Biomechanical rationale for the pathology of rheumatoid arthritis in the craniovertebral junction

Finite Element Analysis of Head-Neck Kinematics in Rear-End Impact Conditions with Headrest

A detailed three-dimensional (3D) head-neck (C0-C7) finite element (FE) model was developed and used to dictate the motions of each cervical spinal segment under static physiological loadings of flexion and extension with a magnitude of 1.0 Nm and rear-end impacts. In this dynamic study, a rear-end impact pulse was applied to C7 to create accelerations of 4.5 G and 8.5 G. The predicted segmental motions and displacements of the head were in agreement with published results under physiological loads of 1.0 Nm. Under rear-end impact conditions, the effects of peak pulse acceleration and headrest angles on the kinematic responses of the head-neck complex showed rates of increase/decrease in the rotational motion of various cervical spinal segments that were different in the first 200 ms. The peak flexion rotation of all segments was lower than the combined ROM of flexion and extension. The peak extension rotation of all segments showed variation compared to the combined ROM of flexion and extension depending on G and the headrest angle. A higher acceleration of C7 increased the peak extension angle of lower levels, but the absolute increase was restricted by the distance between the head and the headrest. A change in the headrest angle from 45° to 30° resulted in a change in extension rotation at the lower C5-C6 segments to flexion rotation, which further justified the effectiveness of having distance between the head and the headrest. This study shows that the existing C0-C7 FE model is efficient at defining the gross reactions of the human cervical spine under both physiological static and simulated whiplash circumstances. The fast rate of changes in flexion and extension rotation of various segments may result in associated soft tissues and bony structures experiencing tolerances beyond their material characteristic limits. It is suggested that a proper location and angle of the headrest could effectively prevent the cervical spine from injury in traumatic vehicular accidents.

Maximum Safety Limits of Laminectomy of the C1 Vertebra for Chiari Malformation Surgery: A Finite Element Analysis

Background

The treatment of Chiari malformations generally consists of posterior fossa decompression. C1 laminectomy is required in selected cases. However, cases of iatrogenic anterior arch fractures at C1 without high-energy trauma have been reported. Developing theoretical models of atlas C1 bones that have undergone a laminectomy can help researchers identify the regions where fractures may occur as a result of sudden loads.

Methods

In this study, we created a detailed three-dimensional solid finite element model of the human atlas bone (C1) using geometric data. The loadings of the laminectomy dimension were evaluated on the basis of three groups. Group I comprised atlas bones that had not undergone a laminectomy. For Group II, the lateral border of the laminectomy was determined as the projection of the lateral mass medial border on the lamina. For Group III, the bilateral sulcus arteriosus was determined as the border for the lateral border of the laminectomy. The analysis results, which are in good agreement with those of previous reports, showed high concentrations of localized stress in the anterior and posterior arches of the atlas bone.

Results

The analysis results showed that the stress increased in the laminectomy models. The maximum stress observed was consistent with the clinical observations of fracture sites in previous studies.

Conclusion

In the treatment of patients with Chiari malformations, C1 laminectomy is often required. The width of this laminectomy can lead to iatrogenic anterior arch fractures. This is the first study to evaluate C1 laminectomy width using finite element modeling.

Endoscopic endonasal odontoidectomy: a long-term follow-up results for a cohort of 21 patients

BACKGROUND

Endoscopic endonasal odontoidectomy (EEO) has been described as a potential approach for craniovertebral junction (CVJ) disease which could cause anterior bulbomedullary compression and encroaching. Due to the atlantoaxial junction's uniqueness and complex biomechanics, treating CVJ pathologies uncovers the challenge of preventing C1-C2 instability. A large series of patients treated with endonasal odontoidectomy is reported, analyzing the feasibility and necessity of whether or not to perform posterior stabilization. Furthermore, the focus is on the long-term follow-up, especially those whom only underwent partial C1 arch preservation without posterior fixation.

METHODS

This study is a retrospective analysis of patients with ventral spinal cord compression for non-reducible CVJ malformation, consecutively treated with EEO from July 2011 to March 2019. Postoperative dynamic X-ray and CT scans were obtained in each case in order to document CVJ decompression as well as to exclude instability. The anterior atlas-dens interval, posterior atlas-dens interval and C1-C2 total lateral overhang were measured as a morphological criteria to determine upper cervical spine stability.

RESULTS

Twenty-one patients (11:10 F:M) with a mean age of 60.6 years old at the time of surgery (range 34-84 years) encountered the inclusion criteria. For all 21 patients, a successful decompression was achieved at the first surgery. In 11 patients, the partial C1 arch integrity did not require a posterior cervical instrumentation on the bases of postoperative and constant follow-up radiological examination. In 13 cases, an improvement of motor function was recorded at the time of discharge. Only one patient had further motor function improvement at follow-up. Among the patients that did not show any significant motor change at discharge, 4 patients showed an improvement at the last follow-up.

CONCLUSIONS

The outcomes, even in C1 arch preservation without posterior fixation, are promising, and it could be said that the endonasal route potentially represents a valid option to treat lesions above the nasopalatine line.

Magnetic resonance imaging anatomy of the craniovertebral ligaments: A radiological study with confirmatory dissection

Background

Descriptions of the radiological appearance of the craniovertebral ligaments often lack detail. This study aimed to provide an accurate description of the morphology and radiological appearance of the alar and cruciform ligaments with confirmation of findings by fine dissection.

Materials and Methods:

Six embalmed human cadaveric specimens were reduced to an osseoligamentous arrangement spanning the C2/3 disc to the occiput. Specimens were imaged on a 4.6T Bruker magnetic resonance (MR) system using a 3D RARE multiple SE sequence with acquisition time 18 h 24 min. Acquired images were viewed in three planes, and detailed descriptions and morphometric measurement of the ligaments were obtained. Specimens were then examined and described using fine dissection. Direct comparison of the descriptions of each method was undertaken.

Results:

From imaging, detailed features of all alar ligaments could be identified in all specimens. Consistency in shape, orientation, and attachments is described. Attachment to the medial aspect of the atlantooccipital joints was evident in all specimens. Five of six alar ligament pairs contained fibers that traversed the dens without attachment. Ascending cruciform ligaments could be clearly identified in four of six specimens. No descending cruciform ligaments could be clearly delineated. Detailed features of the transverse ligaments could be identified and described in all planes. Dissection findings were mostly consistent with descriptions obtained from MR images.

Conclusion:

4.6T MR images provide accurate detail of the structure, dimensions, and attachments of the craniovertebral ligaments. The morphology of the craniovertebral ligaments assessed radiologically was consistent with findings on gross dissection.

Surgical nuances and construct patterns influence construct stiffness in C1-2 stabilizations: a biomechanical study of C1-2 gapping and advanced C1-2 fixation

PURPOSE

Stabilization of C1-2 using a Harms-Goel construct with 3.5 mm titanium (Ti) rods has been established as a standard of reference (SOR). A reduction in craniocervical deformities can indicate increased construct stiffness at C1-2. A reduction in C1-2 can result in C1-2 joint gapping. Therefore, the authors sought to study the biomechanical consequences of C1-2 gapping on construct stiffness using different instrumentations, including a novel 6-screw/3-rod (6S3R) construct, to compare the results to the SOR. We hypothesized that different instrument pattern will reveal significant differences in reduction in ROM among constructs tested.

