Exploring the Influence of Facet Orientation and Tropism On Neutral Zone Properties.

Lumbar spine pathologies have been linked independently to both neutral zone (NZ) properties and facet joint anatomical characteristics; however, the effect of facet joint orientation (FO) and tropism (FT) on NZ properties remains unclear. The aim of the present study was to investigate how axial plane FO and FT relate to NZ range and stiffness in the human lumbar spine and porcine cervical spine. Seven human lumbar functional spine units (FSUs) and 94 porcine cervical FSUs were examined. FO and FT were measured, and in-vitro mechanical testing was used to determine anterior-posterior (AP) and flexion-extension (FE) NZ range and stiffness. FO and FT were found to have no significant relationship with AP and FE NZ range. Increases in FT were associated with greater FE and AP NZ stiffness in human FSUs, with no FT-NZ stiffness relationship observed in porcine specimens. A significant relationship (p < .001) between FO and FE NZ stiffness was observed for both porcine and human FSUs, with a more sagittal orientation of the facet joints being associated with decreased FE NZ stiffness. Given the link between NZ stiffness and pathological states of the lumbar spine, further research is warranted to determine the practical significance of the observed facet joint anatomical characteristic-NZ property relationship.

Implementing an accelerometer-based pelvis segment for low back kinetic analyses during dynamic movement tasks.

An accelerometer-based pelvis has been employed to study segment and joint kinematics during scenarios involving close human-object interface and/or line-of-sight obstructions. However, its accuracy for examining low back kinetic outcomes is unknown. This study compared reaction moments and contact forces of the L5S1 joint calculated with an accelerometer-based and optically tracked pelvis segment. An approach to correct the global pelvis position as a function of thigh angle was developed. One participant performed four dynamic tasks: forward bend, squat, sit-to-stand-to-sit, and forward lunge. A standard bottom-up inverse dynamics approach was used and the root mean square error (RMSE) and coefficient of determination (R2) were calculated to examine kinetic differences between the optical and accelerometer approaches. The RMSE observed for L5S1 reaction flexion-extension moments ranged from 1.32 Nm to 2.20 Nm (R2 ≥ 0.98). The RMSE for net shear and compression reaction forces ranged from 2.13 to 10.45 N and 0.63 - 4.96 N, respectively. Similarly, the RMSE for L5S1 joint contact shear and compression ranged from 13.45 N to 19.51 N (R2 ≥ 0.85) and 31.18 N - 55.97 N (R2 ≥ 0.97), respectively. In conclusion, the accelerometer-based pelvis together with the approach to correct the global pelvis position is a feasible approach for computing low back kinetics with a single equivalent muscle model. The observed error in joint contact forces represents less than 5 % of the NIOSH recommended action limits and is unlikely to alter the interpretation of low back injury risk.

Indentation mechanics and native collagen content in the cartilaginous endplate: A comparison between porcine cervical and human lumbar spines.

This study characterized the regional indentation mechanics and native collagen content in cartilaginous endplates (CEPs) from the porcine cervical spine, young human lumbar spine, and aged human lumbar spine. Seventeen endplates were included in this study: six porcine cervical, nine young human lumbar, and two aged human lumbar. Width and depth measurements were obtained using a digital caliper and used to size-normalize and identify the central, anterior, posterior, and lateral regions. Regional microindentation tests were performed using a serial robot, where surface locations were loaded/unloaded at 0.1 mm/s and held at a constant 10 N force for 30 s. Loading stiffness and creep displacement were obtained from force-displacement data. Immunofluorescence staining for type I and type II collagen was subsequently performed on sagittal sections of all endplate regions. 255 images were obtained from which fluorescence intensity, sub-surface void area, and cartilage thickness were measured. CEPs from the young human lumbar spine were, on average, 27% more compliant, 0.891 mm thicker, had a lower fluorescence intensity for native collagen proteins within the cartilage (-58%) and subchondral bone (-24%), and had a sub-surface void area that was 19.7 times greater than porcine cervical CEPs. Compared to aged human lumbar CEPs, young human lumbar CEPs were 57% stiffer, 0.568 mm thicker, had a higher fluorescence intensity for native collagen proteins within the cartilage (+30%) and subchondral bone (+46%), and had a sub-surface void area that was 10.6 times smaller. Although not a perfect mechanical and structural surrogate, porcine cervical CEPs provided initial conditions that may be more representative of the young and healthy human lumbar spine compared to aged human cadaveric specimens. The indentation properties presented may have further applications to finite element models of the human lumbar spine.

