Rotational Head Movements Research Articles

Development of a Metric for Predicting Brain Strain Responses Using Head Kinematics.

Diffuse brain injuries are caused by excessive brain deformation generated primarily by rapid rotational head motion. Metrics that describe the severity of brain injury based on head motion often do not represent the governing physics of brain deformation, rendering them ineffective over a broad range of head impact conditions. This study develops a brain injury metric based on the response of a second-order mechanical system, and relates rotational head kinematics to strain-based brain injury metrics: maximum principal strain (MPS) and cumulative strain damage measure (CSDM). This new metric, universal brain injury criterion (UBrIC), is applicable over a broad range of kinematics encountered in automotive crash and sports. Efficacy of UBrIC was demonstrated by comparing it to MPS and CSDM predicted in 1600 head impacts using two different finite element (FE) brain models. Relative to existing metrics, UBrIC had the highest correlation with the FE models, and performed better in most impact conditions. While UBrIC provides a reliable measurement for brain injury assessment in a broad range of head impact conditions, and can inform helmet and countermeasure design, an injury risk function was not incorporated into its current formulation until validated strain-based risk functions can be developed and verified against human injury data.

Designing safer composite helmets to reduce rotational accelerations during oblique impacts.

Oblique impact is the most common accident situation that occupants in traffic accidents or athletes in professional sports experience. During oblique impact, the human head is subjected to a combination of linear and rotational accelerations. Rotational movement is known to be responsible for traumatic brain injuries. In this article, composite foam with a column/matrix composite configuration is proposed for head protection applications to replace single-layer uniform foam, to better attenuate rotational movement of the head during oblique impacts. The ability of composite foam in the mitigation of rotational head movement is studied by performing finite element (FE) simulations of oblique impact on flat and helmet shape specimens. The performance of composite foam with respect to parameters such as compliance of the matrix foam and the number, size and cross-sectional shape of the foam columns is explored in detail, and subsequently an optimized structure is proposed. The simulation results show that using composite foam instead of single-layer foam, the rotational acceleration and velocity of the headform can be significantly reduced. The parametric study indicates that using a more compliant matrix foam and by increasing the number of columns in the composite foam configuration, the rotation can be further mitigated. This was confirmed by experimental results. The simulation results were also analyzed based on global head injury criteria such as head injury criterion, rotational injury criterion, brain injury criterion and generalized acceleration model for brain injury threshold which further confirmed the superior performance of composite foam versus single-layer homogeneous expanded polystyrene foam. The findings of simulations give invaluable information for design of protective helmets or, for instance, headliners for the automotive industry.

A reanalysis of football impact reconstructions for head kinematics and finite element modeling

BackgroundHead kinematics generated by laboratory reconstructions of professional football helmet impacts have been applied to computational models to study the biomechanics of concussion. Since the original publication of this data, techniques for evaluating accelerometer consistency and error correction have been developed. This study applies these techniques to the original reconstruction data and reanalyzes the results given the current state of concussion biomechanics. MethodsConsistency checks were applied to the sensor data collected in the head of each test dummy. Inconsistent data were corrected using analytical techniques, and head kinematics were recalculated from the corrected data. Reconstruction videos were reviewed to identify artefactual impacts during the reconstruction to establish the region of applicability for simulations. Corrected head kinematics were input into finite element brain models to investigate strain response to the corrected dataset. FindingsMultiple reconstruction cases had inconsistent sensor arrays caused by a problematic sensor; corrections to the arrays caused changes in calculated rotational head motion. These corrections increased median peak angular velocity for the concussion cases from 35.6 to 41.5 rad/s. Using the original kinematics resulted in an average error of 20% in maximum principal strain results for each case. Simulations of the reconstructions also demonstrated that simulation lengths less than 40 ms did not capture the entire brain strain response and under-predicted strain. InterpretationThis study corrects data that were used to determine concussion risk, and indicates altered head angular motion and brain strain response for many reconstructions. Conclusions based on the original data should be re-examined based on this new study.

Development of a Single-Degree-of-Freedom Mechanical Model for Predicting Strain-Based Brain Injury Responses.

