The facet joint is a crucial anatomic region of the spine owing to its biomechanical role in facilitating articulation of the vertebrae of the spinal column. It is a diarthrodial joint with opposing articular cartilage surfaces that provide a low friction environment and a ligamentous capsule that encloses the joint space. Together with the disc, the bilateral facet joints transfer loads and guide and constrain motions in the spine due to their geometry and mechanical function. Although a great deal of research has focused on defining the biomechanics of the spine and the form and function of the disc, the facet joint has only recently become the focus of experimental, computational and clinical studies. This mechanical behavior ensures the normal health and function of the spine during physiologic loading but can also lead to its dysfunction when the tissues of the facet joint are altered either by injury, degeneration or as a result of surgical modification of the spine. The anatomical, biomechanical and physiological characteristics of the facet joints in the cervical and lumbar spines have become the focus of increased attention recently with the advent of surgical procedures of the spine, such as disc repair and replacement, which may impact facet responses. Accordingly, this review summarizes the relevant anatomy and biomechanics of the facet joint and the individual tissues that comprise it. In order to better understand the physiological implications of tissue loading in all conditions, a review of mechanotransduction pathways in the cartilage, ligament and bone is also presented ranging from the tissue-level scale to cellular modifications. With this context, experimental studies are summarized as they relate to the most common modifications that alter the biomechanics and health of the spine-injury and degeneration. In addition, many computational and finite element models have been developed that enable more-detailed and specific investigations of the facet joint and its tissues than are provided by experimental approaches and also that expand their utility for the field of biomechanics. These are also reviewed to provide a more complete summary of the current knowledge of facet joint mechanics. Overall, the goal of this review is to present a comprehensive review of the breadth and depth of knowledge regarding the mechanical and adaptive responses of the facet joint and its tissues across a variety of relevant size scales.

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Spinal facet joint biomechanics and mechanotransduction in normal, injury and degenerative conditions

Finite element modeling mesh quality, energy balance and validation methods: a review with recommendations associated with the modeling of bone tissue.

In this article, we summarize our work on understanding the influence of cervical sagittal malalignment on the mechanics of the cervical spine. Biomechanical studies were performed using an ex vivo laboratory model to study the kinematic and kinetic response of human cervical spine specimens in the setting of cervical sagittal imbalance. The model allowed controlled variations of C2–C7 Sagittal Vertical Alignment (C2–C7 SVA) and T1-Slope so that clinically relevant sagittally malaligned profiles could be prescribed, while maintaining horizontal gaze, and their biomechanical consequences studied. Our results demonstrated that increasing C2–C7 SVA caused flexion of lower cervical (C2–C7) segments and hyperextension of suboccipital (C0–C1–C2) segments to maintain horizontal gaze. An increase in C2–C7 SVA increased the lower cervical neural foraminal areas. Conversely, increasing T1-slope predominantly influenced subaxial cervical lordosis and, as a result, decreased cervical neural foraminal areas. Therefore, we believe patients with increased upper thoracic kyphosis and radicular symptoms may respond with increased forward head posture (FHP) as a compensatory mechanism to increase their lower cervical neural foraminal area and alleviate nerve root compression as well as reduce the burden on posterior muscles and soft and bony structures of the cervical spine. Increasing FHP (i.e., increased C2–C7 SVA) was associated with shortening of the cervical flexors and occipital extensors and lengthening of the cervical extensors and occipital flexors, which corresponds to C2–C7 flexion and C0–C2 extension. The greatest shortening occurred in the suboccipital muscles, suggesting considerable load bearing of these muscles during chronic FHP. Regardless, there was no evidence of nerve compression within the suboccipital triangle. Finally, cervical sagittal imbalance may play a role in exacerbating adjacent segment pathomechanics after multilevel cervical fusion and should be considered during surgical planning. The results of our biomechanical studies have improved our understanding of the impact of cervical sagittal malalignment on pathomechanics of the cervical spine. We believe this improved understanding will assist in clinical decision-making.

Cervical sagittal balance: a biomechanical perspective can help clinical practice

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Spinal Facet Joint Biomechanics and Mechanotransduction in Normal, Injury and Degenerative

Purpose
Various design concepts have been adopted in cervical disc prostheses, including sliding articulation and standalone configuration. This study aimed to evaluate the biomechanical effects of the standalone U-shaped configuration on the cervical spine.

