Diane E. Gregory

A Comparison of Uniaxial and Biaxial Mechanical Properties of the Annulus Fibrosus: A Porcine Model

The annulus fibrosus of the intervertebral disk experiences multidirectional tension in vivo, yet the majority of mechanical property testing has been uniaxial. Therefore, our understanding of how this complex multilayered tissue responds to loading may be deficient. This study aimed to determine the mechanical properties of porcine annular samples under uniaxial and biaxial tensile loading. Two-layer annulus samples were isolated from porcine disks from four locations: anterior superficial, anterior deep, posterior superficial, and posterior deep. These tissues were then subjected to three deformation conditions each to a maximal stretch ratio of 1.23: uniaxial, constrained uniaxial, and biaxial. Uniaxial deformation was applied in the circumferential direction, while biaxial deformation was applied simultaneously in the circumferential and compressive directions. Constrained uniaxial consisted of a stretch ratio of 1.23 in the circumferential direction while holding the tissue stationary in the axial direction. The maximal stress and stress-stretch ratio (S-S) moduli determined from the biaxial tests were significantly higher than those observed during both the uniaxial tests (maximal stress, 97.1% higher during biaxial; p=0.002; S-S moduli, 117.9% higher during biaxial; p=0.0004) and the constrained uniaxial tests (maximal stress, 46.8% higher during biaxial; S-S moduli, 82.9% higher during biaxial). These findings suggest that the annulus is subjected to higher stresses in vivo when under multidirectional tension.

ISSLS Prize Winner : Adaptations to the Multifidus Muscle in Response to Experimentally Induced Intervertebral Disc Degeneration

Study Design. Basic science study of the rabbit multifidus muscle response to intervertebral disc degeneration. Objective. To assess changes in passive mechanical properties, associated protein structure, and histology of multifidus in response to disc degeneration produced by experimental needle puncture. Summary of Background Data. Relationships have been reported between muscle dysfunction and low back injury; however, little is known about the cause and effect of such relationships. Methods. Twelve rabbits were studied; 4 in each of 3 groups: control, 4-weeks postintervertebral disc injury (4-week disc degeneration), and 12-weeks postintervertebral disc injury (12-week disc degeneration). Single multifidus fibers and bundles of fibers were isolated and tested for slack sarcomere length and elastic modulus. Titin isoform mass, myosin heavy chain distribution, and muscle histology were also examined. Results. Compared to control, individual muscle fibers were 34% stiffer and fiber bundles 107% stiffer in the 12-week disc degeneration group. No changes were detected at 4-week disc degeneration. No statistically significant change was found for MHC distribution in the 12-week disc degeneration group when compared to control, whereas titin isoforms were larger (P < 0.05) in the 12-week disc degeneration group. Histology revealed select regions of multifidus, at 12-week disc degeneration, with increased space between bundles of fibers, which in some instances was partly occupied by adipose tissue. Conclusion. Multifidus becomes stiffer, both in individual fibers and fiber bundles, in response to experimentally induced intervertebral disc degeneration. This cannot be explained by change in fiber-type due to reduced muscle use, nor by the increased size of the protein titin (which would reduce stiffness). We hypothesize that fiber bundles become stiffer by proliferation and/or reorganization of collagen content within the muscle but the basis for fiber stiffening is not known.

Does vibration influence the initiation of intervertebral disc herniation? An examination of herniation occurrence using a porcine cervical disc model.

Study Design. In vitro biomechanics, randomized control trial. Objective. The objectives of this study were 2-fold: first, to determine the effect of exposure to axial vibration on the initiation and progression of disc herniation; second, to determine the effect of vibration exposure and the presence of disc damage on the mechanical properties of individual lamella from the annulus. Summary of Background Data. Vibration exposure has been linked to a higher reporting of low back pain and disc herniation via epidemiological studies. However, these studies are unable to determine causal relationships. In vitro tissue experimentation assists in determining if certain exposures, for example vibration, actually lead to herniation. Methods. A total of 20 porcine (aged, 6–8 months; similar skeletal development as an adolescent human) functional spine units (FSU) were subjected to repetitive flexion-extension (6000 cycles), which has been shown to produce intervertebral disc herniation. While being exposed to the repeated flexion/extension, 10 FSUs were statically compressed under 1400 N (control group) and the other 10 were cyclically compressed (1260–1540 N) at a frequency of 5 Hz (vibration group). Post collection, intervertebral discs were dissected and individual lamella of the annulus was tested under uniaxial tension to failure (tension applied perpendicular to the orientation of the collagen fibers) to isolate the mechanical properties of the intralamellar matrix. Results. Of the 10 control FSUs, 4 had evidence of herniation initiation while 8 of the 10 vibrated FSUs showed herniation initiation (P ∇ 0.01). No significant differences in disc height loss or FSU stiffness were observed between the control and vibrated groups. Further, no significant differences were observed between the 2 groups for any of the single lamella mechanical properties. Conclusion. This study confirmed that vibration is a causal mechanical risk factor that significantly increases the occurrence of herniation.