Linking head kinematics to injury risk has been the focus of numerous brain injury criteria. Although many early forms were developed using mechanics principles, recent criteria have been developed using empirical methods based on subsets of head impact data. In this study, a single-degree-of-freedom (sDOF) mechanical analog was developed to parametrically investigate the link between rotational head kinematics and brain deformation. Model efficacy was assessed by comparing the maximum magnitude of displacement to strain-based brain injury predictors from finite element (FE) human head models. A series of idealized rotational pulses covering a broad range of acceleration and velocity magnitudes (0.1-15krad/s2 and 1-100 rad/s) with durations between 1 and 3000 ms were applied to the mechanical models about each axis of the head. Results show that brain deformation magnitude is governed by three categories of rotational head motion each distinguished by the duration of the pulse relative to the brain's natural period: for short-duration pulses, maximum brain deformation depended primarily on angular velocity magnitude; for long-duration pulses, brain deformation depended primarily on angular acceleration magnitude; and for pulses relatively close to the natural period, brain deformation depended on both velocity and acceleration magnitudes. These results suggest that brain deformation mechanics can be adequately explained by simple mechanical systems, since FE model responses and experimental brain injury tolerances exhibited similar patterns to the sDOF model. Finally, the sDOF model was the best correlate to strain-based responses and highlighted fundamental limitations with existing rotational-based brain injury metrics.

A Public Database of Immersive VR Videos with Corresponding Ratings of Arousal, Valence, and Correlations between Head Movements and Self Report Measures.

Virtual reality (VR) has been proposed as a methodological tool to study the basic science of psychology and other fields. One key advantage of VR is that sharing of virtual content can lead to more robust replication and representative sampling. A database of standardized content will help fulfill this vision. There are two objectives to this study. First, we seek to establish and allow public access to a database of immersive VR video clips that can act as a potential resource for studies on emotion induction using virtual reality. Second, given the large sample size of participants needed to get reliable valence and arousal ratings for our video, we were able to explore the possible links between the head movements of the observer and the emotions he or she feels while viewing immersive VR. To accomplish our goals, we sourced for and tested 73 immersive VR clips which participants rated on valence and arousal dimensions using self-assessment manikins. We also tracked participants' rotational head movements as they watched the clips, allowing us to correlate head movements and affect. Based on past research, we predicted relationships between the standard deviation of head yaw and valence and arousal ratings. Results showed that the stimuli varied reasonably well along the dimensions of valence and arousal, with a slight underrepresentation of clips that are of negative valence and highly arousing. The standard deviation of yaw positively correlated with valence, while a significant positive relationship was found between head pitch and arousal. The immersive VR clips tested are available online as supplemental material.

Real-time Correction of Motion and Imager Instability Artifacts during 3D γ-Aminobutyric Acid-edited MR Spectroscopic Imaging.

Purpose To compare the involuntary head motion, frequency and B0 shim changes, and effects on data quality during real-time-corrected three-dimensional γ-aminobutyric acid-edited magnetic resonance (MR) spectroscopic imaging in subjects with mild cognitive impairment (MCI), patients with Parkinson disease (PD), and young and older healthy volunteers. Materials and Methods In this prospective study, MR spectroscopic imaging datasets were acquired at 3 T after written informed consent was obtained. Translational and rotational head movement, frequency, and B0 shim were determined with an integrated volumetric navigator. Head motion patterns and imager instability were investigated in 33 young healthy control subjects (mean age ± standard deviation, 31 years ± 5), 34 older healthy control subjects (mean age, 67 years ± 8), 34 subjects with MCI (mean age, 72 years ± 5), and 44 patients with PD (mean age, 64 years ± 8). Spectral quality was assessed by means of region-of-interest analysis. Group differences were evaluated with Bonferroni-corrected Mann-Whitney tests. Results Three patients with PD and four subjects with MCI were excluded because of excessive head motion (ie, > 0.8 mm translation per repetition time of 1.6 seconds throughout >10 minutes). Older control subjects, patients with PD, and subjects with MCI demonstrated 1.5, 2, and 2.5 times stronger head movement, respectively, than did young control subjects (1.79 mm ± 0.77) (P < .001). Of young control subjects, older control subjects, patients with PD, and subjects with MCI, 6%, 35%, 38%, and 51%, respectively, moved more than 3 mm during the MR spectroscopic imaging acquisition of approximately 20 minutes. The predominant movements were head nodding and "sliding out" of the imager. Frequency changes were 1.1- and 1.4-fold higher in patients with PD (P = .007) and subjects with MCI (P < .001), respectively, and B0 shim changes were 1.3-, 1.5-, and 1.9-fold higher in older control subjects (P = .005), patients with PD (P < .001), and patients with MCI (P < .001), respectively, compared with those of young control subjects (12.59 Hz ± 2.49, 3.61 Hz · cm-1 ± 1.25). Real-time correction provided high spectral quality in all four groups (signal-to-noise ratio >15, Cramér-Rao lower bounds < 20%). Conclusion Real-time motion and B0 monitoring provides valuable information about motion patterns and B0 field variations in subjects with different predispositions for head movement. Immediate correction improves data quality, particularly in patients who have difficulty avoiding movement. © RSNA, 2017 Online supplemental material is available for this article.

