Inverse kinematics in cervical spine models: Effects of scaling and model degrees of freedom for extension and flexion movements.

Intervertebral kinematics can affect model-predicted loads and strains in the spine; therefore knowledge of expected vertebral kinematics error is important for understanding the limitations of model predictions. This study addressed how different kinematic models of the neck affect the prediction of intervertebral kinematics from markers on the head and trunk. Eight subjects executed head and neck extension-flexion motion with simultaneous motion capture and biplanar dynamic stereo-radiography (DSX) of vertebrae C1-C7. A generic head and neck model in OpenSim was scaled by marker data, and three versions of the model were used with an inverse kinematics solver. The models differed according to the number of independent degrees of freedom (DOF) between the head and trunk: 3 rotational DOF with constraints defining intervertebral kinematics as a function of overall head-trunk motion; 24DOF with 3 independent rotational DOF at each level, skull-T1; 48DOF with 3 rotational and 3 translational DOF at each level. Marker tracking error was lower for scaled models compared to generic models and decreased as model DOF increased. The largest mean absolute error (MAE) was found in extension-flexion angle and anterior-posterior translation at C1-C2, and superior-inferior translation at C2-C3. Model scaling and complexity did not have a statistically significant effect on most error metrics when corrected for multiple comparisons, but ranges of motion were significantly different from DSX in some cases. This study evaluated model kinematics in comparison to gold standard radiographic data and provides important information about intervertebral kinematics error that are foundational to model validity.

The effects of setup parameters on the measured kinetic output of cervical disc prostheses.

Mechanical testing machines are used to evaluate kinematics, kinetics, wear, and efficacy of spinal implants. The simulation of "physiological" spinal loading conditions necessitates the simultaneous use of multiple actuators. The challenge in achieving a desired loading profile lies in achieving close synchronization of these actuators. Errors in load application can be attributed to both the control system and the intrinsic sample response. Moreover, the presence of friction in the setup can have an impact on the measured outcome. The optimization of setup parameters can substantially improve the ability to simulate spinal loading conditions and obtain reliable data on implant performance. In this study, a reproducible kinematic test protocol was developed to evaluate the sensitivity of the kinetic response (i.e., measured loads, moments, and stiffnesses) of a cervical disc prosthesis to several testing parameters. In this context, five ceramic ball and socket sample implants were mounted in a 6 DOF material testing machine and tested with a constant axial compressive force of 100 N in two motion modes: 1) flexion-extension (±7.5°) and 2) lateral bending (±6°). Parameters including rotation rate, slider friction, friction between the samples' articulating surfaces, and moment arm were considered to determine their effects on measured kinetic parameters. The sensitivity analysis indicated that all setup parameters except friction between the samples' articulating surfaces had a substantial effect on the results. The findings were then compared to predictions from a free body diagram to determine the optimal setup parameters. Consequently, the setup with the lowest rotation rate and employing passive sliders yielded results that were consistent with the free body diagram. This study demonstrated the significance of a comprehensive setup evaluation for reliable and reproducible testing of spinal implants, also for comparison between labs.

Cervical Sensorimotor Function Tests Using a VR Headset-An Evaluation of Concurrent Validity.

Sensorimotor disturbances such as disturbed cervical joint position sense (JPS) and reduced reaction time and velocity in fast cervical movements have been demonstrated in people with neck pain. While these sensorimotor functions have been assessed mainly in movement science laboratories, new sensor technology enables objective assessments in the clinic. The aim was to investigate concurrent validity of a VR-based JPS test and a new cervical reaction acuity (CRA) test. Twenty participants, thirteen asymptomatic and seven with neck pain, participated in this cross-sectional study. The JPS test, including outcome measures of absolute error (AE), constant error (CE), and variable error (VE), and the CRA test, including outcome measures of reaction time and maximum velocity, were performed using a VR headset and compared to a gold standard optical motion capture system. The mean bias (assessed with the Bland-Altman method) between VR and the gold standard system ranged from 0.0° to 2.4° for the JPS test variables. For the CRA test, reaction times demonstrated a mean bias of -19.9 milliseconds (ms), and maximum velocity a mean bias of -6.5 degrees per seconds (°/s). The intraclass correlation coefficients (ICCs) between VR and gold standard were good to excellent (ICC 0.835-0.998) for the JPS test, and excellent (ICC 0.931-0.954) for reaction time and maximum velocity for the CRA test. The results show acceptable concurrent validity for the VR technology for assessment of JPS and CRA. A slightly larger bias was observed in JPS left rotation which should be considered in future research.

