A computational simulation study of the influence of helmet wearing on head injury risk in adult cyclists.

Evidence for the effectiveness of cycle helmets has relied either on simplified experiments or complex statistical analysis of patient cohorts or populations. This study directly assesses the effectiveness of cycle helmets over a range of accident scenarios, from basic loss of control to vehicle impact, using computational modelling. Simulations were performed using dynamics modelling software (MADYMO) and models of a 50% Hybrid III dummy, a hybrid cross bicycle and a car. Loss of control was simulated by a sudden turn of the handlebars and striking a curb, side and rear-on impacts by a car were also simulated. Simulations were run over a representative range of cycle speeds (2.0-14.0 m s(-1)) and vehicle speeds (4.5-17.9 m s(-1)). Bicycle helmets were found to be effective in reducing the severity of head injuries sustained in common accidents. They reduced the risk of an AIS>3 injury, in cases with head impacts, by an average of 40%. In accidents that would cause up to moderate (AIS=2) injuries to a non-helmeted rider, helmets eliminated the risk of injury. Helmets were also found to be effective in preventing fatal head injuries in some instances. The effectiveness of helmets was demonstrated over the entire range of cycle speeds studied, up to and including 14 m s(-1). There was no evidence that helmet wearing increased the risk of neck injury, indeed helmets were found to be protective of neck injuries in many cases. Similarly, helmets were found to offer an increase in protection even when an increase in cycle speed due to risk compensation was taken into consideration.

MADYMO simulation of children in cycle accidents: a novel approach in risk assessment.

Head injuries are a significant cause of death and injury to child cyclists both on and off the road. Current evaluations of the effectiveness of cycle helmets rely on simplified mechanical testing or the analysis of aggregated accident statistics. This paper presents a direct evaluation of helmet efficacy by using computational modelling to simulate a range of realistic accident scenarios, including loss of control, collision with static objects and vehicle impact. A 6-year-old cyclist was modelled (as a Hybrid III 6-year-old dummy), in addition to a typical children's bicycle and a vehicle using the MADYMO dynamics software package. Simulations were performed using ranges of cyclist position, cycle speed and vehicle speed with and without a helmet that meets current standards. Wearing a cycle helmet was found to reduce the probability of head injuries, reducing the average probability of fatality over the scenarios studied from 40% to 0.3%. Similarly, helmet wearing reduced the probability of neck injuries (average probability of fatality reduced from 11% to 1%). There was no evidence that helmet wearing increased the severity of brain or neck injuries caused by rotational accelerations; in fact these were slightly reduced. Similarly, there was no evidence that increased cycling speed, such as might result from helmet related risk compensation, increased the probability of head injury.

The sensitivity of the calculation of ΔV to vehicle and impact parameters.

ΔV is frequently used to describe collision severity, and is often used by accident investigators to estimate speeds of vehicles prior to a collision, and by researchers looking for correlations between severity and outcome. This study identifies how ΔV varies over a wide range of input uncertainties allowing the direct comparison of different methods of input data collection in terms of their effect on uncertainty in the calculation of ΔV. Software was developed to implement this sensitivity analysis and was validated against examples presented in the CRASH3 manual. The findings are therefore representative of, and relevant to, commercially available tools such as CRASH3 and AIDamage. It is possible to measure the vehicle and collision parameters with sufficient accuracy to determine ΔV to a level of precision that is useful to predict occupant fatality. In many cases, ΔV is largely insensitive to the input parameter and category values or values determined from photographs may be used. A vehicle specific value of the stiffness parameter B should be used. Direct measurement of crush measurements and vehicle mass (including the best estimates of fluid loss) should be used. Similarly the mass of occupants and cargo should be measured directly rather than estimated from 50th centile values. Calculation of ΔV is sensitive to PDOF which should be measured with a precision of better than ±6°.

The implications of stress patterns in the vertebral body under axial support of an artificial implant.

Clinical studies show an association between changed load patterns both in the disc and its adjacent vertebral body, with painful degenerated discs. This suggests that failure to restore the normal loading pattern on implantation of a disc replacement could be a cause of lower clinical success rate. In the present study the variations of load patterns in the vertebra after disc implantation was studied using a simplified finite element models of natural and artificial discs. The effect of implant size and presence of voids at the implant-bony endplate interface were studied, for the worst case scenario of no bone remodelling. An altered stress pattern was observed in the vertebrae of implanted segments. The use of smaller size implants and presence of voids at the interface caused localized stress concentration in the endplate and adjacent cancellous bone. The study results support the hypothesis that current implants fail to restore normal loading patterns in the vertebral body, and the localized high stress regions could be the source of pain, and the cause of low success rate of TDRs.

