Karin Brolin

Development of a Finite Element Model of the Upper Cervical Spine and a Parameter Study of Ligament Characteristics

Study Design. Numeric techniques were used to study the upper cervical spine. Objectives. To develop and validate an anatomic detailed finite element model of the ligamentous upper cervical spine and to analyze the effect of material properties of the ligaments on spinal kinematics. Summary of Background Data. Cervical spinal injuries may be prevented with an increased knowledge of spinal behavior and injury mechanisms. The finite element method is tempting to use because stresses and strains in the different tissues can be studied during the course of loading. The authors know of no published results so far of validated finite element models that implement the complex geometry of the upper cervical spine. Methods. The finite element model was developed with anatomic detail from computed tomographic images of the occiput to the C3. The ligaments were modeled with nonlinear spring elements. The model was validated for axial rotation, flexion, extension, lateral bending, and tension for 1.5 Nm, 10 Nm, and 1500 N. A material property sensitivity study was conducted for the ligaments. Results. The model correlated with experimental data for all load cases. Moments of 1.5 Nm produced joint rotations of 3° to 23° depending on loading direction. The parameter study confirmed that the mechanical properties of the upper cervical ligaments play an important role in spinal kinematics. The capsular ligaments had the largest impact on spinal kinematics (40% change). Conclusions. The anatomic detailed finite element model of the upper cervical spine realistically simulates the complex kinematics of the craniocervical region. An injury that changes the material characteristics of any spinal ligament will influence the structural behavior of the upper cervical spine.

Neck injuries among the elderly in Sweden

Neck injuries are some of the most important injuries as they have the potential to influence the spinal cord. A previous national survey of neck injuries in Sweden revealed that injury incidence was increasing for the population over 65 years of age, although it was decreasing for the population as a whole. The aim of this study was therefore to further clarify the magnitude, severity, and external causes of neck injuries in the elderly people in Sweden. A national incidence study, with focus on the age group above 65 years, was undertaken with data from the injury surveillance program at the Swedish National Board of Health and Welfare. The investigation includes cervical vertebral fractures reported between 1987 and 1999, and cervical soft tissue injuries from 1997 to 1999. Data in the hospital discharge register were reported in ICD9 from 1987 to 1996, while data from 1997 to 1999 were reported in ICD10. During the study period 4168 cervical injuries occurred of which 341 were fatal. People above 65 years of age made up 17% of the population and sustained 30% of all cervical injuries and 43% of all fatal cervical injuries. Half of the cervical injuries were axis (C2) fractures. Lower vertebral fractures occurred in 16% of the cases and atlas (C1) fractures in 11%. The cervical soft tissue injuries amount to 19% of all injuries. Fall accidents account for the majority (71%) of the accidents. There is an increasing trend for fall accidents resulting in neck injuries. The male population has a higher incidence for neck fractures than females, disregarding the external cause of injury. The upper cervical injuries are the most common, have the longest hospital treatments, and seem to be caused mainly by low energy falls. Further research is needed to understand the mechanisms of these injuries and in this aspect engineering could contribute with valuable knowledge, through accident simulations with numerical models. The increasing incidence of fall injuries calls for further preventive actions. The public sector should implement preventive strategies to reduce the number of extrinsic accidents, while the health care sector should focus on preventing intrinsic accidents with individual actions for each patient.

