A Comparative in vivo Study of Semi-constrained and Unconstrained Cervical Artificial Disc Prostheses

Cervical Disk Arthroplasty: Updated Considerations of an Evolving Technology

For years, anterior cervical diskectomy and fusion has been considered the benchmark for patients with cervical radiculopathy/myelopathy. However, concerns regarding adjacent segment pathology have promoted the popularity of cervical disk arthroplasty (CDA) with its motion-preserving properties. To replicate the natural cervical disk's six degrees of freedom and compressibility in cervical spine implants, designers need to carefully consider the level of constraint for stability and material selection. Recent CDA designs have incorporated strategies to facilitate unrestricted or semirestricted motion, deploying various articulating components and materials with distinct wear and compressibility properties. To optimize outcomes, patient selection considering additional degeneration of the cervical spine is critical. Clinical long-term studies have been reported in industry-funded FDA investigational device exemption and nonindustry-funded data for one-level and two-level CDA. There are limited data available on three-level and four-level CDA. Adverse events such as heterotopic ossification, osteolysis, migration, subsidence, and failure have been described, where analysis from explanted devices yields insight into in vivo wear and impingement performance. CDA has shown short-term cost advantages, such as decreased procedural expenses. Nonetheless, long-term analysis is necessary to assess possible economic tradeoffs. Advancements in designs may lead to improved implant longevity while evidence-based decision making will guide and responsibly manage the rapid advancement in CDA technology.

The move-C cervical artificial disc can restore intact range of motion and 3-D kinematics

BACKGROUND CONTEXT

In contrast to cervical discectomy and fusion, total disc replacement (TDR) aims at preserving the motion at the treated vertebral level. Spinal motion is commonly evaluated with the range of motion (ROM). However, more qualitative information about cervical kinematics before and after TDR is still lacking.

PURPOSE

The aim of this in vitro study was to investigate the influence of cervical TDR on ROM, instantaneous centers of rotation (ICR) and three-dimensional helical axes.

STUDY DESIGN

An in vitro study with human spine specimens under pure moment loading was conducted to evaluate the kinematics of the intact cervical spine and compare it to cervical TDR.

METHODS

Six fresh frozen human cervical specimens (C4-5, median age 28 years, range 19-47 years, two female and four male) were biomechanically characterized in the intact state and after implantation of a cervical disc prosthesis (MOVE-C, NGMedical, Germany). To mimic in vivo conditions regarding temperature and humidity, water steam was used to create a warm and humid test environment with 37°C. Each specimen was quasistatically loaded with pure moments up to ±2.5 Nm in flexion/extension (FE), lateral bending (LB) and axial rotation (AR) in a universal spine tester for 3.5 cycles at 1 °/s. For each third cycle of motion the ROM was evaluated and an established method was used to determine the helical axis and COR and to project them into three planar X-rays. Statistical analysis was conducted using a Friedman-test and post hoc correction with Dunn-Bonferroni-tests (p<.05).

RESULTS

After TDR, total ROM was increased in FE from 19.1° to 20.1°, decreased in LB from 14.6° to 12.6° and decreased in AR from 17.7° to 15.5°. No statistical differences between the primary ROM in the intact condition and ROM after TDR were detected. Coupled rotation between LB and AR were also maintained. The position and orientation of the helical axes after cervical TDR was in good agreement with the results of the intact specimens in all three motion directions. The ICR in FE and AR before and after TDR closely matched, while in LB the ICR after TDR were more caudal. The intact in vitro kinematics we found also resembled in vivo results of healthy individuals.

CONCLUSION

The results of this in vitro study highlight the potential of artificial cervical disc implants to replicate the quantity as well as the quality of motion of the intact cervical spine.

CLINICAL SIGNIFICANCE

Physiological motion preservation was a driving factor in the development of cervical TDR. Our results demonstrate the potential of cervical TDR to replicate in vivo kinematics in all three motion directions.

Preliminary Data of Neck Muscle Morphology With Head-Supported Mass in Male and Female Volunteers

INTRODUCTION

This study quantified parameters related to muscle morphology using a group of upright seated female and male volunteers with a head-supported mass.