Initiation and accumulation of loading induced changes to native collagen content and microstructural damage in the cartilaginous endplate.

BACKGROUND CONTEXT: Injury to the cartilaginous endplate (CEP) is linked to clinically relevant low back disorders, including intervertebral disc degeneration and pain reporting. Despite this link to clinical disorders, the CEP injury pathways and the modulating effect of mechanical loading parameters on the pace of damage accumulation remains poorly understood. PURPOSE: This study examined the effect of cyclic loading on the initiation and accumulation of changes to native collagen content (type I, type II) and microstructural damage in the central region of cadaveric porcine CEPs. STUDY DESIGN: In vitro longitudinal study. METHODS: One hundred fourteen porcine cervical spinal units were included (N=6 per group). The study contained a control group (no cyclic loading) and 18 experimental groups that differed by loading duration (1,000, 3,000, 5,000 cycles), joint posture (flexed, neutral), and cyclic peak compression variation (10%, 20%, 40%). Multicolor immunofluorescence staining was used to quantify loading induced changes to type I (ie, subchondral bone) and type II (ie, endplate) native collagen content (fluorescence area, fluorescence intensity) and microstructural damage (pore area [transverse plane], void area along the CEP-bone border [sagittal plane]). RESULTS: Significant main effects of loading duration and posture were observed for fluorescence area and fluorescence intensity of type I and II collagen. In the transverse plane, type II fluorescence area significantly decreased following 1,000 cycles (-12%), but a significant change in fluorescence intensity was not observed until 3,000 cycles (-17%). Type II fluorescence area (-14%) and intensity (-10%) were both significantly less in flexed postures compared to neutral. Similar trends were observed for type I collagen in the sagittal plane sections. Generally, significant changes to fluorescence area were accompanied by the development of microstructural voids along the endplate-subchondral bone border. CONCLUSIONS: These findings demonstrate that microstructural damage beneath the endplate surface occurs before significant changes to the density of native type I and II collagen fibers. Although flexed postures were associated with greater and accelerated changes to native collagen content, the injury initiation mechanism appears similar to neutral. CLINICAL SIGNIFICANCE: Neutral joint postures can delay the initiation and pace of microdamage accumulation in the CEP during low-to-moderate demand lifting tasks. Furthermore, the management of peak compression exposures appeared relevant only when a neutral posture was maintained. Therefore, clinical low back injury prevention and load management efforts should consider low back posture in parallel with applied joint forces.

Increasing movement during office work at sit-stand workstations: A novel seating device to facilitate transitions.

A novel active office chair (Movably Pro) was designed to facilitate frequent sit-stand movement 1) through auditory and tactile prompts and 2) with minimal-to-no work surface adjustment when transitioning. The purpose of this study was to compare lumbopelvic kinematics, discomfort, and task performance between the novel chair and traditional sitting/standing. Sixteen participants completed three separate 2-h sedentary exposures. Although participants transitioned every 3 min between sitting and standing with the novel chair, productivity was not affected. When standing in the novel chair, the lumbopelvic angles fell in between traditional sitting and standing (p < 0.01). Movement and/or postural changes that occurred with the novel chair reduced low back and leg discomfort for pain developers (PDs) (p < 0.01). All participants classified as PDs in traditional standing were non-PDs with the novel chair. This intervention was effective in reducing sedentary time without the time loss associated with desk movement.

Experimentally dissociating the overuse mechanisms of endplate fracture lesions and Schmorl's node injuries using the porcine cervical spine model.

