In vivo three-dimensional kinematics of the cervical spine during maximal axial rotation

Standard of Care Cervical Spine Flexion/Extension Radiograph Measurements Do Not Predict Multiplanar Intervertebral Motion

Evaluation of patients with neck pain often relies on end-range flexion and extension radiographs that do not capture mid-range or multiplanar motion. The purpose of this study was to determine if end-range flexion/extension range of motion (ROM) predicts axial rotation ROM or mid-range ROM during either flexion/extension or axial rotation in patients with neck pain or in asymptomatic controls. It was hypothesized that end-range flexion/extension ROM would predict mid-range flexion/extension ROM, but not mid-range or end-range axial rotation ROM. Dynamic flexion/extension and axial rotation were performed by 75 patients prior to surgery and 71 asymptomatic controls, while synchronized biplane radiographs were collected at 30 images per second. Intervertebral motion from C2 to C7 was tracked using a validated volumetric model-based tracking process that matched subject-specific computed tomography (CT)-based bone models to the radiographs. The main findings were that intervertebral end-range flexion/extension ROM is a strong to very strong predictor of mid-range flexion/extension at all subaxial motion segments of the cervical spine (all r = 0.61 to 0.91), but, in general, a weak to moderate predictor of axial rotation mid-range (all ρ = 0.002 to 0.50) and end-range (all r = 0.2 to 0.68) ROM. This study suggests that the current standard of care end-range flexion/extension ROM is not sufficient to characterize the multiplanar motion that occurs in the cervical spine during activities of daily living.

The deflection of fatigued neck

The human neck is a unique mechanical structure, highly flexible but fatigue prone. The rising prevalence of neck pain and chronic injuries has been attributed to increasing exposure to fatigue loading in activities such as prolonged sedentary work and overuse of electronic devices. However, a causal relationship between fatigue and musculoskeletal mechanical changes remains elusive. This work aimed to establish this relationship through a unique experiment design, inspired by a cantilever beam mechanical model of the neck, and an orchestrated deployment of advanced motion-force measurement technologies including dynamic stereo-radiographic imaging. As a group of 24 subjects performed sustained-till-exhaustion neck exertions in varied positions-neutral, extended, and flexed, their cervical spine musculoskeletal responses were measured. Data verified the occurrence of fatigue and revealed fatigue-induced neck deflection which increased cervical lordosis or kyphosis by 4-5° to 11°, depending on the neck position. This finding and its interpretations render a renewed understanding of muscle fatigue from a more unified motor control perspective as well as profound implications on neck pain and injury prevention.

Anatomy and mobility in the adult cadaveric craniocervical junction

Genetic diseases with craniofacial malformations can be associated with anomalies of the craniocervical joint (CCJ). The functions of the CCJ are thus impaired, as mobility may be either limited by abnormal bone fusion causing headaches, or exaggerated in the case of hypermobility, which may cause irreparable damage to the spinal cord. Restoring the balance between mobility and stability requires surgical correction in children. The anatomy and biomechanics of the CCJ are quite unique, yet have been overlooked in the past decades. Pediatric evidence is so scarce, that investigating the adult CCJ is our best shot to disentangle the form-function relationships of this anatomical region. The motivation of the present study was to understand the morphological and functional basis of motion in the CCJ, in the hope to find morphological features accessible from medical imaging able to predict mobility. To do so, we have quantified the in-vitro kinematics of the CCJ in nine cadaveric asymptomatic adults, and estimated a wide range of mobility variables covering the complexity of spinal motion. We compared these variables with the shape of the occipital, the atlas and the axis, obtained using a dense geometric morphometric approach. Morphological joint congruence was also quantified. Our results suggest a strong relationship between bone shape and motion, with the overall geometry predicting best the primary movements, and the joint facets predicting best the secondary movements. We propose a functional hypothesis stating that the musculoligamental system determines movements of great amplitude, while the shape and congruence of joint facets determine the secondary and coupled movements, especially by varying the geometry of bone stops and the way ligaments are tensioned. We believe this work will provide valuable insights in understanding the biomechanics of the CCJ. Furthermore, it should help surgeons treating CCJ anomalies by enabling them to translate objectives of functional and clinical outcome into clear objectives of morphological outcome.

Coupled rotation patterns in cervical spine axial rotation can change when the head is kept level

The biomechanical literature describes axial rotation occurring coupled with lateral bending and flexion in the cervical spine. Since the head is kept level during some activities of daily living, we set out to investigate the changes in total and segmental motion that occur when a level gaze constraint is applied to cadaveric cervical spine specimens during axial rotation. 1.5Nm of left and right axial rotation moment was applied to sixteen C2-T1 cadaveric specimens with C2 unconstrained and C2 constrained to simulate level gaze. Overall and segmental motions were determined using optoelectronic motion measurement and specimen-specific kinematic modeling. Without a kinematic constraint on C2, motions were as described in the literature; namely, flexion and lateral bending to the same side as axial rotation. Keeping C2 level reduced that total axial rotation range of motion of the specimens. Changes were also produced in segmental coupled rotation in all specimens. The observed changes included completely opposite coupled motion than in the uncoupled specimens, and traditional coupled behavior at one load extreme and the opposite at the other extreme. Constraining C2 during axial rotation to simulate level gaze can produce coupled motion that differs from the classically described flexion and lateral bending to the same side as axial rotation. Statement of Clinical Significance: Activities of daily living that require the head to be kept level during axial rotation of the cervical spine may produce segmental motions that are quite different from the classically described motions with implications for biomechanical experiments and implant designers.

A kinematics-based method for creating deformed patient-derived head and neck CT scans. ANNUAL INTERNATIONAL CONFERENCE OF THE IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. ANNUAL INTERNATIONAL CONFERENCE 2023;2023:1-4. [PMID: 38083025 DOI: 10.1109/embc40787.2023.10340930] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

CT scans of the head and neck have multiple clinical uses, and simulating deformation of these CT scans allows for predicting patient motion and data augmentation for machine-learning methods. Current methods for creating patient-derived deformed CT scans require multiple scans or use unrealistic head and neck motion. This paper describes the CTHeadDeformation software package which allows for realistic synthetic deformation of head and neck CT scans for small amounts of motion. CTHeadDeformation is a python-based package that uses a kinematics-based approach using anatomical landmarks, and rigid/non-rigid registration to create a realistic patient-derived deformed CT scan. CTHeadDeformation is also designed for simple clinical implementation. The CTHeadDeformation software package was demonstrated on a head and neck CT scan of one patient. The CT scan was deformed in the anterior-posterior, superior-inferior, and left-right directions. Internal organ motion and more complex combination motions were also simulated. The results showed the patient's CT scan was able to be deformed in a way that preserved the shape and location of the anatomy.Clinical Relevance- This method allows for the realistic simulation of head and neck motion in CT scans. Clinical applications including simulating how patient motion affects radiation therapy treatment effectiveness. The CTHeadDeformation software can also be used to train machine-learning networks that are robust to patient motion, or to generate ground truth images for imaging or segmentation grand challenges.

