Christopher R. Dennison

Bicycle helmets are highly effective at preventing head injury during head impact: Head-form accelerations and injury criteria for helmeted and unhelmeted impacts

Cycling is a popular form of recreation and method of commuting with clear health benefits. However, cycling is not without risk. In Canada, cycling injuries are more common than in any other summer sport; and according to the US National Highway and Traffic Safety Administration, 52,000 cyclists were injured in the US in 2010. Head injuries account for approximately two-thirds of hospital admissions and three-quarters of fatal injuries among injured cyclists. In many jurisdictions and across all age levels, helmets have been adopted to mitigate risk of serious head injuries among cyclists and the majority of epidemiological literature suggests that helmets effectively reduce risk of injury. Critics have raised questions over the actual efficacy of helmets by pointing to weaknesses in existing helmet epidemiology including selection bias and lack of appropriate control for the type of impact sustained by the cyclist and the severity of the head impact. These criticisms demonstrate the difficulty in conducting epidemiology studies that will be regarded as definitive and the need for complementary biomechanical studies where confounding factors can be adequately controlled. In the bicycle helmet context, there is a paucity of biomechanical data comparing helmeted to unhelmeted head impacts and, to our knowledge, there is no data of this type available with contemporary helmets. In this research, our objective was to perform biomechanical testing of paired helmeted and unhelmeted head impacts using a validated anthropomorphic test headform and a range of drop heights between 0.5m and 3.0m, while measuring headform acceleration and Head Injury Criterion (HIC). In the 2m (6.3m/s) drops, the middle of our drop height range, the helmet reduced peak accelerations from 824g (unhelmeted) to 181g (helmeted) and HIC was reduced from 9667 (unhelmeted) to 1250 (helmeted). At realistic impact speeds of 5.4m/s (1.5m drop) and 6.3m/s (2.0m drop), bicycle helmets changed the probability of severe brain injury from extremely likely (99.9% risk at both 5.4 and 6.3m/s) to unlikely (9.3% and 30.6% risk at 1.5m and 2.0m drops respectively). These biomechanical results for acceleration and HIC, and the corresponding results for reduced risk of severe brain injury show that contemporary bicycle helmets are highly effective at reducing head injury metrics and the risk for severe brain injury in head impacts characteristic of bicycle crashes.

Mechanisms of cervical spine injury in rugby union: is it premature to abandon hyperflexion as the main mechanism underpinning injury?

Cervical spine injuries in rugby union have received growing worldwide attention owing to their often catastrophic nature.1,–,3 Kuster et al 4 considered recent changes in the epidemiology as well as the ex vivo biomechanics literature on cervical spine injury during head-first impacts. Kuster postulated that the majority of catastrophic cervical spine injuries before year 2000 occurred through a hyperflexion mechanism in the scrum and since the year 2000, these injuries have occurred during tackles via an axial compression mechanism and related ‘buckling’ of the cervical spine. Regulators and other stakeholders in the game continually seek to improve understanding of the scope, true incidence and mechanism of these catastrophic injuries. Ideally, a full comprehension of cervical injury mechanisms occurring during rugby would lead to changes in the game, through rule changes or changes in enforcement and/or player coaching or education, which in turn would reduce the incidence of these injuries.2 Achieving such a full comprehension, however, has been elusive.1,–,3 ,5,–,9 For example, recent changes to the rules have led to reduced time spent in scrum and have altered scrum engagement, which subsequently have been associated with a reduction in incidence of scrum-related cervical spine injury (an injury primarily involving forward positions and accounting for 37%–51% of cervical spine injuries).5 ,7 ,8 ,10 ,11 These changes have therefore affected the distribution of players who get injured, with increased percentage of these injuries now being sustained by back positions than prior to these rule changes, and up to 57% of cervical injuries are now occurring …

Validation of a novel minimally invasive intervertebral disc pressure sensor utilizing in-fiber Bragg gratings in a porcine model: an ex vivo study.

Study Design. Nucleus pressure was measured within porcine intervertebral discs (IVDs) with a novel in-fiber Bragg grating (FBG) sensor (0.4 mm diameter) and a strain gauge (SG) sensor (2.45 mm). Objective. To validate the accuracy of a new FBG pressure sensor designed for minimally invasive measurements of nucleus pressure. Summary of Background Data. Although its clinical utility is controversial, it is possible that the predictive accuracy of discography can be improved with IVD pressure measurements. These measurements are typically obtained using needle-mounted SG sensors inserted into the nucleus. However, by virtue of their size, SG sensors alter disc mechanics, injure anulus fibers, and can potentially initiate or accelerate degenerative changes thereby limiting their utility particularly clinically. Methods. Six functional spinal units were loaded in compression from 0 N to 500 N and back to 0 N; nucleus pressure was measured using the FBG and SG sensors at various locations along anterior and anterolateral axes. Results. On average maximum IVD pressures measured using the FBG and SG sensors were within 9.39% of each other. However, differences between maximum measured pressures from the FBG and SG sensors were larger (22.2%) when the SG sensor interfered with vertebral endplates (P < 0.05). The insertion of the FBG sensor did not result in visible damage to the anulus, whereas insertion of the SG sensor resulted in large perforations in the anulus through which nucleus material was visible. Conclusion. The new FBG sensor is smaller and less invasive than any previously reported disc pressure sensor and gave results consistent with previous disc pressure studies and the SG sensor. There is significant potential to use this sensor during discography while avoiding the controversy associated with disc injury as a result of sensor insertion.

