The association between isoinertial trunk muscle performance and low back pain in male adolescents.

The literature reports inconsistent findings regarding the association between low back pain (LBP) and trunk muscle function, in both adults and children. The strength of the relationship appears to be influenced by how LBP is qualified and the means by which muscle function is measured. The aim of this study was to examine the association between isoinertial trunk muscle performance and consequential (non-trivial) low back pain (LBP) in male adolescents. Healthy male adolescents underwent anthropometric measurements, clinical evaluation, and tests of trunk range of motion (ROM), maximum isometric strength (STRENGTH) and peak movement velocity (VEL), using an isoinertial device. They provided information about their regular sporting activities, history and family history of LBP. Predictors of "relevant/consequential LBP" were examined using multivariable logistic regression. LBP status was reassessed after 2 years and the change from baseline was categorised. At baseline, 33/95 (35%) subjects reported having experienced consequential LBP. BMI, a family history of LBP, and regularly playing sport were each significantly associated with a history of consequential LBP (p < 0.05). 85/95 (89%) boys participated in the follow-up: 51 (60%) reported no LBP at either baseline or follow-up (never LBP); 5 (6%) no LBP at baseline, but LBP at follow-up (new LBP); 19 (22%) LBP at baseline, but none at follow-up; and 10 (12%) LBP at both time-points (recurrent/persistent LBP). The only distinguishing features of group membership in these small groups were: fewer sport-active in the "never LBP" group); worse trunk mobility, in the "persistent LBP" group, lower baseline sagittal ROM in the "never LBP" and "new LBP" (p < 0.05). Regular involvement in sport was a consistent predictor of LBP. Isoinertial trunk performance was not associated with LBP in adolescents.

Validation of a noninvasive dynamic spinal stiffness assessment methodology in an animal model of intervertebral disc degeneration.

STUDY DESIGN: An experimental in vivo ovine model of intervertebral disc degeneration was used to quantify the dynamic motion response of the lumbar spine. OBJECTIVE: The purpose of this study was to: (1) compare invasively measured lumbar vertebral bone acceleration responses to noninvasive displacement responses, and (2) determine the effects of a single level degenerative intervertebral disc lesion on these responses. SUMMARY OF BACKGROUND DATA: Biomechanical techniques have been established to quantify vertebral motion responses, yet their invasiveness limits their use in a clinical setting. METHODS: Twenty-five Merino sheep were examined; 15 with surgically induced disc degeneration at L1-L2 and 10 controls. Triaxial accelerometers were rigidly fixed to the L1 and L2 spinous processes and dorsoventral (DV) mechanical excitation (20-80 N, 100 milliseconds) was applied to L3 using a spinal dynamometer. Peak force and displacement and peak-peak acceleration responses were computed for each trial and a least squares regression analysis assessed the correlation between L3 displacement and adjacent (L2) segment acceleration responses. An analysis of covariance (ANCOVA) was performed to test the homogeneity of slopes derived from the regression analysis and to assess the mean differences. RESULTS: A significant, positive, linear correlation was found between the DV displacement of L3 and the DV acceleration measured at L2 for both normal (R = 0.482, P < 0.001) and degenerated disc groups (R = 0.831, P < 0.001). The L3 DV displacement was significantly lower (ANCOVA, P < 0.001) for the degenerated group (mean: 10.39 mm) in comparison to the normal group (mean: 9.07 mm). Mean peak-peak L2-L1 DV acceleration transfer was also significantly reduced from 12.40 m/s to 5.50 m/s in the degenerated animal group (ANCOVA, P < 0.001). CONCLUSION: The findings indicate that noninvasive displacement measurements of the prone-lying animal can be used to estimate the segmental and intersegmental motions in both normal and pathologic spines.

Early stage disc degeneration does not have an appreciable affect on stiffness and load transfer following vertebroplasty and kyphoplasty.

