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Research Finds Difference between Office Chair Angles on Postural Stress

postural stress, chair, design, seat pan

A study 1 on the impact of different office chair features on spinal posture while sitting found that seat pan tilt and some types of lumbar supports may offer negligible improvement in sitting postures, but overall, sitting for extended periods of time puts stress on the spine and may generate spinal injury. Perhaps something new needs to be invented to minimize end range postural stress and spinal compression over time.

What’s at Stake

Sitting involves flexing the hips and back and moves joints out of their neutral zones, which contributes to stress in the lower back. Even 20 minutes of spinal flexion can cause laxity in the ligaments and change the reflex action of spinal muscles. Over the years, office chair manufacturers have attempted improvements in their products that would lessen stress on the back and improve overall sitting postures, but standing postures that incorporate a neutral posture—where the lower back maintains its convex curvature and the pelvis keeps a slight forward tilt— remains the optimal stance for preventing back damage and pain.

Studies have found that chairs that help the sitter to extend their lumbar spine and anteriorly rotate their pelvis while sitting are the most helpful in reducing the likelihood of damage to the ligaments, muscles, and vertebrae. These types of chair features fall into three categories: thoracic supports, lumbar supports, and seat-pan tilting mechanisms. Of these three modifications, only one type of seat pan mechanism and lumbar supports have been radiographically studied. It is also unclear if men and women have similar lumbar lordosis and pelvic inclination.

This study examined lumbar spine and pelvic postures radiographically using three available seating features to determine which, if any of the three features caused an improvement in spinal or pelvic positioning during sitting. It also looked at posture responses in men and women to see if there were differences between them.

 

The Study

The study involved 28 participants (14 males and 14 females) with no recent history of lower back problems. The volunteers, whose mean age was 25 years, were not told about the conditions being tested or the relevant features of the products being tested. They were each radiographed in the standing and sitting postures using standardized postural and breathing instructions during their x-rays. Their radiographic measures were then compiled into the software program, and a statistical analysis was performed.

Results

Researchers found that the lumbar postures of all participants were significantly flexed and extended in the seating position compared to the standing position. There was trivial improvement between those using postural interventions and the control groups whose chairs did not employ those features.  There were slightly reduced levels of flexion with seat pan tilt features and lumbar support mechanisms, indicating potential value in these accommodations, but lumbar lordosis or intervertebral joint angles were not affected in any significant way across the improvement features.  At the pelvic level, researchers found statistically significant results: The seat pan tilt configuration produced more anterior rotation of the pelvis and less flexion. The male subjects showed greater flexion during sitting than females.

Conclusion

Sitting places strain on the lower back, particularly at the L4/L5 and S5/S1 disc levels. Lumbar supports and some seat pan tilt features may be slightly helpful in reducing the stress put upon the lower back while in the seated position, but overall, there was minimal difference between the various office chair features on reducing lumbar lordosis or IVJ joint strain. What improvement was evident was local to the spinal segment, rather than posture as a whole. Male and female subjects displayed angle differences in the effects of sitting at the L1/L2 and L2/L3 IVJ joints and pelvis.

KEYWORDS: postural stress, different office chair features, spinal posture, seat pan tilt, lumbar support, lumbar lordosis, flexion, joint strain, posture responses in men and women, back damage and pain, lower back, IVJ joint strain

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Study Finds Evidence Asymmetrical Postures During Loading Contribute to Disc Failure

disc failure

A microstructural analysis of how healthy discs respond to compression and complex loading postures—specifically those incorporating flexion and facet-constrained shear—found evidence that the required load contributing to disc failure was reduced when complex postures, rather than simple flexion, were utilized in load-bearing situations. In addition, when asymmetric postures were used during lifting, rather than simple compression or flexion, there occurred more infiltration of the nucleus material as it made its way to the annular periphery. The results of the study indicate that asymmetric postures during lifting are more likely to contribute to disc degeneration and lower back pain and should therefore be avoided.

The Study

The study 1 involved 30 motion segments from 10 sheep spines that had no previous signs of disc degeneration. The discs were frozen, thawed, and then rehydrated fully prior to the compression experiments to be in agreement with previous similar experiments and maximize the annular load. Researchers created a bending, twisting, lifting scenario that involved axial rotation, lateral, anterior, and posterior shear, and flexion, adapting the mechanical rig to compress and rotate the disc segments to failure using compressive force.