The internal pressure and stress environment of the scoliotic intervertebral disc--a review.

The aetiology, in terms of both initiation and progression, of the deformity in idiopathic scoliosis is at present unclear. Even in neuromuscular cases, the mechanisms underlying progression are not fully elucidated. It is thought, however, that asymmetrical loading is involved in the progression of the disease, with evidence mainly from animal studies and modelling. There is, however, very little direct information as to the origin or mechanism of action of these forces in the scoliotic spine. This review describes the concept of intervertebral disc pressure or stress and examines possible measurement techniques. The biological and mechanical consequences of abnormalities in these parameters are described. Future possible studies and their clinical significance are also briefly discussed. Techniques of pressure measurement have culminated in the development of 'pressure profilometry', which provides stress profiles across the disc in mutually perpendicular axes. A hydrated intervertebral disc exhibits mainly hydrostatic behaviour. However, in pathological states such as degeneration and scoliosis, non-hydrostatic behaviour predominates and annular peaks of stress occur. Recent studies have shown that, in scoliosis, high hydrostatic pressures are seen with asymmetrical stresses from concave to convex sides. These abnormalities could influence both disc and endplate cellular activity directly, causing asymmetrical growth and matrix changes. In addition, disc cells could be influenced via nutritional changes consequent to end-plate calcification. Evidence suggests that the stress environment of the scoliotic disc is abnormal, probably generated by high and asymmetrical loading of non-muscular origin. If present in the scoliotic spine during daily activities, this could generate a positive feedback of cellular changes, resulting in curve progression. Future advances in understanding may rely on the development of computer models owing to the difficulties of in-vivo invasive measurements.

The effect of leg fracture level and vehicle front-end geometry on pedestrian knee injury and response.

In this study a MADYMO (mathematical dynamic modelling) model has been used to identify the influence of leg fracture on the injuries sustained by the pedestrian during front end impact with a vehicle. A factorial study of a MADYMO pedestrian and vehicle model are used to investigate the effect of different leg fracture tolerances, geometry, and vehicle compliance on the criteria measured in the European Enhanced Vehicle-safety Committee (EEVC) pedestrian safety tests. These criteria include knee bending, knee shear response, and lower leg bone (tibia) acceleration. The main study examines the spread of typical values of lower limb tolerance based on reported literature and contrasts the response of weaker, low-strength bones, normal tolerance, and limbs which do not fracture. Results show that knee bending angles and therefore ligament strains are significantly increased when fracture does not occur, and are decreased in bones exhibiting a low-strength response. Bone fracture tolerance is shown to be a significant parameter influencing knee bending. The parameters are compared to show that knee shear is significantly influenced by vehicle bumper compliance and that both criteria are heavily influenced by bumper height. Vehicles with more aggressive geometry, higher bumpers, and larger bumper lead were considered for comparative purposes.

A one-dimensional theoretical prediction of the effect of reduced end-plate permeability on the mechanics of the intervertebral disc.

The permeability of the cartilage end-plate (CEP) may play an important role in intervertebral disc (IVD) degeneration by controlling the convective and diffusive transport of metabolites into the nucleus pulposus. A one-dimensional poroelastic model was used to predict the effect of a CEP of lower permeability than the disc tissue on the convective transfer into and out of the IVD. With decreasing CEP permeability, associated with degeneration, the model predicted that the change in disc height with time became more linear; the disc could not rehydrate as quickly; and internal fluid movement was slowed. This study has shown that CEP permeability will only markedly have an effect on fluid movement, and hence convective nutrition, if the permeability of the CEP is reduced to less than that of the disc tissue.

Rigid-body modelling of shaken baby syndrome.

