Three-dimensional motion analysis of the upper cervical spine during axial rotation

Dynamics of atlantoaxial rotation related to age and sex: a cross-sectional study of 308 subjects

BACKGROUND CONTEXT

Physiological ranges and dynamic changes of atlantoaxial rotation (ROTC1/2), total cervical spine rotation (ROTCs) and the percentage of ROTC1/2 from ROTCs (ROTCperc) for different age groups have not yet been investigated in a sufficiently sized cohort. Furthermore, it is not clear whether demographic variables such a sex, smoking status or diabetes affect ROTC1/2, ROTCs and ROTCperc.

PURPOSE

Obtain physiological ranges of ROTC1/2, ROTCs and ROTCperc for different age groups and determine their age-based dynamics. Investigate whether ROTC1/2, ROTCs and ROTCperc are affected by sex, smoking status or diabetes.

DESIGN

Observational cross-sectional study.

PATIENT SAMPLE

Patients undergoing elective CT examinations of the head and neck region between August 2020 and January 2022.

OUTCOME MEASURES

Ranges of motion of ROTC1/2, ROTCs and ROTCperc in degrees.

METHODS

A total of 308 subjects underwent dynamic rotational CT examinations of the upper cervical spine. Patients were divided into three age categories A1 (27-49 years), A2 (50-69 years) and A3 (≥70 years). Category A3 was further divided into B1 (70-79 years) and B2 (≥80 years). Values of ROTC1/2, ROTCs and ROTCperc were compared between all age groups, males and females, smokers and nonsmokers, diabetics a nondiabetics. Dynamics of ROTC1/2, ROTCs related to age and sex were visualized using scatterplot and trendline models.

RESULTS

ROTC1/2 significantly decreased from group A1 (64.4°) to B2 (46.7°) as did ROTCs from A1 (131.2°) to B2 (97.6°). No significant differences of ROTperc were found between groups A1-B2 with values oscillating between 49% and 51%. Smoking and diabetes did not significantly affect ROTC1/2, ROTCs and ROTCperc, females had significantly higher ROTCs than males. Males and females demonstrated a different dynamic of ROTC1/2 and ROTCs demonstrated by out scatterplot and trendline models.

CONCLUSIONS

Both ROTC1/2 and ROTCs significantly decrease with age, whereas ROTCperc remains stable. Females demonstrated higher ROTCs and their decrease of ROTC1/2 and ROTCs occurred in higher age groups compared to males. The functional repercussions atlantoaxial fusion are variable based on patient age and sex and should be taken into account prior to surgery.

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.

Craniocervical Junction Visualization and Radiation Dose Consideration Utilizing Cone Beam Computed Tomography for Upper Cervical Chiropractic Clinical Application a Literature Review

Objectives

To highlight the detail obtained on a Cone Beam Computed Tomography (CBCT) scan of the craniocervical junction and its usefulness to Chiropractors who specialize in the upper cervical spine. A review of the dose considerations to patients vs radiography in a chiropractic clinical setting and to review the effective radiation dose to the patient.

Methods

A review of studies discussing cervical biomechanics, neurovascular structures, and abnormal radiographic findings, was discussed in relation to chiropractic clinical relevance. Further studies were evaluated demonstrating radiation dose to the patient from radiographs compared to CBCT.

Results

Incidental and abnormal findings of the craniocervical junction were shown to have superior visualization with CBCT compared to radiography. The radiation dose to the patient for similar imaging protocols to the craniocervical junction and cervical spine was equal or less utilizing CBCT when compared to radiographs.

Conclusions

The use of CBCT for visualization of the craniocervical junction and cervical spine in the chiropractic clinical setting allows for adjunctive visualization of the osseous structures which is germane to clinical protocol. Further with CBCT the effective dose to the patient is equal or less than similar imaging protocols utilizing radiographs to evaluate the craniocervical junction.

Kinematics of the Cervical Spine Under Healthy and Degenerative Conditions: A Systematic Review

Knowledge of spinal kinematics is essential for the diagnosis and management of spinal diseases. Distinguishing between physiological and pathological motion patterns can help diagnose these diseases, plan surgical interventions and improve relevant tools and software. During the last decades, numerous studies based on diverse methodologies attempted to elucidate spinal mobility in different planes of motion. The authors aimed to summarize and compare the evidence about cervical spine kinematics under healthy and degenerative conditions. This includes an illustrated description of the spectrum of physiological cervical spine kinematics, followed by a comparable presentation of kinematics of the degenerative cervical spine. Data was obtained through a systematic MEDLINE search including studies on angular/translational segmental motion contribution, range of motion, coupling and center of rotation. As far as the degenerative conditions are concerned, kinematic data regarding disc degeneration and spondylolisthesis were available. Although the majority of the studies identified repeating motion patterns for most motion planes, discrepancies associated with limited sample sizes and different imaging techniques and/or spine configurations, were noted. Among healthy/asymptomatic individuals, flexion extension (FE) and lateral bending (LB) are mainly facilitated by the subaxial cervical spine. C4-C5 and C5-C6 were the major FE contributors in the reported studies, exceeding the motion contribution of sub-adjacent segments. Axial rotation (AR) greatly depends on C1-C2. FE range of motion (ROM) is distributed between the atlantoaxial and subaxial segments, while AR ROM stems mainly from the former and LB ROM from the latter. In coupled motion rotation is quantitatively predominant over translation. Motion migrates caudally from C1-C2 and the center of rotation (COR) translocates anteriorly and superiorly for each successive subaxial segment. In degenerative settings, concurrent or subsequent lesions render the association between diseases and mobility alterations challenging. The affected segments seem to maintain translational and angular motion in early and moderate degeneration. However, the progression of degeneration restrains mobility, which seems to be maintained or compensated by adjacent non-affected segments. While the kinematics of the healthy cervical spine have been addressed by multiple studies, the entire nosological and kinematic spectrum of cervical spine degeneration is partially addressed. Large-scale in vivo studies can complement the existing evidence, cover the gaps and pave the way to technological and clinical breakthroughs.

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.

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.

In vivo primary and coupled segmental motions of the healthy female head-neck complex during dynamic head axial rotation

While previous studies have greatly improved our knowledge on the motion capability of the cervical spine, few reported on the kinematics of the entire head-neck complex (C0-T1) during dynamic activities of the head in the upright posture. This study investigated in vivo kinematics of the entire head-neck complex (C0-T1) of eight female asymptomatic subjects during dynamic left-right head axial rotation using a dual fluoroscopic imaging system and 3D-to-2D registration techniques. During one-sided head rotation (i.e., left or right head rotation), the primary rotation of the overall head-neck complex (C0-T1) reached 55.5 ± 10.8°, the upper cervical spine region (C0-2) had a primary axial rotation of 39.7 ± 9.6° (71.3 ± 8.5% of the overall C0-T1 axial rotation), and the lower cervical spine region (C2-T1) had a primary rotation of 10.0 ± 3.7° (18.6 ± 7.2% of the overall C0-T1 axial rotation). Coupled bending rotations occurred in the upper and lower cervical spine regions in similar magnitude but opposite directions (upper: contralateral bending of 18.2 ± 5.9° versus lower: ipsilateral bending of 21.4 ± 5.1°), resulting in a compensatory cervical lateral curvature that balances the head to rotate horizontally. Furthermore, upper cervical segments (C0-1 or C1-2) provided main mobility in different rotational degrees of freedom needed for head axial rotations. Additionally, we quantitatively described both coupled segmental motions (flexion-extension and lateral bending) by correlation with the overall primary axial rotation of the head-neck complex. This investigation offers comprehensive baseline data regarding primary and coupled motions of craniocervical segments during head axial rotation.