Transoral unilateral lag screw osteosynthesis for coronal split fracture of the lateral mass of the atlas - case report, operative technique and review of the literature

Introduction

Atlas ring fractures, which account for 1.3% of all spinal fractures, are predominantly managed conservatively. However, in certain cases, surgical treatment may be necessary depending on the type of fracture, degree of comminution, fracture location, and associated ligamentous injuries. Surgical stabilization frequently results in a posterior C1-2 or C0-2 fusion, which restricts movement, particularly craniocervical rotation. Coronal split fractures of the lateral mass need to be reduced and fixed due to dislocation, instability and secondary osteoarthritis. The preferred treatment approach involves internal fixation of the reduced fracture fragments, while avoiding restriction of the upper cervical spine's range of motion (ROM).

Research question

Is unilateral anterior transoral lag screw for treatment of unstable coronal split fracture of lateral mass of the atlas feasible and a safe treatment option?

Case Report Material and Methods

We report on a 55-year-old female suffering from polytrauma with multiple spinal and extremity injuries.

Results

A coronal split fracture of the lateral mass of the atlas was treated minimally invasive with a transoral lag screw technique to reduce and fix the fracture that has a tendency for fracture gap widening. Stable fixation and fracture union and thus restoration of function was achieved.

Discussion and conclusion

Transoral lag screw osteosynthesis for coronal split fracture of the lateral mass of the atlas is a potential treatment option in selected cases to preserve mobility in the upper cervical spine after spinal trauma.

Effects of occipital-atlas stabilization on the upper cervical spine rotation combinations: an in vitro study

The purpose of this study is to compare axial rotation range of motion for the upper cervical spine during three movements: axial rotation, rotation + flexion + ipsilateral lateral bending and rotation + extension + contralateral lateral bending before and after occiput-atlas (C0-C1) stabilization. Ten cryopreserved C0-C2 specimens (mean age 74 years, range 63-85 years) were manually mobilized in 1. axial rotation, 2. rotation + flexion + ipsilateral lateral bending and 3. rotation + extension + contralateral lateral bending without and with a screw stabilization of C0-C1. Upper cervical range of motion and the force used to generate the motion were measured using an optical motion system and a load cell respectively. The range of motion (ROM) without C0-C1 stabilization was 9.8° ± 3.9° in right rotation + flexion + ipsilateral lateral bending and 15.5° ± 5.9° in left rotation + flexion + ipsilateral lateral bending. With stabilization, the ROM was 6.7° ± 4.3° and 13.6° ± 5.3°, respectively. The ROM without C0-C1 stabilization was 35.1° ± 6.0° in right rotation + extension + contralateral lateral bending and 29.0° ± 6.5° in left rotation + extension + contralateral lateral bending. With stabilization, the ROM was 25.7° ± 6.4° (p = 0.007) and 25.3° ± 7.1°, respectively. Neither rotation + flexion + ipsilateral lateral bending (left or right) or left rotation + extension + contralateral lateral bending reached statistical significance. ROM without C0-C1 stabilization was 33.9° ± 6.7° in right rotation and 28.0° ± 6.9° in left rotation. With stabilization, the ROM was 28.5° ± 7.0° (p = 0.005) and 23.7° ± 8.5° (p = 0.013) respectively. The stabilization of C0-C1 reduced the upper cervical axial rotation in right rotation + extension + contralateral lateral bending and right and left axial rotations; however, this reduction was not present in left rotation + extension + contralateral lateral bending or both combinations of rotation + flexion + ipsilateral lateral bending.

Association between age, sex and cervical and upper cervical rotation tests. Descriptive and correlational study in healthy volunteers

Background

Active cervical spine rotation (ACROM Rot) shows cervical rotation and flexion rotation test (FRT); side-bending rotation test (SBRT) and upper cervical axial rotation test (C0-C2ART) are described to measure upper cervical rotation. The objectives of this study are (1) to describe the normal range of motion (ROM) of ACROM Rot, and the ROM in FRT, SBRT and C0-C2ART tests; (2) to explore the correlation among the four tests and (3) to investigate the influence of age and sex in their ROM.

Methods

A cross-sectional study was carried out with healthy volunteers from 18 to 75 years of age. Tests were measured using a CROM device and a bubble inclinometer. Descriptive analysis was performed to establish normative data for the ROM tests. Correlation analysis was conducted to understand the relation between upper and global cervical rotation ROM and among the three upper cervical rotation tests. Linear regression models were developed to understand the influence of age and sex in the ROM of all tests.

Results

Normative values were obtained from 122 healthy volunteers (50% male), by sex and age strata. The degree of correlation ranged between 0.582 (p < 0.01) for FRT and ACROM Rot left and 0.217 (p < 0.05) for SBRT and C0-C2ART left. Linear regression models showed the influence of sex for ACROM Rot right (men -4.64° less than women), SBRT (men -4.1° less than women) left and C0-C2ART right and left (men -2.24° and -1.78° less than women). The age influenced rotation ROM with a decrease for every 10 years of -2.11° and -1.96° for ACROM Rot right and left, of -1.72° and -1.26° for FRT right and left and -0.58° and -0.41° for C0-C2ART right and left in the linear regression models. No association was found between age and SBRT (p = 0.63 for right SBRT and p = 0.49 for left SBRT).

Conclusion

Weak-to-moderate correlation was found between the upper cervical spine rotation tests and with the ACROM. Women had a larger ROM in ACROM Rot right, SBRT left and C0-C2ART. Decreases in ROM related with age were observed for ACROM Rot, FRT and C0-C2ART but not for SBRT.

Cervical rotational osteotomy for correction of axial deformity in a patient with ankylosing spondylitis

PURPOSE

Severe cervical axial deformity associated with ankylosing spondylitis (AS) is rare in clinic, and there are little concerns about surgical treatment of axial deformity associated with AS. The case study aims to show the surgical technique to perform cervical rotational osteotomy.

METHODS

We present the case of a young AS patient whose neck was fixed in a left-rotational posture at 18°, requiring his trunk to be turned to the right to look forward visually. This made his gait appear to be limping, inconveniencing him with great difficulty. In order to correct this deformity, we performed a novel cervical rotational osteotomy through a one-stage posterior-anterior-posterior approach. Firstly, we performed laminectomies of C7 and T1, followed by a C7/T1 facetectomy with release of the bilateral C8 nerve roots. Next, we performed C7/T1 discectomy, bony resection of the lateral body and uncovertebral joints. The head of the patient was then rotated manually, so that both his face and torso were simultaneously facing frontward. Finally, rods spanning the screws from C6 to T2 were fixed.

