The effect of creep on human lumbar intervertebral disk impact mechanics

Biomechanical response of lumbar intervertebral disc in daily sitting postures: a poroelastic finite element analysis

This study aims to establish and validate a poroelastic L4-L5 finite element model to evaluate the effect of different sitting postures and their durations on the mechanical responses of the disc. During the sustained loading conditions, the height loss, fluid loss and von-Mises stress gradually increased, but the intradiscal pressure decreased. The varying rates of aforementioned parameters were more significant at the initial loading stage and less so at the end. The predicted values in the flexed sitting posture were significantly greater than other postures. The extended sitting posture caused an obvious von-Mises stress concentration in the posterior region of the inter-lamellar matrix. From the biomechanical perspective, prolonged sitting may pose a high risk of lumbar disc degeneration, and therefore adjusting the posture properly in the early stage of sitting time may be useful to mitigate that. Additionally, upright sitting is a safer posture, while flexed sitting posture is more harmful.

Identifying Regional Viscoelastic Properties of the Superficial Muscular Aponeurotic System

BACKGROUND

Suspension of the superficial muscular aponeurotic system (SMAS) is generally believed to be necessary in facelift surgery. Although many techniques have been suggested, all rely on the viscoelastic properties of the SMAS.

OBJECTIVES

The aim of this study was to determine the viscoelastic properties of bursting strength, stress relaxation, and creep in the lateral, mid-cheek, and medial regions of the SMAS.

METHODS

The viscoelastic properties of the SMAS were determined in 12 cadaveric hemifaces. Lateral SMAS was classified as the SMAS overlying the parotid gland; mid-cheek SMAS as anterior to the parotid and overlying the masseter muscle; and medial SMAS as including tissue extending medial from the lateral canthus and ending at the nasolabial fold.

RESULTS

The 3 SMAS regions showed significantly different bursting strengths: 38.9 N for the lateral SMAS, 26.7 N for the mid-cheek SMAS, and 11.9 N for the medial SMAS (P < 0.0001). Stress relaxation was similar in all vertical regions with measurements of 54% in the lateral, 48% in the mid-cheek, and 59% in the medial SMAS. Creep was found to be similar in the lateral and mid-cheek SMAS with values of 18% and 19%, respectively. The medial SMAS was noted to have a higher creep at 22%.

CONCLUSIONS

The lateral SMAS has a stronger bursting strength than the mid-cheek and medial SMAS. Creep appears to be lower in the lateral and mid-cheek SMAS. Stress relaxation appears to be similar in all 3 vertical regions.

Tissue physiology revolving around the clock: circadian rhythms as exemplified by the intervertebral disc

Circadian clocks in the brain and peripheral tissues temporally coordinate local physiology to align with the 24 hours rhythmic environment through light/darkness, rest/activity and feeding/fasting cycles. Circadian disruptions (during ageing, shift work and jet-lag) have been proposed as a risk factor for degeneration and disease of tissues, including the musculoskeletal system. The intervertebral disc (IVD) in the spine separates the bony vertebrae and permits movement of the spinal column. IVD degeneration is highly prevalent among the ageing population and is a leading cause of lower back pain. The IVD is known to experience diurnal changes in loading patterns driven by the circadian rhythm in rest/activity cycles. In recent years, emerging evidence indicates the existence of molecular circadian clocks within the IVD, disruption to which accelerates tissue ageing and predispose animals to IVD degeneration. The cell-intrinsic circadian clocks in the IVD control key aspects of physiology and pathophysiology by rhythmically regulating the expression of ~3.5% of the IVD transcriptome, allowing cells to cope with the drastic biomechanical and chemical changes that occur throughout the day. Indeed, epidemiological studies on long-term shift workers have shown an increased incidence of lower back pain. In this review, we summarise recent findings of circadian rhythms in health and disease, with the IVD as an exemplar tissue system. We focus on rhythmic IVD functions and discuss implications of utilising biological timing mechanisms to improve tissue health and mitigate degeneration. These findings may have broader implications in chronic rheumatic conditions, given the recent findings of musculoskeletal circadian clocks.

Assessment of lumbar spinal disc injury in frontal crashes

Frontal vehicle crashes have been a leading cause of spinal injuries in recent years. Reconstruction of frontal crashes using computational models and spinal load analysis helps us understand the patterns of injury and load propagation during frontal crashes. By reconstructing a real crash test and using a viscoelastic crash dummy model, spinal injury patterns were analyzed. The results indicated that a moderate crash with an impact speed of 56 km/h leads to injuries in L1-L2 and L5-S1 levels (L for lumbar and S for sacral vertebrae). The largest spinal loads and injuries were mainly observed immediately after the airbag deployment when the peak of the crash acceleration transpires. Also, the effects of impulse magnitude on the spinal loads and head injury criterion (HIC) showed that HIC is more sensitive than compressive forces to the magnitude of impulse. Moreover, the effects of disc preconditioning as a major factor in the risk of injury was evaluated. The results demonstrate that as the lumbar spine is subjected to a longer preloading, it will be more vulnerable to injury; preconditioning of the discs more adversely affected the risk of injury than a 10% increase in the crash impulse. Overall the results highlight the importance of spinal injury prevention in frontal crashes.