[A case of proximal-type Hirayama disease associated with neck axial rotation]

Hirayama disease is characterized by juvenile onset of unilateral muscular atrophy of a distal upper extremity. The pathogenic mechanism of Hirayama disease is cervical cord compression by the posterior dura with forward displacement in the neck flexion position. A few cases of 'proximal-type Hirayama disease' have been described as showing muscular weakness and atrophy of the proximal upper extremities caused by the pathogenic mechanism similar to that of Hirayama disease. We report herein the case of a 16-year-old boy with proximal-type Hirayama disease, who developed symptoms after he began kyudo (Japanese traditional archery). Neurological examination revealed bilateral weakness of the muscles innervated by C5 and C6 segments (the deltoid, biceps brachii, brachioradialis), bilateral mild sensory disturbance in the radial side of the forearm, absent tendon reflexes of the biceps brachii and brachioradialis with preserved triceps reflex, pyramidal signs of the bilateral lower extremities (pathologically brisk reflexes of lower extremities, Babinski's signs). MR images in the neck flexion position showing expansion of the posterior extradural space and forward displacement of the spinal cord at the C3/4, C4/5, C5/6 and C6/7 disk levels. CT myelogram revealed spinal cord compression not only in neck flexion but also in neck left axial rotation. His symptoms improved after the restriction of neck flexion and axial rotation. Weakness of the upper extremities improved after 2 months. Pyramidal signs of the lower extremities disappeared after 18 months. The pathogenic mechanism in this case may be associated with not only neck flexion but also neck axial rotation.

Intervertebral range of motion characteristics of normal cervical spinal segments (C0-T1) during in vivo neck motions

The in vivo intervertebral range of motion (ROM) is an important predictor for spinal disorders. While the subaxial cervical spine has been extensively studied, the motion characteristics of the occipito-atlantal (C0-1) and atlanto-axial (C1-2) cervical segments were less reported due to technical difficulties in accurate imaging of these two segments. In this study, we investigated the intervertebral ROMs of the entire cervical spine (C0-T1) during in vivo functional neck motions of asymptomatic human subjects, including maximal flexion-extension, left-right lateral bending, and left-right axial torsion, using previously validated dual fluoroscopic imaging and model registration techniques. During all neck motions, C0-1, similar to C7-T1, was substantially less mobile than other segments and always contributed less than 10% of the cervical rotations. During the axial rotation of the neck, C1-2 contributed 73.2 ± 17.3% of the cervical rotation, but each of other segments contributed less than 10% of the cervical rotation. During both lateral bending and axial torsion neck motions, regardless of primary or coupled motions, the axial torsion ROM of C1-2 was significantly greater than its lateral bending ROM (p < 0.001), whereas the opposite differences were consistently observed at subaxial segments. This study reveals that there are distinct motion patterns at upper and lower cervical segments during in vivo neck motions. The reported data could be useful for the development of new diagnosis methods of cervical pathologies and new surgical techniques that aim to restore normal cervical segmental motion.

Can the Etiopathogenesis of Chiari Malformation Be Craniocervical Junction Stabilization Difference? Morphometric Analysis of Craniocervical Junction Ligaments

OBJECTIVE

The craniocervical junction permits a certain amount of mobility for the cervical spine. The biomechanical properties of occipital bone-atlas joint mainly depend on the bony structure, and atlas-axis joint biomechanical properties mainly depend on ligamentous structure. The underlying etiologic factor of Chiari malformation (CM) is debatable. Nowadays, some researchers argue that stabilization difference is one of the suspicious factors for etiopathogenesis. We aim to analyze the ligamentous morphometric differences of CM.

METHODS

Magnetic resonance images of 93 adult healthy subjects (n = 93) without any craniocervical junction development abnormalities and 25 (n = 25) adult patients with craniocervical junction development abnormalities (Arnold CM) were evaluated. Length, width, and length-width ratios of ligaments were evaluated.

RESULTS

Length of transverse ligament (mean: 23 ± 3.6 [range: 12.1-31.4]) in the normal population was significantly longer than transverse ligament length in CM patients (mean: 21.3 ± 2.5 [range: 17.2-24.9]). Length of alar ligament (mean: 10.7 ± 2 [range: 5.1-15.4]) in the normal population was significantly longer than alar ligament length in CM patients (mean: 8.8 ± 3.8 [range: 1.1-16.6]) (P = 0.007).

CONCLUSIONS

Craniocervical ligaments play an important role in maintaining stability and motion capacity of this region. This study promoted better understanding of craniocervical junction anomalies and provided data that facilitate performing more precise surgical treatment.

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.

Neck function in early hominins and suspensory primates: Insights from the uncinate process

OBJECTIVES

Uncinate processes are protuberances on the cranial surface of subaxial cervical vertebrae that assist in stabilizing and guiding spinal motion. Shallow uncinate processes reduce cervical stability but confer an increased range of motion in clinical studies. Here we assess uncinate processes among extant primates and model cervical kinematics in early fossil hominins.

MATERIALS AND METHODS

We compare six fossil hominin vertebrae with 48 Homo sapiens and 99 nonhuman primates across 20 genera. We quantify uncinate morphology via geometric morphometric methods to understand how uncinate process shape relates to allometry, taxonomy, and mode of locomotion.

RESULTS

Across primates, allometry explains roughly 50% of shape variation, as small, narrow vertebrae feature the relatively tallest, most pronounced uncinate processes, whereas larger, wider vertebrae typically feature reduced uncinates. Taxonomy only weakly explains the residual variation, however, the association between Uncinate Shape and mode of locomotion is robust, as bipeds and suspensory primates occupy opposite extremes of the morphological continuum and are distinguished from arboreal generalists. Like humans, Australopithecus afarensis and Homo erectus exhibit shallow uncinate processes, whereas A. sediba resembles more arboreal taxa, but not fully suspensory primates.

DISCUSSION

Suspensory primates exhibit the most pronounced uncinates, likely to maintain visual field stabilization. East African hominins exhibit reduced uncinate processes compared with African apes and A. sediba, likely signaling different degrees of neck motility and modes of locomotion. Although soft tissues constrain neck flexibility beyond limits suggested by osteology alone, this study may assist in modeling cervical kinematics and positional behaviors in extinct taxa.

An Exploratory Electromyography-Based Coactivation Index for the Cervical Spine

Objective Develop a coactivation index for the neck and test its effectiveness with complex dynamic head motions. Background Studies describing coactivation for the cervical spine are sparse in the literature. Of those in existence, they were either limited to a priori definitions of agonist/antagonist activity that limited the testing to sagittal and lateral planes or consisted of isometric exertions. Multiplanar movements would allow for a more realistic understanding of naturalistic movements in the cervical spine and propensity for neck pain. However, a gap in the literature exists in which a method to describe coactivation during complex dynamic motions does not exist for the cervical spine. Methods An electromyography-based coactivation index was developed for the cervical spine based on previously tested methodology used on the lumbar spine without a high-end model and tested using a series of different postures and speeds. Results Complex motions involving twisting (i.e., flexion and twisting) and higher speed had higher magnitudes of coactivation than uniplanar motions in the sagittal or lateral plane, which was expected. The coupled motion of flexion and twisting showed four to five times higher coactivation than uniplanar (sagittal or lateral) movements. Conclusion The coactivation index developed accommodates multiplanar, naturalistic movements. Testing of the index showed that motions requiring higher degrees of head control had higher effort due to coactivation, which was expected. Application Overall, this coactivation index may be utilized to understand the neuromuscular effort of various tasks in the cervical spine.

The application of a pre-positioned upper cervical traction mobilization to patients with painful active cervical rotation impairment: A case series

BACKGROUND

Cervical mobilization and manipulation have been shown to improve cervical range of motion and pain. Cervical rotatory thrust manipulation has been associated with adverse patient reaction and damage to the V3 segment of the vertebral artery (VA).

OBJECTIVE

To document and describe the effects of an upper cervical (UC) traction based mobilization on participants with restricted and painful cervical rotation and to document if the mobilization changed blood flow velocity through the vertebral artery.

METHODS

This case series examined the effects of a traction based spinal mobilization on two different groups of participants. Group I included 93 participants with restricted bilateral cervical rotation that was also painful at end range. Group II included 30 different participants whose VA blood flow velocity was examined during the same mobilization. Pre- and post-mobilization active cervical rotation, pain intensity levels, and VA blood flow velocity during mobilization was documented.

RESULTS

Paired T-tests were used to determine statistical significance for changes in cervical rotation, and VA blood flow velocity during mobilization. Ninety-three participants in group I demonstrated an average increase of 16 degrees of cervical rotation. No participant demonstrated an increase in pain, and no participant in group II (N= 30) demonstrated a change in VA blood flow velocity.

CONCLUSIONS

The application this UC traction based mobilization improved active cervical rotation, end range rotation pain response, did not cause pain during its application and did not alter blood flow through the VA during application.

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.

Longitudinal Study of the Six Degrees of Freedom Cervical Spine Range of Motion During Dynamic Flexion, Extension, and Rotation After Single-level Anterior Arthrodesis

STUDY DESIGN

A longitudinal study using biplane radiography to measure in vivo intervertebral range of motion (ROM) during dynamic flexion/extension, and rotation.

