Tsuyoshi Yasuki

Head-turned postures increase the risk of cervical facet capsule injury during whiplash.

Study Design. In vitro experiments using cadaveric cervical spine motion segments to quantify facet capsular ligament strain during whiplash-like loading. Objective. To quantify facet capsule strains during whiplash-like loading with an axial intervertebral prerotation simulating an initial head-turned posture and to then compare these strains to previously-published strains for partial failure and gross failure of the facet capsule for these specimens. Summary of Background Data. Clinical data have shown that a head-turned posture at impact increases the severity and duration of whiplash-related symptoms. Methods. Thirteen motion segments were used from 7 women donors (50 ± 10 years). Axial pretorques (±1.5 Nm), axial compressive preloads (45, 197, and 325 N), and quasi-static shear loads (posteriorly-directed horizontal forces from 0 to 135 N) were applied to the superior vertebral body to simulate whiplash kinematics with the head turned. Three-dimensional displacements of markers placed on the right facet capsular ligament were used to estimate the strain field in the ligament during loading. The effects of pretorque direction, compression, and posterior shear on motion segment motion and maximum principal strain in the capsule were examined using repeated-measures analyses of variance. Results. Axial pretorque affected peak capsule strains more than axial compression or posterior shear. Peak strains reached 34% ± 18% and were higher for pretorques toward rather than away from the facet capsule (i.e.—, head rotation to the right caused higher strain in the right facet capsule). Conclusion. Compared to previously-reported data for these specimens, peak capsule strains with a pretorque were double those without a pretorque (17% ± 6%) and not significantly different from those at partial failure of the ligament (35% ± 21%). Thus a head-turned posture increases facet capsular ligament strain compared to a neutral head posture—a finding consistent with the greater symptom severity and duration observed in whiplash patients who have their head turned at impact.

Age-Dependent Factors Affecting Thoracic Response: A Finite Element Study Focused on Japanese Elderly Occupants

Objectives: The ultimate goal of this research is to reduce thoracic injuries due to traffic crashes, especially in the elderly. The specific objective is to develop and validate a full-body finite element model under 2 distinct settings that account for factors relevant for thoracic fragility of elderly: one setting representative of an average size male and one representative of an average size Japanese elderly male. Methods: A new thorax finite element model was developed from medical images of a 71-year-old average Japanese male elderly size (161cm, 60 kg) postmortem human subject (PMHS). The model was validated at component and assembled levels against original series of published test data obtained from the same elderly specimen. The model was completed with extremities and head of a model previously developed. The rib cage and the thoracic flesh materials were assigned age-dependent properties and the model geometry was scaled up to simulate a 50th percentile male. Thereafter, the model was validated against existing biomechanical data for younger and elderly subjects, including hub-to-thorax impacts and frontal impact sled PMHS test data. Finally, a parametric study was conducted with the new models to understand the effect of size and aging factors on thoracic response and risk of rib fractures. Results: The model behaved in agreement with tabletop test experiments in intact, denuded, and eviscerated tissue conditions. In frontal impact sled conditions, the model showed good 3-dimensional head and spine kinematics, as well as rib cage multipoint deflections. When properties representative of an aging person were simulated, both the rib cage deformation and the predicted number of rib fractures increased. The effects of age factors such as rib cortical thickness, mechanical properties, and failure thresholds on the model responses were consistent with the literature. Aged and thereby softened flesh reduced load transfer between ribs; the coupling of the rib cage was reduced. Aged costal cartilage increased the severity of the diagonal belt loading sustained by the lower loaded rib cage. Conclusions: When age-specific parameters were implemented in a finite element (FE) model of the thorax, the rib cage kinematics and thorax injury risk increased. When the effect of size was isolated, 2 factors, in addition to rib material properties, were found to be important: flesh and costal cartilage properties. These 2 were identified to affect rib cage deformation mechanisms and may potentially increase the risk of rib fractures.

Structural response and strain patterns of isolated ribs under lateral loading

Efforts to mitigate thoracic injury under lateral impact require an understanding of the structural and fracture characteristics of individual ribs under lateral loading. While a number of studies have loaded segments of rib under three-point bending, this study is the first to investigate lateral loading onto entire ribs. Fifteen individual ribs were extracted, positioned upright and a lateral displacement at 1 m/sec was applied. Displacements at the time of fracture were relatively constant across rib levels at 18.3 ± 3.7 mm. Fracture forces ranged from 27 N to 270 N at the anterior extremity and 103 N to 326 N at the posterior extremity; this was also insensitive to rib number. The strain gages indicated that the point immediately opposite the loader on the ribs internal surface experienced the highest tensile strains, while elsewhere the internal surface was in compression and the external surface was in tension. This structural-level rib characterisation can help to better understand the mechanisms of thoracic injury under lateral impact.

Kinematics of the unrestrained vehicle occupants in side-impact crashes.

A test series involving direct right-side impact of a moving wall on unsupported, unrestrained cadavers with no arms was undertaken to better understand human kinematics and injury mechanisms during side impact at realistic speeds. The tests conducted provided a unique opportunity for a detailed analysis of the kinematics resulting from side impact. Specifically, this study evaluated the 3-dimensional (3D) kinematics of 3 unrestrained male cadavers subjected to lateral impact by a multi-element load wall carried by a pneumatically propelled rail-mounted sled reproducing a conceptual side crash impact. Three translations and 3 rotations characterize the movement of a solid body in the space, the 6 degrees of freedom (6DoF) kinematics of 15 bone segments were obtained from the 3D marker motions and computed tomography (CT)-defined relationships between the maker array mounts and the bones. The moving wall initially made contact with the lateral aspect of the pelvis, which initiated lateral motion of the spinal segments beginning with the pelvis and moving sequentially up through the lumbar spine to the thorax. Analyzing the 6DoF motions kinematics of the ribs and sternum followed right shoulder contact with the wall. Overall thoracic motion was assessed by combining the thoracic bone segments as a single rigid body. The kinematic data presented in this research provides quantified subject responses and boundary condition interactions that are currently unavailable for lateral impact.

External biofidelity in lateral impact measurement of global and local forces

A study was conducted to develop high-resolution external biofidelity data for the response of post-mortem human surrogates (PMHS) in side-impact loading. This study implemented stationary PMHS (N = 3) impacted by a wall moving at a constant velocity. The wall was subdivided into 15 impact plates, each instrumented to record the normal and shear forces as well as reaction moments about the shear axes. A method to determine the time-history of the centre of pressure (COP) of each load plate was developed and validated in both quasi-static and dynamic loading conditions. A validation test demonstrated that the COP can be predicted to within 1 cm for loads generally achieved by the shoulder and pelvis. The repeatability of COP was very good for the pelvis, where maximum variation was 1.44 cm, but higher for the thorax (3.4 cm) and shoulder (4.1 cm). Patterns of COP motion on the pelvis plate were consistent for all subjects.

Biomechanical response of the clavicle under bending

Over 90% of the clavicle fractures occurring during frontal car impacts are caused by the seat-belt webbing (Kemper et al. 2006; Hynd et al. 2007). Clavicle fractures are not considered as severe but they can cause long-term impairment. To develop and validate accurate models of human thorax for crash simulation and restraint development, previous studies have investigated the response and failure properties of clavicle under bending loading (Kemper et al. 2006). This study aims to augment the existing data set with the response and failure properties of clavicles from relatively young cadavers.