BACKGROUND: Compared to the documented overuse mechanisms of endplate fracture lesions, the cause of Schmorl's node injuries remains unknown, despite existing hypotheses. Therefore, this study aimed to examine and dissociate the overuse injury mechanisms of these spinal pathologies. METHODS: Forty-eight porcine cervical spinal units were included. Spinal units were randomly assigned to groups that differed by initial condition (control, sham, chemical fragility, structural void) and loading posture (flexed, neutral). Chemical fragility and structural void groups involved a verified 49% reduction in localized infra-endplate trabecular bone strength and removal of central trabecular bone, respectively. All experimental groups were exposed to cyclic compression loading that was normalized to 30% of the predicted tolerance until failure occurred. The cycles to failure were examined using a general linear model and the distribution of injury types were examined using chi-squared statistics. FINDINGS: The incidence of fracture lesions and Schmorl's nodes was 31(65%) and 17(35%), respectively. Schmorl's nodes were exclusive to chemical fragility and structural void groups and 88% occurred in the caudal joint endplate (p = 0.004). In contrast, 100% of control and sham spinal units sustained fracture lesions, with 100% occurring in the cranial joint endplate (p < 0.001). Spinal units tolerated 665 fewer cycles when cyclically loaded in flexed postures compared to neutral (p = 0.015). Furthermore, the chemical fragility and structural void groups tolerated 5318 fewer cycles compared to the control and sham groups (p < 0.001). INTERPRETATION: These findings demonstrate that Schmorl's node and fracture lesion injuries can result from pre-existing differences in the structural integrity of trabecular bone supporting the central endplate.

Incidence of Compression-Induced Microinjuries in the Cartilage Endplate of the Spine.

STUDY DESIGN: In vitro biomechanical study. OBJECTIVE: This study investigated the incidence of microstructural endplate injuries caused by cyclic compression loading. The covarying effects of joint posture, loading duration, and peak compression variation were assessed. SUMMARY OF BACKGROUND DATA: The endplate is physiologically and functionally important for the maintenance of spine health. Despite the ability to radiographically diagnose and classify macroscopic endplate injuries, the mechanical mechanisms of injury initiation and progression remain largely unknown. METHODS: One hundred and fourteen porcine cervical spinal units were examined. All spinal units were exposed to preconditioning tests, followed by cyclic compression testing that differed by posture (flexed, neutral), loading duration (1000, 3000, 5000 cycles), and peak compression variation (10%, 20%, 40%). Microstructural injuries were examined via immunofluorescence staining for collagen I ( i.e. , subchondral bone) and collagen II ( i.e. , hyaline cartilage endplate). From the 678 acquired images, the incidence of node, avulsion, cartilage, and circumferential pore microinjuries were determined. The distribution of microinjuries between postures, spinal levels, and vertebrae were evaluated along with the associations of incidence and size of injuries with loading duration and variation. RESULTS: The incidence of avulsion injuries was significantly greater in caudal endplates (92%, P =0.006). No other injuries differed between vertebrae ( P ≥0.804) and no significant differences were observed between spinal units ( P ≥0.158). With respect to posture, 100% ( P <0.001) and 90% ( P <0.001) of avulsion and node injuries, respectively, occurred in flexed postures, whereas 82% ( P <0.001) of cartilage microinjuries occurred with neutral postures. Loading duration was significantly associated with microinjury incidence ( P <0.001) and lesion size ( P ≤0.003). CONCLUSION: Mechanical factors such as posture did not appreciably affect the incidence of endplate injury, but microinjury types were differently distributed between flexed and neutral postures. The duration of compression was shown to have an important role in the incidence of microinjury and lesion size.

Mechanical work and energy of sit-to-stand and stand-to-sit transitions performed with traditional and dynamic office chair designs.

BACKGROUND: Adherence to sit-stand workstation usage has been shown to decrease post-intervention, with the reported reasons related to fatigue, cumbersome workstation adjustments, and focus. OBJECTIVE: To characterize the mechanical work and total energy required to perform transitions from a traditional office chair and a dynamic chair designed specifically for sit-stand workstations. The whole-body, thigh, and shank centre-of-mass (CoM) were evaluated. METHODS: Fifteen participants (8 male; 7 female) performed three intermittent sit-to-stand and stand-to-sit transitions from the traditional and dynamic chairs. Kinematic data of the trunk, pelvis, and lower extremities were collected using an optoelectronic motion capture system and triaxial accelerometers. The change in total energy and work between the sitting and standing postures were evaluated for each CoM point. Lumbar spine range-of-motion was further assessed between chair conditions. RESULTS: Chair designs facilitated opposite work and energy responses for a given transition. Transitions performed from the dynamic chair reduced the work and total energy of the whole-body CoM, by ±8.5J and ±214.6J (p < 0.001), respectively. The work and energy of the thigh CoM differed within transitions (p < 0.001), but the positive and negative components were similar between chairs (work =±0.18J, energy =±0.55J). The dynamic chair increased the total energy (±38.3J, p < 0.001) but not the work of the shank CoM (±1.1J, p≥0.347). CONCLUSION: The required mechanical work and energy of sit-to-stand and stand-to-sit transitions was modified by chair design. These outcomes have the potential to address identified reasons for the disuse of sit-stand workstations.