Vertebroplasty and kyphoplasty have been reported to alter the mechanical behavior of the treated and adjacent-level segments, and have been suggested to increase the risk for adjacent-level fractures. The intervertebral disc (IVD) plays an important role in the mechanical behavior of vertebral motion segments. Comparisons between normal and degenerative IVD motion segments following cement augmentation have yet to be reported. A microstructural finite element model of a degenerative IVD motion segment was constructed from micro-CT images. Microdamage within the vertebral body trabecular structure was used to simulate a slightly (I = 83.5% of intact stiffness), moderately (II = 57.8% of intact stiffness), and severely (III = 16.0% of intact stiffness) damaged motion segment. Six variable geometry single-segment cement repair strategies (models A-F) were studied at each damage level (I-III). IVD and bone stresses, and motion segment stiffness, were compared with the intact and baseline damage models (untreated), as well as, previous findings using normal IVD models with the same repair strategies. Overall, small differences were observed in motion segment stiffness and average stresses between the degenerative and normal disc repair models. We did however observe a reduction in endplate bulge and a redistribution in the microstructural tissue level stresses across both endplates and in the treated segment following early stage IVD degeneration. The cement augmentation strategy placing bone cement along the periphery of the vertebra (model E) proved to be the most advantageous in treating the degenerative IVD models by showing larger reductions in the average bone stresses (vertebral and endplate) as compared to the normal IVD models. Furthermore, only this repair strategy, and the complete cement fill strategy (model F), were able to restore the slightly damaged (I) motion segment stiffness above pre-damaged (intact) levels. Early stage IVD degeneration does not have an appreciable effect in motion segment stiffness and average stresses in the treated and adjacent-level segments following vertebroplasty and kyphoplasty. Placing bone cement in the periphery of the damaged vertebra in a degenerative IVD motion segment, minimizes load transfer, and may reduce the likelihood of adjacent-level fractures.

Evaluation of abduction range of motion and avoidance of inferior scapular impingement in a reverse shoulder model.

The purpose of this study was to determine the effects of prosthetic design and surgical technique of reverse shoulder implants on total abduction range of motion and impingement on the inferior scapular neck. Custom implants in three glenosphere diameters (30, 36, and 42 mm), with 3 different centers of rotation offsets (0, +5, and +10 mm), were placed into a Sawbones scapula (Pacific Research Laboratories, Vashon, WA) in 3 different positions: superior, center, and inferior glenoid. Humeral sockets were manufactured with a 130 degrees , 150 degrees , and 170 degrees neck-shaft angle. Four independent factors (glenosphere diameter, center of rotation offset, glenosphere position on the glenoid, and humeral neck-shaft angle) were compared with the 2 dependent factors of range of motion and inferior scapular impingement. Center of rotation offset had the largest effect on range of motion, followed by glenosphere position. Neck-shaft angle had the largest effect on inferior scapular impingement, followed by glenosphere position. This information may be useful to the surgeon when deciding on the appropriate reverse implant.

Hierarchy of stability factors in reverse shoulder arthroplasty.

Reverse shoulder arthroplasty is being used more frequently to treat irreparable rotator cuff tears in the presence of glenohumeral arthritis and instability. To date, however, design features and functions of reverse shoulder arthroplasty, which may be associated with subluxation and dislocation of these implants, have been poorly understood. We asked: (1) what is the hierarchy of importance of joint compressive force, prosthetic socket depth, and glenosphere size in relation to stability, and (2) is this hierarchy defined by underlying and theoretically predictable joint contact characteristics? We examined the intrinsic stability in terms of the force required to dislocate the humerosocket from the glenosphere of eight commercially available reverse shoulder arthroplasty devices. The hierarchy of factors was led by compressive force followed by socket depth; glenosphere size played a much lesser role in stability of the reverse shoulder arthroplasty device. Similar results were predicted by a mathematical model, suggesting the stability was determined primarily by compressive forces generated by muscles.

Modeling the onset and propagation of trabecular bone microdamage during low-cycle fatigue.