Enhanced sensitivity of an in-fibre Bragg grating pressure sensor achieved through fibre diameter reduction

Contemporary biomedical pressure sensors are based on miniaturized piezo-resistive, strain-gauge or other solid-state sensing technologies. All of these technologies have key limitations, when packaged into miniaturized sensors, including fragility and long term instability. In-fibre Bragg gratings (FBGs) are an attractive alternative to these electronic technologies because FBGs are biocompatible, robust, immune to electromagnetic interference and mechanically compliant. FBGs can also be used to measure multiple physical parameters and distributions of parameters. We present a FBG-based pressure sensor that has pressure sensitivity 20 times greater than that of a bare fibre FBG, and a major diameter and sensing area of only 200 µm and 0.02 mm2, respectively. Increases in pressure sensitivity are achieved by reducing the diameter of the fibre in the region of the Bragg grating, thereby resulting in reduced cross-sectional area and therefore increased axial strains for a given applied pressure. The presented design is an improvement over other FBG pressure sensors that achieve increased sensitivity through mechanical amplification schemes, usually resulting in major diameters and sensing lengths of many millimetres. Calibration results demonstrate the FBG sensors ability to measure pressure with ±0.36 kPa repeatability over a 14 kPa range. To our knowledge, this is the only FBG-based pressure sensor of its size to achieve this repeatability.

An in-fiber Bragg grating sensor for contact force and stress measurements in articular joints

We present an in-fiber Bragg grating-based sensor (240 µm diameter) for contact force/stress measurements in articular joints. The contact force sensor and another Bragg grating-based pressure sensor (400 µm diameter) are used to conduct the first simultaneous measurements of contact force/stress and fluid pressure in intact cadaveric human hips. The contact force/stress sensor addresses limitations associated with stress-sensitive films, the current standard tools for contact measurements in joints, including cartilage modulus-dependent sensitivity of films and the necessity to remove biomechanically relevant anatomy to implant the films. Because stress-sensitive films require removal of anatomy, it has been impossible to validate the mechanical rationale underlying preventive or corrective surgeries, which repair these anatomies, by conducting simultaneous stress and pressure measurements in intact hips. Methods are presented to insert the Bragg grating-based sensors into the joint, while relevant anatomy is left largely intact. Sensor performance is predicted using numerical models and the predicted sensitivity is verified through experimental calibrations. Contact force/stress and pressure measurements in cadaveric joints exhibited repeatability. With further validation, the Bragg grating-based sensors could be used to study the currently unknown relationships between contact forces and pressures in both healthy and degenerated joints.

A minimally invasive in-fiber Bragg grating sensor for intervertebral disc pressure measurements

We present an in-fiber Bragg grating (FBG) based intervertebral disc (IVD) pressure sensor that has pressure sensitivity seven times greater than that of a bare fiber, and a major diameter and sensing area of only 400 µm and 0.03 mm2, respectively. This is the only optical, the smallest and the most mechanically compliant disc pressure sensor reported in the literature. This is also an improvement over other FBG pressure sensors that achieve increased sensitivity through mechanical amplification schemes, usually resulting in major diameters and sensing lengths of many millimeters. Sensor sensitivity is predicted using numerical models, and the predicted sensitivity is verified through experimental calibrations. The sensor is validated by conducting IVD pressure measurements in porcine discs and comparing the FBG measurements to those obtained using the current standard sensor for IVD pressure. The predicted sensitivity of the FBG sensor matched with that measured experimentally. IVD pressure measurements showed excellent repeatability and agreement with those obtained from the standard sensor. Unlike the current larger sensors, the FBG sensor could be used in discs with small disc height (i.e. cervical or degenerated discs). Therefore, there is potential to conduct new measurements that could lead to new understanding of the biomechanics.