RESULTS

Postoperatively, the patient's axial malalignment was significantly improved, and he was able to walk normally. Surgical outcomes were well maintained at a 3-year follow-up.

CONCLUSION

Through this case, we hope to draw the attention to spinal axial deformity and provide a reference point in the surgical treatment of spinal axial deformity.

Spontaneous Osseous Fusion after Remodeling Therapy for Chronic Atlantoaxial Rotatory Fixation and Recovery Mechanism of Rotatory Range of Motion of the Cervical Spine

We aimed to investigate the risk factors of spontaneous osseous fusion (SOF) of the atlantoaxial joint after closed reduction under general anesthesia followed by halo fixation (remodeling therapy) for chronic atlantoaxial rotatory fixation, and to elucidate the recovery mechanism of the rotatory range of motion (ROM) after halo removal. Twelve patients who underwent remodeling therapy were retrospectively reviewed. Five patients with SOF were categorized as the fusion group and seven patients without SOF as the non-fusion group. Three dimensional CT was used to detect direct osseous contact (DOC) of facet joints before and during halo fixation, while dynamic CT at neutral and maximally rotated head positions was performed to measure rotatory ROM after halo removal. The duration from onset to initial visit was significantly longer (3.2 vs. 5.7 months, p = 0.04), incidence of DOC during halo fixation was higher (0/7 [0%] vs. 4/5 [80%], p = 0.004), and segmental rotatory ROM of Occiput/C1 (Oc/C1) at final follow-up was larger (9.8 vs. 20.1 degrees, p = 0.003) in the fusion group. Long duration from the onset to the initial visit might induce irreversible damage to the articular surface of the affected facet, which was confirmed as DOC during halo fixation and resulted in SOF. Long duration from the onset to the initial visit and DOC during halo fixation could be used to suggest the risk for SOF. Nonetheless, rotatory ROM of Oc/C1 increased to compensate for SOF.

The Effect of Axial Compression and Distraction on Cervical Facet Cartilage Apposition During Shear and Bending Motions

During cervical spine trauma, complex intervertebral motions can cause a reduction in facet joint cartilage apposition area (CAA), leading to cervical facet dislocation (CFD). Intervertebral compression and distraction likely alter the magnitude and location of CAA, and may influence the risk of facet fracture. The aim of this study was to investigate facet joint CAA resulting from intervertebral distraction (2.5 mm) or compression (50, 300 N) superimposed on shear and bending motions. Intervertebral and facet joint kinematics were applied to multi rigid-body kinematic models of twelve C6/C7 motion segments (70 ± 13 year, nine male) with specimen-specific cartilage profiles. CAA was qualitatively and quantitatively compared between distraction and compression conditions for each motion; linear mixed-effects models (α = 0.05) were applied. Distraction significantly decreased CAA throughout all motions, compared to the compressed conditions (p < 0.001), and shifted the apposition region towards the facet tip. These observations were consistent bilaterally for both asymmetric and symmetric motions. The results indicate that axial neck loads, which are altered by muscle activation and head loading, influences facet apposition. Investigating CAA in longer cervical spine segments subjected to quasistatic or dynamic loading may provide insight into dislocation and fracture mechanisms.

Evaluation and Treatment of Trigeminal Symptoms of Cervical Origin After a Motor-Vehicle Crash: A Case Report With 9-Month Follow-up

Objective

The purpose of this case report is to describe the management of a patient with trigeminal symptoms of cervical origin after a motor-vehicle crash (MVC).

Clinical Features

After a head-on MVC, a 65-year-old woman presented with complaints of dizziness, headaches, facial tingling, visual disturbance, tinnitus, loss of cervical motion, and pain in the cervical spine.

Intervention and Outcome

The intervention applied was manipulation of the left C1-C2 and right C2-C3, with targeted exercise to strengthen the cervical musculature. After 4 weeks of treatment, the patient reported improvement in functional tasks and reduction in overall pain, headaches, facial tingling, tinnitus, and dizziness. At a 9-month follow-up, the patient had no report of facial tingling, tinnitus, loss of motion, or eye pain.

Conclusion

This patient with trigeminal symptoms of cervical origin after an MVC responded well to manual therapy to the cervical spine as part of a combination of services.

Validation and application of a novel in vivo cervical spine kinematics analysis technique

To validate the accuracy of Cone beam computed tomography (CBCT) cervical spine modeling with three dimensional (3D)-3D registration for in vivo measurements of cervical spine kinematics. CBCT model accuracy was validated by superimposition with computed tomography (CT) models in 10 healthy young adults, and then cervical vertebrae were registered in six end positions of functional movements, versus a neutral position, in 5 healthy young adults. Registration errors and six degrees of freedom (6-DOF) kinematics were calculated and reported. Relative to CT models, mean deviations of the CBCT models were < 0.6 mm. Mean registration errors between end positions and the reference neutral position were < 0.7 mm. During flexion-extension (F-E), the translation in the three directions was small, mostly < 1 mm, with coupled LB and AR both < 1°. During lateral bending (LB), the bending was distributed roughly evenly, with coupled axial rotation (AR) opposite to the LB at C1-C2, and minimal coupled F-E. During AR, most of the rotation occurred in the C1-C2 segment (29.93 ± 7.19° in left twist and 31.38 ± 8.49° in right twist) and coupled LB was observed in the direction opposite to that of the AR. Model matching demonstrated submillimeter accuracy in cervical spine kinematics data. The presently evaluated low-radiation-dose CBCT technique can be used to measure 3D spine kinematics in vivo across functional F-E, AR, and LB positions, which has been especially challenging for the upper cervical spine.

Intersegmental Kinematics of the Upper Cervical Spine: Normal Range of Motion and Its Alteration After Alar Ligament Transection

STUDY DESIGN

Biomechanical study using cadaveric cervical spines.

OBJECTIVE

To evaluate joint mobility and stiffness at the craniovertebral junction.

SUMMARY OF BACKGROUND DATA

Data on the intersegmental kinematics of the craniovertebral joints are available in the literature with a widespread range of values. The effect that alar ligament injuries have on intersegmental kinematics remains unclear and requires further biomechanical investigation.

METHODS

Ten occipito-atlanto-axial (C0-C1-C2) human specimens were articulated to flexion, extension, bilateral lateral bending, and bilateral axial rotation. The moment-rotation response was continuously tracked through the entire range of motion before and after unilateral alar ligament transection of the right side.

