Moment arms of the human neck muscles in flexion, bending and rotation

There is a paucity of data available for the moment arms of the muscles of the human neck. The objective of the present study was to measure the moment arms of the major cervical spine muscles in vitro. Experiments were performed on five fresh-frozen human head-neck specimens using a custom-designed robotic spine testing apparatus. The testing apparatus replicated flexion-extension, lateral bending and axial rotation of each individual intervertebral joint in the cervical spine while all other joints were kept immobile. The tendon excursion method was used to measure the moment arms of 30 muscle sub-regions involving 13 major muscles of the neck about all three axes of rotation of each joint for the neutral position of the cervical spine. Significant differences in the moment arm were observed across sub-regions of individual muscles and across the intervertebral joints spanned by each muscle (p<0.05). Overall, muscle moment arms were larger in flexion-extension and lateral bending than in axial rotation, and most muscles had prominent moment arms in at least 2 out of the 3 joint motions investigated. This study emphasizes the importance of detailed representation of a muscle's architecture in prediction of its torque capacity about the individual joints of the cervical spine. The dataset produced may be useful in developing and validating computational models of the human neck.     

Kinetics of the cervical spine in pediatric and adult volunteers during low speed frontal impacts

Previous research has quantified differences in head and spinal kinematics between children and adults restrained in an automotive-like configuration subjected to low speed dynamic loading. The forces and moments that the cervical spine imposes on the head contribute directly to these age-based kinematic variations. To provide further explanation of the kinematic results, this study compared the upper neck kinetics - including the relative contribution of shear and tension as well as flexion moment - between children (n=20, 6-14 yr) and adults (n=10, 18-30 yr) during low-speed (<4 g, 2.5 m/s) frontal sled tests. The subjects were restrained by a lap and shoulder belt and photo-reflective targets were attached to skeletal landmarks on the head, spine, shoulders, sternum, and legs. A 3D infrared tracking system quantified the position of the targets. Shear force (F(x)), axial force (F(z)), bending moment (M(y)), and head angular acceleration (θ(head)) were computed using inverse dynamics. The method was validated against ATD measured loads. Peak F(z) and θ(head) significantly decreased with increasing age while M(y) significantly increased with increasing age. F(x) significantly increased with age when age was considered as a univariate variable; however when variations in head-to-neck girth ratio and change in velocity were accounted for, this difference as a function of age was not significant. These results provide insight into the relationship between age-based differences in head kinematics and the kinetics of the cervical spine. Such information is valuable for pediatric cervical spine models and when scaling adult-based upper cervical spine tolerance and injury metrics to children.     

Effect of muscle wrapping on model estimates of neck muscle strength

Of the computational models of the cervical spine reported in the literature, not one takes into account the changes in muscle paths due to the underlying vertebrae. Instead, all model the individual muscle paths as straight-line segments. The major aim of this study was to quantify the changes in muscle moment arm, muscle force and joint moment due to muscle wrapping in the cervical spine. Five muscles in a straight-line model of the cervical spine were wrapped around underlying vertebrae, and the results obtained from this model were compared against the original. The two models were then validated against experimental and computational data. Results show that muscle wrapping has a significant effect on muscle moment arms and therefore joint moments and should not be neglected.     

Effect of muscle wrapping on model estimates of neck muscle strength

Of the computational models of the cervical spine reported in the literature, not one takes into account the changes in muscle paths due to the underlying vertebrae. Instead, all model the individual muscle paths as straight-line segments. The major aim of this study was to quantify the changes in muscle moment arm, muscle force and joint moment due to muscle wrapping in the cervical spine. Five muscles in a straight-line model of the cervical spine were wrapped around underlying vertebrae, and the results obtained from this model were compared against the original. The two models were then validated against experimental and computational data. Results show that muscle wrapping has a significant effect on muscle moment arms and therefore joint moments and should not be neglected.     

Finite element analysis of moment-rotation relationships for human cervical spine

A comprehensive, geometrically accurate, nonlinear C0-C7 FE model of head and cervical spine based on the actual geometry of a human cadaver specimen was developed. The motions of each cervical vertebral level under pure moment loading of 1.0 Nm applied incrementally on the skull to simulate the movements of the head and cervical spine under flexion, tension, axial rotation and lateral bending with the inferior surface of the C7 vertebral body fully constrained were analysed. The predicted range of motion (ROM) for each motion segment were computed and compared with published experimental data. The model predicted the nonlinear moment-rotation relationship of human cervical spine. Under the same loading magnitude, the model predicted the largest rotation in extension, followed by flexion and axial rotation, and least ROM in lateral bending. The upper cervical spines are more flexible than the lower cervical levels. The motions of the two uppermost motion segments account for half (or even higher) of the whole cervical spine motion under rotational loadings. The differences in the ROMs among the lower cervical spines (C3-C7) were relatively small. The FE predicted segmental motions effectively reflect the behavior of human cervical spine and were in agreement with the experimental data. The C0-C7 FE model offers potentials for biomedical and injury studies.     

