Locating the Instant Center of Rotation in the Subaxial Cervical Spine with Biplanar Fluoroscopy during In Vivo Dynamic Flexion-Extension

Quantitatively assessing the effect of cervical sagittal alignment on dynamic intervertebral kinematics by video-fluoroscopy technique

BACKGROUND

Cervical sagittal alignment is crucial for distributing the head load to lower cervical segments and maintaining normal cervical spine function, but its biomechanical effect on the cervical spine was not fully elucidated.

OBJECTIVE

To investigate the effect of cervical sagittal alignment on dynamic intervertebral kinematics.

DESIGN

Cross-sectional study.

METHODS

Healthy participants without neck pain were recruited and divided into lordosis, straight and kyphosis groups according to the C2-C7 Cobb angle at the neutral position. The anti-directional and total joint motions were extracted across 10 epochs of dynamic cervical flexion and extension movements.

RESULTS

/findings: The overall anti-directional joint motion during flexion is larger in the kyphosis group when compared with the lordosis group (p = 0.021), while the range of flexion is smaller in the kyphosis group than that in the lordosis group (p = 0.017). The C2/C3 anti-directional joint motion during extension in the straight group is larger than that in the lordosis group (p = 0016). The range of extension in the kyphosis group (p < 0.001) and the straight group (p = 0.002) are larger than that in the lordosis group. The increased range of extension in the kyphosis and straight groups were mainly from the C3/C4, C4/C5, and C5/C6 joints(p < 0.05).

CONCLUSION

Changes in cervical sagittal alignment alter both the quality and quantity of the individual joint motions. More adjustments are required by the cervical joints to complete neck movements with the loss of lordosis. The lordotic curvature is a relatively effort-saving mode for the cervical spine from a biomechanical perspective.

Effect of device constraint: a comparative network meta-analysis of ACDF and cervical disc arthroplasty

BACKGROUND CONTEXT

Clinical trials have demonstrated that cervical disc arthroplasty (CDA) is an effective and safe alternative treatment to anterior cervical discectomy and fusion (ACDF) for cervical degenerative disc disease in the appropriately indicated patient population. Various devices for CDA exist, differing in the level of device constraint.

PURPOSE

To investigate outcomes following Anterior Cervical Discectomy and Fusion (ACDF) versus CDA stratified based on the level of device constraint: Constrained, Semiconstrained, and Unconstrained.

STUDY DESIGN

Systematic review and network meta-analysis.

PATIENT SAMPLE

2,932 CDA patients (979 Constrained, 1,214 Semiconstrained, 739 Unconstrained) and 2,601 ACDF patients from 41 studies that compared outcomes of patients undergoing CDA or ACDF at a single level at a minimum of 2 years follow-up.

OUTCOME MEASURES

Outcomes of interest included the development of adjacent segment degeneration (ASD), index and adjacent segment reoperation rates, range of motion (ROM), high-grade heterotopic ossification (HO, McAfee Grades 3/4), and patient-reported outcomes (NDI/VAS).

METHODS

CDA devices were grouped based on the degrees of freedom (DoF) allowed by the device, as either Constrained (3 DoF), Semiconstrained (4 or 5 DoF), or Unconstrained (6 DoF). A random effects network meta-analysis was conducted using standardized mean differences (SMD) and log relative risk (RR) were used to analyze continuous and categorical data, respectively.

RESULTS

Semiconstrained (p=.03) and Unconstrained CDA (p=.01) demonstrated a significantly lower risk for ASD than ACDF. All levels of CDA constraint demonstrated a significantly lower risk for subsequent adjacent segment surgery than ACDF (p<.001). Semiconstrained CDA also demonstrated a significantly lower risk for index level reoperation than both ACDF and Constrained CDA (p<.001). Unconstrained devices retained significantly greater ROM than both Constrained and Semiconstrained CDA (p<.001). As expected, all levels of device constraint retained significantly greater ROM than ACDF (p<.001). Constrained and Unconstrained devices both demonstrated significantly lower levels of disability on NDI than ACDF (p=.02). All levels of device constraint demonstrated significantly less neck pain than ACDF (p<.05), while Unconstrained CDA had significantly less arm pain than ACDF (p=.02) at final follow-up greater than 2 years.

CONCLUSION

Cervical Disc Arthroplasty, particularly the unconstrained and semiconstrained designs, appears to be more effective than ACDF in reducing the risk of adjacent segment degeneration and the need for further surgeries, while also allowing for greater range of motion and better patient-reported outcomes. Less constrained CDA conferred a lower risk for index level reoperation, while also retaining more range of motion than more constrained devices.