Cyclic loading history alters the joint compression tolerance and regional indentation responses in the cartilaginous endplate.

This study quantified the effect of subthreshold loading histories that differed by joint posture (neutral, flexed), peak loading variation (10%, 20%, 40%), and loading duration (1000, 3000, 5000 cycles) on the post-loading Ultimate Compressive Tolerance (UCT), yield force, and regional Cartilaginous End Plate (CEP) indentation responses (loading stiffness and creep displacement). One hundred and fourteen porcine spinal units were included. Following conditioning and cyclic compression exposures, spinal units were transected and one endplate from each vertebra underwent subsequent UCT or microindentation testing. UCT testing was conducted by compressing a single vertebra at a rate of 3 kN/s using an indenter fabricated to a representative intervertebral disc size and shape. Force and actuator position were sampled at 100 Hz. Non-destructive uniaxial CEP indentation was performed at five surface locations (central, anterior, posterior, right, left) using a Motoman robot and aluminum indenter (3 mm hemisphere). Force and end-effector position were sampled at 10 Hz. A significant three-way interaction was observed for UCT (p = 0.038). Compared to neutral, the UCT was, on average, 1.9 kN less following each flexed loading duration. No effect of variation was observed in flexion; however, 40% variation caused the UCT to decrease by an average of 2.13 kN and 2.06 kN following 3000 and 5000 cycles, respectively. The indentation stiffness in the central CEP mimicked the UCT response. These results demonstrate a profound effect of posture on post-loading UCT and CEP behaviour. Control of peak compression exposures became particularly relevant only when a neutral posture was maintained and beyond the midpoint of the predicated lifespan.

Kinematic adaptations to restricting spine motion during symmetrical lifting.

When lifting an object from the ground a person has many possible whole-body movement solutions to accomplish the task. It is unclear why lifters use most of their available lumbar spine flexion range-of-motion despite many ergonomic guidelines advising against doing so. Experimentally restricting spine motion and observing compensatory movement strategies is one approach to address this knowledge gap. A kinematic analysis was performed on 16 participants who completed symmetrical lifting tasks with and without wearing a spine motion-restricting device. Sagittal trunk, lumbar spine, and lower extremity kinematics, along with stance width and foot orientation in the transverse plane were evaluated between restricted and unrestricted lifting conditions. Restricting spine motion required greater ankle dorsiflexion (p < 0.001), knee flexion (p < 0.001), and hip flexion (p < 0.001) motion in comparison to unrestricted lifting. Motion was reallocated such that hip flexion showed the largest increase in restricted lifting, followed by ankle dorsiflexion, then knee flexion compared to unrestricted lifting. Trunk inclination decreased (i.e., more upright) in restricted compared to unrestricted lifting (p < 0.001). Neither stance width (p = 0.163) nor foot orientation (p = 0.228) were affected by restricting spine motion. These adaptive movements observed indicate lower extremity joint motion must be available and controlled to minimize lumbar spine flexion during lifting.

Mechanically induced histochemical and structural damage in the annulus fibrosus and cartilaginous endplate: a multi-colour immunofluorescence analysis.