MATERIALS AND METHODS

Upright magnetic resonance images (MRIs) were obtained from 23 healthy volunteers after approval from the U.S. DoD. They were asymptomatic for neck pain, with no history of injury. The volunteers were scanned using an upright MRI scanner with a head-supported mass (army combat helmet). T1 and T2 sagittal and axial images were obtained. Measurements were performed by an engineer and a neurosurgeon. The cross-sectional areas of the sternocleidomastoid and multifidus muscles were measured at the inferior endplate in the sub-axial column, and the centroid angle and centroid radius were quantified. Differences in the morphology by gender and spinal level were analyzed using a repeated measures analysis of variance model, adjusted for multiple corrections.

RESULTS

For females and males, the cross-sectional area of the sternocleidomastoid muscle ranged from 2.3 to 3.6 cm2 and from 3.4 to 5.4 cm2, the centroid radius ranged from 4.1 to 5.1 cm and from 4.7 to 5.7 cm, and the centroid angle ranged from 75° to 131° and from 4.8° to 131.2°, respectively. For the multifidus muscle, the area ranged from 1.7 to 3.9 cm2 and from 2.4 to 4.2 cm2, the radius ranged from 3.1 to 3.4 cm and from 3.3 to 3.8 cm, the angle ranged from 15° to 24.4° and 16.2° to 24.4°, respectively. Results from all levels for both muscles and male and female spines are given.

CONCLUSIONS

The cross-sectional area, angulation, and centroid radii data for flexor and extensor muscles of the cervical spine serve as a dataset that may be used to better define morphologies in computational models and obtain segmental motions and loads under external mechanical forces. These data can be used in computational models for injury prevention, mitigation, and readiness.

Perspectives on Cervical Arthroplasty in Navy and Marine Corps Tactical Jet Aircrew

Symptomatic cervical spondylosis is a progressive degenerative condition of the cervical spine commonly resulting in functionally-limiting pain, weakness, and/or limited dexterity. Symptomatic cervical spondylosis is believed to occur at higher rates in military aviators than civilian counterparts and is a disqualifying condition for all Navy and Marine Corps aircrew. This condition is non-waiverable for tactical jet (ejection-seat-based) aviators. Medical attrition of experienced tactical jet aircrew from the military aviation community results in substantial cost to the U.S. Government, reduces fleet combat capability, and adversely impacts career progression and retention. The clinical maturation of cervical total disc replacement (TDR) technology over the last 2 decades has revolutionized the treatment of symptomatic cervical spondylosis and enabled a return to duty for hundreds of military service members in non-aviation fields. TDR studies demonstrate equal or superior functional outcomes, rates of symptom resolution, reduced complication and reoperation rates, and lower long-term cost compared to traditional Anterior Cervical Discectomy and Fusion (ACDF). Although initial computational modeling studies have evaluated cervical arthroplasty performance during rotary-wing crash impacts, safety within the dynamic tactical jet environment has not yet been established. The purpose of this article is to review factors relevant to TDR safety and outcomes and to propose a framework to evaluate the safety of TDR in Navy and Marine Corps tactical jet aircrew, to ultimately inform aeromedical algorithms regarding return to flight after TDR.

Cervical Total Disc Replacement: Indications and Technique

Cervical total disc replacement devices have been marketed in the United States (US) since 2007, with abundant level 1 evidence published on the treatment. Adherence to the strict inclusion/exclusion criteria and the surgical technique training of the US clinical trials remains the consistent and conservative approach to patient selection and implantation technique. However, patient selection and surgical technique remain debated among US surgeons as the published data and available cervical total disc replacements continue to grow.

Vertebral Level-dependent Kinematics of Female and Male Necks Under G+x Loading

INTRODUCTION

Size-matched volunteer studies report gender-dependent variations in spine morphology, and head mass and inertia properties. The objective of this study was to determine the influence of these properties on upper and lower cervical spine temporal kinematics during G+x loading.

METHODS

Parametrized three-dimensional head-neck finite element models were used, and impacts were applied at 1.8 and 2.6 m/s at the distal end. Details are given in the article. Contributions of population-based variations in morphological and mass-related variables on temporal kinematics were evaluated using sensitivity analysis. Influence of variations on time to maximum nonphysiological curve formation, and flexion of upper and extension of the lower spines were analyzed for male-like and female-like spines.