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Biomechanical effects of cervical arthroplasty with U-shaped disc implant on segmental range of motion and loading of surrounding soft tissue

Study design: A parametric finite element investigation of the cervical spine.
Objective: To determine what effect, if any, cervical disc replacement has on kinematics, facet contact parameters, and anterior column loading.
Summary of Background Data: Anterior cervical discectomy and fusion (ACDF) has been a standard treatment for certain spinal degenerative disorders, but evidence suggests that fusion contributes to adjacent-segment degeneration. Motion-sparing disc replacement implants are believed to reduce adjacent-segment degeneration by preserving kinematics at the treated level. Such implants have been shown to maintain the mobility of the intact spine, but the effects on load transfer between the anterior and posterior elements remain poorly understood.
Methods: In order to investigate the effects of disc replacement on load transfer in the lower cervical spine, a finite element model was generated using cadaver-based Computed Tomography (CT) imagery. Mesh resolution was varied in order to establish model convergence, and cadaveric testing was undertaken to validate model predictions. The validated model was altered to include a disc replacement prosthesis at the C4/C5 level. The effect of disc-replacement on range of motion, antero-posterior load distribution, contact forces in the facets, as well as the distribution of contact pressure on the facets were examined. Three sizes of implants were examined.
Results: Model predictions indicate that the properly-sized implant retains the mobility, load sharing, and contact force magnitude and distribution of the intact case. Mobility, load sharing, nuclear pressures, and contact pressures at the adjacent motion segments were not strongly affected by the presence of the properly sized implant, indicating that disc replacement may not be a significant cause of post-operative adjacent-level degeneration. Implant size affected certain mechanical parameters, such as antero-posterior load sharing, and did not affect compliance or range of motion.
Conclusions: The results of this work support the continued use of motion sparing implants in the lower cervical spine. Load sharing data indicate that implant size may be an important factor that merits further study, although the deleterious effects of improper size selection may be less significant than those of fusion.

Finite element modeling of kinematic and load transmission alterations due to cervical intervertebral disc replacement.

Correlation of mechanical properties within the equine third metacarpal with trabecular bending and multi-density micro-computed tomography data.

The effects of ligamentous injury in the human lower cervical spine.

This study examined the cervical spine range of motion (ROM) resulting from whiplash-type hyperextension and hyperflexion type ligamentous injuries, and sought to improve the accuracy of specific diagnosis of these injuries. The study was accomplished by measurement of ROM throughout axial rotation, lateral bending, and flexion and extension, using a validated finite element model of the cervical spine that was modified to simulate hyperextension and/or hyperflexion injuries. It was found that the kinematic difference between hyperextension and hyperflexion injuries was minimal throughout the combined flexion and extension ROM measurement that is commonly used for clinical diagnosis of cervical ligamentous injury. However, the two injuries demonstrated substantially different ROM under axial rotation and lateral bending. It is recommended that other bending axes beyond flexion and extension are incorporated into clinical diagnosis of cervical ligamentous injury.

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Addition of lateral bending range of motion measurement to standard sagittal measurement to improve diagnosis sensitivity of ligamentous injury in the human lower cervical spine.

The assessment of cervical spine instability following traumatic injury is controversial [1, 4, 5, 8]. Typical definitions of cervical instability are based on the presence of several key detectable injuries using simple radiographs, computed tomography (CT), and magnetic resonance (MR) imaging. Although these imaging modalities have been shown to be relatively reliable for detection of fractures and very large soft tissue injuries, they are largely deficient for determining the presence of smaller soft tissue injuries, such as hyperstrained ligaments [1, 3]. Soft tissue injuries of this nature may be revealed with dynamic range of motion (ROM) assessment, such as a flexion and extension test with radiography. However, these tests are currently inadequate for determining the existence of specific injuries. Cervical soft tissue injuries demand further analysis, given the risk of severe and permanent neurological impairment that may accompany these injuries [2, 5].© 2011 ASME

P. Devin Leahy

Papers

Finite element modeling of kinematic and load transmission alterations due to cervical intervertebral disc replacement.

Correlation of mechanical properties within the equine third metacarpal with trabecular bending and multi-density micro-computed tomography data.

The effects of ligamentous injury in the human lower cervical spine.

Addition of lateral bending range of motion measurement to standard sagittal measurement to improve diagnosis sensitivity of ligamentous injury in the human lower cervical spine.

Mechanical Properties of Injured Human Cervical Spine Ligaments and Corresponding Effect on Spinal Kinematics