The annulus fibrosus (AF) and endplate (EP) are collagenous spine tissues that are frequently injured due to gradual mechanical overload. Macroscopic injuries to these tissues are typically a by-product of microdamage accumulation. Many existing histochemistry and biochemistry techniques are used to examine microdamage in the AF and EP; however, there are several limitations when used in isolation. Immunofluorescence may be sensitive to histochemical and structural damage and permits the simultaneous evaluation of multiple proteins-collagen I (COL I) and collagen II (COL II). This investigation characterized the histochemical and structural damage in initially healthy porcine spinal joints that were either unloaded (control) or loaded via biofidelic compression loading. The mean fluorescence area and mean fluorescence intensity of COL II significantly decreased (- 54.9 and - 44.8%, respectively) in the loaded AF (p ≤ 0.002), with no changes in COL I (p ≥ 0.471). In contrast, the EP displayed similar decreases in COL I and COL II fluorescence area (- 35.6 and - 37.7%, respectively) under loading conditions (p ≤ 0.027). A significant reduction (-31.1%) in mean fluorescence intensity was only observed for COL II (p = 0.043). The normalized area of pores was not altered on the endplate surface (p = 0.338), but a significant increase (+ 7.0%) in the void area was observed on the EP-subchondral bone interface (p = 0.002). Colocalization of COL I and COL II was minimal in all tissues (R < 0.34). In conclusion, the immunofluorescence analysis captured histochemical and structural damage in collagenous spine tissues, namely, the AF and EP.

Characterizing Lumbar Spine Kinematics and Kinetics During Simulated Low-Speed Rear Impact Collisions.

BACKGROUND: Recent work has demonstrated that low back pain is a common complaint following low-speed collisions. Despite frequent pain reporting, no studies involving human volunteers have been completed to examine the exposures in the lumbar spine during low-speed rear impact collisions. METHODS: Twenty-four participants were recruited and a custom-built crash sled simulated rear impact collisions, with a change in velocity of 8 km/h. Randomized collisions were completed with and without lumbar support. Inverse dynamics analyses were conducted, and outputs were used to generate estimates of peak L4/L5 joint compression and shear. RESULTS: Average (SD) peak L4/L5 compression and shear reaction forces were not significantly different without lumbar support (compression = 498.22 N [178.0 N]; shear = 302.2 N [98.5 N]) compared to with lumbar support (compression = 484.5 N [151.1 N]; shear = 291.3 N [176.8 N]). Lumbar flexion angle at the time of peak shear was 36° (12°) without and 33° (11°) with lumbar support. CONCLUSION: Overall, the estimated reaction forces were 14% and 30% of existing National Institute of Occupational Safety and Health occupational exposure limits for compression and shear during repeated lifting, respectively. Findings also demonstrate that, during a laboratory collision simulation, lumbar support does not significantly influence the total estimated L4/L5 joint reaction force.

Reaction Forces and Flexion-Extension Moments Imposed on Functional Spinal Units With Constrained and Unconstrained In Vitro Testing Systems.

A mechanical goal of in vitro testing systems is to minimize differences between applied and actual forces and moments experienced by spinal units. This study quantified the joint reaction forces and reaction flexion-extension moments during dynamic compression loading imposed throughout the physiological flexion-extension range of motion. Constrained (fixed base) and unconstrained (floating base) testing systems were compared. Sixteen porcine spinal units were assigned to both testing groups. Following conditioning tests, specimens were dynamically loaded for 1 cycle with a 1 Hz compression waveform to a peak load of 1 kN and 2 kN while positioned in five different postures (neutral, 100% and 300% of the flexion and extension neutral zone), totaling ten trials per functional spinal unit (FSU). A six degree-of-freedom force and torque sensor was used to measure peak reaction forces and moments for each trial. Shear reaction forces were significantly greater (25.5 N-85.7 N) when the testing system was constrained compared to unconstrained (p < 0.029). The reaction moment was influenced by posture (p = 0.037), particularly in C5C6 spinal units. In 300% extension (C5C6), the reaction moment was, on average, 9.9 N·m greater than the applied moment in both testing systems and differed from all other postures (p < 0.001). The reaction moment error was, on average, 0.45 N·m at all other postures. In conclusion, these findings demonstrate that comparable reaction moments can be achieved with unconstrained systems, but without inducing appreciable shear reaction forces.

Strain Response in the Facet Joint Capsule During Physiological Joint Rotation and Translation Following a Simulated Impact Exposure: An In Vitro Porcine Model.