MR Elastography-derived Stiffness: A Biomarker for Intervertebral Disc Degeneration

Purpose To determine the repeatability of magnetic resonance (MR) elastography-derived shear stiffness measurements of the intervertebral disc (IVD) taken throughout the day and their relationship with IVD degeneration and subject age. Materials and Methods In a cross-sectional study, in vivo lumbar MR elastography was performed once in the morning and once in the afternoon in 47 subjects without current low back pain (IVDs = 230; age range, 20-71 years) after obtaining written consent under approval of the institutional review board. The Pfirrmann degeneration grade and MR elastography-derived shear stiffness of the nucleus pulposus and annulus fibrosus regions of all lumbar IVDs were assessed by means of principal frequency analysis. One-way analysis of variance, paired t tests, concordance and Bland-Altman tests, and Pearson correlations were used to evaluate degeneration, diurnal changes, repeatability, and age effects, respectively. Results There were no significant differences between morning and afternoon shear stiffness across all levels and there was very good technical repeatability between the morning and afternoon imaging results for both nucleus pulposus (R = 0.92) and annulus fibrosus (R = 0.83) regions. There was a significant increase in both nucleus pulposus and annulus fibrosus MR elastography-derived shear stiffness with increasing Pfirrmann degeneration grade (nucleus pulposus grade 1, 12.5 kPa ± 1.3; grade 5, 16.5 kPa ± 2.1; annulus fibrosus grade 1, 90.4 kPa ± 9.3; grade 5, 120.1 kPa ± 15.4), and there were weak correlations between shear stiffness and age across all levels (R ≤ 0.32). Conclusion Our results demonstrate that MR elastography-derived shear stiffness measurements are highly repeatable, weakly correlate with age, and increase with advancing IVD degeneration. These results suggest that MR elastography-derived shear stiffness may provide an objective biomarker of the IVD degeneration process. ^© RSNA, 2017 Online supplemental material is available for this article.

The Contribution of Biomechanical-Biological Interactions of the Spine to Low Back Pain

OBJECTIVE

The objective of this mini-review is to examine a subset of literature that demonstrates multiple interactions between mechanics and biology within the spine and propose how incorporation of these mechano-biologic interactions can be applied to improve the conceptual understanding of tissue tolerances.

BACKGROUND

Low back pain represents a major musculoskeletal problem in the workplace. Traditional biomechanical assessments have employed tissue tolerances as an approach for reducing workplace injuries; however, development of more universal biologically sensitive tolerances requires incorporation of mechano-biologic interactions.

METHODS

A focused literature review addressing the interactions between mechanical loading and biology in the spine.

RESULTS

Mechanical loads applied to the body are distributed across all spatial scales from the body to the tissues to the cells. These mechanical loads regulate cellular metabolism and over time can lead to tissue strengthening or weakening. Mechanical loading also interacts with the biologic environment (e.g., tissue inflammation, nerve sensitization) to influence the perception of pain, thereby changing the risk of experiencing pain. Biologic tissues also exhibit time-dependent changes in mechanical behaviors that occur throughout the day and with disease, suggesting tissue tolerances are time dependent.

CONCLUSION

Incorporating mechano-biologic interactions into the traditional tissue tolerance paradigm through describing tissue tolerances as a function of multiple factors (e.g., preexisting risk factors, underlying pathology, and time) may lead to the development of tissue tolerances that are more representative of the in vivo situation.

APPLICATION

Efforts must work toward incorporating biological concepts into tissue tolerances in order to improve risk assessment tools.

The effect of six degree of freedom loading sequence on the in-vitro compressive properties of human lumbar spine segments

The complex, direction-dependent, poro-viscoelastic properties of the intervertebral disc (disc) suggest that investigations of the six degree of freedom (6DOF) behaviour may be susceptible to inter-test variation in mechanical response if the disc does not return to initial conditions between loading directions. No studies have quantified the effects of sequential multi-directional loading on the consistency of the compressive response of the disc throughout a 6DOF testing protocol. Therefore, the objective of this study was to determine the effect of 6DOF loading on the compressive properties (stiffness and phase angle) of human discs, as evaluated by a reference compression test performed after each single DOF test. Fourteen intact human functional spinal units (FSU) were tested in each of ±6DOFs (shear directions followed by bending and compression) across four orders of magnitude loading frequencies (0.001-1Hz), followed by reference compression tests while subjected to physiological preload, hydration, and body temperature conditions in a hexapod robot. Repeated measures ANOVA revealed significant within-subjects effects between the reference compression tests for modulus (p<0.001), stiffness (p<0.001), and phase angle (p=0.008). Significant post-hoc pairwise comparisons were initially seen between the control and other reference compression tests for stiffness and modulus after the shear DOFs, however, no significant differences were present after the final reference compression test compared to control. More pronounced effects were seen for stiffness in comparison to modulus and phase angle. These effects may be due to three potentials factors, which include the sequence of testing, the cohort of degenerative specimens, and/or cumulative creep due to the constant application of a follower load. While the sequence of test directions was chosen to minimise the biphasic effect, there may be other sequences, which could result in minimal changes in compressive properties.