RESULTS

The intersegmental (C0-C1/C1-C2) moment-rotation response was continuously quantified in full flexion (7.2 ± 6.6°/12.1 ± 5.8°), extension (11.1 ± 6.4°/3.0 ± 2.8°), lateral bending to the right (3.1 ± 2.2°/1.6 ± 1.2°) and left sides (3.3 ± 1.6°/2.1 ± 1.5°), and axial rotation to the right (1.2 ± 3.5°/32.3 ± 9.3°) and left sides (2.7 ± 2.6°/25.3 ± 8.3°). Unilateral alar ligament transection increased the range of motion of C0-C2 in the three planes of movement; however, intersegmental motion alterations were not always observed.

CONCLUSION

Increases in the range of extension and lateral bending at C0-C1, which had not been reported previously, were observed. Further, the range of rotation on the right and left sides increased, in conjunction with the increased ranges at C0-C1 and C1-C2.Level of Evidence: N/A.

In vitro upper cervical spine kinematics: Rotation with combined movements and its variation after alar ligament transection

Previous studies indicate that maximum upper cervical axial rotation occurs only through a combination of transverse, frontal, and sagittal plane motions. This study explores the relationship between transection of the alar ligament and combined upper cervical axial rotation movements. Ten cryopreserved upper cervical spines were manually mobilized in bilateral axial rotation and two different motion combinations with simultaneous motion in the three anatomical planes: rotation in extension (extension + axial rotation + contralateral lateral bending) and rotation in flexion (flexion + axial rotation + ipsilateral lateral bending). These three motions were performed before and after right alar ligament transection. The occiput-axis axial rotation was measured using an optical motion capture system while measuring the applied load. With intact alar ligament, the axial rotation in flexion showed the lowest range of motion (right, R: 9.81 ± 3.89°; left, L: 15.54 ± 5.89°). Similar results were found between the other two mobilizations: axial rotation (R: 33.87 ± 6.64°; L: 27.99 ± 6.90°) and rotation in extension (R: 35.15 ± 5.97°; L: 28.96 ± 6.47°). After right alar ligament transection, rotation in flexion (particularly in left rotation) showed the largest increase in motion: rotation in flexion (R: 13.78 ± 9.63°; L: 23.04 ± 5.59°), rotation in extension (R: 36.39 ± 7.10°; L: 31.71 ± 7.67°), and axial rotation (R: 38.50 ± 9.47°; L: 31.59 ± 6.55°). Different combinations of movements should be evaluated when analyzing the maximum axial rotation of the upper cervical spine, as axial rotation alone and rotation in extension showed a larger range of motion than rotation in flexion. After unilateral alar ligament injury, rotation to the non-injured side in flexion demonstrates the most movement increase.

Estimating Facet Joint Apposition with Specimen-Specific Computer Models of Subaxial Cervical Spine Kinematics

Computational models of experimental data can provide a noninvasive method to estimate spinal facet joint biomechanics. Existing models typically consider each vertebra as one rigid-body and assume uniform facet cartilage thickness. However, facet deflection occurs during motion, and cervical facet cartilage is nonuniform. Multi rigid-body computational models were used to investigate the effect of specimen-specific cartilage profiles on facet contact area estimates. Twelve C6/C7 segments underwent non-destructive intervertebral motions. Kinematics and facet deflections were measured. Three-dimensional models of the vertebra and cartilage thickness estimates were obtained from pre-test CT data. Motion-capture data was applied to two model types (2RB: C6, C7 vertebrae each one rigid body; 3RB: left and right C6 posterior elements, and C7 vertebrae, each one rigid body) and maximum facet mesh penetration was compared. Constant thickness cartilage (CTC) and spatially-varying thickness cartilage (SVTC) profiles were applied to the facet surfaces of the 3RB model. Cartilage apposition area (CAA) was compared. Linear mixed-effects models were used for all quantitative comparisons. The 3RB model significantly reduced penetrating mesh elements by accounting for facet deflections (p = 0.001). The CTC profile resulted in incongruent facet articulation, whereas realistic congruence was observed for the SVTC profile. The SVTC profile demonstrated significantly larger CAA than the CTC model (p < 0.001).

An Electromyographically Driven Cervical Spine Model in OpenSim

Relatively few biomechanical models exist aimed at quantifying the mechanical risk factors associated with neck pain. In addition, there is a need to validate spinal-rhythm techniques for inverse dynamics spine models. Therefore, the present investigation was 3-fold: (1) the development of a cervical spine model in OpenSim, (2) a test of a novel spinal-rhythm technique based on minimizing the potential energy in the passive tissues, and (3) comparison of an electromyographically driven approach to estimating compression and shear to other cervical spine models. The authors developed ligament force-deflection and intervertebral joint moment-angle curves from published data. The 218 Hill-type muscle elements, representing 58 muscles, were included and their passive forces validated against in vivo data. Our novel spinal-rhythm technique, based on minimizing the potential energy in the passive tissues, disproportionately assigned motion to the upper cervical spine that was not physiological. Finally, using kinematics and electromyography collected from 8 healthy male volunteers, the authors calculated the compression at C7-T1 as a function of the head-trunk Euler angles. Differences from other models varied from 25.5 to 368.1 N. These differences in forces may result in differences in model geometry, passive components, number of degrees of freedom, or objective functions.

Effects of occipital-atlas stabilization in the upper cervical spine kinematics: an in vitro study

This study compares upper cervical spine range of motion (ROM) in the three cardinal planes before and after occiput-atlas (C0–C1) stabilization. After the dissection of the superficial structures to the alar ligament and the fixation of C2, ten cryopreserved upper cervical columns were manually mobilized in the three cardinal planes of movement without and with a screw stabilization of C0–C1. Upper cervical ROM and mobilization force were measured using the Vicon motion capture system and a load cell respectively. The ROM without C0–C1 stabilization was 19.8° ± 5.2° in flexion and 14.3° ± 7.7° in extension. With stabilization, the ROM was 11.5° ± 4.3° and 6.6° ± 3.5°, respectively. The ROM without C0–C1 stabilization was 4.7° ± 2.3° in right lateral flexion and 5.6° ± 3.2° in left lateral flexion. With stabilization, the ROM was 2.3° ± 1.4° and 2.3° ± 1.2°, respectively. The ROM without C0–C1 stabilization was 33.9° ± 6.7° in right rotation and 28.0° ± 6.9° in left rotation. With stabilization, the ROM was 28.5° ± 7.0° and 23.7° ± 8.5° respectively. Stabilization of C0–C1 reduced the upper cervical ROM by 46.9% in the sagittal plane, 55.3% in the frontal plane, and 15.6% in the transverse plane. Also, the resistance to movement during upper cervical mobilization increased following C0–C1 stabilization.