Relatively small amounts of microdamage have been suggested to have a major effect on the mechanical properties of bone. A significant reduction in mechanical properties (e.g. modulus) can occur even before the appearance of microcracks. This study uses a novel non-linear microdamaging finite-element (FE) algorithm to simulate the low-cycle fatigue behavior of high-density trabecular bone. We aimed to investigate if diffuse microdamage accumulation and concomitant modulus reduction, without the need for complete trabecular strut fracture, may be an underlining mechanism for low-cycle fatigue failure (defined as a 30% reduction in apparent modulus). A microCT constructed FE model was subjected to a single cycle monotonic compression test, and constant and variable amplitude loading scenarios to study the initiation and accumulation of low-cycle fatigue microdamage. Microcrack initiation was simulated using four damage criteria: 30%, 40%, 50% and 60% reduction in bone element modulus (el-MR). Evaluation of structural (apparent) damage using the four different tissue level damage criteria resulted in specimen fatigue failure at 72, 316, 969 and 1518 cycles for the 30%, 40%, 50% and 60% el-MR models, respectively. Simulations based on the 50% el-MR model were consistent with previously published experimental findings. A strong, significant non-linear, power law relationship was found between cycles to failure (N) and effective strain (Deltasigma/E(0)): N=1.394x10(-25)(Deltasigma/E(0))(-12.17), r(2)=0.97, p<0.0001. The results suggest that microdamage and microcrack propagation, without the need for complete trabecular strut fracture, are mechanisms for high-density trabecular bone failure. Furthermore, the model is consistent with previous numerical fatigue simulations indicating that microdamage to a small number of trabeculae results in relatively large specimen modulus reductions and rapid failure.

Effects of disc degeneration on neurophysiological responses during dorsoventral mechanical excitation of the ovine lumbar spine.

Mechanisms of spinal manipulation and mobilization include the elicitation of neuromuscular responses, but it is not clear how these responses are affected or altered by disc degeneration. We studied the neurophysiological responses of the normal and degenerated ovine spine subjected to mechanical excitation (varying force amplitude and duration) consistent with spinal manipulative therapy (SMT). Needle electromyographic (EMG) multifidus muscle activation adjacent to the L3 and L4 spinous processes and compound action potentials (CAPs) of the L4 nerve roots were measured during the application of dorsoventral mechanical excitation forces designed to mimic SMT force-time profiles used routinely in clinical practice. The magnitude and percentage of positive EMG responses increased with increasing SMT force magnitude, but not SMT pulse duration, whereas CAP responses were greatest for shorter duration pulses. Disc degeneration was associated with a reduction (20-25%) in positive EMG responses, and a concomitant increase (4.5-10.2%) in CAP responses.

Predicting trabecular bone microdamage initiation and accumulation using a non-linear perfect damage model.

Studies evaluating the mechanical behavior of the trabecular microstructure play an important role in our understanding of pathologies such as osteoporosis, and in increasing our understanding of bone fracture and bone adaptation. Understanding of such behavior in bone is important for predicting and providing early treatment of fractures. The objective of this study is to present a numerical model for studying the initiation and accumulation of trabecular bone microdamage in both the pre- and post-yield regions. A sub-region of human vertebral trabecular bone was analyzed using a uniformly loaded anatomically accurate microstructural three-dimensional finite element model. The evolution of trabecular bone microdamage was governed using a non-linear, modulus reduction, perfect damage approach derived from a generalized plasticity stress-strain law. The model introduced in this paper establishes a history of microdamage evolution in both the pre- and post-yield regions.

Mechanical properties of open-cell foam synthetic thoracic vertebrae.

This study presents comprehensive morphological and mechanical properties (static, dynamic) of open-cell rigid foams (Pacific Research Laboratories Inc. Vashon, WA) and a synthetic vertebral body derived from each of the foams. Synthetic vertebrae were comprised of a cylindrical open-cell foam core enclosed by a fiberglass resin cortex. The open-cell rigid foam was shown to have similar morphology and porosity as human vertebral cancellous bone, and exhibited a crush or fracture consolidation band typical of open-celled materials and cancellous bone. However, the foam material density was 40% lower than natural cancellous bone resulting in a lower compressive apparent strength and apparent modulus in comparison to human bone. During cyclic, mean compression fatigue tests, the synthetic vertebrae exhibited an initial apparent modulus, progressive modulus reduction, strain accumulation and S-N curve behaviour similar to human and animal vertebral cancellous bone. Synthetic open-cell foam vertebrae offer researchers an alternative to human vertebral bone for static and dynamic biomechanical experiments, including studies examining the effects of cement injection.