Cerebro-spinal flow pattern in the cervical subarachnoid space of healthy volunteers: Influence of the spinal cord morphology.

INTRODUCTION: Toward further cerebro-spinal flow quantification in clinical practice, this study aims at assessing the variations in the cerebro spinal fluid flow pattern associated with change in the morphology of the subarachnoid space of the cervical canal of healthy humans by developing a computational fluid dynamics model. METHODS: 3D T2-space MRI sequence images of the cervical spine were used to segment 11 cervical subarachnoid space. Model validation (time-step, mesh size, size and number of boundary layers, influences of parted inflow and inflow continuous velocity) was performed a 40-year-old patient-specific model. Simulations were performed using computational fluid dynamics approach simulating transient flow (Sparlart-Almaras turbulence model) with a mesh size of 0.6, 6 boundary layers of 0.05 mm, a time step of 20 ms simulated on 15 cycles. Distributions of components velocity and WSS were respectively analyzed within the subarachnoid space (intervertebral et intravertebral levels) and on dura and pia maters. RESULTS: Mean values cerebro spinal fluid velocity in specific local slices of the canal range between 0.07 and 0.17 m.s-1 and 0.1 and 0.3 m.s-1 for maximum values. Maximum wall shear stress values vary between 0.1 and 0.5 Pa with higher value at the middle of the cervical spine on pia mater and at the lower part of the cervical spine on dura mater. Intra and inter-individual variations of the wall shear stress were highlighted significant correlation gwith compression ratio (r = 0.76), occupation ratio and cross section area of the spinal cord. CONCLUSION: The inter-individual variability in term of subarachnoid canal morphology and spinal cord position influence the cerebro-spinal flow pattern, highlighting the significance of canal morphology investigation before surgery.

Dynamic in vivo 3D atlantooccipital kinematics during multiplanar physiologic motions.

Normal biomechanics of the upper cervical spine, particularly at the atlantooccipital joint, remain poorly characterized. The purpose of this study was to determine the intervertebral kinematics of the atlantooccipital joint (occiput-C1) during three-dimensional in vivo physiologic movements. Twenty healthy young adults performed dynamic flexion/extension, axial rotation, and lateral bending while biplane radiographs were collected at 30 images per second. Motion at occiput-C1 was tracked using a validated volumetric model-based tracking process that matched subject-specific CT-based bone models to the radiographs. The occiput-C1 total range of motion (ROM) and helical axis of motion (HAM) was calculated for each movement. During flexion/extension, the occiput-C1 moved almost exclusively in-plane (ROM: 17.9 ± 6.9°) with high variability in kinematic waveforms (6.3°) compared to the in-plane variability during axial rotation (1.4°) and lateral bending (0.9°) movements. During axial rotation, there was small in-plane motion (ROM: 4.2 ± 2.5°) compared to out-of-plane flexion/extension (ROM: 12.7 ± 5.4°). During lateral bending, motion occurred in-plane (ROM: 9.0 ± 3.1°) and in the plane of flexion/extension (ROM: 7.3 ± 2.7°). The average occiput-C1 axis of rotation intersected the sagittal and coronal planes 7 mm to 18 mm superior to the occipital condyles. The occiput-C1 axis of rotation pointed 60° from the sagittal plane during axial rotation but only 10° from the sagittal plane during head lateral bending. These novel results are foundational for future work on upper cervical spine kinematics.

Human Cervical Epidural Spinal Electrogram Topographically Maps Distinct Volitional Movements.

Little is known about the electrophysiologic activity of the intact human spinal cord during volitional movement. We analyzed epidural spinal recordings from a total of five human subjects of both sexes during a variety of upper extremity movements and found that these spinal epidural electrograms contain spectral information distinguishing periods of movement, rest, and sensation. Cervical epidural electrograms also contained spectral changes time-locked with movement. We found that these changes were primarily associated with increased power in the theta (4-8 Hz) band and feature increased theta phase to gamma amplitude coupling, and this increase in theta power can be used to topographically map distinct upper extremity movements onto the cervical spinal cord in accordance with established myotome maps of the upper extremity. Our findings have implications for the development of neurostimulation protocols and devices focused on motor rehabilitation for the upper extremity, and the approach presented here may facilitate spatiotemporal mapping of naturalistic movements.