The typical failure was lower under complex loading conditions than in conditions of simple flexion. Microstructural damage included fractures of the vertebrae and three variations of annular damage, including mid-span direct tearing, non-continuous mid-span tearing, and annular-endplate tearing. Combinations of all three types of damage occurred, as well as circumferential failure, in all 30 discs.

The complex postures utilized in the study lessened the discs abilities to withstand compressive loading and contributed to failures. The complex loading conditions contributed to instances of dual modes of failure, including the circumferential (circuitous tracking of nuclear materials towards the annular periphery) evident in all study samples. This suggests that the lateral parts of the disc may be especially vulnerable during flexion because of shear loading in the area. Circumferential damage was evident in all 30 discs involved in this study, which suggests that it is likely an important type of damage involved in disc failure under complex loading conditions.

Conclusion

Complex postures during load-lifting may contribute to herniation and disc failure. Asymmetrical postures (in addition to flexion) should be avoided during lifting to reduce the likelihood of sustaining a lower back injury.

 

KEYWORDS: complex loading, disc failure, asymmetrical postures during loading, compression and complex loading postures, flexion and facet-constrained shear, disc degeneration, annular damage, disc herniation

Study Provides Numerical Model that may Predict IVD Failure Under All Loading Conditions

loading conditions

A new study 1 to determine which mechanical conditions were unsafe for the intervertebral disc (IVD) culminated with the presentation of a numerical model that can be used to predict disc failure under all loading conditions.  Using a series of complex loading conditions on ovine lumbar IVD segments, researchers were able to examine the state of stress of the IVD and AF, combining numerical results with that of a parallel in vitro study.

 

The Study

Researchers used an FE model of the ovine lumbar intervertebral disc. The geometry of this model was centered around reconstruction of the endplates (Eps), caudal and cranial vertebrae from a 1.3-4 spinal segment. They used a custom Python script to generate the disc. This construct was created to represent the lumbar spine. This disc model contained an annulus fibrosus (AF), nucleus pulposus (NP), and a bony and cartilaginous endplate (EP). Researchers divided the AF in the posterior, lateral, and anterior sections. They composed a mesh of 56,496 hexahedral elements and 60,145 nodes. They then compared its flexibility with previous collected data to validate that the model was in agreement with the results of that study.

The study’s authors simulated five loading scenarios in near-static states to replicate the previous study’s conditions. They chose loading angles double that of those used in the previous study and applied briefly 3.75 Nm without posterior elements. They used a kinetic energy that was less than 10 percent of the total energy in the simulations. They then analyzed the stresses created in the circumferential, axial, and radial direction, as well as the interface stress between the Eps and the three defined, divided, and subdivided sections of the AF. They averaged all stress spatially over the cranial, middle, caudal, inner, and outer AF and calculated the interface stress between the AF and the EP as the ratio between the cross-sectional area and nodal forces. They then studied the role of each type of stress in disc failure to determine a numerical formula for predicting IVD failure.

Results

The results of the study demonstrated that tensile axial stress greater than 10 MPa and a positive circumferential stress greater than 8MPa may cause the annulus fibrosis (AF) to fail and that flexion is the loading condition most often associated with disc failure. The results were in agreement with those of the previous study and were useful in developing a model to predict disc failure. In short, the highest stress states were created by the application of rotations in the main-plains. This observation was most prevalent in the posteo-lateral and anterior regions of the circumferential and axial directions.

loading conditions

Loading for Lifting

 

Conclusion

By comparing the results of this investigation of how complex loading conditions generate stress in the IVD or failure of the AF to a parallel in vitro study, researchers determined that a tensile axial stress of 10MPs or above and a positive circumferential stress of over 8 MPs may cause AF failure. The most unsafe type of loading for the disc is flexion. This numerical model can be used to predict the risk of disc failure in any type of loading condition. The formula can also be used to determine risks in implantable devices and models of entire motion segments.

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Increasing Gradients of Compressive Stress Can Lead to Annular Delamination, Collapse, and IVD Degeneration

delamination, annulus

A ISSLS Prize-winning study 1 examined how increasing gradients of compressive stress within the intervertebral disc (IVD) contributed to the progress of dis degeneration. The research findings suggest that an increased grade of disc degeneration created decreased nucleus pressure and compressive annulus stress, but anterior annular stress gradients increased by approximately 75 percent, and by 108 percent in the posterior annulus—findings that are clinically significant.

The neural arch may provide a stress-shield for the degenerating disc during mechanical loading, but delamination and collapse of the annulus are most likely caused not by loading, but by increasing gradients of compressive stress, leading to advanced disc degeneration, despite the stress-shield.