METHODS

The range of motion (ROM) of instrumented C1-2 polyamide models was analyzed in a six-degree-of-freedom spine tester. The models were loaded with pure moments (2.0 Nm) in axial rotation (AR), flexion extension (FE), and lateral bending (LB). Comparisons of C1-2 construct stiffness among the constructs included variations in rod diameter (3.5 mm vs. 4.0 mm), rod material (Ti. vs. CoCr) and a cross-link (CLX). Construct stiffness was tested with C1-2 facets in contact (Contact Group) and in a 2 mm distracted position (Gapping Group). The ROM (°) was recorded and reported as a percentage of ROM (%ROM) normalized to the SOR. A difference > 30% between the SOR and the %ROM among the constructs was defined as significant.

RESULTS

Among all constructs, an increase in construct stiffness up to 50% was achieved with the addition of CLX, particularly with a 6S3R construct. These differences showed the greatest effect for the CLX in AR testing and for the 6S3R construct in FE and AR testing. Among all constructs, C1-2 gapping resulted in a significant loss of construct stiffness. A protective effect was shown for the CLX, particularly using a 6S3R construct in AR and FE testing. The selection of rod diameter (3.5 mm vs. 4.0 mm) and rod material (Ti vs. CoCr) did show a constant trend but did not yield significance.

CONCLUSION

This study is the first to show the loss of construct stiffness at C1-2 with gapping and increased restoration of stability using CLX and 6S3R constructs. In the correction of a craniocervical deformity, nuances in the surgical technique and advanced instrumentation may positively impact construct stability.

Transarticular Screw Fixation in the Treatment of Severe C1-C2 Dislocation: A Case Series Report

Background

To aim of the present paper was to evaluate the results of halo traction and transarticular screw fixation combined with bone autoplasty in patients with severe atlantoaxial dislocation.

Case presentation

This is a retrospective study of severe cases of atlantoaxial dislocation in nine patients (six men and three women) treated with preoperative halo traction and posterior C1–C2 transarticular screw fixation combined with bone autoplasty from June 2006 to June 2011 at the Saint Paul Hospital (Hanoi). The mean age of patients was 37.48 ± 13.753 years (range, 26–50 years). The possibility of fixing dislocation using a halo apparatus was investigated through a series of preoperative halo corrections performed within a span of 1–2 weeks. For transarticular screw fixation, two transarticular screws were used that were positioned according to the Magerl technique. For bone autoplasty, an iliac crest bone graft approximately 3 × 2 cm in size was used. The postoperative assessment of clinical improvement was performed using the neck disability index (NDI), the American Spinal Injury Association (ASIA) impairment scale, and the visual analog scale (VAS) measurement instruments, through the gradation of atlantoaxial dislocation, and via the clivoaxial angle(CAA) index and the space available for cord (SAC) index after 6 months. The image diagnosis demonstrates that all the cases of atlantoaxial dislocations are unstable and correspond to the Fielding and Hawkins type III dislocation. Eight patients underwent complete reduction using the halo fixation device. In one patient, the C1–C2 displacement was manually reduced during surgery. CT scanning revealed that the accuracy of screw placement was 94.4%. The bone fusion rate was 100% after 6 months. Based on the ASIA impairment scale, the preoperative examination of patients revealed grade C injuries in seven patients and grade D injuries in two patients. After surgery, all patients had grade D injuries. Six months after surgery, four patients had moderate self‐reported neck disability (30%–48%) and five patients reported mild disability (10%–28%); that is, the patient perception of the neck problem improved. In the postoperative phase, all patients showed an improvement in VAS pain scores and the SAC score returned to the normal range in all patients. The CAA returned to normal in only seven patients; in the other two patients, the CAA returned to a value that was close to normal (145° and 149°).

Conclusion

Through halo traction combined with transarticular screw fixation and bone autoplasty, noticeable postoperative improvements were attained based on the clinical scores for NDI, ASIA, and VAS, as well as SAC and CAA.

Cervical spine manifestations of rheumatoid arthritis: a review

Rheumatoid arthritis (RA) is a progressive autoimmune inflammatory disease affecting 1% of the population with three times as many women as men. As many as 86% of patients suffering from RA have cervical spine involvement. Synovial inflammation in the cervical spine causes instability and injuries including atlantoaxial subluxation, retroodontoid pannus formation, cranial settling, and subaxial subluxation. While many patients with cervical spine involvement are asymptomatic, symptomatic patients often present with nonspecific symptoms resulting from inflammation and additional secondary symptoms that are due to compression of the brainstem, cranial nerves, vertebral artery, and spinal cord. Radiographs are the imaging modality used most often, while MRI and CT are used for assessment of neural element involvement and surgical planning. Multiple classification systems exist. Early diagnosis and treatment of cervical spine involvement is critical. Surgical management is indicated when patients experience symptoms from cervical involvement that result in biomechanical instability and, or a neurological deficit. Atlantoaxial instability managed with atlantoaxial fusion, retroodontoid pannus with neural element compression is managed with posterior decompression and atlantoaxial fusion or occipitocervical fusion. Cranial settling is managed can be managed with anterior decompression and posterior fusion or with dorsal only approaches. Subaxial subluxation is managed with circumferential fusion or posterior only decompression and fusion. Patients with atlantoaxial instability have better functional and neurologic outcomes. RA patients have higher complication rates and more frequent need for revision surgery than the general population of spine surgery patients.

Development and Validation of Finite Element Analysis Model (FEM) of Craniovertebral Junction: Experimental Biomechanical Cadaveric Study

STUDY DESIGN

Experimental Cadaveric Biomechanical Study.

OBJECTIVE

To establish an experimental procedure in cadavers to estimate joint stiffness/stability at craniovertebral junction (CVJ) region with various implant systems and to develop/validate an indigenous cost effective 3D-FEM (three-dimensional finite element model) of CVJ region.

SUMMARY OF BACKGROUND DATA

Finite element analysis (FEM) tools can provide estimates of internal stress and strain in response to external loading of various implant systems used in CVJ fixations.

METHODS

Experimental setup for conducting biomechanical movements on CVJ region of cadaver was developed using cost effective innovative tools. A manually actuated seven- degrees of freedom parallel manipulator motion testing system (MA7DPM) was designed and developed to impart designed trajectories and to conduct various biomechanical motion studies at CVJ region for the present study.

RESULTS

FEM model of CVJ region was developed and subsequently validated with CVJ morphometry data of 15 human subjects of Asian origin. Validated FEM was subjected to force motion studies at the CVJ region. The force-motion maps obtained from the FEM studies were subsequently validated against biomechanical experiment results from cadaveric experiment results obtained with three different implant fixations.

CONCLUSIONS

A cost effective biomechanical tool (which did not require decapitation of cadaveric head) and a customised chair (to place cadaver in sitting position during conduct of biomechanical movements simulating real-life scenario) was indigenously designed and developed. Developed biomechanical tool (MA7DPM) for this study is likely to be useful for stress-testing analysis of various implant systems for individual patients undergoing surgery at CVJ region in near future.