OBJECTIVE

To longitudinally compare intervertebral maximal ROM and midrange motion in asymptomatic control subjects and single-level arthrodesis patients.

SUMMARY OF BACKGROUND DATA

In vitro studies consistently report that adjacent segment maximal ROM increases superior and inferior to cervical arthrodesis. Previous in vivo results have been conflicting, indicating that maximal ROM may or may not increase superior and/or inferior to the arthrodesis. There are no previous reports of midrange motion in arthrodesis patients and similar-aged controls.

METHODS

Eight single-level (C5/C6) anterior arthrodesis patients (tested 7 ± 1 months and 28 ± 6 months postsurgery) and six asymptomatic control subjects (tested twice, 58 ± 6 months apart) performed dynamic full ROM flexion/extension and axial rotation whereas biplane radiographs were collected at 30 images per second. A previously validated tracking process determined three-dimensional vertebral position from each pair of radiographs with submillimeter accuracy. The intervertebral maximal ROM and midrange motion in flexion/extension, rotation, lateral bending, and anterior-posterior translation were compared between test dates and between groups.

RESULTS

Adjacent segment maximal ROM did not increase over time during flexion/extension, or rotation movements. Adjacent segment maximal rotational ROM was not significantly greater in arthrodesis patients than in corresponding motion segments of similar-aged controls. C4/C5 adjacent segment rotation during the midrange of head motion and maximal anterior-posterior translation were significantly greater in arthrodesis patients than in the corresponding motion segment in controls on the second test date.

CONCLUSION

C5/C6 arthrodesis appears to significantly affect midrange, but not end-range, adjacent segment motions. The effects of arthrodesis on adjacent segment motion may be best evaluated by longitudinal studies that compare maximal and midrange adjacent segment motion to corresponding motion segments of similar-aged controls to determine if the adjacent segment motion is truly excessive.

LEVEL OF EVIDENCE

3.

Narrative review of the in vivo mechanics of the cervical spine after anterior arthrodesis as revealed by dynamic biplane radiography

Arthrodesis is the standard of care for numerous pathologic conditions of the cervical spine and is performed over 150,000 times annually in the United States. The primary long-term concern after this surgery is adjacent segment disease (ASD), defined as new clinical symptoms adjacent to a previous fusion. The incidence of adjacent segment disease is approximately 3% per year, meaning that within 10 years of the initial surgery, approximately 25% of cervical arthrodesis patients require a second procedure to address symptomatic adjacent segment degeneration. Despite the high incidence of ASD, until recently, there was little data available to characterize in vivo adjacent segment mechanics during dynamic motion. This manuscript reviews recent advances in our knowledge of adjacent segment mechanics after cervical arthrodesis that have been facilitated by the use of dynamic biplane radiography. The primary observations from these studies are that current in vitro test paradigms often fail to replicate in vivo spine mechanics before and after arthrodesis, that intervertebral mechanics vary among cervical motion segments, and that joint arthrokinematics (i.e., the interactions between adjacent vertebrae) are superior to traditional kinematics measurements for identifying altered adjacent segment mechanics after arthrodesis. Future research challenges are identified, including improving the biofidelity of in vitro tests, determining the natural history of in vivo spine mechanics, conducting prospective longitudinal studies on adjacent segment kinematics and arthrokinematics after single and multiple-level arthrodesis, and creating subject-specific computational models to accurately estimate muscle forces and tissue loading in the spine during dynamic activities.

Anatomy and biomechanics of the craniovertebral junction

The craniovertebral junction (CVJ) has unique anatomical structures that separate it from the subaxial cervical spine. In addition to housing vital neural and vascular structures, the majority of cranial flexion, extension, and axial rotation is accomplished at the CVJ. A complex combination of osseous and ligamentous supports allow for stability despite a large degree of motion. An understanding of anatomy and biomechanics is essential to effectively evaluate and address the various pathological processes that may affect this region. Therefore, the authors present an up-to-date narrative review of CVJ anatomy, normal and pathological biomechanics, and fixation techniques.

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.

Posterior C1-C2 screw and rod instrument for reduction and fixation of basilar invagination with atlantoaxial dislocation

PURPOSE

To report the surgical technique and preliminary clinical results for the treatment of basilar invagination (BI) with atlantoaxial dislocation (AAD) by posterior C1-C2 pedicle screw and rod instrument.

METHODS

Between July 2012 and August 2013, 33 patients who had BI with AAD underwent surgery at our institution. Pre and postoperative three-dimensional computed tomographic (CT) scans were performed to assess the degree of dislocation. Magnetic resonance (MR) imaging was used to evaluate the compression of the medulla oblongata. For all patients, reduction of the AAD was conducted by two steps: fastening nuts and rods was performed to achieve the horizontal reduction. Distraction between C1 and C2 screws was performed to obtain the vertical reduction.

RESULTS

No neurovascular injury occurred during surgery. Follow-up ranged from 6 to 15 months (mean 10.38 months) in 32 patients. Post-operative three-dimensional CT showed that complete horizontal reduction was obtained in 30/33 (90.9%), and complete vertical reduction was obtained in 31/33 (93.9%). The repeated three-dimensional CT and MR image demonstrated that bony fusion and the decompression of the medulla oblongata were obtained in all patients. Clinical symptoms improved significantly 3 months after surgery.

CONCLUSIONS

This C1-C2 pedicle screw and rod instrument is a promising technique for the treatment of BI with AAD.

Validation protocol for assessing the upper cervical spine kinematics and helical axis: An in vivo preliminary analysis for axial rotation, modeling, and motion representation

Context:

The function of the upper cervical spine (UCS) is essential in the kinematics of the whole cervical spine. Specific motion patterns are described at the UCS during head motions to compensate coupled motions occurring at the lower cervical segments.

Aims:

First, two methods for computing in vitro UCS discrete motions were compared to assess three-dimensional (3D) kinematics. Secondly, the same protocol was applied to assess the feasibility of the procedure for in vivo settings. Also, this study attempts to expose the use of anatomical modeling for motion representation including helical axis.

Settings and Design:

UCS motions were assessed to verify the validity of in vitro 3D kinematics and to present an in vivo procedure for evaluating axial rotation.

Materials and Methods:

In vitro kinematics was sampled using a digitizing technique and computed tomography (CT) for assessing 3D motions during flexion extension and axial rotation. To evaluate the feasibility of this protocol in vivo, one asymptomatic volunteer performed an MRI kinematics evaluation of the UCS for axial rotation. Data processing allowed integrating data into UCS 3D models for motion representation, discrete joint behavior, and motion helical axis determination.

Results:

Good agreement was observed between the methods with angular displacement differences ranging from 1° to 1.5°. Helical axis data were comparable between both methods with axis orientation differences ranging from 3° to 6°. In vivo assessment of axial rotation showed coherent kinematics data compared to previous studies. Helical axis data were found to be similar between in vitro and in vivo evaluation.

Conclusions:

The present protocol confirms agreement of methods and exposes its feasibility to investigate in vivo UCS kinematics. Moreover, combining motion analysis, helical axis representation, and anatomical modeling, constitutes an innovative development to provide new insights for understanding motion behaviors of the UCS.

Normative data on axial rotation of atlanto-occipital joint on 3 Tesla MRI using a simple and reliable method of calculation

BACKGROUND

Various methods have been used to image and measure the normal range of axial rotation of the atlanto-occipital joint (AOJ), but a simple, precise, and reliable method is needed for everyday practice.

PURPOSE

To generate normative ranges for AOJ rotation in various in-vivo positions and to investigate the reliability of a simple imaging method for measurement using routine high-field magnetic resonance imaging (MRI).

MATERIAL AND METHODS

One hundred healthy volunteers were imaged on 3 T MRI with the AOJ in the center of the field of view. The scans were uniformly performed in seven different positions. The range of axial rotation was calculated by the angle between the craniofacial midline and the line linking the anterior and posterior tubercles of the atlas. The angle was defined as positive when it was angled right, and negative when it was angled left. The actual normative range of axial rotation was the difference between the angle in the supine neutral position and in the other positions.