Cervical injuries in Sweden, a national survey of patient data from 1987 to 1999

Neck injuries are one of the most important injuries as they have the potential to influence the spinal cord. Data from most parts of the world are not sufficient to define a comprehensive view of mortality, morbidity, disability and handicap due to neck injuries. In Sweden, there are no data on the incidence of neck injuries. The aim of this study is to define the national incidence and causes of neck injuries in Sweden. An incidence study was undertaken with data from the injury surveillance program at the Swedish National Board of Health and Welfare. The investigation includes cervical vertebral fractures reported between 1987 and 1999, and cervical soft tissue injuries over a period of three years, from 1997 to 1999. Data between 1987 and 1996 were reported in ICD 9, while data from 1997 to 1999 were reported in ICD 10. During the study period, 14,310 non-fatal and 782 fatal cervical injuries occurred. A decreasing incidence for cervical fractures can be seen for the Swedish population, except for the elderly that have a slight increase in incidence. The incidence for cervical soft tissue injuries is almost constant. Cervical fractures demand longer periods of hospitalization than the soft tissue injuries. Transportation-related cervical fractures have dropped since 1991, while soft tissue injuries increased slowly between 1997 and 1999. Fall accidents are now the largest external cause of cervical fractures, and the population above 65 years accounts for almost 50% of the fall accidents. The male population has a higher incidence of cervical fractures, disregarding age. It is concluded that safety programs for transportation-related injuries in Sweden have been successful, while fall accidents are still substantial. Much more can be done to prevent neck injuries; especially to reduce the number of transportation-related cervical soft tissue injuries and fall injuries in the elderly population.

The Occupant Response to Autonomous Braking: A Modeling Approach That Accounts for Active Musculature

Objective: The aim of this study is to model occupant kinematics in an autonomous braking event by using a finite element (FE) human body model (HBM) with active muscles as a step toward HBMs that can be used for injury prediction in integrated precrash and crash simulations. Methods: Trunk and neck musculature was added to an existing FE HBM. Active muscle responses were achieved using a simplified implementation of 3 feedback controllers for head angle, neck angle, and angle of the lumbar spine. The HBM was compared with volunteer responses in sled tests with 10 ms−2 deceleration over 0.2 s and in 1.4-s autonomous braking interventions with a peak deceleration of 6.7 ms−2. Results: The HBM captures the characteristics of the kinematics of volunteers in sled tests. Peak forward displacements have the same timing as for the volunteers, and lumbar muscle activation timing matches data from one of the volunteers. The responses of volunteers in autonomous braking interventions are mainly small head rotations and translational motions. This is captured by the HBM controller objective, which is to maintain the initial angular positions. The HBM response with active muscles is within ±1 standard deviation of the average volunteer response with respect to head displacements and angular rotation. Conclusions: With the implementation of feedback control of active musculature in an FE HBM it is possible to model the occupant response to autonomous braking interventions. The lumbar controller is important for the simulations of lap belt–restrained occupants; it is less important for the kinematics of occupants with a modern 3-point seat belt. Increasing head and neck controller gains provides a better correlation for head rotation, whereas it reduces the vertical head displacement and introduces oscillations.

A Human Body Model With Active Muscles for Simulation of Pretensioned Restraints in Autonomous Braking Interventions

Objective: The aim of this work is to study driver and passenger kinematics in autonomous braking scenarios, with and without pretensioned seat belts, using a whole-body finite element (FE) human body model (HBM) with active muscles. Methods: Upper extremity musculature for elbow and shoulder flexion–extension feedback control was added to an HBM that was previously complemented with feedback controlled muscles for the trunk and neck. Controller gains were found using a radial basis function metamodel sampled by making 144 simulations of an 8 ms−2 volunteer sled test. The HBM kinematics, interaction forces, and muscle activations were validated using a second volunteer data set for the passenger and driver positions, with and without 170 N seat belt pretension, in 11 ms−2 autonomous braking deceleration. The HBM was then used for a parameter study in which seat belt pretension force and timing were varied from 170 to 570 N and from 0.25 s before to 0.15 s after deceleration onset, in an 11 ms−2 autonomous braking scenario. Results: The model validation showed that the forward displacements and interaction forces of the HBM correlated with those of corresponding volunteer tests. Muscle activations and head rotation angles were overestimated in the HBM when compared with volunteer data. With a standard seat belt in 11 ms−2 autonomous braking interventions, the HBM exhibited peak forward head displacements of 153 and 232 mm for the driver and passenger positions. When 570 N seat belt pretension was applied 0.15 s before deceleration onset, a reduction of peak head displacements to 60 and 75 mm was predicted. Conclusions: Driver and passenger responses to autonomous braking with standard and pretensioned restraints were successfully modeled in a whole-body FE HBM with feedback controlled active muscles. Variations of belt pretension force level and timing revealed that belt pretension 0.15 s before deceleration onset had the largest effect in reducing forward head and torso movement caused by the autonomous brake intervention. The displacement of the head relative to the torso for the HBM is quite constant for all variations in timing and belt force; it is the reduced torso displacements that lead to reduced forward head displacements.