Analysis of three-dimensional facet joint displacement during two passive upper cervical mobilizations

BACKGROUND

Understanding the 3D-kinematics of the upper cervical spine during manual mobilization is essential for clinical examination and therapy. Some information about rotational motion is available in literature but translational components are often ignored, complicating the understanding of the complex inter-segmental motions.

OBJECTIVES

This study aims to describe the amount, trajectories and reproducibility of atlanto-occipital facet joints' displacement during a flexion-extension mobilization and of the atlanto-axial facet joints during an axial rotation mobilization.

DESIGN

Original research using quantitative data.

METHODS

20 fresh frozen human cervical specimens were examined with a Zebris® CMS20 ultrasound-based motion tracking system. Two physiotherapists performed regionalmobilizations in flexion-extension and axial rotation. The amount of displacement and the trajectories were calculated along the XYZ axes. Difference between measurements was evaluated with the Friedman two-way ANOVA test. Intra- and inter-rater reliability were estimated through ICC scores.

RESULTS

3D-displacement (2.6-23.4 mm) was larger at C1-C2 during axial rotation, Atlanto-occipital flexion displayed the greatest variability in the C0 trajectory. During a right rotation, the left C1 facet moved mainly forward, and the right C1 facet moved backward. During a left rotation, the left C1 facet moved backward, while the right C1 facet moved forward. Intra-tester and Inter-tester ICCs varied between 0.5 and 0.90 (p < 0.005).

CONCLUSIONS

During passive spinal motion, there is an important variability in magnitude and trajectory of joints' displacement. Nevertheless, different clinicians may be able to achieve the same position at the end of the mobilization.

Upper cervical range of rotation during the flexion-rotation test is age dependent: an observational study

Background:

The flexion-rotation test (FRT) is widely used to detect movement dysfunction in the spinal segment C1/C2, especially in patients with cervicogenic headache. The current published literature indicates that range recorded during the FRT is not age dependent. This is questionable, considering the well documented relationship between aging and degeneration in the cervical spine and loss of cervical movement in older people. The present study therefore aims to examine the influence of age on FRT mobility, and to provide normative values for different age groups. An additional aim is to examine the influence of age on the ratio between lower and upper cervical rotation mobility.

Methods:

For this cross-sectional, observational study, healthy subjects aged from 18 to 90 years were recruited. The upper cervical range of rotation during the FRT was measured using a digital goniometer. Personal data including age, weight, height, and lifestyle factors were also assessed.

Results:

A total of 230 (124 male) healthy, asymptomatic subjects, aged between 18 and 87 years were included. Regression analysis showed that 27.91% (p < 0.0001) of the variance in FRT mobility can be explained by age alone, while 41.28% (p < 0.0001) of the variance in FRT mobility can be explained by age and total cervical range of motion (ROM). Normative values for different age decades were calculated using regression analysis. No significant influence of age on the ratio between ROM of lower and upper cervical rotation was found. There was no relevant impact of personal (gender, height, and weight) and lifestyle (smartphone and PC use) factors on ROM during the FRT.

Conclusion:

Upper cervical rotation mobility determined by the FRT correlates strongly with age; hence, the results of the FRT have to be interpreted taking into account the individual age of the tested subject. The ratio between lower and upper cervical rotation mobility is maintained in all age groups.

Measurement of three-dimensional cervical segmental kinematics: Reliability of whole vertebrae and facet-based approaches

BACKGROUND

Previous studies have used orientation and translation of whole-vertebrae to describe three-dimensional cervical segmental kinematics. Describing kinematics using facet joint movement may be more relevant to pathology and effects of interventions but has not been investigated in the cervical spine. This study compared the reliability of two different methods (whole-vertebrae vs facet joint) to evaluate cervical kinematics.

METHODS

Two healthy adults each had six cervical (C1 to T1) magnetic resonance imaging scans, two each in neutral and left and right rotation. A semi-automated method of segmentation and alignment determined the relative orientation and translation of each whole-vertebrae and translation of each facet joint. Intra-rater and inter-rater reliability was determined using limits of agreement (LOA) with 95% confidence intervals and intraclass correlation coefficients (ICC_3,1 for intra- and ICC_2,1 for inter-rater).

RESULTS

The LOA for intra-rater evaluation of facet movement was superior to whole vertebra translation. Both methods showed excellent intra-rater ICC_3,1 (0.80-0.99) and inter-rater ICC_2,1 (0.79-0.85) for all variables except for Euler angle for flexion/extension which was good (0.65). Intra-and inter-rater ICCs were better for facet movement than all measures of whole of vertebrae movement except Euler angles of axial rotation where no difference was detected.

CONCLUSIONS

Measurement of three-dimensional segmental kinematics using either the facet joint or the whole-vertebrae method demonstrated excellent and comparable reliability. These findings support the use of the facet joint method as an option for describing and investigating cervical segmental kinematics.

In vivo three-dimensional kinematics of the cervical spine during maximal active head rotation

Objective

The aim of this study was to measure the movement of the cervical spine in healthy volunteers and patients with cervical spondylosis (CS) and describe the actual motion of the cervical spine using a three-dimensional (3D) CT reconstruction method. The results can enrich current biomechanical data of cervical spine and help to find the differences between the noted two groups.

Materials and methods

20 healthy volunteers underwent CT examination ranging from the clivus of the occiput (Oc) to the top of first thoracic vertebrae (T1) in a neutral position with left or right maximal axial rotation, while 26 CS patients received the same CT scan procedures in the neutral position with left and right maximum rotation. Subsequently, the three-dimensional images of the occiput and every cervical vertebrae (C1-C7) were reconstructed using medical software. 3 virtual non-collinear markers were placed on the prominent structures of foramen magnum and every cervical vertebrae. Then, the 3D orthogonal spatial coordinates were defined with these anatomical markers to represent the orientation and position of every vertebra. Segmental relative motions were calculated using Cardan angles in the 3D spatial coordinates. Finally, the differences between the two groups were analyzed with statistical software SPSS.

Results

The cervical spine exhibited complicated 3D movements, which could be adequately described using the three-dimensional CT reconstruction method. Reliability analysis of the 3D CT reconstruction method showed inter-rater ICC of 0.90–0.99 and intra-rater ICC of 0.91–0.98, suggesting very good consistency. Besides, the rotation at the upper cervical spine (Oc-C2) took up at least 60% of the total cervical rotation. The coupled lateral bending movement of the upper cervical spine was opposite to the major motion, while the movement of the lower cervical spine followed the same direction as that of the major motion. Oc to C5 segments were all coupled with the back-extension movement. The relative translations of all adjacent segments in each direction were minimal. CS patients showed a significant decrease in the movement of the C4-C5 segment compared with healthy volunteers.