Compressive Follower Load Influences Cervical Spine Kinematics and Kinetics During Simulated Head-First Impact in an in Vitro Model

Current understanding of the biomechanics of cervical spine injuries in head-first impact is based on decades of epidemiology, mathematical models, and in vitro experimental studies. Recent mathematical modeling suggests that muscle activation and muscle forces influence injury risk and mechanics in head-first impact. It is also known that muscle forces are central to the overall physiologic stability of the cervical spine. Despite this knowledge, the vast majority of in vitro head-first impact models do not incorporate musculature. We hypothesize that the simulation of the stabilizing mechanisms of musculature during head-first osteoligamentous cervical spine experiments will influence the resulting kinematics and injury mechanisms. Therefore, the objective of this study was to document differences in the kinematics, kinetics, and injuries of ex vivo osteoligamentous human cervical spine and surrogate head complexes that were instrumented with simulated musculature relative to specimens that were not instrumented with musculature. We simulated a head-first impact (3 m/s impact speed) using cervical spines and surrogate head specimens (n = 12). Six spines were instrumented with a follower load to simulate in vivo compressive muscle forces, while six were not. The principal finding was that the axial coupling of the cervical column between the head and the base of the cervical spine (T1) was increased in specimens with follower load. Increased axial coupling was indicated by a significantly reduced time between head impact and peak neck reaction force (p = 0.004) (and time to injury (p = 0.009)) in complexes with follower load relative to complexes without follower load. Kinematic reconstruction of vertebral motions indicated that all specimens experienced hyperextension and the spectrum of injuries in all specimens were consistent with a primary hyperextension injury mechanism. These preliminary results suggest that simulating follower load that may be similar to in vivo muscle forces results in significantly different impact kinetics than in similar biomechanical tests where musculature is not simulated.

Superstructured fiber-optic contact force sensor with minimal cosensitivity to temperature and axial strain

In this work a new superstructured, in-fiber Bragg grating (FBG)-based, contact force sensor is presented that is based on birefringent D-shape optical fiber. The sensor superstructure comprises a polyimide sheath, a stress-concentrating feature, and an alignment feature that repeatably orients the sensor with respect to contact forces. A combination of plane elasticity and strain-optic models is used to predict sensor performance in terms of sensitivity to contact force and axial strain. Model predictions are validated through experimental calibration and indicate contact force, axial strain, and temperature sensitivities of 169.6 pm/(N/mm), 0.01 pm/με, and -1.12 pm/°C in terms of spectral separation. The sensor addresses challenges associated with contact force sensors that are based on FBGs in birefringent fiber, FBGs in conventional optical fiber, and tilted FBGs. Relative to other birefringent fiber sensors, the sensor has contact force sensitivity comparable to the highest sensitivity of commercially available birefringent fibers and, unlike other birefringent fiber sensors, is self-aligning with respect to contact forces. Unlike sensors based on Bragg gratings in conventional fiber and tilted Bragg gratings, the sensor has minimal cosensitivity to both axial strain and changes in temperature.

Sensitivity of Bragg gratings in birefringent optical fiber to transverse compression between conforming materials

A theoretical and experimental investigation of the transverse load sensitivity of Bragg gratings in birefringent fibers to conforming contact is presented. A plane elasticity model is used to predict the contact dimensions between a conforming material and optical fiber and the principal stresses, indicating birefringence, created as a result of this contact. The transverse load sensitivity of commercially available birefringent fiber is experimentally measured for two cases of conforming contact. Theoretical and experimental results show that birefringent optical fiber can be used to make modulus-independent measurements of contact load. Therefore, Bragg gratings could be applied to conforming contact load measurements while avoiding some of the complications associated with existing contact sensors: specifically, the necessity to precalibrate by using materials with mechanical properties identical to those found in situ.

A novel helmet-mounted device for reducing the potential of catastrophic cervical spine fractures and spinal cord injuries in head-first impacts

Background: Head‐first impacts with an aligned cervical spine cause some of the most severe types of injuries due to the risk of fractures and associated spinal cord injury. Sports, such as football, mountain biking and horseback riding, contribute to the incidence of spinal cord injury but there is potential to reduce the risk of these injuries through a helmet‐mounted device. Methods: A novel device, the Pro‐Neck‐Tor mechanism, was incorporated into a commercial football helmet and tested in head‐first impact experiments. The Pro‐Neck‐Tor connects an inner and outer helmet shell, which upon head‐first impact of a certain load, induces motion of the head away from the path of the following torso. Impacts were performed onto three impact surface angles with a flexion‐inducing Pro‐Neck‐Tor mechanism. Findings: Based on averaged data, the Pro‐Neck‐Tor provided a significant and consistent reduction in peak compressive neck forces compared to the unmodified football helmet in the conditions tested. In some impact conditions, the Pro‐Neck‐Tor increased the peak sagittal plane neck bending moments and impulse over that observed for the unmodified helmet. Interpretation: The Pro‐Neck‐Tor with flexion escape is capable of lowering axial neck forces in head‐first impacts compared to a conventional helmet by guiding the cervical column away from an aligned posture and into an eccentric loading scenario which published studies suggests frequently leads to no injury or to a less severe injury. Continued development and testing of the device are needed to optimize the altered neck loading and to drive the design toward a commercial configuration. HIGHLIGHTSThe Pro‐Neck‐Tor device alters the head and neck loading in head‐first impacts.Compressive neck forces were consistently reduced due to the Pro‐Neck‐Tor.Neck bending moments were increased due to the Pro‐Neck‐Tor in some circumstances.Such loading changes may reduce the potential for catastrophic cervical injury.