 

The Study

Using 191 motion segments from 42 cadavers of varied ages, researchers measured the intradiscal stresses under 1 kN of compression. A pressure transducer was pulled along the midsagittal diameter of the disc to measure the intradiscal stresses. Stress gradients in the annulus were quantified using a formula that averaged the rate of increase in compressive stress between the area of maximum stress in the anterior or posterior annuls, and the nucleus. Measurements were compared before and after applied creep-loading, as well as in flexed or erect postures. A scale of 1to 4 was used to describe the amount of macroscopic disc degeneration observed.

 

Results

An increase of disc degeneration from 2 to 4 decreased by 68 percent the amount of pressure in the nucleus, and compressive stress in the annulus was decreased by 48-64 percent, depending on the simulated posture of the segment and the location of the disc. However, anterior annular stress gradients showed an average 75 percent increase in the flexion position, and posterior annular stress gradients increased 108 percent in upright posture.

 

Conclusion

The neural-arch provides stress-shielding, but compressive stress gradients are significantly increased with an increasing grade of disc degeneration. Adjacent lamellae are sheared by the stress gradients, which may contribute to the delamination and collapse of the annulus.

Kinematics Study of Lumbar Disc Herniation Impact on Activities Involving Lower Extremities, Pelvis, and Multi-segmental Spine

kinematic effects of LDH

A study of 26 healthy and seven male patients with lower back disc herniation (LDH) investigated the kinematic effects of LDH on the multi-segmental spine, pelvis, and lower extremities during activities of daily living (ADL). The intent of the study was to help establish a clearer understanding of how LDH affects the movement of the spinal segments and to assist in the future development of more effective rehabilitation regimens for patients suffering from LDH.

The Study

The study 1 used 3D active markers strategically-placed at the thorax, pelvis, hip, knee, ULx, and LLx to track the motion of spinal segments during five pre-determined ADLs. The subjects were monitored during three rounds of level walking, stair climbing, trunk flexion, ipsilateral pickup, and contralateral pickup, and the data was collected and calculated using the AnyBody managed model repository (AMMR, version 1.6 and version 6.0.6) systems. Computer models then analyzed kinematic angles, and the range of motion (ROM) was calculated for joint or segmental angles in three planes during flexion-extension or gait cycles. A custom program was created and utilized in MATLAB (The MathWorks, Inc) and each of the variables and stats were compared to independent studies involving control groups.

 

Results

Subjects with LDH demonstrated statistically greater pelvic and LLx rotation during stair-climbing than subjects in the control group. The LDH patients also had a reduced ROM during thoracic flexion, and had more hip abduction, and greater degrees of pelvic tilt during the activities. There was a slight difference in thoracic flexion ROM between the control and LDH groups, but the LDH group had a significantly decreased ROM in lumbar flexion, particularly in ULx without sagittal angular displacement. When performing ipsilateral or contralateral pickups, the LDH group compensated the lack of lumbar flexion with a tilted pelvis. The LDH patients had increased ROM during trunk flexion in pelvic rotation the frontal and transverse planes. During ipsilateral pickup, they demonstrated greater pelvic rotation and hip abduction, but decreased ROM of LLx when bending laterally during trunk flexion.

 

The study indicates that patients will alter their kinetics to avoid pain from LDH. These adaptive strategies during ADLs occurred in the thoracic, ULx, LLx, pelvis, and right-side lower extremities, indicating different compensations in the two segments of the lumbar region. The lumbar region during the five ADLs utilized a small amount of intersegmental movement, and the LDH patients demonstrated less pelvic rotation during stair climbing and contralateral pickup, but more during other ADLs.  As mentioned, fear of pain may be responsible for a greater pelvic rotation in LDH patients during activities.

Conclusion

This study of kinetic characteristics and range of motion and direction in subjects with, and without LHD should contribute to increased understanding of how LDH influences spinal movement and will hopefully lead to enhanced rehabilitation techniques in the clinical treatment of LDH. Further research of the kinetics of the lower extremities, pelvis, spinal segments in reaction to LDH might include an investigation of the inverse dynamics of loads, particularly at the disc, facet joint, hip, pelvis, knee, and ankle. An examination of the ligament and muscular forces involved during kinetic adjustment could also benefit clinicians.