Intervertebral disc degeneration reduces vertebral motion responses.

STUDY DESIGN: A prospective in vivo experimental animal study. OBJECTIVE: To determine the effects of disc degeneration and variable pulse duration mechanical excitation on dorsoventral lumbar kinematic responses. SUMMARY OF BACKGROUND DATA: In vitro and in vivo biomechanical studies have examined spine kinematics during posteroanterior loading mimicking spinal manipulation therapy (SMT), but few (if any) studies have quantified SMT loading-induced spinal motion responses in the degenerated intervertebral disc. METHODS: Fifteen sheep underwent a survival surgical procedure resulting in chronic disc degeneration of the L1-L2 disc. Ten age- and weight-matched animals served as controls. Uniform pulse dorsoventral mechanical forces (80 N) were applied to the L3 spinous processes using 10-, 100-, and 200-ms duration pulses mimicking SMT. L3 displacement and L2-L1 acceleration in the control group were compared with the degenerated disc group. RESULTS: Dorsoventral displacements increased significantly (fivefold, P < 0.001) with increasing mechanical excitation pulse duration (control and degenerated disc groups). Displacements and L2-L1 acceleration transfer were significantly reduced (approximately 19% and approximately 50%, respectively) in the degenerated disc group compared with control (100- and 200-ms pulse duration protocols, P < 0.01). CONCLUSION: Dorsoventral vertebral motions are dependent on mechanical excitation pulse duration and are significantly reduced in animals with degenerated discs.

Center of rotation affects abduction range of motion of reverse shoulder arthroplasty.

Although clinical outcomes of the reverse shoulder replacement have noted improvements in pain and function, evaluation of these outcomes reveals concerns regarding progressive scapular notching and variability of functional improvements in range of motion. Therefore, an apparatus was designed to examine differences in abduction range of motion for seven configurations of reverse shoulder arthroplasty. An electronic goniometer was used to measure abduction range of motion, and digital video analysis was used to determine impingement points. Finally, a correlation analysis between range of motion and the effect of changing the center of rotation of the glenosphere was performed. As the center of rotation was moved more lateral from the glenoid, abduction range of motion increased. The greatest range of motion was 97 degrees +/- 0.9 degrees using a glenoid component with a center of rotation offset 10 mm +/- 0.4 mm from the glenoid. The smallest range of motion was 67 degrees +/- 1.8 degrees using a glenosphere with a center of rotation offset 0.5 mm +/- 0.1 mm from the glenoid surface. Range of motion always was limited by impingement points on the scapula. Inferiorly, adduction was limited by impingement on either the inferior scapular border or the glenoid. Superiorly, abduction was limited by impingement on the acromion. A positive linear correlation was found between abduction range of motion and center of rotation offset relative to the glenoid.

Muscular contributions to dynamic dorsoventral lumbar spine stiffness.