Interactions between arm and leg neuronal circuits following paired cervical and lumbosacral transspinal stimulation in healthy humans.

Transspinal (or transcutaneous spinal cord) stimulation is a promising noninvasive method that may strengthen the intrinsic spinal neural connectivity in neurological disorders. In this study we assessed the effects of cervical transspinal stimulation on the amplitude of leg transspinal evoked potentials (TEPs), and the effects of lumbosacral transspinal stimulation on the amplitude of arm TEPs. Control TEPs were recorded following transspinal stimulation with one cathode electrode placed either on Cervical 3 (21.3 ± 1.7 mA) or Thoracic 10 (23.6 ± 16.5 mA) vertebrae levels. Associated anodes were placed bilaterally on clavicles or iliac crests. Cervical transspinal conditioning stimulation produced short latency inhibition of TEPs recorded from left soleus (ranging from - 6.11 to -3.87% of control TEP at C-T intervals of -50, -25, -20, -15, -10, 15 ms), right semitendinosus (ranging from - 11.1 to -4.55% of control TEP at C-T intervals of -20, -15, 15 ms), and right vastus lateralis (ranging from - 13.3 to -8.44% of control TEP at C-T intervals of -20 and - 15 ms) (p < 0.05). Lumbosacral transspinal conditioning stimulation produced no significant effects on arm TEPs. We conclude that in the resting state, cervical transspinal stimulation affects the net motor output of leg motoneurons under the experimental conditions used in this study. Further investigations are warranted to determine whether this protocol may reactivate local spinal circuitry after stroke or spinal cord injury and may have a significant effect in synchronization of upper and lower limb muscle synergies during rhythmic activities like locomotion or cycling.

Concurrent Validity and Reliability of a New Smartphone Application for Measuring Cervical Range of Motion in Healthy Individuals.

BACKGROUND: Range of motion (ROM) measurement is an important part of physical therapy assessment and patient progress. Smartphones are user-friendly instruments and if proven to be reliable and valid, clinicians can use them for a variety of tasks including ROM measurement. OBJECTIVES: To determine concurrent validity and intra- and inter-rater reliability of the PhysioMaster application in measuring cervical ROM in both Android and iOS operating systems. METHODS: Forty-five healthy individuals (age 31.75 ± 11.94 yrs; 18 men, 27 women) completed this study. Two raters measured cervical ROM, three times each, using an Android phone for intra-rater and inter-rater reliability. With an interval time of 1-7 days after the first session, measurements were repeated by one of the raters once to measure intersession reliability. Validity was estimated by one of the raters using iPhone and Android phones one at a time while 3D motion analysis (3DMA) recorded cervical movements simultaneously. For reliability, intraclass correlation coefficient (ICC), and for validity, Pearson correlation coefficient and Bland-Altman plots were used. RESULTS: ICC values of ≥0.76 and ≥0.84 demonstrated excellent intra-rater and inter-rater reliability, respectively. For concurrent validity, correlation between each phone and 3DMA was nearly perfect for all movements (0.93 ≤ r ≤ 0.97). CONCLUSION: PhysioMaster appears to be a valid and reliable application for measuring cervical ROM in healthy individuals.

Advanced subject-specific neck musculoskeletal modeling unveils sex differences in muscle moment arm and cervical spine loading.