RESULTS

Upper and lower spines responded with initial flexion and extension, resulting in a nonphysiological curve. Time to maximum nonphysiological curve and range of motions (ROMs) of the cervical column ranged from 45 to 66 ms, and 30 to 42 deg. Vertebral depth and location of the head center of gravity (cg) along anteroposterior axis were most influential variables for the upper spine flexion. Location of head cg along anteroposterior axis had the greatest influence on the time of the curve. Both anteroposterior and vertical locations of head cg, disc height, vertebral depth, head mass, and size were influential for the lower spine extension kinematics.

CONCLUSIONS

Models with lesser vertebral depth, that is, female-like spines, experienced greater range of motions and pronounced nonphysiological curves. This results in greater distraction/stretch of the posterior upper spine complex, a phenomenon attributed to suboccipital headaches. Forward location of head cg along anteroposterior axis had the greatest influence on upper and lower spine motions and time of formation of the curve. Any increased anteroposterior location of cg attributable to head supported mass may induce greater risk of injuries/neck pain in women during G+x loading.

Retrieval analysis of an explanted Mobi-C cervical disc replacement: A case study

Ex vivo analysis of artificial discs is essential to better understand their ability to replace degenerated intervertebral discs. The Mobi-C differs from some other contemporary disc designs in that it has a mobile polyethylene insert that is sandwiched between superior and inferior cobalt chromium endplates. While some studies claim the Mobi-C to have restored normal cervical spinal biomechanics, others have noted high levels of migration. Our objective was to contribute to this debate by, for the first time, analysing an explanted Mobi-C cervical disc which was removed due to worsening myelopathy at the nano and macro scales. Intraoperatively, the insert was found to have excessively migrated and it compressed the spinal cord. Roughness was measured as 0.016 ± 0.006 μm (Sa) and 0.055 ± 0.020 μm (Sa) for the superior and inferior plates, and 1.210 ± 0.154 μm (Sa) and 0.446 ± 0.083 μm (Sa) for the superior and inferior surfaces of the insert. Compared to unworn surfaces, the roughness increased for the superior and inferior plates and decreased for both surfaces of the insert. However, the only statistically significant change occurred on the articulating surface of the inferior plate (p = 0.04). At the nanoscale, valleys dominated the articulating surfaces. The superior plate had a burnished appearance whereas the inferior plate appeared matt. Impingement was observed on the endplates. The insert was severely damaged, burnished and had scratches. Additionally, subsurface whitening and internal cracking were observed on the insert.

Mechanical failure of the Mobi-C implant for artificial cervical disc replacement: report of 4 cases

Cervical spondylosis is one of the most commonly treated conditions in neurosurgery. Increasingly, cervical disc replacement (CDR) has become an alternative to traditional arthrodesis, particularly when treating younger patients. Thus, surgeons continue to gain a greater understanding of short- and long-term complications of arthroplasty. Here, the authors present a series of 4 patients initially treated with Mobi-C artificial disc implants who developed postoperative neck pain. Dynamic imaging revealed segmental kyphosis at the level of the implant. All implants were locked in the flexion position, and all patients required reoperation. This is the first reported case series of symptomatic segmental kyphosis after CDR.

Mid- to long-term rates of symptomatic adjacent-level disease requiring surgery after cervical total disc replacement compared with anterior cervical discectomy and fusion: a meta-analysis of prospective randomized clinical trials

Background

Thus far, no meta-analysis focusing on the mid- to long-term incidence of adjacent segment disease requiring surgery after cervical total disc replacement and anterior cervical discectomy and fusion has been published yet. This study aimed to compare mid- to long-term rates of symptomatic adjacent-level disease requiring surgery after cervical disc replacement and anterior cervical fusion.

Methods

A meta-analysis was performed, and only randomized controlled trials with a follow-up period of more than 48 months reporting rates of symptomatic adjacent-level disease requiring surgery after cervical total disc replacement and anterior cervical discectomy and fusion were included.