Low back pain (LBP) is frequently reported following rear impact collisions. Knowledge of how the facet joint capsule (FJC) mechanically behaves before and after rear impact collisions may help explain LBP development despite negative radiographic evidence of gross tissue failure. This study quantified the Green strain tensor in the facet joint capsule during rotation and translation range-of-motion tests completed before and following an in vitro simulation of a rear impact collision. Eight FSUs (4 C3-C4, 4 C5-C6) were tested. Following a preload test, FSUs were flexed and extended at 0.5 deg/s until an ±8 N·m moment was achieved. Anterior and posterior joint translation was then applied at 0.2 mm/s until a target ±400 N shear load was imposed. Markers were drawn on the facet capsule surface and their coordinates were tracked during pre- and postimpact range-of-motion tests. Strain was defined as the change in point configuration relative to the determined neutral joint posture. There were no significant differences (p > 0.05) observed in all calculated FJC strain components in rotation and translation before and after the simulated impact. Our results suggest that LBP development resulting from the initiation of strain-induced mechanoreceptors and nociceptors with the facet joint capsule is unlikely following a severe rear impact collision within the boundaries of physiological joint motion.

Regulating Movement Frequency and Speed: Implications for Lumbar Spine Load Management Strategies Demonstrated Using an In Vitro Porcine Model.

The relationship between internal loading dose and low-back injury risk during lifting is well known. However, the implications of movement parameters that influence joint loading rates-movement frequency and speed-on time-dependent spine loading responses remain less documented. This study quantified the effect of loading rate and frequency on the tolerated cumulative loading dose and its relation to joint lifespan. Thirty-two porcine spinal units were exposed to biofidelic compression loading paradigms that differed by joint compression rate (4.2 and 8.3 kN/s) and frequency (30 and 60 cycles per minute). Cyclic compression testing was applied until failure was detected or 10,800 continuous cycles were tolerated. Instantaneous weighting factors were calculated to evaluate the cumulative load and Kaplan-Meier survival probability functions were examined following nonlinear dose normalization of the cyclic lifespan. Significant reductions in cumulative compression were tolerated when spinal units were compressed at 8.3 kN/s (P < .001, 67%) and when loaded at 30 cycles per minute (P = .008, 45%). There was a positive moderate relationship between cumulative load tolerance and normalized cyclic lifespan (R2 = .52), which was supported by joint survivorship functions. The frequency and speed of movement execution should be evaluated in parallel to loading dose for the management of low-back training exposures.

Exploring the influence of impact severity and posture on vertebral joint mechanics in an in-vitro porcine model.

To date, no in vitro studies have been conducted to explore lumbar soft tissue injury potential and altered mechanical properties from exposure to impact forces. After a motor vehicle collision (MVC), the cause of reported acute onset low back pain is difficult to associate with potential soft tissue strain injury sites including the facet joint and innervated facet joint capsule ligament (FJC). Thus, the purpose of this investigation was to quantify intervertebral anterior-posterior (AP) translation and facet joint capsule strain under varying postures and impact severities. Seventy-two porcine spinal units were exposed to three levels of impact severity (4 g, 8 g, 11 g), and posture (Neutral, Flexion, Extension). Impacts were applied using a custom-built impact track that replicated parameters experienced in low to moderate speed rear-end MVCs. Flexion-extension and anterior-posterior shear neutral zone testing were completed prior to impact. AP intervertebral translation and the strain tensor of the facet capsule ligament were measured during impacts. A significant main effect of collision severity was observed for peak AP intervertebral translation (4 g-2.8 ±0.53 mm; 8 g-6.4 ±2.9 mm; 11 g-8.3 ±0.45 mm) and peak FJC shear strain (2.37% strain change from 4 g to 11 g impact severity). Despite observed main effects of impact severity, no influence of posture was observed. This lack of influence of posture and small FJC strain magnitudes suggest that the FJC does not appear to undergo injurious or permanent mechanical changes in response to low-to-moderate MVC impact scenarios.

Reconstructing an accelerometer-based pelvis segment for three-dimensional kinematic analyses during laboratory simulated tasks with obstructed line-of-sight.