In vivo cervical vertebrae kinematic studies based on dual fluoroscopic imaging system measurement: A narrative review

Understanding the motion characteristics of cervical spine through biomechanical analysis aids in the identification of abnormal joint movements. This knowledge is essential for the prevention, diagnosis, and treatment of related disorders. However, the anatomical structure of the cervical spine is complex, and traditional medical imaging techniques have certain limitations. Capturing the movement characteristics of various parts of the cervical spine in vivo during motion is challenging. The dual fluoroscopic imaging system (DFIS) is able to quantify the motion and motion patterns of individual segments. In recent years, DFIS has achieved accurate non-invasive measurements of dynamic joint movements in humans. This review assesses the research findings of DFIS about the cervical spine in healthy and pathological individuals. Relevant study search was conducted up to October 2023 in Web of Science, PubMed, and EBSCO databases. After the search, a total of 30 studies were ultimately included. Among them, 13 studies focused on healthy cervical spines, while 17 studies focused on pathological cervical spines. These studies mainly centered on exploring the vertebral bodies and associated structures of the cervical spine, including intervertebral discs, intervertebral foramina, and zygapophyseal joints. Further research could utilize DFIS to investigate cervical spine motion in different populations and under pathological conditions.

Biomechanics of Cervical Disk Replacement: Classifying Arthroplasty Implants

Cervical disk arthroplasty has been employed with increased frequency over the past 2 decades as a motion-preserving alternative to anterior cervical discectomy and fusion in select patients with myelopathy or radiculopathy secondary to degenerative disk disease. As indications continue to expand, an understanding of cervical kinematics and materials science is helpful for optimal implant selection. Cervical disk arthroplasty implants can be classified according to the mode of articulation and df , articulation material, and endplate construction. The incorporation of translational and rotational df allows the implant to emulate the dynamic and coupled centers of movement in the cervical spine. Durable and low-friction interfaces at the articulation sustain optimal performance and minimize particulate-induced tissue reactions. Endplate materials must facilitate osseous integration to ensure implant stability after primary fixation. These cardinal considerations underlie the design of the 9 implants currently approved by the FDA and serve as the foundation for further biomimetic research and development.

Finite element analysis of optimized novel additively manufactured non-articulating prostheses for cervical total disc replacement

Ball-and-socket designs of cervical total disc replacement (TDR) have been popular in recent years despite the disadvantages of polyethylene wear, heterotrophic ossification, increased facet contact force, and implant subsidence. In this study, a non-articulating, additively manufactured hybrid TDR with an ultra-high molecular weight polyethylene core and polycarbonate urethane (PCU) fiber jacket, was designed to mimic the motion of normal discs. A finite element (FE) study was conducted to optimize the lattice structure and assess the biomechanical performance of this new generation TDR with an intact disc and a commercial ball-and-socket Baguera^®C TDR (Spineart SA, Geneva, Switzerland) on an intact C5-6 cervical spinal model. The lattice structure of the PCU fiber was constructed using the Tesseract or the Cross structures from the IntraLattice model in the Rhino software (McNeel North America, Seattle, WA) to create the hybrid I and hybrid II groups, respectively. The circumferential area of the PCU fiber was divided into three regions (anterior, lateral and posterior), and the cellular structures were adjusted. Optimal cellular distributions and structures were A2L5P2 in the hybrid I and A2L7P3 in the hybrid II groups. All but one of the maximum von Mises stresses were within the yield strength of the PCU material. The range of motions, facet joint stress, C6 vertebral superior endplate stress and path of instantaneous center of rotation of the hybrid I and II groups were closer to those of the intact group than those of the Baguera^®C group under 100 N follower load and pure moment of 1.5 Nm in four different planar motions. Restoration of normal cervical spinal kinematics and prevention of implant subsidence could be observed from the FE analysis results. Superior stress distribution in the PCU fiber and core in the hybrid II group revealed that the Cross lattice structure of a PCU fiber jacket could be a choice for a next-generation TDR. This promising outcome suggests the feasibility of implanting an additively manufactured multi-material artificial disc that allows for better physiological motion than the current ball-and-socket design.

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.

Surgical Outcomes for C2 Tear Drop Fractures: Clinical Relevance to Hangman's Fracture and C2-3 Discoligamentous Injury

Objective

To analyze characteristics of surgically managed tear drop (TD) fractures of the C₂ axis associated with other injuries such as hangman's fracture and C_2‐3 discoligamentous injury as well as treatment outcomes.

Methods

A total of 14 patients (eight men and six women) with TD fractures of the C₂, who were surgically treated at four national trauma centers of tertiary university hospitals from January 2000 to December 2017, were included in this retrospective study. The mean age of the patients was 45.5 years (ranging from 19 to 74 years). The characteristics, surgical treatment methods (anterior fusion vs posterior fusion), and results of 14 TD fractures of the C₂ were analyzed retrospectively. And the clinical relevance between C₂ TD fracture and hangman's fracture and C_2‐3 discoligamentous injury was investigated through the co‐occurrence between injuries. The mean follow‐up time after surgery was 22.6 months (ranging from 12 to 60 months).