RESULTS

The normative axial rotation range of the AOJ in different positions was between -4.8° and +5.0°. The mean values of the actual rotation angles in the right supine position with maximum bending, the right supine position maximum rotation, and the right prostrate position maximum rotation were 0.1°, 1.70°, and 0.8°, respectively. The mean values of actual rotation angles in the left supine position with maximum bending, the left supine position with maximum rotation, and the left prostrate positive with maximum rotation were 0.1°, -1.7°, and -1.1°, respectively. The inter-observer reliability tested.

CONCLUSION

A simple and reliable method of measurement on 3.0 T MRI demonstrated the normative axial rotation range of the AOJ in different positions to be between -4.8° and +5.0° and it was different from zero in neutral rotation. This method could be practically used to precisely diagnose AOJ rotary subluxation or dislocation.

Relationship between biomechanical changes at adjacent segments and number of fused bone grafts in multilevel cervical fusions: a finite element investigation

OBJECT

Biomechanical studies have shown that anterior cervical fusion construct stiffness and arthrodesis rates vary with different reconstruction techniques; however, the behavior of the adjacent segments in the setting of different procedures is poorly understood. This study was designed to investigate the adjacent-segment biomechanics after 3 different anterior cervical decompression and fusion techniques, including 3-level discectomy and fusion, 2-level corpectomy and fusion, and a corpectomy-discectomy hybrid technique. The authors hypothesized that biomechanical changes at the segments immediately superior and inferior to the multilevel fusion would be inversely proportional to the number of fused bone grafts and that these changes would be related to the type of fusion technique.

METHODS

A previously validated 3D finite element model of an intact C3-T1 segment was used. Three C4-7 fusion models were built from this intact model by varying the number of bone grafts used to span the decompression: a 1-graft model (2-level corpectomy), a 2-graft model (C-5 corpectomy and C6-7 discectomy), and a 3-graft model (3-level discectomy). The corpectomy and discectomy models were also previously validated and compared well with the literature findings. Range of motion, disc stresses, and posterior facet loads at the segments superior (C3-4) and inferior (C7-T1) to the fusion construct were assessed.

RESULTS

Motion, disc stresses, and posterior facet loads generally increased at both of the adjacent segments in relation to the intact model. Greater biomechanical changes were noted in the superior C3-4 segment than in the inferior C7-T1 segment. Increasing the number of bone grafts from 1 to 2 and from 2 to 3 was associated with a lower magnitude of biomechanical changes at the adjacent segments.

CONCLUSIONS

At segments adjacent to the fusion level, biomechanical changes are not limited solely to the discs, but also propagate to the posterior facets. These changes in discs and posterior facets were found to be lower for discectomy than for corpectomy, thereby supporting the current study hypothesis of inverse relationship between the adjacent-segment variations and the number of fused bone grafts. Such changes may go on to influence the likelihood of adjacent-segment degeneration accordingly. Further studies are warranted to identify the causes and true impact of these observed changes.

Alterations in axial curvature of the cervical spine with a combination of rotation and extension in the conventional anterior cervical approach

PURPOSE

Alterations of three-dimensional cervical curvature in conventional anterior cervical approach position are not well understood. The purpose of this study was to evaluate alignment changes of the cervical spine in the position. In addition, simulated corpectomy was evaluated with regard to sufficiency of decompression and perforation of the vertebral artery canal.

METHODS

Fifty patients with cervical spinal disorders participated. Cervical CT scanning was performed in the neutral and supine position (N-position) and in extension and right rotation simulating the conventional anterior approach position (ER-position). Rotation at each vertebral level was measured. With simulation of anterior corpectomy in a vertical direction with a width of 17 mm, decompression width at the posterior wall of the vertebrae and the distance from each foramen of the vertebral artery (VA) were measured.

RESULTS

In the ER-position, the cervical spine was rotated rightward by 37.2° ± 6.2° between the occipital bone and C7. While the cervical spine was mainly rotated at C1/2, the subaxial vertebrae were also rotated by several degrees. Due to the subaxial rotation, the simulated corpectomy resulted in smaller decompression width on the left side and came closer to the VA canal on the right side.

CONCLUSIONS

In the ER-position, the degrees of right rotation of subaxial vertebrae were small but significant. Therefore, preoperative understanding of this alteration of cervical alignment is essential for performing safe and sufficient anterior corpectomy of the cervical spine.

Biomechanics of adjacent segments after a multilevel cervical corpectomy using anterior, posterior, and combined anterior-posterior instrumentation techniques: a finite element model study

BACKGROUND CONTEXT

Adjacent segment degeneration (ASD) after cervical fusion is a clinical concern. Despite previous studies documenting the biomechanical effects of multilevel cervical fusion on segments immediately superior and inferior to the operative segments, the pathogenesis of the initiation of degeneration progression in neighboring segments is still poorly understood.

PURPOSE

To test the hypothesis that changes in range of motion, disc stresses, and facet loads would be highest at the superior adjacent segment (C3-C4) after anterior C4-C7 corpectomy and fusion and that these changes would be the least in anterior fixation and the greatest in posterior or combined anterior-posterior instrumentation techniques.

STUDY DESIGN

A finite element (FE) analysis of adjacent vertebral segment biomechanics after a two-level corpectomy fusion with three different fixation techniques (anterior, posterior, and combined anterior-posterior).

METHODS

A previously validated three-dimensional FE model of an intact C3-T1 segment was used. From this intact model, three additional instrumentation models were constructed using anterior (rigid screw-plate), posterior (rigid screw-rod), and combined anterior-posterior fixation techniques after a C4-C7 corpectomy and fusion. Motion patterns, disc stresses, and posterior facet loads at the levels cephalad and caudal to the fusion were assessed.

RESULTS

Range of motion, disc stresses, and posterior facet loads increased at the adjacent segments. Use of posterior fixation, whether alone or in combination with anterior fixation, infers higher changes in segmental motion, disc stresses, and posterior facet loads at adjacent segments compared with the use of anterior fixation alone. The superior C3-C4 motion was most affected during lateral bending and the inferior C7-T1 motion was most affected during flexion, whereas both superior C3-C4 and inferior C7-T1 motions were least affected during extension. However, disc stresses and facet loads were most affected during extension. Hence, it is speculated that the most remodeling changes in discs and facets might be related to the least changes in extension motion.

CONCLUSIONS

Biomechanical factors such as increased mechanical demand and motion that have been associated with the development of ASD progression are highest in the segment immediately superior to the fusion. These changes are even more pronounced when the fixation technique involves the addition of posterior instrumentation, thereby supporting the hypothesis of the present study. Increased degrees of stiffening of the fused segments not only may lead to degenerative changes in the disc but may also predispose the segments to premature facet degeneration. Over subsequent time period, any remaining construct micro-motion is further eliminated with fusion of the posterior facet joints and the remaining regions in the disc space also filled in with bone, which eventually results in a circumferential type of fusion. After a circumferential fusion, authors, however, speculate that the role of instrumentation in ASD progression might not be significant. In fact, sufficient evidence to support this speculation is still lacking in the literature.

Six-degrees-of-freedom cervical spine range of motion during dynamic flexion-extension after single-level anterior arthrodesis: comparison with asymptomatic control subjects

BACKGROUND

The etiology of adjacent-segment disease following cervical spine arthrodesis remains controversial. The objective of the current study was to evaluate cervical intervertebral range of motion during dynamic flexion-extension in patients who had undergone a single-level arthrodesis and in asymptomatic control subjects.

METHODS

Ten patients who had undergone a single-level (C5/C6) anterior arthrodesis and twenty asymptomatic control subjects performed continuous full range-of-motion flexion-extension while biplane radiographs were collected at thirty images per second. A previously validated tracking process determined three-dimensional vertebral position on each pair of radiographs with submillimeter accuracy. Six-degrees-of-freedom kinematics between adjacent vertebrae were calculated throughout the entire flexion-extension movement cycle over multiple trials for each participant. Cervical kinematics were also calculated from images collected during static full flexion and static full extension.

RESULTS

The C4/C5 motion segment moved through a larger extension range of motion and a smaller flexion range of motion in the subjects with the arthrodesis than in the controls. The extension difference between the arthrodesis and control groups was 3.8° (95% CI [confidence interval], 0.9° to 6.6°; p = 0.011) and the flexion difference was -2.9° (95% CI, -5.3° to -0.5°; p = 0.019). Adjacent-segment posterior translation was greater in the arthrodesis group than in the controls, with a C4/C5 difference of 0.8 mm (95% CI, 0.0 to 1.6 mm) and a C6/C7 difference of 0.4 mm (95% CI, 0.0 to 0.8 mm; p = 0.016). Translation range of motion and rotation range of motion were consistently larger when measured on images collected during dynamic functional movement as opposed to images collected at static full flexion or full extension. The upper 95% CI limit for anterior-posterior translation range of motion was 3.45 mm at C3/C4 and C4/C5, but only 2.3 mm at C6/C7.