Abstracts from The 35th Annual National Neurotrauma Symposium July 7-12, 2017 Snowbird, Utah.

Abstracts from The 35th Annual National Neurotrauma Symposium July 7-12, 2017 Snowbird, Utah.

脳の変形挙動に対する頭部の回転運動の影響

脳の変形挙動に対する頭部の回転運動の影響

Biomechanical Assessment of Cervical Spine with Artificial Disc during Axial Rotation, Flexion and Extension

Background: The cervical spine is the most vulnerable part of the vertebral column and the rotational movements are the most dangerous movements which may cause damages to cervical spine. A good treatment option for the cervical disc disease is the replacement of a damaged disc with an artificial disc that has shown satisfactory clinical results.Methods: The C4 to C6 vertebrae of a normal subject and a person with an artificial disc between the vertebrae C5 and C6 were 3d modelled and then analyzed using FEM. The results of stress and deformationin both subjects were calculated and compared for three rotational head movements: axial rotation, flexion and extension. A distributed load of 73.6 N was used to simulate the head weight and a moment of 1.8 N.m was used to create all three rotational movements.Results: The maximum Von Mises stress in the normal subject during the axial rotation was respectively 2.2 and 1.8 times greater than the maximum stress during flexion and extension. These numbers were 2.6 and 2.3 in the subject with artificial disc.Following the artificial disc replacement, the cervical spine strength against the extension improved about 2.7%, however, the strength in axial rotation and flexion decreased 6.9% and 24.3%, respectively. The maximum values of deformation in the normal subject during flexion, extension and axial rotation were 2.8, 2.8 and 2 times of the values in the subject with artificial disc during the similar movements.Conclusion: The flexion and extension involve risks of hurting the cervical spine, however, the axial rotation is much more dangerous regarding the damages it may cause especially to the C5/6 intervertebral disc. Numerically, there is a much greater possibility of cervical spine injury during axial rotation.

Assessment of Kinematic Brain Injury Metrics for Predicting Strain Responses in Diverse Automotive Impact Conditions.

Numerous injury criteria have been developed to predict brain injury using the kinematic response of the head during impact. Each criterion utilizes a metric that is some mathematical combination of the velocity and/or acceleration components of translational and/or rotational head motion. Early metrics were based on linear acceleration of the head, but recent injury criteria have shifted towards rotational-based metrics. Currently, there is no universally accepted metric that is suitable for a diverse range of head impacts. In this study, we assessed the capability of fifteen existing kinematic-based metrics for predicting strain-based brain response using four different automotive impact conditions. Tissue-level strains were obtained through finite element model simulation of 660 head impacts including occupant and pedestrian crash tests, and pendulum head impacts. Correlations between head kinematic metrics and predicted brain strain-based metrics were evaluated. Correlations between brain strain and metrics based on angular velocity were highest among those evaluated, while metrics based on linear acceleration were least correlative. BrIC and RVCI were the kinematic metrics with the highest overall correlation; however, each metric had limitations in certain impact conditions. The results of this study suggest that rotational head kinematics are the most important parameters for brain injury criteria.

901 脳の変形挙動に対する頭部回転および並進運動の影響(OS10-1 バイオエンジニアリング(2):生体計測・評価)

901 脳の変形挙動に対する頭部回転および並進運動の影響(OS10-1 バイオエンジニアリング(2):生体計測・評価)

SU‐E‐J‐05: Use of a 6D Couch for Real‐Time Full Translational and Rotational Head Motion Correction During Stereotactic Radiosurgery: A Feasibility Study

