Assessing the biofidelity of in vitro biomechanical testing of the human cervical spine

Quantifying the Importance of Active Muscle Repositioning a Finite Element Neck Model in Flexion Using Kinematic, Kinetic, and Tissue-Level Responses

PURPOSE

Non-neutral neck positions are important initial conditions in impact scenarios, associated with a higher incidence of injury. Repositioning in finite element (FE) neck models is often achieved by applying external boundary conditions (BCs) to the head while constraining the first thoracic vertebra (T1). However, in vivo, neck muscles contract to achieve a desired head and neck position generating initial loads and deformations in the tissues. In the present study, a new muscle-based repositioning method was compared to traditional applied BCs using a contemporary FE neck model for forward head flexion of 30°.

METHODS

For the BC method, an external moment (2.6 Nm) was applied to the head with T1 fixed, while for the muscle-based method, the flexors and extensors were co-contracted under gravity loading to achieve the target flexion.

RESULTS

The kinematic response from muscle contraction was within 10% of the in vivo experimental data, while the BC method differed by 18%. The intervertebral disc forces from muscle contraction were agreeable with the literature (167 N compression, 12 N shear), while the BC methodology underpredicted the disc forces owing to the lack of spine compression. Correspondingly, the strains in the annulus fibrosus increased by an average of 60% across all levels due to muscle contraction compared to BC method.

CONCLUSION

The muscle repositioning method enhanced the kinetic response and subsequently led to differences in tissue-level responses compared to the conventional BC method. The improved kinematics and kinetics quantify the importance of repositioning FE neck models using active muscles to achieve non-neutral neck positions.

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.

Study on biomechanical analysis of two-level cervical Mobi-C and arthrodesis

OBJECTIVE

To investigate the range of motion (ROM) index of a two-level cervical arthroplasty.

METHODS

Seven human cadaveric spines were biomechanically examined from C2 level to T1 level under intact status and the following conditions: 2-level arthroplasty (C4-C6) employing Mobi-C devices (MM group), 2-level anterior cervical discectomy and fusions (2-ACDFs) (FF group), and both as a hybrid surgery (HS) (MF group and FM group). Multidirectional flexibility examination was conducted according to the Panjabi hybrid testing protocol. Unconstrained intact moments of ±1.5 NM were performed for axial rotation (AR) flexion/extension (FE), and lateral bending (LB).

RESULTS

No statistical differences were found between the intact spine and MM group at the operative- and adjacent-level kinematics in the three loading conditions, except that C4-C5 ROM significantly increased in the axial rotation loading (P<0.05). Compared with the intact spine, MF group led to a significant decrease at the arthrodesis segment ROM C5-C6 in the three loading (P<0.05), with corresponding significantly increased at C4-C5 in FE and AR (P<0.05). FM group resulted in a significant decrease in ROM C4-C5 (P<0.05) with corresponding significantly increased at C5-C6 in FE, AR and LB (P<0.05). There was not any difference for non-operative level kinematics between MF group and FM group and intact spine. Compared with the intact spine, FF group led to a significant decrease at the arthrodesis-levels (P<0.05) and marked increase at the non-operative level kinematics.

CONCLUSION

A two-level Mobi-C and Hybrid construct generated better biomechanical conditions. This study suggested that two-level cervical total disc replacement or HS could become an alternative approach for therapy of two-level consecutive cervical spondylosis.