CONCLUSIONS

C5/C6 arthrodesis does not affect the total range of motion in adjacent vertebral segments, but it does alter the distribution of adjacent-segment motion toward more extension and less flexion superior to the arthrodesis and more posterior translation superior and inferior to the arthrodesis during in vivo functional loading. Range of motion measured from static full-flexion and full-extension images underestimates dynamic range of motion. Clinical evaluation of excessive anterior-posterior translation should take into account the cervical vertebral level.

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

The cervical spine exhibits considerable mobility, especially in axial rotation. Axial rotation exerts stress on anatomical structures, such as the vertebral artery which is commonly assessed during clinical examination. The literature is rather sparse concerning the in vivo three-dimensional segmental kinematics of the cervical spine. This study aimed at investigating the three-dimensional kinematics of the cervical spine during maximal passive head rotation with special emphasis on coupled motion. Twenty healthy volunteers participated in this study. Low-dose CT scans were conducted in neutral and in maximum axial rotation positions. Each separated vertebra was segmented semi automatically in these two positions. The finite helical-axis method was used to describe 3D motion between discrete positions. The mean (±SD) maximum magnitude of axial rotation between C0 and C1 was 2.5 ± 1.0° coupled with lateral flexion to the opposite side (5.0 ± 3.0°) and extension (12.0 ± 4.5°). At the C1-C2 level, the mean axial rotation was 37.5 ± 6.0° associated with lateral flexion to the opposite side (2.5 ± 6.0°) and extension (4.0 ± 6.0°). For the lower levels, axial rotation was found to be maximal at C4-C5 level (5.5 ± 1.0°) coupled with lateral flexion to the same side (-4.0 ± 2.5°). Extension was associated at levels C2-C3, C3-C4 and C4-C5, whereas flexion occurred between C5-C6 and C6-C7. Coupled lateral flexion occurred to the opposite side at the upper cervical spine and to the same side at the lower cervical spine.

A geometrical model of vertical translation and alar ligament tension in atlanto-axial rotation

INTRODUCTION

While allowing the greatest range of axial rotation of the entire spine with 40° to each side, gradual restraint at the extremes of motion by the alar ligaments is of vital importance. In order for the ligaments to facilitate a gradual transition from the neutral to the elastic zone, a complex interaction of axial rotation and vertical translation via the biconvex articular surfaces is essential. The aim of this investigation is to establish a geometrical model of the intricate interaction of the alar ligaments and vertical translatory motion of C1/C2 in axial rotation.

METHODS

Bilateral alar ligaments including the odontoid process and condylar bony entheses were removed from six adult cadavers aged 65-89 years within 48 h of death. All specimens were judged to be free of abnormalities with the exception of non-specific degenerative changes. Dimensions of the odontoid process and alar ligaments were measured. Graphical multiplanar reconstruction of atlanto-axial rotation was done in the transverse and frontal planes for the neutral position and for rotation to 40° with vertical translation of 3 mm. The necessary fibre elongation of the alar ligaments in the setting with and without vertical translation of the atlas was calculated.

RESULTS

The mean diameter of the odontoid process in the sagittal plane was 10.6 mm (SD 1.1). The longest fibre length was measured from the posterior border of the odontoid enthesis to the posterior border of the condylar enthesis with an average of 13.2 mm (SD 2.5) and the shortest between the lateral (anterior) border odontoid enthesis and the anterior condylar enthesis with an average of 8.2 mm (SD 2.2). In graphical multiplanar reconstruction of atlanto-axial rotation to 40° without vertical translation of C1/C2, theoretical alar fibre elongation reaches 27.1% for the longest fibres, which is incompatible with the collagenous structure of the alar ligaments. Allowing 3 mm caudal translation of C1 on C2 at 40° rotation, as facilitated by the biconvex atlanto-axial joints, reduces alar fibre elongation to 23.3%.

CONCLUSION

The biconvex configuration of the atlanto-axial joints is an integral feature of the functionality of upper cervical spine as it allows gradual vertical translation of the atlas against the axis during axial rotation, with gradual tensing of the alar ligaments. Vertical translation on its own, however, does not explain the tolerance of the alar ligaments towards the maximum of 40° of rotation and is most likely synergistic with the effects of the coupled motion of occipitocervical extension during rotation.

Validation of a noninvasive technique to precisely measure in vivo three-dimensional cervical spine movement

STUDY DESIGN

In vivo validation during functional loading.

OBJECTIVE

To determine the accuracy and repeatability of a model-based tracking technique that combines subject-specific computed tomographic (CT) models and high-speed biplane x-ray images to measure three-dimensional (3D) in vivo cervical spine motion.

SUMMARY OF BACKGROUND DATA

Accurate 3D spine motion is difficult to obtain in vivo during physiological loading because of the inability to directly attach measurement equipment to individual vertebrae. Previous measurement systems were limited by two-dimensional (2D) results and/or their need for manual identification of anatomical landmarks, precipitating unreliable and inaccurate results. All previous techniques lack the ability to capture true 3D motion during dynamic functional loading.

METHODS

Three subjects had 1.0-mm-diameter tantalum beads implanted into their fused and adjacent vertebrae during anterior cervical discectomy and fusion surgery. High-resolution CT scans were obtained after surgery and used to create subject-specific 3D models of each cervical vertebra. Biplane x-ray images were collected at 30 frames per second while the subjects performed flexion/extension and axial rotation movements 6 months after surgery. Individual bone motion, intervertebral kinematics, and arthrokinematics derived from dynamic radiostereophotogrammetric analysis served as a gold standard to evaluate the accuracy of the model-based tracking technique.

RESULTS

Individual bones were tracked with an average precision of 0.19 and 0.33 mm in nonfused and fused bones, respectively. Precision in measuring 3D joint kinematics in fused and adjacent segments averaged 0.4 mm for translations and 1.1° for rotations, while anterior and posterior disc height above and below the fusion were measured with a precision ranging between 0.2 and 0.4 mm. The variability in 3D joint kinematics associated with tracking the same trial repeatedly was 0.02 mm in translation and 0.06° in rotation.

CONCLUSION

The 3D cervical spine motion can be precisely measured in vivo with submillimeter accuracy during functional loading without the need for bead implantation. Fusion instrumentation did not diminish the accuracy of kinematic and arthrokinematic results. The semiautomated model-based tracking technique has excellent repeatability.

Simulation of inhomogeneous rather than homogeneous poroelastic tissue material properties within disc annulus and nucleus better predicts cervical spine response: a C3-T1 finite element model analysis under compression and moment loadings

STUDY DESIGN

A finite element (FE) modeling of homogeneous and inhomogeneous poroelastic tissue material properties within disc anulus fibrosus (AF) and nucleus pulposus (NP).

OBJECTIVE

To test the hypothesis that simulation of inhomogeneous poroelastic tissue material properties within AF and NP quadrants, rather than homogeneous properties within regions of AF and NP without quadrants, would better predict the cervical spine biomechanics.

SUMMARY OF BACKGROUND DATA

In order to represent tissue swelling and creep deformation behavior more physiologically in FE models, disc poroelastic tissue material properties should be modeled appropriately. Past studies show an existence of inhomogeneous rather than homogeneous nature of the tissue properties in various quadrants of AF and NP, and this has been simulated in a single-segment FE lumbar model with only compression analysis. This article simulated these tissue properties in a multisegmental cervical spine and reported the results of both compression and moment loads.

METHODS

Two three-dimensional FE models of a C3-T1 segment were developed. Model I included homogeneous poroelastic tissue properties in AF and NP, whereas Model II included inhomogeneous poroelastic tissue properties in AF and NP quadrants. Biomechanical responses of the FE models under diurnal compression and moment loads were compared with corresponding in vivo published studies.

RESULTS

Model II with disc quadrant-based inhomogeneous poroelastic tissue properties predicted better, mainly in flexion and extension, than the Model I with homogeneous tissue properties when compared with the corresponding in vivo results, thereby confirming the current study hypothesis. Inhomogeneous tissue properties govern segmental behavior mainly during sagittal plane motions, with a root-mean-square difference of nearly 50% across the motion segments.