Study Emphasizes Importance in Educating Patients About Cavitation Sounds

Educating Patients About Cavitation Sounds

A study 1 of 100 individuals indicates most spinal patients have false beliefs regarding cavitation sounds during spinal manipulation and that those incorrect beliefs can inhibit patient wellness when the patient either refuses potentially beneficial spinal manipulation or develops the nocebo effect when no sound is produced during a procedure. The study, which examined typical beliefs regarding the cracking sounds that frequently occur during high-velocity low-amplitude (HVLA) thrust spinal manipulation, determined that healthcare providers who practice spinal manipulation could benefit their patients by educating them regarding the processes behind cavitation sounds and their degree of importance as regards to successful manipulation outcomes.

 

About Cavitation and Spinal Manipulation

When a practitioner uses HVLA to push a spinal joint beyond its usual range of motion, they are using a spinal manipulation technique that is ancient (first recorded by Hippocrates in 400 BCE) and widely-used today to treat back pain. Though many studies have been conducted as to the efficacy and safety of the procedure and the origin and results of the frequently-associated cavitation sounds that may occur during spinal manipulation, few studies have been conducted on how beliefs about cavitation may affect the outcome of a patient’s spinal care.

The cracking sounds associated with cavitation occur as the result of the drop of pressure when a mechanical adjustment creates a vacuum within a joint, resulting in the creation of a cavity within synovial fluid. Historically, the unstable bubble was thought to collapse shortly after its formation, but more recent research in PLOS One returned the origin of the sound from the cavity formation. Unfounded beliefs about the nature of the cavitation sounds may lead patients to resist spinal manipulation therapy or contribute to the nocebo effect when the sounds do not occur during therapy.

 

The Study

Patient volunteers were recruited via social media, advertising campaigns, hospitals, and connections to the investigators. In all, 100 individuals participated in the study—60 with no prior history of spinal manipulation and 40 with prior history of the procedure. Of the 60 with no prior history of spinal manipulation, 40 were asymptomatic at the time of the study, and 20 were experiencing nonspecific spinal pain. Of the 40 patients who had previously experienced spinal manipulations, half were experiencing spinal pain at the time of the study, and half were asymptomatic. All 100 volunteer patients completed a questionnaire and an in-person interview to determine their beliefs about cavitation sounds and their origin and effects on spinal manipulation. The mean age of the study participants was 43.5 years.

 

Results and Conclusion

Roughly half of the volunteer participants believed that the cavitation noises were a result of vertebral repositioning. Nearly one quarter of the volunteers ascribed the noises to vertebral friction. Only 9 percent of the study’s participants correctly assumed that the cavitation sounds were created due to a release of gas during manipulation. Forty percent of the participants assumed (incorrectly) that the cavitation noises were integral to a successful procedure and that no sounds were indicative of a failed procedure. The belief statistics were similar across both groups of participants—those with a history of spinal manipulation procedures, and those with no history of HVLA. Further, the study results suggest that most spinal patients perceive the spine as a fragile, unstable entity that may be subject to damage as the result of spinal manipulation and associated cavitation sounds. Because they may be more likely to resist treatment or experience the nocebo effect when the cracking sounds are not audible during treatment, it would be beneficial for spinal practitioners and physicians to educate their patients about cavitation noises and the process of spinal manipulation prior to treatment.

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Hydraulic Recovery with Recumbency

hydraulic recovery of the cervical discs through lying down

The spine undergoes an accordion-like cycle of compression and decompression. This variation occurs not exclusively with the sleep-wake cycle but is influenced by gravity and load orientation of the spine. Researchers have long known that the spine undergoes a diurnal variation with compression changes of water exchange in and out of the intervertebral discs. Much of the attention has been on the discs of the lumbar spine, presumably because of the degree of lower back pain on this planet, but the intervertebral discs are throughout the spine.

In a paper published this month in The Spine Journal, researchers looked at how much the cervical and thoracic discs change with the simple act of lying down. They looked at 101 healthy individuals and found significant volume changes in the cervical and thoracic intervertebral disc heights when the subjects laid down, on average, for 29 minutes. It would seem obvious that this kind of research has been conducted in the past but no.

cervical hydraulic recovery with recumbancy

Research shows how the cervical and thoracic discs fill with the simple act of recumbency.

This basic science research is fundamental if we are to try to figure out the mechanics of optimal load and off-load environments for the spine. Lying down is also a behaviour patients perform when visiting physical therapists, chiropractors, massage therapists, and acupuncturists. Is there a mechanical therapeutic factor of recumbency?


Dynamic Disc Designs develop dynamic spine models to help in the basic understanding of core science in the pursuit of finding the best mechanical strategies for disc height and hydraulic regeneration. Explore.