Results

The analysis revealed that the overall rate of symptomatic adjacent-level disease requiring surgery in the cervical disc replacement group was significantly lower than that of the anterior cervical fusion group at 48–120 months’ follow-up. The subgroup analysis of different follow-up periods also yielded the same results. The rate of symptomatic adjacent-level disease requiring surgery in the cervical disc replacement group using unrestricted prosthesis was significantly lower than that of the anterior cervical fusion group (p < 0.001); however, the cervical disc replacement group using semi-restricted prosthesis showed no statistical difference compared with the fusion group.

Conclusions

Our review suggests that cervical disc replacement is preferable to anterior cervical fusion in reducing the incidence of symptomatic adjacent-level disease requiring surgery at mid- to long-term follow-up. A review of the literature also demonstrated that randomized controlled trials investigating the rate of symptomatic adjacent-level disease requiring surgery were insufficient; therefore, studies focusing on this subject with longer-term follow-up are warranted.

Cervical Disc Arthroplasty Migration Following Mechanical Intubation: A Case Presentation and Review of the Literature

BACKGROUND

Cervical arthroplasty has established itself as a safe and efficacious alternative to fusion in management of symptomatic cervical degenerative disease. Recent literature has indicated a trend toward decreased risk of reoperation with cervical arthroplasty, and reoperation in this subset commonly occurs secondary to recurrent pain and device-related complications. The instance of cervical arthroplasty migration, particularly in the setting of trauma, is particularly rare. Here, we report the first case of implant migration secondary to iatrogenic trauma following neck manipulation during direct laryngoscopy for mechanical intubation.

CASE DESCRIPTION

A 53-year-old smoker with cervical spondylosis underwent a cervical 3/4 arthroplasty with a ProDisc-C implant. About a month postoperatively, he was intubated via direct laryngoscopy for community acquired pneumonia and began experiencing new dysphonia and dysphagia after extubation. Delayed imaging revealed anterior migration of the implant. The patient immediately underwent removal of the implant and conversion to anterior cervical discectomy and fusion.

CONCLUSIONS

Supraphysiologic forces exerted through neck manipulation in mechanical intubation mimicked low-energy trauma, and in the setting of ligamentous resection necessary for cervical arthroplasty and inadequate osseous integration, led to migration of the implant. We recommend the integration of fiberoptic technique or video laryngoscopy with manual in line stabilization for intubation of post cervical arthroplasty patients when airway management is necessary within 10 months after cervical arthroplasty. Clinicians and anesthesiologists should have a high clinical suspicion for prompt and early workup with spine imaging in the setting of persistent postintubation symptoms such as dysphonia and/or dysphagia.

Uncertainty Evaluations for Risk Assessment in Impact Injuries and Implications for Clinical Practice

Injury risk curves (IRCs) represent the quantification of risk of adverse outcomes, such as a bone fracture, quantified by a biomechanical metric such as force or deflection. From a biomechanical perspective, they are crucial in crashworthiness studies to advance human safety. In clinical settings, they can be used as an assistive tool to aid in the decision-making process for surgical or conservative treatment. The estimation of risk corresponding to a level of biomechanical metric is done using a regression technique, such as a parametric survival regression model. As with any statistical procedure, error measures are computed for the IRC, representing the quality of the estimated risk. For example, confidence intervals (CIs) are recommended by the International Standards Organization, and the normalized confidence interval width (NCIW) is computed based on the width of the CI. This is a surrogate for the quality of the risk curve. A 95% CI means that if the same experiment were hypothetically repeated 100 times, at least 95 of the computed CIs should contain the true risk curve. Such an interpretation is problematic in most biomechanical contexts as rarely the same experiment is repeated. The notion that a wider confidence interval implies a poorer quality risk curve can be misleading. This article considers the evaluation of CIs and its implications in biomechanical settings for safety engineering and clinical practice. Alternatives are suggested for future studies.