Neck pain and injuries are growing healthcare burdens with women having a higher incidence rate and poorer treatment outcomes than males. A better understanding of sex differences in neck biomechanics, foundational for more targeted, effective prevention or treatment strategies, calls for more advanced subject-specific musculoskeletal modeling. Current neck musculoskeletal models are based on generic anatomy, lack subject specificity beyond anthropometric scaling, and are unable to accurately reproduce neck strengths exhibited in vivo without arbitrary muscle force scaling factors or residual torque actuators. In this work, subject-specific neck musculoskeletal models of 23 individuals (11 male, 12 female) were constructed by integrating multi-modality imaging and biomechanical measurements. Each model simulated maximal voluntary neck static exertions in three postures: neck flexion in a neutral posture, flexion in a 40° extended posture, and extension in a 40° flexed posture. Quantitative model validation showed close agreement between model-predicted muscle activation and EMG measurement. The models unveiled that (1) males have greater moment arms in one flexor muscle group and five extensor muscle groups, (2) females exhibited higher cervical spinal compression per unit exertion force in the flexed posture, and (3) the variability of compression force was much greater in females in all three exertions but most notably in the extension with a flexed "dropped head" position. These insights illuminated a plausible pathway from sex differences in neck biomechanics to sex disparities in the risk and prevalence of neck pain.

The role of the deep cervical extensor muscles in multi-directional isometric neck strength.

Clinical management of whiplash-associated disorders is challenging and often unsuccessful, with over a third of whiplash injuries progressing to chronic neck pain. Previous imaging studies have identified muscle fat infiltration, indicative of muscle weakness, in the deep cervical extensor muscles (multifidus and semispinalis cervicis). Yet, kinematic and muscle redundancy prevent the direct assessment of individual neck muscle strength, making it difficult to determine the role of these muscles in motor dysfunction. The purpose of this study was to determine the effects of deep cervical extensor muscle weakness on multi-directional neck strength and muscle activation patterns. Maximum isometric forces and associated muscle activation patterns were computed in 25 test directions using a 3-joint, 24-muscle musculoskeletal model of the head and neck. The computational approach accounts for differential torques about the upper and lower cervical spine. To facilitate clinical translation, the test directions were selected based on locations where resistance could realistically be applied to the head during clinical strength assessments. Simulation results reveal that the deep cervical extensor muscles are active and contribute to neck strength in directions with an extension component. Weakness of this muscle group leads to complex compensatory muscle activation patterns characterized primarily by increased activation of the superficial extensors and deep upper cervical flexors, and decreased activation of the deep upper cervical extensors. These results provide a biomechanistic explanation for movement dysfunction that can be used to develop targeted diagnostics and treatments for chronic neck pain in whiplash-associated disorders.

Exploring the Influence of Facet Orientation and Tropism on Neutral Zone Properties.

Lumbar spine pathologies have been linked independently to both neutral zone (NZ) properties and facet joint anatomical characteristics; however, the effect of facet joint orientation (FO) and tropism (FT) on NZ properties remains unclear. The aim of the present study was to investigate how axial plane FO and FT relate to NZ range and stiffness in the human lumbar spine and porcine cervical spine. Seven human lumbar functional spine units (FSUs) and 94 porcine cervical FSUs were examined. FO and FT were measured, and in vitro mechanical testing was used to determine anterior-posterior (AP) and flexion-extension (FE) NZ range and stiffness. FO and FT were found to have no significant relationship with AP and FE NZ range. Increases in FT were associated with greater FE and AP NZ stiffness in human FSUs, with no FT-NZ stiffness relationship observed in porcine specimens. A significant relationship (p < 0.001) between FO and FE NZ stiffness was observed for both porcine and human FSUs, with a more sagittal orientation of the facet joints being associated with decreased FE NZ stiffness. Given the link between NZ stiffness and pathological states of the lumbar spine, further research is warranted to determine the practical significance of the observed facet joint anatomical characteristic-NZ property relationship.

Altered Cervical Spine Position Results in Decreased Eccentric Shoulder Rotation Strength.

Overhead athletes require strenuous shoulder activity in nonneutral cervical spine positions to eccentrically decelerate the throwing/striking arm following ball release/contact. We therefore aimed to compare eccentric shoulder rotation strength through a 90° arc between neutral and rotated positions. Fifty-two participants (19 M, 34 F 170±10 cm; 73±18 kg, 21.9±2.9 years) without shoulder or cervical spine pathology participated. Isokinetic eccentric shoulder rotation strength was measured through a 90° arc with the shoulder elevated 90° in frontal plane (frontal plane), and 45° anterior to the frontal plane (scapular plane) in neutral and rotated cervical spine positions. Cervical spine position was obtained by instructing participants to maximally rotate their respective side. Frontal plane eccentric external rotated strength differed between neutral and contralaterally positions in the first 10° of the motion, near forearm vertical (p+=+0.029). Internal rotation strength differed between neutral and contralaterally rotated positions from 55-60° external rotation (p+=+0.004). Scapular plane eccentric external rotation differed between cervical positions between 21-67° shoulder external rotation (p<0.001). Scapular plane internal rotation strength differed between cervical positions between 22-60° shoulder external rotation (p<0.001). In populations requiring strenuous use of their shoulders in altered cervical spine positions, sports medicine clinicians should consider including strength testing that reflects the functional positions of their patients during an orthopedic examination.