Purpose:This work presents a methodology for correcting full translational and rotational head motion deviations during frameless image‐guided stereotactic radiosurgery (SRS) procedures with a robotic 6D couch. The main objective is to design control algorithms which optimize the correction motion of a 6D couch when sub‐millimeter/sub‐degree accuracy requirements are necessary in SRS.Methods:The system includes 6D position tracking and a 6D couch with three serially connected platforms. The bottom platform controls yaw compensation (around the z axis), the middle one generates full 3D translational motion, and the top platform controls both pitch (around X) and roll (around Y) motion with two linear actuators. Based on the 6D position of the head and the couch at each moment, the desired 6D couch position for correction is calculated, with the required 6 actuator outputs computed by an inverse algorithm accordingly. To minimize their responsive motion, the actuators take action only after the head position deviates beyond certain predetermined thresholds, and for each action the head is returned to the center of desired 6D treatment position.Results:With computer simulation, head motion of an SRS patient was modeled and the simulated couch was used to compensate for this motion in real‐time. For a 15‐minute test with a 0.3 mm and 0.15 degree target position tolerance, the corrected 6D head position was within tolerance over 99.5% of the time. The actuators moved approximately 80 times during the simulation, and the total actuator motion for translational correction was around 205 mm.Conclusion:We investigated a control approach for full translational and rotational head motion correction with a 6D couch. The simulations demonstrated that with moderate correction motion of a 6D couch, the control performance meets real‐time submillimeter and sub‐degree accuracy requirement of frameless SRS.The research is funded by the American Cancer Society (ACS) grant. The grant name is: Frameless SRS Based on Robotic Head Motion Cancellation. The grant number is: RSG‐13‐313‐01‐CCE

Robotic real-time translational and rotational head motion correction during frameless stereotactic radiosurgery.

To develop a control system to correct both translational and rotational head motion deviations in real-time during frameless stereotactic radiosurgery (SRS). A novel feedback control with a feed-forward algorithm was utilized to correct for the coupling of translation and rotation present in serial kinematic robotic systems. Input parameters for the algorithm include the real-time 6DOF target position, the frame pitch pivot point to target distance constant, and the translational and angular Linac beam off (gating) tolerance constants for patient safety. Testing of the algorithm was done using a 4D (XY Z + pitch) robotic stage, an infrared head position sensing unit and a control computer. The measured head position signal was processed and a resulting command was sent to the interface of a four-axis motor controller, through which four stepper motors were driven to perform motion compensation. The control of the translation of a brain target was decoupled with the control of the rotation. For a phantom study, the corrected position was within a translational displacement of 0.35 mm and a pitch displacement of 0.15° 100% of the time. For a volunteer study, the corrected position was within displacements of 0.4 mm and 0.2° over 98.5% of the time, while it was 10.7% without correction. The authors report a control design approach for both translational and rotational head motion correction. The experiments demonstrated that control performance of the 4D robotic stage meets the submillimeter and subdegree accuracy required by SRS.

Crista egregia: a geometrical model of the crista ampullaris, a sensory surface that detects head rotations.

The crista ampullaris is the epithelium at the end of the semicircular canals in the inner ear of vertebrates, which contains the sensory cells involved in the transduction of the rotational head movements into neuronal activity. The crista surface has the form of a saddle, or a pair of saddles separated by a crux, depending on the species and the canal considered. In birds, it was described as a catenoid by Landolt et al. (J Comp Neurol 159(2):257-287, doi: 10.1002/cne.901590207 ,1972). In the present work, we establish that this particular form results from principles of invariance maximization and energy minimization. The formulation of the invariance principle was inspired by Takumida (Biol Sci Space 15(4):356-358,2001). More precisely, we suppose that in functional conditions, the equations of linear elasticity are valid, and we assume that in a certain domain of the cupula, in proximity of the crista surface, (1) the stress tensor of the deformed cupula is invariant under the gradient of the pressure, (2) the dissipation of energy is minimum. Then, we deduce that in this domain the crista surface is a minimal surface and that it must be either a planar, or helicoidal Scherk surface, or a piece of catenoid, which is the unique minimal surface of revolution. If we add the hypothesis that the direction of invariance of the stress tensor is unique and that a bilateral symmetry of the crista exists, only the catenoid subsists. This finding has important consequences for further functional modeling of the role of the vestibular system in head motion detection and spatial orientation.