Recent reassessment of the literature on the shaken baby syndrome (SBS) has revealed a lack of scientific evidence and understanding of all aspects of the syndrome. In particular, studies have been unable to clarify the mechanisms of injury, indicating that impact, rather than shaking alone, is necessary to cause the type of brain damage observed. Rigid-body modelling (RBM) was used to investigate the effect of neck stiffness on head motion and head-torso impacts as a possible mechanism of injury. Realistic shaking data obtained from an anthropometric test dummy (ATD) was used to simulate shaking. In each study injury levels for concussion were exceeded, though impact-type characteristics were required to do so in the neck stiffness study. Levels for the type of injury associated with the syndrome were not exceeded. It is unlikely that further gross biomechanical investigation of the syndrome will be able to significantly contribute to the understanding of SBS. Current injury criteria are based on high-energy, single-impact studies. Since this is not the type of loading in SBS it is suggested that their application here is inappropriate and that future studies should focus on injury mechanisms in low-energy cyclic loading.

Demonstration of the appearance of the paraspinal musculoligamentous structures of the cervical spine using ultrasound.

The application of ultrasound in the imaging of the neck has primarily focussed on anterior structures (e.g., thyroid gland). Structures located on the posterior aspect of the neck have received little attention. This study illustrates the capability of modern ultrasound equipment in visualising the musculoligamentous structures of the neck, particularly the paraspinal musculature. Ten healthy adult volunteers (6 female; 4 male) underwent ultrasound examination of the cervical spine. A standardised technique for transducer placement was adopted and successive images of the neck of each subject were obtained. Spatial compound (extended field of view) images were obtained in subjects using one of two different ultrasound systems. Images of structures produced by ultrasound were compared to those achieved with magnetic resonance imaging in three subjects. Identification of key landmarks aided orientation and identification of structures. The internal architecture of the musculoligamentous structures of the cervical spine, especially the posterior neck muscles, was demonstrated well using ultrasound. Our study showed that modern ultrasound equipment is capable of producing clear images of the posterior cervical spine musculature and certain bony features.

Musculoskeletal motion flow fields using hierarchical variable-sized block matching in ultrasonographic video sequences.

We examine tissue deformations using non-invasive dynamic musculoskeletal ultrasonograhy, and quantify its performance on controlled in vitro gold standard (groundtruth) sequences followed by clinical in vivo data. The proposed approach employs a two-dimensional variable-sized block matching algorithm with a hierarchical full search. We extend this process by refining displacements to sub-pixel accuracy. We show by application that this technique yields quantitatively reliable results.

The internal mechanics of the intervertebral disc under cyclic loading.

The mechanics of the intervertebral disc (IVD) under cyclic loading are investigated via a one-dimensional poroelastic model and experiment. The poroelastic model, based on that of Biot (J. Appl. Phys. 12 (1941) 155; J. Appl. Mech. 23 (1956) 91), includes a power-law relation between porosity and permeability, and a linear relation between the osmotic potential and solidity. The model was fitted to experimental data of the unconfined IVD undergoing 5 cyclic loads of 20 min compression by an applied stress of 1MPa, followed by 40 min expansion. To obtain a good agreement between experiment and theory, the initial elastic deformation of the IVD, possibly associated with the bulging of the IVD into the vertebral bodies or laterally, was removed from the experimental data. Many combinations of the permeability-porosity relationship with the initial osmotic potential (pi(i)) were investigated, and the best-fit parameters for the aggregate modulus (H(A)) and initial permeability (k(i)) were determined. The values of H(A) and k(i) were compared to literature values, and agreed well especially in the context of the adopted high-stress testing regime, and the strain related permeability in the model.

Knoop microhardness anisotropy of the ovine radius.

The Knoop indenter has been used to characterise fully the Knoop microhardness (H(K)) anisotropy of compact bone. 2120 indentations were performed on mature ovine radii and a linear relationship was found between H(K) and the angle between the major diagonal of the indenter and the lamella boundaries (p<<0.001). H(K) increased significantly with ash fraction (p<0.001), but decreased with atmospheric vapour pressure (p<0.05). A significant interaction was found between ash fraction and atmospheric vapour pressure (p<0.01). H(K) significantly varied with indentation position along the diaphysis and around the cortex (both p<<0.001), however radial variation in H(K) was not statistically significant. The variation of ash fraction showed similar trends. These data show that H(K) varies similarly to Vickers microhardness, but in addition, can provide clear information on the anisotropy of Haversian bone without the need for excising many different indentation planes. A large number of indentations are required to obtain low type I and type II errors in the statistical analysis.

Intervertebral disc structure: observation by a novel use of ultrasound imaging.