External and internal responses of cervical disc arthroplasty and anterior cervical discectomy and fusion: A finite element modeling study

Surgical treatment for spinal disorders, such as cervical disc herniation and spondylosis, includes the removal of the intervertebral disc and replacement of biological or artificial materials. In the former case, bone graft is used to fill the space, and this conventional procedure is termed anterior cervical discectomy and fusion (ACDF). The latter surgery is termed as artificial disc replacement ADR) or cervical disc arthroplasty (CDA). Surgeries are most commonly performed at one or two levels. The present study was designed to determine the external (range of motion, ROM) and internal (anterior and posterior load sharing) responses of the spines with one-level and two-level surgeries in both models (ACDF and CDA) using a previously validated finite element model (FEM) of the subaxial cervical spinal column. The FEM simulated the vertebra (cancellous core and cortical shell of the body, posterior elements - laminae, pedicles and spinous processes), discs (anulus fibers, ground substance, and nucleus pulposus), anterior and posterior ligaments of the disc and facet joints, and interspinous and supraspinous ligaments. Appropriate material properties were assigned to the spinal components. The United States Food Drug Administration-approved Mobi-C was used for the CDA option. The FEM was exercised under pure flexion and extension moment loading of 2 Nm in the intact state. The overall ROM of the column was obtained. The hybrid loading protocol applied moments that matched the ROM in the intact spine for both one-level (C5-C6) and two-level (C5-C7) ACDF and CDA surgeries. ROM at the level(s) of surgery, termed the index level was obtained. These data along with anterior column load (ACL) and posterior column load (PCL) sharing were obtained for all surgical options at superior and inferior segments (termed adjacent segment outputs). Results for both one-level and two-level surgeries showed that ACDFs decreases ROM at the index level, while CDAs increase motions compared to the intact normal spine. The ROM, ACL, and PCL increased at both adjacent levels for the ACDF while CDA showed a decrease. Although two-level surgeries resulted in increased these biomechanical variables, greater changes to adjacent segment biomechanics in ACDF may accelerate adjacent segment disease. Decreased ROM and lower load sharing in CDAs may limit adjacent segment effects such as accelerated degeneration. Their increased posterior load sharing, however, may need additional attention for patients with suspected facet joint disease.

Comparison of 10-year Outcomes of Bryan Cervical Disc Arthroplasty for Myelopathy and Radiculopathy

Objective

To evaluate the long‐term efficacy of Bryan cervical disc arthroplasty in the treatment of myelopathy patients compared with radiculopathy patients.

Methods

This study is a prospective study. Sixty‐six patients (38 patients in myelopathy group and 28 patients in radiculopathy group) who were treated with Bryan cervical disc arthroplasty between 2004 and 2007 and followed for 10 years were included in this study. The Japanese Orthopaedic Association (JOA) score, neck disability index (NDI), and Odom's criteria were used to evaluate the clinical outcomes. X‐ray, computed tomography (CT), and magnetic resonance imaging (MRI) were used to evaluate the radiographic outcomes including the global range of motion (ROM), segmental ROM, and segment alignment before the surgery and at last follow‐up. The incidence of segmental kyphosis, segmental mobility lost, and the grade of paravertebral ossification (PO) were also evaluated at last follow‐up.

Results

The JOA score and NDI improved in both groups. Thirty‐three of 38 patients in myelopathy group and all patients in radiculopathy group reported good or excellent outcomes according to Odom's criteria. The segmental ROM was (9.5° ± 4.4°) before surgery and maintained at (9.0° ± 5.5°) at last follow‐up in myelopathy group. The segmental ROM was (9.5° ± 4.6°) and (9.0° ± 5.3°) before surgery and at last follow‐up in radiculopathy group, respectively. The Bryan prosthesis remained mobile at last follow‐up for 30 patients (78.9%) in the myelopathy group and 22 patients (78.6%) in the radiculopathy group. Of the patients in the myelopathy group, 21.1% developed segmental kyphosis, as did 21.4% of patients in the radiculopathy group. The incidence of PO and high‐grade PO was 92.1 and 28.9% in the myelopathy group, and was 92.9 and 32.1% in the radiculopathy group. There was no significant difference between both groups.

Conclusions

Bryan cervical disc arthroplasty was an effective and safe technique in treating patients with myelopathy. The clinical and radiographic outcomes in the myelopathy group were similar to those in the radiculopathy group at the 10‐year follow‐up.