TH-A-9A-09: Optimal Feedback Control with Feed Forward for a Robotic 4D Stage in Frameless Stereotactic Radiosurgery

Purpose: A novel 4D robotic system was developed to correct both translational and rotational head motion deviations during frameless image-guided stereotactic radiosurgery (SRS) procedures. The work presents a systematic approach to optimize correction performance. Methods: The system included a robotic 4D (XYZ+Pitch) stage, a head position sensing unit and a control computer. The measured head position signal was processed and a resulting command was sent to the interface of a 4-axis motor controller, through which 4 stepper motors were driven to perform motion compensation. Due to the displacement between the lesion and the pivot of the 4D stage, the rotation of the 4D stage leads to both intended rotational and undesired translational motion of the lesion. To address the issue, feedback control with feed forward was utilized in control law design, and coupling coefficients were online tuned to improve control performance. The synchronization of all 4 axes motion was found to be crucial in the decoupling control. Results: A dynamic 6D head phantom was placed on the top of the 4D stage and was used both to perform pure 4D displacements together with simulating actual framless SRS patient head motion. The 4D stage was applied to compensate such motion. The translational displacement 7 mm and pitch displacement 0.8 degree were corrected in 5 seconds without overshoot, and the steady errors were less than 0.15 mm and 0.04 degree, respectively. For a 15 minutes test with 0.25 mm and 0.07 degree target position tolerance, a subject within tolerance in 1% of the time without motion correction, while this was increased to 99.5% with motion correction. Conclusion: We report a general control design approach for both translational and rotational head motion correction. The experiments demonstrated that control performance of the 4D robotic stage meets real time sub-millimeter accuracy requirement of frameless SRS. Research was partially funded by NIH Grant T32 EB002103-21 from NIBIB. Contents are solely the responsibility of the authors and do not necessarily represent the official views of the NIBIB or NIH.

The Pinwheel Sign: Artifact from Head Rotation During SPECT Acquisition for Dopamine Transporter Imaging

This case study illustrates the pattern and significance of the pinwheel sign, a SPECT artifact from rotational head motion that can complicate the diagnosis of Parkinson disease. Monitoring the patient during image acquisition is crucial since rotational motion artifacts are difficult to identify on raw or processed images and may lead to incorrect interpretation of findings.

J0250102 回転性脳損傷に関わる頭蓋内構造因子の検討([J025-01]傷害防止工学セッション(1),バイオエンジニアリング部門)

Diffused Axonal Injury is a type of head injury which occurs frequently in traffic accidents. It is said that it is caused by rotational head movement, and elucidation of the mechanism is necessary. There are three factors affecting the occurrence of DAI based on the intracranial structure; A complicated brain and skull structure, relative motion of the brain and skull with cerebrospinal fluid present?, and the restraint of the brain motion due to cerebral flax and tentorium cerebelli. Although various studies are done based on these three factors, it is difficult to reproduce in solid-model and simulation model, and while the influence of three factors in computational dynamics has been unknown, the simplification of the model is performed in the simulation. In this study, constructed the head neck solid-models which the intracranial structure factors differ, and considered the influence of an intracranial structure factors. The experiment which imitated the collision from the side was conducted on three sorts of the head neck solid-models, and brain depths distortion was measured. It turned out that the experimental result of three sorts of head neck substance models is compared, and it has influence with relative motion important for brain depths distortion between a brain slot and a brain-skull.

A Geometric Approach to Head/Eye Control

In this paper, we study control problems that can be directly applied to controlling the rotational motion of eye and head. We model eye and head as a sphere, or ellipsoid, rotating about its center, or about its south pole, where the axes of rotation are physiologically constrained, as was proposed originally by Listing and Donders. The Donders' constraint is either derived from Fick gimbals or from observed rotation data of adult human head. The movement dynamics is derived on SO(3) or on a suitable submanifold of SO(3) after describing a Lagrangian. Using two forms of parametrization, the axis-angle and Tait-Bryan, the motion dynamics is described as an Euler-Lagrange's equation, which is written together with an externally applied control torque. Using the control system, so obtained, we propose a class of optimal control problem that minimizes the norm of the applied external torque vector. Our control objective is to point the eye or head, toward a stationary point target, also called the regulation problem. The optimal control problem has also been analyzed by writing the dynamical system as a Newton-Euler's equation using angular velocity as part of the state variables. In this approach, explicit parametrization of SO(3) is not required. Finally, in the appendix, we describe a recently introduced potential control problem to address the regulation problem.