Effect of smartphone use on cervical spine stability.

Using a smartphone often involves a sustained head-forward tilt posture, which may deteriorate the mechanism of muscle reaction efficiency or reduce the stiffness of connective tissues of the cervical spine. These changes in muscular and connective tissues can impair cervical spine stability and contribute to developing neck pain symptoms. In this experiment, change in the cervical spine stability associated with a sustained smartphone use posture was evaluated by quantifying the effective stiffness and the reflexive responses of the head to sudden perturbations. Seventeen young smartphone users maintained their heads tilted forward approximately 30° for 30 min while watching videos on their smartphones in sitting. Data show that the measures of cervical spine stability did not change significantly after the smartphone use task despite developing mild to moderate neck and upper body discomfort symptoms. Study findings imply that keeping the head tilt posture for 30 min for smartphone use did not significantly alter spinal stability, rejecting its association with neck discomfort.

Automatic Tracking of Hyoid Bone Displacement and Rotation Relative to Cervical Vertebrae in Videofluoroscopic Swallow Studies Using Deep Learning.

The hyoid bone displacement and rotation are critical kinematic events of the swallowing process in the assessment of videofluoroscopic swallow studies (VFSS). However, the quantitative analysis of such events requires frame-by-frame manual annotation, which is labor-intensive and time-consuming. Our work aims to develop a method of automatically tracking hyoid bone displacement and rotation in VFSS. We proposed a full high-resolution network, a deep learning architecture, to detect the anterior and posterior of the hyoid bone to identify its location and rotation. Meanwhile, the anterior-inferior corners of the C2 and C4 vertebrae were detected simultaneously to automatically establish a new coordinate system and eliminate the effect of posture change. The proposed model was developed by 59,468 VFSS frames collected from 1488 swallowing samples, and it achieved an average landmark localization error of 2.38 pixels (around 0.5% of the image with 448 × 448 pixels) and an average angle prediction error of 0.065 radians in predicting C2-C4 and hyoid bone angles. In addition, the displacement of the hyoid bone center was automatically tracked on a frame-by-frame analysis, achieving an average mean absolute error of 2.22 pixels and 2.78 pixels in the x-axis and y-axis, respectively. The results of this study support the effectiveness and accuracy of the proposed method in detecting hyoid bone displacement and rotation. Our study provided an automatic method of analyzing hyoid bone kinematics during VFSS, which could contribute to early diagnosis and effective disease management.

Biomechanical analysis of cervical spine (C2-C7) at different flexed postures.

Musculoskeletal diseases are often related with postural changes in the neck region that can be caused by prolonged cervical flexion. This is one of the contributing factors. When determining the prevalence, causes, and related risks of neck discomfort, having a solid understanding of the biomechanics of the cervical spine (C1-C7) is absolutely necessary. The objective of this study is to make predictions regarding the intervertebral disc (IVD) stress values across C2-C7 IVD, the ligament stress, and the variation at 0°, 15°, 30°, 45°, and 60° of cervical neck angle using finite element analysis (FEA). In order to evaluate the mechanical properties of the cervical spine (particularly, C2-C7), this investigation makes use of computed tomography (CT) scans to develop a three-dimensional FEA model of the cervical spine. A preload of 50 N compression force was applied at the apex of the C2 vertebra, and all degrees of freedom below the C7 level were constrained. The primary objective of this investigation is to assess the distribution of von Mises stress within the IVDs and ligaments spanning C2-C7 at various flexion angles: 0°, 15°, 30°, 45°, and 60°, utilizing FEA. The outcomes derived from this analysis were subsequently compared to previously published experimental and FEA data to validate the model's ability to replicate the physiological motion of the cervical spine across different flexion angles.