LEVEL OF EVIDENCE

5.

Design a novel integrated screw for minimally invasive atlantoaxial anterior transarticular screw fixation: a finite element analysis

PURPOSE

To design a new type of screw for minimally invasive atlantoaxial anterior transarticular screw (AATS) fixation with a diameter that is significantly thicker than that of traditional screws, threaded structures at both ends, and a porous metal structure in the middle. The use of a porous metal structure can effectively promote bone fusion and compensate for the disadvantages of traditional AATSs in terms of insufficient fixation strength and difficulty of bone fusion. The biomechanical stability of this screw was verified through finite element analysis. This instrument may provide a new surgical option for the treatment of atlantoaxial disorders.

METHODS

According to the surgical procedure, the new type of AATS was placed in a three-dimensional atlantoaxial model to determine the setting of relevant parameters such as the diameter, length, and thread to porous metal ratio of the structure. According to the results of measurement, the feasibility and safety of the new AATS were verified, and a representative finite element model of the upper cervical vertebrae was chosen to establish, and the validity of the model was verified. Then, finite element-based biomechanical analysis was performed using three models, i.e., atlantoaxial posterior pedicle screw fixation, traditional atlantoaxial AATS fixation, and atlantoaxial AATS fixation with the new type of screw, and the biomechanical effectiveness of the novel AATS was verified.

RESULTS

By measuring the atlantoaxial parameters, the atlantoaxial CT data of the representative 30-year-old normal adult male were selected to create a personalized 3D printing AATS screw. In this case, the design parameters of the new screw were determined as follows: diameter, 6 mm; length of the head thread structure, 10 mm; length of the middle porous metal structure, 8 mm (a middle porous structure containing an annular cylinder ); length of the tail thread structure, 8 mm; and total length, 26 mm. Applying the same load conditions to the atlantoaxial complex along different directions in the established finite element models of the three types of atlantoaxial fusion modes, the immediate stability of the new AATS is similar with Atlantoaxial posterior pedicle screw fixation.They are both superior to traditional atlantoaxial anterior screw fixation.The maximum local stress on the screw head in the atlantoaxial anterior surgery was less than those of traditional atlantoaxial anterior surgery.

CONCLUSIONS

By measuring relevant atlantoaxial data, we found that screws with a larger diameter can be used in AATS surgery, and the new AATS can make full use of the atlantoaxial lateral mass space and increase the stability of fixation. The finite element analysis and verification revealed that the biomechanical stability of the new AATS was superior to the AATS used in traditional atlantoaxial AATS fixation. The porous metal structure of the new AATS may promote fusion between atlantoaxial joints and allow more effective bone fusion in the minimally invasive anterior approach surgery.

Three dimensional finite element analysis used to study the influence of the stress and strain of the operative and adjacent segments through different foraminnoplasty technique in the PELD: Study protocol clinical trial (SPIRIT Compliant)

INTRODUCTION

Percutaneous endoscopic lumbar disectomy (PELD) is one of the most popular minimally invasive techniques of spinal surgery in recent years. At present, there are 2 main surgical approaches in PELD: foraminal approach and interlaminar approach. What's more, foraminoplasty is a necessary step for both approaches. However, there are few biomechanical studies on the formation of different parts of the intervertebral foramen. The aim of this study is to explore the effects of different foraminoplasty methods on the biomechanics of the corresponding and adjacent segments of the lumbar through a 3-dimensional finite element model analysis.

METHODS

We established a normal 3-dimensional finite element mode of L3 to L5, simulated lumbar percutaneous endoscopy by doing cylindrical excision of bone whose diameter was 7.5 mm on the L5 superior articular process and the L4 inferior articular process, respectively, so that we obtained 3 models: the first one was normal lumbar model, the second one was the L4 inferior articular process shaped model, and the third one was the L5 superior articular process shaped model. We compared the biomechanics of the intervertebral disc of L3/4 and L4/5 when they were in the states of forward flexion, backward extension, left and right flexion, and left and right rotation on specific loading condition.

DISCUSSION

If the outcomes indicate the trial is feasible and there is evidence that one of the foraminoplasty technique may make few differences in biomechanics of corresponding lumbar intervertebral disc, we will proceed to a definitive trial to test the best way to foraminplasty, which could make biomechanical influence as little as possible.

TRIAL REGISTRATION

Chinese Clinical Trial Registry, ChiCTR1900026973. Registered on September 27, 2019.

Cell migration: implications for repair and regeneration in joint disease

Connective tissues within the synovial joints are characterized by their dense extracellular matrix and sparse cellularity. With injury or disease, however, tissues commonly experience an influx of cells owing to proliferation and migration of endogenous mesenchymal cell populations, as well as invasion of the tissue by other cell types, including immune cells. Although this process is critical for successful wound healing, aberrant immune-mediated cell infiltration can lead to pathological inflammation of the joint. Importantly, cells of mesenchymal or haematopoietic origin use distinct modes of migration and thus might respond differently to similar biological cues and microenvironments. Furthermore, cell migration in the physiological microenvironment of musculoskeletal tissues differs considerably from migration in vitro. This Review addresses the complexities of cell migration in fibrous connective tissues from three separate but interdependent perspectives: physiology (including the cellular and extracellular factors affecting 3D cell migration), pathophysiology (cell migration in the context of synovial joint autoimmune disease and injury) and tissue engineering (cell migration in engineered biomaterials). Improved understanding of the fundamental mechanisms governing interstitial cell migration might lead to interventions that stop invasion processes that culminate in deleterious outcomes and/or that expedite migration to direct endogenous cell-mediated repair and regeneration of joint tissues.

Biomechanical Rationale for the Development of Atlantoaxial Instability and Basilar Invagination in Patients with Occipitalization of the Atlas: A Finite Element Analysis

OBJECTIVE

Occipitalization of the atlas (OA) often is associated with atlantoaxial dislocation and basilar invagination. The purpose of this study is to determine the biomechanical difference between normal and OA conditions in the craniovertebral junction and to further explore the rationale for development of atlantoaxial dislocation and basilar invagination using the finite element model (FEM).

METHODS

A ligamentous, nonlinear, sliding-contact, 3-dimensional FEM of the occipitoatlantoaxial complex was generated. Validation of the model was accomplished by comparing kinematic predictions with experimental data. We defined the atlantooccipital joint as a tie contact to simulate the OA deformity. The range of motion and the value of the maximum Von Mises stress were compared between the intact and OA models.

RESULTS

We found all of the predicted data in the intact FEM fell within 1 standard deviation of the cadaver data for all 6 loadings. The OA simulation significantly reduced the overall range of motion of the occipitoatlantoaxial complex at all loadings. The maximum Von Mises stress was predicted to increase at the transverse ligament and the superior articular facet of the axis for all the flexion, extension, lateral bending, and axial rotation loadings.

CONCLUSIONS

The OA could result in hypermobility of the atlantoaxial segment and cause overstress in the transverse ligament and the lateral atlantoaxial joints. These changes explain the pathogenesis of atlantoaxial dislocation and basilar invagination associated with OA. Follow-up should be scheduled regularly due to the nature of the dynamic development of atlantoaxial dislocation and basilar invagination.