Should We Be More on the Ball? The Efficacy of Accommodation Training on Lumbar Spine Posture, Muscle Activity, and Perceived Discomfort During Stability Ball Sitting

Objective: The aim of this study was to evaluate the efficacy of a 9-day accommodation protocol on reducing perceived discomfort while sitting on a stability ball (SB); trunk muscle activity levels and lumbar spinal postures were also considered. Background: Previous studies have compared SB sitting with office chair sitting with few observed differences in muscle activity or posture; however, greater discomfort during SB sitting has been reported. These findings may indicate an accommodation period is necessary to acclimate to SB sitting. Method: For this study, 6 males and 6 females completed two separate, 2-hr sitting sessions on an SB. Half the participants completed a 9-day accommodation period between the visits, whereas the other half did not use an SB during the time. On both occasions, self-reported perceived discomfort ratings were collected along with erector spinae and abdominal muscle activity and lumbar spinal postures. Results: Discomfort ratings were reduced in female participants following the accommodation; no effects on muscle activation or lumbar spine postures were observed. Conclusion: Accommodation training may reduce perceived low-back discomfort in females. Trunk muscle activity and lumbar spine postures during seated office work on an SB did not differ between groups; however, greater sample power was required to conclusively address these variables. Application: Regarding whether to use an SB in place of a standard office chair, this study indicates that females electing to use an SB can decrease discomfort by following an accommodation protocol; no evidence was found to indicate that SB chair use will improve trunk strength or posture, even following an accommodation period.

The use of intermittent trunk flexion to alleviate low back pain during prolonged standing.

The current study examined of the effect of intermittent, short-term periods of full trunk flexion on the development of low back pain (LBP) during two hours of standing. Sixteen participants completed two 2-h standing protocols, separated by one week. On one day, participants stood statically for 2h (control day); on the other day participants bent forward to full spine flexion (termed flexion trials) to elicit the flexion relaxation (FR) phenomenon for 5s every 15min (experimental day). The order of the control and experimental day was randomized. During both protocols, participants reported LBP using a 100mm visual analogue scale every 15min. During the flexion trials, lumbar spine posture, erector spinae and gluteus medius muscle activation was monitored. Ultimately, intermittent trunk flexion reduced LBP by 36% (10mm) at the end of a 2-h period of standing. Further, erector spinae and gluteus medius muscle quietening during FR was observed in 91% and 65% of the flexion trials respectively, indicating that periods of rest did occurred possibly contributing to the reduction in LBP observed. Since flexion periods do not require any aids, they can be performed in most workplaces thereby increasing applicability.

A comparison between porcine, ovine, and bovine intervertebral disc anatomy and single lamella annulus fibrosus tensile properties

This project aimed to compare gross anatomical measures and biomechanical properties of single lamellae from the annulus fibrosus of ovine and porcine lumbar vertebrae, and bovine tail vertebrae. The morphology of the vertebrae of these species differ significantly both from each other and from human, yet how these differences alter biomechanical properties is unknown. Geometric parameters measured in this study included: 1) absolute and relative intervertebral (IVD) and vertebral body height and 2) absolute and relative intervertebral disc (IVD) anterior‐posterior (AP) and medial‐lateral (ML) widths. Single lamella tensile properties included toe‐region stress and stretch ratio, stiffness, and tensile strength. As expected, the bovine tail IVD revealed a more circular shape compared with both the ovine and porcine lumbar IVD. The bovine tail also had the largest IVD to vertebral body height ratio (due to having the highest absolute IVD height). Bovine tail lamellae were also found to be strongest and stiffest (in tension) while ovine lumbar lamellae were weakest and most compliant. Histological analysis revealed the greatest proportion of collagen in the bovine corroborating findings of increased strength and stiffness. The observed differences in anatomical shape, connective tissue composition, and tensile properties need to be considered when choosing an appropriate model for IVD research. J. Morphol. 277:244–251, 2016.

