Mechanical role of the posterior column components in the cervical spine

Biomechanical evaluation of subaxial lateral mass prothesis: a finite element analysis study

Pathologies of the lateral masses could lead to bone destruction of the cervical spine. Their treatment includes lesion resection and fixation. However, the resulting bone defect of a lateral mass is often neglected, resulting in difficulty in bone fusion. Therefore, we designed a subaxial lateral mass prosthesis to achieve lateral mass joint fusion. This study aims to evaluate the role of a new subaxial lateral mass prosthesis using finite element analysis. Five finite element models (intact, lateral mass resection, screw-rod fixation, prosthesis implantation, and prosthesis fusion groups) were compared in terms of the range of motion (ROM), prosthesis von Mises stress, and screw-rod von Mises stress during flexion, extension, lateral bending, and rotation. The ROM of the model increased significantly after lateral mass resection, and was significantly reduced after fixation with screws and rods. Screw-rod fixation combined with prosthesis implantation further reduced the ROM. After bone fusion in the prosthesis, the ROM can also be reduced slightly. The von Mises stress of the bilateral screws and rods significantly decreased after prosthesis implantation. The von Mises stress of the prosthesis further decreased during the right bending after bone fusion was achieved. Subaxial lateral mass prosthesis can help restore the stability of the cervical spine after lateral mass resection and can reduce the stress on the bilateral screws and rods. Reconstruction of a lateral mass is more consistent with the mechanical transmission of the three-column spine and contributes to interfacet fusion of the lateral mass joint.

Which traumatic spinal injury creates which degree of instability? A systematic quantitative review

BACKGROUND CONTEXT

Traumatic spinal injuries often require surgical fixation. Specific three-dimensional degrees of instability after spinal injury, which represent criteria for optimum treatment concepts, however, are still not well investigated.

PURPOSE

The aim of this review therefore was to summarize and quantify multiplanar instability increases due to spinal injury from experimental studies.

STUDY DESIGN/SETTING

Systematic review.

METHODS

A systematic review of the literature was performed using keyword-based search on PubMed and Web of Science databases in order to detect all in vitro studies investigating the destabilizing effect of simulated and provoked traumatic injury in human spine specimens. Together with the experimental designs, the instability parameters range of motion, neutral zone and translation were extracted from the studies and evaluated regarding type and level of injury.

RESULTS

A total of 59 studies was included in this review, of which 43 studies investigated the effect of cervical spine injury. Range of motion increase, which was reported in 58 studies, was generally lower compared to the neutral zone increase, given in 37 studies, despite of injury type and level. Instability increases were highest in flexion/extension for most injury types, while axial rotation was predominantly affected after cervical unilateral dislocation injury and lateral bending solely after odontoid fracture. Whiplash injuries and wedge fractures were found to increase instability equally in all motion planes.

CONCLUSIONS

Specific traumatic spinal injuries produce characteristic but complex three-dimensional degrees of instability, which depend on the type, level, and morphology of the injury. Future studies should expand research on the cervicothoracic, thoracic, and lumbosacral spine and should additionally investigate the destabilizing effects of the injury morphology as well as concomitant rib cage injuries in case of thoracic spinal injuries. Moreover, neutral zone and translation should be measured in addition to the range of motion, while mechanical injury simulation should be preferred to resection or transection of structures to ensure high comparability with the clinical situation.

Numerical investigation on the stability of human upper cervical spine (C1-C3)

BACKGROUND

The finite element method (FEM) is an efficient and powerful tool for studying human spine biomechanics.

OBJECTIVE

In this study, a detailed asymmetric three-dimensional (3D) finite element (FE) model of the upper cervical spine was developed from the computed tomography (CT) scan data to analyze the effect of ligaments and facet joints on the stability of the upper cervical spine.

METHODS

A 3D FE model was validated against data obtained from previously published works, which were performed in vitro and FE analysis of vertebrae under three types of loads, i.e. flexion/extension, axial rotation, and lateral bending.

RESULTS

The results show that the range of motion of segment C1-C2 is more flexible than that of segment C2-C3. Moreover, the results from the FE model were used to compute stresses on the ligaments and facet joints of the upper cervical spine during physiological moments.

CONCLUSION

The anterior longitudinal ligaments (ALL) and interspinous ligaments (ISL) are found to be the most active ligaments, and the maximum stress distribution is appear on the vertebra C3 superior facet surface under both extension and flexion moments.