Spinal musculature plays a major role in spine stability, but its importance to spinal stiffness is poorly understood. We studied the effects of graded trunk muscle stimulation on the in vivo dynamic dorsoventral (DV) lumbar spine stiffness of 15 adolescent Merino sheep. Constant voltage supramaximal electrical stimulation was administered to the L3-L4 interspinous space of the multifidus muscles using four stimulation frequencies (2.5, 5, 10, and 20 Hz). Dynamic stiffness was quantified at rest and during muscle stimulation using a computer-controlled testing apparatus that applied variable frequency (0.46-19.7 Hz) oscillatory DV forces (13-N preload to 48-N peak) to the L3 spinous process of the prone-lying sheep. Five mechanical excitation trials were randomly performed, including four muscle stimulation trials and an unstimulated or resting trial. The secant stiffness (k (y) = DV force/L3 displacement, kN/m) and loss angle (phase angle, deg) were determined at 44 discrete mechanical excitation frequencies. Results indicated that the dynamic stiffness varied 3.7-fold over the range of mechanical excitation frequencies examined (minimum resting k (y) = 3.86 +/- 0.38 N/mm at 4.0 Hz; maximum k (y) = 14.1 +/- 9.95 N/mm at 19.7 Hz). Twenty hertz muscle stimulation resulted in a sustained supramaximal contraction that significantly (P < 0.05) increased k (y) up to twofold compared to rest (mechanical excitation at 3.6 Hz). Compared to rest, k (y) during the 20 Hz muscle stimulation was significantly increased for 34 of 44 mechanical excitation frequencies (mean increase = 55.1%, P < 0.05), but was most marked between 2.55 and 4.91 Hz (mean increase = 87.5%, P < 0.05). For lower frequency, sub-maximal muscle stimulation, there was a graded change in k (y), which was significantly increased for 32/44 mechanical excitation frequencies (mean increase = 40.4%, 10 Hz stimulus), 23/44 mechanical excitation frequencies (mean increase = 10.5%, 5 Hz stimulus), and 11/44 mechanical excitation frequencies (mean increase = 4.16%, 2.5 Hz stimulus) when compared to rest. These results indicate that the dynamic mechanical behavior of the ovine spine is modulated by muscle stimulation, and suggests that muscle contraction plays an important role in stabilizing the lumbar spine.

Dynamic dorsoventral stiffness assessment of the ovine lumbar spine.

Posteroanterior spinal stiffness assessments are common in the evaluating patients with low back pain. The purpose of this study was to determine the effects of mechanical excitation frequency on dynamic lumbar spine stiffness. A computer-controlled voice coil actuator equipped with a load cell and LVDT was used to deliver an oscillatory dorsoventral (DV) mechanical force to the L3 spinous process of 15 adolescent Merino sheep. DV forces (48 N peak, approximately 10% body weight) were randomly applied at periodic excitation frequencies of 2.0, 6.0, 11.7 and a 0.5-19.7 Hz sweep. Force and displacement were recorded over a 13-22 s time interval. The in vivo DV stiffness of the ovine spine was frequency dependent and varied 3.7-fold over the 0.5-19.7 Hz mechanical excitation frequency range. Minimum and maximum DV stiffness (force/displacement) were 3.86+/-0.38 and 14.1+/-9.95 N/mm at 4.0 and 19.7 Hz, respectively. Stiffness values based on the swept-sine measurements were not significantly different from corresponding periodic oscillations (2.0 and 6.0 Hz). The mean coefficient of variation in the swept-sine DV dynamic stiffness assessment method was 15%, which was similar to the periodic oscillation method (10-16%). The results indicate that changes in mechanical excitation frequency and animal body mass modulate DV spinal stiffness.

Three-dimensional vertebral motions produced by mechanical force spinal manipulation.

OBJECTIVE: The aim of this study was to quantify and compare the 3-dimensional intersegmental motion responses produced by 3 commonly used chiropractic adjusting instruments. METHODS: Six adolescent Merino sheep were examined at the Institute for Medical and Veterinary Science, Adelaide, Australia. In all animals, triaxial accelerometers were attached to intraosseous pins rigidly fixed to the L1 and L2 spinous processes under fluoroscopic guidance. Three handheld mechanical force chiropractic adjusting instruments (Chiropractic Adjusting Tool [CAT], Activator Adjusting Instrument IV [Activator IV], and the Impulse Adjusting Instrument [Impulse]) were used to randomly apply posteroanterior (PA) spinal manipulative thrusts to the spinous process of T12. Three force settings (low, medium, and high) and a fourth setting (Activator IV only) were applied in a randomized repeated measures design. Acceleration responses in adjacent segments (L1 and L2) were recorded at 5 kHz. The multiaxial intersegmental (L1-L2) acceleration and displacement response at each force setting was computed and compared among the 3 devices using a repeated measures analysis of variance (alpha = .05). RESULTS: For all devices, intersegmental motion responses were greatest for axial, followed by PA and medial-lateral (ML) measurement axes for the data examined. Displacements ranged from 0.11 mm (ML axis, Activator IV low setting) to 1.76 mm (PA axis, Impulse high setting). Compared with the mechanical (spring) adjusting instruments (CAT, Activator IV), the electromechanical Impulse produced the most linear increase in both force and intersegmental motion response and resulted in the greatest acceleration and displacement responses (high setting). Significantly larger magnitude intersegmental motion responses were observed for Activator IV vs CAT at the medium and high settings (P < .05). Significantly larger-magnitude PA intersegmental acceleration and displacement responses were consistently observed for Impulse compared with Activator IV and CAT for the high force setting (P < .05). CONCLUSIONS: Larger-magnitude, 3D intersegmental displacement and acceleration responses were observed for spinal manipulative thrusts delivered with Impulse at most force settings and always at the high force setting. Our results indicate that the force-time characteristics of impulsive-type adjusting instruments significantly affects spinal motion and suggests that instruments can and should be tuned to provide optimal force delivery.