CONCLUSION

The current data justify the need to simulate inhomogeneous tissue properties within disc quadrants for any FE model analysis. Model II can be further used to understand the biomechanical effects of quadrant-based degenerative poroelastic tissue properties on cervical spine behavior. Future experiments are necessary to support the current study results.

Relative contributions of strain-dependent permeability and fixed charged density of proteoglycans in predicting cervical disc biomechanics: a poroelastic C5-C6 finite element model study

Disc swelling pressure (P(swell)) facilitated by fixed charged density (FCD) of proteoglycans (P(fcd)) and strain-dependent permeability (P(strain)) are of critical significance in the physiological functioning of discs. FCD of proteoglycans prevents any excessive matrix deformation by tissue stiffening, whereas strain-dependent permeability limits the rate of stress transfer from fluid to solid skeleton. To date, studies involving the modeling of FCD of proteoglycans and strain-dependent permeability have not been reported for the cervical discs. The current study objective is to compare the relative contributions of strain-dependent permeability and FCD of proteoglycans in predicting cervical disc biomechanics. Three-dimensional finite element models of a C5-C6 segment with three different disc compositions were analyzed: an SPFP model (strain-dependent permeability and FCD of proteoglycans), an SP model (strain-dependent permeability alone), and an FP model (FCD of proteoglycans alone). The outcomes of the current study suggest that the relative contributions of strain-dependent permeability and FCD of proteoglycans were almost comparable in predicting the physiological behavior of the cervical discs under moment loads. However, under compression, strain-dependent permeability better predicted the in vivo disc response than that of the FCD of proteoglycans. Unlike the FP model (least stiff) in compression, motion behavior of the three models did not vary much from each other and agreed well within the standard deviations of the corresponding in vivo published data. Flexion was recorded with maximum P(fcd) and P(strain), whereas minimum values were found in extension. The study data enhance the understanding of the roles played by the FCD of proteoglycans and strain-dependent permeability and porosity in determining disc tissue swelling behavior. Degenerative changes involving strain-dependent permeability and/or loss of FCD of proteoglycans can further be studied using an SPFP model. Future experiments are necessary to support the current study results.

Reduction in segmental flexibility because of disc degeneration is accompanied by higher changes in facet loads than changes in disc pressure: a poroelastic C5-C6 finite element investigation

BACKGROUND CONTEXT

Nerve fiber growth inside the degenerative intervertebral discs and facets is thought to be a source of pain, although there may be several other pathological and clinical reasons for the neck pain. It, however, remains difficult to decipher how much disc and facet joints contribute to overall degenerative segmental responses. Although the biomechanical effects of disc degeneration (DD) on segmental flexibility and posterior facets have been reported in the lumbar spine, a clear understanding of the pathways of degenerative progression is still lacking in the cervical spine.

PURPOSE

To test the hypothesis that after an occurrence of degenerative disease in a cervical disc, changes in the facet loads will be higher than changes in the disc pressure.

STUDY DESIGN

To understand the biomechanical relationships between segmental flexibility, disc pressure, and facet loads when the C5-C6 disc degenerates.

METHODS

A poroelastic, three-dimensional finite element (FE) model of a normal C5-C6 segment was developed and validated. Two degenerated disc models (moderate and severe) were built from the normal disc model. Biomechanical responses of the three FE models (normal, moderate, and severe) were further studied under diurnal compression (at the end of the daytime activity period) and moment loads (at the end of 5 seconds) in terms of disc height loss, angular motions, disc pressure, and facet loads (average of right and left facets).

RESULTS

Disc deformation under compression and segmental rotational motions under moment loads for the normal disc model agreed well with the corresponding in vivo studies. A decrease in segmental flexibility because of DD is accompanied by a decrease in disc pressure and an increase in facet loads. Biomechanical effects of degenerative disc changes are least in flexion. Segmental flexibility changes are higher in extension, whereas changes in disc pressure and facet loads are higher in lateral bending and axial rotation, respectively.

CONCLUSIONS

The results of the present study confirmed the hypothesis of higher changes in facet loads than in disc pressure, suggesting posterior facets are more affected than discs because of a decrease in degenerative segmental flexibility. Therefore, a degenerated disc may increase the risk of overloading the posterior facet joints. It should be clearly noted that only after degeneration simulation in the disc, we recorded the biomechanical responses of the facets and disc. Therefore, our hypothesis does not suggest that facet joint osteoarthritis may occur before degeneration in the disc. Future cervical spine-based experiments are warranted to verify the conclusions presented in this study.

Axial head rotation increases facet joint capsular ligament strains in automotive rear impact

Axial head rotation prior to low speed automotive rear impacts has been clinically identified to increase morbidity and symptom duration. The present study was conducted to determine the effect of axial head rotation on facet joint capsule strains during simulated rear impacts. The study was conducted using a validated intact head to first thoracic vertebra (T1) computational model. Parametric analysis was used to assess effects of increasing axial head rotation between 0 and 60° and increasing impact severity between 8 and 24 km/h on facet joint capsule strains. Rear impacts were simulated by horizontally accelerating the T1 vertebra. Characteristics of the acceleration pulse were based on the horizontal T1 acceleration pulse from a series of simulated rear impact experiments using full-body post mortem human subjects. Joint capsule strain magnitudes were greatest in ipsilateral facet joints for all simulations incorporating axial head rotation (i.e., head rotation to the left caused higher ligament strain at the left facet joint capsule). Strain magnitudes increased by 47-196% in simulations with 60° head rotation compared to forward facing simulations. These findings indicate that axial head rotation prior to rear impact increases the risk of facet joint injury.

Motion changes in adjacent segments due to moderate and severe degeneration in C5-C6 disc: a poroelastic C3-T1 finite element model study

STUDY DESIGN

Biomechanics of normal vertebral segments adjacent to a degenerated segment in the cervical spine.

OBJECTIVE

To test the hypothesis of higher motion changes in the segment immediately inferior to a degenerated segment.

SUMMARY OF BACKGROUND DATA

Past research has shown how disc degeneration (DD) affects adjacent segments; however, these studies are conducted only on the lumbar spine or the experimental protocols used are characterized by the presence of degeneration in adjacent segments. The question arises as to how much of the degenerative effect in a particular segment is internal to degeneration at that segment and how much is caused by degeneration at adjacent segments. It would be clinically relevant to analyze biomechanical changes in adjacent segments in the cervical spine by considering DD at only one segment, where adjacent segments remain normal.

METHODS

A poroelastic, 3-dimensional finite element model of a normal C3-T1 segment was validated and then used for the degenerative study. Two additional C3-T1 models were developed with moderate and severe degenerative C5-C6 disc grades. Disc geometry and tissue material properties were modified to simulate C5-C6 DD. Intersegmental rotational motions (C4-C5, C5-C6, and C6-C7) for the 3 C3-T1 models were computed under moment loads.

RESULTS

With progressive C5-C6 DD, motion decreased at that segment. At adjacent segments, higher motion changes were observed mainly in flexion/extension. Inferior C6-C7 motion changes were higher than superior C4-C5 motion changes. The inferior C6-C7 motion was affected even when C5-C6 DD was moderate, and it was further affected by severe C5-C6 DD. The superior C4-C7 motion, however, was mostly affected by severe C5-C6 DD.

CONCLUSION

The hypothesis of higher motion changes in the normal C6-C7 segment immediately inferior to a degenerated C5-C6 segment was found to be true. Future experiments on multisegmental cervical spines are recommended to verify the current data.

Craniovertebral Junction

THE CRANIOVERTEBRAL JUNCTION is a complex region that incorporates the occiput–C1–C2 portions of the spine. It is a transition between the cranium and the mobile cervical spine that permits significant motion. The motions afforded and the anatomy are vastly different at the occiput–C1 and C1–C2 articulations. These differences make treating pathology in this region very difficult. Problems include bony fixation of the cranium and the cervical spine (specifically C1 and C2), which limits complex motions, and limited bony sites available for arthrodesis. A thorough knowledge of the normal anatomy and biomechanics is required for fixation of this region. Moreover, an understanding of pathologic motions and the biomechanics of fixation is needed for successful construct design and good patient outcome.

Screw angulation affects bone-screw stresses and bone graft load sharing in anterior cervical corpectomy fusion with a rigid screw-plate construct: a finite element model study

BACKGROUND CONTEXT

Anterior corpectomy and reconstruction with bone graft and a rigid screw-plate construct is an established procedure for treatment of cervical neural compression. Despite its reliability in relieving symptoms, there is a high rate of construct failure, especially in multilevel cases.

