consistently more severe than those induced by the low velocity, with behavioral correlations ranging between 0.46 and 0.58 (p < 0.05). Longer duration of residual compression did not produce significantly more severe injuries as measured by functional tests and histology. These findings demonstrate that the velocity of the initial traumatic impact may be a more important factor than duration of residual compression in determining SCI severity in a dislocation model of SCI.

Shear stiffness in the lower cervical spine: Effect of sequential posterior element injury.

The aim of this study was to determine the effect of the posterior ligaments and facet joints on the shear stiffness of lower cervical functional spinal units in anterior, posterior, and lateral shear. Five functional spinal units were loaded in anterior, posterior, and right lateral shear up to 100 N using a custom-designed apparatus in a materials testing machine. Specimens were tested in three conditions: intact, with the posterior ligaments severed, and with the facet joints removed. There was a significant decrease in anterior stiffness in the 20-100 N load range from 186 (range: 98-327) N/mm in the intact condition to 105 (range: 78-142) N/mm in the disc-only condition (p = 0.03). Posterior stiffness between these condition decreased significantly from 134 (range: 92-182) N/mm to 119 (range: 83-181) N/mm (p = 0.03). There was no significant effect of posterior ligament removal on shear stiffness. No significant differences were found in the lateral direction or in the 0-20 N range for any direction. Under a 100-N shear load, the facet joints played a significant role in the stiffness of the cervical spine in the anterior-posterior direction, but not in the lateral direction.