The internal structure of intervertebral discs is clinically important in the management of back pain. No current routine imaging modality is able to image disc structure satisfactorily. The aim of this work was to investigate and validate ultrasound imaging so that it might be applied to assessment of structural integrity and degree of degeneration. The optimum imaging technique was determined using a 3.5 MHz probe in one female subject. The applicability of this technique to investigate disc structure in the entire thoracolumbar spine was further investigated in 13 subjects. The optimum disc imaging technique was found to be a posterolateral approach, 1 to 2 cm lateral of the dorsal midline, that revealed structure within the disc not apparent using other approaches. It was demonstrated that posterolateral imaging introduces a smaller reproducibility error in measurements of linear dimensions close to the disc. It is possible to observe internal structure within the disc between T11 and L3 in at least 54% of individuals.

Determination of a standard site for the measurement of bone mineral density of the human calcaneus.

Ultrasound of the calcaneus may be used as a cheap, ionising radiation-free and easy to use indicator of skeletal status, and hence of osteoporotic fracture risk. At present ultrasound is not widely used as it suffers from high precision errors. As ultrasound parameters are determined in part by bone mineral density (BMD), an increase in the accuracy and precision of BMD measurements should reduce the precision error associated with ultrasound measurements. The aim of this study was to define an anatomical site on the calcaneus at which accurate and precise measurements of BMD can be made. Ten dry calcanei and 10 cadaveric feet were scanned using a DXA scanner; 9 anatomically defined regions (1 cm2) were selected in the posterior part of the calcaneus for analysis. The centre of region 1 was positioned halfway along the line joining the anterior border of the calcaneal tubercle and the peak of the posterior superior tubercle, and the remaining 8 regions were placed around this central area. The BMD in these 9 regions was compared with the whole bone BMD and the variability of BMD within each of the 9 regions was measured. The reproducibility of the technique was assessed by taking 10 repeated measurements of 2 bone and 2 cadaveric specimens, each specimen being removed and repositioned between measurements. Region 1 was found to be the most representative of total BMD in cadaveric feet. This region also showed the least variability of BMD and consistently gave the lowest coefficients of variation in the reproducibility study both in the bone and the cadaveric specimens. This region is hence the most suitable site on the calcaneus for measuring absolute values of and changes in BMD. The surface position of region 1 was found to be consistently 5/9 along the line at 45 degrees to the vertical, from the lateral malleolus to the heel. The identification of the surface location of region 1 relative to anatomical landmarks of the foot has enabled the same anatomical site to be measured in all subjects. This allows meaningful intersubject comparisons to be made. Preliminary data suggest that precision errors using ultrasound are also reduced when measurements are taken at this region of the calcaneus. The reduction in the precision error of ultrasound assessment of skeletal status may provide a cheap and safe way to identify individuals at risk from osteoporotic fracture.

Microhardness anisotropy of lamellar bone.

The Knoop microhardness test has been utilised to observe in-plane microhardness anisotropy of rat tibiae. The elongated rhombohedral geometry of the Knoop indenter enables the Knoop microhardness (HK) to be calculated for a given indenter orientation. Two indenter orientations were used: the major axis of the indenter was aligned along the length of, and across the mid-sagittal section. The statistical analysis demonstrated that the variation in HK was primarily due to the orientation of the Knoop indenter (p < 0.001). HK was consistently greater when the indenter was aligned with the major diagonal radial on the mid-sagittal section.

The effects of posterior fixation on internal intervertebral disc mechanics.

Posterior fixation of intervertebral discs is used to treat, and occasionally diagnose, discogenic pain since it is thought that it will reduce the internal loading of the discs in vitro. We measured the internal loading of ten intervertebral discs using stress profilometry under simulated physiological loads and then after posterior fixation. Partial discectomies were performed to simulate advanced disc degeneration and the sequence repeated. Posterior fixation had very little effect on the magnitude of the loads acting on the disc and none when disc degeneration was simulated. It did, however, reduce bulging of the anterior annulus under combined bending and compression (p < 0.03). Recent experiments in vivo have shown that discogenic pain is associated with abnormal bulging of the annulus which suggests that the clinical benefit of fixation may be due to this.

'Stress' distributions inside intervertebral discs. The effects of age and degeneration.