Prognostic value of cervical ligamentum flavum thickness as a morphological parameter to predict cervical stenosis

One of major causes of cervical central stenosis (CCS) is thickened change of cervical ligament flavum (CLF). The association of a morphological parameter called cervical ligament flavum thickness (CLFT) with CCS has not been reported yet. Thus, the purpose of this research was to investigate the relationship between CCS and CFJT.Data were obtained from 88 patients with CCS. A total of 87 normal controls also underwent cervical spine magnetic resonance imaging (CSMRI). All subjects underwent axial T2-weighted CSMRI. Using our picture archiving and communications system, thickness of ligament flavum of the cervical spine at C6/7 level was analyzed.The mean CLFT was 1.41 ± 0.24 mm in normal subjects and 2.09 ± 0.39 mm in patients with CCS. The CCS group was found to have significantly (P < .001) higher rate of CLFT than normal subjects. ROC curves were used to assess the usefulness of CLFT as a predictor of CCS. In the CCS group, the best practical cut off-point of CLFT was 1.71 mm (sensitivity = 90.9%; specificity = 90.8%), with AUC of 0.94 (95% confidence interval: 0.90--0.98).Greater CLFT values were associated with greater possibility of CCS. Thus, treating physician should carefully examine CLFT, as it can help diagnose CCS.

Influences of functional structures on the kinematic behavior of the cervical spine

BACKGROUND CONTEXT

A few studies have already investigated the influences of functional structures of the cervical spine on its biomechanical behavior. In most studies, this has been done by measuring the range of motion. However, this parameter lacks of qualitative information about the overall kinematic behavior, such as coupled motions or translations. These data are essential for future development of cervical implants and surgical techniques.

PURPOSE

An investigation of the influences of cervical structures on the kinematic behavior of the cervical spine under in vivo conditions is almost impossible due to ethical reasons. Therefore, an in vitro study was conducted which allowed the analysis of these influences using three-dimensional helical axes.

STUDY DESIGN/SETTING

An in vitro test applying pure moments on mono-segmental specimens was designed in order to investigate the influences of a series of structures on the kinematic behavior of the cervical spine using three-dimensional helical axes.

METHODS

In this study we extracted motion segments C2-C3, C4-C5, and C6-C7 from 6 human cadaveric specimens with an average age of 48 years. The specimens were carefully selected using X-ray images. For the in vitro experiments, seven states were defined. The first state represented the intact state of each specimen. The remaining six states correspond with the subsequent resection of the following structures in the given order: interspinous ligament, ligamentum flavum, facet capsule, vertebral arch, posterior longitudinal ligament, and anterior longitudinal ligament. Each state was tested using a well-established spine tester. Each test sequence included 3.5 quasi-static motion cycles in all three bending directions using pure moments of 1 Nm. All motions were recorded using a motion tracking device and six reflective markers which were attached to the specimens. The recordings were then used to calculate the 3D helical axes, which were matched with the X-ray images. Due to the small number of specimens, qualitative results, such as the helical axes, were analyzed using descriptive statistics.

RESULTS

In general, the overall range of motion was increased in all loading directions due to the resection steps. The least change in the kinematic behavior of the cervical spine was observed during flexion/extension. For lateral bending and axial rotation the greatest change in the pattern of the helical axes was observed during the resection of the vertebral arch. For some specimens, however, typical patterns regarding the orientation of the helical axes remained until the last state. For lateral bending, it could be observed that the deviation in the axes' orientation increased whereas for axial rotation it decreased.

CONCLUSION

Resection of the cervical ligaments are much less crucial than the removal of guiding structures such as the facet joint. Furthermore, coupled motions not only result from the orientations of the articular surfaces of the facet joints but also from the overall shape of the cervical vertebrae including the uncinate processes.

CLINICAL SIGNIFICANCE

It is well-known that coupled motions play a substantial role in cervical kinematics. However, the influences of cervical structures on the overall kinematic behavior of the cervical spine are not yet fully understood. Knowledge of these influences could help to reduce or even prevent iatrogenic degeneration after surgical intervention. Furthermore, the data provided by this study can be helpful for future developments of cervical implants as well as finite element models for more advanced numerical investigations.

Experimental cervical interspinous ligament pain altered cervical joint motion during dynamic extension movement

BACKGROUND

Although the cervical interspinous ligament is a potential source of neck pain, the effects on cervical joint motion and pressure pain sensitivity has never been investigated. The understanding of the relationship will broaden our understanding of cervical biomechanics and improve diagnosis and treatment of neck pain.

METHODS

Fluoroscopy videos of cervical flexion and extension movements and pressure pain thresholds over bilateral C2/C3 and C5/C6 facet joints were collected in fifteen healthy subjects before and after injections of hypertonic and isotonic saline in C4/C5 ISL. The videos were divided into 10 even epochs and the motion of individual joints during each epoch was extracted. Joint motion parameters including anti-directional motion, pro-directional motion, total joint motion and joint motion variability were extracted across epochs. Joint motion parameters and PPTs were compared before and after injection of hypertonic and isotonic saline separately.