PURPOSE

There has been no study evaluating the biomechanical effects of screw angulation on construct stability; this study investigates the C4-C7 construct stability and load-sharing properties among varying screw angulations in a rigid plate-screw construct.

STUDY DESIGN

A finite element model of a two-level cervical corpectomy with static anterior cervical plate.

METHODS

A three-dimensional finite element (FE) model of an intact C3-T1 segment was developed and validated. From this intact model, a fusion model (two-level [C5, C6] anterior corpectomy) was developed and validated. After corpectomy, allograft interbody fusion with a rigid anterior screw-plate construct was created from C4 to C7. Five additional FE models were developed from the fusion model corresponding to five different combinations of screw angulations within the vertebral bodies (C4, C7): (0 degrees, 0 degrees), (5 degrees, 5 degrees), (10 degrees, 10 degrees), (15 degrees, 15 degrees), and (15 degrees, 0 degrees). The fifth fusion model was termed as a hybrid fusion model.

RESULTS

The stability of a two-level corpectomy reconstruction is not dependent on the position of the screws. Despite the locked screw-plate interface, some degree of load sharing is transmitted to the graft. The load seen by the graft and the shear stress at the bone-screw junction is dependent on the angle of the screws with respect to the end plate. Higher stresses are seen at more divergent angles, particularly at the lower level of the construct.

CONCLUSION

This study suggests that screw divergence from the end plates not only increases load transmission to the graft but also predisposes the screws to higher shear forces after corpectomy reconstruction. In particular, the inferior screw demonstrated larger stress than the upper-level screws. In the proposed hybrid fusion model, lower stresses on the bone graft, end plates, and bone-screw interface were recorded, inferring lower construct failure (end-plate fractures and screw pullout) potential at the inferior construct end.

In vitro 3D-kinematics of the upper cervical spine: helical axis and simulation for axial rotation and flexion extension

PURPOSE

Registration of 3D-anatomical model and kinematics data is reported to be an accurate method to provide 3D-joint simulation. We applied this approach to discrete kinematics analysis of upper cervical spine (UCS) during axial rotation (AR) and flexion extension (FE) to create anatomical models with movement simulation including helical axis.

METHODS

Kinematics and CT imaging data were sampled in ten anatomical specimens. Using technical and anatomical marker digitizing, spatial position of segments was computed for five discrete positions of AR and FE using a 3D-digitizer. Computerized tomography was used to create anatomical models and to assure kinematics and imaging data registration for simulation. Kinematics was processed using orientation vector and helical axis (HA) computation.

RESULTS

Maximal standard error on marker digitizing was 0.47 mm. Range of motion and coupled movement during AR was in agreement with previous in vitro studies. HA location and orientation have shown low variation at the occipitoaxial and atlantoaxial levels for FE and AR, respectively.

CONCLUSIONS

We developed a protocol to create UCS anatomical model simulations including three-dimensional discrete kinematics, using previously validated methods. In this study, simultaneous segmental movement simulation and display of HA variations was shown to be feasible. Although partially confirming previous results, helical axis computation showed variations of motion patterns dependent on movement, level and specimen. Further in vivo investigations are needed to confirm relevance of this method in the clinical field.

Validity of a computer postural analysis to estimate 3-dimensional rotations and translations of the head from three 2-dimensional digital images

OBJECTIVE

The purpose of this study is to describe and evaluate the validity/accuracy of the computerized system PosturePrint for measuring head posture.

METHODS

Computer analysis was compared with 125 measured positions of a mannequin head in 5 degrees of freedom. For each mannequin position, 3 digital photographs were obtained (left lateral, anteroposterior, and right lateral) and were processed through the PosturePrint computer system. For the head analysis, a headgear with 3 reflective markers was placed on a subject; and there were additional click-on markers at the ear tragus, upper lip, acromioclavicular joints, and episternal notch. Head postures were calculated as lateral translation (T(x)), lateral flexion (R(z)), axial rotation (R(y)), flexion-extension (R(x)), and anterior-posterior translation (T(z)). For an error analysis, PosturePrint algorithm calculations were compared with the true mannequin head positions. Furthermore, average head posture was determined in student volunteers (n = 40).

RESULTS

Mean computational errors were R(x) = 1.3 degrees (SD 0.6 degrees) and T(z) = 1.1 mm (SD 0.5 mm) for sagittal displacements and R(y) = 1.1 degrees (SD 0.7 degrees), R(z) = 0.6 degrees (SD 0.4 degrees), and T(x) = 1.1 mm (SD 0.5 mm) for frontal view displacements. For the normal group, mean head displacements were 1.1 degrees or less for all rotations and 1 mm or less for lateral translations (T(x)); and forward head posture (T(z)) averaged 3 cm.

CONCLUSION

From the mannequin positions, small mean errors indicate that the PosturePrint system is accurate. In the future, statistical research determining the correlation between head displacements, neck pain, function, and health status should be performed.

Anatomical, biomechanical, and practical considerations in posterior occipitocervical instrumentation

BACKGROUND CONTEXT

Patients with cervical myelopathy secondary to craniocervical instability commonly present with spinal cord compression secondary to a combination of static forces and gross instability. Craniocervical arthrodesis is therefore indicated in the treatment of the majority of these conditions. In order to facilitate arthrodesis, techniques for occipitocervical instrumentation have been developed.

PURPOSE

To systematically review the anatomy, biomechanics, and practical considerations involved in posterior occipitocervical instrumentation.

STUDY DESIGN

Retrospective literature review.

PATIENT SAMPLE

Not applicable.

OUTCOME MEASURES

Not applicable.

METHODS

Retrospective literature review.

RESULTS

The anatomic elements of the craniocervical junction include the occipital bone, occipital condyles, atlas (C1), and axis (C2). The occiput-C1 and C1-C2 motion segments possess unique mechanical properties. Occipitocervical instrumentation constructs are comprised of points of fixation and longitudinal elements, each with characteristic strengths and weaknesses.

CONCLUSIONS

Analysis of the anatomy, available points of fixation, and the movements to be controlled leads to the choice of a longitudinal element which can control movement by incorporating the strongest points of fixation. By going through this process for each patient, an informed decision may be made regarding the optimal occipitocervical instrumentation construct.

Coupling behavior of the cervical spine: a systematic review of the literature

OBJECTIVE

The purpose of this study was to investigate evidence of consistency of reported directional coupling patterns among selected studies and to determine its use in manual medical treatment.

METHODS

The study was a systematic literature review of English-only journals using PubMed and CINAHL. The keywords included "cervical vertebrae," "biomechanics," "coupling," and "three-dimensional movement" and required coupling directional assessment of individual spine segments.

RESULTS

Four 2-dimensional and 8 3-dimensional studies met inclusion criteria. This study found 100% agreement in coupling direction (side flexion and rotation to the same side) in lower cervical vertebral segments (C2-3 and lower) and variation in coupling patterns in the upper cervical segments of occiput-C1 (during side flexion initiation) and C1-2. Dissimilarities may be explained by differences in measurement devices, movement initiation, in vivo vs in vitro specimens, and anatomical variations.

CONCLUSIONS

These findings suggest that use of 3-dimensional analyzed cervical coupling patterns for the lower cervical vertebral during apposition and treatment application may show clinical use for manual clinicians. The use of directional coupling based on 2-dimensional cervical coupling patterns or upper cervical spine coupling that addresses C1-2 should be questioned.

Kinematics of the cervical spine in lateral bending: in vivo three-dimensional analysis

STUDY DESIGN

Kinematics of the cervical spine during lateral bending were investigated using a novel system of three-dimensional motion analysis.

OBJECTIVES

To demonstrate in vivo intervertebral coupled motions of the cervical spine during lateral bending of the neck.

SUMMARY OF BACKGROUND DATA

No previous studies have successfully documented in vivo three-dimensional intervertebral motions of the cervical spine during lateral bending.

METHODS

Twelve healthy volunteers underwent three-dimensional magnetic resonance imaging (MRI) of the cervical spine in 7 positions with 10 degrees increments of lateral bending. Relative motions of the cervical spine were calculated automatically by superimposing a segmented three-dimensional-MRI of the vertebra in the neutral position over images of each position using volume registration.