We investigated the distribution of compressive 'stress' within cadaver intervertebral discs, using a pressure transducer mounted in a 1.3 mm diameter needle. The needle was pulled along the midsagittal diameter of a lumbar disc with the face of the transducer either vertical or horizontal while the disc was subjected to a constant compressive force. The resulting 'stress profiles' were analysed in order to characterise the distribution of vertical and horizontal compressive stress within each disc. A total of 87 discs from subjects aged between 16 and 87 years was examined. Our results showed that age-related degenerative changes reduced the diameter of the central hydrostatic region of each disc (the 'functional nucleus') by approximately 50%, and the pressure within this region fell by 30%. The width of the functional annulus increased by 80% and the height of compressive 'stress peaks' within it by 160%. The effects of age and degeneration were greater at L4/L5 than at L2/L3, and the posterior annulus was affected more than the anterior. Age and degeneration were themselves closely related, but the stage of degeneration had the greater effect on stress distributions. We suggest that structural changes within the annulus and endplate lead to a transfer of load from the nucleus to the posterior annulus. High 'stress' concentrations within the annulus may cause pain, and lead to further disruption.

In vivo stress measurement can predict pain on discography.

STUDY DESIGN: An in vivo experimental investigation of internal disc mechanics and discogenic pain. OBJECTIVES: To test the hypotheses: 1) The pattern of internal loading of intervertebral discs in vivo is similar to that measured previously in vitro; 2) stress concentrations also are found in clinically degenerate discs in vivo; and stress concentrations are associated with discogenic pain. SUMMARY OF BACKGROUND DATA: Stress concentrations corresponding to potentially painful loading patterns of the intervertebral disc and endplate have been observed in vitro. METHODS: The distribution of stress within the lumbar intervertebral discs of patients with chronic discogenic pain was measured using stress profilometry. The severity of their pain was assessed using provocative discography. RESULTS: Discogenic pain was found to be associated with anomalous loading of the posterolateral anulus (P < 0.001) and nucleus (P < 0.01). Painful discs were found to have a 38% wider posterolateral anulus (P < 0.023) than painless discs and to have a 63% lower mean nuclear stress (P < 0.017). CONCLUSIONS: Stress profilometry is an effective investigation of the mechanics of intervertebral discs in vivo. Discogenic pain is caused by changes in the pattern of loading of the posterolateral anulus or nucleus pulposus.

Effects of hydrostatic pressure on matrix synthesis in different regions of the intervertebral disk.

The intervertebral disk is routinely subjected to compressive loads that alter with posture and muscle activity and can produce pressures > 2 MPa in human lumbar disks in vivo (A. Nachemson and G. Elfstrom. Scand. J. Rehabil. Med. 2, Suppl. 1:1-40, 1979; A. Nachemson and J. M. Morris. J. Bone Jt. Surg. Am. Vol. 46A: 1077-1092, 1964). We measured the effect of load on hydrostatic pressures in bovine caudal disks. With increase in applied load, pressure increased linearly in the nucleus and inner annulus. The resting pressure measured after slaughter (0.19 +/- 0.05 MPa) and the pressure at failure (34 MPa, estimated from the vertebrae/disk segment failure load of 7,430 +/- 590 N) define the limits that can occur in vivo. Because hydrostatic pressure influences matrix synthesis in articular cartilage, we have examined the effects of pressures in the range 1-10 MPa applied for 20 s or 2 h on proteoglycan synthesis in bovine caudal and human lumbar intervertebral disks in vitro. In the nucleus pulposus and inner annulus of bovine disks, application of hydrostatic pressure in the range of 1-7.5 MPa for only 20 s stimulated matrix synthesis over the following 2 h at atmospheric pressure. The maximum stimulation in the bovine disks was seen in the inner annulus after application of 2.5 MPa, where proteoglycan synthesis rates doubled. Exposure to 2.5 MPa also stimulated synthesis in the nucleus pulposus of human disks taken at surgery, whereas 7.5 MPa inhibited synthesis in five out of six specimens. With 2-h continuous exposure to the same levels of pressure, no stimulation was seen in the nucleus of bovine disks, and significant stimulation was only observed at 5.0 MPa in the inner annulus. Exposure to 10 MPa for either 20 s or 2 h inhibited proteoglycan synthesis in these regions of the disks. In contrast, in the outer annulus, where loading does not lead to a rise in hydrostatic pressure in vivo, there was no significant response to hydrostatic pressure over the range of 1-10 MPa in bovine or human disks.