Endoscopic Transnasal Odontoidectomy With Anterior C1 Arch Preservation and Anterior Vertebral Column Reconstruction in Patients With Irreducible Bulbomedullary Compression by Complex Craniovertebral Junction Abnormalities: Operative Nuance

BACKGROUND

During the past decades, the transoral transpharyngeal approach has been advocated as the standard route for the removal of odontoid causing an irreducible symptomatic neural compression. However, it may be potentially associated with a significant built-in morbidity because of the splitting of the soft palate for an adequate working angle, tracheostomy, and incision of the oral mucosa, causing exposure to a higher risk of infection by oral flora.

OBJECTIVE

To describe our experience with the minimally invasive pure endoscopic transnasal odontoidectomy in patients with bulbomedullary compression affected by complex anterior craniovertebral junction abnormalities.

METHODS

Five patients underwent a pure endoscopic neuronavigation-assisted transnasal odontoidectomy with anterior C1 arch preservation. Moreover, the anterior cervical spine column was reconstructed by filling the gap between the C1 arch and the residual C2 body with autologous/artificial bone. Neither tracheostomy nor enteral tube feeding were needed in any case.

RESULTS

A postoperative neurological improvement was observed in all patients. Postoperative imaging confirmed a satisfactory spinal cord decompression with cervical anterior column arthrodesis, and without evidence of instability at follow-up, so far.

CONCLUSION

The endoscopic transnasal approach seems to represent an efficient and safe alternative to the transoral route for the resection of odontoid process causing irreducible bulbomedullary compression. It provides a straightforward and minimally invasive natural surgical corridor to the anterior craniocervical junction, allowing a better working angle with preservation of spine biomechanics, while minimizing potential comorbidities.

Development and initial evaluation of a finite element model of the pediatric craniocervical junction

OBJECT There is a significant deficiency in understanding the biomechanics of the pediatric craniocervical junction (CCJ) (occiput-C2), primarily because of a lack of human pediatric cadaveric tissue and the relatively small number of treated patients. To overcome this deficiency, a finite element model (FEM) of the pediatric CCJ was created using pediatric geometry and parameterized adult material properties. The model was evaluated under the physiological range of motion (ROM) for flexion-extension, axial rotation, and lateral bending and under tensile loading. METHODS This research utilizes the FEM method, which is a numerical solution technique for discretizing and analyzing systems. The FEM method has been widely used in the field of biomechanics. A CT scan of a 13-month-old female patient was used to create the 3D geometry and surfaces of the FEM model, and an open-source FEM software suite was used to apply the material properties and boundary and loading conditions and analyze the model. The published adult ligament properties were reduced to 50%, 25%, and 10% of the original stiffness in various iterations of the model, and the resulting ROMs for flexion-extension, axial rotation, and lateral bending were compared. The flexion-extension ROMs and tensile stiffness that were predicted by the model were evaluated using previously published experimental measurements from pediatric cadaveric tissues. RESULTS The model predicted a ROM within 1 standard deviation of the published pediatric ROM data for flexion-extension at 10% of adult ligament stiffness. The model's response in terms of axial tension also coincided well with published experimental tension characterization data. The model behaved relatively stiffer in extension than in flexion. The axial rotation and lateral bending results showed symmetric ROM, but there are currently no published pediatric experimental data available for comparison. The model predicts a relatively stiffer ROM in both axial rotation and lateral bending in comparison with flexion-extension. As expected, the flexion-extension, axial rotation, and lateral bending ROMs increased with the decrease in ligament stiffness. CONCLUSIONS An FEM of the pediatric CCJ was created that accurately predicts flexion-extension ROM and axial force displacement of occiput-C2 when the ligament material properties are reduced to 10% of the published adult ligament properties. This model gives a reasonable prediction of pediatric cervical spine ligament stiffness, the relationship between flexion-extension ROM, and ligament stiffness at the CCJ. The creation of this model using open-source software means that other researchers will be able to use the model as a starting point for research.

C1 anterior arch preservation in transnasal odontoidectomy using three-dimensional endoscope: A case report

BACKGROUND

The transoral ventral corridor is the most common approach used to reach the craniovertebral junction (CVJ). Over the last decade, many case reports have demonstrated the transnasal corridor to the odontoid peg represents a practicable route to remove the tip of the odontoid process. The biomechanical consequences of the traditional odontoidectomy led to the necessity of a cervical spine stabilization. Preserving the inferior portion of the C1 anterior arch should prevent instability.

CASE DESCRIPTION

This is the first report in which the technique to remove the tip of the odontoid while preserving the C1 anterior arch is described by means of a three-dimensional (3D) endoscope. A 53-year-old man underwent a transnasal 3D endoscopic approach because of a complex CVJ malformation. The upper-medial portion of the C1 anterior arch was removed preserving its continuity, and the odontoidectomy was performed. After surgery, a dynamic X-ray scan showed no difference in CVJ motility in comparison with the preoperative one.

CONCLUSIONS

The stereoscopic perception augmented the precision of the surgical gesture in the deep field. The importance of a 3D view relates to the depth of field, which a two-dimensional endoscopy cannot provide. This affects the preservation of the C1 anterior arch because of the presence of critical structures that are exposed to potential damage if not displayed.

Biomechanical comparison of a novel transoral atlantoaxial anchored cage with established fixation technique - a finite element analysis

BACKGROUND

The transoral atlantoaxial reduction plate (TARP) fixation has been introduced to achieve reduction, decompression, fixation and fusion of C1-C2 through a transoral-only approach. However, it may also be associated with potential disadvantages, including dysphagia and load shielding of the bone graft. To prevent potential disadvantages related to TARP fixation, a novel transoral atlantoaxial fusion cage with integrated plate (Cage + Plate) device for stabilization of the C1-C2 segment is designed. The aims of the present study were to compare the biomechanical differences between Cage + Plate device and Cage + TARP device for the treatment of basilar invagination (BI) with irreducible atlantoaxial dislocation (IAAD).

METHODS

A detailed, nonlinear finite element model (FEM) of the intact upper cervical spine had been developed and validated. Then a FEM of an unstable BI model treated with Cage + Plate fixation, was compared to that with Cage + TARP fixation. All models were subjected to vertical load with pure moments in flexion, extension, lateral bending and axial rotation. Range of motion (ROM) of C1-C2 segment and maximum von Mises Stress of the C2 endplate and bone graft were quantified for the two devices.

RESULTS

Both devices significantly reduced ROM compared with the intact state. In comparison with the Cage + Plate model, the Cage + TARP model reduced the ROM by 82.5 %, 46.2 %, 10.0 % and 74.3 % in flexion, extension, lateral bending, and axial rotation. The Cage + Plate model showed a higher increase stresses on C2 endplate and bone graft than the Cage + TARP model in all motions.

CONCLUSIONS

Our results indicate that the novel Cage + Plate device may provide lower biomechanical stability than the Cage + TARP device in flexion, extension, and axial rotation, however, it may reduce stress shielding of the bone graft for successful fusion and minimize the risk of postoperative dysphagia. Clinical trials are now required to validate the reproducibility and advantages of our findings using this anchored cage for the treatment of BI with IAAD.