Kinematics of Child Volunteers and Child Anthropomorphic Test Devices During Emergency Braking Events in Real Car Environment

Objectives: The objective of this study was to present, compare, and discuss the kinematic response of children and child anthropomorphic test devices (ATDs) during emergency braking events in different restraint configurations in a passenger vehicle. Methods: A driving study was conducted on a closed-circuit test track comprising 16 children aged 4 to 12 years old and the Q3, Hybrid III (HIII) 3-year-old, 6-year-old, and 10-year-old ATDs restrained on the right rear seat of a modern passenger vehicle. The children were exposed to one braking event in each of the 2 restraint systems and the ATDs were exposed to 2 braking events in each restraint system. All events had a deceleration of 1.0 g. Short children (stature 107–123 cm) and the Q3, HIII 3-year-old, and 6-year-old were restrained on booster cushions as well as high-back booster seats. Tall children (stature 135–150 cm) and HIII 10-year-old were restrained on booster cushions or restrained by 3-point belts directly on the car seat. Vehicle data were collected and synchronized with video data. Forward trajectories for the forehead and external auditory canal (ear) were determined as well as head rotation and shoulder belt force. Results: A total of 40 trials were analyzed. Child volunteers had greater maximum forward displacement of the head and greater head rotation compared to the ATDs. The average maximum displacement for children ranged from 165 to 210 mm and 155 to 195 mm for the forehead and ear target, respectively. Corresponding values for the ATDs were 55 to 165 mm and 50 to 160 mm. The change in head angle was greater for short children than for tall children. Shoulder belt force was within the same range for short children when restrained on booster cushions or high-back booster seats. For tall children, the shoulder belt force was greater when restrained on booster cushions compared to being restrained by seat belts directly on the car seat. Conclusions: The forward displacement was within the same range for all children regardless of stature and restraint system. However, the maximum forward position depended on the initial seated posture and shoulder belt position on the shoulder. Differences could also be seen in the curvature of the neck and spine. Short children exhibited a greater flexion motion of the head, whereas a more upright posture at maximum forward position was exhibited by the tall children. The ATDs displayed less forward displacement compared to the children. Supplemental materials are available for this article. Go to the publishers online edition of Traffic Injury Prevention to view the supplemental file.

Active muscle response using feedback control of a finite element human arm model

Mathematical human body models (HBMs) are important research tools that are used to study the human response in car crash situations. Development of automotive safety systems requires the implementation of active muscle response in HBM, as novel safety systems also interact with vehicle occupants in the pre-crash phase. In this study, active muscle response was implemented using feedback control of a nonlinear muscle model in the right upper extremity of a finite element (FE) HBM. Hill-type line muscle elements were added, and the active and passive properties were assessed. Volunteer tests with low impact loading resulting in elbow flexion motions were performed. Simulations of posture maintenance in a gravity field and the volunteer tests were successfully conducted. It was concluded that feedback control of a nonlinear musculoskeletal model can be used to obtain posture maintenance and human-like reflexive responses in an FE HBM.