Close interface between humans and inanimate objects (furniture, assistive devices, and external loads) can obstruct line-of-sight in biomechanics studies that utilize optoelectronic motion capture systems. This specific problem is frequently encountered with the pelvis segment. This study sought to compare joint and pelvis angles computed from a pelvis-fixed local coordinate system (LCS) that was constructed from optically tracked pelvis landmarks (gold standard) and landmarks derived from angular deviations calculated from triaxial accelerometer data. One participant performed seven tasks: sitting, forward bend, sit-to-stand-to-sit, forward lunge, symmetrical squat, asymmetrical squat, and gait. The root mean square error (RMSE) and coefficient of determination (R2) were examined for the pelvis, lumbar spine, and hip joint angles calculated using the standard and accelerometer-based methods for creating a LCS. The RMSE values for global pelvis angles ranged from 2.2° (gait; R2 = 0.47) to 4.9° (sit-to-stand-to-sit; R2 = 0.98), 0.6° (sitting; R2 = 0.88) to 7.4° (gait; R2 = 0.39), and 1.5° (forward bend; R2 = 0.99) to 2.9° (sit-to-stand-to-sit; R2 = 0.99) for motion about the X, Y, and Z axes, respectively. The magnitude of error observed for adjacent joint motion was lowest about the Z axis for all tasks. In conclusion, the accelerometer-based LCS offers an alternative method for computing pelvis and adjacent joint angles without the reliance on a visual line-of-sight. For motion about the X and Y axes, time-series data derived with the accelerometer-based method may be less representative of discrete events, particularly for gait and lunging tasks.

Partitioning the total seatback reaction force amongst the lumbar spine motion segments during simulated rear-impact collisions.

Purpose. This study aimed to determine how the seatback force is distributed across lumbar spine motion segments during a simulated low-velocity rear-impact collision with and without the application of mechanical lumbar support. Methods. A ferroresistive pressure-sensing system was used during simulated rear-impact collisions (ΔV = 7.66 km/h). Total seatback reaction force was derived from pressure recordings as the product of calibrated pressure outputs and sensel areas. The three-dimensional position of the pressure mat and the lumbar spinous processes were tracked and then used to extract the seatback force that was applied to the lumbar motion segments. Results. On average, 77% (637 N) and 53% (430 N) of the total seatback force was applied directly to the lumbar spine with and without lumbar support, respectively (p < 0.001). In addition to four of five individual motion segments bearing a greater force with lumbar support (p < 0.029), the distribution of the total lumbar force was found to be significantly different between support type conditions. Conclusions. Although lumbar supports can alter the magnitude and distribution of shear force applied to the lumbar spine during low-velocity rear-impact collisions, they do not appear to elevate the injury risk.

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

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

Strain of the facet joint capsule during rotation and translation range-of-motion tests: an in vitro porcine model as a human surrogate.

BACKGROUND CONTEXT: Prior data about the modulating effects of lumbar spine posture on facet capsule strains are limited to small joint deviations. Knowledge of facet capsule strain during rotational and translational intervertebral joint motion (ie, large joint deviations) under physiological loading could be useful as it may help explain why visually normal lumbar spinal joints become painful. PURPOSE: This study quantified the strain tensor of the facet capsule during rotation and translation range-of-motion tests. STUDY DESIGN/SETTING: Strain was calculated in isolated porcine functional spinal units. Following a preload, each specimen underwent a flexion/extension rotation (F/E) followed by an anterior/posterior translation (A/P) range-of-motion test while under a 300 N compression load. METHODS: Twenty porcine spinal units (10 C3-C4, 10 C5-C6) were tested. Joint flexion/extension was imposed by applying a ±8 Nm moment at a rate of 0.5°/s, and translation was facilitated by loading the caudal vertebra with a ±400 N shear force at a rate of 0.2 mm/s. Points were drawn on the exposed capsule surface and their coordinates were optically tracked throughout each test. Strain was calculated as the displacement of the point configuration with respect to the configuration in a neutral joint position. RESULTS: Compared to a neutral posture, superior-inferior strain increased and decreased systematically during flexion and extension, respectively. Posterior displacement of the caudal vertebra by more than 1.3 mm was associated with negative strains, which was significantly lower than the +4.6% strain observed during anterior displacement (p≥.199). The shear strain associated with anterior translation was, on average, -1.1% compared to a neutral joint posture. CONCLUSIONS: These results demonstrate that there is a combination of strain types within the facet capsule when spinal units are rotated and translated. The strains documented in this study did not reach the thresholds associated with nociception. CLINICAL RELEVANCE: The magnitude of flexion-extension rotation and anterior-translation may glean insight into the facet capsule deformation response under low compression (300 N) loading scenarios. Further, intervertebral joint motion alone, even under low compression loading, does not appear to initiate a clinically relevant pain response in the lumbar facet capsule of a nondegenerated spinal joint.