Endoscopic endonasal atlantoaxial transarticular screw fixation technique: an anatomical feasibility and biomechanical study

OBJECT The primary disadvantage of the posterior cervical approach for atlantoaxial stabilization after odontoidectomy is that it is conducted as a second-stage procedure. The goal of the current study is to assess the surgical feasibility and biomechanical performance of an endoscopic endonasal surgical technique for C1-2 fixation that may eliminate the need for posterior fixation after odontoidectomy. METHODS The first step of the study was to perform endoscopic endonasal anatomical dissections of the craniovertebral junction in 10 silicone-injected fixed cadaveric heads to identify relevant anatomical landmarks. The second step was to perform a quantitative analysis using customized software in 10 reconstructed adult cervical spine CT scans to identify the optimal screw entry point and trajectory. The third step was biomechanical flexibility testing of the construct and comparison with the posterior C1-2 transarticular fixation in 14 human cadaveric specimens. RESULTS Adequate surgical exposure and identification of the key anatomical landmarks, such as C1-2 lateral masses, the C-1 anterior arch, and the odontoid process, were provided by the endonasal endoscopic approach in all specimens. Radiological analysis of anatomical detail suggested that the optimal screw entry point was on the anterior aspect of the C-1 lateral mass near the midpoint, and the screw trajectory was inferiorly and slightly laterally directed. The custommade angled instrumentation was crucial for screw placement. Biomechanical analysis suggested that anterior C1-2 fixation compared favorably to posterior fixation by limiting flexion-extension, axial rotation, and lateral bending (p > 0.3). CONCLUSIONS This is the first study that demonstrates the feasibility of an endoscopic endonasal technique for C1-2 fusion. This novel technique may have clinical utility by eliminating the need for a second-stage posterior fixation operation in certain patients undergoing odontoidectomy.

Endoscopic endonasal odontoidectomy with anterior C1 arch preservation in elderly patients affected by rheumatoid arthritis

BACKGROUND CONTEXT

Rheumatoid arthritis is the most common inflammatory disease involving the spine with predilection for the craniovertebral segment. Surgery is usually reserved to patients with symptomatic craniovertebral junction (CVJ) instability, basilar invagination, or upper spinal cord compression by rheumatoid pannus. Anterior approaches are indicated in cases of irreducible ventral bulbo-medullary compression. Classically performed through the transoral approach, the exposure of this region can be now achieved by a minimally invasive endonasal endoscopic approach (EEA).

PURPOSE

The aim of this article is to demonstrate the feasibility of performing an odontoidectomy and a rheumatoid pannus removal by a minimally invasive EEA, preserving the anterior C1 arch continuity and avoiding a posterior fixation procedure.

STUDY DESIGN

Technical description and cohort report.

METHODS

We report three cases of elderly patients with a long history of rheumatoid arthritis and irreducible anterior bulbo-medullary compression secondary to basilar invagination and/or rheumatoid pannus. Anterior decompression was achieved by an endonasal image-guided fully endoscopic approach.

RESULTS

Neurological improvement and adequate bulbo-medullary decompression were obtained in all cases. The anterior C1 arch continuity was preserved, and none of the patients required a subsequent posterior fixation.

CONCLUSIONS

Anterior decompression by a minimally invasive EEA could represent an innovative option for the treatment of irreducible ventral CVJ lesions in elderly patients with rheumatoid arthritis. This approach permits the preservation of the anterior C1 arch and the avoidance of a posterior fixation, thus preserving the rotational movement at C0-C2 segment and reducing the risk of a subaxial instability development.

Treatment of thoracolumbar fracture with pedicle screws at injury level: a biomechanical study based on three-dimensional finite element analysis

The aim of this study was to investigate the biomechanical mechanisms of treatment of thoracolumbar compression fracture with pedicle screws at injury level based on a three-dimensional finite element method. We constructed one three-dimensional finite element model of T11-L1 in a patient with a compression fracture of the T12 vertebral body(anterior edges of vertebral body were compressed to 1/2, and kyphosis Cobb angle was 18.6°) fixed by four pedicle screws and another model fixed by six pedicle screws at the injured vertebrae, and then assigned different forces to the two models to account for axial compression, flexion, extension, left lateral bending, and rightward axial rotation by Ansys software. After different loading forces were applied to the models, we recorded stress measurements on the vertebral pedicle screws, as well as the maximum displacement of T11. The stress distribution suggested that stress concentration was appreciable at the root of the pedicle screws under different loading modalities. Under axial compression, flexion, extension, left lateral bending, and rightward axial rotation load, the stress for the superior screw was significantly greater than the stress for the inferior screw (P < 0.05). The stress in the six pedicle screw fixation model was significantly decreased compared to the four screw interbody fusion model (P < 0.05), but the maximum displacement of T11 between two models under different loadings was not statistically different. The use of pedicle screws at injured vertebral bodies may optimize internal fixation load and reduce the incidence of broken screws.

EFFECT OF MUSCLES ACTIVATION ON HEAD-NECK COMPLEX UNDER SIMULATED EJECTION

A detailed three-dimensional head-neck (C0–C7) finite element (FE) model developed based on the actual geometry of an embalmed human cadaver specimen was exercised to dictate the motions of the cervical spine under dynamic loadings. The predicted results analyzed under vertex drop impact were compared against experimental study to validate the FE model. The validated C0–C7 FE model was then further analyzed to investigate the response of the whole head-neck complex under 10G-ejection condition. From the simulation of ejection process, obvious hyper-flexion of the head-neck complex could be found. The peak flexion angles of all the lower motion segments were beyond physiological tolerance indicating a potential injury in these regions. Furthermore, the stress values in the spine were also related to the magnitudes of rotation of the motion segments. During the acceleration onset stage, the maximum stresses in the bone components were low. After that, the stress values increased sharply into the dangerous range with increased rotational angles. The effect of muscles in alleviating the potential damage in the neck is significant. It was implied that it is important for pilots to stiffen the neck before ejection to avoid severe cervical injury.

A biomechanical rationale for C1-ring osteosynthesis as treatment for displaced Jefferson burst fractures with incompetency of the transverse atlantal ligament