Assessment of the stiffness of the upper trapezius muscle in a group of asymptomatic people with cervical spine rotation asymmetry.

This study investigated the relationship between the stiffness of the upper trapezius muscle and the range of rotational movement of the cervical spine. A total of 60 right-handed asymptomatic students participated in the study. Participants (N = 22) characterised by asymmetry in rotational movements were selected for the experimental group. A difference of ≥10° between right and left rotation of the cervical spine was considered asymmetrical. The control group (N = 38) included participants whose rotation difference was < 10°. Belonging to the experimental or control group did not significantly differentiate trapezius muscle stiffness. The rotation side differentiated the stiffness of the right and left trapezius muscles only in the group of people with rotational movement asymmetry. There were high correlation coefficients between right cervical rotation and the stiffness of the muscle on the right side, and between rotation to the left and the stiffness of the muscle on the left side. There is a relationship between the stiffness of the right and left upper trapezius muscles and the range of right and left rotational motion of the cervical spine. Stiffness of the upper trapezius correlates more strongly with rotation to the side on which the muscle lies than to the opposite side. Increased stiffness of the upper trapezius muscle on the side of limited cervical spine rotation is likely to be determined by the muscle fibre stretching mechanism.

Are owls technically capable of making a full head turn?

The three-dimensional configuration of the neck that produces extreme head turn in owls was studied using the Joint Coordinate System. The limits of planar axial rotation (AR), lateral, and sagittal bending in each vertebral joint were measured. They are not extraordinary among birds, except probably for the extended ability for AR. The vertebral joint angles involved in the 360° head turn do not generally exceed the limits of planar mobility. Rotation in one plane does not expand the range of motion in the other, with one probable exception being extended dorsal bending in the middle of the neck. Therefore, the extreme 360° head turn can be presented as a simple combination of the three planar motions in the neck joints. Surprisingly, certain joints are always laterally bent or axially rotated to the opposite side than the head was turned. This allows keeping the anterior part of the neck parallel to the thoracic spine, which probably helps preserve the ability for peering head motions throughout the full head turn. The potential ability of one-joint muscles of the owl neck, the mm. intertransversarii, to ensure the 360° head turn was addressed. It was shown that the 360° head turn does not require these muscles to shorten beyond the known contraction limit of striated vertebrate muscles. Shortening by 50% or less is enough for the mm. intertransversarii in the middle neck region for the 360° head turn. This study has broad implications for further research on vertebral mobility and function in a variety of tetrapods, providing a new method for CT scan-based measurement of intervertebral angles.

Head tilt as a clinical sign of cervical spinal or paraspinal disease in dogs: 15 cases (2000-2021).

OBJECTIVES: To characterise head tilt as a rare clinical sign of cervical spinal or paraspinal disease in dogs. MATERIALS AND METHODS: Retrospective single-centre case-series study of dogs with head tilt and cervical spinal or paraspinal disease in the absence of intracranial abnormalities. Descriptive statistics were used. RESULTS: Fifteen dogs met the inclusion criteria of this study. Median age at onset was 6 years (range 2.5 to 12 years). Onset of neurological signs was mainly chronic (9/15, 60%). Most common presenting complaints included head tilt (9/15, 60%) and cervical hyperaesthesia (8/15, 53%). Most common neurological findings included head tilt (15/15, 100%), generalised proprioceptive ataxia and tetraparesis (6/15, 40%) and cervical hyperaesthesia (8/15, 53%). Diagnoses included post-operative complication of C2 spinal nerve root mass removal (2/15, 13%), C3-C4 intervertebral disc extrusion (2/15, 13%), cervical paraspinal myositis (2/15, 13%) and one of each: C2 vertebral malformation, C2 spinal nerve root mass, C1-C2 meningioma, C2 vertebral fracture, C4-C5 intervertebral disc extrusion, C4 vertebral body mass, C5-C7 osseous-associated cervical spondylomyelopathy, and concurrent C5-C6 and C6-C7 intervertebral disc protrusions. Two dogs were euthanased shortly after diagnosis and two of 15 were dogs lost to follow-up. No post-mortem examination was performed for these cases. For the 11 of 15 remaining dogs, head tilt resolved in eight of 15 (53%) dogs after treatment of the underlying condition and in three of 15 (20%) dogs, it remained static. CLINICAL SIGNIFICANCE: Head tilt can be a rare clinical sign of cervical spinal or paraspinal disease in dogs.

