Lynne E. Bilston

In vivo brain viscoelastic properties measured by magnetic resonance elastography

Magnetic resonance elastography (MRE) is a non‐invasive imaging technique used to visualise and quantify mechanical properties of tissue, providing information beyond what can be currently achieved with standard MR sequences and could, for instance, provide new insight into pathological processes in the brain. This study uses the MRE technique at 3 T to extract the complex shear modulus for in vivo brain tissue utilizing a full three‐dimensional approach to reconstruction, removing contributions of the dilatational wave by application of the curl operator. A calibrated phantom is used to benchmark the MRE measurements, and in vivo results are presented for healthy volunteers. The results provide data for in vivo brain storage modulus (G′), finding grey matter (3.1 kPa) to be significantly stiffer than white matter (2.7 kPa). The first in vivo loss modulus (G″) measurements show no significant difference between grey matter (2.5 kPa) and white matter (2.5 kPa). Copyright

The mechanical properties of the human cervical spinal cord in vitro

The response of spinal cord tissue to mechanical loadings is not well understood. In this study, isolated fresh cervical spinal cord samples were obtained from cadavers at autopsy and tested in uniaxial tension at moderate strain rates. Stress relaxation experiments were performed with an applied strain rate and peak strain in the physiological range, similar to those seen in the spinal cord during voluntary motion. The spinal cord samples exhibited a nonlinear stress-strain response with increasing strain increasing the tangent modulus. In addition, significant relaxation was observed over 1 min. A quasilinear viscoelastic model was developed to describe the behavior of the spinal cord tissue and was found to describe the material behavior adequately. The data also were fitted to both hyperelastic and viscoelastic fluid models for comparison with other data in the literature.

Optimal Timing of a Single Dose of Zoledronic Acid to Increase Strength in Rat Fracture Repair

We hypothesized that ZA treatment would bolster fracture repair. In a rat model for closed fracture healing, a single dose of ZA at 0, 1, or 2 wk after fracture significantly increased BMC and strength of the healed fracture. Delaying the dose (1 or 2 wk after fracture) displayed superior results compared with dosing at the time of fracture.

Rheological properties of the tissues of the central nervous system: A review

Knowledge of the biomechanical properties of central nervous system (CNS) tissues is important for understanding mechanisms and thresholds for injury, and aiding development of computer or surrogate models of these tissues. Many investigations have been conducted to estimate the properties of CNS tissues including under shear, compressive and tensile loading, however there is much variability in this body of literature, making it difficult to separate the material properties from effects that result from a given experimental protocol. This review summarises previous studies of brain and spinal cord properties; discussing their main findings and points of difference, and displays the reported data on comparable scales. Additionally, based on the observed effects of methodological choices on reported tissue properties, recommendations for future studies of brain and spinal cord properties are made.

Effects of Proteins, Blood Cells and Glucose on the Viscosity of Cerebrospinal Fluid

It has long been assumed that cerebrospinal fluid (CSF) is a newtonian fluid with viscosity similar to water, yet high protein content, has been postulated to increase the viscosity of CSF in vivo. Such an increase in viscosity may have serious implications for the effectiveness of surgical shunts implanted to re-establish the CSF flow in cases of abnormal CSF circulation. In this study, glucose content, total protein content and blood cell count in the CSF of 23 patients undergoing brain surgery were measured. Viscosity measurements were performed on duplicate CSF samples over a range of shear strain rates of 25–1,460 s–1. The results indicated that high protein or high cell concentration in CSF does not significantly affect the viscosity of the cerebral fluid at those shear rates. CSF is clearly newtonian, and its viscosity at 37°C is in the range of 0.7–1 mPa·s.

Linear viscoelastic properties of bovine brain tissue in shear

We report the results from a series of rheological tests of fresh bovine brain tissue. Using a standard Bohlin VOR shear rheometer, shear relaxation and oscillating strain sweep experiments were performed on disks of brain tissue 30 mm in diameter, with a thickness of 1.5-2 mm. The strain sweep experiment showed that the viscoelastic strain limit is of the order of 0.1% strain. Shear relaxation data do not indicate the presence of a long-term elastic modulus, indicating fluid-like behavior. A relaxation spectrum was calculated by inverting the experimental data and used to predict oscillatory response, which agreed well with measured data.

Zoledronic Acid Prevents Osteopenia and Increases Bone Strength in a Rabbit Model of Distraction Osteogenesis

Prolonged healing times and stress‐shielding osteopenia remain problematic in distraction osteogenesis. In this study of 30 rabbits, zoledronic acid increased regenerate volume, mineralization, and tibial strength and prevented osteopenia over a 6‐week period. Translation to the clinical setting, if safe, could improve outcomes in distraction osteogenesis in children.

In vivo passive mechanical behaviour of muscle fascicles and tendons in human gastrocnemius muscle-tendon units

Non‐Technical Summary  Relaxed skeletal muscles behave like springs that resist joint motion. There have been few in vivo studies of the spring‐like properties of relaxed muscles. In this study, ultrasound was used to image human calf muscles while muscle length was changed by rotating the ankle of relaxed subjects. The muscles of some subjects buckled at short lengths. At short lengths most muscle fascicles (bundles of muscle cells) are slack. As the muscle is lengthened the slack is progressively taken up, first in some fascicles then in others. The increase in muscle length is due partly to increases in the length of muscle fascicles but most of the increase in muscle length occurs in the tendons.

Serious injury is associated with suboptimal restraint use in child motor vehicle occupants

Aim:  To investigate the relationship between restraint usage and injury outcome in child motor vehicle occupants aged 2–8 years.

Movement of the tongue during normal breathing in awake healthy humans

Electromyographic (EMG) activity of the airway muscles suggest that genioglossus is the primary upper airway dilator muscle. However, EMG data do not necessarily translate into tissue motion and most imaging modalities are limited to assessment of the surfaces of the upper airway. In this study, we hypothesized that genioglossus moves rhythmically during the respiratory cycle and that the motion within is inhomogeneous. A ‘tagged’ magnetic resonance imaging technique was used to characterize respiratory‐related tissue motions around the human upper airway in quiet breathing. Motion of airway tissues at different segments of the eupnoeic respiratory cycle was imaged in six adult subjects by triggering the scanner at the end of inspiration. Displacements of the ‘tags’ were analysed using the harmonic phase method (HARP). Respiratory timing was monitored by a band around the upper abdomen. The genioglossus moved during the respiratory cycle. During expiration, the genioglossus moved posteriorly and during inspiration, it moved anteriorly. The degree of motion varied between subjects. The maximal anteroposterior movement of a point tracked on the genioglossus was 1.02 ± 0.54 mm (mean ±s.d.). The genioglossus moved over the geniohyoid muscle, with minimal movement in other muscles surrounding the airway at the level of the soft palate. Local deformation of the tongue was analysed using two‐dimensional strain maps. Across the respiratory cycle, positive strains within genioglossus reached peaks of 17.5 ± 9.3% and negative strains reached peaks of −16.3 ± 9.3% relative to end inspiration. The patterns of strains were consistent with elongation and compression within a constant volume structure. Hence, these data suggest that even during respiration, the tongue behaves as a muscular hydrostat.