Nonsurgical treatment of Jefferson burst fractures (JBF) confers increased rates of C1-2 malunion with potential for cranial settling and neurologic sequels. Hence, fusion C1-2 was recognized as the superior treatment for displaced JBF, but sacrifies C1-2 motion. Ruf et al. introduced the C1-ring osteosynthesis (C1-RO). First results were favorable, but C1-RO was not without criticism due to the lack of clinical and biomechanical data serving evidence that C1-RO is safe in displaced JBF with proven rupture of the transverse atlantal ligament (TAL). Therefore, our objectives were to perform a biomechanical analysis of C1-RO for the treatment of displaced Jefferson burst fractures (JBF) with incompetency of the TAL. Five specimens C0-2 were subjected to loading with posteroanterior force transmission in an electromechanical testing machine (ETM). With the TAL left intact, loads were applied posteriorly via the C1-RO ramping from 10 to 100 N. Atlantoaxial subluxation was measured radiographically in terms of the anterior antlantodental interval (AADI) with an image intensifier placed surrounding the ETM. Load-displacement data were also recorded by the ETM. After testing the TAL-intact state, the atlas was osteotomized yielding for a JBF, the TAL and left lateral joint capsule were cut and the C1-RO was accomplished. The C1-RO was subjected to cyclic loading, ramping from 20 to 100 N to simulate post-surgery in vivo loading. Afterwards incremental loading (10-100 N) was repeated with subsequent increase in loads until failure occurred. Small differences (1-1.5 mm) existed between the radiographic AADI under incremental loading (10-100 N) with the TAL-intact as compared to the TAL-disrupted state. Significant differences existed for the beginning of loading (10 N, P = 0.02). Under physiological loads, the increase in the AADI within the incremental steps (10-100 N) was not significantly different between TAL-disrupted and TAL-intact state. Analysis of failure load (FL) testing showed no significant differences among the radiologically assessed displacement data (AADI) and that of the ETM (P = 0.5). FL was Ø297.5 +/- 108.5 N (range 158.8-449.0 N). The related displacement assessed by the ETM was Ø5.8 +/- 2.8 mm (range 2.3-7.9). All specimens succeeded a FL >150 N, four of them >250 N and three of them >300 N. In the TAL-disrupted state loads up to 100 N were transferred to C1, but the radiographic AADI did not exceed 5 mm in any specimen. In conclusion, reconstruction after displaced JBF with TAL and one capsule disrupted using a C1-RO involves imparting an axial tensile force to lift C0 into proper alignment to the C1-2 complex. Simultaneous compressive forces on the C1-lateral masses and occipital condyles allow for the recreation of the functional C0-2 ligamentous tension band and height. We demonstrated that under physiological loads, the C1-RO restores sufficient stability at C1-2 preventing significant translation. C1-RO might be a valid alternative for the treatment of displaced JBF in comparison to fusion of C1-2.

["Isolated injury" of the alar ligaments: MRI diagnosis and surgical therapy]

Spinal distortions caused by traffic collisions play a large role in medical expert opinions. Prolonged or chronic conditions present particular difficulties. The radiologist E. Volle developed and published a system for the classification of isolated injuries of the alar ligaments. As a result, surgery on the craniocervical junction was carried out in a large number of patients and the results published on multiple occasions. This article describes the anatomy of the alar ligaments, complicated injuries, the concept of the isolated lesion of the alar ligaments and their surgical management. German and international publications are evaluated.

RESULT

It was impossible to substantiate isolated injuries to alar ligaments. According to current knowledge, the published results are based on a misinterpretation of MRI findings. These results are to be considered as artefacts. There is no anatomical correlation for the classification of isolated injuries to alar ligaments. Surgical stabilisation due to an allegedly isolated injury to the alar ligaments is therefore not indicated. This statement does not apply to injuries sustained in high-speed trauma in combination with complex injuries of the atlanto-occipital and atlanto-dental-joint (joint capsules, atlanto-occipital membrane) with clear signs of instability.

The kinematic relationships of the upper cervical spine

STUDY DESIGN

A retrospective radiographic study.

OBJECTIVE

To elucidate the kinematic relationships of the upper cervical spine.

SUMMARY OF BACKGROUND DATA

To our knowledge, few reports have described the kinematic relationships of the upper cervical spine in patients with general age-related cervical spondylosis.

METHODS

We performed Kinetic magnetic resonance imaging for 295 consecutive patients experiencing neck pain without neurologic symptoms. Subjects with rheumatoid arthritis, traumatic history, and severe degenerative changes in the upper cervical spine were excluded. Anterior atlantodens interval (AADI) and the cervicomedullary angle in 3 different postures were measured, and the variations in each value between flexion and neutral (F-N), neutral and extension (N-E), and flexion and extension (F-E) were calculated. The subjects were classified into 3 groups according to the space available for the cord values (A: <or=14 mm, B: 14-15 mm, C: >or=15 mm).

RESULTS

AADI significantly increased from extension to flexion posture, however, no significant differences were observed in every posture among the groups. F-N variation in AADI showed no significant differences among the groups; however, N-E variation between Groups A and C and between Groups B and C and F-E variation between Groups A and C showed significant differences. The cervicomedullary angle significantly increased from flexion to extension posture, however, no significant differences were observed in every posture among the groups. Angle variations among the groups showed no significant differences, except for F-N angle variation between Groups B and C. None of the variations in AADI and the cervicomedullary angle were significantly correlated.

CONCLUSION

Our results suggest that only the kinematics of the atlantoaxial movement, especially the posterior movement, was greatly affected by the narrowing of space available for the cord. The central atlantoaxial joint may be closely related to the mechanisms for protecting the spinal cord by restriction of the atlantoaxial movement.

Finite element analysis of head-neck kinematics under simulated rear impact at different accelerations

The information on the variation of ligament strains over time after rear impact has been seldom investigated. In the current study, a detailed three-dimensional C0-C7 finite element model of the whole head-neck complex developed previously was modified to include T1 vertebra. Rear impact of half sine-pulses with peak values of 3.5g, 5g, 6.5g and 8g respectively were applied to the inferior surface of the T1 vertebral body to validate the simulated variations of the intervertebral segmental rotations and to investigate the ligament tensions of the cervical spine under different levels of accelerations. The simulated kinematics of the head-neck complex showed relatively good agreement with the experimental data with most of the predicted peak values falling within one standard deviation of the experimental data. Under rear impact, the whole C0-T1 structure formed an S-shaped curvature with flexion at the upper levels and extension at the lower levels at early stage after impact, during which the lower cervical levels might experience hyperextensions. The predicted high resultant strain of the capsular ligaments, even at low impact acceleration compared with other ligament groups, suggests their susceptibility to injury. The peak impact acceleration has a significant effect on the potential injury of ligaments. Under higher accelerations, most ligaments will reach failure strain in a much shorter time immediately after impact.

Anatomy and biomechanics of normal craniovertebral junction (a) and biomechanics of stabilization (b)

INTRODUCTION

A knowledge of the bony configuration, ligamentous attachments, joint articulations, vascular supply, muscle function, and lymphatic drainage as well as the kinetic anatomy of the craniocervical junction is necessary to understand the etiology of abnormalities in this area and their treatment.

RESULTS AND DISCUSSION

The craniovertebral junction (CVJ) is the most mobile of the upper cervical spine especially in children. It is uniquely adapted for stability and motion. The bony anatomy and the normal biomechanics of the CVJ in children are presented and subsequently the biomechanics of complex stabilization. Our review of more than 600 children who required stabilization is presented.

Development and validation of a finite element model of the occipito-atlantoaxial complex under physiologic loads

STUDY DESIGN

Numerical techniques were used to study the occipito-atlantoaxial complex.

OBJECTIVES

To improve previous upper cervical spine finite element models and validate both the range of motion (ROM) and neutral zone of the model.

SUMMARY OF BACKGROUND DATA

Finite element modeling is an important tool for studying the cervical spine. It has been theorized that the neutral zone may be a more sensitive parameter of spinal instability than ROM. However, the authors know of no published results by far that have validated the neutral zone of an occipito-atlantoaxial finite element complex.