The move-C cervical artificial disc can restore intact range of motion and 3-D kinematics.

BACKGROUND CONTEXT: In contrast to cervical discectomy and fusion, total disc replacement (TDR) aims at preserving the motion at the treated vertebral level. Spinal motion is commonly evaluated with the range of motion (ROM). However, more qualitative information about cervical kinematics before and after TDR is still lacking. PURPOSE: The aim of this in vitro study was to investigate the influence of cervical TDR on ROM, instantaneous centers of rotation (ICR) and three-dimensional helical axes. STUDY DESIGN: An in vitro study with human spine specimens under pure moment loading was conducted to evaluate the kinematics of the intact cervical spine and compare it to cervical TDR. METHODS: Six fresh frozen human cervical specimens (C4-5, median age 28 years, range 19-47 years, two female and four male) were biomechanically characterized in the intact state and after implantation of a cervical disc prosthesis (MOVE-C, NGMedical, Germany). To mimic in vivo conditions regarding temperature and humidity, water steam was used to create a warm and humid test environment with 37°C. Each specimen was quasistatically loaded with pure moments up to ±2.5 Nm in flexion/extension (FE), lateral bending (LB) and axial rotation (AR) in a universal spine tester for 3.5 cycles at 1 °/s. For each third cycle of motion the ROM was evaluated and an established method was used to determine the helical axis and COR and to project them into three planar X-rays. Statistical analysis was conducted using a Friedman-test and post hoc correction with Dunn-Bonferroni-tests (p<.05). RESULTS: After TDR, total ROM was increased in FE from 19.1° to 20.1°, decreased in LB from 14.6° to 12.6° and decreased in AR from 17.7° to 15.5°. No statistical differences between the primary ROM in the intact condition and ROM after TDR were detected. Coupled rotation between LB and AR were also maintained. The position and orientation of the helical axes after cervical TDR was in good agreement with the results of the intact specimens in all three motion directions. The ICR in FE and AR before and after TDR closely matched, while in LB the ICR after TDR were more caudal. The intact in vitro kinematics we found also resembled in vivo results of healthy individuals. CONCLUSION: The results of this in vitro study highlight the potential of artificial cervical disc implants to replicate the quantity as well as the quality of motion of the intact cervical spine. CLINICAL SIGNIFICANCE: Physiological motion preservation was a driving factor in the development of cervical TDR. Our results demonstrate the potential of cervical TDR to replicate in vivo kinematics in all three motion directions.

Assessment of axial rotation movement in cervical dystonia using cone-beam computed tomography.

BACKGROUND: Cervical dystonia is a neurological disorder characterized by involuntary muscle contractions and abnormal postures of the head and neck. Botulinum neurotoxin injection is the first-line treatment. Imaging determination of the cervical segments involved (lower or upper according to the torticollis-torticaput [COL-CAP] Classification) is an aid in determining the muscles to be injected. We aimed to clarify the impact of dystonia on posture and rotational movement of cervical vertebrae in the transverse plane. METHODS: A comparative study was conducted in a movement disorders department. Ten people with cervical dystonia and 10 matched healthy subjects (without cervical dystonia) were recruited. 3-D images of posture and cervical range of motion in axial rotation in the sitting position were recorded by using a cone-beam CT scanner. Range of rotational motion of the upper cervical spine from the occipital bone to fourth cervical vertebra was measured and compared between the two groups. FINDINGS: The head posture analysis showed that the total cervical spine position was more significantly distant from the neutral position for people with dystonia than healthy subjects (p = 0.007). The rotational range of motion of the cervical spine was significantly lower in cervical dystonia participants than in healthy subjects for the total (p = 0.026) and for upper cervical spine (p = 0.004). INTERPRETATION: We demonstrated, by means of cone-beam CT, that the disorganization of movements due to cervical dystonia affected the upper cervical spine and mostly the atlantoaxial joint. The involvement of rotator muscles at this cervical level should be considered more in treatments.