An examination of the mechanical properties of the annulus fibrosus: The effect of vibration on the intra-lamellar matrix strength

Vibration has been associated with low back pain and disc herniation; however the mechanism responsible for this association is unclear. It is theorized that herniation propagates through annular layers via clefts that form in the extracellular matrix between collagen fibres (intra-lamellar matrix) within each lamella. The effect of cyclic compressive loading at 5 Hz, applied to porcine functional spine units, on the mechanical properties of excised single annular lamellae was examined. These lamellae were tested under tension applied perpendicular to the collagen fibre orientation, effectively isolating the intra-lamellar matrix. Vibration affected the deformation magnitude at the end of the toe region of the stress-stretch ratio curve; specifically vibrated tissues showed larger toe regions (stretch ratio of 1.50 as compared to 1.31 observed in the control tissues, p=0.027). It is hypothesized that this result may be due to damaged elastin-a protein responsible for minimizing deformation and assisting with returning tissues to normal length following tension.

An Examination of the Influence of Strain Rate on Subfailure Mechanical Properties of the Annulus Fibrosus

Disk herniation is often considered a cumulative injury in that repetitive stress on the posterior annulus can result in the nucleus pulposus penetrating the annulus fibrosus and eventually extruding posteriorly. Further, it has been documented that the nucleus pulposus works its way through the annulus through clefts, which form as a result of repetitive tensile strain. The annulus fibrosus is viscoelastic in nature and therefore could express different mechanical responses to applied strain at varying rates. Other viscoelastic tissues, including tendons and ligaments, have shown altered mechanical responses to different rates of applied strain, but the response of the annulus to varying rates of strain is largely unknown. The present study examined the mechanical properties of 20 two-layered samples of porcine annulus fibrosus tissue at three distinct rates of applied 20% biaxial strain (20% strain over 20 s (slow), over 10 s (medium), and over 5 s (fast)); these three rates are considered applicable to nontraumatic loading. No differences in the stiffness or maximum stress in each of the two directions of applied strain were observed between the three strain rates. Specifically, the average (standard deviation) moduli calculated at the fast, medium, and slow rates, respectively, in the axial direction were 7.42 MPa (6.06), 7.77 MPa (6.61), and 7.63 MPa (6.67) and 8.22 MPa (8.4), 8.63 MPa (9.00), and 8.49 MPa (8.69) in the circumferential direction. The maximum stress values reached during the fast, medium, and slow rates, respectively, in the axial direction were 0.40 (0.36) MPa, 0.40 (0.36) MPa, and 0.39 (0.35) MPa and 0.45 (0.47) MPa, 0.44 (0.46) MPa, and 0.43 (0.46) MPa in the circumferential direction. At submaximal strain magnitudes over a range of nontraumatic rates likely to result in clefts in the annulus and potentially leading to disk herniation, any strain rate dependence is not significant.

Prevalence of occupation-related pain among baristas and an examination of low back and shoulder demand during the preparation of espresso-based beverages

Many baristas complain of low back pain (LBP) and upper extremity discomfort while at work. This study documented the prevalence of LBP and shoulder pain, via questionnaire, among a population of baristas to determine whether cumulative low back loads and shoulder moments are associated with pain reporting. Fifty-nine baristas completed the questionnaire; ten were also video-recorded for biomechanical analysis while making espresso beverages and cumulative and peak low back loads and shoulder moments were calculated. Seventy-three percent of those who completed the questionnaire reported having experienced LBP, and half attributed this pain to their job as a barista. Furthermore, 68% reported having experienced shoulder pain and half also attributed this pain to their job. Those who suffered from LBP had higher peak low back compression and those with shoulder pain had, in general, higher moments about their dominant shoulder. Practitioner Summary: This study found that, of those surveyed, at least half of baristas who suffer from low back or shoulder pain attributed this pain to their job. Furthermore, these individuals, in general, have higher loads in their low back and higher moments about their shoulders.

Perceived Risk of Low-Back Injury Among Four Occupations.

Objective: This study aimed to assess the perception of risk of low-back injury of individuals from four groups: office/administrative employees, dental workers (dentists/dental hygienists), firefighters, and undergraduate students. Background: The concept of worker’s perception of injury risk has been used to set safe material-handling limits and to determine compliance with health and safety regulations but has not been used to identify perceptual differences among occupations or potential deficiencies in risk awareness. Method: Participants (N = 232) were presented with eight images of different low-back postures/tasks and were required to rate their perceived magnitude of low-back risk on a scale from 0 (no risk) to 10 (extreme risk). Results: Office/administrative and dental workers rated postures higher than firefighters and students. Individuals from all groups perceived kyphotic postures as having a higher low-back risk than lordotic postures. Further, office and dental workers, compared to firefighters and students, perceived sitting postures to have a relatively higher level of risk, likely due to these postures being typically adopted by these individuals at work. No relationship between previous low-back pain and risk rating was observed in this study. Conclusion: Low-back injury risk perception varies between occupations/groups and may be a result of different exposures. Application: The results of this study can potentially be used to implement occupation-specific training programs to ensure that the scientific research regarding low-back injuries is being properly conveyed to employees across all sectors.