METHODS

An anatomic detailed, nonlinear finite element model based on the Visible Human Male data set was developed in this study. ROM and the neutral zone were compared with published experimental data in the analysis of the motions of each vertebral level under physiologic static loadings to simulate the movements of upper cervical spine under axial rotation, flexion, extension, and lateral bending. In addition, the loads of each ligament were also recorded.

RESULTS

The moment-rotation relationship predicted by this model was apparently nonlinear, and the largest rotation was predicted in horizontal plane, followed by median plane and coronal plane. The ligaments across the complex were generally lax, and, therefore, the complex exhibited large ROMs and high proportions of the neutral zone to ROM.

CONCLUSIONS

The findings from the validation of this newly developed model coincide with the experimental studies so that its application helps contribute to a more comprehensive understanding of the biomechanics of the craniocervical region.

Medical Screening for Red Flags in the Diagnosis and Management of Musculoskeletal Spine Pain

When a patient presents with pain in the different regions of the spine, the clinician executes a region-appropriate basic examination that includes appropriate historical cues and specific physical examination tests that can be used to identify red flags. The clinical tests include a specific examination of the sensory and motor systems. Test outcomes are best interpreted in context with the entire examination profile, where the sensitivity and specificity of these tests can influence their utility in uncovering red flags. These red flags can be categorized based on the nature and severity or the specific elements of the patient's presentation. Many general red flags can be observed in any region of the spine, while specific red flags must be categorized and discussed for each spinal region. This categorization can guide the clinician in the direction of management, whether that management is aimed at redirecting the patient's care to another specialist, reconsidering the presentation and observing for clusters of findings that may suggest red flags, or managing the patient within the clinician's specialty in context with the severity of the patient's presentation.

Finite element analysis of head–neck kinematics during motor vehicle accidents: Analysis in multiple planes

In this study, a detailed three-dimensional head-neck (C0-C7) finite element (FE) model developed previously based on the actual geometry of a human cadaver specimen was used. Five simulation analyses were performed to investigate the kinematic responses of the head-neck complex under rear-end, front, side, rear- and front-side impacts. Under rear-end and front impacts, it was predicted that the global and intervertebral rotations of the head-neck in the sagittal plane displayed nearly symmetric curvatures about the frontal plane. The primary sagittal rotational angles of the neck under direct front and rear-end impact conditions were higher than the primary frontal rotational angles under other side impact conditions. The analysis predicted early S-shaped and subsequent C-shaped curvatures of the head-neck complex in the sagittal plane under front and rear-end impact, and in the frontal plane under side impact. The head-neck complex flexed laterally in one direction with peak magnitude of larger than 22 degrees and a duration of about 130 ms before flexing in the opposite direction under both side and rear-side impact, compared to the corresponding values of about 15 degrees and 105 ms under front-side impact. The C0-C7 FE model has reasonably predicted the effects of impact direction in the primary sagittal and frontal segmental motion and curvatures of the head-neck complex under various impact conditions.

Development and validation of a CO-C7 FE complex for biomechanical study

In this study, the digitized geometrical data of the embalmed skull and vertebrae (C0-C7) of a 68-year old male cadaver were processed to develop a comprehensive, geometrically accurate, nonlinear C0-C7 FE model. The biomechanical response of human neck under physiological static loadings, near vertex drop impact and rear-end impact (whiplash) conditions were investigated and compared with published experimental results. Under static loading conditions, the predicted moment-rotation relationships of each motion segment under moments in midsagittal plane and horizontal plane agreed well with experimental data. In addition, the respective predicted head impact force history and the S-shaped kinematics responses of head-neck complex under near-vertex drop impact and rear-end conditions were close to those observed in reported experiments. Although the predicted responses of the head-neck complex under any specific condition cannot perfectly match the experimental observations, the model reasonably reflected the rotation distributions among the motion segments under static moments and basic responses of head and neck under dynamic loadings. The current model may offer potentials to effectively reflect the behavior of human cervical spine suitable for further biomechanics and traumatic studies.

Finite element analysis of moment-rotation relationships for human cervical spine

A comprehensive, geometrically accurate, nonlinear C0-C7 FE model of head and cervical spine based on the actual geometry of a human cadaver specimen was developed. The motions of each cervical vertebral level under pure moment loading of 1.0 Nm applied incrementally on the skull to simulate the movements of the head and cervical spine under flexion, tension, axial rotation and lateral bending with the inferior surface of the C7 vertebral body fully constrained were analysed. The predicted range of motion (ROM) for each motion segment were computed and compared with published experimental data. The model predicted the nonlinear moment-rotation relationship of human cervical spine. Under the same loading magnitude, the model predicted the largest rotation in extension, followed by flexion and axial rotation, and least ROM in lateral bending. The upper cervical spines are more flexible than the lower cervical levels. The motions of the two uppermost motion segments account for half (or even higher) of the whole cervical spine motion under rotational loadings. The differences in the ROMs among the lower cervical spines (C3-C7) were relatively small. The FE predicted segmental motions effectively reflect the behavior of human cervical spine and were in agreement with the experimental data. The C0-C7 FE model offers potentials for biomedical and injury studies.

Atlantoaxial instability in neck retraction and protrusion positions in patients with rheumatoid arthritis

STUDY DESIGN

Radiographic analysis of the upper cervical spine was performed in patients with rheumatoid arthritis who had C1-C2 instability.

OBJECTIVE

To assess whether neck retraction or neck protrusion movements can cause C1-C2 subluxation in patients with C1-C2 instability.

SUMMARY OF BACKGROUND DATA

Cervical protrusion is the position where the head is maximally translated anteriorly with zero sagittal rotation, and this position has been shown to produce maximal C1-C2 extension. In contrast, cervical retraction is the position where the head is maximally translated posteriorly, and this position produces maximal C1-C2 flexion. To date, there have been no studies evaluating the effects of these two positions on C1-C2 status in patients with C1-C2 instability.

METHODS

Twenty-four patients with rheumatoid arthritis who showed an atlantodental interval of at least 5 mm during neck flexion were evaluated in this study. These patients were instructed to actively hold the neck in protrusion and retraction positions, as well as in flexion and extension positions. Lateral cervical radiographs were taken to measure the C1-C2 angle and the atlantodental interval in the sagittal plane in each position.

RESULTS

Retraction produced both maximal C1-C2 flexion and anterior C1-C2 subluxation, of a degree just the same as that produced by cervical flexion. Protrusion reversely produced maximal C1-C2 extension. However, 9 of 24 patients exhibited C1-C2 subluxation even in this protrusion position, in marked contrast to the cervical extension position in which only 2 of 24 patients showed C1-C2 subluxation. The patients who showed C1-C2 subluxation in the protrusion position tended to have more severe C1-C2 instability and less capacity for C1-C2 extension than the other patients who achieved a reduction of C1-C2 in the protrusion position.

CONCLUSION

In patients with C1-C2 instability, not only cervical flexion but also cervical retraction constantly led to both maximal C1-C2 flexion and subluxation. In some patients with severe C1-C2 instability, protrusion also resulted in C1-C2 subluxation, even though the C1-C2 was maximally extended.

