Biomechanics of the cervical spine. I: Normal kinematics

doi:10.1016/S0268-0033(00)00034-6

Clinical Biomechanics

Volume 15, Issue 9, November 2000, Pages 633-648

https://doi.org/10.1016/S0268-0033(00)00034-6 Get rights and content

Abstract

This review constitutes the first of four reviews that systematically address contemporary knowledge about the mechanical behavior of the cervical vertebrae and the soft-tissues of the cervical spine, under normal conditions and under conditions that result in minor or major injuries. This first review considers the normal kinematics of the cervical spine, which predicates the appreciation of the biomechanics of cervical spine injury. It summarizes the cardinal anatomical features of the cervical spine that determine how the cervical vertebrae and their joints behave. The results are collated of multiple studies that have measured the range of motion of individual joints of the cervical spine. However, modern studies are highlighted that reveal that, even under normal conditions, range of motion is not consistent either in time or according to the direction of motion. As well, detailed studies are summarized that reveal the order of movement of individual vertebrae as the cervical spine flexes or extends. The review concludes with an account of the location of instantaneous centres of rotation and their biological basis.
Relevance

The facts and precepts covered in this review underlie many observations that are critical to comprehending how the cervical spine behaves under adverse conditions, and how it might be injured. Forthcoming reviews draw on this information to explain how injuries might occur in situations where hitherto it was believed that no injury was possible, or that no evidence of injury could be detected.

Introduction

Amongst its several functions, the head can be regarded as a platform that houses the sensory apparatus for hearing, vision, smell, taste and related lingual and labial sensations. In order to function optimally, these sensory organs must be able to scan the environment and be delivered towards objects of interest. It is the cervical spine that subserves these facilities. The cervical spine constitutes a device that supports the sensory platform, and moves and orientates it in three-dimensional space.

The movements of the head are executed by muscles but the type of movements possible depend on the shape and structure of the cervical vertebrae and interplay between them. The kinematics of the cervical spine are, therefore, predicated by the anatomy of the bones that make up the neck and the joints that they form.

Section snippets

Functional anatomy

For descriptive purposes, the cervical spine can be divided and perceived as consisting of four units, each with a unique morphology that determines its kinematics and its contribution to the functions of the complete cervical spine. In anatomical terms the units are the atlas, the axis, the C2–3 junction and the remaining, typical cervical vertebrae. In metaphorical, functional terms these can be perceived as the cradle, the axis, the root, and the column.

Atlanto-occipital joint

Studies of the atlanto-occipital joint in cadavers found the range of flexion–extension to be about 13°; that of axial rotation was 0°; but about 8° was possible when the movement was forced [15]. A detailed radiographic study of cadaveric specimens [16], [17] found the mean ranges (SD) to be flexion–extension: 18.6°(0.6), axial rotation 3.4°(0.4), and lateral flexion $3.9° (0.6)$ . It also revealed that when flexion–extension was executed, it was accompanied by negligible movements in the other