Differential scaling patterns of vertebrae and the evolution of neck length in mammals

Almost all mammals have seven vertebrae in their cervical spines. This consistency represents one of the most prominent examples of morphological stasis in vertebrae evolution. Hence, the requirements associated with evolutionary modifications of neck length have to be met with a fixed number of vertebrae. It has not been clear whether body size influences the overall length of the cervical spine and its inner organization (i.e., if the mammalian neck is subject to allometry). Here, we provide the first large‐scale analysis of the scaling patterns of the cervical spine and its constituting cervical vertebrae. Our findings reveal that the opposite allometric scaling of C1 and C2–C7 accommodate the increase of neck bending moment with body size. The internal organization of the neck skeleton exhibits surprisingly uniformity in the vast majority of mammals. Deviations from this general pattern only occur under extreme loading regimes associated with particular functional and allometric demands. Our results indicate that the main source of variation in the mammalian neck stems from the disparity of overall cervical spine length. The mammalian neck reveals how evolutionary disparity manifests itself in a structure that is otherwise highly restricted by meristic constraints.     

Spinal constraint modulates head instantaneous center of rotation and dictates head angular motion

The head is kinematically constrained to the torso through the spine and thus, the spine dictates the amount of output head angular motion expected from an input impact. Here, we investigate the spinal kinematic constraint by analyzing the head instantaneous center of rotation (HICOR) with respect to the torso in head/neck sagittal extension and coronal lateral flexion during mild loads applied to 10 subjects. We found the mean HICOR location was near the C5-C6 intervertebral joint in sagittal extension, and T2-T3 intervertebral joint in coronal lateral flexion. Using the impulse-momentum relationship normalized by subject mass and neck length, we developed a non-dimensional analytical ratio between output angular velocity and input linear impulse as a function of HICOR location. The ratio was 0.65 and 0.50 in sagittal extension and coronal lateral flexion respectively, implying 30% greater angular velocities in sagittal extension given an equivalent impulse. Scaling to subject physiology also predicts larger required impulses given greater subject mass and neck length to achieve equivalent angular velocities, which was observed experimentally. Furthermore, the HICOR has greater motion in sagittal extension than coronal lateral flexion, suggesting the head and spine can be represented with a single inverted pendulum in coronal lateral flexion, but requires a more complex representation in sagittal extension. The upper cervical spine has substantial compliance in sagittal extension, and may be responsible for the complex motion and greater extension angular velocities. In analyzing the HICOR, we can gain intuition regarding the neck’s role in dictating head motion during external loading.     

Finite element model of the human neck during omni-directional impacts. Part II: relation between cervical curvature and risk of injury

A detailed 3D FE model of the human neck was used to assess a possible relationship between risk of injury and cervical spine curvature for various impacts. A FE model was previously developed, representing the head and neck of a 50th percentile human with a normal lordotic curvature. The model behaviour was omni-directionally validated for various impacts using published results. For the present study, the model was deformed in order to obtain a straight and a kyphotic curvature, and for each geometry, rear-end, frontal, lateral and oblique impact were simulated. Although results showed similar kinematic patterns, significant differences were found in the distribution and peak values of ligament elongations, forces and moments along the cervical spine for the three configurations. It was concluded that the variability observed on the curvature of the human cervical spine may have a significant influence both on the behaviour and on the risk of injury of the neck during impact.     

Finite element model of the human neck during omni-directional impacts. Part II: relation between cervical curvature and risk of injury

A detailed 3D FE model of the human neck was used to assess a possible relationship between risk of injury and cervical spine curvature for various impacts. A FE model was previously developed, representing the head and neck of a 50th percentile human with a normal lordotic curvature. The model behaviour was omni-directionally validated for various impacts using published results. For the present study, the model was deformed in order to obtain a straight and a kyphotic curvature, and for each geometry, rear-end, frontal, lateral and oblique impact were simulated. Although results showed similar kinematic patterns, significant differences were found in the distribution and peak values of ligament elongations, forces and moments along the cervical spine for the three configurations. It was concluded that the variability observed on the curvature of the human cervical spine may have a significant influence both on the behaviour and on the risk of injury of the neck during impact.     