FINDINGS

Compared with baselines: hypertonic saline injection 1) decreased anti-directional motion and joint motion variability at C4/C5 (P < 0.05) and increased at C2/C3 (P < 0.05) during extension; 2) increased total joint motion of C0/C1 during first half range (P < 0.05) and decreased during second half range of extension, and total joint motion of C2/C3 increased during second half range of extension (P < 0.05) and; 3) increased pressure pain thresholds over left C2/C3 facet joint (P < 0.01).

INTERPRETATION

The cervical interspinous ligament pain redistributed anti-directional motion between C4/C5 and C2/C3 during dynamic extension and decreased pressure pain sensitivity over the left C2/C3 facet joint.

Comparable clinical and radiological outcomes between skipped-level and all-level plating for open-door laminoplasty

PURPOSE

To compare the clinical and radiological outcomes between skipped-level and all-level plating for cervical laminoplasty.

METHODS

Patients with cervical spondylotic myelopathy (CSM) treated by open-door laminoplasty with minimum 2-year postoperative follow-up were included. All patients had opening from C3-6 or C3-7 and were divided into skipped-level or all-level plating groups. Japanese Orthopaedic Association (JOA) scores and canal measurements were obtained preoperatively, immediate (within 1 week) postoperatively, and at 2, 6 weeks, 3, 6 and 12 months postoperatively. Paired t test was used for comparative analysis. Receiver operating characteristic analysis was used to determine the canal expansion cutoff for spring-back closure.

RESULTS

A total of 74 subjects were included with mean age of 66.1 ± 11.3 years at surgery. Of these, 32 underwent skipped-level plating and 42 underwent all-level plating. No significant differences were noted between the two groups at baseline and follow-up. Spring-back closure was observed in up to 50% of the non-plated levels within 3 months postoperatively. The cutoff for developing spring-back closure was 7 mm canal expansion for C3-6. No differences were observed in JOA scores and recovery rates between the two groups. None of the patients with spring-back required reoperation.

CONCLUSIONS

There were no significant differences between skipped-level and all-level plating in terms of JOA or recovery rate, and canal diameter differences. This has tremendous impact on saving costs in CSM management as up to two plates per patient undergoing a standard C3-6 laminoplasty may be omitted instead of four plates to every level to achieve similar clinical and radiological outcomes.

LEVEL OF EVIDENCE

III. These slides can be retrieved under Electronic Supplementary Material.

Adaptation of a clinical fixation device for biomechanical testing of the lumbar spine

In-vitro biomechanical testing is widely performed for characterizing the load-displacement characteristics of intact, injured, degenerated, and surgically repaired osteoligamentous spine specimens. Traditional specimen fixture devices offer an unspecified rigidity of fixation, while varying in the associated amounts and reversibility of damage to and "coverage" of a specimen - factors that can limit surgical access to structures of interest during testing as well as preclude the possibility of testing certain segments of a specimen. Therefore, the objective of this study was to develop a specimen fixture system for spine biomechanical testing that uses components of clinically available spinal fixation hardware and determine whether the new system provides sufficient rigidity for spine biomechanical testing. Custom testing blocks were mounted into a robotic testing system and the angular deflection of the upper fixture was measured indirectly using linear variable differential transformers. The fixture system had an overall stiffness 37.0, 16.7 and 13.3 times greater than a typical human functional spine unit for the flexion/extension, axial rotation and lateral bending directions respectively - sufficient rigidity for biomechanical testing. Fixture motion when mounted to a lumbar spine specimen revealed average motion of 0.6, 0.6, and 1.5° in each direction. This specimen fixture method causes only minimal damage to a specimen, permits testing of all levels of a specimen, and provides for surgical access during testing.

Influence of variations in stiffness of cervical ligaments on C5-C6 segment

The ligaments of the cervical spine each play a critical role in maintaining stability. Large variations in the mechanical behavior of each ligament have been reported, but it remains unclear how these variations influence cervical biomechanics. The objective of this study was to investigate the mechanical response of the cervical spine to variations in the properties of each cervical ligament. A finite element model of the C5-C6 spine was constructed with the average material properties. The stiffness of each ligament was then changed in turn by increasing or decreasing it per its designated maximum or minimum stiffness. The range of motion (ROM) and intradiscal pressure (IDP) were calculated and compared among the different models under pure moments. The results showed that the capsular ligament with the greatest stiffness caused a lower ROM in all anatomical planes. Varying the stiffness of the anterior longitudinal ligament had the greatest influence on ROM in extension, while the interspinous ligament was the most influential in flexion. During lateral bending or axial rotation, the capsular ligament with the minimum stiffness resulted in a higher IDP, while the capsular ligament with the maximum stiffness resulted in a lower IDP. Varying the capsular ligament stiffness had the greatest role on the C5-C6 ROM and therefore care must be taken to assign appropriate material properties. This study showed a less influence on the intervertebral disc with smaller ROM, especially when the ligaments were relaxed. This suggested that the control of the neck posture may be beneficial for patients with a degenerated cervical spine.