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The cervical spinal canal has a wide range of motion and specific biomechanics involved with different pathologies that may cause dynamic cord compressions. This study has introduced new protocol for acquiring an extension view of cervical MRI to assess dynamic cervical spinal canal compromise. We posit that dynamic MRI comprising extension view in prone position could be a practical option when deciding the best approach in treating challenging patients.
Coupled rotation patterns in cervical spine axial rotation can change when the head is kept level
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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 randomized controlled trial with extended long-term follow-up: Quality of cervical spine motion after anterior cervical discectomy (ACD) or anterior cervical discectomy with arthroplasty (ACDA)
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In previous research, a consistent sequence of segmental contributions during dynamic extension X-rays of the cervical spine was observed in 80–90% of healthy participants.
To investigate whether this previously defined ‘normal’ sequence of segmental contributions was present in patients who underwent anterior cervical discectomy with arthroplasty (ACDA) or anterior cervical discectomy (ACD).
A randomized controlled trial with extended follow-up was conducted. Patients with single level cervical degenerative radiculopathy with a surgical indication were included and randomized. Dynamic X-ray recordings were made before surgery, one-year post-operative, and at long term follow-up.
A total of 27 patients were included, three in an ACDA pilot group and 24 were randomized to receive ACDA (N = 12) or ACD (N = 12). A total of 20 patients were available for follow-up. Preoperatively, 16.7% of patients in the ACDA group and 58.3% of patients in the ACD group showed a normal sequence. One-year post-operative, 66.7% showed a normal sequence in the ACDA group versus 30.0% in the ACD group (p = 0.036). After an average of 11-years follow-up, a normal sequence was observed in 9.1% of patients in the ACDA group and in none of the patients in the ACD group (p = 0.588).
These findings suggest that while ACDA can restore and preserve a normal sequence of segmental contributions in the short term, this effect is not maintained in the long term. Throughout the process of ageing, not only the quantity, but also the quality of motion changes.
Range of motion after 1, 2, and 3 level cervical disc arthroplasty
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Motion of a solid body involves translation and rotation. Few investigations examine the isolated translational and rotational components associated with disc arthroplasty devices. This study investigates single- and multi-level cervical disc arthroplasty with respect to index and adjacent level range of motion. The investigators hypothesized that single- and multilevel cervical disc replacement will lead to comparable or improved motion at implanted and adjacent levels.
Seven human cervical spines from C2 to C7 were subjected to displacement-controlled loading in flexion, extension, and lateral bending under intact, 1-Level (C5–C6), 2-Level (C5–C6, C6–C7) and 3-Level (C5–C6, C6–C7, C4–C5) conditions. 3D motions sensors were mounted at C4, C5, and C6. Motion data for translations and rotations at each level for each surgical condition and loading mode were compared to intact conditions.
1-Level: The index surgery resulted in statistically increased translations in extension and lateral bending at all levels with statistically increased translation observed in flexion in the superior and inferior levels. In rotation, the index surgeries decreased rotation under flexion, with remaining levels not statistically different to intact conditions.
A device placed inferiorly resulted in statistically increased translations at all levels in extension with statistically increased translations superior and inferior to the index level in flexion. Lateral bending resulted in increased nonsignificant translations. Rotations were elevated or comparable to the intact level for all loading.
Translations were statistically increased for all levels in all loading modes while rotations were elevated or were comparable to the intact level for all loading modes and levels.
Micromotion sensors permitted monitoring and recording of small magnitude angulations and translations using a loading mechanism that did not over constrain cervical segmental motion. Multilevel cervical disc arthroplasty yielded comparable or increased overall motion at the index and adjacent levels compared to intact conditions.
Novel Radiographic Parameters for Posterior Atlantoaxial Dislocation Secondary to Os Odontoideum and Its Clinical Significance
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To explore correlations of 3 novel radiographic parameters with myelopathy induced by posterior atlantoaxial dislocation (PAAD) secondary to os odontoideum (OO) and assess their reproducibility.
Of the 51 patients with OO with PAAD enrolled in this study, 28 developed PAAD-induced myelopathy (myelopathy group), and the other 23 patients had no myelopathy (control group). Neurologic function was evaluated by the neurologic function rating system and the Japanese Orthopaedic Association score system. Three novel radiographic parameters (OP [median sagittal diameter of the spinal canal from the posteroinferior edge of the ossicle to the anterosuperior edge of the spinous process of C2]/C4 SAC [space available for spinal cord] ratio, C1 posterior inclination angle, and posterior dislocation index) were measured by lateral cervical dynamic radiography. Their correlations with neurologic function were analyzed, and their reproducibility was assessed by the intraclass correlation coefficient (ICC). In addition, receiver operating characteristic curve analysis was performed.
A significant correlation was observed between the OP/C4 SAC ratio and the neurologic function (P < 0.01), and between the C1 posterior inclination angle and the neurologic function (P < 0.01). Furthermore, their interobserver and intraobserver reliability was excellent (ICC ≥ 0.912). Receiver operating characteristic curve analysis showed that the optimal threshold value relating to myelopathy of the OP/C4 SAC ratio and C1 posterior inclination angle was 0.93 and 20°, respectively.
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Review paperBiomechanics of the cervical spine. I: Normal kinematics

Abstract

Introduction

Section snippets

Functional anatomy

Atlanto-occipital joint

Clin Biomech

Clin Biomech

J Biomech

Clin Rheum Dis

Clin Biomech

Clin Biomech

Clin Biomech

J Biomech

Morphological and functional studies on the lateral joints of the first and second cervical vertebrae in man

Anat Embryol

Intra-articular inclusions of the cervical synovial joints

Brit J Rheumatol

Cervical spine motion in the sagittal plane. (I). Range of motion of actually performed movements, an X-ray cinematographic study

Eur J Morph

Transections of the C1–C2 joint capsular ligaments in the cadaveric spine

Spine

Tears of the transverse ligament of the atlas

J Bone Joint Surg

Lateral subluxation of the atlas in rheumatoid arthritis: a case report and post-mortem study

Ann Rheum Dis

Morphological aspects and biomechanical properties of the vertebro-axial joint (C2-C3)

Acta Morphol Neerl-Scand

Rotation of the cervical spine. A CT study in normal subjects

Spine

The ligaments and anulus fibrosus of human adult cervical intervertebral discs

Spine

Intervertebral disc changes with aging of human cervical vertebra. From the neonate to the eighties

Spine

Biomechanical analysis of clinical stability in the cervical spine

Clin Orthop

The possibilities of movement in the craniovertebral joints

Acta Orthop Scandinav

Messungen von Vor- and Ruckbeuge in den Kopfgelenken

Fortsch Röntgenstr

Cineroentgenography of the normal cervical spine

J Bone Joint Surg

Range of mobility of the cervical spine

Arch Phys Med Rehab

Normal ranges of motion of the cervical spine

Arch Phys Med Rehabil

The lateral movement in the atlanto-axial joints and its clinical significance

Acta Radiol

The atlanto-axial joint

J Bone Joint Surg

Three-dimensional motion analysis of the cervical spine with special reference to the axial rotation

Spine

Histology of the alar and transverse ligaments

Spine

Functional anatomy of the alar ligaments

Spine

Review paper
Biomechanics of the cervical spine. I: Normal kinematics