TH-A-137-06: Development of a 4DOF Robotic Frameless SRS System for Both Translational and Rotational Head Motion Cancellation

Purpose: Frameless techniques have not been able to match or achieve spatial target accuracies better than traditional frame based SRS. This is due to the fact that decreased invasiveness will lead to decreased accuracy. Here, relaxation of the immobilization parameter by replacement of the head ring (strong immobilization) with a more comfortable immobilization device (weak immobilization) will lead to expression of a patient's natural intrinsic small sub‐millimeter involuntary motions. Current frameless techniques do not offer a means for correcting this intrinsic motion. To address this issue, we propose a novel non‐immobilization SRS approach based on continuous cancellation of a patient's intrinsic head motion throughout the radiation delivery process. The work presents for the first time a system where both translational (XYZ) and rotational (pitch) head motion cancellation is performed. Methods: A 4D (xyz+pitch) robotic motion cancellation device was constructed using 4 stepper motors and a 4 axis motor controller (PCI‐7344, National Instruments). Real‐time 6D head motion was optically monitored using either IR markers (Polaris, NDI) or a 3D surface camera (AlignRT, VisionRT). The measured head motion was inputted into a Labview algorithm based on a proportional‐integrative‐derivative (PID) controller used to guide the motors for performing real‐time head motion cancellation. Results: The real‐time (15 fps) 6D head motion of a human volunteer was monitored with and without the use of robotic cancellation. For all axes where motion cancellation was applied (XYZ+pitch), the mean translational and rotational motion never exceeded 0.001mm and 0.01deg., respectively (essentially reduced to small fluctuations around zero). Without cancellation, this increased to 0.8mm and 0.4deg., respectively. Conclusion: It is found that substantial reductions in both translational and rotational head motion can be realized through real‐time robotic motion cancellation. This has potential to allow frameless SRS to be performed at accuracies equal or greater than frame based SRS.

Editorial: Leather football helmets

At the request of the editor we are providing this editorial to Rowson et al.’s “Biomechanical performance of leather and modern football helmets. Technical note.”5 In their paper, Rowson et al. have attempted to provide additional data and insight to clarify certain conclusions reported in an earlier American football helmet study by Bartsch et al.1 In the real world, American football collisions always involve combinations of linear and rotational motion that may pose risk to the participant’s short-term and longterm brain health. When Rowson et al.’s rigid drop tests are contrasted with Bartsch et al.’s common “on-field” laboratory impacts, it is of utmost importance to consider the testing methodologies and head motions induced. Rowson et al. drop-tested helmets against a rigid surface with infinite mass and stiffness under standard conditions.4 These standard drop tests allowed mainly linear head motion and benchmarked skull fracture risk. Their data showed that modern helmets, which are designed to perform under these standardized testing conditions, performed significantly better than leather helmets. A realworld analogy to these results is that one’s skull would be significantly better protected when running headfirst into a brick wall while wearing a modern American football helmet versus a vintage leather helmet. In contrast, Bartsch et al. tested a helmeted head form striking a second helmeted head form that was mounted on a flexible neck, permitting linear and rotational head motion. These laboratory tests represented an approximation of two players colliding headfirst while wearing modern and vintage helmets under common on-field conditions that could induce skull, brain, and neck loading. Bartsch et al.’s data demonstrated that in these common on-field impact scenarios, modern and vintage leather helmets frequently protected the skull and brain comparably. The two studies used disparate methodologies, examined different injury risk metrics, and hence produced divergent results. Therefore, Rowson et al.’s additional data do not clarify the limitations of, but rather stand in contrast to, the data presented in Bartsch et al.’s study. Rowson et al. are correct in recharacterizing the conclusions of Bartsch et al.’s experiments by stating “leather helmets performed similarly to modern helmets when struck by a Riddell VSR4 helmet....” However, even with this recharacterization, the resulting differences between the studies reported by Rowson et al. and by Bartsch et al. provide ample motivation to reassess the rigid drop test standard. These differences also suggest the need to continue physics-based helmet performance investigations under on-field conditions that induce linear and rotational head motion. Furthermore, Rowson et al. as well as others have recently published on-field impact data2,3,6 validating the fact that Bartsch et al.’s “on-field” laboratory impact conditions generated linear and rotational head motions similar to those commonly occurring in the real world. It is emphasized that these common linear and rotational real-world head motions have now been conclusively proven to be markedly different from Rowson et al.’s drop test head motions. We now have two studies—by Rowson et al. and by Bartsch et al.—that provide dissimilar results in spite of apparent similarities in head forms, helmets, impact energy, and impact momenta. The difference in results can be conclusively explained by the two very different testing methodologies used and the head motions induced. We also can conclude that not all impact tests are created equal. Therefore, we must continue examining experimental protocols to achieve better quantification of helmet performance under conditions in which on-field physics testing methodologies and relevant injury risk metrics are considered. (http://thejns.org/doi/abs/10.3171/2012.12.JNS122174)