Conclusion

The motion of the cervical spine was complicated and three-dimensional. The CT reconstruction method employed here was good at describing such movement. The 3D CT reconstruction method exhibited high reproducibility when measuring cervical spine movement. CS patients and healthy volunteers showed significant differences in the movement of some segments.

The effect of axial compression and distraction on cervical facet mechanics during anterior shear, flexion, axial rotation, and lateral bending motions

The subaxial cervical facets are important load-bearing structures, yet little is known about their mechanical response during physiological or traumatic intervertebral motion. Facet loading likely increases when intervertebral motions are superimposed with axial compression forces, increasing the risk of facet fracture. The aim of this study was to measure the mechanical response of the facets when intervertebral axial compression or distraction is superimposed on constrained, non-destructive shear, bending and rotation motions. Twelve C6/C7 motion segments (70 ± 13 yr, nine male) were subjected to constrained quasi-static anterior shear (1 mm), axial rotation (4°), flexion (10°), and lateral bending (5°) motions. Each motion was superimposed with three axial conditions: (1) 50 N compression; (2) 300 N compression (simulating neck muscle contraction); and, (3) 2.5 mm distraction. Angular deflections, and principal and shear surface strains, of the bilateral C6 inferior facets were calculated from motion-capture data and rosette strain gauges, respectively. Linear mixed-effects models (α = 0.05) assessed the effect of axial condition. Minimum principal and maximum shear strains were largest in the compressed condition for all motions except for maximum principal strains during axial rotation. For right axial rotation, maximum principal strains were larger for the contralateral facets, and minimum principal strains were larger for the left facets, regardless of axial condition. Sagittal deflections were largest in the compressed conditions during anterior shear and lateral bending motions, when adjusted for facet side.

Comparison of range of motion during the cervical flexion rotation versus the side-bending rotation test in individuals with and without hyperlaxity

Objective: The flexion rotation test (FRT) is used to determine C1-2 involvement in individuals with neck pain and headaches. Some individuals present with generalized joint hyperlaxity (GJH) which could influence the results of this test, which relies on a soft tissue locking mechanism. The purpose of this study was to examine the side-bend rotation test (SBRT), which utilizes osseous locking, compared to the FRT. Methods: Thirty-eight healthy individuals (25 female, 26.03 years) were assessed for GJH via the Beighton Hypermobility Index (BHI). A blinded examiner performed the FRT and SBRT bilaterally, measuring ROM using a digital goniometer device. Results: Statistically significant differences in ROM were present for the FRT based on negative (0-3) and positive (4-9) BHI score: (Right 46.4±3.6, 49.6±4.8, p=.031), (Left 45.5±3.5, 49.0±5.2, p=.023); no differences were observed for the SBRT (Right 37.6±4.3, 38.9±3.4), (Left 37.7±4.2, 37.6±3.4).  When further stratifying the groups, a one-way ANOVA and post-hoc testing revealed significant differences of FRT range of motion between the BHI 7-9 group(52.4 ± 4.4 -53.9 ± 3.4) compared to BHI 0-3 (45.4 ± 3.6-46.2 ± 3.5) and 4-6 groups (46.0 ± 3.7-46.4 ± 2.2), p < .001; there were no significant differences between the 0-3 and 4-6 groups. There were no between group differences for the SBRT, BHI 0-3 (37.5 ± 4.4-37.7 ± 4.3), BHI 7-9 (39.9 ± 3.7-39.2 ± 3.5). Discussion: Individuals with GJH demonstrated significant differences in ROM for the FRT, but not the SBRT. The SBRT may be a useful alternative to the FRT for individuals with hyperlaxity. However, further research needs to be conducted to assess the diagnostic ability of this test in individuals with cervical pathology.

Prediction of in vivo lower cervical spinal loading using musculoskeletal multi-body dynamics model during the head flexion/extension, lateral bending and axial rotation

Cervical spine diseases lead to a heavy economic burden to the individuals and societies. Moreover, frequent post-operative complications mean a higher risk of neck pain and revision. At present, controversy still exists for the etiology of spinal diseases and their associated complications. Knowledge of in vivo cervical spinal loading pattern is proposed to be the key to answer these questions. However, direct acquisition of in vivo cervical spinal loading remains challenging. In this study, a previously developed cervical spine musculoskeletal multi-body dynamics model was utilized for spinal loading prediction. The in vivo dynamic segmental contributions to head motion and the out-of-plane coupled motion were both taken into account. First, model validation and sensitivity analysis of different segmental contributions to head motion were performed. For model validation, the predicted intervertebral disk compressive forces were converted into the intradiskal pressures and compared with the published experimental measurements. Significant correlations were found between the predicted values and the experimental results. Thus, the reliability and capability of the cervical spine model was ensured. Meanwhile, the sensitivity analysis indicated that cervical spinal loading is sensitive to different segmental contributions to head motion. Second, the compressive, shear and facet joint forces at C3–C6 disk levels were predicted, during the head flexion/extension, lateral bending and axial rotation. Under the head flexion/extension movement, asymmetric loading patterns of the intervertebral disk were obtained. In comparison, symmetrical typed loading patterns were found for the head lateral bending and axial rotation movements. However, the shear forces were dramatically increased during the head excessive extension and lateral bending. Besides, a nonlinear correlation was seen between the facet joint force and the angular displacement. In conclusion, dynamic cervical spinal loading was both intervertebral disk angle-dependent and level-dependent. Cervical spine musculoskeletal multi-body dynamics model provides an attempt to comprehend the in vivo biomechanical surrounding of the human head-neck system.

A model-based tracking method for measuring 3D dynamic joint motion using an alternating biplane x-ray imaging system

PURPOSES

To propose a new model-based tracking method for measuring three-dimensional (3D) dynamic joint kinematics using a clinical alternating biplane x-ray imaging system; and to quantify in vitro its errors in measuring ankle and knee motions at different motion speeds.

METHODS

A new model-based tracking method based on motion component partition and interpolation (MCPI) was developed for measuring 3D dynamic joint kinematics based on a clinical alternating biplane x-ray imaging system. Two detectors of the biplane imaging system placed perpendicular to each other were operated to collect alternating fluoroscopic images of the targeted joint during tasks. The CT data of the joint were also acquired for the reconstruction of volumetric and surface models of each of the associated bones. The CT-based models of the bones were first registered to the alternating images using a model-to-single-plane fluoroscopic image registration method, and the resulting bone poses were then refined using a two-level optimization with motion component partition and model vertex trajectory interpolation. The MCPI method was evaluated in vitro for measurement errors for an ankle and a knee specimen moving at different speeds against a standard reference provided by a highly accurate motion capture system. The positional and rotational errors of the measured bone poses were quantified in terms of the bias, precision, and root-mean-squared errors (RMSE), as well as the mean target registration error (mTRE), a final mTRE less than 2.5 mm indicating a successful registration.