Examining motion in the cervical spine II: characterization of coupled joint motion using an opto-electronic device to track skin markers

Analysis of coupled motion in the cervical spine may be useful in helping to identify injuries. In order to investigate this possibility, the nature of coupled motion in the spine and previous investigations on this subject are reviewed here. An enhanced set of displays are developed for an existing opto-electronic device employed for the non-invasive measurement of movement in the upper spine. This instrument consists of a high resolution motion analysis system which tracks small infrared emitting diodes (IREDs). Kinematic data for the motion of the markers is processed and absolute coordinates for the location of each IRED at any time are tabulated; coupled motion with respect to a fixed calibration frame, as well as for vertebrae relative to each other, is deduced from these. Overall analysis provided by the original device includes assessment of cervical lordosis, thoracic kyphosis, and inter-segmental mobility. Characterization of coupled motion, in particular, involves a series of plots showing principal versus secondary motion. Principal movements include flexion-extension, lateral bending, and axial rotation, corresponding to motion in the sagittal, transverse, and horizontal planes, respectively. Mobility is represented in terms of the direction angles made by virtual vectors orthogonal to the planes made by markers on the head, neck, and shoulders. Development of the enhanced displays and the required refinements are described. Precision of the deduced angles is found to be approximately 1°. This representation of coupled motion is expected to be valuable in improving the accuracy of attempts to identify normal versus pathological motion in the cervical spine.     

Biomechanical contributions of upper cervical ligamentous structures in Type II odontoid fractures

Fractures of the odontoid present frequently in spinal trauma, and Type II odontoid fractures, occurring at the junction of the odontoid process and C2 vertebrae, represent the bulk of all traumatic odontoid fractures. It is currently unclear what soft-tissue stabilizers contribute to upper cervical motion in the setting of a Type II odontoid fracture, and evaluation of how concomitant injury contributes to cervical stability may inform surgical decision-making as well as allow for the creation of future, accurate, biomechanical models of the upper cervical spine. The objective of the current study was to determine the contribution of soft-tissue stabilizers in the upper cervical spine following a Type II odontoid fracture. Eight cadaveric C0-C2 specimens were evaluated using a robotic testing system with motion tracking. The unilateral facet capsule (UFC) and anterior longitudinal ligament (ALL) were serially resected to determine their biomechanical role following odontoid fracture. Range of motion (ROM) and moment at the end of intact specimen replay were the primary outcomes. We determined that fracture of the odontoid significantly increases motion and decreases resistance to intact motion for flexion–extension (FE), axial rotation (AR), and lateral bending (LB). Injury to the UFC increased AR by 3.2° and FE by 3.2°. ALL resection did not significantly increase ROM or decrease end-point moment. The UFC was determined to contribute to 19% of intact flexion resistance and 24% of intact AR resistance. Overall, we determined that Type II fracture of the odontoid is a significant biomechanical destabilizer and that concurrent injury to the UFC further increases upper cervical ROM and decreases resistance to motion in a cadaveric model of traumatic Type II odontoid fractures.     

Development and validation of a CO-C7 FE complex for biomechanical study

In this study, the digitized geometrical data of the embalmed skull and vertebrae (C0-C7) of a 68-year old male cadaver were processed to develop a comprehensive, geometrically accurate, nonlinear C0-C7 FE model. The biomechanical response of human neck under physiological static loadings, near vertex drop impact and rear-end impact (whiplash) conditions were investigated and compared with published experimental results. Under static loading conditions, the predicted moment-rotation relationships of each motion segment under moments in midsagittal plane and horizontal plane agreed well with experimental data. In addition, the respective predicted head impact force history and the S-shaped kinematics responses of head-neck complex under near-vertex drop impact and rear-end conditions were close to those observed in reported experiments. Although the predicted responses of the head-neck complex under any specific condition cannot perfectly match the experimental observations, the model reasonably reflected the rotation distributions among the motion segments under static moments and basic responses of head and neck under dynamic loadings. The current model may offer potentials to effectively reflect the behavior of human cervical spine suitable for further biomechanics and traumatic studies.     

Contact analysis of a posterior cervical spine plate using a three-dimensional canine finite element model

The development of a three-dimensional finite element model of a posteriorly plated canine cervical spine (C3-C6) including contact nonlinearities is described. The model was created from axial CT scans and the material properties were derived from the literature. The model demonstrated sufficient accuracy from the results of a mesh convergence test. Significant steps were taken toward establishing model validation by comparison of plate surface strains with a posteriorly plated canine cervical spine under three-point bending. This model was developed to better characterize the contact pressures at the various interfaces under average physiologic canine loading. The analysis showed that the screw-plate interfaces had the highest values of all the mechanical parameters evaluated.     