Results

Among 14 patients with TD fracture of the C₂, four patients (28.6%) had anterior TD fracture and 10 patients (71.4%) had posterior TD fracture. All 10 posterior TD fracture patients had anterior C_2‐3 displacement. While two of four anterior TD fracture patients had posterior C_2‐3 displacement, the remaining two did not. All 14 patients of TD fracture had at least two or more other associated C₂ injuries as well as C_2‐3 discoligamentous injuries. About 92.9% (13/14) of the patients had typical or atypical hangman's fracture; 100% (10/10) of the posterior TD fracture patients had hangman's fracture, but 75% (3/4) of the anterior TD fracture had hangman's fracture. At admission, 13 patients were neurologically intact. However, the remaining patient had spinal cord injury with American Spinal Injury Association (ASIA) impairment scale B with C_2‐3 bilateral facet dislocation. All four anterior TD fracture patients underwent posterior C_2‐3 fusion. While four of 10 posterior TD fracture patients underwent C_2‐3 anterior fusion, the remaining six underwent posterior fusion. At last follow‐up, 100% (14/14) of the patients achieved solid fusion, and visual analog scale for neck pain was significantly improved (5.9 vs 2.2, P < 0.001). One patient with ASIA impairment scale B had significantly improved to scale D. No major complications occurred.

Conclusion

Our study showed that surgically managed TD fractures of the C₂ showed a high incidence of other associated spine injuries including hangman's fracture and C_2‐3 discoligamentous injury. Therefore, special attention and careful radiologic evaluation are needed to investigate the presence of other associated spine injuries including hangman's fracture and C_2‐3 discoligamentous injury, which are likely to require surgery.

The effect of cervical intervertebral disc degeneration on the motion path of instantaneous center of rotation at degenerated and adjacent segments: A finite element analysis

BACKGROUND

The motion path of instantaneous center of rotation (ICR) is a crucial kinematic parameter to dynamically characterize cervical spine intervertebral patterns of motion; however, few studies have evaluated the effect of cervical disc degeneration (CDD) on ICR motion path. The purpose of this study was to investigate the effect of CDD on the ICR motion path of degenerated and adjacent segments.

METHOD

A validated nonlinear three-dimensional finite element (FE) model of a healthy adult cervical spine was used. Progressive degeneration was simulated with six FE models by modifying intervertebral disc height and material properties, anterior osteophyte size, and degree of endplate sclerosis at the C5-C6 level. All models were subjected to a pure moment of 1 Nm and a compressive follower load of 73.6 N to simulate physical motion. ICR motion paths were compared among different models.

RESULTS

The normal FE model results were consistent with those of previous studies. In degenerative models, average ICR motion paths shifted significantly anterior at the degenerated segment (β = 0.27 mm; 95% CI: 0.22, 0.32) and posterior at the proximal adjacent segment (β = -0.09 mm; 95% CI: -0.15, -0.02) than those of the normal model.

CONCLUSION

CDD significantly affected ICR motion paths at the degenerated and proximal adjacent segments. The changes at adjacent segments may be a result of compensatory mechanisms to maintain the balance of the cervical spine. Surgical treatment planning should take into account the restoration of ICR motion path to normal. These findings could provide a basis for prosthesis design and clinical practice.

Comparison of anterior-only versus combined anterior and posterior fusion for unstable subaxial cervical injuries: a meta-analysis of biomechanical and clinical studies

OBJECTIVE

The purpose of the present study was to perform a meta-analysis comparing biomechanical and clinical outcomes between anterior-only and combined anterior and posterior fusions to determine which method of cervical fusion yielded better results for unstable cervical injuries.

METHODS

The MEDLINE, EMBASE, Cochrane Central Register of Controlled Trials, Web of Science and SCOPUS electronic databases were searched for relevant articles published through 2000-2019 that compared the biomechanical and clinical outcomes of anterior-only and combined anterior and posterior fusion for unstable cervical fracture.

RESULTS

Eight biomechanical and four clinical studies were included in the analysis. There were significant biomechanical differences between the groups with respect to flexion-extension, axial rotation and lateral bending. Combined fusion provided better biomechanical stability for unstable cervical injuries than anterior-only fusion, regardless of the number of corpectomies or the presence of a posterior column injury. However, despite significant biomechanical differences, there were no significant differences in clinical outcomes, such as the degree of neurologic improvement and complications between the two groups.

CONCLUSION

Anterior-only and combined anterior and posterior fusions for unstable subaxial cervical injuries can both restore cervical stability. Although combined fusion might have some advantages in terms of stability biomechanically, there were no significant differences in clinical outcomes, such as the degree of neurologic improvement and perioperative complications. Therefore, rather than the routine use of combined fusion for unstable cervical injuries, the selective use of anterior-only or combined fusion according to the type of injury is recommended.