RESULTS

The new method was found to have RMSE of bone pose measurements of less than 0.18 mm for translations and 0.72° for rotations for the ankle, and 0.33 mm and 0.74° for the knee with a high successful registration rate (>97%), and did not appear to be affected by joint motion speeds. Given the same alternating fluoroscopic images, the MCPI method outperformed the typical biplane analysis method assuming zero time offset between the two fluoroscopic views. The differences in performance between the methods were increased with increased joint motion speed. With the accurate bone pose data, the new method enabled talocrural, subtalar, and tibiofemoral kinematics measurements with submillimeter and subdegree accuracy, except for an RMSE of 1.04° for the internal/external rotation of the talocrural joint.

CONCLUSIONS

A new model-based tracking method based on MCPI has been developed for measuring dynamic joint motions using an alternating biplane x-ray imaging system widely available in medical centers. The MCPI method has been demonstrated in vitro to be highly accurate in determining the 3D kinematics of the bones of both the ankle joint complex and the knee. The current results suggest that the MCPI method would be an effective approach for measuring in vivo 3D kinematics of dynamic joint motion in a clinical setting equipped with an alternating biplane x-ray imaging system.

Three-dimensional kinematics of the cervical spine using an electromagnetic tracking device. Differences between healthy subjects and subjects with non-specific neck pain and the effect of age

BACKGROUND

A cross-sectional observational study of three-dimensional cervical kinematics in 35 non-specific neck pain patients and 100 asymptomatic controls. To compare qualitative and quantitative aspects of cervical kinematics between healthy subjects and subjects with non-specific neck pain and to determine the effect of age on cervical kinematics in healthy subjects.

METHODS

Three-dimensional kinematics of active lateral bending and flexion-extension of 35 patients and 100 controls were registered by means of an electromagnetic tracking system. The means of several kinematic parameters were compared using t-tests. In addition, we assessed the age-dependency of the three-dimensional kinematic parameters by stratifying the 100 control subjects in 6 age categories.

FINDINGS

Comparison of the patient group with the control group reveals no statistically significant differences in qualitative and quantitative parameters. Analysis of the effect of age showed that the range of motion decreases significantly (p < 0.01) with increasing age. In lateral bending, the ratio between axial rotation and lateral bending increases significantly (p < 0.01) among older subjects. Differences in acceleration, jerk and polynomial fit are seen between the age categories, but are not significant.

INTERPRETATION

This study demonstrates no significant differences in kinematic parameters between healthy subjects and subjects with non-specific neck pain. Healthy subjects in higher age categories demonstrate higher ratios of coupled movements and lower ranges of motion. Future research should focus on classifying patients with non-specific neck pain in order to gain a better insight on possible subgroup specific differences in kinematics. More studies on this subject are warranted.

LEVEL OF EVIDENCE

4.

Morphometric changes of the cervical intervertebral foramen: A comparative analysis of pre-manipulative positioning and physiological axial rotation

BACKGROUND

Cervical foraminal impingement has been described as a source of radicular pain. Clinical tests and head motions have been reported for affecting the intervertebral foramen (IVF) dimensions. Although manual approaches are proposed in the management of cervical radiculopathy, their influence on the foraminal dimensions remains unclear.

OBJECTIVES

To investigate the influence of pre-manipulative positioning versus cervical axial rotation on the foraminal dimensions of the lower cervical spine.

METHODS

Thirty asymptomatic volunteers underwent CT scan imaging in neutral position and axial rotation or pre-manipulative positioning. The manipulation task was performed at C4-C5 following a multiple components procedure. 3D kinematics and IVF (height, width and area) were computed for each cervical segment.

RESULTS

The results showed that foraminal changes are dependent on motion types and cervical levels. With reference to head rotation, IVF opening occurred on the ipsilateral side during pre-manipulative positioning while axial rotation involved the contralateral side. Regardless of the side considered, magnitudes of opening were similar between both attitudes while narrowing was lower at the target and adjacent levels during the pre-manipulative positioning. Some associations between segmental motion and IVF changes were observed for the target level and the overlying level.

CONCLUSIONS

The present study demonstrated that pre-manipulative positioning targeting C4-C5 modified IVF dimensions differently than the passive axial rotation. The findings suggest that techniques which incorporate combined movement positioning influence segmental motion and IVF dimensions differently at the target segment, compared to unconstrained rotation. Further investigations are needed to determine the clinical outcomes of such an approach.

Comparison of three-dimensional helical axes of the cervical spine between in vitro and in vivo testing

BACKGROUND CONTEXT

The range of motion is a well-accepted parameter for the assessment and evaluation of cervical motion. However, more qualitative data of the kinematics of the cervical spine are needed for the development and success of cervical disc arthroplasty.

PURPOSE

The aim of this study was to provide basic information about helical axes of human cervical spine under in vitro conditions. Furthermore, it should clarify whether the three-dimensional helical axes of cervical motion gained from in vitro experiments are in agreement with those gained from in vivo experiments, and therefore to prove its reliability.

STUDY DESIGN/SETTING

An in vitro test with pure moments and mono-segmental specimens was designed to investigate and compare the helical axes of the cervical spine.

METHODS

Six human cadaveric specimens (three male and three female) with an average age of 47.5 years (range: 34-58 years) were carefully selected. Each specimen was divided into three motion segments: C2-C3, C4-C5, and C6-C7. We performed 3.5 full cycles of rotation about all axes, flexion-extension, lateral bending, and axial rotation, by applying pure moments of 1.5 Nm without any preload. Following the in vitro tests, the three-dimensional helical axes were calculated and projected into the x-ray images.

RESULTS

Rotation analysis of all three directions revealed similar results for all six specimens. All calculated helical axes were similar to the published in vivo data. Furthermore, the instantaneous centers of rotation were in agreement with in vivo data.

CONCLUSIONS

The data gained from this study verify cervical kinematics during in vitro testing using pure moments. It can be assumed that other soft tissue such as muscles are not necessarily needed to simulate cervical kinematics in vitro.