Cervical spine posteroanterior stiffness differs with neck position

PurposeSpinal stiffness is commonly considered when treating patients with neck pain, but there are few studies reporting the objective measurement of cervical spine stiffness or the possible kinesiological factors that may affect its quantification. The aim of this study was to determine if the position of the neck affects cervical spine stiffness.MethodsAn instrumented stiffness assessment device measured posteroanterior cervical spine stiffness at C4 of 25 prone-lying asymptomatic subjects in three neck positions in randomised order: maximal flexion, maximal extension, and neutral. The device applied five standardised mechanical oscillatory pressures while measuring the applied force and concurrent displacement, defining stiffness as the slope of the linear portion of the force–displacement curve. Repeated measures analysis of variance with Bonferroni-adjusted post hoc comparisons determined whether stiffness differed between neck positions.ResultsThere was a significant difference in cervical spine stiffness between different neck positions (F_(1.6,38.0) = 16.6, P < 0.001). Stiffness was least in extension with a mean of 3.09 N/mm (95% CI 2.59, 3.58) followed by neutral (3.94, 95% CI 3.49, 4.39), and then flexion (4.32, 95% CI 3.96, 4.69).ConclusionWhen assessing cervical spine stiffness, neck position should be standardised to ensure maximal reliability and utility of stiffness judgments.     

Cervical helical axis characteristics and its center of rotation during active head and upper arm movements-comparisons of whiplash-associated disorders, non-specific neck pain and asymptomatic individuals

The helical axis model can be used to describe translation and rotation of spine segments. The aim of this study was to investigate the cervical helical axis and its center of rotation during fast head movements (side rotation and flexion/extension) and ball catching in patients with non-specific neck pain or pain due to whiplash injury as compared with matched controls. The aim was also to investigate correlations with neck pain intensity. A finite helical axis model with a time-varying window was used. The intersection point of the axis during different movement conditions was calculated. A repeated-measures ANOVA model was used to investigate the cervical helical axis and its rotation center for consecutive levels of 15 degrees during head movement. Irregularities in axis movement were derived using a zero-crossing approach. In addition, head, arm and upper body range of motion and velocity were observed. A general increase of axis irregularity that correlated to pain intensity was observed in the whiplash group. The rotation center was superiorly displaced in the non-specific neck pain group during side rotation, with the same tendency for the whiplash group. During ball catching, an anterior displacement (and a tendency to an inferior displacement) of the center of rotation and slower and more restricted upper body movements implied a changed movement strategy in neck pain patients, possibly as an attempt to stabilize the cervical spine during head movement.     

Inertial properties and loading rates affect buckling modes and injury mechanisms in the cervical spine

Cervical spine injuries continue to be a costly societal problem. Future advancements in injury prevention depend on improved physical and computational models which, in turn, are predicated on a better understanding of the responses of the neck during dynamic loading. Previous studies have shown that the tolerance of the neck is dependent on its initial position and its buckling behavior. This study uses a computational model to examine the mechanical factors influencing buckling behavior during impact to the neck. It was hypothesized that the inertial properties of the cervical spine influence the dynamics during compressive axial loading. The hypothesis was tested by performing parametric analyses of vertebral mass, mass moments of inertia, motion segment stiffness, and loading rate. Increases in vertebral mass resulted in increasingly complex kinematics and larger peak loads and impulses. Similar results were observed for increases in stiffness. Faster loading rates were associated with higher peak loads and higher-order buckling modes. The results demonstrate that mass has a great deal of influence on the buckling behavior of the neck, particularly with respect to the expression of higher-order modes. Injury types and mechanisms may be substantially altered by loading rate because inertial effects may influence whether the cervical spine fails in a compressive mode, or a bending mode.     