RESULTS

Mean maximum lateral bending of the cervical spine to one side was 1.6 degrees to 5.7 degrees at each level. Coupled axial rotation opposite to lateral bending was observed in the upper cervical levels (Oc-C1, 0.2 degrees ; C1-C2, 17.1 degrees ), while in the subaxial cervical levels, it was observed in the same direction as lateral bending except for at C7-T1. Coupled flexion-extension motion was small at all vertebral levels (<1.1 degrees).

CONCLUSIONS

We succeeded in identifying in vivo coupled motions of the cervical spine in lateral bending for the first time.

Decreased isometric neck strength in women with chronic neck pain and the repeatability of neck strength measurements11No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the author(s) or upon any organization with which the author(s) is/are associated

OBJECTIVES

To evaluate neck flexion, extension, and, especially, rotation strength in women with chronic neck pain compared with healthy controls and to evaluate the repeatability of peak isometric neck strength measurements in patients with neck pain.

DESIGN

Cross-sectional.

SETTINGS

Rehabilitation center and physical and rehabilitation medicine department at a Finnish hospital.

PARTICIPANTS

Twenty-one women with chronic neck pain and healthy controls matched for sex, age, anthropometric measures, and occupation.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Peak isometric strength of the cervical muscles was tested in rotation, flexion, and extension.

RESULTS

Significantly lower flexion (29%), extension (29%), and rotation forces (23%) were produced by the chronic neck pain group compared with controls. When the repeated test results were compared pairwise against their mean, considerable variation was observed in the measures on the individual level. Intratester repeatability of the neck muscle strength measurements was good in all the 4 directions tested in the chronic neck pain group (intraclass correlation coefficient range,.74-.94). The coefficient of repeatability was 15N, both in flexion and extension, and 1.8 Nm in rotation. On the group level, improvement up to 10% due to repeated testing was observed.

CONCLUSIONS

The group with neck pain had lower neck muscle strength in all the directions tested than the control group. This factor should be considered when planning rehabilitation programs. Strength tests may be useful in monitoring training progress in clinical settings, but training programs should be planned so that the improvement in results is well above biologic variation, measurement error, and learning effect because of repeated testing.

Atlanto-odontoid osteoarthritis in rheumatoid arthritis: dynamic CT findings

We analyzed the CT appearances of degenerative change in the atlanto-odontoid joint (AOJ) in patients with rheumatoid arthritis (RA) and evaluated the effect of these changes on atlanto-axial joint (AAJ) rotation by dynamic CT. This revealed that 9 patients (24%) treated with methotrexate had degenerative features in the AOJ. The ratio of AAJ rotation to the total rotation of the cervical spine was significantly higher in normal subjects (54 +/- 3%) than in patients (38 +/- 12%). The degree of AAJ rotation was significantly lower in the patient group with degenerative features in the AOJ (20.9 +/- 8.4 degrees ) than in patients without degenerative features (28.5 +/- 7.4 degrees ). RA patients with a history of longstanding disease and treatment with antirheumatic drugs may develop AO OA. Although secondary OA was described as healing phenomena in the joints of RA patients, it can limit rotation in the AAJ and cause suboccipital neck pain. A regular check-up of the AAJ and AOJ by means of dynamic CT in all RA patients is proposed to avoid possible antirheumatic drug complications.

Kinematics of the upper cervical spine in rotation: in vivo three-dimensional analysis

STUDY DESIGN

Kinematics of the upper cervical spine during head rotation were investigated using three-dimensional magnetic resonance imaging (MRI) in healthy volunteers.

OBJECTIVES

To demonstrate in vivo intervertebral coupled motions of the upper cervical spine.

SUMMARY OF BACKGROUND DATA

Although various in vivo and in vitro studies have identified the normal movement patterns of the upper cervical spine, no previous studies have accurately analyzed in vivo three-dimensional intervertebral motions of the upper cervical spine during head rotation.

METHODS

Fifteen healthy volunteers underwent three-dimensional MRI of the upper cervical spine using a 1.0-T imager in progressive 15 degrees steps during head rotation. Segmented three-dimensional MRIs of each vertebra in the neutral position were superimposed over images taken at other positions, using voxel-based registration. Relative motions between occiput (Oc) and atlas (C1) and between C1 and axis (C2) were measured and described with 6 degrees of freedom by rigid body Euler angles and translations.

RESULTS

Mean (+/- SD) maximum angles of axial rotation in Oc-C1 and C1-C2 were 1.7 +/- 1.5 degrees and 36.2 +/- 4.5 degrees to each side, respectively. Increases in angle of axial rotation in C1-C2 became smaller with increased head rotation, indicating axial rotation in C1-C2 displayed nonlinear motion. Coupled lateral bending with axial rotation was observed in the direction opposition to that of axial rotation in Oc-C1 (mean, 4.1 +/- 1.4 degrees) and C1-C2 (mean, 3.8 +/- 3.0 degrees). Coupled extension with axial rotation occurred at both C0-C1 (mean, 13.3 +/- 4.9 degrees) and C1-C2 (mean, 6.9 +/- 3.0 degrees).

CONCLUSIONS

We developed an innovative in vivo three-dimensional motion analysis system using three-dimensional MRI. In vivo coupled motions of the upper cervical spine investigated using this system supported the results of the previous in vitro study.

Mechanical properties of the human cervical spine as shown by three-dimensional load-displacement curves

STUDY DESIGN

The mechanical properties of multilevel human cervical spines were investigated by applying pure rotational moments to each specimen and measuring multidirectional intervertebral motions.

OBJECTIVES

To document intervertebral main and coupled motions of the cervical spine in the form of load-displacement curves.

SUMMARY OF BACKGROUND DATA

Although a number of in vivo and in vitro studies have attempted to delineate normal movement patterns of the cervical spine, none has explored the complexity of the whole cervical spine as a three-dimensional structure.

METHODS

Sixteen human cadaveric specimens (C0-C7) were used for this study. Pure rotational moments of flexion-extension, bilateral axial torque, and bilateral lateral bending were applied using a specially designed loading fixture. The resulting intervertebral motions were recorded using stereophotogrammetry and depicted as a series of load-displacement curves.

RESULTS

The resulting load-displacement curves were found to be nonlinear, and both rotation and translation motions were coupled with main motions. With flexion-extension moment loading, the greatest degree of flexion occurred at C1-C2 (12.3 degrees), whereas the greatest degree of extension was observed at C0-C1 (20.2 degrees). With axial moment loading, rotation at C1-C2 was the largest recorded (56.7 degrees). With lateral bending moments, the average range of motion for all vertebral levels was 7.9 degrees.

CONCLUSIONS

The findings of the present study are relevant to the clinical practice of examining motions of the cervical spine in three dimensions and to the understanding of spinal trauma and degenerative diseases.

Cervical coupling during lateral head translations creates an S-configuration

OBJECTIVE

To determine cervical coupling during the posture of lateral head translation relative to a fixed thoracic cage.

DESIGN

Digitized measurements from anteroposterior cervical radiographs of 20 volunteers were obtained in neutral, left, and right lateral translation posture of the head compared to a fixed thorax.

BACKGROUND DATA

Clinically, lateral translation of the head is a common posture. Ranges of motion and spinal coupling have not been reported for this movement.

METHODS

Vertebral body corners, mid-lateral articular pillars and the superior spinous-lamina junction of C3-T4 were digitized on 60 radiographs. Using the orthogonal axis of positive x-direction to the left, vertical as positive y and anterior as positive z, digitized points were used to measure projected segmental z-axis rotation, y-axis rotation, and segmental lateral translations of each vertebra.

RESULTS

Subjects translated their heads laterally a mean of 51 mm. The major coupled motion was lateral bending (z-axis rotation), which changed direction at the C4-C5 disc space creating an S-shape. Upper cervical (C3-C4) lateral bending was contralateral to the main motion of head translation direction. Lower cervical and upper thoracic lateral bending were ipsilateral. Other segmental motions averaged less than 1 mm and 1 degrees.

CONCLUSIONS

Lateral head translations (x-axis) compared to a fixed thoracic cage can be large with a mean of 51 mm to one side. The major spinal coupling was lateral bending which changed direction at C4-C5 resulting in an S-configuration. This might have application in side impacts. All other segmental movements were small, less than 1 mm and 1 degrees.

RELEVANCE

The clinically common posture of lateral head translation results in an S-shaped cervical spine and may occur in side impact trauma. This posture has not been studied for cervical coupling patterns or range of motion (ROM).