Dynamic in vivo 3D atlantoaxial spine kinematics during upright rotation

Diagnosing dysfunctional atlantoaxial motion is challenging given limitations of current diagnostic imaging techniques. Three-dimensional imaging during upright functional motion may be useful in identifying dynamic instability not apparent on static imaging. Abnormal atlantoaxial motion has been linked to numerous pathologies including whiplash, cervicogenic headaches, C2 fractures, and rheumatoid arthritis. However, normal C1/C2 rotational kinematics under dynamic physiologic loading have not been previously reported owing to imaging difficulties. The objective of this study was to determine dynamic three-dimensional in vivo C1/C2 kinematics during upright axial rotation. Twenty young healthy adults performed full head rotation while seated within a biplane X-ray system while radiographs were collected at 30 images per second. Six degree-of-freedom kinematics were determined for C1 and C2 via a validated volumetric model-based tracking process. The maximum global head rotation (to one side) was 73.6±8.3°, whereas maximum C1 rotation relative to C2 was 36.8±6.7°. The relationship between C1/C2 rotation and head rotation was linear through midrange motion (±20° head rotation from neutral) in a nearly 1:1 ratio. Coupled rotation between C1 and C2 included 4.5±3.1° of flexion and 6.4±8.2° of extension, and 9.8±3.8° of contralateral bending. Translational motion of C1 relative to C2 was 7.8±1.5mm ipsilaterally, 2.2±1.2mm inferiorly, and 3.3±1.0mm posteriorly. We believe this is the first study describing 3D dynamic atlantoaxial kinematics under true physiologic conditions in healthy subjects. C1/C2 rotation accounts for approximately half of total head axial rotation. Additionally, C1 undergoes coupled flexion/extension and contralateral bending, in addition to inferior, lateral and posterior translation.

Three-dimensional intervertebral range of motion in the cervical spine: Does the method of calculation matter?

Intervertebral range of motion (ROM) is commonly calculated using ordered rotations or projection angles. Ordered rotations are sequence-dependent, and projection angles are dependent upon on which orientation vectors are projected. This study assessed the effect of calculation method on intervertebral ROM in the subaxial cervical spine (C3-C7) during in vivo dynamic, three-dimensional, functional movement. Biplane radiographs were collected at 30 images per second while 29 participants performed full ROM flexion/extension, axial rotation and lateral bending movements of their cervical spine. In vivo bone motion was tracked with sub-millimeter accuracy using a validated volumetric model-based tracking technique. Intervertebral rotations were calculated using six Cardan angle sequences and two projection angle combinations. Within-subject comparisons revealed significant differences in intervertebral ROM among calculation methods (all p<0.002). Group mean ROM differences were small, but significantly different among calculation methods (p<0.001). A resampling technique demonstrated that as group size increases, the differences between calculation methods decreases substantially. It is concluded that the method used to calculate intervertebral rotations of the sub-axial cervical spine can significantly affect within-subject and between group comparisons of intervertebral ROM.

An immediate effect of axial neck rotation training with real time visual feedback using a smartphone inclinometer on improvement in axial neck rotation function

OBJECTIVES

The purpose of this study was to compare the immediate effects of axial neck rotation training (Axi-NRT) with and without real-time visual feedback (VF) using a smartphone inclinometer on the range of motion (ROM) for axial neck rotation and the onset of compensatory neck lateral bending and extension during active neck rotation.

METHODS

Twenty participants with restricted ROM for neck rotation but no neck pain (21.1 ± 1.6 years and 8 males, 12 females) were recruited for Axi-NRT with VF, and twenty age- and gender-matched participants with restricted ROM for neck rotation were recruited for Axi-NRT without VF. Changes in ROM for neck rotation and the onset time of compensatory neck movement during active neck rotation were measured using an electromagnetic tracking system.

RESULTS

Axi-NRT with VF was more effective in increasing ROM for neck rotation and decreasing and delaying the onset of compensatory neck movements during active neck rotation compared with Axi-NRT without VF.

CONCLUSIONS

Repeated Axi-NRT using VF is useful to educate participants in maintaining the axis of the cervical spine and to increase ROM for axial neck rotation with less compensatory neck motion in participants with a restricted range of neck rotations.

Measurement and Geometric Modelling of Human Spine Posture for Medical Rehabilitation Purposes Using a Wearable Monitoring System Based on Inertial Sensors

This paper presents a mathematical model that can be used to virtually reconstruct the posture of the human spine. By using orientation angles from a wearable monitoring system based on inertial sensors, the model calculates and represents the curvature of the spine. Several hypotheses are taken into consideration to increase the model precision. An estimation of the postures that can be calculated is also presented. A non-invasive solution to identify the human back shape can help reducing the time needed for medical rehabilitation sessions. Moreover, it prevents future problems caused by poor posture.

In vivo three-dimensional intervertebral kinematics of the subaxial cervical spine during seated axial rotation and lateral bending via a fluoroscopy-to-CT registration approach

Accurate measurement of the coupled intervertebral motions is helpful for understanding the etiology and diagnosis of relevant diseases, and for assessing the subsequent treatment. No study has reported the in vivo, dynamic and three-dimensional (3D) intervertebral motion of the cervical spine during active axial rotation (AR) and lateral bending (LB) in the sitting position. The current study fills the gap by measuring the coupled intervertebral motions of the subaxial cervical spine in ten asymptomatic young adults in an upright sitting position during active head LB and AR using a volumetric model-based 2D-to-3D registration method via biplane fluoroscopy. Subject-specific models of the individual vertebrae were derived from each subject's CT data and were registered to the fluoroscopic images for determining the 3D poses of the subaxial vertebrae that were used to obtain the intervertebral kinematics. The averaged ranges of motion to one side (ROM) during AR at C3/C4, C4/C5, C5/C6, and C6/C7 were 4.2°, 4.6°, 3.0° and 1.3°, respectively. The corresponding values were 6.4°, 5.2°, 6.1° and 6.1° during LB. Intervertebral LB (ILB) played an important role in both AR and LB tasks of the cervical spine, experiencing greater ROM than intervertebral AR (IAR) (ratio of coupled motion (IAR/ILB): 0.23-0.75 in LB, 0.34-0.95 in AR). Compared to the AR task, the ranges of ILB during the LB task were significantly greater at C5/6 (p=0.008) and C6/7 (p=0.001) but the range of IAR was significantly smaller at C4/5 (p=0.02), leading to significantly smaller ratios of coupled motions at C4/5 (p=0.0013), C5/6 (p<0.001) and C6/7 (p=0.0037). The observed coupling characteristics of the intervertebral kinematics were different from those in previous studies under discrete static conditions in a supine position without weight-bearing, suggesting that the testing conditions likely affect the kinematics of the subaxial cervical spine. While C1 and C2 were not included owing to technical limitations, the current results nonetheless provide baseline data of the intervertebral motion of the subaxial cervical spine in asymptomatic young subjects under physiological conditions, which may be helpful for further investigations into spine biomechanics.