Increased multiaxial lumbar motion responses during multiple-impulse mechanical force manually assisted spinal manipulation.

BACKGROUND: Spinal manipulation has been found to create demonstrable segmental and intersegmental spinal motions thought to be biomechanically related to its mechanisms. In the case of impulsive-type instrument device comparisons, significant differences in the force-time characteristics and concomitant motion responses of spinal manipulative instruments have been reported, but studies investigating the response to multiple thrusts (multiple impulse trains) have not been conducted. The purpose of this study was to determine multi-axial segmental and intersegmental motion responses of ovine lumbar vertebrae to single impulse and multiple impulse spinal manipulative thrusts (SMTs). METHODS: Fifteen adolescent Merino sheep were examined. Tri-axial accelerometers were attached to intraosseous pins rigidly fixed to the L1 and L2 lumbar spinous processes under fluoroscopic guidance while the animals were anesthetized. A hand-held electromechanical chiropractic adjusting instrument (Impulse) was used to apply single and repeated force impulses (13 total over a 2.5 second time interval) at three different force settings (low, medium, and high) along the posteroanterior axis of the T12 spinous process. Axial (AX), posteroanterior (PA), and medial-lateral (ML) acceleration responses in adjacent segments (L1, L2) were recorded at a rate of 5000 samples per second. Peak-peak segmental accelerations (L1, L2) and intersegmental acceleration transfer (L1-L2) for each axis and each force setting were computed from the acceleration-time recordings. The initial acceleration response for a single thrust and the maximum acceleration response observed during the 12 multiple impulse trains were compared using a paired observations t-test (POTT, alpha = .05). RESULTS: Segmental and intersegmental acceleration responses mirrored the peak force magnitude produced by the Impulse Adjusting Instrument. Accelerations were greatest for AX and PA measurement axes. Compared to the initial impulse acceleration response, subsequent multiple SMT impulses were found to produce significantly greater (3% to 25%, P < 0.005) AX, PA and ML segmental and intersegmental acceleration responses. Increases in segmental motion responses were greatest for the low force setting (18%-26%), followed by the medium (5%-26%) and high (3%-26%) settings. Adjacent segment (L1) motion responses were maximized following the application of several multiple SMT impulses. CONCLUSION: Knowledge of the vertebral motion responses produced by impulse-type, instrument-based adjusting instruments provide biomechanical benchmarks that support the clinical rationale for patient treatment. Our results indicate that impulse-type adjusting instruments that deliver multiple impulse SMTs significantly increase multi-axial spinal motion.

Spinal manipulation force and duration affect vertebral movement and neuromuscular responses.