Acute atlanto-axial post-operative subluxation following posterior C1/2 fusion

Two cases referred with acute post-operative C1/2 subluxation following posterior fusion are reported. Both cases had initial treatment for atlanto-axial instability with posterior cable (Brooks and interspinous) and graft techniques, and placed immediately in a Philadelphia collar. One case was found to have subluxed immediately post-operatively when failing to breathe following reversal of anaesthetic agents, and despite immediate realignment and reoperation was left with a significant quadriparesis. The other patient was noted to have subluxed on routine X-ray on day 4, and had no neurological deficit before or after reoperation. Risk factors for this dangerous complication are discussed and the techniques of C1/2 posterior fusion and stabilization are reviewed in detail. Surgeons performing atlanto-axial stabilization procedures should be familiar with and have expertize in the complete range of techniques described and choose the one most appropriate for the patient's individual requirements.

A retrospective radiographic analysis of subaxial sagittal alignment after posterior C1-C2 fusion

STUDY DESIGN

Subaxial sagittal alignment following atlantoaxial (A-A) posterior fusion was investigated retrospectively in patients with A-A subluxation.

OBJECTIVES

To evaluate the association between A-A fusion angle and postoperative subaxial sagittal alignment and to determine the optimal fusion angle for preservation of physiologic subaxial alignment.

SUMMARY OF BACKGROUND DATA

A-A posterior fusion has been used for patients with A-A instability and provided satisfactory clinical results. However, there are patients showing unexpected development of subaxial kyphosis after surgery. The reasons for subaxial kyphosis after A-A fusion remain unclear.

METHODS

Seventy-six patients with A-A subluxation who underwent several types of posterior A-A fusion were involved. There were 46 women and 30 men. The causes of A-A subluxation were rheumatoid arthritis in 47, trauma in 16, os odontoideum in 8, and unknown in 5. The methods of posterior fusion consisted of Magerl procedure with posterior wiring in 51, Brooks wiring in 18, and Halifax clamp in 7. Angles at C1-C2, C2-C7, and C1-C7 in the neural position were measured before surgery and at the final follow-up to find out any association between postoperative C2-C7 angle and the other radiologic parameters. The association between O-C1 range of motion and C2-C7 angle was also investigated.

RESULTS

The mean angles of C1-C2, C2-C7, and C1-C7 before surgery were 18.4 degrees, 14.5 degrees, and 32.9 degrees, respectively. Those at the final follow-up were 26.0 degrees, 5.5 degrees, and 31.5 degrees, respectively. These results indicated that C1-C2 fixation in a hyperlordotic position led to a subaxial kyphosis after surgery. Statistics showed that there was a linear association between the C1-C2 lordotic fixation angle and the C2-C7 kyphotic angle.

CONCLUSIONS

Surgical fixation of A-A joint in a hyperlordotic position will lead the lower cervical spine to a kyphotic sagittal alignment after surgery. To maintain the physiologic sagittal alignment of the subaxial cervical spine, C1-C2 should not be fixed in a hyperlordotic position.

A finite element investigation of upper cervical instrumentation

STUDY DESIGN

The finite element technique was used to predict changes in biomechanics that accompany the application of a novel instrumentation system designed for use in the upper cervical spine.

OBJECTIVE

To determine alterations in joint loading, kinematics, and instrumentation stresses in the craniovertebral junction after application of a novel instrumentation system. Specifically, this design was used to assess the changes in these parameters brought about by two different cervical anchor types: C2 pedicle versus C2-C1 transarticular screws, and unilateral versus bilateral instrumentation.

SUMMARY OF BACKGROUND DATA

Arthrodesis procedures can be difficult to obtain in the highly mobile craniovertebral junction. Solid fusion is most likely achieved when motion is eliminated. Biomechanical studies have shown that C1-C2 transarticular screws provide good stability in craniovertebral constructs; however, implantation of these screws is accompanied by risk of vertebral artery injury. A novel instrumentation system that can be used with transarticular screws or with C2 pedicle screws has been developed. This design also allows for unilateral or bilateral implantation. However, the authors are unaware of any reports to date on the changes in joint loading or instrumentation stresses that are associated with the choice of C2 anchor or unilateral/bilateral use.

METHODS

A ligamentous, nonlinear, sliding contact, three-dimensional finite element model of the C0-C1-C2 complex and a novel instrumentation system was developed. Validation of the model has been previously reported. Finite element models representing combinations of cervical anchor type (C1-C2 transarticular screws vs. C2 pedicle screws) and unilateral versus bilateral instrumentation were evaluated. All models were subjected to compression with pure moments in either flexion, extension, or lateral bending. Kinematic reductions with respect to the intact (uninjured and without instrumentation) case caused by instrumentation use were reported. Changes in loading profiles through the right and left C0-C1 and C1-C2 facets, transverse ligament-dens, and dens-anterior ring of C1 articulations were calculated by the finite element model. Maximum von Mises stresses within the instrumentation were predicted for each model variant and loading scenario.

RESULTS

Bilateral instrumentation provided greater motion reductions than the unilateral instrumentation. When used bilaterally, C2 pedicle screws approximate the kinematic reductions and instrumentation stresses (except in lateral bending) that are seen with C1-C2 transarticular screws. The finite element model predicted that the maximum stress was always in the region in which the plate transformed into the rod.

CONCLUSIONS

To the best of the authors' knowledge, this is the first report of predicting changes in loading in the upper cervical spine caused by instrumentation. The most significant conclusion that can be drawn from the finite element model predictions is that C2 pedicle screw fixation provides the same relative stability and instrumentation stresses as C1-C2 transarticular screw use. C2 pedicle screws can be a good alternative to C2-C1 transarticular screws when bilateral instrumentation is applied.

Pathomechanisms of failures of the odontoid

STUDY DESIGN

A finite element investigation to determine the causal mechanisms that lead to odontoid fracture.

OBJECTIVES

To elucidate which loading scenarios, including rotational moments, compression-tension, and lateral and anteroposterior shear, can result in Type I, Type II, and Type III odontoid failures.

SUMMARY OF BACKGROUND DATA

There is considerable controversy about the major loading path that causes odontoid fractures. A review of the clinical and laboratory research literature did not provide a consensus on this issue.

METHODS

A three-dimensional, nonlinear finite element model of the occipito-atlantoaxial (C0-C1-C2) complex was generated from human cadaveric data. Force loads were applied at the posterior margin of the occiput and were applied as lone entities or after the model was prepositioned in flexion, extension, or lateral-bending moments through applied rotation moments. Intraosseous stresses were reported to characterize the probability of fracture due to the applied loadings.

RESULTS

The data indicate that hyperextension can lead to failure of the odontoid at its superior tip (Type I). Finite element model predictions also demonstrated the propensity of loads that induce axial rotation to create relatively high maximum von Mises stress in the Type II fracture region. Flexion prepositioning reduced the stress response of the odontoid.

CONCLUSIONS

Force loading that puts the head in extension coupled with lateral shear or compression leads to Type I fractures, whereas axial rotation and lateral shear can produce Type II fractures. The model failed to elucidate causal mechanisms for Type III fractures. Flexion seems to provide a protective mechanism against force application that would otherwise cause a higher risk of odontoid failure.