Influence of the crash pulse shape on the peak loading and the injury tolerance levels of the neck in in vitro low-speed side-collisions

The aim of the present in vitro study was to investigate the effect of the crash pulse shape on the peak loading and the injury tolerance levels of the human neck. In a custom-made acceleration apparatus 12 human cadaveric cervical spine specimens, equipped with a dummy head, were subjected to a series of incremental side accelerations. While the duration of the acceleration pulse of the sled was kept constant at 120 ms, its shape was varied: Six specimens were loaded with a slowly increasing pulse, i.e. a low loading rate, the other six specimens with a fast increasing pulse, i.e. a high loading rate. The loading of the neck was quantified in terms of the peak linear and angular acceleration of the head, the peak shear force and bending moment of the lower neck and the peak translation between head and sled. The shape of the acceleration curve of the sled only seemed to influence the peak translation between head and sled but none of the other four parameters. The neck injury tolerance level for the angular acceleration of the head and for the bending moment of the lower neck was almost identical for both, the high and the low loading rate. In contrast, the injury tolerance level for the linear acceleration of the head and for the shear force of the lower neck was slightly higher for the low loading rate as compared to the high loading rate. For the translation between head and sled this difference was even statistically significant. Thus, if the shape of the crash pulse is not known, solely the peak bending moment of the lower neck and the peak angular acceleration of the head seem to be suitable predictors for the neck injury risk but not the peak shear force of the lower neck, the peak linear acceleration of the head and the translation between head and thorax.     

Sensitivity of lumbar spine response to follower load and flexion moment: finite element study

The follower load (FL) combined with moments is commonly used to approximate flexed/extended posture of the lumbar spine in absence of muscles in biomechanical studies. There is a lack of consensus as to what magnitudes simulate better the physiological conditions. Considering the in-vivo measured values of the intradiscal pressure (IDP), intervertebral rotations (IVRs) and the disc loads, sensitivity of these spinal responses to different FL and flexion moment magnitudes was investigated using a 3D nonlinear finite element (FE) model of ligamentous lumbosacral spine. Optimal magnitudes of FL and moment that minimize deviation of the model predictions from in-vivo data were determined. Results revealed that the spinal parameters i.e. the IVRs, disc moment, and the increase in disc force and moment from neutral to flexed posture were more sensitive to moment magnitude than FL magnitude in case of flexion. The disc force and IDP were more sensitive to the FL magnitude than moment magnitude. The optimal ranges of FL and flexion moment magnitudes were 900–1100 N and 9.9–11.2 Nm, respectively. The FL magnitude had reverse effect on the IDP and disc force. Thus, magnitude for FL or flexion that minimizes the deviation of all the spinal parameters together from the in-vivo data can vary. To obtain reasonable compromise between the IDP and disc force, our findings recommend that FL of low magnitude must be combined with flexion moment of high intensity and vice versa.     

A biomechanical evaluation of whiplash using a multi-body dynamic model

This paper presents a biomechanical evaluation of whiplash injury potential during the initial extension motion of the head in a rear-end collision. A four-segment dynamic model is developed in the sagittal plane for the analysis. The model response is validated using the existing experimental data and is shown to simulate the "S-shape" kinematics of the cervical spine and the resulting dynamics observed in human and cadaver experiments. The model is then used to evaluate the effects of parameters such as collision severity, head/headrest separation, and the initial head orientation in the sagittal plane on the "S-shape" kinematics of the cervical spine and the resulting neck loads. It is shown, for example, that the cervical spine forms an "S-shape" for a range of change in speeds and that at lower and higher speeds changes the spine does not form the "S-shape." Furthermore, it is shown that the "S-shape" formation also depends on the head to headrest separation distance.     

Flexion and extension structural properties and strengths for male cervical spine segments

New vehicle safety standards are designed to limit the amount of neck tension and extension seen by out-of-position motor vehicle occupants during airbag deployments. The criteria used to assess airbag injury risk are currently based on volunteer data and animal studies due to a lack of bending tolerance data for the adult cervical spine. This study provides quantitative data on the flexion-extension bending properties and strength on the male cervical spine, and tests the hypothesis that the male is stronger than the female in pure bending. An additional objective is to determine if there are significant differences in stiffness and strength between the male upper and lower cervical spine. Pure-moment flexibility and failure testing was conducted on 41 male spinal segments (O-C2, C4-C5, C6-C7) in a pure-moment test frame and the results were compared with a previous study of females. Failures were conducted at approximately 90 N-m/s. In extension, the male upper cervical spine (O-C2) fails at a moment of 49.5 (s.d. 17.6)N-m and at an angle of 42.4 degrees (s.d. 8.0 degrees). In flexion, the mean moment at failure is 39.0 (s.d. 6.3 degrees) N-m and an angle of 58.7 degrees (s.d. 5.1 degrees). The difference in strength between flexion and extension is not statistically significant. The difference in the angles is statistically significant. The upper cervical spine was significantly stronger than the lower cervical spine in both flexion and extension. The male upper cervical spine was significantly stiffer than the female and significantly stronger than the female in flexion. Odontoid fractures were the most common injury produced in extension, suggesting a tensile mechanism due to tensile loads in the odontoid ligamentous complex.