BACKGROUND: Previous study in human subjects has documented biomechanical and neurophysiological responses to impulsive spinal manipulative thrusts, but very little is known about the neuromechanical effects of varying thrust force-time profiles. METHODS: Ten adolescent Merino sheep were anesthetized and posteroanterior mechanical thrusts were applied to the L3 spinous process using a computer-controlled, mechanical testing apparatus. Three variable pulse durations (10, 100, 200 ms, force = 80 N) and three variable force amplitudes (20, 40, 60 N, pulse duration = 100 ms) were examined for their effect on lumbar motion response (L3 displacement, L1, L2 acceleration) and normalized multifidus electromyographic response (L3, L4) using a repeated measures analysis of variance. FINDINGS: Increasing L3 posteroanterior force amplitude resulted in a fourfold linear increase in L3 posteroanterior vertebral displacement (p < 0.001) and adjacent segment (L1, L2) posteroanterior acceleration response (p < 0.001). L3 displacement was linearly correlated (p < 0.001) to the acceleration response over the 20-80 N force range (100 ms). At constant force, 10 ms thrusts resulted in nearly fivefold lower L3 displacements and significantly increased segmental (L2) acceleration responses compared to the 100 ms (19%, p = 0.005) and 200 ms (16%, p = 0.023) thrusts. Normalized electromyographic responses increased linearly with increasing force amplitude at higher amplitudes and were appreciably affected by mechanical excitation pulse duration. INTERPRETATION: Changes in the biomechanical and neuromuscular response of the ovine lumbar spine were observed in response to changes in the force-time characteristics of the spinal manipulative thrusts and may be an underlying mechanism in related clinical outcomes.

Comparison of mechanical force of manually assisted chiropractic adjusting instruments.

OBJECTIVE: To quantify the force-time and force-delivery characteristics of six commonly used handheld chiropractic adjusting devices. METHODS: Four spring-loaded instruments, the Activator Adjusting Instrument; Activator II Adjusting Instrument, Activator III Adjusting Instrument, and Activator IV Adjusting Instrument, and two electromechanical devices, the Harrison Handheld Adjusting Instrument and Neuromechanical Impulse Adjusting Instrument, were applied to a dynamic load cell. A total of 10 force-time histories were obtained at each of three force excursion settings (minimum to maximum) for each of the six adjusting instruments at preload of approximately 20 N. RESULTS: The minimum-to-maximum force excursion settings for the spring-loaded mechanical adjusting instruments produced similar minimum-to-maximum peak forces that were not appreciably different for most excursion settings. The electromechanical adjusting instruments produced short duration ( approximately 2-4 ms), with more linear minimum-to-maximum peak forces. The force-time profile of the electromechanical devices resulted in a more uniform and greater energy dynamic frequency response in comparison to the spring-loaded mechanical adjusting instruments. CONCLUSIONS: The handheld, electromechanical instruments produced substantially larger peak forces and ranges of forces in comparison to the handheld, spring-loaded mechanical devices. The electromechanical instruments produced greater dynamic frequency area ratios than their mechanical counterparts. Knowledge of the force-time history and force-frequency response characteristics of spinal manipulative instruments may provide basic benchmarks and may assist in understanding mechanical responses in the clinical setting.

Vertebroplasty and kyphoplasty affect vertebral motion segment stiffness and stress distributions: a microstructural finite-element study.

STUDY DESIGN: The mechanical behavior of a thoracic motion segment following cement augmentation was studied using the finite-element method. OBJECTIVE: To examine effects of cement augmentation on motion segment stiffness and load transfer. SUMMARY OF BACKGROUND DATA: Vertebroplasty and kyphoplasty procedures are meant to stiffen and strengthen the vertebral body, but the optimal cement volume and placement to achieve these goals without altering load transfer to adjacent segments are unknown. METHODS: A microstructural finite-element model of a vertebral motion segment was constructed from micro-CT images. Microdamage within the vertebral body trabecular structure was modeled using an elasto-plastic modulus reduction scheme. Three motion segment damage models were created: I = 18% apparent modulus reduction (least damage), II = 45%, and III = 85% (most damage); and several one- and two-segment polymethylmethacrylate cement repair strategies (partial fill kyphoplasty, replacement of bone and marrow; and both partial fill and complete fill vertebroplasty, replacement of marrow only) were studied. Average disc and bone stresses and motion segment apparent compressive stiffness were compared with baseline (undamaged and untreated) simulation results. RESULTS: In terms of maximizing stiffness and minimizing stress alterations in the adjacent vertebral body and increasing motion segment apparent stiffness, we found that, other than complete fill, the most effective single-segment cement repair strategy was vertebroplasty on the periphery of the superior segment overlying the disc anulus (<0.1% overall vertebral body bone stress alteration and 83% stiffness increase, respectively, damage Model III). Two-segment vertebroplasty (all repair models) restored motion segment stiffness to baseline levels in all damage models, while single-segment vertebroplasty (all repair models) restored stiffness to baseline levels only in damage Model I. Single- and two-segment kyphoplasty was effective in restoring stiffness to baseline levels for Model I only. Compared with the baseline model, cement augmentation decreased average treated segment bone stresses (up to 66%, complete fill vertebroplasty elasto-plastic modulus reduction Model III), increased average intervertebral disc nucleus stresses (up to 59%, kyphoplasty elasto-plastic modulus reduction Model III), and increased average adjacent segment, endplate region stresses (up to 2.8%, kyphoplasty elasto-plastic modulus reduction Model II). Adjacent (untreated) segment peak bone stresses were increased (up to 45%, kyphoplasty, Model III) in endplate regions underlying the intervertebral disc nucleus. CONCLUSIONS: The damage-repair simulations indicated that cement augmentation improves motion segment stiffness but substantially alters bone stress distributions in treated and adjacent segments.