Dynamic Spatial Tuning of Cervical Muscle Reflexes to Multidirectional Seated Perturbations

Study Design. Human volunteers were exposed experimentally to multidirectional seated perturbations. Objective. To determine the activation patterns, spatial distribution and preferred directions of reflexively activated cervical muscles for human model development and validation. Summary of Background Data. Models of the human head and neck are used to predict occupant kinematics and injuries in motor vehicle collisions. Because of a dearth of relevant experimental data, few models use activation schemes based on in vivo recordings of muscle activation and instead assume uniform activation levels for all muscles within presumed agonist or antagonist groups. Data recorded from individual cervical muscles are needed to validate or refute this assumption. Methods. Eight subjects (6 males, 2 females) were exposed to seated perturbations in 8 directions. Electromyography was measured with wire electrodes inserted into the sternocleidomastoid, trapezius, levator scapulae, splenius capitis, semispinalis capitis, semispinalis cervicis, and multifidus muscles. Surface electrodes were used to measure sternohyoid activity. Muscle activity evoked by the perturbations was normalized with recordings from maximum voluntary contractions. Results. The multidirectional perturbations produced activation patterns that varied with direction within and between muscles. Sternocleidomastoid and sternohyoid activated similarly in forward and forward oblique directions. The semispinalis capitis, semispinalis cervicis, and multifidus exhibited similar spatial patterns and preferred directions, but varied in activation levels. Levator scapulae and trapezius activity generally remained low, and splenius capitis activity varied widely between subjects. Conclusion. All muscles showed muscle- and direction-specific contraction levels. Models should implement muscle- and direction-specific activation schemes during simulations of the head and neck responses to omnidirectional horizontal perturbations where muscle forces influence kinematics, such as during emergency maneuvers and low-severity crashes. Level of Evidence: N/A

Simulation of active skeletal muscle tissue with a transversely isotropic viscohyperelastic continuum material model

Human body models with biofidelic kinematics in vehicle pre-crash and crash simulations require a constitutive model of muscle tissue with both passive and active properties. Therefore, a transversely isotropic viscohyperelastic continuum material model with element-local fiber definition and activation capability is suggested for use with explicit finite element codes. Simulations of experiments with New Zealand rabbit’s tibialis anterior muscle at three different strain rates were performed. Three different active force–length relations were used, where a robust performance of the material model was observed. The results were compared with the experimental data and the simulation results from a previous study, where the muscle tissue was modeled with a combination of discrete and continuum elements. The proposed material model compared favorably, and integrating the active properties of the muscle into a continuum material model opens for applications with complex muscle geometries.

A Female Ligamentous Cervical Spine Finite Element Model Validated for Physiological Loads

Mathematical cervical spine models allow for studying of impact loading that can cause whiplash associated disorders (WAD). However, existing models only cover the male anthropometry, despite the female population being at a higher risk of sustaining WAD in automotive rear-end impacts. The aim of this study is to develop and validate a ligamentous cervical spine intended for biomechanical research on the effect of automotive impacts. A female model has the potential to aid the design of better protection systems as well as improve understanding of injury mechanisms causing WAD. A finite element (FE) mesh was created from surface data of the cervical vertebrae of a 26-year old female (stature 167 cm, weight 59 kg). Soft tissues were generated from the skeletal geometry and anatomical literature descriptions. Ligaments were modeled with nonlinear elastic orthotropic membrane elements, intervertebral disks as composites of nonlinear elastic bulk elements, and orthotropic anulus fibrosus fiber layers, while cortical and trabecular bones were modeled as isotropic plastic-elastic. The model has geometrical features representative of the female cervical spine-the largest average difference compared with published anthropometric female data was the vertebral body depth being 3.4% shorter for the model. The majority the cervical segments compare well with respect to biomechanical data at physiological loads, with the best match for flexion-extension loads and less biofidelity for axial rotation. An average female FE ligamentous cervical spine model was developed and validated with respect to physiological loading. In flexion-extension simulations with the developed female model and an existing average male cervical spine model, a greater range of motion (ROM) was found in the female model.