Influence of spine morphology on intervertebral disc loads and stresses in asymptomatic adults: implications for the ideal spine.

BACKGROUND CONTEXT: Sagittal profiles of the spine have been hypothesized to influence spinal coupling and loads on spinal tissues. PURPOSE: To assess the relationship between thoracolumbar spine sagittal morphology and intervertebral disc loads and stresses. STUDY DESIGN: A cross-sectional study evaluating sagittal X-ray geometry and postural loading in asymptomatic men and women. PATIENT SAMPLE: Sixty-seven young and asymptomatic subjects (chiropractic students) formed the study group. OUTCOME MEASURES: Morphological data derived from radiographs (anatomic angles and sagittal balance parameters) and biomechanical parameters (intervertebral disc loads and stresses) derived from a postural loading model. METHODS: An anatomically accurate, sagittal plane, upright posture, quadrilateral element model of the anterior spinal column (C2-S1) was created by digitizing lateral full-spine X-rays of 67 human subjects (51 males, 16 females). Morphological measurements of sagittal curvature and balance were compared with intervertebral disc loads and stresses obtained using a quadrilateral element postural loading model. RESULTS: In this young (mean 26.7, SD 4.8 years), asymptomatic male and female population, the neutral posture spine was characterized by an average thoracic angle (T1-T12) = +43.7 degrees (SD 11.4 degrees ), lumbar angle (T12-S1) = -63.2 degrees (SD 10.0 degrees ), and pelvic angle = +49.4 degrees (SD 9.9 degrees ). Sagittal curvatures exhibited relatively broad frequency distributions, with the pelvic angle showing the least variance and the thoracic angle showing the greatest variance. Sagittal balance parameters, C7-S1 and T1-T12, showed the best average vertical alignment (5.3 mm and -0.04 mm, respectively). Anterior and posterior disc postural loads were balanced at T8-T9 and showed the greatest difference at L5-S1. Disc compressive stresses were greatest in the mid-thoracic region of the spine, whereas shear stresses were highest at L5-S1. Significant linear correlations (p < .001) were found between a number of biomechanical and morphological parameters. Notably, thoracic shear stresses and compressive stresses were correlated to T1-T12 and T4-hip axis (HA) sagittal balance, respectively, but not to sagittal angles. Lumbar shear stresses and body weight (BW) normalized shear loads were correlated with T12-S1 balance, lumbar angle, and sacral angle. BW normalized lumbar compressive loads were correlated with T12-S1 balance and sacral angle. BW normalized lumbar disc shear (compressive) loads increased (decreased) significantly with decreasing lumbar lordosis. Cervical compressive stresses and loads were correlated with all sagittal balance parameters except S1-HA and T12-S1. A neutral spine sagittal model was constructed from the 67 subjects. CONCLUSIONS: The analyses suggest that sagittal spine balance and curvature are important parameters for postural load balance in healthy male and female subjects. Morphological predictors of altered disc load outcomes were sagittal balance parameters in the thoracic spine and anatomic angles in the lumbar spine.